20. geochemistry of the cretaceous/tertiary boundary at ...€¦ · proceedings of the ocean...

Weissel, J., Peirce, J., Taylor, E., Alt, J., et al., 1991Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 121

20. GEOCHEMISTRY OF THE CRETACEOUS/TERTIARY BOUNDARY AT HOLE 752B,BROKEN RIDGE1

Robert M. Owen2 and Andrew R. B. Zimmerman2

ABSTRACT

Statistical analyses of sediment geochemical data from a Cretaceous/Tertiary (KT) boundary section at Broken Ridgeindicate that the sediments here are composed of three major geochemical end-members: (1) a biogenic/dissolution residuecomponent; (2) a volcanogenic component; and (3) a diagenetic/alteration product component. The KT boundary at BrokenRidge is overlain by a 6-m-thick ash-rich layer. Relative flux comparisons of the three geochemical end-members suggest,however, this layer is the result of a dramatic (11-fold) decrease in biological productivity immediately following the KTboundary event, while both the flux and composition of the volcanogenic component remain constant across the KTboundary. The volcanogenic component apparently is derived from a local source, and we find no evidence of a causal linkbetween volcanic activity and the KT boundary event. The flux of the diagenetic component, which is enriched in Au,increases sharply at the KT boundary. All other elements associated with this component are present in significant amountsin the other two geochemical end-members. With the possible exception of Au, then, geochemical anomalies in thesediments just below the KT boundary appear to result from diagenetic alteration of biogenic and volcanogenic sedimentprecursors.

INTRODUCTION

An unexpected accomplishment of drilling at Broken Ridgewas the recovery of an expanded section of sediment across theCretaceous/Tertiary boundary (KT boundary) at Hole 752B. Wehave conducted a geochemical analysis of this sedimentary sec-tion to obtain information relevant to the ongoing discussionconcerning the probable causes and ultimate effects of global-scale processes which occurred during this period of earth history.

Analyses of KT boundary sediments from many locations arecharacterized by geochemical anomalies involving enrichmentsin Ir and other noble metals (Au, Pd, Pt), as well as certainsiderophile (Cr, Co, Ni) and chalcophile (As, Sb, Zn, Cu, Se)elements (Alvarez et al., 1980; Kyte et al., 1980; Ganapathy,1980; Gilmore et al., 1984; Smit and ten Kate, 1982; Schmitz etal., 1988), and depletions in rare earth element (REE) concentra-tions (Smit and ten Kate, 1982; DePaolo et al., 1983). At presentno consensus exists concerning the significance of these geo-chemical anomalies, although two major hypotheses have beenproposed. Some authors (Alvarez et al., 1980, 1982) have sug-gested that the positive Ir anomalies may reflect the impact of alarge meteorite upon the earth, while others (McLean, 1985;Officer and Drake, 1983; Officer et al., 1987) have argued thatthe Ir anomalies and enrichments of Se, Sb, and As in KT bound-ary sediments could have resulted from a period of widespreadvolcanism. The resolution of this debate is complicated by the factthat the observed geochemical anomalies involve both meteoriticand non-meteoritic elements, the meteoritic elements do not pointto a single known meteorite composition (Kyte et al., 1980; Kyteand Wasson, 1982; Crocket et al., 1988), and the possibility thatthe sediments may have been "contaminated" by ejecta material(Kastner et al., 1984) or altered by diagenetic effects (Schmitz,1985; Schmitz et al., 1988; Dyer et al., 1989).

Weissel, J., Peirce, J., Taylor, E., Alt, J., et al., 1991. Proc. ODP, Sci. Results,121: College Station, TX (Ocean Drilling Program).

2 Department of Geological Sciences, The University of Michigan, Ann Arbor,MI 48109-1063, U.S.A.

Whatever the cause of the KT boundary event, there is generalagreement that its effects were both pronounced and ubiquitous.Analyses of KT boundary sections from both Atlantic and PacificDSDP cores reveal significant reductions in carbonate accumula-tion rates and a drastic reduction in the diversity of marinephytoplankton and other microfossil species (Tappan, 1968;Fischer and Arthur, 1977; Arthur et al., 1987). Considering thehigh degree of preservation of these calcareous microfossils, thesechanges can not, in many cases, be attributed simply to increaseddissolution (Thierstein, 1981; Zachos and Arthur, 1986). Further-more, negative perturbations in the δ 1 3 C of marine surface watersand surface to deepwater δ 1 3 C gradients are considered as evi-dence of a primary productivity minimum which may have lasteda million years following the boundary event (Zachos and Arthur,1986).

The recovery of a KT boundary section during ODP Leg 121provides an opportunity to add to our knowledge about this periodby developing information from the Indian Ocean and from anaustral/temperate setting. The sample suite examined here repre-sents the time interval between 67.5 and 64.2 Ma, and thus spansthe KT boundary (66.4 Ma, Peirce, Weissel, et al., 1989). We notethat these analyses cannot be construed as a high resolution studyof a complete sedimentary section because the boundary sedi-ments were recovered in the form of "drilling biscuits" (seefollowing discussion) and it is likely that some sedimentary ma-terial is missing. Consequently, the primary aim of this study hasbeen to highlight similarities and/or differences, and to delineatesignificant changes in sediment geochemistry which occurredduring the period represented by the onset, occurrence, and recov-ery from the KT boundary event at Broken Ridge. Our analysesinclude many of those elements which are significant to the debateconcerning the possible cause(s) of the mass extinctions at the KTboundary, as well as those which can serve as proxy indicators ofthe effects of the biological productivity crisis.

LITHOLOGIC SETTING

The KT boundary was recovered at Hole 752B in Core 121-752B-11R (52% recovery) at about 358 m below the seafloor(mbsf). Preliminary shipboard analyses indicate a major decreasein Cretaceous calcareous microfossil diversity at 358.75 mbsf and

423

R. M. OWEN, A.R.B. ZIMMERMAN

the first appearance of Tertiary nannofossils at 358.53 mbsf(Peirce, Weissel, et al., 1989). Based on global lithologic corre-lation, the location of the KT boundary is assumed to occur withinthe interval between 358.7 and 358.75 mbsf (Peirce, Weissel, etal., 1989) in this core. The boundary occurs within an ash-chert-chalk sequence which is underlain by light grey Cretaceous chalksand overlain by 6 m of dark green, ash-rich sediment (see Peirce,Weissel, et al., 1989 for a stratigraphic section diagram). Thisthick ash layer is, in turn, overlain by Paleocene chalks. Finelaminations, burrows, and mottles are found in the ash and chalklayers of the 60 cm boundary section. Because the boundarysection was recovered in the form of "drilling biscuits," we mustkeep in mind that some sediment, most likely that associated withchert layers, may be missing. The flora of the boundary sectionand the overlying ash-rich layer exhibit a sharp decrease in speciesdiversity and contain an assemblage of opportunistic "survivor"species (Peirce, Weissel, et al., 1989). The lithostratigraphy ofthis KT boundary section is atypical of other studied boundarysections only with respect to the voluminous ash deposits withwhich it is associated. Shipboard estimates of sediment fluxacross the boundary suggest the ash-rich layer is primarily areflection of reduced carbonate flux rather than a sudden sharpinflux of volcanic debris (Rea et al., 1990). An increase in Tertiarynannofossil diversity occurs at 354.5 mbsf, about 66 Ma (Peirce,Weissel, et al., 1989) and is followed by a return to normal (i.e.,Maestrichtian) productivity levels between 63.8 and 64.8 Ma (Reaet al., 1990).

METHODS

Geochemical analyses were performed on a suite of 44 samplestaken from sediment deposited over 3.3 m.y. (64.2-67.5 Ma)ranging from the upper Maastrichtian, through the KT boundaryand the 6 m ash-rich layer, and ending in the lower Paleocene.Except as noted below, samples were collected at 1.5 m intervalsfrom 30 m above to 27 m below the KT boundary (328-385 mbsf).A higher resolution suite of 24 samples was collected from theinterval between 66 and 66.4 Ma. This suite corresponds tosampling intervals of 10 cm and 30 cm, respectively, for theboundary layer sediments (Section 121-752B-11R-3), and for thetwo overlying core sections. We emphasize that our analyses donot include material from the presumed KT boundary layer (121-752B-11R-3, 90—5 cm); analyses of this layer are discussedelsewhere in this volume. All samples, consisting of 1-2 cmsections of core, were weighed, freeze-dried, and reweighed sothat dry bulk density could be calculated. Samples were thenground with mortar and pestle and homogenized. Carbonate con-tent was determined using a conventional pressure bomb tech-nique. All elemental abundances were determined by instrumentalneutron activation analysis (INAA) at the Phoenix MemorialLaboratory, The University of Michigan, using standard INAAprocedures (Gordon et al., 1968; Dams and Robbins, 1970).Replicate analyses of NBS-1633A standard as well as StandardPacific Sediment (Owen and Ruhlin, 1986) were performed toevaluate analytical precision (Table 1).

The statistical methods employed in this study are designed todiscriminate between different types of sediment in the samplesand to determine the relative contribution of each type to anygiven sediment sample. Both the number and the chemical com-position of the major geochemical end-members in our samplesare determined by a quantitative sediment partitioning procedure,which involves a mathematical manipulation of the geochemicaldata set using multivariate Q-mode factor analysis and vectorrotation according to the technique of Leinen and Pisias (1984).The chemical significance (i.e., the nature of the source material)of each end-member identified by this procedure is determined bycomparing key inter-element ratios within each end-member with

analogous ratios reported in the literature and known to be char-acteristic of pure geochemical source materials (Dymond, 1981;Graybeal and Heath, 1984; Leinen and Pisias, 1984). The relativeamount of each end-member in each sample is determined by alinear programming procedure which provides an optimum solu-tion (minimum error) for each sample. The computer programsneeded to implement this technique were kindly provided to us byDrs. Leinen and Pisias.

The biostratigraphy used here is based on the nannofossilstratigraphy as determined on board ship and as slightly revisedat the Leg 121 post-cruise meeting in January of 1989 (Peirce,Weissel, et al., 1989). For Cenozoic materials, we have used thezonations of Okada and Bukry (1980) as tied to the time scale ofBerggren et al. (1985). Mesozoic stratigraphy is based on thezonation of Sissingh (1977) as tied to the magnetic reversal timescale of Harland et al. (1982) using the revisions proposed by Kentand Gradstein (1985). The total sediment mass accumulation rate(MAR) for each sample (Table 2) is a quantification of the truemass flux of sedimentary material to the seafloor, and is calcu-lated as the product of the dry bulk density and the linear sedi-mentation rate. When the total MAR value for each sample ismultiplied by the weight percent of any sediment component inthat sample, the flux of that component is also quantified, and sothe pitfalls of interpreting relative abundance data can be avoided.Dry bulk density (DBD) for each sample was determined from ourporosity measurements, where DBD = 2.65 (100 - % porosity).Linear sedimentation rates are based on the nannofossil stratigra-phy. A summary of the age-depth-LSR values for the core inter-vals discussed here are reported in Rea et al. (1990) and, forconvenience, are also shown here in Table 3. We note that becauseof the limitations of LSR data, which are determinable for everynannofossil zone, interpretations of the resulting MAR values areuseful for deciphering phenomena that occur on tectonic timescales. Rea et al. (1990) have estimated the accuracy of the MARdata to be ±20%; minor changes may not be significant.

RESULTS AND DISCUSSION

Compositional End-members

The downcore distributions of 28 elemental concentrations(calculated on a carbonate-free basis), CaCθ3 content, and wholesediment MAR's are listed in Table 2. Several preliminary statis-tical analyses were performed to identify groups of elementswhich exhibit a coherent chemical behavior. Where appropriate,a single representative element (e.g., Ba, for Ba, Sr, and Br) wasused in the final factor analysis to avoid significant redundancyin the data set.

The factor analysis identified three factors which collectivelyaccount for 91.8% of the total variance in the data (Table 4). Weinterpret Factor 1 as representing biogenic material and the disso-lution residue of this material. Several inter-elemental ratios inthis factor are typical of those reported in the literature for marineplankton (Mn/Ba, Al/B a, Co/Zn, Al/Mn; Martin and Knauer,1973; Dymond, 1981; Graybeal and Heath, 1984; Palmer, 1985),foraminifera tests (La/Yb, La/Sm, Sm/Eu; Palmer, 1985) and/orfor the dissolution residue of carbonate and siliceous organisms(Zn/Al, Mn/Al; Dymond, 1981; Graybeal and Heath, 1984). Bar-ium, which is believed to be strongly associated with biogenicmaterial (Gurvich et al., 1978; Dehairs et al., 1980) and can beused as a proxy for enhanced biological productivity (Boström etal., 1973; Schmitz, 1987), is most heavily loaded in Factor 1.Manganese, also loaded in Factor 1, has been found to be enrichedin phytoplankton (Riley and Roth, 1971; Martin and Knauer,1973). In preliminary statistical runs, Sr, which known to substi-tute for Ca in carbonates, and Br, an element associated withorganic carbon (Price and Calvert, 1977; Mayer et al., 1981) were

424

GEOCHEMISTRY OF K/T BOUNDARY, HOLE 752B

Table 1. Summary of replicate analyses of Pacific Mud and NBS-1633A Standard(data in ppm).

AlMgFeMnKTiBaSr

ZnCrBrSeVSbCoHfThSe

LaCeNdSmEuTbDyYbLu

aU.M.

Pacific Mud Standard

aMean

2460-

6220011400

--443

2020

1044.0342.21.54183

2.4418.7

0.5720.432

-

28.010.730.04.25

1.080.927

4.653.13

0.410

aStandardDev.

696-

2520514--77.1336

7.721.463.62

0.12921.2

0.3260.7650.1790.122

-

1.681.734.98

0.335

0.0670.2460.5700.2460.030

"Publish.Mean

19502800

6120011800

500200368

1500

1422.3044.81.89204

2.3120.0

0.300-

-

25.79.3217.55.26

1.350.680

-2.73

0.460

INAA, based upon 17 analyses.bOwen and Ruhlin (1986).CU. M. INAA, based upon 13 analyses.d Gladney et al. (1987).

cMean

1410004560

93600187

19500839014901080

2122011.9737.83016.9941.27.3126.017.1

84.9187

84.317.4

3.182.7014.88.281.15

NBS-1633A

°StandardDev.

109001740329015.1

1060442

80.6219

19.36.41

0.4201.1812.7

0.3261.92

0.3240.751

1.60

2.135.925.49

0.886

0.1730.4940.9470.2850.082

Standard

dPublish.Mean

1430004550

94000179

1880081001400830

220196

2.3039.0297

6.8043.07.4024.710.3

84.0175

74.017.0

3.702.5015.67.401.12

also strongly associated with this factor. Consequently, we haveused downcore variations in Factor 1 as a proxy indicator ofpaleoproductivity trends.

Factor 2, which accounts for 36.7% of the variability in thedata set, is interpreted as a volcanogenic sediment component. Itis well-documented that there are significant variations in thecomposition of volcanogenic materials in marine sedimentsthroughout the world ocean (e.g., Chester and Aston, 1976). Thus,while our interpretation of Factor 2 is consistent with the largeamounts of ash present in this core section, it cannot be evaluatedin terms of any universally accepted set of geochemical criteriafor identifying volcanogenic materials, because no such criteriaexist. In this case, however, several inter-element ratios of Factor2 do compare well (Table 5) with those of known volcanogenicmaterials from the same region, including southern NinetyeastRidge volcanogenic sediment (DSDP Site 253, Fleet et al., 1976;Fleet and McKelvey, 1978), a Santonian ash layer (ODP Hole121-755A), and Kerguelen Plateau basalts (Storey et al., 1988).

The composition of Factor 3, which accounts for 12.1% of thetotal variance, is similar in some respects to that of both biogenicmaterials (Factor 1) and volcanogenic materials (Factor 2). UnlikeFactors 1 or 2, however, Factor 3 is enriched in the noble metal

Au, although it has been suggested that noble metals may beenriched in both volcanic ash (Goldschmidt, 1954; Zoller et al.,1983; Olmez et al., 1986) and organic materials (Crocket and Kuo,1979; Schmitz et al., 1988) through diagenetic processes. Forthese reasons, we interpret Factor 3 as reflecting a diageneticalteration product of the biogenic and volcanogenic sedimentrepresented by the first two end-members. Although both Factor2 and 3 are enriched in Fe, the Mn/Fe ratios of these end-membersappear to be too low to be indicative of the presence of anysignificant amounts of hydrothermal or authigenic metalliferoussedimentary components (Dymond, 1981; Graybeal and Heath,1984). These sediments also do not display As enrichments (Mar-chig et al., 1982) or the very negative Ce anomalies and heavyREE enriched patterns typical of hydrothermal sediments (Piper,1974a; Ruhlin and Owen, 1986), nor do they display the positiveCe anomalies found in authigenic ferromanganous sediments(Piper, 1974b; Elderfield et al., 1981).

The REE patterns of individual sediment samples (Fig. 1) aregrossly similar. All show distinct light REE enrichment andabsolute abundances similar to those of basalts from the Ker-guelen Plateau and Ninetyeast Ridge (Fleet et al., 1976; Reddy etal., 1978), or more generally, from an ocean island or alkalic

425


Table 2. Major and trace element data determined by INAA (data in ppm except where indicated), sampleages, whole sediment mass accumulation rates (MAR), and carbonate content. For statistical analysis, weassume elemental concentrations equal to one-half the detection limit where indicated by the (<) symbol.The data presented here are averages of two analyses and are calculated on a carbonate-free basis.

Sample

ODP Hole 121-752B

8R-03, 132-135 cm8R-04, 132-135 cm8R-05, 131-134 cm8R-06, 132-135 cm8R-07, 132-135 cm9R-01, 129-132 cm10R-01, 54-57 cm10R-02, 54-57 cm10R-03, 54-57 cm10R-04, 53-56 cm10R-05, 53-56 cm10R-06, 55-58 cm11R-01, 30-32 cm11R-01, 70-72 cm11R-01, 110-112 cm11R-01, 140-142 cm11R-02, 30-32 cm11R-02, 70-72 cm11R-02, 110-112 cm11R-02, 140-142 cm11R-03, 20-21 cm11R-03, 35-36 cm11R-03, 50-51 cm11R-03, 65-66 cm11R-03, 80-81 cm11R-03, 97-98 cm11R-03, 105-106 cm11R-03, 115-116 cm11R-03, 125-126 cm11R-03, 135-136 cm12R-01, 139-142 cm12R-02, 136-139 cm12R-03, 70-73 cm12R-04, 68-71 cm12R-05, 37-40 cm12R-06, 48-51 cm13R-01,2-5cm13R-02, 51-54 cm13R-03, 142-145 cm13R-04, 43-46 cm13R-05, 91-94 cm13R-06, 47-50 cm14R-01, 57-60 cm14R-02, 48-51 cm

mbsf

328.72330.22331.71333.22334.72336.69345.64347.14348.64350.13351.63353.15355.10355.50355.90356.20356.60357.00357.40357.70358.00358.15358.30358.45358.60358.77358.85358.95359.05359.15365.79367.26368.10369.58370.77372.38374.02376.01378.42378.93380.91381.97384.17385.58

Age(Ma)

64.1864.2464.2964.3564.4164.4864.8765.0665.2465.4365.6265.8166.0366.0766.1266.1566.1966.2466.2866.3166.3566.3666.3866.3966.4066.4166.4166.4266.4266.4366.7066.7666.7966.8566.9066.9767.0367.1167.2167.2367.3167.3667.4567.50

Al(%)

3.511.824.503.325.472.213.876.085.113.921.275.715.695.664.634.904.434.674.854.314.364.162.384.871.732.193.992.912.632.815.986.024.194.734.404.354.357.146.025.405.957.005.677.36

Mg(%)

2.130.865

3.571.313.171.301.513.243.282.79

0.3593.383.272.942.772.693.703.032.912.953.633.451.504.551.461.271.562.151.271.201.833.981.112.151.381.952.062.573.544.932.922.323.953.13

Fe(%)

3.191.816.483.147.202.313.387.096.056.50

0.9757.428.676.736.876.428.346.766.386.317.267.981.978.301.152.573.742.291.901.754.408.292.143.902.915.332.434.827.266.308.166.315.374.01

Mn

2940158012131500743

17302051181022002800

762621705590611531601540619849674809

355015705170214023402960277023402160230025703010228016202340134026202600

676122036005000

K(%)

<0.806<0.437

1.982.532.332.501.782.40

0.8472.02

0.5092.111.831.801.831.861.572.271.762.151.441.96

<0.4511.57

<0.6781.742.482.031.921.643.512.072.043.552.423.102.602.982.792.182.703.042.82

<1.53

Ti(%)

0.530<0.1320.7980.3980.8370.5200.634

1.020.6390.7020.2020.859

1.291.00

0.9470.8880.3680.952

1.120.6560.5500.8910.3290.741

<0.1780.3550.7150.6060.6930.1490.783

1.100.715

1.090.3460.6970.5700.8980.7040.7790.7680.896

1.12<0.487

Ba(%)

0.7420.4240.5860.6390.3250.5790.6030.4680.5150.6080.2460.3970.0770.1620.3170.2610.2730.3220.3590.3310.4050.6270.5920.3890.3700.4460.4470.5260.5250.4970.5640.4410.8040.7820.6290.5660.6820.5550.5550.4910.3510.3040.7610.816

Sr(%)

0.6190.2520.2340.2630.1780.4050.7200.5200.4940.7650.1360.1370.1290.2030.1460.1970.1740.1270.1830.2130.2280.2570.1880.1980.2810.4830.6140.7130.6570.4590.7670.7250.8360.9300.6880.6440.7790.5830.8840.5860.3320.533

1.161.68

basaltic source. Submarine weathering of basalt has been shown

to produce light REE enrichment (Ludden and Thompson, 1978;

Despraires and Bonnot-Courtois, 1980); however, the effect of

diagenetic alteration on the REE's of volcanogenic sediment is

relatively unknown (Fleet et al., 1976). Some of the REE patterns

also display slightly negative Ce anomalies, which may be due to

the presence of calcareous or siliceous biogenic sediments which

are commonly Ce depleted (Piper, 1974a, b; Shimizu and Masuda,

1977; Palmer, 1985). The most negative Ce anomalies occur in

those samples which best reflect the composition of Factor 1, and

thus they support our interpretation of this factor being composed

of biogenic material derived from seawater, which is charac-

terized by a pronounced negative Ce anomaly (Fleet, 1984).

End-member Distributions

Downcore profiles of both the relative amount (Fig. 2A-C) and

MAR (Fig. 3A-C) of each geochemical end-member are used here

as an aid in interpreting the paleoceanography of the KT section.

The profile of the biogenic end-member (Factor 1) closely follows

that of carbonate content (Fig. 4). There is a drastic reduction in

both the relative abundance (Fig. 2A) and MAR (Fig. 3 A) of this

end-member at 358.45 mbsf (66.39 Ma) just above the KT bound-

ary. We report an 11-fold reduction in the MAR of the biogenic

component (from 3.3 to 0.3 g/cm2/k.y.) across the KT boundary.

The MAR of the biogenic component does not return to Maas-

trichtian levels (2-3 g/cm2/k.y.) until sometime between 64.5 and

426


Table 2 (continued).

Ce

41.114.425.030.929.424.432.537.333.041.316.428.535.925.427.726.516.632.849.654.035.164.481.326.227.640.249.934.043.752.756.647.349.984.392.852.957.688.473.873.036.571.4164

70.4

Nd

85.915.831.946.216.753.138.547.547.044.321.221.823.717.718.218.921.112.733.429.918.242.339.826.337.669.261.759.142.863.058.943.375.3141102

62.371.362.847.645.739.431.9123

59.8

Sm

10.74.905.576.195.025.127.398.146.538.262.864.705.783.904.294.003.014.286.015.713.8310.78.775.884.418.8711.010.49.839.6413.713.719.822.021.214.312.413.213.49.957.359.5020.615.1

Eu

2.811.211.571.361.501.231.852.152.162.75

0.7461.341.671.161.281.17

0.9011.181.571.01

0.7962.451.311.111.151.542.581.981.811.652.892.602.893.762.922.782.962.423.522.411.642.104.033.96

Tb

4.070.356

1.200.936

2.690.416

1.501.631.231.68

0.5681.031.141.43

0.8770.695

<0.3440.760

1.460.7460.8692.371.631.29

<0.3871.051.991.901.801.332.491.663.372.314.992.293.282.133.662.892.752.214.328.20

Dy

12.66.896.697.525.328.299.439.069.769.073.736.397.336.256.525.585.285.637.286.294.769.387.457.309.058.7613.09.0612.48.3312.112.917.619.713.811.313.215.913.27.3010.013.624.825.9

Yb

7.30<3.984.174.083.63

<3.914.705.364.285.372.023.324.692.953.403.142.793.084.034.142.876.675.564.183.566.007.576.286.556.068.779.0512.913.112.39.229.268.4610.48.295.766.8516.114.1

Lu

1.160.4240.5120.5520.4920.4240.5940.7290.5820.8300.2900.4310.5860.3730.4440.3980.3660.3950.5580.5990.3630.8730.6370.6380.4590.7340.9570.8360.8510.801

1.231.131.371.721.651.031.201.111.351.18

0.6780.9622.041.93

Carb.(%)

82.969.858.070.139.474.878.771.167.876.140.217.82.92.75.97.28.5

10.417.029.227.230.166.342.480.872.975.679.479.275.278.178.984.285.081.275.681.071.282.281.957.866.488.292.0

MAR(g/cm2ky)

5.605.975.105.864.265.451.591.531.511.641.811.071.211.171.181.241.221.221.291.331.341.331.821.345.674.704.464.524.724.534.924.904.795.084.844.855.704.945.485.504.974.995.225.35

64.9 Ma; it is not possible to further constrain the timing of thisrecovery because of the 9 m gap in core recovery between 336.7and 345.6 mbsf. This suggests that the recovery time for biologi-cal productivity following the KT event at Site 752B, then at apaleolatitude of about 50144S, lasted between 1.5 and 2.0 m.y. Incontrast, a significantly faster recovery time (0.5-1.0 m.y.) hasbeen noted for some lower latitude Tethyan KT boundary sections(Smit and ten Kate, 1982; Smit and Romein, 1985) and on theShatsky Rise (Zachos et al., 1989). Whether this result indicatesthat significant differences exist between the timing of responsesto ecological stress of high vs. low latitude biota, or is simply alocal phenomenon, awaits further study. It is also possible that themagnitude and duration of the stress applied by the KT event,whatever it was, varied latitudinally.

The relative abundance profile (Fig. 2B) of the volcanogeniccomponent is essentially the mirror image of that of carbonatecontent (Fig. 4). This is consistent with the fact that biogenic andvolcanogenic materials are the two dominant sediment compo-nents, and thus should be inversely related when displayed asfractions of whole sediment. Estimates of the flux of biogenic andvolcanogenic materials across the KT boundary section are im-portant because they have a direct bearing on the paleoceano-graphic significance of the 6 m thick ash sequence whichimmediately overlies the boundary. Shipboard estimates of sedi-ment flux, based on the visual examination of smear slides,suggest there is a marked decrease in carbonate accumulation(3.94-0.34 g/cm2/k.y.) but relatively little change in ash flux(0.73-0.93 g/cm2/k.y.) across the KT boundary (Rea et al., 1990).

427


Table 2 (continued).

Zn

13960.615410515869.013819915718843.914114315516215012915251915099.317434.416211267.235411318293.817917211518526717578.2215213240247164170169

Cr

71.317.264.148.882.944.866.260.192.010313.295.856.466.360.959.376.760.359.4

43.268.843.030.679.727.830.910236.929.542.779.511758.245.110576.356.197.170.956.911564.6118101

Br

57.318.018.122.617.329.234.023.723.628.26.6519.016.118.918.516.417.720.020.4

21.821.229.532.228.434.542.748.463.057.657.036.445.092.464.658.647.637.527.836.833.419.228.977.7124

Se

20.28.7123.616.328.411.820.429.827.931.25.0727.430.028.028.727.429.527.629.917.619.122.311.626.46.0810.819.212.713.010.226.630.921.020.721.223.719.0

25.026.627.731.525.027.229.2

V

19762.017298.021361.114827220520747.420234033224823223825728815015018989.923490.777.315288.092.862.6206211122143188194141230230229273189181259

Sb

<0.903<0.510<0.483<0.517

1.732.23

<0.702<0.5702.283.391.571.331.721.191.11

<0.282<0.322

1.231.901.822.030.8842.39

<0.3583.96

<0.4966.39

<0.5603.071.96

<0.806<0.767

1.79<0.8630.413<0.7152.60

3.503.832.292.75

<0.585<1.23<2.05

Co

17.9

10.929.311.831.49.8818.232.727.530.96.8729.346.234.334.230.337.527.631.827.624.119.310.229.515.86.861186.8812.07.0023.832.117.717.122.031.113.4

17.826.014.347.421.319.1

19.2

Hf

2.561.291.781.633.151.403.933.492.323.221.072.313.933.233.432.992.442.803.669.485.695.621.622.971.571.684.422.273.371.384.291.745.824.903.732.973.88

6.764.914.003.184.2312.56.83

Th

1.580.7791.281.331.32

<0.4361.852.761.561.571.352.281.701.311.981.601.251.552.4111.66.236.281.401.431.362.245.612.053.233.623.524.583.035.855.083.333.39

7.465.69<1.891.956.7916.4

<1.35

Se

<8.24<4.74<4.36<5.25<3.48<5.63<6.99<6.22<5.59<6.694.202.915.766.43<2.693.423.732.594.73

6.395.215.626.32

<3.75<6.86<5.3916.2

<7.09<6.91<6.005.89<8.10<9.50<9.96<8.42<6.81<8.26

6.0016.9

<8.9512.1

<5.63<13.5<18.3

Au

<0.0327<0.01750.0195<0.02110.00925<0.02140.02350.02800.0194<0.02170.0369

0.01390.107

<0.0099<0.01030.1150.05190.0218<0.01160.02180.01060.113

0.007720.03110.04390.2140.03320.07920.02160.01970.0258<0.02420.0209<0.0288<0.02610.02990.0942<0.0215<0.0304<0.0282<0.01780.07500.0381<0.0561

La

68.932.228.135.420.833.243.341.833.848.918.017.014.410.313.211.78.514.223.427.818.040.061.729.439.658.166.668.367.466.486.886.713215715195.190.5

72.591.874.637.562.8151140

Our geochemical factor analysis results are in substantial agree-ment with these flux estimates. As noted above, we report an11-fold decrease in accumulation of the biogenic sediment com-ponent, which compares well with the 9-fold decrease in carbon-ate accumulation determined from shipboard studies. Ourestimate of ash flux across the KT boundary is more complicatedbecause factor analysis distinguishes between volcanic ash (Fac-tor 2) and diagenetic alteration products (Factor 3). However,even if we assume that all of the material represented by Factor 3was originally volcanic ash, and thus estimate ash flux as the sumof the MAR's of Factors 2 and 3, (Figs. 3B and 3C), our resultsalso suggest there was little change in ash flux (about 2.5-1.5g/cm2/k.y.) across the KT boundary. In both cases, then, inde-pendent estimates of sediment flux suggest that the ash layer didnot result from enhanced volcanic activity, but instead is the resultof a dramatic decrease in planktonic productivity immediatelyfollowing the KT boundary event.

The MAR's determined for Factor 3 typically fall within therange 0.1-1.0 g/cm2/k.y., except for two spikes (1.5 and 2.2g/cm2/k.y.) below the KT boundary, both of which correspond todistinct ash layers, and a pronounced spike (3.5 g/cm2/k.y.) at theKT boundary (Fig. 3C). Among all of the elements examined, onlyAu is almost exclusively associated with Factor 3 (Table 3). Thehighest measured Au concentration (214 ppb) was determined forthe sample collected just below the KT boundary (358.77 mbsf,Table 1), a finding similar to that reported by Alvarez (1984). NoIr concentrations above the detection limit (3-8 ppb) of the INAAwere measured; however, INAA is less sensitive than radiochemi-cal neutron activation analysis, the preferred method for Ir analy-ses, and there may be Ir in the lowest boundary clay which we didnot sample. Except for Au, all other elements associated withFactor 3 also display strong associations with the biogenic and/orvolcanogenic sediment end-members. For example, the non-me-teoritic elements Mn and Se are strongly associated with both the

428


Table 3. Age-depth correlation and linear sedimentation rates (LSR) forHole 752B (from Rea et al., 1990).

Age(m.y.)

63.8-64.8

64.8-65.9

65.9-66.4

66.4-69.0

Time interval(m.y.)

1.0

1.1

0.5

2.6

Depth(mbsf)

318.7-345.1

345.1-353.9

353.9-358.5

358.5-422.3

Depth interval(m)

26.4

8.8

4.6

3.85

LSR(cm/k.y.)

2.64

0.80

0.92

2.4

Table 4. End-member compositions deter-mined by factor analysis (in ppm except whereindicated).

Element

Al (%)Fe (%)Mn (%)Ba (%)

ZnSeSbCoThSeAu

LaCeSmEuYbLu

% of totalvariance =

Factor 1

4.850.00

0.8161.60

21015.45.332.907.8021.0

0.0120

279106

31.05.6422.22.92

43.0

Cummulative variance =

Factor 2

11.918.60.00

0.470

43167.12.7088.93.967.12

0.0010

0.00192

23.210.420.12.65

36.7

91.8

Factor 3

11.815.4

0.4251.24

25859.10.0049.37.4515.2

0.472

181310

25.12.4718.62.28

12.1

biogenic/dissolution residue end-member (Factor 1) and the sedi-ment diagenetic end-member (Factor 3). This association is con-sistent with the suggestions of Schmitz (1985) and Schmitz et al.(1988) that these elements may become enriched via a mechanisminvolving redox-controlled and/or microbially mediated precipi-tation processes. Similarly, high concentrations of Fe, Co, Zn, andSe are associated with both the volcanic ash (Factor 2) anddiagenesis (Factor 3) end-members (Table 4). Diagenetic enrich-ment of certain elements in volcanogenic sediment has beenproposed as the origin of the boundary signal by workers whohave examined the clay mineralogy and microstratigraphy ofother boundary sections (Wezel et al., 1981; Rampino and Rey-nolds, 1983; Crocket et al., 1988; Elliott et al., 1989). While theseelemental associations do not rule out the possibility of contribu-tions from an exotic source, they do suggest that diagenesis ofbiogenic and volcanogenic sediment may be a viable source formany of the elements anomalously enriched at the KT boundary.

Some authors have suggested that a period of global-scalevolcanism may have produced changes in ocean chemistry suffi-cient to cause the mass extinctions at the KT boundary (Keith,1982; McLean, 1985; Officer et al., 1987). Our results do not ruleout, but certainly do not support this hypothesis. Each of thegeochemical end-members identified in this study spans the KTboundary and none is associated exclusively with the sediment

above or below the boundary. We infer from this, and from theflux data discussed above, that both the amount and compositionof the volcanogenic material deposited at Broken Ridge remainedessentially constant before and after the KT boundary event,whereas the composition of the biogenic component remainedconstant, but the flux was drastically reduced.

SUMMARY

A Q-mode factor analysis of geochemical data obtained froman expanded section of sediments across the KT boundary atBroken Ridge indicates the sediments here are composed of threemajor geochemical end-members. The significance of each ofthese end-members was determined by comparing key inter-ele-ment ratios for each end-member with analogous ratios that areconsidered to be characteristic of specific types of sedimentarymaterial. Based on these comparisons, we have interpreted thethree end-members as representing (1) a biogenic/dissolutionresidue component, (2) a volcanogenic component, and (3) adiagenetic or alteration product component.

We have examined both composition and relative flux of eachof these end-members as a means to gain insight into the processesassociated with the time represented by the onset, occurrence, andrecovery from the KT boundary event at Broken Ridge. Forexample, the KT boundary section at Broken Ridge is overlain bya 6-m-thick ash-rich layer. A significant question here is whetherthis layer reflects a sudden increase in ash deposition or, alterna-tively, a sharp reduction in biological productivity. Our analysesindicate there was an 11-fold decrease in the flux of the biogenicend-member, while the flux of the volcanogenic end-memberremained essentially unchanged. This result is in good agreementwith preliminary shipboard estimates of sediment flux rates, andsuggests the ash layer at Broken Ridge is the result of a dramaticdecrease in planktonic productivity immediately following theKT boundary event. The flux of the biogenic component does notreturn to Maastrichtian levels until 1.5-2.0 m.y. after the KTboundary event, a recovery time that is about 1.0 m.y. longer thanthat reported for lower latitude KT boundary sections.

The inter-element ratios and REE patterns of the volcanogeniccomponent suggest that the volcanogenic sediments at BrokenRidge are derived from a local source. Both the composition andflux of the volcanogenic component remain constant across theKT boundary, while the composition of the biogenic end-memberremains constant but its flux is significantly reduced. Conse-quently, our results do not indicate that volcanism played a criticalrole in the KT boundary event.

There is a sharp increase in the flux of the diagenetic end-mem-ber coincident with the KT boundary. This end-member containsenriched amounts of Au, and the highest Au concentration occursin a sample recovered from just below the KT boundary. Otherworkers have also reported Au enrichments in sediments fromimmediately below the KT boundary (Crocket and Kao, 1979),but no agreement exists concerning whether these enrichmentsrepresent evidence for the impact hypothesis, or if they are simplythe result of diagenetic processes. Except for Au, all other ele-ments that are enriched at the KT boundary are also stronglyassociated with either or both the biogenic and/or the volcano-genic end-members. We believe the most plausible explanationfor geochemical anomalies in the sediments immediately belowthe KT boundary at Broken Ridge is that they occur as a result ofenhanced diagenetic processing which serves to concentrate cer-tain elements originally present in biogenic and volcanogenicsediment precursors.

ACKNOWLEDGMENTS

We thank our colleagues on board the Resolution during ODPLeg 121 for their assistance in obtaining the samples and gener-

429


Table 5. Inter-elemental ratios of Factor 2, Factor 3, and literature values for volcanogenicsediment.

Element Factor 2 Factor 3

aSantonian bKerguelen cSite 253Ash Basalt Vole. Sed.

Al%/Fe%Mn%/Fe%Ba ppm/Fe%Zn ppm/Fe%Se ppm/Fe%Sb ppm/Fe%Co ppm/Fe%Th ppm/Fe%

Co ppm/AI%Zn ppm/AI%

La/YbSm/LuSm/EuSm/Yb

0.6370.00252

23.13.60

0.1454.77

0.213

7.5036.0

.

8.682.211.14

0.7670.0279

80616.73.830.003.19

0.483

0.60043.0

9.7011.010.01.34

0.6190.0140

10917.53.83

0.2513.83

0.190

6.1926.9

5.069.623.311.30

0.76/0.950.030/0.033

122/45-

6.18/8.37--

0.94/0.38

_

-

24.0/7.9522.9/12.53.69/2.913.29/1.81

0.7960.01015.6

---

6.60•

8.2617.5

4.469.483.711.42

aOur mean analyses of a Santonian ash layers in ODP Sections 121-755A-5-1 to 121-755A-9-2.bKerguelen alkali basalt/tholeiite data from Storey et al. (1988).c Middle Eocene volcanogenic sediment sequence DSDP Site 253, mean of Units I-VI. REE's are

from Fleet et al. (1976). All other elements from Fleet and McKelvey (1978).

21

•E

331.71 mbsf348.64353.15355.50355.90358.30358.45

(fi

.157 59 61 63 65 67La Ce Nd Sm Eu Tb Dy

Atomic number

Figure 1. Typical REE patterns of KT section samples. All data on a carbon-ate-free basis and normalized to a North American shale composite (Grometet al., 1984).

ating the shipboard data used in this study, and for making a longcruise a very pleasant one. M. Lyle, J. Weissel, and an anonymousreviewer provided helpful comments and suggestions based on areview of an earlier version of this paper. This research wassupported by Contract No. 20252 from the Texas A&M ResearchFoundation awarded to R. M. Owen.

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Date of initial receipt: 14 December 1989Date of acceptance: 14 August 1990Ms 121B-147

A. Factor 1

(% composition)

40 60

B. Factor 2

(% composition)

40 60

c. Factor 3

(% composition)

40 60

Figure 2. A. Downcore relative contribution of Factor 1 (biogenic) sediment. B. Factor 2 (volcanogenic). C. Factor 3 (diagenetic). Placement of the KT boundaryis as suggested by Peirce, Weissel, et al. (1989).

A. Factor 1 - MAR

(g/cm2 k.y.)

B. Factor 2 - MAR

(g/cm2 k.y.)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

C. Factor 3 - MAR

(g/cm2k.y.)

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Figure 3. A. Downcore mass accumulation rate of Factor 1 (biogenic) sediment. B. Factor 2 (volcanogenic). C. Factor 3 (diagenetic). Factor MAR's were calculatedas the product of whole sediment mass accumulation rate (for each sample) and the relative contribution of each factor (to each sample).

432

Carbonate content (%)

20 40 60

325


100

Figure 4. Downcore carbonate content determined by carbonate bomb tech-nique.

433

20. geochemistry of the cretaceous/tertiary boundary at ...€¦ · proceedings of the ocean...

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