Organic geochemistry of late Cenozoic sediments from the subtropical South Atlantic Ocean

Abstract

Organic matter has been characterized in samples of Pleistocene, Pliocene, and Miocene sediments from seven Deep Sea Drilling Project sites in the subtropical South Atlantic Ocean. Organic carbon concentrations average 0.3% for most samples, and n-alkanoic acid, n-alkanol, and alkane biomarkers indicate extensive microbial reworking of organic matter in these organic-carbon-lean sediments. Samples from the easternmost parts of the South Atlantic contain an average of 4.1% organic carbon and reflect the high productivity associated with the Benguela Current. Lipid biomarkers show less microbial reworking in these sediments. Eolian transport of land-derived hydrocarbons is evident at most of these oceanic locations.

Organic Geochemistry of Cretaceous Black Shales and Adjacent Strata from the Galicia Margin, North Atlantic Ocean

“…These long-chain-length alcohols are especially dominant in the black shale in Samples 103-638B-26R-1, 14-19 cm (Hauterivian), 103-641C-2R-4, 3-6 cm (Albian), and 103-641A-6X, CC (3-6 cm) (Cenomanian). The most abundant w-alkanol in eight of the 10 samples in Figure 5 is C 22 , which has been postulated to arise from microbial processing of sediment organic matter (Cranwell, 1981;Keswani et al, 1984), a possibility consistent with the fl-alkanoic acid data. Like the rt-acids, the even-over-odd carbon-chain-length predominance of these w-alkanols is high, indicating good preservation of the biological character of this geolipid fraction.…”

“…Concentrations of w-alkanoic acids, however, are generally lower in these eastern margin samples than in the Site 603 rocks, perhaps reflecting more diagenetic reworking of the organic matter in the eastern location, inasmuch as fatty acids are more sensitive to such losses than are the other classes of geolipids (cf. Keswani et al, 1984). The Cenomanian samples from Hole 641A have notably higher absolute concentrations of n-alkanes than the other Leg 103 samples and contain more of these hydrocarbons than the Cenomanian samples from Site 603.…”

Geolipids of Late Cenozoic Sediments from the Western Flank of the East Pacific Rise, Deep Sea Drilling Project Leg 92

“…Sample 600-1-4, 40-46 cm, is the only one of the four in this study to contain sufficient organic nitrogen to yield a C/N ratio. This value, 7.4, is similar to C/N ratios found in other organic-carbon-poor DSDP samples of Pleistocene age (Waples and Sloan, 1980;Keswani et al, 1984). Values in this neighborhood are sometimes considered indicative of marine origin (Premuzic et al, 1982), yet differences in diagenetic losses of carbon and nitrogen from organic matter result in alterations of C/N values over time (Waples and Sloan, 1980).…”

“…In DSDP samples having 1% or more of organic carbon, terrigenous C24, C26, and C28 /2-alkanols are often the major components (cf. Keswani et al, 1984) as a result of overall better preservation of organic matter. The general dominance of «-C22 is consistent with other indicators of microbial alteration of geolipids in these Leg 92 sediments.…”

Anthropogenic Activities Have Contributed Moderately to Increased Inputs of Organic Materials in Marginal Seas off China

“…These TOC contents were much greater than those from the subtropical Atlantic Ocean (0.2%− 0.32%). 28 They were also slightly higher than or comparable to those from the Nansha Sea (0.7% and 0.53%), 12 and those in seven short sediment cores from the southern Okinawa Trough (0.55%, 0.68%, 0.38%, 0.61%, 0.61%, 0.72%, and 0.63%). 16 However, they were much lower than those from inland lakes, e.g., Lakes Ontario and Erie (1.5%−5%) 13 which have both a Abbreviations: ND = not detected, CPI = carbon preference index, ACL = average chain length, Pr = pristane, Ph = phytane, %waxCn = plant wax n-alkanes, OEP = odd−even carbon number preference, TAR = terrigenous to aquatic ratio, TOC = total organic carbon, and UCM = unresolved complex mixture.…”