Myhre, A.M., Thiede, J., Firth, J.V., et al , 1995Proceedings of the Ocean Drilling Program, Initial Reports, Vol. 151

10. SITE 9121

Shipboard Scientific Party2

HOLE 912A

Date occupied: 27 August 1993

Date departed: 28 August 1993

Time on hole: 20 hr, 45 min

Position: 79°57.557'N, 5°27.360'E

Bottom felt (drill pipe measurement from rig floor, m): 1047.9

Distance between rig floor and sea level (m): 11.14

Water depth (drill pipe measurement from sea level, m): 1036.8

Total depth (from rig floor, m): 1193.3

Penetration (m): 145.4

Number of cores (including cores with no recovery): 16

Total length of cored section (m): 145.4

Total core recovered (m): 118.37

Core recovery (%): 81.4

Oldest sediment cored:Depth (mbsf): 145.4Nature: silty clayEarliest age: Pliocene

HOLE 912B

Date occupied: 28 August 1993

Date departed: 28 August 1993

Time on hole: 10 hr, 45 min

Position: 79°57.533'N, 5°27.397'E

Bottom felt (drill pipe measurement from rig floor, m): 1048.6

Distance between rig floor and sea level (m): 11.14

Water depth (drill pipe measurement from sea level, m): 1037.5

Total depth (from rig floor, m): 1089.1

Penetration (m): 40.5

Number of cores (including cores with no recovery): 5

Total length of cored section (m): 40.5

Total core recovered (m): 41.77

Core recovery (%): 103.1

Oldest sediment cored:Depth (mbsf): 40.5Nature: silty clayEarliest age: Quaternary

'Myhre, A.M., Thiede, J., Firth, J.V., et al., 1995. Proc. ODP, Init. Repts., 151: Col-lege Station, TX (Ocean Drilling Program).

2 Shipboard Scientific Party is as given in the list of participants preceding the Tableof Contents.

HOLE 912C

Date occupied: 12 September 1993

Date departed: 13 September 1993

Time on hole: 1 day, 30 min

Position: 79°57.523'N, 5°27.363'E

Bottom felt (drill pipe measurement from rig floor, m): 1048.0

Distance between rig floor and sea level (m): 11.38

Water depth (drill pipe measurement from sea level, m): 1036.6

Total depth (from rig floor, m): 1257.1

Penetration (m): 209.1

Number of cores (including cores with no recovery): 12

Total length of cored section (m): 115.6

Total core recovered (m): 5.91

Core recovery (%): 5.1

Oldest sediment cored:Depth (mbsf): 209.1Nature: silty clayEarliest age: Pliocene

Principal results: Site 912 is located on the shallow southwestern edge of theYermak Plateau. The site was selected to study trends in Neogene andQuaternary sediment accumulation on the Yermak Plateau and to investi-gate the glacial history of the Arctic gateway. Site 912 had been plannedfor penetration through a thick sedimentary section of Quaternary andNeogene age with evidence for the glacial history of the Arctic Ocean, thehistory of the North Atlantic (West Spitsbergen Current) water influx intothe Arctic Ocean. It also was to be an intermediate member of a depthtransect.

When we arrived at the planned location, which was easily verified byseismic reflection profiling during the approach, sea ice was detected at asufficient distance to begin drilling operations. However, during APC-coring of the first hole, the ice edge advanced rapidly, forcing us to aban-don drilling operations and pull back to close to the seafloor. After the iceedge had settled down, we renewed our efforts by starting a second APChole, which to our dismay also had to be abandoned after a few cores, thistime permanently because of the eastward advancing ice.

Hole 912A reached 145.4 mbsf, and Hole 912B only 40.5 mbsf. To-ward the end of Leg 151, we returned to Site 912 and drilled Hole 912C.The aim was to RCB-core the deeper part of the section; however, thishole (with very low recoveries) also had to be abandoned after it reached209.1 mbsf, because of advancing ice. The stratigraphic record of this site,therefore, is too short to reach all scientific objectives.

Sediments recovered at Site 912 are predominantly very dark gray, un-lithified, slightly to moderately bioturbated silty clays and clayey silts ofPliocene to Quaternary age. Faint color banding, possibly of diagenetic or-igin, is present in intervals throughout. Sediment texture and mineralabundances exhibit variations of approximately 20%-30% throughout thesequence, but no major trends occur. A single lithologic unit was definedat Site 912 based on the relative uniformity in sediment type, texture, andmineralogy. The unit is characterized by higher abundances and greater

319

SITE 912

AWI-9131

Figure 1. Multichannel seismic line AWI-9131, showing location of Site 912 at shotpoint 790.

sizes of dropstones as compared with Sites 910 and 911 on the inner Yer-mak Plateau. Most of the dropstones consist of silt-, sand-, and mudstones,but metamorphic and igneous rocks are also common. Two lithologic sub-units were defined on the basis of changes in dropstone abundance, withthe Subunit IA/IB boundary placed at 40 mbsf (within the Quaternary).The uphole increase in dropstone abundance at about 40 mbsf indicates anenhancement of glacio-derived sediment transport potentially related toan intensification of Northern Hemisphere glaciation. Minor differencesin dropstone lithologies between the subunits could indicate slight chang-es in ice circulation and source areas.

Three holes recovered a Quaternary and Pliocene sequence of ice-raft-ed sediments with scattered occurrences of calcareous microfossils. Sili-ceous microfossils are absent, with the exception of rare reworkeddiatoms and silicoflagellates, rare and poorly preserved radiolarians, andrare diatoms indicative of uppermost Pliocene to lower upper Quaternary.Dinoflagellates are rare and non-age-diagnostic; terrestrial palynomorphsare common throughout. Ice-rafted Inoceramus prisms are found inPliocene sediments.

The magnetostratigraphic results yield a fairly robust age vs. depthmodel, which is consistent with biostratigraphic age determinations. Be-cause of poor core recovery, estimation of Pliocene linear sedimentationrates was not possible. Pre-Jaramillo sedimentation rates vary from 80 to100 m/m.y., but they decrease to about 30 m/m.y. during the last 1 m.y. ofdeposition.

The physical properties allowed subdivision of the sequence into threegeotechnical units. The upper one (geotechnical Unit I, to 8 mbsf) com-prises highly variable, but generally low bulk density and high porositysilty clay without dropstones. Geotechnical Unit II (8-50 mbsf) consistsof silty clay to mud with variable amounts of dropstones and increased,

but highly variable, bulk densities, as well as inversely related trends inwater content and porosity, hi Geotechnical Unit III (below 50 mbsf), re-covery is reduced, but physical properties appear more constant. The re-covered sediments suggest a hemipelagic, glacio-marine depositionalenvironment, the dominant proportion of the material being relativelyfine-grained.

BACKGROUND AND OBJECTIVES

Site 912 (proposed site YERM 2A) (see Fig. 17 of "Introduction"chapter, this volume) is located on the southwestern slope of the Yer-mak Plateau in water depths of about 1050 m, on the upper part of theslope that dips into the Molloy Rift and Spitsbergen Fracture Zone(see Figs. 6 and 7 of "Introduction" chapter, this volume). Site 912 isabout 45 km Southwest of Site 910, which is the most shallow site onthe Yermak Plateau. Site 912 is located on multichannel seismic lineAWI-9131, shotpoint 790, which has been recorded running almostNorth-South parallel to the major features of the margin, (Fig. 1). Astrong seafloor multiple is masking the deeper part of the section, buta sequence of sediments probably more than 1300 m thick can be rec-ognized, although basement is not observed. Sequences of continu-ous, strong amplitude, high-frequency, and gently southwestwarddipping reflectors can be observed in the upper part of the seismicsection. These are alternating with sequences characterized by con-tinuous reflectors of very low amplitude that sometimes contain shorthigh-amplitude segments. In the deeper part of the section the reflec-tors have a more low-frequency character with varying amplitude,and the reflectors are difficult to trace over longer distances. Several

320

SITE 912

Table 1. Coring summary, Holes 912A, 912B, and 912C.

Core

151-912A-1H2H3H4H5H6H7H

9X10X11X12X13X14X15X16X

Coring totals

151-912B-1H2H3H4H5H

Coring totals

151-912C-1R2R3R4R5R6R7R8R9R10R11R12R

Coring totals

Date(1993)

Aug. 27Aug. 27Aug. 27Aug. 27Aug. 27Aug. 27Aug. 27Aug. 27Aug. 27Aug. 27Aug. 28Aug. 28Aug. 28Aug. 28Aug. 28Aug. 28

Aug. 28Aug. 28Aug. 28Aug. 28Aug. 28

Sept. 12Sept. 12Sept. 12Sept. 12Sept. 12Sept. 12Sept. 12Sept. 12Sept. 12Sept. 12Sept. 12Sept. 12

Time(UTC)

1750180518251915194020102055212022502335003502150245032504150510

10351055111511451255

115012401345142015201615165017251820190520202115

Depth(mbsf)

0.0-4.04.0-13.5

13.5-23.023.0-32.532.5^-2.042.0-51.551.5-61.061.0-70.570.5-78.078.0-87.687.6-97.297.2-106.9

106.9-116.5116.5-126.1126.1-135.8135.8-145.4

0.0-5.35.3-14.8

14.8-24.324.3-31.031.0-40.5

93.5-103.1103.1-112.7112.7-122.4122.4-132.0132.0-141.7141.7-151.3151.3-160.9160.9-170.6170.6-180.2180.2-189.9189.9-199.5199.5-209.1

Lengthcored(m)

4.09.59.59.59.59.59.59.57.59.69.69.79.69.69.79.6

145.4

5.39.59.56.79.5

39.5

9.69.69.79.69.79.69.69.79.69.79.69.6

115.6

Lengthrecovered

(m)

4.009.939.90

10.0610.0510.2310.3310.288.728.019.620.008.440.008.650.15

118.37

5.319.889.946.669.98

41.77

0.150.080.080.030.300.030.280.760.080.000.723.40

5.91

Recovery(%)

100.0104.5104.2105.9105.8107.7108.7108.2116.383.4

100.20.0

87.90.0

89.21.6

81.4

100.2104.0104.699.4

105.1

103.1

1.60.80.80.33.10.32.97.80.80.07.5

35.4

5.1

sedimentary wedges and unconformities can be mapped, some indi-cating erosional features. As the multichannel seismic line is obliqueto the general dip direction, it is difficult to find any obvious patternto the variation in the thicknesses of the sequences. This site is locat-ed in an area swept by the West Spitsbergen Current, resulting in ahigh temporal variability of sedimentation (Manley et al., 1992).

Based on deep multichannel seismic data, Eileen (1993) suggeststhat the continent/ocean boundary is located under the upper slope inthis region, and Site 912 might be situated on oceanic crust. We referto the "Site 910" chapter (this volume) for general geological back-ground information of this region.

Scientific Objectives

Site 912 was planned as an intermediate member of a bathymetrictransect across the Yermak Plateau. The purpose of the site was tostudy the Neogene and Quaternary glacial history of the Arctic Oceanand the history of North Atlantic Ocean surface water influx to theArctic. The site was also suggested as an alternate site for YERM-1if that was not accessible. Site 912, however, was not to be drilled tobasement, and its water depth is 150 m deeper than YERM-1.

OPERATIONS

Transit to Site 912

The 35-nmi transit to Site 912 required 3.6 hr for an average speedof 9.7 kt. The seismic survey started at 1136 hr, 27 August, and cov-ered 13 nmi in 2.3 hr at 5.6 kt. A Datasonics 354B beacon wasdropped at 1256 hr, the seismic gear retrieved, and the ship returnedto location.

Hole 912A

The Fennica found some scattered ice on location and pushed itaway before the Resolution arrived. The closest ice to the locationwas a 15-nmi-long by 0.1-nmi-wide ice tongue about 5.2 nmi fromthe location at heading 335°.

An advanced hydraulic piston corer/extended core barrel (APC/XCB) bottom-hole assembly (BHA) was run. The precision depth re-corder (PDR) indicated a water depth of 1048.4 mbrf. Core 151-912A-1H established the water depth as 1047.9 mbrf. Cores 151-912A-1H through -8H were taken from 0.0 to 70.5 mbsf, with 70.5 mcored and 74.8 m recovered (106.2% recovery) (Table 1). APC-cor-ing was terminated after Core 8H because it was a very hard clay-stone. Cores were not oriented because of the high latitude. TheAdara temperature shoe was run on Cores 4H and 7H. The formationwas extremely stiff, compacted, gray, silty clay with occasional ice-rafted debris (IRD) pebbles and a hard claystone at the bottom.

The XCB-coring assembly was run, and Cores 151-912A-9Xthrough -16X were taken from 70.5 to 145.4 mbsf, with 74.9 m coredand 43.59 m recovered (58.2% recovery). The shoe of Core 151-912A-16X contained about 0.15 m of dolostone (probably a large ice-rafted dropstone).

The ice tongue had been moving South-Southwest at about 0.5 ktwith the current from the North and wind from the East. The Fennicaattempted to push and wash the ice away from the Resolution withsome success, but the ice tongue was too large and closing back to-gether. The Fennica reported it could not move one ice block 20 m ×30 m × 15 m. It appeared the ice would pass along a line 0.5 nmiSouth-Southwest of the ship, and at 0515, 28 August, the ice tongueapproached within 1.5 nmi of the ship moving at 0.3 kt. The bit waspulled to within one stand of the seafloor as a precaution. The ice

321

SITE 912

moved South-Southeast of the ship, stopped, reversed course, andseemed to stall about 1 nmi North of the ship.

Hole 912B

Hole 912B was spudded in the interim after 1 hr observing the ice.The ice seemed to be slowing down to 0.2 kt, which would allowenough time to core a second shallow APC hole for high-resolutionstudies. Core 151-912B-1H established the water depth as 1048.6mbrf. Cores 151-912B-1H through -5H were taken from 0.0 to 40.5mbsf, with 40.5 m cored and 41.77 m recovered (103.1% recovery).No cores were oriented because of the high latitude, and no tempera-tures were measured. The formation was extremely stiff, compacted,gray, silty clay with occasional IRD pebbles.

The ice had continued to move toward the ship at 0.2 kt despiteattempts by the Fennica to push and wash it back. APC-coring wasterminated after Core 151-912B-5H because the ice moved to within0.3 nmi of the Resolution, and the Fennica was unable to open a pathfor the ship or control the many 6 m × 2 m × 2 m blocks advancingon a 3-nmi front. One block of ice 20 m × 30 m × 15 m could not becontrolled at all. Additional large ice floes were seen on radar in thearea.

The pipe was pulled above the seafloor at 1350 hr. The Resolutionmoved one mile away in dynamic positioning (DP) mode to monitorthe ice, which stayed on the location until 1800 hr, when a decisionwas made to return to Site 910. The bit was pulled through the rotarytable at 2017 hr, 28 August, ending Hole 912B. Two beacons wereleft on location because they were covered by the ice field.

Return to Site 912

After Site 909 was completed, another attempt was made to coreon the Yermak Plateau. At the time, only Site 912 was open. The 87-nmi transit from Site 909 to Site 912 required 7.75 hr for an averagespeed of 11.2 kt. The site had been surveyed previously, so no seis-mic survey was conducted. The original Datasonics beacons werestill operating, but a single Benthos 210 beacon was dropped (in casethe ice returned and forced the ship off location again) at the originalglobal positioning system (GPS) position (South and East of theHoles 912A and 912B). The two Datasonic beacons that were origi-nally left on site were recalled and recovered. The Fennica reportedthat the edge of the pack ice was to the Northwest, 4.9 nmi away andmoving Southeast at 0.3 kt. The ice consisted of heavy ice up to 4 mthick, with 100% cover and ridges 2 m high.

Hole 912C

An RCB BHA was run. The water depth was 1048.0 mbrf, andHole 912C was spudded at 0700 hr, 12 September. The hole wasdrilled from 0.0 to 93.5 mbsf. Cores 151-912C-1R through -12R weretaken from 93.5 to 209.1 mbsf, with 115.6 m cored and 5.91 m recov-ered (5.1% recovery). Heat flow measurements performed in Hole912A indicated a temperature gradient of 64.8°C/km.

The ice edge continued to advance to the East-Southeast, and at2115 hr forced us to pull up to 60 mbsf with the ice only 1.69 nmiaway, moving toward us at 0.6 kt. At 2215 hr, the ice edge had ad-vanced to within 1.25 nmi, and the bit was pulled above the seafloor.The ice edge continued to advance, and it was obvious that the loca-tion could not be drilled; therefore, the bit was pulled up to the ship,clearing the rotary table at 0120 hr, 13 September.

Fennica Leaves

An ice breaker would probably not be required at the next site(EGM-2), so the Fennica was called back to the Resolution for a finalfuel gauge check, and released at 0140 hr, 13 September.

LITHOSTRATIGRAPHYIntroduction

The lithostratigraphic summary is largely based on Hole 912A(0-145.4 mbsf) because Hole 912B only reached 40.5 mbsf and Hole912C (93.5-209.1 mbsf) had a recovery of only 5.1%. Data from be-low 140 mbsf are not presented in the lithologic summary chart (Fig.2) because of the low recovery in this depth interval.

Sediments recovered at Site 912 (Holes 912A, 912B, and 912C)are predominantly very dark gray, unlithified, structureless to moder-ately bioturbated silty clays and clayey silts of Pliocene to Quaternaryage. Faint color banding, probably of diagenetic origin, is present inintervals throughout. Sediment texture and particle abundances (Fig.2) exhibit variations of ~20%-30% throughout the sequence, but nomajor trends occur.

Silty mud and clayey mud, mainly interbedded with finer-grainedsediment, occur as minor lithologies throughout the sequence (Fig.2). The mud layers are characterized by a more brownish very darkgray color and a slightly coarser texture on the split core surface. Lay-ers of dark gray clay occur only in the upper 15 m of the sequence.Inorganic carbonate grains form distinct layers, with up to 40% car-bonate, and occur as a background concentration of ~5% throughoutthe sequence (Fig. 2). Carbonate grains are typically silt-sized andsimilar to carbonates described in the site chapters for Sites 908-911.Nevertheless, in some mud layers the carbonate grains are more ir-regular in shape, and sand-sized grains occur.

A single lithostratigraphic unit was defined at Site 912 based onthe relative uniformity in sediment type, texture, and mineralogy.The unit is characterized by higher abundance of dropstones andgreater sizes of dropstones compared with Sites 910 and 911 on theinner Yermak Plateau. The amount of dropstones varies between 0and 28 per core, with the highest abundances above 40 mbsf (Fig. 3).The size distribution is fairly constant throughout the sequence, andno major trends are recorded (Fig. 4 and Table 2). Dropstones occurin all lithologies, although they are most common in mud layers.Most of the dropstones consist of siltstones, sandstones, and mud-stones, but metamorphic and igneous rocks are also common. Theabundance of biogenic material in the recovered sediments is verylow, and most intervals are barren of microfossils according to smearslide analyses. Biocarbonate is found from 0 to 38 mbsf, and biosilicais present in trace amounts in the interval 80-209.1 mbsf. No ash lay-ers are present in the sediments from this site.

Two lithologic subunits were defined on the basis of changes indropstone (>l cm) abundance (Table 3). The subunit boundary isplaced at 40 mbsf, where the dropstone abundance decreases down-hole from about 15 to

SITE 912

Hole 912A

Inorganiccarbonate

particles (%)20 40 60 80

Dropstones(per core)

5 10 15 20

Figure 2. Texture, particle abundance, inorganic carbonate particle content, and dropstone abundance (per core) vs. depth for Hole 912A. Recovery, lithostrati-graphic units, and age based on paleontology are shown.

CD

2 8O £

Hole 912ADropstones

(number per core)10 20

2 8O CD

30 ü GC

912BDropstones

(number per core)10 20 30

Figure 3. Abundance of dropstones (>l cm) in Holes 912A and 912B. The values are not normalized to 100% recovery. Note the downhole decrease in drop-stone abundance at 40 mbsf, which marks the boundary between the lithostratigraphic subunits.

mud and clayey mud, predominantly very dark gray, are interbeddedwith silty clay throughout the subunit (Fig. 5). As a result of biotur-bation, most contacts are gradational, but some mud layers havesharp bases (Fig. 5). In general, the silt and sand component of themud layers are similar to the major lithology in particle abundance.Dark gray clay occurs in some intervals, commonly as part of a nor-

mally graded fining-upward sequence from clayey mud through siltyclay. Fining-upward sequences are most common above 12 mbsf,typically with thicknesses of about 20 cm. Clay- and silt-sized car-bonate grains are present in all lithologies, varying in abundancefrom 2% to 40% (Fig. 2). The highest abundances of inorganic car-bonate particles are found in mud layers, including up to 5% sand-

323

SITE 912

Hole 912ADropstone size

(cm)CD

Hole912BDropstone size

(cm)

Figure 4. Size of dropstones (>l cm) in Holes 912A and 912B. The single dropstone found in Hole 912C is marked by a cross. Note the range of dropstone sizesbetween 1 and 8 cm and the large dropstone at 70.6 mbsf, with a length of one axis of 28 cm.

sized grains. In all lithologies the coarse fraction is dominated byquartz (Fig. 2). Feldspar, mica, opaques, and accessory minerals oc-cur in minor amounts.

The number of dropstones (diameter >l cm) per core varies be-tween 10 and 28 with an average of about 15 (Fig. 3). No obvious cor-relation is observed between dropstone abundances in Hole 912A andthose in Hole 912B, although the highest concentration seems to benear 30 mbsf and the lowest at the top of Subunit IA. The size of thedropstones (>l cm) is variable, with a diameter up to 7 cm (Fig. 4).However, most dropstones have sizes between 1 and 3 cm. The drop-stones are mainly sedimentary in origin, but igneous and metamor-phic rocks are also fairly common in this subunit (Table 2). Smear-slide data typically reveal trace abundances of foraminifers and nan-nofossils, but 2%-6% is found in four distinct layers of carbonatemud (0.65-1.3 mbsf, 12.6-12.8 mbsf, 19.56-20.0 mbsf, and 37.0-37.4 mbsf).

Lithologic Subunit IB:

Sections 151-912A-5H-6, 0 cm, through -16H-1 (40.0-145.4 mbsf), 151-912C-1R-1 through -12R-CC (93.5-209.1 mbsf)

Thickness: 169.1 mAge: Pliocene to Quaternary

Subunit IB consists of silty clay and clayey silt, in most intervalsalternating with silty mud and clayey mud. All lithologies are struc-tureless to slightly bioturbated and very dark gray in color, but themud layers tend to be more brownish. Contacts are typically grada-tional, but a few mud layers exhibit sharp bases. Color banding israre, and black iron-monosulfide pods are only scarcely present insome sections throughout Subunit IB. Sandy pods, probably burrows,composed of dark gray, gray, and olive gray sediment are scatteredthroughout. Quartz, feldspar, and accessory minerals dominate the

coarse fraction. The inorganic carbonate particle content varies from1% to 12% (Fig. 2).

The subunit is primarily distinguished by a relatively low drop-stone abundance accompanied by the absence of biocarbonate andthe presence of biosilica. The number of dropstones varies from 0 to11 per core (Fig. 3). The diameter of the dropstones is variable and ingeneral less than 8 cm (Fig. 4), but one dropstone located at 70.6 mbsfhas a thickness of 28 cm (Fig. 6). This dropstone is a sandstone, withmm-scale lamination of varying frequency and cross-stratified lami-nae. The darker laminae consist of compacted aggregates of clay-sized material, rich in organic matter. The coarse fraction (-80% ofthe grains) is dominated by quartz and feldspar in a carbonate cement.Sandstones and siltstones are the predominant type of dropstonesthroughout. Igneous and metamorphic rocks are rare compared toSubunit IA (Table 2). Smear-slide data reveal up to 2% siliceous mi-crofossils (diatoms and spicules) in this subunit, concentrated be-tween 80 and 130 mbsf and below 189 mbsf.

Interpretation

The recovered sediments suggest a hemipelagic, glacial-marinedepositional environment characterized by a large input of silt- tosand-sized siliciclastics. The input of siliciclastics has varied throughtime, but no major trends are recorded in the sequence (Fig. 2). Be-cause of the relatively shallow water depth (1040 m) and the locationon the southern Yermak Plateau, several transport modes for silici-clastic particles are possible. The siliciclastics can be deposited bymelting icebergs, from sea ice, or by downslope sediment transportand redeposition. Three observations indicate that the coarser mate-rial generally was not deposited by downslope sediment transport: (1)dropstones were observed in all lithologies, (2) coarse- and fine-grained lithologies have similar particle composition, and (3) no clearevidence is found for sedimentary structures indicative of gravity

324

SITE 912

Table 2. Occurrences of dropstones at Site 912.

Core, section,interval top

(cm)

151-912A-1H-2, 201H-3, 362H-1.212H-1.212H-1, 1182H-1, 1322H-2, 182H-3, 1052H-3, 1332H-3, 1332H-4, 642H-4, 802H-4, 1102H-4, 1362H-5, 162H-5, 252H-5, 252H-5, 1142H-6, 22H-6, 172H-6, 242H-6, 522H-6, 702H-CC, 153H-1, 183H-1.213H-1.743H-1, 843H-1, 1013H-1, 1273H-2, 343H-2, 663H-2, 1363H-3, 103H-3, 103H-3, 1353H-4,473H-5, 663H-5, 1363H-6, 873H-7, 474H-1,864H-1,924H-2, 74H-2, 234H-2, 294H-2, 1024H-2, 1374H-2, 1464H-3, 334H-3,484H-3,484H-3, 1044H-4, 164H-4, 194H-4, 204H-4, 674H-5, 514H-5, 994H-5, 1404H-5, 1404H-5, 1404H-6, 524H-6, 604H-6, 665H-1,55H-1,55H-1,55H-1.705H-1, 1315H-1, 1345H-1, 1345H-2, 555H-2, 935H-4, 105H-4, 1175H-5, 275H-5, 1415H-7, 456H-1, 1206H-1, 1216H-2, 506H-3, 1086H-4, 346H-4,476H-6, 736H-7, 76

Depth(mbsf)

1.703.364.214.215.185.325.688.058.338.339.149.309.609.86

10.1610.2510.2511.1411.5211.6711.7412.0212.2013.7813.6813.7114.2414.3414.5114.7715.3415.6616.3616.6016.6017.8518.4720.1620.9221.8722.9723.8623.9224.5724.7324.7925.5225.8725.9626.3326.4826.4827.0427.6627.6927.7028.1729.5129.9930.4030.4030.4031.0231.1031.1632.5532.5532.5533.2033.8133.8433.8434.5534.9337.1038.1738.7739.9141.9543.2043.2144.0046.0846.8446.9750.2351.76

Size(cm)

1.22.02.91.31.31.63.22.51.22.01.42.41.71.31.23.21.61.31.53.01.52.55.52.02.01.71.91.71.01.01.51.32.11.91.65.01.01.02.51.31.71.02.03.02.02.51.02.01.02.71.01.01.31.81.51.71.21.11.02.42.22.01.01.03.12.51.31.01.12.82.61.31.51.52.53.02.51.42.52.01.55.92.51.01.51.01.0

Lithology

IgneousSiltstonePlutonicPlutonicQuartzPlutonicSandstoneSiltstoneIgneousMudstonePlutonicSandstoneSiltstoneSandstoneMudstoneMudstoneSandstoneCoalSandstoneMetamorphicMudstoneSandstoneSiltstoneShaleChertLimestoneQuartzSiltstoneCalcareous siltstoneShaleSiltstoneSiltstoneSiltstoneQuartziteShaleSandstoneSiltstoneSiltstoneSandstoneCalcareous sandstoneIgneousCarbonateSandstoneSchistSlateMetamorphicMudstoneMudstoneMudstoneQuartzitePlutonicSandstonePyritic sedimentaryPlutonicSandstoneCoalSandstoneSandstoneMudstoneMetamorphicMetamorphicMetamorphicMetamorphicMetamorphicMetamorphicSiltstoneSiltstoneSiltstoneMudstoneSandstoneMudstoneSandstoneSiltstoneSiltstoneSiltstonePhylliteSiltstoneLimestoneSiltstoneSiltstoneSiltstoneSiltstoneSiltstoneSiltstoneSiltstoneQuartziteQuartzite

Core, section,interval top

(cm)

7H-1,327H-l,407H-4, 87H-4, 937H-5, 1097H-6, 237H-6, 577H-6, 707H-7, 607H-CC, 137H-CC, 308H-2, 1208H-3, 958H-4, 298H-4, 768H-6, 428H-6, 1149X-1,89X-1,729X-3, 209X-3, 7510X-3, 1410X-3, 12510X-3, 13611X-1,4711X-1,6611X-2, 5411X-2, 13111X-3, 10511X-3, 12011X-6, 37HX-6,7211X-6, 112HX-7,213X-1, 12215X-1, 13615X-2, 10616X-1,516X-1,10

151-912B-1H-1,921H-2, 121H-2, 631H-2, 651H-2, 701H-2, 1501H-3, 5lH-3,221H-3, 221H-3, 342H-2, 1402H-3, 212H-3, 242H-3, 572H-3, 1132H-4, 252H-5, 112H-5, 452H-5, 552H-5, 642H-5, 932H-6, 472H-6, 1082H-6, 1402H-7, 463H-l,303H-1,533H-1, 1293H-1, 1473H-2, 43H-2, 223H-2, 393H-3, 1453H-4, 1233H-4, 1363H-5, 93H-5, 383H-5, 863H-5, 1313H-6, 583H-6, 993H-7, 193H-7, 243H-7, 393H-7, 513H-7, 514H-2, 254H-2, 334H-2,40

Depth(mbsf)

51.8251.9056.0856.9358.5959.2359.5759.7061.1061.4761.6463.7064.9565.7966.2668.9269.6470.5871.2273.7074.2581.1482.2582.3688.0788.2690.0490.4191.6591.8095.4795.8296.2296.43

108.12127.46128.58135.85135.90

0.921.622.132.152.203.003.053.223.223.348.208.518.548.879.43

10.0511.4111.7511.8511.9412.2313.2713.8814.2014.7615.1015.3316.0916.2716.3416.5216.6919.2520.5320.6620.8921.1821.6622.1122.8823.2923.9924.0424.1924.3124.3126.0526.1326.20

Size(cm)

1.61.56.51.61.01.21.21.84.54.04.51.11.53.51.63.71.2

28.02.21.21.35.02.01.03.01.21.32.02.53.22.01.01.21.32.61.61.27.08.0

2.51.71.31.41.91.01.01.31.52.22.91.12.72.12.43.21.51.51.01.52.51.01.31.54.53.01.02.52.02.01.51.02.01.01.01.01.01.51.01.01.01.01.01.01.02.01.26.33.1

Lithology

PhylliteQuartziteShalePhylliteDolostoneShaleSiltstoneSiltstoneSlateSiltstoneLimestoneSandstoneSandstoneSandstoneSandstoneSandstoneShaleSandstoneSandstoneSandstoneSandstoneSandstoneSiltstoneChertCoalFeldsparSchistChertSiltstoneSandstoneGranitePhylliteMudstoneMudstoneShaleLimestoneSandstoneDolostoneSandstone

PlutonicPlutonicSiltstoneSiltstoneSiltstoneSandstoneFeldsparSchistSiltstoneSandstoneSiltstoneQuartziteSiltstoneSiltstoneSiltstoneSandstoneMetamorphicQuartziteSiltstoneQuartziteSiltstoneSiltstoneCoalSiltstoneSiltstoneGneissCarbonateSandstoneSiltstoneCarbonateMudstoneShaleSiltstoneSiltstoneSiltstoneSiltstoneSiltstoneMetamorphicSiltstoneSiltstoneSiltstoneMetamorphicSiltstoneMetamorphicSiltstoneSiltstoneGneissSandstoneSandstone

325

SITE 912

Table 2 (continued).

Core, section,interval top

(cm)

4H-2, 574H-2, 984H-2, 1224H-2, 1394H-2, 1424H-3, 24H-3, 144H-3, 214H-3, 254H-3, 274H-3, 344H-3, 414H-3, 524H-3, 694H-3, 904H-3, 1384H-4, 74H-4, 114H-4, 374H-4, 504H-4, 504H-4, 594H-4, 704H-4, 944H-4, 1395H-1.855H-1.945H-2, 85H-2, 105H-3, 285H-3, 455H-3, 835H-3, 1355H-4, 535H-4, 545H-4, 565H-4, 625H-4, 1325H-4, 1445H-5, 845H-5, 895H-5, 1085H-5, 1245H-7,51

Depth(mbsf)

26.3726.7827.0227.1927.2227.3227.4427.5127.5527.5727.6427.7127.8227.9928.2028.6828.8728.9129.1729.3029.3029.3929.5029.7430.1930.8530.9431.5831.6033.2833.4533.8334.3535.0335.0435.0635.1235.8235.9436.8436.8937.0837.2439.51

Size(cm)

1.02.62.31.12.43.56.53.81.41.21.21.21.41.71.01.61.51.04.51.01.01.01.01.51.01.61.21.01.22.01.11.02.51.12.12.43.12.52.81.01.02.52.51.0

Lithology

?

SandstoneSandstoneSandstoneSlateGneissSandstoneSandstoneFeldsparSiltstoneSandstoneSchist?

SandstoneFeldsparGneissSiltstoneBasalt?SiltstoneSiltstoneSiltstoneSiltstoneSiltstoneSiltstoneSiltstoneSandstoneSiltstoneSiltstoneSandstoneSiltstoneSandstoneSiltstoneSandstoneQuartziteSandstoneSandstoneChertQuartziteSiltstoneSiltstoneSiltstoneSiltstoneSiltstone

flows, contourites, or current-related deposition. Thus, we considerthe siliciclastic material to be deposited by melting icebergs and seaice. The dropstones are interpreted as deposited primarily from ice-bergs, because sea ice incorporates sand- and gravel-sized materialonly when formed near shore (Drewry, 1986).

The uphole increase in dropstone abundance at about 40 mbsf(Fig. 3) indicates an enhancement of glacially derived sedimenttransport, which could be related to an intensification of the NorthernHemisphere glaciations. The increases are mainly interpreted as in-creases in transport and melting of icebergs in the area. The minordifference in dropstone lithology between the subunits may indicateslight changes in ice circulation and source areas.

In the upper Quaternary, the number of dropstones per core isabout twice as high at this site as at Sites 910 and 911 on the innerYermak Plateau. All three sites are located at almost the same dis-

tance from Svalbard, so this difference cannot be attributed to differ-ences in distance from source areas. Instead it could indicate that Site912 was located closer to a major iceberg trajectory. The southernpart of the Yermak Plateau might be more influenced by the EastGreenland Current, which transports icebergs southward from theArctic Ocean (Thiede et al., 1990a). If Site 912 were located close tothe front between polar and North Atlantic water masses along whichmelting of icebergs occur, this could explain enhanced ice-rafted de-bris (IRD) deposition at this site.

In Subunit IA (0-40 mbsf) distinctive color banding is present insilty clay. Typically the color banding cannot be correlated to grainsize, which makes a diagenetic origin most plausible. However, someminor changes in color and lithology could possibly be attributed tovariations in climate. This is supported by the fact that the highestabundances of calcareous microfossils (2%-6% of total sediment)are restricted to four dark gray carbonate mud layers, although theabundance is too low to clearly indicate interglacial deposits.

The low content of microfossils in the sediment could have result-ed from a combination of any of the following processes: (1) low pro-ductivity, (2) removal by dissolution or bottom currents, (3)recrystallization, and (4) dilution by siliciclastic material. Low pro-ductivity related to extensive sea-ice cover and dilution by siliciclas-tics are plausible explanations, whereas the amount of carbonatedissolution and recrystallization is more uncertain and difficult totest. Inorganic carbonate particles are a minor component in all lithol-ogies (Fig. 2). One possible origin is recrystallization of calcareousnannofossils, which is supported by the presence of high abundancesof inorganic carbonate particles in the layers marked by 2%-6% cal-careous microfossils. However, at this site the inorganic carbonate ismore irregular in shape and size and often found in mud layers, whichsupports ice-rafting as a depositional mechanism for at least some ofthe carbonate grains.

The presence of iron-monosulfide throughout the sequence indi-cates strongly reducing conditions in the sediments. This correspondsto a high content of sedimentary organic matter (see "OrganicGeochemistry" section, this chapter). Bioturbation gives evidence forenough O2 near the sediment-water interface to support benthic life.Bioturbation is most pronounced in Subunit IA, which may indicateincreasing O2 levels in the bottom water during the Quaternary.

BIOSTRATIGRAPHY

Introduction

A Quaternary and Pliocene sequence of ice-rafted sediments withscattered occurrences of calcareous microfossils was recovered fromthree holes at Site 912. Figure 7 summarizes biostratigraphic results.Siliceous microfossils are absent, with the exception of rare reworkeddiatoms and silicoflagellates, and rare and poorly preserved radiolar-ians. In Sample 151-912A-13X-CC, there are rare diatoms indicativeof the uppermost Pliocene to lower upper Quaternary. Dinoflagel-lates are rare and non-age-diagnostic, whereas terrestrial pollen andspores are common throughout. Ice-rafted Inoceramus prisms (Cre-

Table 3. Summary of lithologic subunits, Site 912.

SubunitDominantlithology

Interval, mbsf(thickness, m)

Occurrence(hole-core-section)

IA Predominantly silty clay with intervals 0-40.0 Quaternary 912A-1H-1 toof clayey mud. Defined by high (40.0) 5H-5, 150 cmabundances of dropstones (10-28 per core). 912B-1H-1 to

5H-CC

IB Predominantly silty clay and clayey silt 40.0-209.1 Pliocene to 912A-5H-6,with intervals of silty mud and clayey (169.1) Quaternary 0 cm to 16H-1mud. Defined by low abundances of 912C-1R-1 todropstones (0-11 per core). 12R-3

326

SITE 912

cm

9 0 -

1 0 0 -

110—'

1 2 0 -

cm

1 0 -

Gradational' contact

2 0 -

Clayeymud

3 0 -

. Sharpcontact

Siltyclay

Figure 5. Example from Section 151-912B-1H-3 of alternating layers of darkgray silty clay and very dark gray clayey mud. The clayey mud layer exhibitsa sharp base (117-118 cm) and a gradational, bioturbated upper contact(101-104 cm).

4 0 - J

Figure 6. Close-up of a large laminated sandstone (dropstone) found at 70.6mbsf (151-912A-9X-1, 8-35 cm). The black laminae consist of compacted,black, clay-sized particles rich in organic matter. Cross-stratified laminae arepresent between 33 and 35 cm, and a water-release or load structure can beseen at ~14 cm.

327

SITE 912

taceous) are found in Samples 151-912C-4R-CC through -11R-CC inPliocene sediments.

Diatoms

Core-catcher and shipboard samples from Cores 151-912A-1Hthrough -8H are barren of diatoms, as are all core catchers from Hole909B (Cores 1H through 5H). Samples 151-912A-9X-CC (78.0mbsf) and -10X-CC (87.6 mbsf) contain rare reworked diatoms, in-cluding Eocene and upper Oligocene to Miocene forms. Sample 151-912A-13X-CC (126.1 mbsf) contains reworked forms, as above, butalso contains diatoms that may be in situ. These include few speci-mens of Thalassiosira oestrupii, T. nidulus, Proboscia barboi, andActinocyclus occulatus, which suggest the P. barboi Zone of Schrad-er and Fenner (1976), upper upper Pliocene to lower upper Quaterna-ry according to some correlations (Baldauf, 1984; 1987), althoughthe age of the P. barboi Zone/T, oestrupii Zone transition is not wellestablished. Schrader and Fenner (1976) suggest the boundary is inthe upper Pliocene. The diatom assemblage at Site 907 seems to sup-port a Pliocene age for this zonal boundary.

In addition to the above taxa, diatoms of the Pliocene T. kryophilaZone occur in Sample 151-912A-13X-CC, including single speci-mens of T. convexa, Thalassiosira aff. T. complicata, T. zabellinae,Stephanopyxis horridus, and Stephanogonium hanzawae. These taxaare interpreted as reworked from older Pliocene strata. Older re-worked diatoms in Sample 151-912A-13X-CC include the upper Oli-gocene to lower Miocene marker Rocella praenitida and numerouslong-ranging taxa.

The interpretation of/3, barboi Zone in Core 151-912A-13X is ingeneral agreement with calcareous nannofossils and planktonic fora-minifers, which suggest the Pliocene/Quaternary boundary in Core151-912A-15X-CC (135.8 mbsf) (Fig. 7), unless further studies pro-vide clear evidence that the P. barboi Zone/T, oestrupii Zone transi-tion is in the Pliocene. There was no recovery in Cores 151-912A-12X and -14X, and Cores 151-912A-15X and -16X are barren of di-atoms.

Continuous rotary coring in Hole 912C began at 93.5 mbsf, al-though recovery throughout the cored interval was very poor. The in-terval containing diatoms of the P. barboi Zone, in Core 151-912A-13X (106.9-116.5 mbsf) is not recognized in Hole 912C, probablybecause of the poor recovery in this interval. Cores 151-912C-1R and151-912C-5R through -11R are barren of diatoms. Cores 151-912C-2R through -4R contain rare, presumably reworked non-age-diagnos-tic diatoms. Core 151-912C-12R, including Samples 151-912C-12R-1, 30-32 cm, 151-912C-12R-1, 99-100 cm, and 151-912C-12R-CC(209.1 mbsf), contains rare to few diatoms, all of which may be re-worked. The assemblage in Core 151-912C-12R includes severalspecimens of the lower Eocene diatom Trinacria exculpta and onespecimen of T. simulacrum, as well as numerous long-ranging heavi-ly silicified taxa.

Silicoflagellate*

Rare reworked Eocene to Miocene silicoflagellates and unidenti-fiable silicoflagellate fragments occur in all samples from Site 912that contain reworked diatoms (see previous "Diatoms" section).Sample 151-912A-13X-CC contains Mesocena cf. M. elliptica, Nav-iculopsis sp., Dictyocha fischeri(l), and Corbisema tricantha. Sam-ple 151-912C-12R-CC contains C. tricantha, D. deflandrei, and/λfrenguellii.

Radiolarians

Radiolarians recovered from core-catcher samples at Site 912 arerare, poorly preserved, and predominantly non-age-diagnostic. Therecrystallized assemblage recovered from all three holes at this site

Hole 912A

ü

5 0 -

mbs

f)

£ 1 0 0 -

Uep

-

150-

-

200-

Hole912C

Co

re |

1R

2ROR

4R

5R6R7R8R

9R

10R

11R12R

Rec

I

• •

I Co

re

1H2H3H4H5H6H7H

8H9X10X

_MΛJ

12X

13X14Xj

16X

g l Nanno.

••

-

NN19-21

NN19

NN19

NN18/NN19

MM1R/MM1Q

ForaminifersPlanktonic

N.pachydermasin. Zπ.

N. pachydermasin. Zn.

M.oach. sin. Zn.N. sc.iuv.dex.

N. atlant sin. Zn

Benthic

Cassidulinareniforme

6µm) are found in Samples 151-912A-8H-CC and -9X-CC. On the ba-sis of these assemblages, Samples 151-912A-1H-CC through -4H-CC are correlated to upper Quaternary NN19 to NN21 Zones. Sam-ples 151-912A-5H-CC through -9X-CC are assigned to QuaternaryZone NN19 based on the co-occurrence of G. caribbeanica with P.lacunosa. Furthermore, the occurrence of large Gephyrocapsa inboth Samples 151-912A-8H-CC and -9X-CC indicates an age be-tween the Cobb Mountain Event and Olduvai Event in the MatuyamaReversed Subchron (between datums 8 and 10 of Sato et al., 1991).Below Sample 151-912A-9X-CC, calcareous nannofossil assem-blages are characterized by the presence of small Gephyrocapsa,Crenalithus doronicoides, and Coccolithus pelagicus and the ab-sence of G. caribbeanica and G. oceanica. This indicates that Core151-912A-15X lies between datums 12 and 14 (Sato et al., 1991), inor just below the Olduvai Subchron (uppermost Pliocene; NN18 toNN19 Zones).

SITE 912

Calcareous nannofossils occur in only one sample (151-912B-3H-CC) among five core-catcher samples from Hole 912B, and consistof poorly preserved and rare specimens of Gephyrocapsa sp. and G.caribbeanica. This indicates that the sample is correlated to the Qua-ternary NN19 to NN21 Zones. Nannofossils are also rare to absent inHole 912C except in Sample 151-912C-3R-CC (122.4 mbsf), whichcontains small (

SITE 912

Intensity(10-3 Am-1)

10-2 10 1020

Susceptibility(10-5 SI units)

10 102 104

GRAPE(g/cm3)

2

140

Figure 8. The variation in NRM intensity after 30-mT AF demagnetization,magnetic susceptibility, and GRAPE density with depth in Hole 912A.

Intensity Susceptibility(10-3 Am-1) (10-5 SI units)

0 1 100 1 100

Figure 9. The variation in NRM intensity after 30-mT demagnetization treat-ment and magnetic susceptibility in Hole 912B.

acteristic magnetization. Further shore-based study of discrete sam-ples from this hole should determine which magnetic phases arepresent and establish their importance as carriers of the magneto-stratigraphic signal. A step-like discontinuity in magnetic suscepti-bility also is observed in Hole 912A at about 78 mbsf (Fig. 8), nearlycoincident with the abrupt occurrence of silty mud in the lithostrati-graphic record. In this case the discontinuity probably represents adepositional change in sediment type rather than a chemical reactionprocess.

AF Demagnetization Behavior and Rock Magnetism

To examine the composition of the ferromagnetic mineralogy ofthe sediments, a combination of saturation isothermal remanent mag-netization (SIRM) and two partial anhysteretic remanent magnetiza-tions (PARMs) was imparted to the samples along three orthogonalaxes and followed by stepwise thermal demagnetization. An SIRMwas applied with a pulsed field (maximum intensity = 1.2 T) alongthe Z axis, a PARM was applied along the Y axis in the 30-mT to 95-mT coercivity window, and a final PARM was applied along the Xaxis in the 0-mT to 30-mT coercivity window. In all cases the finalremanence of the sample was dominated by the SIRM signal (the Zaxis, see Fig. 11 A). Thermal demagnetization indicated that the highcoercivity mineral that dominates the remanent-carrying potential ofthese sediments has a very discrete unblocking temperature between275°C and 350°C consistent with the magnetic properties of iron sul-fides (Fig. HB). A very small (

SITE 912

-Z.-Y

NRM2 0 0 ° c

0 300

J o = 16 A m -1

600

1.0x/XO (pARM o_ 3 0 m T )

y/Y0(pARM 3 0 _ 9 5 m T )

z/ZO (SIRM)

¥ ¥ 1200 400

Temperature (°C)

600

+Z,+YFigure 11. The results of the thermal demagnetization of composite PARMs and an SIRM applied to the x, y, and z axes of the sample (details described in thetext). A. The variation in intensity with thermal treatment. The SIRM is unblocked in an extremely narrow temperature range consistent with the SIRM beingcarried by iron sulfides. B. Orthogonal axis demagnetogram showing thermal demagnetization of sediment from 112 mbsf in Hole 912A. Open circles are theprojection of vector endpoints onto the vertical plane perpendicular to the split face of the core (xz plane). Filled circles are the projection onto the horizontalplane (xy plane). C. Plot showing the thermal demagnetization of each of the separate components of SIRM and PARM. Each component has been normalizedto its initial, pre-heating value.

low the sediment water interface down to 135 mbsf, and on Hole912B archive halves from just below the seafloor to a depth of 40mbsf. Consistent inclination vs. depth records between Holes 912Aand 912B in the region of their overlap support the notion that the in-clinations from the two holes are reliable recorders of the geomagnet-ic field. In Figure 12 we propose an interpretation of the inclinationrecord in terms of normal and reversed polarity zones. The inclina-tion record was fairly easily interpreted for most of Holes 912A and912B except at a depth of about 25 to 30 mbsf in both cores, where azone of intermediate inclinations is observed. Scattered intervals ofpoor recovery complicate the interpretation of the magnetostratigra-phy, especially in Hole 912A. Pass-through measurements on Hole912C cores did not add significantly to the magnetostratigraphicrecord because of poor recovery in that hole. The magnetic polaritystratigraphy was interpreted from the inclination of the NRM after the30-mT treatment (Fig. 12 and Table 4). In both holes the Brunhes/Matuyama (Cln/Clr) boundary is placed at the base of zones of con-sistently steep positive inclinations that are observed at the tops ofboth cores. The exact placement is complicated in both records byzones of intermediate inclinations. The Jaramillo subchronozone(Clr.ln) is observed in both holes, its top boundary being poorly de-fined at about 30 m. However, the lower boundary of the Jaramillo iswell defined by a relatively sharp transition from steep upward tosteep downward inclinations at about 35 mbsf. Below 40 mbsf, therecord from Hole 912A is predominantly of reversed polarity withonly three relatively thin normal-polarity magnetozones. We inter-pret the upper zone (54.7 to 53.8 mbsf) as the Cobb Mountain sub-chronozone and the lowermost zone (recorded only in Core 151-912A-13X) as the Olduvai subchronozone. Because of the near zerorecovery in Cores 151-912A-12 and -14, the boundaries of the Oldu-vai in Table 4 are placed at the centers of these missed recovery in-tervals. Between about 75 and 78 mbsf we observe a fairly welldefined interval of normal polarity that is difficult to reconcile withthe geomagnetic polarity time scale (GPTS) of Cande and Kent(1992). The bottom of this zone is bounded by a core break; however,the upper boundary falls within Section 151-912A-9X-3, so it is dif-ficult to explain this zone as a simple coring artifact. The reversal oc-curs at an abrupt change in susceptibility and NRM intensity. This isalso at approximately the same depth as the sharp increase in the siltfraction and the appearance of dropstones (see "Lithostratigraphy"section, this chapter). Further shore-based study will be necessary todetermine whether this magnetozone has any chronologic signifi-cance or is merely reflecting the selective remagnetization of this sed-iment interval.

Comparison Between Magnetostratigraphyand Biostratigraphy

Figure 13 indicates the age vs. depth relationship implied by thecorrelations discussed above and the GPTS of Cande and Kent(1992). According to the magnetostratigraphy, the base of the section

> Inclination -^2? o (degrees) i5O Φ Oü CC - 9 0 0 90 Q-

> Inclination| o (degrees) ^ü

SITE 912

studied paleomagnetically in Hole 912A (135 mbsf) was depositedshortly before the Olduvai subchron at approximately 2 Ma. As dis-cussed in the "Biostratigraphy" section (this chapter), the calcareousnannofossils are particularly age diagnostic at this locale, and the bio-stratigraphic ages are completely consistent with Figure 13. The oc-currence of large Gephyrocapsa in Sections 151-912A-8H-CC and151-912A-9X-CC (70.5 and 78 mbsf) is also consistent with the lo-cations of the base of the Brunhes Chronozone (Cln.o at 24.6 mbsf)and the top of the Cobb Mountain subchronozone (Clr.2r-ln.t at 53.8mbsf) in Hole 912A and the range determined for this form by Satoand Takayama (1992). Likewise, position of the nannofossil assem-blage obtained from Core 151-912A-15X, just below the Olduvaisubchronozone (the base of which is placed at 121.6 mbsf within thepoorly recovered Core 151-912A-14X) is consistent with the ob-served range of important elements of this nannofossil assemblage(Sato and Takayama, 1992). Thus the Pliocene/Pleistocene boundaryis placed at similar levels on both biostratigraphic and magnetostrati-graphic grounds. These preliminary results indicate a relatively shorttime lag between depositional processes and the acquisition of thecharacteristic magnetization. This is surprising because of the rela-tively deep diagenetic front at Site 912 (15.5 mbsf). Nevertheless, theclose agreement of the paleomagnetic and biostratigraphic ages sug-gests a more complex chemical remanence acquisition process thangenerally admitted in most previous paleomagnetic studies.

Table 4. Magnetozone boundaries and apparent sedimentation rates inHoles 912A and 912B.

Depth(mbsf)

Hole912A0

24.6

31.2

36.5

53.8

54.774.6

78107.1

121.6

Hole 912B0

24.3

30.2

36.7

Age(Ma)

0

0.780

0.984

1.049

1.201

1.212?

?

1.757

1.983

0

0.78

0.984

1.049

Sed. rate(m/m.y.)

31.5

32.4

81.5

113.8

81.8

>64.2

31.2

28.9

100.0

Chron

Cln

Clr.ln

Clr.2r-ln

?

C2n

Cln

Clr.ln

Sedimentation Rates

The poor recovery below 140 mbsf makes it impossible to esti-mate Pliocene sedimentation rates magnetostratigraphically. The av-erage pre-Jaramillo sedimentation rates depend on the exact positionof the Olduvai subchronozone boundaries, and vary from about 80 m/m.y. to 100 m/m.y. Apparent sedimentation rates at Site 912 de-creased to about 30 m/m.y. during the last 1 m.y. of deposition (seeTable 4 and Fig. 13).

Conclusions

The magnetostratigraphic results from Site 912 yield a fairly ro-bust age vs. depth model for the sediments at this site. Poor recoverywithin a couple of key zones near the Brunhes/Matuyama boundaryand surrounding the Olduvai subchronozone prevents an exact spec-ification of the ages in the section. However, despite the presence ofhigh coercive diagenetic iron sulfides in the sediment, the magneto-stratigraphy is entirely consistent with the available biostratigraphicinformation.

INORGANIC GEOCHEMISTRY

Interstitial Water

Table 5 and Figure 14 show the composition of interstitial waterscollected from Holes 912A and 912C. Sodium shows a modest dilu-tion due to early clay diagenesis, and potassium is depleted by thesame process. Magnesium decreases from the seawater value in the

150

Age (Ma)

Note: Sedimentation rates are calculated by linear interpolation between two successiveages.

Figure 13. Age-depth model based on the magnetostratigraphy of sedimentsin Hole 912A (circles) and 912B (squares). There is a large uncertainty in theOlduvai subchronozone boundaries due to poor recovery in Cores 151-912A-12Xandl51-912A-14X.

Table 5. Composition of interstitial waters in Holes 912A and 912C.

Core,section

151-912A-1H-24H-77H-710X-413X-3

151-912C-12R-1

Depth(mbsf)

2.9532.6061.2983.95

111.22

Na(mmol/L)

475442418420427

K(mmol/L)

13.410.48.58.98.5

Mg(mmol/L)

51.733.430.026.724.5

Ca(mmol/L)

10.42.43.13.53.9

Cl(mmol/L)

542539514509498

SO4(mmol/L)

26.40.00.00.00.0

NH4(µmol/L)

218519

104416002049

SiO2(µmol/L)

192387453546579

pH

7.447.727.757.757.76

7.97

Alkalinity(meq/L)

4.45112.19312.71813.52613.201

19.381

SITE 912

Na (mmol/L)

250 500 oMg (mmol/L)

30 60 0

NH4 (µmol/L)

1250 2500

ε60 -

CL

Q

120

I 1 ' / *

/ *

f '

K j Naji *

T e a ,' M g

i i

k\ \ SiO2

NH V- \4 - \

i 1 i

K (mmol/L)

pH

7.5

150

80

Ca (mmol/L)150

Cl (mmol/L)

300 6000

300 600SiO2 (µmol/L)

SO4 (mmol/L)

15 30

CO

f60 -

Q.

CDo

120

1 < l . 1 1

^ ^ A l k a l i n i t y

t 0 ~

* r

PH! "

* i

1 | > .

•

i ~

è

é1 1 1 • i l l .

0 15Alkalinity (meq/L) Figure 14. Interstitial water compositions at Site 912.

uppermost core sampled to about one half of that value at 100 mbsf.Calcium shows a rapid decrease from well above the level of ambientseawater in the first core samples down to the seawater value at about30 mbsf, and then increases slowly with increasing depth.

Ammonia increases to levels above 1000 µmol/L below 100mbsf. Silica increases to around 600 µmol/L below 100 mbsf. Thegeneral dearth of siliceous microfossils suggests that dissolved silicais controlled by the formation of opal CT and that amorphous silicahas been exhausted in these sediments.

Alkalinity and pH both rise rapidly near the surface and then leveloff, indicating that carbonate dissolution has reached some steadystate. It is worth noting that the high calcium concentration observedin these interstitial waters at the sediment-water interface is not re-flected in the alkalinity or pH profiles and may reflect an unexplainedartifact. It does not appear to be caused by carbonate dissolution, asalkalinity does not rise by an equivalent amount.

Chloride shows the same slight dilution effect as sodium becauseof the dehydration of clays. Sulfate shows a rapid decrease from sea-water levels to zero below 20 mbsf, and methane (see "OrganicGeochemistry" section, this chapter) becomes abundant at about thesame depth.

Sediment Geochemistry

Table 6 shows major element abundances measured on samplesfrom Holes 912A and 912C by X-ray fluorescence on pressed pellets.Silica (Fig. 15) is relatively constant in these sediments, and the av-erage concentration, about 60 wt%, is high. TiO2 is also constant atabout 1 wt%, indicating little variation in composition caused by di-lution by exotic components. Alumina and total iron (Fig. 15) simi-larly show little variation.

Magnesium and calcium (Fig. 15) show no major changes, al-though magnesium appears to decrease gradually with increasingdepth in the section. Sodium and potassium (Fig. 15) also appear toshow no trend, although a slight decrease in potassium below approx-imately 150 mbsf may signal the start of the shift to lower potassiumabundances noted at other sites in the region. Unfortunately, therecord is not long enough to clearly show this change.

Discussion

The chemistry of sediments at Site 912 shows only hints of varia-tion. Potassium in the solids may be shifting to lower (preglacial?)values, but the record stops too soon to tell. Sulfate reduction is pro-ceeding at a relatively rapid pace, and the formation of diagenetic sul-fides, mostly "mono-sulfides" (greigite, mackinawite andpyrrhotite),in the upper parts of the section appears to be rapid and may contrib-ute to the remanent magnetism.

The initial high calcium value mentioned previously appears inother cores as well and is not matched by an equivalent increase inalkalinity. This steep calcium gradient in the uppermost cores sug-gests the diffusion of calcium into the core tops, an unlikely process.There appears to be an artifact in the analysis of calcium in these sur-face cores, perhaps associated with the sulfate abundance. As yet, thisartifact is unexplained.

ORGANIC GEOCHEMISTRY

The shipboard organic geochemistry program at Site 912 includedanalyses of volatile hydrocarbons and determinations of inorganiccarbon, total nitrogen, total carbon, and total sulfur (for methods, see"Explanatory Notes" chapter, this volume).

SITE 912

Table 6. Major element composition of sediments in Holes 912A and 912C.

Core,section

Depth(mbsf)

SiO2(wt%)

TiO2(wt%)

A12O3(wt%)

Fe2O3(wt%)

MnO(wt%)

MgO(wt%)

CaO(wt%)

Na2O(wt%)

K2O(wt%)

P2O5(wt%)

Total(wt%)

151-912A-1H-32H-33H-34H-35H-36H-37H-38H-39X-310X-311X-313X-315X-3

151-912C-5R-CC7R-18R-111R-112R-2

SiO2

3.637.63

17.1126.6436.1045.6155.1164.5774.1481.6089.72

110.53129.28

132.17151.47161.50190.46202.16

(wt%)

58.9956.2960.1560.0160.2960.4459.2761.8459.2859.1961.6259.7759.45

61.3861.3160.0265.2860.18

0.881.000.900.910.950.950.950.970.890.900.941.001.01

0.930.950.980.781.00

15.7517.6115.8015.7316.1516.3016.3415.8915.8415.1915.8215.6917.28

16.3315.5816.8913.8617.34

AI2O3 (wt%)

6.508.846.215.806.866.468.166.616.947.116.027.297.96

6.367.627.765.297.05

0.040.080.060.040.050.050.070.040.060.050.040.060.07

0.050.070.060.040.05

3.193.583.133.313.383.233.243.132.793.162.993.402.90

3.042.972.892.422.75

2.101.922.002.102.041.941.951.932.302.011.872.161.82

2.121.881.901.631.79

MgO (wt%)

1.962.132.091.981.991.992.172.011.802.052.062.261.92

1.852.062.062.182.03

3.343.302.863.323.373.223.073.312.903.202.963.043.05

3.312.972.912.602.87

0.290.160.190.170.150.180.180.130.630.160.190.150.17

0.170.170.290.160.15

93.0394.9193.3793.3895.2294.7595.4095.8593.4493.0194.5194.8195.63

95.5495.5795.7594.2495.20

Na2O (wt%)

CD

a

250

125 -

TiO2 (wt%) Fe2O3 (wt%) CaO (wt%)

Figure 15. Sediment compositions at Site 912.

K2O (wt%)

Methane (ppm)

101 103 105Ethane (ppm)

10° 102Propane (ppm)

110L 10 102Methane/ethane

104

ë, 100Q.CD

Q

200

J J

1 i r r m T

I I , 1 1 I

Figure 16. Records of headspace and vacutainer methane (Q), ethane (C2), and propane (C3) concentrations and Q/C2 ratios in sediments from Hole 912A.Open circles = vacutainer data, and closed circles headspace data.

Volatile Hydrocarbons

As part of the shipboard safety and pollution monitoring program,concentrations of methane (Q), ethane (C2), and propane (C3) gaseswere monitored every core using standard ODP vacutainer and head-space-sampling techniques.

According to the concentrations of headspace methane, the 145.4-m-thick sedimentary section of Hole 912A can be divided into twointervals (Fig. 16, Table 7). The upper about 11.5 mbsf is character-ized by very low methane concentrations of 7-18 ppm. Between 11.5and 23 mbsf, methane concentration distinctly increased from 18 to

almost its maximum value of 40,000 ppm. First minor amounts ofheadspace ethane (2-8 ppm) and propane (1-3 ppm) were deter-mined below about 20 mbsf. The vacutainer concentrations of meth-ane are about 600,000 ppm; ethane and propane vary between 67 and138 ppm and between 3 and 5 ppm, respectively, increasing down-hole (Fig. 16, Table 7).

Q/C2 ratios derived from headspace as well as vacutainer analy-ses are high, ranging from about 4300 to 13,000 ppm, with a decreas-ing trend downhole (Fig. 16). The high Q/C2 ratios suggest in-situmicrobial production of methane from marine organic carbon, whichis probably present in major quantities (see following section). At

334

Table 7. Results of headspace and vacutainer gas analysis from Hole 912A.

Core, section,interval top

(cm)

151-912A-1H-3, 02H-6, 03H-6, 04H-1.05H-6, 06H-6, 07H-6, 08H-6, 09X-6, 010X-6, 011X-6, 013X-4, 015X-6, 0

Depth(mbsf)

3.0311.5321.0323.0340.0349.5359.0368.5378.0385.5395.13

111.43133.63

C,-HS(ppm)

718

1389139225378984428837571266452768322473271213776436602

CrHS(ppm)

0023677455478

C3-HS(ppm)

0031111111010

C,/CrHS

694613075631663275367666155374495678053954575

Q-VAC(ppm)

538672568207597994597734

C2-VAC(ppm)

678194

138

C3-VAC(ppm)

3345

C,/C2-VAC

8040701563624331

Table 8. Summary of geochemical analyses of sediments in Hole 912A.

Core, section,interval top

(cm)

151-912A-1H-1,751H-1, 1351H-2, 201H-2, 531H-2, 1242H-1.462H-3, 1022H-3, 252H-5, 182H-5, 922H-7, 213H-1.933H-3, 183H-5, 293H-7, 354H-1, 1064H-2, 1194H-3, 1044H-4, 944H-5, 1024H-6, 904H-7, 345H-1, 145H-3, 685H-5, 705H-7,416H-1, 566H-3, 1306H-5, 1086H-7, 217H-1.997H-3, 997H-5, 847H-7, 498H-1.888H-3, 878H-5, 708H-7, 269X-1, 1019X-3, 1129X-5, 2110X-1, 1810X-3, 1910X-5, 1811X-1,2411X-2, 24HX-3,2411X-5, 2411X-7, 2413X-1, 11913X-3, 11913X-5, 11915X-1.5115X-3, 2015X-5, 20

Depth(mbsf)

0.751.351.702.032.744.468.028.55

10.1812.2213.2114.4316.6819.7922.8524.0625.6927.0428.4430.0231.4032.3432.6436.1839.2041.9142.5646.3049.0851.2152.4955.4958.3460.9961.8864.8767.7070.2671.5174.6276.7178.1881.1984.1887.8489.3490.8493.8496.65

108.09111.09113.93126.61128.85131.80

TC(%)

1.281.881.160.691.481.500.492.170.741.590.550.571.951.271.281.090.910.910.691.231.281.031.281.351.570.851.441.450.951.281.221.481.441.661.131.401.731.070.771.151.461.261.331.150.871.381.430.911.161.451.041.291.651.491.00

IC(%)

0.980.550.710.450.320.890.230.460.330.400.210.241.380.300.310.280.170.200.220.340.210.210.220.280.420.220.280.210.310.220.290.290.220.210.280.200.280.290.180.220.340.500.620.160.150.260.530.180.290.390.210.290.330.270.26

TOC(%)

0.301.330.450.241.160.610.261.710.411.190.340.330.570.970.970.810.740.710.470.891.070.821.061.071.150.631.161.240.641.060.931.191.221.450.851.201.450.780.590.931.120.760.710.990.721.120.900.730.871.060.831.001.321.220.74

CaCO3(%)

8.24.65.93.72.77.41.93.82.73.31.72.0

11.52.52.62.31.41.71.82.81.71.71.82.33.51.82.31.72.61.82.42.41.81.72.31.72.32.41.51.82.84.25.21.31.22.24.41.52.43.21.72.42.72.22.2

TN(%)

0.000.100.070.060.130.080.050.120.070.110.070.070.080.090.080.100.080.080.080.080.140.100.120.110.110.090.110.130.060.110.110.110.130.120.120.120.140.080.100.120.140.130.120.140.090.130.090.090.090.120.110.110.140.130.11

TS(%)

0.140.220.140.100.170.110.050.270.050.140.000.130.440.400.760.610.850.380.871.080.640.380.380.580.320.310.320.360.410.190.340.330.570.520.110.380.120.100.050.170.140.160.130.190.110.550.150.300.200.120.190.190.200.110.07

C/N

13.36.44.08.97.65.2

14.25.8

10.84.84.77.1

11.012.08.19.28.95.9

11.07.68.28.89.7

10.47.0

10.59.5

10.09.68.4

10.89.4

12.17.1

10.010.39.75.97.78.05.85.97.18.08.6

10.08.19.68.87.59.19.49.46.7

C/S

2.16.03.22.46.85.55.26.38.28.5

2.51.32.41.31.30.91.80.50.81.72.12.81.83.62.03.63.41.55.62.73.62.12.87.73.2

12.17.8

12.05.58.04.75.45.26.52.06.02.44.38.84.35.36.6

11.110.0

Notes: IC = inorganic carbon; CaCO3 = carbonate; TC = total carbon; TOC = total organic carbon; TN = total nitrogen, TS = total sulfur; C/N = total organic carbon/total nitrogenratios; C/S = total organic carbon/total sulfur ratios.

SITE 912

335

SITE 912

Hole 912ATotal organic carbon C/N

(%)

Figure 17. Carbonate contents, total organic carbon contents, organic carbon/total nitrogen (C/N) ratios, and total sulfur contents in sediments from Hole 912A.Shaded area in the C/N record marks range of dominantly marine organic matter; C/N ratios >IO may suggest significant amounts of terrigenous organic matter.

Hole 912A, the methanogenesis has begun at shallow depths around15 mbsf, as clearly indicated by the sharp increase in methane con-centrations and the contemporaneous sharp cessation of sulfate re-duction (see "Inorganic Geochemistry" section, this chapter).Unfortunately, the sampling density is not close enough to determinemore accurately the depth of the transition between sulfate reductionand methane production.

Carbon, Nitrogen, and Sulfur Concentrations

The results of determinations of inorganic carbon, carbonate, totalcarbon, total nitrogen, and total sulfur are summarized in Table 8 andpresented in Figures 17 and 18.

Most of the carbonate values of the 145.4-m-thick sedimentarysequence of Hole 912A vary between 1.5% and 4%, with the highervalues occurring between 77 and 92 mbsf and in the upper 18m (Fig.17). Single peaks of carbonate contents as high as 7.4% to 11.5% oc-cur at 0.8,4.5, and 16.7 mbsf (Fig. 17, Table 8). The origin of the car-bonate might be inorganic and/or biogenic, because in the upper partof the hole significant amounts of foraminifers (see "Biostratigra-phy" section, this chapter) as well as inorganic carbonate (see"Lithostratigraphy" section, this chapter) exist.

The total organic carbon (TOC) values vary between 0.3% and1.7% (Fig. 17). In the upper 15 m, both the minimum and maximumvalues occur, reflecting high-amplitude variations in organic carbon

typical for this interval. According to increased C/N ratios, the peakTOC values suggest increased proportions of terrigenous organicmatter present in these sediments. Between 15 and 131.8 mbsf, TOCvalues range from 0.6% to 1.4%, with the lower values more typicalin the middle part of this interval. C/N ratios between 6 and 11 pointto the presence of significant amounts of marine organic carbon(Figs. 17 and 18; for use of C/N ratios as organic-carbon type indica-tor see Stein [1991a] and further references therein). Before an inter-pretation of the organic carbon data in terms of paleoenvironment andits change through time is possible, however, much more data aboutthe composition of the organic matter as well as flux rates are re-quired.

In general, the total sulfur values vary between 0.1% and 1.1%(Fig. 17). Based on this sulfur data, the sedimentary sequences ofHole 912A can be divided into three intervals. Sulfur values of dom-inantly 15mbsf)

Total organic carbon (%)1

Figure 18. Total organic carbon vs. total nitrogen diagram. Lines of C/Nratios of 5, 10, and 20 are indicated.

PHYSICAL PROPERTIES

Introduction

The shipboard physical properties program at Site 912 includednondestructive measurements of bulk density, bulk magnetic suscep-tibility, and total natural gamma activity on whole sections of core us-ing the multi-sensor track (MST), as well as discrete measurementsof thermal conductivity, compressional-wave velocity, shearstrength, and index properties. The downhole temperature measure-ments are also reported here. Methodology is discussed in the "Ex-planatory Notes" chapter (this volume).

Whole-core Measurements

Multi-sensor Track

Whole-core magnetic susceptibility, GRAPE density, and naturalgamma activity were measured on all of the cores from Holes 912A

336

SITE 912

| GRAPE density8 (g/cm3)DC 1.6 2

Magneticsusceptibility

(cgs)1.2 1.6

Natural gammaactivity (counts)

600 1000 1400

2 0 -

4 0 -

3H

4H

5H

Figure 19. Downhole GRAPE density (g/cm3), magnetic susceptibility (unitsare uncorrected cgs Bartington meter units), and total natural gamma activity(units are arbitrary but are based on total gamma counts recorded by a detec-tor) in Hole 912A. All data have been low-pass filtered.

and 912B using the sensors on the multi-sensor track. No MST mea-surements were performed on the whole cores from Hole 912C be-cause of poor core recovery. The MST data are displayed vs. depth inFigures 19 and 20, for Holes 912A and 912B, respectively. The MSTrecords were low-pass filtered to highlight the gross trends.

A notable feature of the GRAPE density record in Hole 912A isthe abrupt increase in bulk density from about 1.65 g/cm3 to about2.05 g/cm3 near 9 mbsf (Fig. 19). Additional abrupt changes in den-sity are observed in the upper 90 mbsf of Hole 912A, where the datacoverage is optimum; below 90 mbsf the core recovery limits the dataquality. The magnetic susceptibility is initially high, decreasesabruptly at about 15 mbsf, fluctuates above an average value of about1.2 (cgs units) in the interval from 15 to 42 mbsf. The natural gammaactivity seems to mimic the general trend of the GRAPE densitycurve, but exhibits smaller-scale fluctuations relative to the densityrecord.

The MST measurements in Hole 912B (Fig. 20) are quite similarto the data from Hole 912A in the upper 40 mbsf (Fig. 19). GRAPEdensity increases abruptly at 8 mbsf, and a well-defined density peakbetween 27 and 30 mbsf corresponds to similar features in Hole912A. Similarly, both the magnetic susceptibility decrease at 15 mbsfand the double peak near 30 mbsf are observed in the records fromeach hole; the natural gamma record is likewise comparable betweenHoles 912A and 912B.

Thermal Conductivity

Thermal conductivities measured at Site 912 are given in Table 9and are illustrated in Figures 21 and 22, for Holes 912A and 912B,respectively. The conductivity data are plotted along with downholetrends in laboratory values of dry density, void ratio, and shearstrength. Thermal conductivity measurements typically were per-formed at about 75 cm in four whole-round core sections from eachcore; a black rubber standard (cond. = 0.54 ± 0.02 W/m K) was usedeach time to evaluate the degree of error in the measurements. Theconductivities measured from the seafloor to 132.4 mbsf range from1.0 to 1.9 W/m K. Scatter in the data generally corresponds to chang-es in water content, the relative abundance of the various sedimento-logic components, and to changes in the sand/silt/clay ratio.

Q

o-

-

2 0 -

4 0 -

oü

—11H

2H

3H

4H

5H

-

m

GRAPE density(g/cm 3 )

1.6 2

Magneticsusceptibility

(cgs)1.4 1.8

Natural gammaactivity (counts)

600 1000

Figure 20. Downhole GRAPE density (g/cm3), magnetic susceptibility (unitsare uncorrected cgs Bartington meter units), and total natural gamma activity(units are arbitrary but are based on total gamma counts recorded by a detec-tor) in Hole 912B. All data have been low-pass filtered.

Downhole Temperature Measurements and Geothermal Gradient

Downhole temperatures were determined at the seafloor and attwo depths in Hole 912A using the Adara Temperature Tool; an ad-ditional measurement was performed using the downhole water sam-pler and temperature probe (WSTP). The temperature at the seafloorwas used to intercalibrate the two tools. The equilibrium temperaturewas calculated as explained in the "Explanatory Notes" chapter (thisvolume). The results are as follows: Mud line, -0.41 °C; 32.5 mbsf,1.46°C; 61.0 mbsf, 3.23°C; and 97.2 mbsf, 6.88°C. The geothermalgradient determined by a least squares fit of these data was estimatedat 64.8°C/km (Fig. 23). This thermal gradient and a range of thermalconductivity values from 1.0 to 1.9 W/m K were used to estimate theheat flow at Site 912 as between 65 and 123 W/m2.

Split-core Measurements

Compressional-wave Velocity

Relatively few velocity measurements were performed at Site912, due to poor quality of the material and its gassy nature. Thesedata are given in Table 10.

Shear Strength

Shear strength measurements were routinely performed in thesediment from split half-cores from Hole 912A and 912B using themechanical vane throughout, and using a hand-held penetrometer af-ter the sediment attained a certain minimum stiffness. The results ofthe vane and penetrometer strength measurements are given in Table11 and are illustrated in Figures 21 and 22. Strength using the vanevaried from 5 to 90 kPa in Hole 912A, and up to 250 kPa in Hole912B. Penetrometer strength measurements reported are the averageof two to three measurements at each depth.

Index Properties

Gravimetric determinations of index properties were made forsamples in Holes 912A and 912B at the rate of about 2 samples/coresection (see Table 12). The representative index properties are shown

337

SITE 912

Table 9. Thermal conductivity measurements, Site 912.

Core, section,interval top

(cm)

151-912A-1H-1,751H-2, 751H-3, 352H-2, 752H-3, 752H-5, 752H-6, 753H-2, 753H-3, 753H-5, 753H-6, 754H-2, 754H-3, 754H-6, 755H-2, 755H-3, 755H-4, 755H-5, 756H-2, 756H-3, 756H-4, 756H-5, 757H-2, 757H-2, 757H-3, 757H-4, 757H-5, 758H-2, 758H-3, 758H-4, 758H-5, 759X-2, 759X-3, 759X-4, 759X-5, 759X-5, 7510X-2, 7510X-3, 7510X-4, 7510X-5, 75HX-3,75HX-5,75HX-6,7513X-2, 7513X-3, 7513X-4, 7515X-2, 7515X-3, 7515X-4, 7515X-5, 75

151-912B-1H-1.751H-2, 751H-3, 751H-4, 282H-2, 752H-3, 752H-5, 752H-6, 753H-2, 753H-3, 753H-5, 753H-6, 754H-2, 754H-3, 754H-4, 755H-2, 755H-3, 755H-4, 755H-5, 75

Depth(mbsf)

0.752.253.356.257.7510.7512.2515.7517.2520.2521.7525.2526.7531.2534.7536.2537.7539.2544.2545.7547.2548.7553.7553.7555.2556.7558.2563.2564.7566.2567.7572.7574.2575.7577.2577.2580.2581.7583.2584.7591.3594.3595.85109.15110.65112.07128.27129.40130.90132.35

0.752.253.754.787.559.0512.0513.5517.0518.5521.5523.0526.5528.0529.5533.2534.7536.2537.75

M

FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF

FFFFFFFFFFFFFFFFFFF

P#

332338339332338339350332

339350332338350332338350339332338339350332332338339350350339

332332338339350350332338339350338339350332338339332338339350

332338339350332338339350332338339350332338339332338339350

TCcorr(W/m°K)

1.3821.3561.4111.0291.4231.5421.7781.3231.4571.1731.4321.4211.1781.1321.3961.1891.6281.2651.2221.1651.7321.8031.1011.0891.3421.5081.5351.3151.0141.3471.1901.4391.5561.0061.4521.4671.4051.2271.4411.4841.3051.3771.6681.2641.2181.4121.5591.0711.2391.342

1.1891.3631.2991.3781.0741.5291.6641.7731.5001.3351.5751.7971.3711.4461.6991.2711.4561.3751.292

Std dev(W/m K)

0.003310.004360.002560.003210.003190.002080.002430.002660.003280.009630.002780.002080.002630.003010.001760.002420.001980.002710.002000.001980.002740.002000.002720.002200.003720.002750.002550.001550.002410.001160.002090.001610.002230.005990.002290.002000.002270.002900.002760.001620.002820.003530.003130.002370.003620.003630.002320.003240.003150.00196

0.003020.002600.002660.003510.002950.004180.002000.003370.002550.003030.003890.003880.002010.001990.004560.002270.001060.002960.00200

Drift(°C/min)

0.024-0.003-0.0010.0050.0180.0040.0150.0250.0210.0100.0140.0210.0100.0370.016

0.0270.0130.025

0.0230.0270.0230.0200.0100.0240.0160.0100.0110.0070.0220.0130.029

-0.0300.0210.0240.0280.020

-0.0010.0260.0330.0340.0390.0210.0300.0110.025

-0.0030.0180.003

0.0200.0180.0070.0210.0110.0070.035

-0.0030.0150.0060.0350.0200.0110.0100.0160.0190.0020.011

-0.012

Notes: M = method used, F = full-space or H = half-space; P # = probe number for test; TCcorr = thermal conductivity corrected for drift; Std dev = standard deviation of measurement.

in Figures 21 and 24 for Hole 912A (0-145.4 mbsf) and Figures 22and 25 for Hole 912B (0^0.5 mbsf).

Geotechnical Units

Statistical data abstracted from the index properties determined inthe laboratory were used to construct a series of units with respect togeotechnical properties. Three geotechnical units were identified inHole 912A: an upper zone of highly variable but generally low bulkdensities and high porosities, consisting of silty clay without drop-

stones from the seafloor to ~8 mbsf (geotechnical Unit G-I); a zoneof silty clay to silty mud having variable amounts of dropstones be-tween 8 and about 50 mbsf, with increased but highly variable bulkdensity values and inversely related trends in water content and po-rosity (geotechnical Unit G-II); and a zone below about 50 mbsfwhere the physical properties seem to become more constant and thecore recovery is reduced (geotechnical Unit G-III).

Ms 151IR-110

338

SITE 912

Dry density

(g/cm3)1.0 1.5 2.0

Thermal conductivity(W/m K)

1.0 1.5 2.0

50

Q.CD

a100

150

Method Ci i i i

Void ratio

1.0 2.0 0I

Strength(kPa)

50 100

J®

tl OFigure 21. Comparison of laboratory index property mea-surements, thermal conductivity, and peak shear strengthvs. depth in Hole 912A. Shear strength was measuredusing the mechanical vane (closed circles) and using thehand-held penetrometer (open squares).

Dry density Thermal conductivity(g/cm3) (W/m K) Void ratio

0.6 1.4 2.21.0 1.5 2.00.0 1.0 2.0

10 -

3.00

Strength(kPa)125 250

CDQ

20 -

3 0 -

40

P "

B'

Φ

\

? -Method C ^

— n . - —

to -

•?

1

1 V 1

. i i '

•

• -

•

•

•

1 1 1

1 1 ' B-•-

%

α.

— •ú

1P

a,

S

\- i

p Method Bi A i 1 i

•

— —••m

an

»

v i i i I i i i i

Figure 22. Comparison of laboratory index property mea-surements, thermal conductivity, and peak shear strengthvs. depth in Hole 912B. Shear strength was measuredusing the mechanical vane (closed circles) and using thehand-held penetrometer (open squares).

Geothermal gradientHole 912A (0-97.2 mbsf)

Temperature (°C), - 1 . 1 3 5

Table 10. Compressional-wave velocity measurements, Site 912.

20

j§40E

á60

80

100

64.8 °C/ km -

Y = -0.5371 + 0.0648X

Figure 23. Downhole temperature measurements in Hole 912A.

Core, section,interval

(cm)

151-912A-1H-1, 100-1071H-2, 50-571H-3, 24-312H-3,45-523H-2,42-49

151-912B-1H-1,50-571H-2, 50-571H-3,43-502H-1,50-572H-2, 50-572H-3, 50-572H-5, 50-572H-6, 50-573H-1,40-473H-2, 53-603H-3, 32-39

Depth(mbsf)

1.002.003.247.45

15.42

0.502.003.435.807.308.80

11.8013.3015.2016.8318.12

Tool

dsvdsvdsvdsvdsv

dsvdsvdsvdsvdsvdsvdsvdsvdsvdsvdsv

Velocity (m/s)

Perp Par

15651543159915561599

15581567151015601539156814841626161515921586

Note: Tool = device used for measurement, either dsv = digital sound velocimeter orham = Hamilton Frame. Perp = perpendicular, Par = parallel.

339

SITE 912

Table 11. Shear strength measurements, Site 912.

Core, sectioninterval (cm)

151-912A-1H-1, 105-1061H-2, 55-561H-3, 28-292H-1, 84-852H-1, 122-1232H-2, 115-1162H-4, 4 5 ^ 62H-6, 119-1203H-2, 4 5 ^ 63H-4, 33-343H-6, 53-544H-3, 65-664H-4, 92-934H-6, 35-365H-2, 103-1045H-6, 110-1116H-1, 110-1116H-4, 95-966H-7, 47^187H-1, 100-1017H-3, 100-1017H-5, 86-877H-7, 25-268H-1,85-868H-3, 85-868H-5, 10-118H-7, 65-669H-1, 136-1379H-2, 110-1119H-3, 111-1129H-3, 120-1219H-4, 110-1119H-5, 118-1199H-6, 50-5110H-1, 110-11110H-2, 110-11111H-3, 110-11111H-4, 110-11111H-5, 110-11111H-1, 110-11111H-2,25-2611H-2,110-11111H-3, 92-9311H-4,45^1611H-4, 110-11111H-5, 110-11113H-1, 110-11113H-2, 110-11113H-3, 110-11113H-4, 110-111

Depth(mbsf)

1.052.053.284.845.226.658.95

12.6915.4518.3321.5326.6528.4230.8535.0341.1043.1047.4551.4752.5055.5058.3660.7561.8564.8567.1070.6571.8673.1074.7074.6176.1077.6878.5079.1080.6088.7090.2089.3591.7091.5293.2093.2092.5594.7094.70

108.00109.50111.00112.42

SN

1

:::

lllllllll3*33333333333333333333;5

Vane(kPa)

7889

179

2016203147213815913359393940495331456459834148503743538432324532302324385233565125384451

Res(kPa)

4456

102

1089

1620

9207

60141918151418

1316581827191823201817

151419141412121927151523131420

Pen(kPa)

7251473736

2651667239414225266964

3240

27

44

3662

Core, sectioninterval (cm)

13H-5, 110-11113H-5, 125-12615H-2, 100-10115H-3, 111-11215H-4, 110-11115H-5, 110-11115H-6, 110-111

151-912B-1H-1, 110-1111H-2, 110-1111H-3, 110-1111H-4, 40-412H-1, 112-1132H-2, 110-1112H-3, 110-1112H-4, 110-1112H-5, 116-1172H-6, 118-1193H-1,43^43H-2, 55-563H-3, 35-363H-4, 76-783H-5, 66-673H-6, 90-913H-7,6-74H-1, 111-1124H-2, 30-314H-2, 74-754H-2, 109-1104H-3, 40-414H-3, 76-834H-3, 113-1144H-3, 120-1214H-4,44-^54H-4, 69-704H-4, 102-1034H-4, 125-1265H-1,61-625H-2, 12-135H-2, 78-795H-4, 85-865H-6, 82-83

Depth(mbsf)

113.99113.84128.52129.76131.25132.70134.20

1.102.604.104.906.427.909.40

10.9012.4613.9815.2316.8518.1520.0621.4623.2023.8625.4126.1026.5426.8927.7028.0628.4328.5029.2429.4929.8230.0531.6132.6233.2836.3539.32

SN

3333333

11

::::::lllllllllll

34

4

111111

Vane(kPa)

77646147856476

67

15121110

2115222029423663

6804831185922

178129

245

213799442631

Res(kPa)

25252520242123

577665

12106

12108

23192730

91040

6

8060

100

91567191317

Pen(kPa)

695459

697274

101

1018374

52231212

Note: SN = numbered spring used to make the measurement. Vane = undrained shearstrength; Res = residual shear strength; Pen = unconfined shear strength as mea-sured by the penetrometer. To obtain more accurate penetrometer values, multiplyPen value by 0.98067 (see "Physical Properties" section in the "Explanatory Notes"chapter, this volume).

NOTE: For all sites drilled, core-description forms ("barrel sheets") and core photographs canbe found in Section 6, beginning on page 465. Forms containing smear-slide data can be foundin Section 7, beginning on page 849. Thin-section descriptions are given in Section 8, beginningon page 885, and sediment thin sections in Section 9, beginning on page 895. Dropstone descrip-tions are included in Section 10, beginning on page 903.

340

SITE 912

Table 12. Index property measurements, Site 912.

Core, section,interval (cm)

151-912A-1H-1,77-791H-1,132-1341H-2, 18-201H-2, 54-561H-2, 122-1241H-3, 39-412H-1,47-492H-1, 122-1242H-2, 14-162H-2, 37-392H-2, 114-1162H-3, 104-1062H-3, 123-1252H-4, 15-172H-4,45^*72H-4, 142-1442H-5, 20-222H-5, 127-1292H-6, 26-282H-6, 118-1202H-7, 22-243H-1,94-963H-1, 125-1273H-2,45^73H-2, 125-1273H-3, 18-203H-3, 106-1083H-4, 34-363H-4, 143-1453H-5, 28-303H-5, 100-1023H-6, 53-553H-6, 136-1383H-7, 35-374H-1,33-354H-1, 105-1074H-2,41-434H-2, 118-1204H-3, 23-254H-3, 104-1064H-4, 36-384H-4, 96-984H-5, 17-194H-5, 103-1054H-6, 52-544H-6, 90-924H-7, 35-375H-1.9-115H-1,60-625H-2, 15-175H-2, 102-1045H-3, 65-675H-3, 136-1385H-4, 74-765H-4, 114-1165H-5, 67-695H-6, 58-605H-7, 3 8 ^ 06H-1, 53-556H-2, 30-326H-2, 117-1196H-3, 55-576H-3, 129-1316H-4,43-456H-4, 81-836H-5, 37-396H-5, 107-1096H-6, 50-526H-6, 116-1186H-7, 25-277H-1, 101-1037H-2, 101-1037H-3, 101-1037H-4, 101-1037H-5, 85-877H-6, 101-1037H-7, 51-538H-1,86-888H-2, 86-888H-3, 86-888H-4, 107-1098H-5, 68-708H-6, 69-718H-7, 23-259X-1, 100-1029X-2, 20-229X-2, 110-1129X-3, 20-22

Depth(mbsf)

0.771.321.682.042.723.394.475.225.645.876.648.048.238.658.959.92

10.2011.2711.7612.6813.2214.4414.7515.4516.2516.6817.5618.3419.4319.7820.5021.5322.3622.8523.3324.0524.9125.6826.2327.0427.8628.4629.1730.0331.0231.4032.3532.5933.1034.1535.0236.1536.8637.7438.1439.1740.5841.8842.5343.8044.6745.5546.2946.9347.3148.3749.0750.0050.6651.2552.5154.0155.5157.0158.3560.0161.0161.8663.3664.8666.5767.6869.1970.2371.5072.2073.1073.70

WC-d(ft)

63.3342.3743.0946.8572.2355.9866.9232.1643.3933.4464.3630.6044.6942.1628.6443.8836.1428.5445.5941.7546.5244.8925.7130.0125.2035.4633.0644.6641.3925.9226.0231.2130.5329.5321.0742.1330.8433.2837.9337.7123.9137.1133.9321.0421.4638.7138.0320.9838.4734.1529.2743.9332.1032.5734.2536.7028.7934.0348.2421.4729.2941.3137.8329.8125.0536.4321.8528.8824.0832.2037.7041.0738.8637.3233.9132.5431.0743.6539.5936.2029.5234.8425.7921.3634.2734.7233.0733.48

WC-w(%)

38.7729.7630.1131.9041.9435.8940.0924.3330.2625.0639.1623.4330.8929.6622.2630.5026.5522.2031.3129.4531.7530.9820.4523.0820.1326.1824.8530.8729.2720.5820.6523.7823.3922.8017.4029.6423.5724.9727.5027.3819.2927.0725.3317.3817.6727.9127.5517.3427.7825.4622.6430.5224.3024.5725.5126.8522.3625.3932.5417.6722.6629.2327.4522.9620.0326.7017.9322.4119.4024.3527.3829.1227.9927.1825.3224.5523.7130.3928.3626.5822.7925.8420.5017.6025.5225.7724.8525.08

WBDa

(g/cm3)

1.631.841.811.791.631.751.631.981.831.831.611.951.851.842.021.741.952.001.781.801.811.741.981.871.881.781.861.621.612.042.031.941.972.022.121.761.791.791.812.012.071.761.962.142.121.891.912.171.811.732.021.861.841.741.941.932.001.951.612.131.991.891.912.002.081.932.152.042.101.911.921.901.981.411.961.962.031.681.921.952.031.972.062.171.961.941.771.96

WBDb

(g/cm3)

1.681.821.811.811.621.731.641.951.831.921.681.951.811.851.971.811.901.961.821.821.781.792.021.912.011.861.921.741.791.992.001.911.951.971.971.821.921.921.861.782.041.871.932.092.091.861.882.121.861.911.951.831.941.931.901.881.981.911.692.071.961.861.861.982.031.892.111.982.071.941.881.851.841.861.921.911.961.821.861.891.961.902.032.091.931.911.951.92

GDa

(g/cm3)

2.592.762.692.762.822.902.712.822.772.492.532.692.902.772.802.512.882.742.672.622.832.532.602.472.372.422.542.182.112.742.722.702.752.822.742.512.322.392.553.162.752.402.842.782.752.792.832.832.582.272.822.882.462.252.802.852.752.822.232.772.752.902.832.802.802.852.822.862.812.642.862.933.091.642.842.792.922.322.922.902.862.892.782.852.862.822.322.81

GDb

(g/cm3)

2.832.722.692.812.772.832.762.742.772.702.862.692.762.802.692.742.752.662.812.682.692.692.692.592.652.632.702.542.602.632.662.612.692.722.442.712.642.702.692.462.682.702.752.672.692.712.742.732.712.702.662.802.722.722.692.722.712.692.452.642.672.812.692.752.702.732.742.712.742.732.742.762.662.672.722.652.732.742.742.712.692.712.712.692.762.722.772.70

DBDa

(g/cm3)

1.001.291.261.220.941.120.981.501.271.380.981.491.281.291.571.211.431.551.221.271.241.201.571.431.501.321.401.121.141.621.611.481.511.561.751.241.371.341.311.461.671.281.461.771.741.361.381.791.311.291.561.291.391.311.451.411.551.461.091.751.541.341.381.541.661.421.761.591.691.441.391.351.421.031.461.481.551.171.371.431.571.461.641.791.461.441.331.47

DBDb

(g/cm3)

1.031.281.261.230.941.110.991.471.281.441.021.491.251.301.541.261.401.531.251.281.211.241.611.471.601.381.441.211.271.581.591.451.491.521.631.281.471.441.351.291.651.371.441.731.721.341.361.751.341.421.511.271.471.461.421.381.541.421.141.701.511.321.351.531.631.381.731.541.671.471.361.311.321.351.431.441.501.271.331.381.511.411.611.731.431.411.461.44

Por3

(%)

61.5453.2953.1155.7766.5661.2863.8746.9753.9444.8661.3444.5455.8553.2543.9351.8250.4143.2854.3251.6056.2052.5739.5142.0136.8345.5345.0848.7245.9540.9640.8645.0845.0644.8636.0250.7641.1643.6548.5353.7439.0546.4448.4836.3436.5051.3451.2236.7249.2043.0444.6055.2743.5641.6848.3150.5543.6148.3551.2036.7344.0453.9351.0844.8840.6050.3537.5744.6639.7845.3051.2454.0153.9937.3348.4546.9346.9749.6653.0050.5645.1949.5541.1537.2548.8648.8642.8247.88

Porb

(%)

63.6252.9353.1156.2066.1260.6964.3446.1953.9946.8664.2144.5354.6353.5042.9053.9649.2342.5555.5352.2155.0054.0940.2643.1239.4147.5946.5552.5451.2639.9140.3444.2944.5043.9133.4452.6544.2646.7249.9047.5338.4549.4447.6435.4436.0050.5950.4435.8650.4347.4043.1854.5545.9746.3647.3449.3543.2047.1953.5535.6543.2453.0749.7944.4339.7549.2136.8943.3139.1246.1350.1552.5450.2149.2847.3645.6745.3253.8951.4248.9043.6247.9540.5135.9547.9647.9147.2246.90

VRa

1.601.141.131.261.991.581.770.8