Diagenetic alteration of organic matter in DSDP Leg 57 Holes

I analyzed Leg 57 sediments organogeochemically and spectroscopically. Organic carbon and extractable organic matter prevail from the Pliocene to the Miocene. Humic acids occur widely from the Pleistocene to the lower Miocene and one portion of the Oligocene. The absence of humic acids in Oligocene and Cretaceous samples suggests that humic acids had changed to kerogen. Visible spectroscopic data reveal that humic acids in this study have a low degree of condensed aromatic-ring system, which is a feature of anaerobic conditions during deposition, and that chlorophyll derivatives that had at first combined with humic acids moved to the solvent- soluble fraction during diagenesis. The elemental compositions of humic acids show high H/C and O/C ratios, which seem appropriate to a stage before transformation to kerogen. The relation between the linewidths and g-values on the electron spin resonance data indicates that the free radicals in humic acids are quite different from those in kerogen. The low spin concentrations of kerogen and the yields of humic acids up to the lower Miocene demonstrate that organic matter in these sediments is immature. The foregoing indicate the necessity to isolate humic acids even in ancient rocks in the study of kerogen.

Organic geochemistry of ODP Sites 128-798 and 128-799

Miocene to Quaternary sediments from the Oki Ridge (Site 798) and the Kita-Yamato Trough (Site 799) in the Japan Sea contain organic carbon ranging from about 0.6% in light-colored layers to almost 6% in dark layers. The organic matter consists of a variable mixture of marine and terrigenous contributions, the ratio of which is not correlated to the total organic carbon content. Marine organic particles clearly dominate in the deeper section of Hole 799B. The extractable bitumen is strongly dominated by long-chain alkenones from microalgae in the shallower sediments, whereas bishomohopanoic acid (C32) of eubacterial origin is the single most abundant compound in deeper samples. Normal alkanes and straight-chain carboxylic acids, both of which show a bimodal distribution with odd and even carbon-number predominance, respectively, are two other groups of compounds which are important constituents of the extracts. The deepest samples at Site 799 contain a considerable amount of short-chain components, which probably migrated upward from thermally more altered deeper sediments.

Organic geochemistry of DSDP Leg 41 holes

The black shale encountered in Cretaceous cores of the Cape Verde area during the DSDP Leg 41 are of marine origin and correspond to excellent potential oil source rocks. They have a low content of humic compounds. Pyrolysis assays, chloroformic extracts, and kerogen data attest to a relatively low stage of evolution for samples at Site 367 (Cape Verde Basin). The samples from Site 368 (Cape Verde Rise) are more evolved, and the deeper ones would be located at the beginning of the principal zone of oil formation.

Contents of organic compounds in sediments from DSDP Legs 1 and 4

Fifteen sediment samples were studied from five drill sites recovered by the Glomar Challenger on Legs I and IV in the Gulf of Mexico and western Atlantic. This study concentrated on compounds derived from biogenic precursors, namely: (1) hydrocarbons, (2) fatty acids, (3) pigments and (4) amino acids. Carbon isotope (dC13) data [values <(-26)?, relative to PDB], long-chain n-alkyl hydrocarbons (>>C27) with odd carbon numbered molecules dominating even carbon numbered species, and presence of perylene proved useful as possible indicators for terrigenous contributions to the organic matter in some samples. Apparently land-derived organic matter can be transported for distances over 1000 km into the ocean and their source still recognized. The study was primarily designed to investigate: (i) the sources of the organic matter present in the sediment, (ii) their stability with time of accumulation and (iii) the conditions necessary for in situ formation of new compounds.

Organic geochemistry of sediments at DSDP Leg 63 Holes

Selected core samples from the California Continental Borderland (Sites 467-469) were analyzed to evaluate the nature and composition of the lipids and kerogens in terms of their genetic origin and geological maturity. The lipids were of a multiple origin. On the basis of the homolog distributions of the n-alkanes and n-fatty acids, with the shape and magnitude of the unresolved branched and cyclic hydrocarbons, and the structural and stereochemical compositions of the molecular markers, these lipids were derived from primary autochthonous marine (microbial), from allochthonous terrigenous (higher plant wax), and from recycled (geologically mature organic matter) sources. The kerogens were composed of principally marine microbial detritus with a minor input of allochthonous terrestrial material. For the most part, the samples had undergone a thermal maturation according to a normal geothermal gradient, except in the proximity of intrusives. Such additional thermal stress was evident for the samples from Site 469 and to some extent for Site 467 at about a sub-bottom depth of 700 to 800 meters.

Composition of organic matter from samples of massive sulfides, bottom sediments, and seawater collected within the Logachev and Broken Spur hydrothermal fields, Mid-Atlantic Ridge

Lipid components of hydrothermal deposits from the unusual field at 14°45'N MAR and from the typical field at 29°N MAR were studied. For the first time mixed nature of organic matter (OM) from hydrothermal sulfide deposits was established with use of biochemical, gas chromatographic, and molecular methods of studies. In composition of OM lipids of phytoplankton, those of chemosynthesis bacteria and non-biogenic synthesis lipids were determined. Specific conditions of localization of sulfide deposits originated from ''black smokers'' (reducing conditions, absence of free oxygen, presence of reduced sulfur preventing OM from decomposition) let biogenic material, including bacterial one, be preserved in sulfide deposits. The hydrothermal system at 14°45'N MAR is characterized by geological, geochemical and thermodynamic conditions allowing abiogenic synthesis of methane and petroleum hydrocarbons. For sulfide deposits at 29°N and other active hydrothermal fields known at MAR, abiogenic synthesis of hydrocarbons occurs in lower scales.

Organic geochemistry of sediments of the Gulf of California

The effects of intrusive thermal stress have been studied on a number of Pleistocene sediment samples obtained from Leg 64 of the DSDP-IPOD program in the Gulf of California. Samples were selected from Sites 477, 478 and 481 where the organic matter was subjected to thermal stress from sill intrusions. For comparison purposes, samples from Sites 474 and 479 were selected as representative of unaltered material. The GC and GC-MS data show that lipids of the thermally unaltered samples were derived from microbial and terrestrial higher-plant detritus. Samples from sill proximities were found to contain thermally-derived distillates and those adjacent to sills contained essentially no lipids. Curie point pyrolysis combined with GC and GC-MS was used to show that kerogens from the unaltered samples reflected their predominantly autochthonous microbial origin. Pyrograms of the altered kerogens were much less complex than the unaltered samples, reflecting the thermal effects. The kerogens adjacent to the sills produce little or no pyrolysis products since these intrusions into unconsolidated, wet sediments resulted in in situ pyrolysis of the organic matter. Examination of the kerogens by ESR showed that spin density and line width pass through a maximum during the course of alteration but ESR g-values show no correlation with maturity. Stable carbon isotope (d13C) values of kerogens decrease by 1-1.5 per mil near the sills at Sites 477 and 481 and the atomic N/C decreases slightly with proximity to a smaller sill at Site 478. Differences in maturation behavior between Site 477 and 481 and Site 478 are attributed to dissimilarities in thermal stress and to chemical and isotopic heterogeneity of Guaymas Basin protokerogen.

Elemental contents, non-carbonate, fatty acids and alcohols analysis from different Holes of IODP Expedition 310

During IODP Expedition 310 (Tahiti Sea Level), drowned Pleistocene-Holocene barrier-reef terraces were drilled on the slope of the volcanic island. The deglacial reef succession typically consists of a coral framework encrusted by coralline algae and later by microbialites; the latter make up < 80% of the rock volume. Lipid biomarkers were analyzed in order to identify organisms involved in reef-microbialite formation at Tahiti, as the genesis of deglacial microbialites and the conditions favoring their formation are not fully understood. Sterols plus saturated and monounsaturated short-chain fatty acids predominantly derived from both marine primary producers (algae) and bacteria comprise 44 wt% of all lipids on average, whereas long-chain fatty acids and long-chain alcohols derived from higher land plants represent an average of only 24 wt%. Bacterially derived mono-O-alkyl glycerol ethers (MAGEs) and branched fatty acids (10-Me-C16:0; iso- and anteiso-C15:0 and -C17:0) are exceptionally abundant in the microbial carbonates (average, 19 wt%) and represent biomarkers of intermediate-to-high specificity for sulfate-reducing bacteria. Both are relatively enriched in 13C compared to eukaryotic lipids. No lipid biomarkers indicative of cyanobacteria were preserved in the microbialites. The abundances of Al, Si, Fe, Mn, Ba, pyroxene, plagioclase, and magnetite reflect strong terrigenous influx with Tahitian basalt as the major source. Chemical weathering of the basalt most likely elevated nutrient levels in the reefs and this fertilization led to an increase in primary production and organic matter formation, boosting heterotrophic sulfate reduction. Based on the observed biomarker patterns, sulfate-reducing bacteria were apparently involved in the formation of microbialites in the coral reefs off Tahiti during the last deglaciation.

Organic- and carbonate-carbon values and extract data for DSDP Holes 63-467 and 63-471

Quantity, type, and maturity of the organic matter of middle Miocene to Quaternary sediments from the eastern North Pacific (Deep Sea Drilling Project Leg 63) were determined. Hydrocarbons and fatty acids in lipid extracts were analyzed by capillary column gas chromatography and combined gas chromatography/mass spectrometry. Kerogens were investigated by Rock-Eval pyrolysis and microscopy, and vitrinite reflectance values were determined. At Site 467, in the San Miguel Gap of the outer California Continental Borderland, organic carbon contents range from 1.46% to 5.40%. Normalized to organic carbon, total extracts increase from about 10 to 36 mg/g Corg with depth. The organic matter is a mixture of both marine and terrestrial origin, with the marine organic matter representing a high proportion in some of the samples. Steroid hydrocarbons - sterenes and steradienes in the upper part of the section and steranes in the deepest sample - are the most abundant compounds in the nonaromatic hydrocarbon fractions. Perylene, alkylated thiophenes, and aromatic steroid hydrocarbons dominate in the aromatic hydrocarbon fractions of the shallower samples; increasing maturation is indicated by a more petroleumlike aromatic hydrocarbon distribution. Microscopy revealed a high amount of liptinitic organic matter and confirmed the maturation trend as observed from analysis of the extracts. The vitrinite reflectance may be extrapolated to a bottom-hole value of nearly 0.5% Ro. The liquid hydrocarbon potential of the sediments at higher maturity levels is rated to be good to excellent. At Site 471, off Baja California, organic carbon values are between 0.70% and 1.12%. Extract values increase with depth, as at Site 467. The investigation of the soluble and insoluble organic matter, despite some compositional similarities, consistently revealed a more terrigenous influx compared with Site 467. Thus the potential for liquid hydrocarbon generation is lower, the organic matter being more gas-prone. The deepest sample analyzed indicates the onset of hydrocarbon generation. At this site, frequent sand intercalations offer pathways for migration and possibly reservoir formation.

Petrography and geochemistry of organic matter in Triassic and Cretaceous sediments from the Wombat and Exmouth Plateau

Triassic (Carnian-Rhaetian) continental margin sediments from the Wombat Plateau off northwest Australia (Sites 759, 760, 761, and 764) contain mainly detrital organic matter of terrestrial higher plant origin. Although deposited in a nearshore deltaic environment, little liptinitic material was preserved. The dominant vitrinites and inertinites are hydrogen-lean, and the small quantities of extractable bitumen contain w-alkanes and bacterial hopanoid hydrocarbons as the most dominant single gas-chromatography-amenable compounds. Lower Cretaceous sediments on the central Exmouth Plateau (Sites 762 and 763) farther south in general have an organic matter composition similar to that in the Wombat Plateau sediments with the exception of a smaller particle size of vitrinites and inertinites, indicating more distal transport and probably deposition in deeper water. Nevertheless, organic matter preservation is slightly better than in the Triassic sediments. Long-chain fatty acids, as well as aliphatic ketones and alcohols, are common constituents in the Lower Cretaceous sediments in addition to n-alkanes and hopanoid hydrocarbons. Thin, black shale layers at the Cenomanian/Turonian boundary, although present at several sites (Sites 762 and 763 on the Exmouth Plateau, Site 765 in the Argo Abyssal Plain, and Site 766 on the continental margin of the Gascoyne Abyssal Plain), are particularly enriched in organic matter only at Site 763 (up to 26%). These organic-matter-rich layers contain mainly bituminite of probable fecal-pellet origin. Considering the high organic carbon content, the moderate hydrogen indices of 350-450 milligrams of hydrocarbon-type material per gram of Corg, the maceral composition, and the low sedimentation rates in the middle Cretaceous, we suggest that these black shales were accumulated in an area of oxygen-depleted bottom-water mass (oceanwide reduced circulation?) underlying an oxygen-rich water column (in which most of the primary biomass other than fecal pellets is destroyed) and a zone of relatively high bioproductivity. Differences in organic matter accumulation at the Cenomanian/Turonian boundary at different sites off northwest Australia are ascribed to regional variations in primary bioproductivity.

Organic carbon, age, lithology, and CPI-alkenes at DSDP Sites 57-440 and 56-436

"Bound" and "free" solvent-extractable lipids have been examined from Sections 440A-7-6, 440B-3-5, 440B-8-4, 440B-68-2, and 436-11-4. The compound classes studied include aliphatic and aromatic hydrocarbons, ketones, alcohols, and carboxylic acids. Carotenoids and humic acids have also been examined. The quantitative results are considered in terms of input indicators, diagenesis parameters, and structural classes. A difference in input is deduced across the Japan Trench, with a higher proportion of autochthonous components on the western inner trench slope compared with the more easterly, outer trench, wall and greater input in the early Pleistocene than in the Miocene. A variety of diagenetic transformations is observed at Site 440 as sample depth increases. Results are compared with those of samples from Atlantic Cretaceous sediments and from the Walvis Bay high productivity area.

Organic matter preservation in turbidites on the Madeira Abyssal Plain

Oxidized intervals of five organic-rich Madeira Abyssal Plain (MAP) turbidites deposited during the Miocene, Pliocene, and Pleistocene all displayed comparable major loss of total organic carbon (TOC) (84 ± 3.1%) accompanied by a negative isotopic (d13C) shift ranging from -0.3 to -2.9 per mil. Major but significantly lower loss of total nitrogen (Ntot, 61 ± 7.1%) also occurred, leading to a decrease in TOC relative to Ntot (C/Ntot) and a +1.3 to 2.7 per mil Ntot isotopic (d15N) shift. Compound specific isotopic measurements on plant wax n-alkanes indicate the terrestrial organic component in the unoxidized deposits is 13C-enriched owing to significant C4 contribution. Selective preservation of terrestrial relative to marine organic carbon could account for the d13C behavior of TOC upon oxidation but only if a 13C-depleted component of the bulk terrestrial signal is selectively preserved in the process. Although the C/Ntot decrease and positive d15N shift seems inconsistent with selective terrestrial organic preservation, results from analysis of a Modern eolian dust sample collected in the vicinity indicate these observations are compatible. Regardless of the specific explanation for these isotopic observations, however, our findings provide evidence that paleoreconstruction of properties such as pCO2 using the d13C of TOC is a goal fraught with uncertainty whether or not the marine sedimentary record considered is 'contaminated' with significant terrestrial input. Nonetheless, despite major and selective loss of both marine and terrestrial components as a consequence of postdepositional oxidation, intensive organic geochemical proxies such as the alkenone unsaturation index, UK'37, appear resistant to change and thereby retain their paleoceanographic promise.

Organic matter from black shales collected in DSDP Holes 41-367 and 41-368

It is shown that microscopic algae dominate in source material of organic matter of black shales, and admixture of residues of organisms and terrestrial humic material is contained. The main direction of source material transformation during syngenesis and sedimentogenesis is associated with jellofication resulting to formation of organic matter of significantly sapropelic type. Low reflectance of vitrinite and alginite from organic matter refer to the primary and secondary lignite stages of its carbonification. Significantly sapropel type of organic matter and low stage of carbonification are reliable criteria for assigning black shales to the category of potential oil source strata.