Chemical and isotopic analysis of bulk organic matter and hydrocarbon biomarkers from IODP Holes 311-U1327C, 311-U1328B and 311-U1328C

The chemical and isotopic compositions of sedimentary organic matter (SOM) from two mid-slope sites of the northern Cascadia margin were investigated during Integrated Ocean Drilling Program (IODP) Expedition 311 to elucidate the organic matter origins and identify potential microbial contributions to SOM. Gas hydrate is present at both locations (IODP Sites U1327 and U1328), with distinct patterns of near-seafloor structural accumulations at the cold seep Site U1328 and deeper stratigraphic accumulations at the slope-basin Site U1327. Source characterization and evidence that some components of the organic matter have been diagenetically altered are determined from the concentrations and isotopic compositions of hydrocarbon biomarkers, total organic carbon (TOC), total nitrogen (TN) and total sulfur (TS). The carbon isotopic compositions of TOC (d13C TOC = -26 to -22 per mil) and long-chain n-alkanes (C27, C29 and C31, d13C = -34 to -29 per mil) suggest the organic matter at both sites is a mixture of 1) terrestrial plants that employ the C3 photosynthetic pathway and 2) marine algae. In contrast, the d15N TN values of the bulk sediment (+4 to +8 per mil) are consistent with a predominantly marine source, but these values most likely have been modified during microbial organic matter degradation. The d13C values of archaeal biomarker pentamethylicosane (PMI) (-46.4 per mil) and bacterial-sourced hopenes, diploptene and hop-21-ene (-40.9 to -34.7 per mil) indicate a partial contribution from methane carbon or a chemoautotrophic pathway. Our multi-isotope and biomarker-based conclusions are consistent with previous studies, based only on the elemental composition of bulk sediments, that suggested a mixed marine-terrestrial organic matter origin for these mid-slope sites of the northern Cascadia margin.

Biomarkers in surface sediments from the Ob and Yenisei estuaries and the southern Kara Sea

Organic-geochemical bulk parameter (Total organic carbon contents, C/N ratios and d13Corg values), biogenic opal and biomarkers (n-alkanes, fatty acids, sterols and amino acids) were determined in surface sediments from the Ob and Yenisei estuaries and the adjacent southern Kara Sea. Maximum TOC contents were determined in both estuaries, reaching up to 3 %. Relatively high C/N ratios around 10, light d13Corg values of -26.5 per mil (Yenisei) and -28 to -28.7 per mil (Ob), and maximum concentrations of long-chain n-alkanes of up to about 10 µg/g Sed clearly show the predominance of terrigenous organic matter in the sediments from the estuaries. Towards the open Kara Sea, all p arameters indicate a decrease in terrigenous organic carbon. Brassicasterol as well as the short-chain n-alkanes parallel this trend, suggesting that these biomarkers are probably also related to a terrigenous (fresh-water phytoplankton) source. Amino acid spectra show characteristic trends from the Yenisei Estuary to the open Kara Sea revealing increasing state of degradation. Sedimentary organic matter in the Yenisei Estuary is relatively less degraded compared to the Ob Estuary and the open Kara Sea.

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 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.

Geochemistry and stable carbon isotope composition of kerogen of DSDP Site 124-767

Celebes Basin sediments from Ocean Drilling Program Site 767 (Leg 124) containing both marine and terrestrial organic matter have been investigated through palynofacies and geochemical analyses. The main degradation processes affecting or having affected organic matter are recorded in the sedimentary column as shown by ammonium, phosphate and sulfate pore-water profiles, and by petrographic and geochemical analyses of sediments. In the upper part of the sedimentary section (down to 200 mbsf), the decrease of the ratio of total organic carbon to sulfur (TOC/S) with depth, generally related to the sulfate reduction process, is accompanied by an increase of framboidal pyrite content in the marine organic matter, and by an increasing amount of amorphous marine organic matter relative to the total organic matter. However, as the terrestrial organic input also varies with depth, dilution effects are superimposed on diagenesis. This continental supply affects the TOC/S ratio by increasing total organic carbon and decreasing the ability of the bulk organic matter to be metabolized through sulfate reduction. A positive relationship between the TOC/P ratio and the amount of degraded organic matter of marine origin clearly displays the effect of an organic source on the composition of the sediment. Each lithostratigraphic unit possesses its own characteristics in terms of composition and preservation of organic matter. The effects of diagenesis can only be appreciated within a single lithostratigraphic unit and mainly affect the less-resistant marine organic matter.

Composition of organic matter in suspended matter and bottom sediments of the southeastern Baltic Sea

Data on contents and compositions of hydrocarbons (HCs)-aliphatic (AHCs) and polycyclic aromatic (PAHs) are provided in comparison with contents of total organic carbon (Corg), lipids in suspended matter, and Corg in bottom sediments. Particular attention is paid to distribution of HCs in the area of the Kravtsov oil field. It is established that concentrations of AHCs in water are governed by concentrations of suspended matter and elevated AHC concentrations are confined to coastal areas. In the area of D-6 platform sandy bottom sediments are notable for great variability of HC concentrations, both laterally and from year to year. In summer of 2010 average content of AHCs was 40 ppm (19% of Corg) and that of PAHs was 0.023 ppm. Natural seepage from sediment mass is considered to be a source of HCs along with oil contamination.

Geochemistry and petrography of organic matter at DSDP Site 93-603

A series of 22 sediment samples of Cretaceous and Cenozoic age from DSDP Holes 603, 603B, and 603C at the continental rise off the northeastern American coast near Cape Hatteras was investigated by organic geochemical methods including organic carbon determination, Rock-Eval pyrolysis, gas chromatography and combined gas chromatography/mass spectrometry of extractable hydrocarbons, and kerogen microscopy. An abundance of terrigenous organic matter, including larger coal particles (almost exclusively consisting of huminite/vitrinite macerals), is the dominant characteristic of the organofacies types at Site 603. Marine organic matter, mostly structurally degraded and in the form of fecal pellets, was preserved in the Valanginian laminated marls and in Cenomanian black claystone turbidites. Long-chain nalkanes reflect the terrigenous imprint in the nonaromatic hydrocarbon fractions, whereas a second maximum at lower carbon numbers in most cases is caused by the presence of more mature recycled organic matter. Abundant isoprenoid and steroid hydrocarbons were found in sediments containing mainly marine organic matter, whereas hopanoids reflect the ubiquitous microbial activity. The organic matter in the Site 603 sediments, in so far as it is not recycled, is thermally immature.

Organic geochemistry of Cretaceous black shales

Organic-carbon-rich 'black shales' and adjacent organic-carbon-poor rocks from three different Cretaceous settings encountered during ODP Leg 103 have been studied by organic geochemical methods. Rock-Eval analysis, carbon isotope data, and lipid biomarkers show organic matter to contain varying proportions of marine and continental materials. In Hauterivian-Barremian organic-carbon-rich marlstone turbidites, large amounts of land-derived organic matter are found. Aptian-Albian black-colored shales are interspersed within green claystones, from which they differ by containing more marine organic matter. An abbreviated layer of black shale from the Cenomanian/Turonian boundary is dominated by well-preserved marine organic matter. Downslope transport and rapid reburial within a predominantly oxygenated deep-water setting created most of these examples of black shales, except for the Cenomanian-Turonian deposits in which deep-water anoxia may have been involved.

Mineral and organic geochemistry from Lake Anterne sediments (North French Alps)

A high-resolution sedimentological and geochemical study was performed on a 20 m long core from the alpine Lake Anterne (2063 m a.s.l., NW French Alps) spanning the last 10 ka. Sedimentation is mainly of minerogenic origin. The organic matter quantity (TOC%) as well as its quality (hydrogen (HI) and oxygen (OI) indices) both indicate the progressive onset and subsequent stabilization of vegetation cover in the catchment from 9950 to 5550 cal. BP. During this phase, the pedogenic process of carbonate dissolution is marked by a decrease in the calcium content in the sediment record. Between 7850 and 5550 cal. BP, very low manganese concentrations suggest anoxic conditions in the bottom-water of Lake Anterne. These are caused by a relatively high organic matter (terrestrial and lacustrine) content, a low flood frequency and longer summer stratification triggered by warmer conditions. From 5550 cal. BP, a decrease in TOC, stabilization of HI and higher sedimentation rates together reflect increased erosion rates of leptosols and developed soils, probably due to a colder and wetter climate. Then, three periods of important soil destabilization are marked by an increased frequency and thickness of flood deposits during the Bronze Age and by increases in topsoil erosion relative to leptosols (HI increases) during the late Iron Age/Roman period and the Medieval periods. These periods are also characterized by higher sedimentation rates. According to palynological data, human impact (deforestation and/or pasturing activity) probably triggered these periods of increased soil erosion.

Organic chemistry of sediments from the Tyrrhenian Sea

Thirty sediment samples from Tortonian to Pleistocene age of five ODP locations (Holes 650A, 651A, and 652A, and Sites 654 and 655) in the Marsili Basin, Vavilov Basin, and Sardinia Margin (Tyrrhenian Sea) were studied by organic geochemical methods including total organic carbon determination, Rock-Eval pyrolysis, bitumen extraction, pyrolysis-gas chromatography, and organic petrography. Six organic facies, including open ocean anoxia with variable terrestrial input, oxic open ocean, oxic tidal flat, mildly oxic lagoon, and anoxic lacustrine algal-bacterial mat environments, have been recognized in these sediments. The sediments below 500 m in Sardinia Margin are mature for significant hydrocarbon generation. Possible mature source-rock (Type I and IIB/III kerogen) and migrated bitumen occur in the deeper part of the section in Vavilov Basin and Sardinia Margin sediments. Sporadic sapropel formation observed in the studied Pliocene-Pleistocene sediment section is probably controlled by organic productivity due to nutrient supply by the rivers and terrestrial input associated with open ocean anoxia or anoxia caused by the material balance between rate of organic matter supplied by turbidites and organic matter consumption. Pliocene and Pleistocene sapropels are mostly immature and lie within Type II-III (precisely as IIA-IIB and IIB source rocks) kerogen maturation path.

Organic geochemistry of Lower Cretaceous sediments from the northwetsern Pacific guyits

Organic geochemical data of Lower Cretaceous shallow water sediments from two sites (865 and 866) drilled during ODP leg 143 are presented. The organic matter is mainly terrestrial at the bottom of the sedimentary column at site 865, whereas algal and/or bacterial organic matter is dominant at site 866. This is the first evidence of shallow water deposition of organic matter during the Early Cretaceous in the Northwestern Pacific. The lower Aptian organic carbon-rich layers from the shallow water sediments of site 866 are geochemically similar to coeval mid-water sediments of site 463.

Stable carbon isotopic composition of biomarker lipids of DSDP Hole 31-302

The middle Paleocene through early Eocene long-term gradual warming was superimposed by several transient warming events, such as the Paleocene-Eocene Thermal Maximum (PETM) and Eocene Thermal Maximum 2 (ETM2). Both events show evidence for extreme global warming associated with a major injection of carbon into the ocean-atmosphere system, but the mechanisms of carbon injection and many aspects of the environmental response are still poorly understood. In this study, we analyzed the concentration and stable carbon isotopic (d13C) composition of several sulfur-bound biomarkers derived from marine photoautotrophs, deposited in the Arctic Ocean at ~85°N, during ETM2. The presence of sulfur-bound biomarkers across this event points toward high primary productivity and anoxic bottom water conditions. The previously reported presence of isorenieratene derivatives indicates euxinic conditions in the photic zone, likely caused by a combination of enhanced primary productivity and salinity stratification. The negative carbon isotope excursion measured at the onset of ETM2 for several biomarkers, ranges between 3 per mil and 4.5 per mil, much larger than the ~1.4 per mil recorded in marine carbonates elsewhere, suggesting substantial enhanced isotopic fractionation by the primary producers likely due to a significant rise in pCO2. In the absence of biogenic carbonates in the ETM2 section of our core we use coeval planktonic d13C from elsewhere to estimate surface water d13C in the Arctic Ocean and then apply the relation between isotopic fractionation and pCO2, originally calibrated for haptophyte alkenones, to three selected organic biomarkers (i.e., S-bound phytane, C35 hopane, and a C25 highly branched isoprenoid). This yields pCO2 values potentially in the range of four times preindustrial levels. However, these estimates are uncertain because of a lack of knowledge on the importance of pCO2 on photosynthetic isotopic fractionation.