18. ORGANIC GEOCHEMISTRY, LEG 64: INTRODUCTION AND SUMMARY1

Bernd R. T. Simoneit,2 Institute of Geophysics and Planetary Physics, University of California,Los Angeles, California

INTRODUCTION

Organic geochemical studies were carried out on sam-ples from all sites of Leg 64 off Cabo San Lucas and inthe Central Gulf of California (Fig. 1). Short sections ofcore were cut on board, sealed with mylar under thecore caps, taped with black vinyl type, and then keptfrozen until the sub-sampling for the various investiga-tions. Some samples that had extruded from the end ofthe core liner because of excessive gas pressure weresealed in Kapak bags. A listing of all frozen samplestaken for organic geochemistry is found in Table 1, andtheir locations down-hole are indicated by the sequentialnumbers on the lithologic columns of each site (Figs. 2and 3). Selected interstitial gas (in vacutainers) andsmall CHN samples were also analyzed in greater detail,and some interstitial-water samples (sealed in glass am-poules) were examined for organic matter.

The 22 papers that follow in Pt. 2 of Volume 64 pre-sent the results of the organic geochemical studies. Thesemajor aspects of organic geochemistry were addressed:(1) the effects of the various intrusives on the unconsoli-dated sediments in terms of gas and liquid products andof residual kerogen; (2) the maturity, diagenesis, andcatagenesis of the organic matter; and (3) the varioussources of that organic matter and environmental condi-tions of sedimentation.

In order to keep within the general arrangement ofthis volume, I have placed the papers in the followingorder: first, the background data are presented; fol-lowed by, second, papers presenting overall regionaldata; third, papers confined to the three regions—mouth of the Gulf, Guaymas Basin and Guaymas Con-tinental Slope; and, fourth, miscellaneous papers, in-cluding a synthesis.

SUMMARYThe first four papers present general background

material. The shipboard organic geochemical and safetymonitoring data for gas, organic carbon and nitrogen,and bitumen fluorescence are summarized (Simoneit,this volume). Gas was present in all sites except 475 and476, and the biogenic components become admixed withthermogenic products at depth. Petroliferous bitumen ispresent near sills. Detailed optical analyses of the paly-

115°

105°

Curray, J. R., Moore, D. G., et al., Init. Repts. DSDP, 64: Washington (U.S. Govt.Printing Office).

2 Present address: School of Oceanography, Oregon State University, Corvallis,Oregon.

Figure 1. Location map for the Leg 64 drill sites at the mouth of and inthe central Gulf of California.

nological residues (humin, kerogen, and palynomorphs)are presented (Rueda-Gaxiola et al., this volume). Thesedata are utilized to evaluate the biostratigraphic faciesdistributions, and the type and origin of sedimentaryorganic matter with its diagenesis and catagenesis. Sam-ples from Sites 474, 476 to 479, and 481 were dated bythe radiocarbon method to supplement the sedimenta-tion rate based on the biostratigraphic record (Spikerand Simoneit, this volume). These data indicated higherrates; sedimentation is relatively greatest in GuaymasBasin > mouth of the Gulf of California > Guaymasslope. Methanogenic bacterial activity could be demon-strated to sediment depths of about 12 meters at some ofthe sites (Oremland et al., this volume).

The next nine papers discuss results on a regionalscope and are presented in the order of gas, bitumen,and kerogen data. The C2-C8 hydrocarbons from Sites474 and 476 are present at low concentrations and aretypical of immature Recent sediments (Whelan andHunt, this volume). In contrast, some of the samplesfrom Sites 477 to 479 and 481 exhibited C2-C8 composi-tions characteristic of thermogenesis. Interstitial gaswas analyzed in terms of the stable-isotope composi-

717

B. R. T. SIMONEIT

Table 1. Listing of core samples taken on Leg 64for shore-based organic geochemistry.

Sample Number(cf. Figs. 2 and 3)

123456789

10111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596061626364656667686970717273

DSDP Sample(interval in cm)

474-2-3, 120-150474-5-3, 120-150474-6-5, 0-30474-17-6, 100-125474-17-6, 125-150474A-7-2, 110-140474A-21-6, 48-78474A-28-2, 120-150474A-31-2, 120-150474A-32-2, 120-150474A-40-3, 120-150474A-41-3, 120-150475-2-2, 120-150475-6-5, 120-150475-12-4, 120-150475-16-2, 120-150476-3-4, 120-150476-7-6, 110-140476-11-4, 110-140476-15-2, 110-140476-19-1, 120-150477-5-1, 120-140477-7-1, 132-142477-16-5, 58-88477-20-1, 115-135477B-1-2, 84-98478-2-5, 120-150478-4-3, 120-150478-6-3, 120-150478-7-5, 120-150478-9-3, 110-140478-11-4, 120-150478-13-2, 120-150478-14-4, 120-150478-16-4, 120-150478-17-4, 120-150478-20-4, 108-138478-28-4, 120-150478-35-5, 110-140481-2-2, 125-150481-8-2, 110-140481-11-1, 125-150481A-4-2, 110-140481A-8-2, 110-150481A-10-2, 110-150481A-12-2, 120-150481A-13-6, 0-14481A-20-1, 140-145481A-22-4, 120-150481A-24-5, 110-140481A-26-5, 120-150481A-27.CC481A-30-5, 110-140480-20-3, 62-82479-3-2, 105-110479-3-2, 110-130479-5-3, 120-125479-5-3, 125-150479-7-5, 110-140479-9-2, 110-115479-9-2, 115-140479-13-1, 110-140479-15-5, 110-140479-17-5, 120-150479-19-5, 115-140479-22-5, 110-140479-27-4, 120-150479-29-5, 120-150479-34-5, 110-140479-37-5, 130-150479-39-4, 110-140479-43-1, 120-140479.47-4, 110-140

Sub-bottomDepth(m)a

6.3534.8546.15

153.13153.38223.25352.13413.35441.85451.35528.85538.35

8.8551.35

106.85141.8524.3565.25

100.25135.25171.8530.8049.87

121.73154.25

2.41P10.8526.8545.8558.3574.2594.85

110.85123.35142.35151.85170.73246.85305.25

7.63P36.OOP48.88P73.25

111.30130.30149.35153.57223.93247.35267.75286.85298.98324.7598.72P15.07B15.2035.7335.8857.7572.13B72.28

108.75133.75152.85171.78200.10246.35266.85314.25342.90360.25393.80436.25

a Depths are given in the DSDP convention at the top ofthe core sample; P indicates piston core, and B indi-cates extruded sample sealed in Kapak bag.

tions of methane and carbon dioxide (Galimov andSimoneit, this volume; Schoell, this volume). The con-tribution of thermogenic components at depth could beconfirmed by the enrichment of carbon-13 methane atSites 477, 478, and 481. The CO2 at depth was derivedfrom thermal carbonate decomposition (Galimov andSimoneit, this volume), except at Site 479, where itssource ranged from biogenic to decarbonylation reac-tions (Schoell, this volume). The 18O and D concentra-tions of interstitial waters from Sites 479 and 481 werealso determined and compared with lithology (Schoell,this volume).

Bitumen was analyzed for pigments and hydrocarboncompositions to evaluate sources, diagenesis, and cata-genesis (Baker and Louda, this volume; Galimov et al.,this volume; Rullkötter et al., this volume). Sedimentsamples from shallow depths contained primarily chlor-ophyll derivatives, formed under reductive diagenesis atSite 479 and in early oxidative conditions at Site 481,with traces of possible allochthonous porphyrins (Bakerand Louda, this volume). Porphyrins were observed insediments approaching sill intrusions, but not at closeproximity (Galimov et al., this volume; Baker and Lou-da, this volume). Carotenoid concentrations decreasedrapidly with sub-bottom depth, and perylene was ubiq-uitous (Baker and Louda, this volume). The hydrocar-bon yield and distributions and the stable carbon iso-tope compositions reflect the effect of thermal stressand indicate a primarily marine origin at Sites 474 and477, with a stronger terrestrial influence at Sites 478,479, and 481 (Galimov et al., this volume). This is alsoconfirmed by some of the data of Rullkötter et al. (thisvolume), based on the hydrocarbon distributions, mo-lecular markers, and maceral analysis.

Kerogen was analyzed to evaluate its maturity, petro-leum-generating potential, facies and sources (Rullköt-ter et al., this volume; Deroo et al., this volume; Gilbertand Summerhayes, this volume; Kendrick, this volume;Galimov et al., this volume). The kerogen was primarilyof an autochthonous marine origin at all sites, exceptthat at Sites 474, 478, and 481 a significant allochtho-nous component of terrigenous detritus was also detect-able (Rullkötter et al., this volume; Deroo et al., thisvolume; Galimov et al., this volume). The kerogens areimmature, except in the vicinity of sills, and have a fairto good petroleum-generating potential (Gilbert andSummerhayes, this volume; Kendrick, this volume; De-roo et al., this volume).

The sediments from the mouth of the Gulf that wereanalyzed contained organic matter derived from pri-marily marine autochthonous sources, and two lignitefragments were identified as driftwood (Simoneit, thisvolume). Carbohydrate residues were present in samplesfrom Site 474 and 476 back into the Pliocene and indi-cated sources including bacteria, marine algae, and ter-restrial plants (Swain and Bratt, this volume).

In Guaymas Basin, the sedimentary organic matterwas analyzed to examine sources, depositional environ-ment, and thermal stress from intrusions. The bitumen(lipids) and kerogen of a set of small samples (residuesfrom the shipboard CHN analyses) from Sites 477, 478,

718

ORGANIC GEOCHEMISTRY: INTRODUCTION AND SUMMARY

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Figure 2. Lithologic columns for Sites 474 to 476 at the mouth of the Gulf of California. The organic geochemistry samples are indicated by the se-quential numerals (on the left of each column), as listed in Table 1.

and 481 were characterized (Simoneit and Philp, thisvolume; Jenden et al., this volume). The bitumen of theunaltered samples was derived primarily from autoch-thonous marine sources, the allochthonous terrigenousinflux being greatest at Site 481, and lowest at Site 477.The samples approaching sills contained bitumen pyro-lysate, and those adjacent to contacts had essentially nobitumen. The pyrograms of the kerogens exhibited theinverse; unaltered material yielded complex bitumen py-rolysate, and the thermally stressed samples producedsimpler mixtures of bitumen, if any, as the intrusive

temperature increased. This is also confirmed by theN/C data and ESR spin density and line width. The δ13Cdata, however, indicated differences in maturation be-havior, possibly due to kerogen heterogeneity and dis-similar thermal stress (Jenden et al., this volume). A de-tailed lipid-with-molecular-marker analysis of Sample481-2-2, 125-150 cm was carried out (Thomson et al.,this volume). The major lipids were derived from algae,but lesser components from bacterial and terrestrialsources were also present, and all reflected a very earlystage of diagenesis.

719

B. R. T. SIMONEIT

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Figure 3. Lithologic columns for Sites 477 to 481 in the central Gulf of California. The organic geochemistry samples are in-dicated by the sequential numerals, as listed in Table 1.

The organic matter in samples from Sites 479 and 480on the Guaymas Slope has been analyzed to examinepossible differences between laminated and nonlami-nated facies (Simoneit, this volume; Peters and Simo-neit, this volume). The lipid composition of both faciesis very similar and is of an autochthonous microbial ori-gin, with traces of terrestrial influx. The non-laminatedsections contain relatively more terrigenous detritus anda much greater amount of perylene. The kerogen analy-ses also indicate a primarily marine source and imma-ture nature, and the laminated, anoxic facies containmore hydrogen-rich, oxygen-poor organic matter thanthe oxic, homogeneous facies.

The organic geochemistry of interstitial waters fromSites 474 and 479 was examined (Michaelis et al., thisvolume). Dissolved organic carbon, amino acids, andcarbohydrates were determined, and significant compo-sitional changes were observed versus sub-bottom depth.

Results of the organic carbon and nitrogen and thecarbonate analyses carried out both on board andashore are given in Appendix II (Simoneit and Bode,this volume).

These organic geochemical studies have been dis-cussed in the perspective of an overall synthesis whichfollows at the end of this section (Simoneit et al., thisvolume).

720

ORGANIC GEOCHEMISTRY: INTRODUCTION AND SUMMARY

GUAYMAS BASIN SLOPE/OXYGEN MINIMUM ZONEHOLE 481

Water depth = 1998 mHOLE481A SITE 480

Water depth = 655 mSITE 479

Water depth - 747 m

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Figure 3. (Continued).

721