Biogeochemistry of the Darwin Mounds, NE Atlantic

The Darwin Mounds are a series of small (<=5 m high, 75-100 m diameter) sandy features located in the northern Rockall Trough. They provide a habitat for communities of Lophelia pertusa and associated fauna. Suspended particulate organic matter (sPOM) reaching the deep-sea floor, which could potentially fuel this deep-water coral (DWC) ecosystem, was collected during summer 2000. This was relatively "fresh" (i.e. dominated by labile lipids such as polyunsaturated fatty acids) and was derived largely from phytoplankton remains and faecal pellets, with contributions from bacteria and microzooplankton. Labile sPOM components were enriched in the benthic boundary layer (~10 m above bottom (mab)) relative to 150 mab. The action of certain benthic fauna that are exclusively associated with the DWC ecosystem (e.g. echiuran worms) leads to the subduction of fresh organic material into the sediments. The mound surface sediments are enriched in organic carbon, relative to off-mound sites. There is no evidence for hydrocarbon venting at this location.

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.

Fatty acid content of POM, sediment, foraminifer and peracarid samples obtained during POLARSTERN cruise ANT-XXIV/2 (PS71)

In order to investigate the diversity of diet composition in macrobenthic peracarid crustaceans from the Antarctic shelf and deep sea, the fatty acid (FA) composition of different species belonging to the orders Isopoda, Amphipoda, Cumacea and Tanaidacea was analysed. Multivariate analyses of the FA composition confirmed general differences between the orders, but also distinct differences within these orders. To gain information on the origin of the FAs found, the potential food sources sediment, POM and foraminiferans were included in the study. Most of the analysed amphipod species displayed high 18:1(n-9)-18:1(n-7) ratios, widely used as an indicator for a carnivorous component in the diet. Cumaceans were characterised by increased phytoplankton FA markers such as 20:5(n-3) (up to 29% of total FAs), suggesting a diet based on phytodetritus. High values of the FA 20:4(n-6) were found in some munnopsid isopods (up to 21% of total FAs) and some tanaidacean species (up to 19% of total FAs). 20:4(n-6) also occurred in high proportions in some foraminiferan samples (up to 21% of total fatty acids), but not in sediment and POM, possibly indicating the ingestion of foraminiferans by some peracarid crustaceans.

Occurrence of metazoans, and gut pigments and fatty acid composition of Acartia bifilosa in ice and under-ice water samples from Santala Bay

A field study was conducted in Santala Bay with weekly samplings during February and March 2000. Ice thickness was 20-28 cm, snow cover 0-1 cm. The under-ice water column was stratified with a cold (-0.3 - 0.2°C) and less saline (S = 2.1-4.9) interface layer. Concentrations of particulate organic carbon (0.5-5.8 mg POC/l) and algal pigments (0.3-18.2 µg chlorophyll a/l) were higher in the ice than in the water (0.2-0.5 mg POC/l, 1.6-7.1 µg chlorophyll a/l) and peaked mostly in the bottom part of the ice. The thin ice and almost lacking snow cover had favoured an early ice-algal and phytoplankton bloom. The diversity of metazoans was low, with six species in the ice and eight species in the under-ice water. The rotifer Synchaeta cf. littoralis dominated both in ice and water, with maximum abundances of 230 individuals/l in the bottom part of the ice. Rotifer eggs were also observed in the ice. Baltic sea ice seems to be a suitable habitat for rotifers. Nauplii and copepodids of the calanoid Acartia longiremis in the under-ice water showed some herbivorous feeding (<0.1-0.23 ng gut pigment/individual), but analysis of fatty acids, fatty alcohols and biomarker ratios indicated a more omnivorous/carnivorous diet. Despite low temperatures, this copepod showed growth and development below the ice, doubling in numbers (mainly CI, CII) from 118 to 230 individuals m during the third week of March.

Lipid biomarkers in urface sediments of the Southeast Atlantic

Surface sediments from the eastern South Atlantic were investigated for their lipid biomarker contents and bulk organic geochemical characteristics to identify sources, transport pathways and preservation processes of organic components. The sediments cover a wide range of depositional settings with large differences in mass accumulation rates. The highest marine organic carbon (OC) contributions are detected along the coast, especially underlying the Benguela upwelling system. Terrigenous OC contributions are highest in the Congo deep-sea fan. Lipid biomarker fluxes are significantly correlated to the extent of oxygen exposure in the sediment. Normalization to total organic carbon (TOC) contents enabled the characterization of regional lipid biomarker production and transport mechanisms. Principal component analyses revealed five distinct groups of characteristic molecular and bulk organic geochemical parameters. Combined with information on lipid sources, the main controlling mechanisms of the spatial lipid distributions in the surface sediments are defined, indicating marine productivity related to river-induced mixing and oceanic upwelling, wind-driven deep upwelling, river-supply of terrigenous organic material, shallow coastal upwelling and eolian supply of plant-waxes.

Sediment descriptions, stage, calcium carbonate, organic carbon, delta 13C and geolipids at DSDP Hole 93-603B

Organic matter contents of black shales from the Cretaceous Hatteras and Blake-Bahama formations have been compared to those from surrounding organic-poor strata using C/N ratios, d13C values, and distributions of extractable and nonsolvent-extractable, long-chain hydrocarbons, acids, and alcohols. The proportion of marine and land-derived organic matter varies considerably among all samples, although terrigenous components generally dominate. Most black shales are hydrocarbon-poor relative to their organic-carbon concentrations. Deposition of the black shales in Hole 603B evidently occurred through turbiditic relocation from shallower landward sites and rapid reburial at this outer continental rise location under generally oxygenated bottom-water conditions.

General descriptions, organic contents and ratios of geolipid components at DSDP Holes 75-530B and 75-532

Distributions of free and bound n-alkanes, n-alkanoic acids, and n-alkanols were determined in order to compare the character of organic matter contained in organic-carbon-rich sediments from two sites sampled by the hydraulic piston corer. Two diatomaceous debris-flow samples of Pleistocene age were obtained from Hole 530B in the Angola Basin. A sample of bioturbated Pleistocene diatomaceous clay and another of bioturbated late Miocene nannofossil clay were collected from Hole 532 on the Walvis Ridge. Geolipid distributions of all samples contain large terrigenous contributions and lesser amounts of marine components. Similarities in organic matter contents of Hole 530B and Hole 532 sediments suggest that a common depositional setting, probably on the Walvis Ridge, was the original source of these sediments through Quaternary, and possibly late Neogene, times and that downslope relocation of these biogenic deposits has frequently occurred.