Organic geochemistry of sediment cores from the Arbian Sea

Two deep-sea sediment cores from the northeastern and the southeastern Arabian Sea were studied in order to reconstruct the palaeoenvironments of the past glacial cycles. Core 136KL was recovered from the high-productivity area off Pakistan within the modern oxygen-minimum zone (OMZ). By contrast, modern primary productivity at the site of MD900963 close to Maldives is moderate and bottom waters are today well oxygenated. For both cores, we reconstructed the changes in palaeoproductivity using a set of biomarkers (alkenones, dinosterol and brassicasterol); the main result is that primary productivity is enhanced during glacial stages and lowered during interstadials. The proxies associated with productivity show a 23 kyr cyclicity corresponding to the precession-related insolation cycle. Palaeoredox conditions were studied in both cores using a new organic geochemical parameter (C35/C31-n-alkane ratio) developed by analysing surface sediments from a transect across the OMZ off Pakistan. The value of this ratio in core 136KL shows many variations during the last 65 kyr, indicating that the OMZ was not stable during this time: it disappeared completely during Heinrich- and the Younger Dryas events, pointing to a connection between global oceanic circulation and the stability of the OMZ. The C35/C31 ratio determined in sediments of core MD900963 shows that bottom waters remained rather well oxygenated over the last 330 kyr, which is confirmed by comparison with authigenic metal concentrations in the same sediments. A zonally averaged, circulation-biogeochemical ocean model was used to explore how the intermediate Indian Ocean responds to a freshwater flux anomaly at the surface of the North Atlantic. As suggested by the geochemical time series, both the abundance of Southern Ocean Water and the oxygen concentration are significantly increased in response to this freshwater perturbation.

Organic geochemistry of two sediment cores from the Kara Sea

Within the Russian-German research project on "Siberian River Run-off (SIRRO)" devoted to the freshwater discharge and its influence on biological, geochemical, and geological processes in the Kara Sea, sedimentological and organic-geochemical investigations were carried-out on two well-dated sediment cores from the Yenisei Estuary area. The main goal of this study was to quantify the terrigenous organic carbon accumulation based on biomarker and bulk accumulation rate data, and its relationship to Yenisei river discharge and climate change through Holocene times. The biomarker data at both cores clearly indicate the predominance of terrigenous organic matter, reaching 70 to 100% and 50 to 80% of the total organic carbon within and directly north of the estuary, respectively. During the last about 9 Cal. kyrs. BP represented in the studied sediment section, siliciclastic sediment and (terrigenous) organic carbon input was strongly influenced by postglacial sea-level rise and climate-related changes in river discharge. The mid-Holocene Climatic Optimum is documented by maximum river discharge between 8.2 and 7.3 Cal. kyrs. BP. During the last 2000 years river discharge probably became reduced, and accumulation of both terrigenous and marine organic carbon increased due to increased coagulation of fine-grained material.

Electromagnetic, rock magnetic, and geochemical properties of surficial sediments in Eckernförde Bay

Submarine groundwater discharge in coastal settings can massively modify the hydraulic and geochemical conditions of the seafloor. Resulting local anomalies in the morphology and physical properties of surface sediments are usually explored with seismo-acoustic imaging techniques. Controlled source electromagnetic imaging offers an innovative dual approach to seep characterization by its ability to detect pore-water electrical conductivity, hence salinity, as well as sediment magnetic susceptibility, hence preservation or diagenetic alteration of iron oxides. The newly developed electromagnetic (EM) profiler Neridis II successfully realized this concept for a first time with a high-resolution survey of freshwater seeps in Eckernförde Bay (SW Baltic Sea). We demonstrate that EM profiling, complemented and validated by acoustic as well as sample-based rock magnetic and geochemical methods, can create a crisp and revealing fingerprint image of freshwater seepage and related reductive alteration of near-surface sediments. Our findings imply that (1) freshwater penetrates the pore space of Holocene mud sediments by both diffuse and focused advection, (2) pockmarks are marked by focused freshwater seepage, underlying sand highs, reduced mud thickness, higher porosity, fining of grain size, and anoxic conditions, (3) depletion of Fe oxides, especially magnetite, is more pervasive within pockmarks due to higher concentrations of organic and sulfidic reaction partners, and (4) freshwater advection reduces sediment magnetic susceptibility by a combination of pore-water injection (dilution) and magnetite reduction (depletion). The conductivity vs. susceptibility biplot resolves subtle lateral litho- and hydrofacies variations.

Heterocyst glycolipids in surface sediments and water samples of the Amazon shelf

Marine endosymbiotic heterocystous cyanobacteria make unique heterocyst glycolipids (HGs) containing pentose (C5) moieties. Functionally similar HGs with hexose (C6) moieties found in free-living cyanobacteria occur in the sedimentary record, but C5 HGs have not been documented in the natural environment. Here we developed a high performance liquid chromatography multiple reaction monitoring (MRM) mass spectrometry (HPLC-MS2) method specific for trace analysis of long chain C5HGs and applied it to cultures of Rhizosolenia clevei Ostenfeld and its symbiont Richelia intracellularis which were found to contain C5 HGs and no C6 HGs. The method was then applied to suspended particulate matter (SPM) and surface sediment from the Amazon plume region known to harbor marine diatoms carrying heterocystous cyanobacteria as endosymbionts. C5 HGs were detected in both marine SPM and surface sediments, but not in SPM or surface sediment from freshwater settings in the Amazon basin. Rather, the latter contained C6 HGs, established biomarkers for free-living heterocystous cyanobacteria. Our results indicate that the C5 HGs may be potential biomarkers for marine endosymbiotic heterocystous cyanobacteria.

Age determination and n-Alkane concentration of Arctic Ocean sediment cores

We have reconstructed the surface water environment of the Arctic Ocean over the last ? 50,000 years using measurements of the organic nitrogen and carbon isotope ratios, carbonate and total organic carbon concentrations (TOC), and terrestrial biomarkers (lignin and long-chain n-alkanes) in four multicores. Variations in nitrogen isotope ratios that are concordant with TOC and carbonate concentrations (representing foraminifera and excluding ice-rafted-debris) reflect differences in relative nutrient utilization of phytoplankton in the surface waters. However, d15N variations also appear to be dependent on the stratification of the water column and therefore potentially track the exchange of nutrients between deep and surface waters. Low Last Glacial Maximum (LGM) d15N values and higher Holocene values are opposite to those recorded in the Southern Ocean. The Arctic Ocean with higher nutrient utilization today compared to the LGM therefore acts as a counterpart to the Southern Ocean, although the global impact on carbon dioxide variations compared to the Southern Ocean is probably low.

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.

Biomarker proxies on sediment cores in the central Arctic Ocean

Although the permanently to seasonally ice-covered Arctic Ocean is a unique and sensitive component in the Earth's climate system, the knowledge of its long-term climate history remains very limited due to the restricted number of pre-Quaternary sedimentary records. During Polarstern Expedition PS87/2014, we discovered multiple submarine landslides over a distance of >350 km along Lomonosov Ridge. Removal of younger sediments from steep headwalls has led to exhumation of Miocene to early Quaternary sediments close to the seafloor, allowing the retrieval of such old sediments with gravity cores. Multi-proxy biomarker analyses of these gravity cores reveal for the first time that the late Miocene central Arctic Ocean was relatively warm (4-7°C) and ice-free during summer, whereas sea ice occurred during spring and autumn/winter. A comparison of our proxy data with Miocene climate simulations seems to favour relatively high late Miocene atmospheric CO2 concentrations. These new findings from the Arctic region provide new benchmarks for groundtruthing global climate reconstructions and modeling.

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 Cretaceous black shales at DSDP Hole 75-530A

Five-hundred ten meters of Cretaceous sediments were drilled north of the Walvis escarpment in Hole 530A during Leg 75. An immature stage of evolution for organic matter can be assigned to all the samples studied. Black shales are interbedded with red and green claystone in the bottom sedimentary unit, Unit 8, which is of Coniacian to late Albian age. The richest organic carbon contents and petroleum potentials occur in the black shales. Detrital organic matter is present throughout the various members of a sequence, mixed with largely oxidized organic matter in the gray and green claystone or marlstone members on both sides. Detrital organic matter also characterizes the black streaks observed in the claystones. Vertical discontinuities in organic matter distribution are assigned to slumping. Several types of black shales can be identified, according to their content of detrital organic matter, the more detrital black levels corresponding to the Albian-Cenomanian period. Cyclic variations of organic matter observed for a sequence can occur for a set of sequences and even for some consecutive sets of sequences. Climatic factors are proposed to account for the cyclic sedimentation and distribution of organic matter for every sequence that includes a black bed.

Lipids and polycyclic aromatic hydrocarbons in bottom sediments from the Atlantic sector of the Southern Ocean

Distribution and composition of lipids and contents of alkanes and polycyclic aromatic hydrocarbons(PAHs) in bottom sediments of the Scotia and Weddell seas are discussed. Comparatively low concentrations of organic carbon (average 0.35%) and lipids (average 0.024%) result from rapid decomposition of organic matter in upper layers of the water column. Composition of alkanes indicates that lipids are of autochthonous origin, and stable concentrations of PAHs (average 25.8 ppb, sigma 15.3 ppb) indicate that they represent the background level for bottom sediments. Higher concentrations of PAHs in sediments near the King George Island (252.1 ppb) and different distributions of individual polyarenes are produced there by the heating systems of the Polish Antarctic Station.

A time-slice study in Heinrich Stadial 1 and Mid Holocene sediment cores off northern Senegal

Sediment dynamics in limnic, fluvial and marine environments can be assessed by granulometric and rock-magnetic methodologies. While classical grain-size analysis by sieving or settling mainly bears information on composition and transport, the magnetic mineral assemblages reflect to a larger extent the petrology and weathering conditions in the sediment source areas. Here, we combine both methods to investigate Late Quaternary marine sediments from five cores along a transect across the continental slope off Senegal. This region near the modern summer Intertropical Convergence Zone is particularly sensitive to climate change and receives sediments from several aeolian, fluvial and marine sources. From each of the investigated five GeoB sediment cores (494-2956 m water depth) two time slices were processed which represent contrasting climatic conditions: the arid Heinrich Stadial 1 (~ 15 kyr BP) and the humid Mid Holocene (~ 6 kyr BP). Each sediment sample was split into 16 grain-size fractions ranging from 1.6 to 500 µm. Concentration and grain-size indicative magnetic parameters (susceptibility, SIRM, HIRM, ARM and ARM/IRM) were determined at room temperature for each of these fractions. The joint consideration of whole sediment and magnetic mineral grain-size distributions allows to address several important issues: (i) distinction of two aeolian sediment fractions, one carried by the north-easterly trade winds (40-63 µm) and the other by the overlying easterly Harmattan wind (10-20 µm) as well as a fluvial fraction assigned to the Senegal River (< 10 µm); (ii) identification of three terrigenous sediment source areas: southern Sahara and Sahel dust (low fine-grained magnetite amounts and a comparatively high haematite content), dust from Senegalese coastal dunes (intermediate fine-grained magnetite and haematite contents) and soils from the upper reaches of the Senegal River (high fine-grained magnetite content); (iii) detection of partial diagenetic dissolution of fine magnetite particles as a function of organic input and shore distance; (iv) analysis of magnetic properties of marine carbonates dominating the grain-size fractions 63-500 µm.

Low-molecular-weight hydrocarbon concentrations, organic carbon contents, and Rock-Eval pyrolysis hydrogen indices at DSDP Holes 75-530A and 75-532

A series of C2-C8 hydrocarbons (including saturated, aromatic, and olefinic compounds) from deep-frozen core samples taken during DSDP Leg 75 (Holes 530A and 532) were analyzed by a combined hydrogen-stripping/thermovaporization method. Concentrations representing both hydrocarbons dissolved in the pore water and adsorbed on the mineral surfaces vary in Hole 530A from about 10 to 15,000 ng/g of dry sediment weight depending on the lithology (organic-carbon-lean calcareous oozes versus "black shales"). Likewise, the organic-carbon-normalized C2-C8 hydrocarbon concentrations vary from 3,500 to 93,100 ng/g Corg, reflecting drastic differences in the hydrogen contents and hence the hydrocarbon potential of the kerogens. The highest concentrations measured of nearly 10**5 ng/g Corg are about two orders of magnitude below those usually encountered in Type-II kerogen-bearing source beds in the main phase of petroleum generation. Therefore, it was concluded that Hole 530A sediments, even at 1100 m depth, are in an early stage of evolution. The corresponding data from Hole 532 indicated lower amounts (3,000-9,000 ng/g Corg), which is in accordance with the shallow burial depth and immaturity of these Pliocene/late Miocene sediments. Significant changes in the light hydrocarbon composition with depth were attributed either to changes in kerogen type or to maturity related effects. Redistribution pheonomena, possibly the result of diffusion, were recognized only sporadically in Hole 530A, where several organic-carbon lean samples were enriched by migrated gaseous hydrocarbons. The core samples from Hole 530A were found to be severely contaminated by large quantities of acetone, which is routinely used as a solvent during sampling procedures on board Glomar Challenger.