23. low-molecular-weight hydrocarbons in …bons in sediments of dsdp legs 71, 75, 79, 89, and 93...

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

23. LOW-MOLECULAR-WEIGHT HYDROCARBONS IN SEDIMENTS OF SITES 752, 754,755(BROKEN RIDGE), 757 AND 758 (NINETYEAST RIDGE), CENTRAL INDIAN OCEAN1

R. G. Schaefer,2 R. Littke,2 and D. Leythaeuser2

ABSTRACT

Core samples taken during Leg 121 drilling aboard the JOIDES Resolution in the central Indian Ocean were analyzed for theirlow-molecular-weight hydrocarbon contents. Forty-three samples from the Broken Ridge and 39 samples from the Ninetyeast Ridgedrill sites, deep-frozen on board immediately after recovery, were studied by a dynamic headspace technique (hydrogen-strip-ping/thermovaporization). Light hydrocarbons (saturated and olefinic) with two to four carbon atoms, and toluene as a selectedaromatic compound, were identified. Total C2-C4 saturated hydrocarbon yields vary considerably from virtually zero in a Paleogenecalcareous ooze from Hole 757B to nearly 600 nanogram/gram of dry-weight sediment (parts per billion) in a Cretaceous claystonefrom Hole 758A. An increase of light-hydrocarbon yields with depth, and hence with sediment temperature, was observed fromHole 758A samples down to a depth of about 500 meters below seafloor. Despite extreme data scatter due to lithological changesover this depth interval, this increased yield indicates the onset of temperature-controlled hydrocarbon formation reactions.

Toluene contents are also extremely variable (generally between 10 and 100 ppb) and reach more than 300 ppb in two samplesof tuffaceous lithology (Sections 121-755A-17R-4 and 121-758A-48R-4). As for the saturated hydrocarbons, there was also anincrease of toluene yields with increasing depth in Hole 758A.

INTRODUCTION

During the cruise of the JOIDES Resolution in the centralIndian Ocean (in May and June 1988) a total of 82 sedimentsamples were collected for this study from five drill sites on Leg121 of the Ocean Drilling Program (ODP). These samples werecollected on board with special precautions taken immediatelyupon recovery from the core barrel. Precautions included collec-tion of samples only from the ends of the 1.5-m sections cut fromthe core liner. By these means, the contamination of the samplesby acetone, which is usually applied in large quantities duringcore-cutting procedures aboard ship, was minimized. This con-tamination problem has been discussed in detail in our previousstudies of Deep Sea Drilling Project (DSDP) samples (cf., Schae-fer et al., 1983b; Schaefer and Leythaeuser, 1985, 1987). Allsamples for the present study were stored in sealed vials anddeep-frozen on board for analysis in our laboratory. This studyfocuses on organic geochemical aspects and examines the condi-tions for the generation of low-molecular-weight hydrocarbons asdetermined in 43 core samples from the Broken Ridge and 39samples from the Ninetyeast Ridge drill sites of Leg 121 (Fig. 1).

Light hydrocarbons are widespread constituents in most deep-sea sediments (Hunt, 1975, 1984; Whelan and Hunt, 1981, 1982;Jasper et al., 1984; Whelan, 1984; Whelan et al., 1984). A mainreason for studying the occurrence and distribution of these com-pounds is that they represent sensitive indicators of the onset ofthermal hydrocarbon generation and are indicative of redistribu-tion processes in the pore spaces of sedimentary rocks (e.g., bydiffusion). Previous analyses of low-molecular-weight hydrocar-bons in sediments of DSDP Legs 71, 75, 79, 89, and 93 (Schaeferet al., 1983a, 1983b, 1984; Schaefer and Leythaeuser, 1984,1985,1987) were designed to address such objectives. The same appliesto the present study, although it was clear from data collected onboard (low organic carbon contents, low maturity, etc.) that light

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

2 Forschungszentrum Jülich GmbH, Institut für Erdöl und Organische Geoche-mie, Postfach 1913, D-5170 Jülich, Federal Republic of Germany.

hydrocarbons would be only trace constituents at these IndianOcean sites. Our initial results also led to the decision that thisstudy be limited to hydrocarbons with two to four carbon atoms(saturated and unsaturated compounds) and to toluene, as anexample of a low-molecular-weight aromatic hydrocarbon. Re-gardless of this limitation, the data presented are believed to benot only of documentary value in the framework of deep-seadrilling research, but reconfirm and extend previous observationsconcerning geochemical behavior of light hydrocarbons in diage-netically young sediments.

METHODS

SamplesEighty-two sediment samples (approximately 5-10 g) were

collected from the cores immediately after they had been cut intosections on deck. Samples were stored in 20-mL screw-cappedglass flasks, sealed with aluminum foil, and placed in a deepfreezer. The samples remained in a deep-frozen condition (ap-proximately -20°C) until the analyses were performed in thelaboratory at KFA Jülich.

Determination of Organic Carbon Contents andLight-Hydrocarbon Yields

Organic carbon contents were measured on the "thermovapor-ized" sediment samples (see below) after treatment with hot 6NHC1 by a combustion method (LECO Carbon Analyzer IR 112).The accuracy of this method is still rather high at organic carbonlevels of 0.1 wt% (dry weight) or even less (10% relative standarddeviation, or coefficient of variation).

The low-molecular-weight hydrocarbons (total molecularrange C2 to C7) were analyzed by a slightly modified dynamicheadspace technique ("hydrogen stripping/thermovaporization")combined with capillary gas chromatography described in detailin our previous studies of DSDP Legs 71, 75, 79, 89, and 93sediments (Schaefer et al., 1983a, 1983b, 1984; Schaefer andLeythaeuser, 1984, 1985, 1987).

The analytical method used comprises both the mobilizationof the hydrocarbons from the sediment samples and subsequentcapillary gas chromatography in a single-step procedure carried

457

R. G. SCHAEFER, R. LITTKE, D. LEYTHAEUSER

30°

Figure 1. A. General location map showing ODP drill sites of the study area. B. Map showing Broken Ridge sites in more detail.Bathymetry in meters.

458

LOW-MOLECULAR-WEIGHT HYDROCARBONS

out in a closed gas-flow system. Briefly, a small portion of thefrozen sediment sample (generally less than 1 g) is placed in theflow system of the gas chromatograph, the hydrogen carrier gasserving as the stripping gas. The rock sample is heated to 150°Cfor 10 min in a gas flow of 10 mL/min. Prior to the gas chroma-tographic separation, the thermovaporized (or "thermodesorbed")compounds are passed through a filter-section of dry CaCh inorder to prevent the pore water from entering the gas chromatog-raphic column.

In contrast to our previous studies, we decided to restrict thisanalysis to the gaseous compounds except methane (i.e., ethane,ethene, propane, propene, isobutane, isobutene and butene-1(chromatographically not resolved), n-butane, trans-butene-2,and cis-butene-2), and toluene. This selection of compounds re-sulted mainly from the fact that most other hydrocarbons (includ-ing benzene, for instance) could not be determined with sufficientaccuracy because of peaks overlapping with, and limited separa-tion of, unknown compounds in the molecular weight range ofinterest. In addition to the gaseous compounds, only tolueneconcentrations could be determined. Owing to analytical limita-tions, the method does not include the measurement of methanebecause of inefficiency of the cold trap involved. Hydrocarbonyields (Tables 1-3) were obtained by external standardization ofthe flame-ionization detector signal with n-butane and represent,therefore, n-butane weight equivalents. Different response factorsfor the various compounds were not considered owing to the lackof reliable data. We can assume that this simplification is accept-able for the kind of data presented here.

As discussed later, we have indications that the yields obtainedby our thermovaporization method represent the total quantitiesof saturated and aromatic light hydrocarbons both dissolved in thepore water and adsorbed at, or distributed in, the mineral/kerogenmatrix. In contrast, part of the olefinic compounds might begenerated by low-temperature "hydrous pyrolysis" under the con-ditions applied. In addition, the "artificial" formation of olefinsby the drilling process cannot completely be ruled out (Faber etal., 1988). This possibility could not apply, however, to samplesobtained by hydraulic piston coring.

Concentration values obtained by our method should not becompared with corresponding data obtained by other methods,e.g., static headspace analysis of DSDP Leg 22 sediments (Hunt,1974), unless partition coefficients are known.

Relative Standard Deviation of Hydrocarbon YieldMeasurements

The relative standard deviation (or coefficient of variation) ofhydrocarbon stripping yields was determined to be about 10%-20% for rock samples containing relatively low to moderate lighthydrocarbon stripping yields of 40-3000 ng/g dry-weight sedi-ment (Schaefer et al., 1978). There are reasons to believe, how-ever, that the pronounced scatter of the low thermovaporization

yields in the present study is not an instrumental problem, but ismainly caused by heterogeneities in the rock samples (see alsofollowing discussion about the influence of the thermovaporiza-tion temperature on hydrocarbon yield).

As a result of the anticipated low light-hydrocarbon concen-trations in the sediment samples, the standard deviation of theconcentration values was determined for a selected sample. Themeasurements of 10 aliquots of Sample 121-757B-20X-5, 147-150 cm, resulted in the following relative standard deviations:ethane (±51%), propane (54%), isobutane (91%), «-butane (53%),and toluene (69%). The standard deviations for the olefinic hy-drocarbons were even higher (exceeding even 100% for propene,trans-, and cis-butene-2) and, therefore, the olefin concentrationsin Table 3 should be considered with much more caution.

They will not be discussed in detail and are included in thistable only for documentary purposes. Although these results areonly valid for the particular sample studied, we believe that suchfluctuations are at least typical for the reliability of the individualmeasurements discussed below. The organic carbon determina-tion for the aliquots of this particular sample gave a standarddeviation of ±29%. There was no clear relationship betweenorganic carbon contents and light-hydrocarbon yields.

Influence of Thermovaporization Temperature onHydrocarbon Yield

In order to investigate the influence of the thermovaporizationtemperature as an important analytical parameter for the determi-nation of the hydrocarbon yield, we selected a sediment samplefor a systematic study. To this end, aliquots of Sample 121-757B-17H-6, 0-1 cm, a nannofossil calcareous ooze from 156.60 mbelow seafloor (mbsf), were introduced in the sampling tube andheated to temperatures between ambient (22°C) and 150°C. Theanalytical results show that, despite considerable data scatter,there is a significant increase with temperature for all compounds,with the highest yields, in general, at 150°C (Table 1). The lowyields at 22°C mainly represent those hydrocarbons dissolved inthe pore water, whereas at higher temperatures additional amountsof the same compounds are mobilized from more strongly adsorb-ing sites or from closed pores in the sediment matrix. In thisparticular example, the increase for the saturated hydrocarbonsand toluene is nearly 10-fold; the olefinic compounds increase bya factor of 11-15. The corresponding change in the hydrocarboncomposition is exemplified in Figure 2, where the relevant sec-tions of the gas chromatograms between ethene and cis-butene-2are depicted for the measurements at 22°, 80°, and 150°C, respec-tively. At this stage of our investigation it is very difficult todetermine whether part of the olefinic compounds are formed bypossible chemical reactions during thermovaporization at thehigher temperatures (e.g., 150°C) from the immature organicmatter. However, in view of the occurrence of olefins at 22°C andsimilar concentration ratios in relation to their saturated homologs

Table 1. Thermovaporization hydrocarbon yield as a function of temperature of Sample 121-757B-17H-6, 0-1 cm.

^-org(<Po)

0.060.060.070.070.070.120.050.140.11

"Thermodesorption"temperature

( ° Q

22406080

100110120130150

2Ethane

2.191.663.964.34

18.836.56

30.674.67

22.27

3Propane

2.783.785.566.34

18.7310.1219.617.50

25.96

4Methylpropane

0.681.871.741.573.812.925.092.337.48

5n-Butane

1.312.452.713.408.735.12

13.194.20

15.29

Toluene

2.002.703.042.524.948.97

10.576.59

15.41

7Ethene

0.500.230.831.191.774.894.642.707.00

8Propene

0.870.832.112.022.484.554.863.70

11.47

9Isobutene + Butene-1

1.452.263.433.262.207.63

14.188.73

15.67

10trans-Butene-2

0.230.310.480.670.611.211.631.292.81

11cis-Butene-2

0.160.350.360.510.551.111.121.192.40

Note: Hydrocarbon yield expressed in nanograms per gram of dry-weight sediment. Numbers 2-5 and 7-11 refer to peak numbering in Figure 2.

459


Table 2. Organic carbon contents in percent of dry-weight sediment from selected coresamples.

Core, section,interval (cm) Age Lithology

Depth(mbsf)

121-752A-

19X-3, 149-15019X-4, 0-129X-5, 149-150

121-752B-

6R-4, 0-17R-3, 147-1508R-6, 0-19R-1, 0-110R-5, 149-15011R-2, 0-112R-2, 149-15014R-2, 149-15017R-4, 147-15019R-2, 149-150

121-754A-

1H-4, 0-17H-6, 0-111H-6, 0-112H-6, 0-113X-3, 0-114X-4, 0-120N-1, 149-15021N-1, 0-1

121-754B-

7R-2, 149-1508R-1, 0-19R-4, 0-110R-4, 149-15011R-2, 149-15012R-2, 0-113R-6, 149-15014R-7, 0-115R-5, 0-116R-2, 149-15017R-2, 0-124R-2, 39-4025R-1, 0-1

121-755A-

1R-4, 149-1506R-2, 149-1509R-1, 149-15011R-1, 67-6812R-1, 0-113R-4, 51-5215R-4, 0-117R-4, 38-3919R-5, 149-150

PaleogenePaleogenePaleogene

PaleogenePaleogenePaleogenePaleogenePaleogenePaleogeneCretaceousCretaceousCretaceousCretaceous

NeogeneNeogeneNeogenePaleogenePaleogeneCretaceousCretaceousCretaceous

CretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceous

NeogeneCretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceous

Nannofossil calcareous chalkNannofossil calcareous chalkNannofossil calcareous chalk

Calcareous chalkCalcareous chalkCalcareous chalkCalcareous chalkCalcareous chalkVolcanic ash with micriteCalcareous chalkCalcareous chalkCalcareous chalkCalcareous chalk

Foraminifer nannofossil oozeForaminifer nannofossil oozeNannofossil oozeNannofossil oozeNannofossil oozeNannofossil oozeCalcareous chalkCalcareous chalk

Calcareous chalkCalcareous chalkLimestoneLimestoneLimestoneLimestoneLimestoneLimestoneLimestoneLimestoneLimestoneLimestone and chertLimestone and chert

Foraminifer nannofossil oozeAshy limestoneTuff with micriteTuff with micriteTuff with micritePorcellaniteTuff with micriteTuff with glauconiteTuff

175.59175.60275.19

311.10320.57331.90335.40352.59356.30367.39386.59418.57434.59

4.5061.60

100.40110.10115.30126.50164.09167.10

173.99180.60194.80205.99212.59220.70236.85247.40254.20260.89269.10337.19345.00

5.9875.09

102.69121.17131.10145.61164.38184.05206.19

0.070.130.13

0.050.060.080.080.070.130.070.130.070.13

0.130.130.040.070.130.130.080.09

0.110.080.080.080.080.130.100.130.060.080.130.170.23

0.130.130.130.180.260.130.130.130.13

over the large temperature range, it is assumed that the yield datashown in this study represent mainly thermally desorbed hydro-carbons.

RESULTS AND DISCUSSION

Sites 752, 754, and 755

Regional Setting

Broken Ridge is a rifted fragment of an oceanic platform (theKerguelen-Heard Plateau) now situated at 30°S in the centralIndian Ocean. Sites 752 to 755 form a north-south transect alongthe crest of Broken Ridge (for details see Fig. 1). The transect wasdesigned to sample the entire dipping truncated sequence at Bro-ken Ridge (Peirce, Weissel, et al., 1989).

Light-Hydrocarbon Yields at Broken Ridge

The low-molecular-weight hydrocarbon yields and the organiccarbon contents of the samples are summarized in Tables 2 and 3.Organic carbon contents are close to 0.1% in all samples (0.04%-0.13% range), except for a few samples with slightly higher valuesat the bottom of Hole 754B in Cores 121-754B-24R and -25R(0.17% and 0.23%) and in Cores 121-755A-1 IR and -12R of Hole755A (0.18% and 0.26%). For a detailed discussion of organiccarbon contents at these sites see Littke et al. (this volume).

The variation of light-hydrocarbon yields is extreme. For ex-ample, the sum of C2-C4 saturated hydrocarbons (sum of ethane,propane, isobutane, and «-butane) reaches nearly 400 ng/g ofdry-weight sediment (ppb) in a nannofossil ooze of Paleogene age(Section 121-754A-12H-6) and is nearly two orders of magnitude

460


Table 2 (continued).

Core, section,interval (cm)

121-757B-

14H-6, 0-117H-6, 0-118H-6, 0-119H-4, 0-320X-5, 147-15021X-6, 0-322X-4, 0-323X-4, 0-326X-2, 0-331X-1, 0-135X-3, 0-3

121-758A-

1H-4, 0-32H-5, 0-13H-6, 144-1454H-6, 0-16H-5, 147-15011H-5, 0-114X-3, 0-122X-3, 0-126X-5, 0-329X-3, 0-131X-5, 0-133X-3, 0-135X-2, 0-139X-1, 149-15041X-5, 0-142X-2, 0-144X-CC, 0-545X-CC, 0-248R-4, 147-14849R-1, 49-5050R-2, 0-151R-1.0-152R-1,0-152R-1, 145-15054R-1, 145-15055R-4, 141-14256R-2, 0-157R-1, 139-140

Age

PaleogenePaleogenePaleogenePaleogenePaleogenePaleogenePaleogenePaleogenePaleogenePaleogenePaleogene

NeogeneNeogeneNeogeneNeogeneNeogeneNeogeneNeogenePaleogenePaleogenePaleogeneCretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceousCretaceous

Lithology

Nannofossil calcareous oozeNannofossil calcareous oozeNannofossil calcareous oozeNannofossil calcareous oozeNannofossil calcareous oozeCalcareous oozeCalcareous oozeCalcareous oozeLapilli tuffTuffTuff

Nannofossil ooze with clayNannofossil ooze with clayNannofossil ooze with clayNannofossil ooze with clayNannofossil ooze with clayNannofossil oozeNannofossil chalkNannofossil chalkCalcareous nannofossil chalkCalcareous nannofossil ooze and chalkCalcareous nannofossil chalkCalcareous nannofossil chalkForaminiferal calcareous chalkCalcareous chalkClay with foraminifers, nannofossils, and ashClay with foraminifers and nannofossilsPorcellanitePorcellaniteTuffTuffTuffTuffTuffTuffCalcareous tuffBasaltClayey tuffClayey tuff

Depth(mbsf)

127.60156.60166.30173.00182.17190.20196.90206.50232.50279.40320.90

4.5012.0024.5432.7051.8798.80

124.70202.00243.70269.59291.90308.30326.10359.09383.01388.10399.60405.60437.17441.39452.10460.20469.90471.35490.65504.59509.90519.29

corg(%)

0.080.110.080.130.050.130.080.070.130.080.08

0.130.130.130.130.130.130.140.130.110.130.040.130.090.180.130.130.130.130.130.130.130.130.130.900.080.090.130.13

lower in the Cretaceous limestone of the same site (Section121-754B-9R-4).

Light-hydrocarbon yields appear to vary with geological age.The Neogene samples of Site 754 (three samples) and Site 755(one sample) have rather low saturated hydrocarbon yields (about50-100 ppb). The toluene content ranges from 8 to nearly 50 ppb.A comparison of these values with corresponding data of DSDPSite 586 (Ontong-Java Plateau), where a section of Neogenenannofossil ooze was analyzed by the same technique (Schaeferand Leythaeuser, 1985), shows that the saturated hydrocarbonyield at Sites 754 and 755 is higher by a factor of 3 to 5, whereasthe toluene content is similar.

The samples of Paleogene age of Site 752 and 754 reveal astrong variation in light-hydrocarbon yields. Most yields, how-ever, are higher than in the Neogene sediments. Saturated hydro-carbon yields reach nearly 400 ppb (Sample 121-754A-12H-6,0-1 cm) as a result of enhanced ethane and propane contents, andare between 100 and nearly 300 ppb in most other samples of thisage. The toluene contents are generally also higher than in theNeogene. Values exceeding 100 ppb were found in samples from175.79 and 331.90 mbsf at Site 752. A possible explanation forthe difference in yields between the Neogene and Paleogenesamples could be an elevated primary production and/or morefavorable conditions for preservation of organic matter in the

Paleogene. In addition, the volcanic ash content is lower in thePaleogene section relative to the Neogene section.

The Cretaceous rocks at Sites 752 and 754 (mainly calcareouschalks and limestones with varying ash contents) have low satu-rated hydrocarbon yields (in general, between 10 and 100 ppb)and toluene contents between 7 and about 70 ppb. Particularly lowyields are observed at about 195 mbsf and between 220 and 248mbsf at Site 754. This low yield may be a result of the elevatedvolcanic ash contents at these depths.

The saturated hydrocarbon yields of the Cretaceous sedimentsof Site 755 are of a similar range as those from the other two sites.Very low yields, however, were found in samples from 121.17and 131.10mbsf (as at 194.80 mbsf of Site 754). The most strikingresult in this sediment section is the very high toluene content(315 ppb) in a tuff sample obtained from 184.05 mbsf. In addition,high toluene contents appear to be generally correlated with higholefin yields. We are not certain at this stage of our investigationwhether this correlation of high toluene and olefin yield is due tomere coincidence or reflects a chemical relationship. Moreover,in view of the low organic-carbon content of Sample 121-755A-17R-4, 38-39 cm, with 0.13% Corg and the very low maturity levelof the organic matter, the origin of such high toluene contentscannot be explained as a generation feature. In a previous studywe interpreted high relative toluene concentrations to be caused

461


Table 3. Low-molecular-weight hydrocarbon yield in nanograms per gram of dry-weight sediment from selected core samples.


121-752A-

19X-3, 149-15019X-4, 0-129X-5, 149-150

121-752B-

6R-4, 0-17R-3, 147-1508R-6, 0-19R-1, 0-110R-5, 149-15011R-2, 0-112R-2, 149-15014R-2, 149-15017R-4, 147-15019R-2, 149-150

121-754A-

1H-4, 0-17H-6, 0-111H-6, 0-112H-6, 0-113X-3, 0-114X-4, 0-120N-1, 149-15021N-1, 0-1

121-754B-

7R-2, 149-1508R-1, 0-19R-4, 0-110R-4, 149-15011R-2, 149-15012R-2, 0-113R-6, 149-15014R-7, 0-115R-5, 0-116R-2, 149-15017R-2, 0-124R-2, 39-4025R-1, 0-1

121-755A-

1R-4, 149-1506R-2, 149-1509R-1, 149-15011R-1, 67-6812R-1, 0-113R-4, 51-5215R-4, 0-117R-4, 38-3919R-5, 149-150

Ethane

112.636.233.6

47.161.536.713.652.525.338.911.014.85.3

20.649.716.9

133.929.2

6.831.614.6

45.125.8

1.425.517.28.01.43.3

11.312.416.414.512.8

35.610.914.4

1.91.9

20.918.110.610.9

Propane

62.926.526.9

51.372.258.862.153.226.443.9

8.917.17.7

21.528.621.0

139.618.36.59.2

20.6

58.034.0

1.832.015.611.75.03.3

38.419.125.716.814.0

27.19.15.62.22.5

22.422.836.017.8

Methylpropane

35.57.97.6

14.921.216.87.5

14.26.89.12.94.61.8

6.39.56.4

35.37.12.75.17.3

16.39.40.67.67.53.41.71.2

13.26.77.84.33.4

10.63.53.80.80.87.96.6

16.06.3

n-Butane

55.319.810.9

31.440.423.8

8.430.916.821.3

6.510.33.9

12.614.89.6

70.614.65.8

11.715.6

30.217.9

1.116.411.75.82.92.6

24.511.212.78.99.2

21.56.97.51.41.6

12.612.58.5

10.4

Toluene

106.416.47.4

28.336.3

120.273.945.4

7.814.87.28.64.9

14.28.39.2

48.110.710.121.118.8

68.719.39.8

18.041.4

8.78.85.5

28.026.534.015.513.9

48.137.655.713.84.3

55.940.1

315.674.6

Ethene

47.212.02.3

13.114.840.017.421.44.33.13.65.91.8

10.39.76.3

15.510.13.76.64.8

25.55.20.9

14.90.44.10.41.50.9

11.210.62.53.4

13.84.44.30.70.68.56.1

20.42.5

Propene

58.77.20.7

17.428.8

108.40.9

42.67.34.33.88.51.9

14.08.6

16.727.6

7.33.85.4

11.3

53.16.90.9

12.220.6

5.02.21.5

21.920.416.23.45.2

14.35.52.10.50.6

17.26.81.14.5

Isobutene + Butene-1

222.416.43.4

45.158.0

440.5179.2136.4

5.74.65.1

13.93.8

19.110.838.230.313.65.7

13.538.6

103.712.36.8

65.376.79.9

15.25.0

41.248.169.4

5.05.0

21.69.5

11.21.51.2

98.618.8

269.411.2

trans-Butene-2

27.52.30.8

10.813.0

107.943.177.9

1.41.10.91.90.7

3.01.74.65.61.91.31.7

11.5

15.02.30.4

16.910.81.10.50.88.7

11.9—0.81.1

5.71.31.40.20.3

14.41.9

79.91.3

cis-Butene-2

1.90.8

—78.429.1—1.31.00.71.70.6

2.21.75.85.31.71.21.4

12.1

1.80.3

——1.00.30.6

———0.70.8

6.21.01.10.10.1

10.11.4

53.31.1

by the preferential removal of polar compounds from movingcompaction waters by adsorption on the mineral surfaces or in thekerogen matrix (Schaefer et al., 1983a, 1983b). This hypothesis,however, needs further clarification and experimental support.

Sites 757 and 758

Regional Setting

Ninetyeast Ridge is interpreted as the 80- to 40-Ma-old vol-canic trace of a hotspot now located below the Kerguelen Plateau(Peirce, Weissel, et al., 1989). The ridge extends over 4500 km ina north-south direction and is covered by pelagic, carbonate-richsediments thickening northward. Site 757 is located in the centralpart of the ridge at 17°S, and Site 758 is at the northern end ofNinetyeast Ridge, where recent sedimentation is influenced byterrestrial debris from the Bengal Fan.

Light-Hydrocarbon Yields at Ninetyeast Ridge

The low-molecular-weight hydrocarbon yields and the organiccarbon contents of the samples are summarized in Tables 2 and 3.Organic carbon contents are close to 0.1% (0.04%-0.18% range)except for one tuff sample (121-758A-52R-1, 145-150 cm) of471.35 mbsf depth with 0.9% Corg.

The Paleogene section of Site 757 is characterized by low-molecular-weight hydrocarbon yields (sum of ethane, propane,isobutane, and n-butane) of 40-200 ppb (with one exception ofvirtually zero in Sample 121-757B-14H-6, 0-1 cm). The yieldsare, except for samples below 232 mbsf, significantly lower thanthose of the Paleogene section from Sites 752, 754, and 758.Similarly, toluene content is generally lower in the NinetyeastRidge sediments, although these concentrations are variable,ranging from 8 to about 140 ppb. The lapilli tuff (Sample 121-

462


Table 3 (continued).


121-757B-

14H-6, 0-117H-6, 0-118H-6, 0-119H-4, 0-320X-5, 147-15021X-6, 0-322X-4, 0-323X-4, 0-326X-2, 0-331X-1.0-135X-3, 0-3

121-758A-

1H-4, 0-32H-5, 0-13H-6, 144-1454H-6, 0-16H-5, 147-15011H-5, 0-114X-3, 0-122X-3, 0 -126X-5, 0-329X-3, 0-131X-5, 0-133X-3, 0-135X-2, 0-139X-1, 149-15041X-5, 0-142X-2, 0-144X-CC, 0-545X-CC, 0-248R-4, 147-14849R-1,49-5050R-2, 0-151R-1.0-152R-1, 0-152R-1, 145-15054R-1, 145-15055R-4, 141-14256R-2, 0-157R-1, 139-140

Ethane

1.022.38.29.8

29.815.727.648.955.637.264.3

26.313.628.321.617.819.060.238.830.754.736.115.656.6

108.0254.9

7.915.552.285.842.9

128.4105.8100.453.40.73.0

87.921.8

Propane

0.326.014.321.421.720.527.927.082.419.969.8

14.99.4

23.229.030.526.449.047.714.967.324.617.574.725.6

202.05.2

17.859.991.834.0

117.599.1

109.265.1

1.04.1

68.118.0

Methylpropane

7.54.3

12.87.6

10.16.67.0

25.19.3

15.0

9.02.89.49.1

12.916.310.817.25.4

22.55.75.8

20.64.3

47.72.65.5

14.490.911.327.421.323.118.40.40.6

15.45.7

«-Butane

0.115.39.38.6

16.212.912.516.146.225.238.2

14.57.3

13.915.914.811.631.230.214.744.714.79.5

42.729.766.74.39.3

35.252.224.050.856.261.434.60.74.4

43.514.1

Toluene

_15.47.7

19.411.416.89.6

13.6137.8

13.317.9

48.38.6

14.217.947.972.319.227.317.835.88.0

12.246.670.376.513.79.2

34.9340.0

32.042.735.739.361.24.4

20.282.978.7

Ethene

0.27.02.88.22.8

11.63.57.4

33.96.77.9

108.04.6

27.610.865.190.5

9.88.37.9

27.54.17.5

11.520.721.44.41.9

29.092.710.916.816.619.823.30.61.2

17.18.9

Propene

_11.56.0

34.94.9

20.32.69.2

127.06.0

14.6

64.22.2

24.715.547.764.714.912.05.2

56.33.6

12.718.315.435.94.12.9

42.3415.0

10.917.022.728.725.30.91.0

29.56.8

Isobutene + Butene-1

15.712.099.010.053.111.619.4

384.29.4

10.8

94.94.4

35.839.0

110.0217.0

18.626.76.9

158.74.8

30.842.759.691.310.85.9

80.01931.2

23.280.837.853.629.7

3.11.4

37.517.9

trans-Butene-2

2.82.1

22.22.1

15.21.02.7

84.22.22.8

1.04.24.2

14.3—3.93.32.0

33.31.22.95.25.96.11.11.2

—299.4

3.04.35.26.95.60.40.36.03.0

cis-Butene-2

2.41.8

35.01.7

15.70.92.0

61.61.92.5

0.93.83.1

——3.32.91.8

—0.82.54.54.55.41.0

——

236.52.73.44.75.84.80.40.34.52.6

Note: Dash = not determined (value below or near detection limit, or uncertain because of peak overlapping).

757B-26X-2, 0-3 cm) from 232.5 mbsf is certainly an exceptionto the low toluene contents. A similar reason as that for Sample121-755A-17R-4, 38-39 cm, may explain its high toluene con-centration. The lower light-hydrocarbon yields of the samplesfrom the interval between 127 to 207 mbsf can be explained bythe fact that these sediments consist of pure carbonates (96%carbonate content). They were deposited in a similar setting as theNeogene carbonates of the Broken Ridge area (Sites 754 and 755)and reveal similar hydrocarbon yields. The tuff sample from 232.5mbsf mentioned above reflects the lithologic change from mainlycarbonate to a tuffaceous section. In the upper part of the lattersection, elevated porosities were measured (50%-60% vs. about40% in the carbonate section).

Drilling at Site 758 penetrated a section of Neogene, Paleo-gene, and Cretaceous sediments. As indicated in Table 3, light-hydrocarbon yields have a tendency to increase with depthalthough the data scatter is significant (Figs. 3 and 4). The highestvalue of about 570 ppb for the sum of saturated hydrocarbonsoccurs at 383.01 mbsf in the Cretaceous. As at Site 755, thehighest toluene content at Site 758 is also encountered in a sampleof tuffaceous lithology (121-758A-48R-4, 147-148 cm) from437.17 mbsf. As mentioned previously, a toluene content of 340ppb is remarkably high in light of the low hydrocarbon potential

of these sediments. The toluene is not, most probably, of autoch-thonous origin in this particular sample.

The saturated hydrocarbon yields from Site 758 sedimentsremain rather high down to 470 mbsf, but are drastically reducedat 490.65 and 504.59 mbsf. This reduction roughly correlates withthe occurrence of a basalt layer around these depths. The light-hydrocarbon yields, including toluene, in the Cretaceous sedi-ments at Ninetyeast Ridge are considerably higher than in thesediments of corresponding age at Broken Ridge. This differencemay be due to the fact that the sediments at Site 758, in contrastto the Broken Ridge sites, were not uplifted, and the hydrocarbonswere retained as far as possible. The lower yields in the Creta-ceous sections of Sites 752 and 754 could be due, therefore, topressure release during uplift processes. The rather high yields inHole 758A at 383.01 and 437.17 mbsf correlate with high-poros-ity intervals near these depths. Below 490 mbsf the porosities arestrongly reduced with an exception at 510 mbsf, where a saturatedhydrocarbon yield of 215 ppb was measured.

CONCLUSIONSTrace amounts of low-molecular-weight hydrocarbons oc-

curred in all samples investigated in this study. Quantities ofindividual C2 to C4 hydrocarbons (saturated and olefinic), as well

463


2

1.5

1.5

2 min

B

1.5 fFigure 2. Partial gas chromatograms showing C2-C4 hydrocarbon composition of Sample121-757B-17H-6, 0-1 cm, as a function of thermovaporization temperature. For peak identifi-cation see Table 1. The compound marked by a triangle is an unknown compound. A. 22°C. B.80°C. C.150°C.

464


50-

100^

150^

200^

250^

300^

350^

400^

450^

500-

1~i 1—r—rr

10

C2-C4 saturated hydrocarbons (ng/g of rock)

Figure 3. Sum of C2-C4 saturated hydrocarbon yield (ng/g of dry-weightsediment) plotted against depth for core samples of Hole 758A. Open circles= Neogene, solid circles = Paleogene, and triangles = Cretaceous.

as toluene as a selected aromatic compound, varied considerablyin the sample series. Part of this data scatter was due to inhomo-geneities in the rock samples, as exemplified by analysis of aselected sample of Core 121-757B-20X. Relative standard devia-tions (coefficients of variation) of about 50% were measured inthis sample for ethane, propane, and n-butane.

Apart from these fluctuations, other systematic differencesappeared to be associated with changes in geological age and/orlithology of the sediments. For example, hydrocarbon yields inthe Paleogene sediments of Broken Ridge (Sites 752 and 754)were generally higher than in the Neogene section (Sites 754 and755). This difference may be due to a reduced primary productionof organic matter and higher volcanic ash contents in the latter.Like the Neogene, the Cretaceous calcareous chalks and lime-stones from Broken Ridge also revealed lower hydrocarbonyields. In contrast to Broken Ridge, most of the Paleogene sectionat Ninetyeast Ridge (Site 757) revealed considerably lower yields,which may be explained by the similar lithologic composition(i.e., by the occurrence of practically pure carbonates) as of theNeogene at Broken Ridge.

Despite considerable data scatter, a depth-related increase ofhydrocarbon yields was found in Hole 758A sediments wheretotal C2-C4 saturated hydrocarbon yields increased from about 50ppb to more than 300 ppb (maximum of 570 ppb reached at 383mbsf) The light-hydrocarbon yields, including toluene, in theCretaceous sediments at Ninetyeast Ridge are considerably higherthan in corresponding sediments from Broken Ridge. This differ-ence was tentatively interpreted to be a result of the lack of uplift

10 100Toluene (ng/g of rock)

1000

Figure 4. Toluene yield (ng/g of dry-weight sediment) plotted against depth forcore samples of Hole 758A. Open circles = Neogene, solid circles = Paleogene,and triangles = Cretaceous.

at Site 758. The hydrocarbons in these sediments, therefore, wereretained in the pore network more efficiently than in the samesection at Broken Ridge. Accordingly, the lower yields at Sites752, 754, and 755 could be due to pressure release during upliftprocesses associated with rifting.

The highest toluene yields were observed in two tuff samplesof Cretaceous age (Sections 121-755A-17R-4 and 121-758A-48R-4) with 315 and 340 ppb, respectively. In view of the factthat tuffs do not contain indigenous organic matter and the lowmaturity of these sediments, such high toluene contents cannot beexplained as a generation feature. In a previous study of DSDPLeg 71 (Schaefer et al., 1983a, 1983b) we interpreted high toluenecontents in relation to its saturated homolog by the preferentialremoval of polar compounds from moving compaction waters byretention on particularly adsorptive mineral surfaces, such aszeolitic claystones. It is uncertain, however, whether this specu-lative interpretation applies to the high toluene concentrations asobserved in certain sediments of tuffaceous lithology in the pre-sent study.

ACKNOWLEDGMENTSWe gratefully acknowledge the financial support of the

Deutsche Forschungsgemeinschaft, Schwerpunktprogramm"Ocean Drilling Program/Deep Sea Drilling Project" (grant no.We 346/27), and of Bundesanstalt für Geowissenschaften undRohstoffe, Hannover, the German member organization of theJoint Oceanographic Institutions for Deep Earth Sampling (JOI-DES), for providing the travel funds of one of the authors (R. L.)

465


who participated in the ODP Leg 121 cruise. Thanks are also dueto the U.S. National Science Foundation for supplying the sam-ples for this study.

Organic carbon values were kindly provided by Dr. B. Hors-field (KFA Jülich). Technical assistance by Mrs. E. Biermanns,Mr. R. Harms, and Mr. H.-G. Sittardt is gratefully appreciated.

REFERENCES

Faber, E., Gerling, P., and Dumke, I., 1988. Gaseous hydrocarbons ofunknown origin found while drilling. In Mattavelli, L., and Novelli,L. (Eds.), Advances in Organic Geochemistry 1987: Oxford (Perga-mon Press), 875-879.

Hunt, J. M., 1974. Hydrocarbon and kerogen studies. In von der Borch,C. C , Sclater, J. G., et al., Init. Repts. DSDP, 22: Washington (U.S.Govt. Printing Office), 673-676.

, 1975. Origin of gasoline range alkanes in the deep sea. Nature,254:411-413.

, 1984. Generation and migration of light hydrocarbons. Science,226:1265-1270.

Jasper, J. P., Whelan, J. K., and Hunt, J. M., 1984. Migration of Cj to C8

volatile organic compounds in sediments from the Deep Sea DrillingProject, Leg 75, Hole 530A. In Hay, W. W., Sibuet, J.-C, et al., Init.Repts. DSDP, 75 (Pt. 2): Washington (U.S. Govt. Printing Office),1001-1008.

Peirce, J., Weissel, J., et al., 1989. Proc. ODP, Init. Repts., 121: CollegeStation, TX (Ocean Drilling Program).

Schaefer, R. G., and Leythaeuser, D., 1984. C2-C8 Hydrocarbons insediments from Deep Sea Drilling Project Leg 75, Holes 530A,Angola Basin, and 532, Walvis Ridge. In Hay, W. W., Sibuet, J.-C,et al., Init. Repts. DSDP, 75 (Pt. 2): Washington (U.S. Govt. PrintingOffice), 1055-1067.

, 1985. Low-molecular-weight hydrocarbons in sediments ofDeep Sea Drilling Project Leg 89, Sites 585, East Mariana Basin, and586, Ontong-Java Plateau. In Moberly, R., Schlanger, S. O., et al.,Init. Repts. DSDP, 89: Washington (U.S. Govt. Printing Office),577-586.

, 1987. Low-molecular-weight hydrocarbons in sediments ofDeep Sea Drilling Project Leg 93, Hole 603B, off the east coast of

North America. In van Hinte, J. E., Wise, S. W., Jr., et al., Init. Repts.DSDP, 93: Washington (U.S. Govt. Printing Office), 1237-1244.

Schaefer, R. G., Leythaeuser, D., and Gormly, J., 1984. Generation andmigration of low-molecular-weight hydrocarbons in sediments ofDeep Sea Drilling Project Leg 79, Sites 544, 545, and 547, offshoreMorocco. In Hinz, K., Winterer, E. L., et al., Init. Repts. DSDP, 79:Washington (U.S. Govt. Printing Office), 743-773.

Schaefer, R. G., Leythaeuser, D., and von der Dick, H., 1983a. Generationand migration of low-molecular-weight hydrocarbons in sedimentsfrom Site 511 of DSDP/IPOD Leg 71, Falkland Plateau, South Atlan-tic. In Bjor0y, M., et al. (Eds.), Advances in Organic Geochemistry1981: Chichester (Wiley), 164-174.

Schaefer, R. G., von der Dick, H., and Leythaeuser, D., 1983b. C2-C8

hydrocarbons in sediments from Deep Sea Drilling Project Leg 71,Site 511, Falkland Plateau, South Atlantic. In Ludwig, W. J., Krash-eninnikov, V. A., et al., Init. Repts. DSDP, 71: Washington (U.S.Govt. Printing Office), 1033-1043.

Schaefer, R. G., Weiner, B., and Leythaeuser, D., 1978. Determinationof sub-nanogram per gram quantities of light hydrocarbons (C2-C9)in rock samples by hydrogen stripping in the flow system of a capillarygas chromatograph. Anal. Chern., 50:1848-1854.

Whelan, J. K., 1984. Volatile C r C g compounds in marine sediments. InOdham, G., Larsson, L., and Márdh, P.-A. (Eds.), Gas Chromatogra-phy/Mass Spectrometry. Applications in Microbiology Ser., NewYork (Plenum), 381—414.

Whelan, J. K., and Hunt, J. M., 1981. Q-Cg Hydrocarbons in IPOD Leg63 sediments from outer California and Baja California borderlands.In Yeats, R. S., Haq, B. U., et al., Init. Repts. DSDP, 63: Washington(U.S. Govt. Printing Office), 775-784.

, 1982. Cj-Cg Hydrocarbons in Leg 64 sediments, Gulf of Cali-fornia. In Curray, J. R., Moore, D. G., et al., Init. Repts. DSDP, 64(Pt. 2): Washington (U.S. Govt. Printing Office), 763-780.

Whelan, J. K., Hunt, J. M., Jasper, J., and Hue, A., 1984. Migration ofC^Cg hydrocarbons in marine sediments. In Schenck, P. A., deLeeuw, J. W., and Lijmbach, G.W.M. (Eds.), Advances in OrganicGeochemistry 1983: Oxford (Pergamon Press), 683-694.

Date of initial receipt: 19 March 1990Date of acceptance: 13 November 1990Ms 121B-176

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23. low-molecular-weight hydrocarbons in …bons in sediments of dsdp legs 71, 75, 79, 89, and 93...

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