Organic carbon, reduced sulfur, and iron in Miocene to Holocene sediments from the Oman and Beguela upwelling systems

We examined sediments from Neogene and Quaternary sections of the Benguela and Oman upwelling systems (DSDP Site 532, ODP Sites 723 and 722) to determine environmental and geochemical factors which control and limit pyrite formation in organic-carbon-rich marine sediments. Those samples from the upwelling sites, which contained low to moderate concentrations of total organic carbon (0.7%-3%), had C/S ratios typical of normal marine sediments, i.e., around 2.8. In these sediments, TOC availability probably limited pyrite formation. Results that do not conform with accepted models were found for the sediments high in TOC (3^0-12.4%). The organic matter was of marine origin and contained considerable pyrolytic hydrocarbons, a fact that we take as a sign of low degradation, yet significant concentrations of dissolved sulfate coexisted with it (> 5 mmol/L in the case of Sites 532 and 723). Detrital iron was probably not limiting in either case, because the degree of pyritization was always less than 0.65. Therefore, controls on sulfate reduction and pyrite formation in the organic matter-rich sediments do not appear to conform simply to generally accepted diagenetic models. The data from these thermally immature, old, and organic-rich marine sediments imply that (1) the total reduced sulfur content of organic-rich marine upwelling sediments rarely exceeds an approximate boundary of 1.5% by weight, (2) the C/S ratio of these sediments is not constant and usually much higher than the empirical values proposed for marine sediments. We conclude that sedimentary pyrite formation in upwelling sediments is limited by an as yet unknown factor, and that caution is advised in using C/S ratios and C vs. S diagrams in paleoenvironmental reconstructions for organic-rich sediments.

Organic geochemistry at DSDP Leg 80 Holes

The four sites drilled on the Irish continental margin (Goban Spur) yielded sediments ranging in age from Holocene to Barremian. Most of the sediments were deposited in well oxygenated waters, and the small amounts of organic matter they contain are highly oxidized. During a few time intervals from the Cenomanian to earliest Turonian, however, the oxygen content of the bottom waters reached very low levels, resulting in the deposition of homogeneous or laminated black sediments containing from 0.5 to 11% total organic carbon (TOC). The original organic matter was of mixed marine and terrestrial origin. The oxidizing-reducing cycles represented by interbedded black and light sediments are probably a result of changes in both circulation and productivity. The black sediments at Sites 550, 551, and 549 were probably deposited near the lower end, middle, and upper end, respectively, of an expanded oxygen-minimum layer. The oil and gas source potential of the laminated black sediments is very good to excellent. The organic-carbon-lean sediments deposited under oxidizing conditions have no oil or gas source potential. The thermal maturity of all sediments is low.

Hydrocarbons in water, suspended matter, and bottom sediments of the White Sea

Data are presented on concentration and composition of aliphatic and polycyclic aromatic hydrocarbons (HC) in water, suspended matter (collected with a Juday net and by a separator), and in bottom sediments of the White Sea. It was found that during the last years the level of aliphatic HC concentrations in waters of the White Sea (aver. 18 µg/l) practically did not change and was comparable with average concentrations in shelf areas of the World Ocean. In water and bottom sediments distribution of HC is determined by discharge of river marginal filters. Here sedimentation of the bulk of anthropogenic HC occurs. That is why a grain-size controlling factor is not active in the zone of the river depocenter (in particular, of the North Dvina River). The same reasons most probably may explain differences in degree of geochemical relationships between contents of TOC and HC in suspended matter and bottom sediments. After passing through marginal filters natural HC are dominant in all migration forms.

(Table 1) Organic carbon, hydrogen and oxygen index and hydrocarbons at DSDP Hole 93-603B

C2-C8 hydrocarbon concentrations (about 35 compounds identified, including saturated, aromatic, and olefinic compounds) from 27 shipboard-sealed, deep-frozen core samples of DSDP Hole 603B off the east coast of North America were determined by a gas-stripping/thermovaporization method. Total yields representing the hydrocarbons dissolved in the pore water and adsorbed on the mineral surfaces of the sediments vary from 22 to 2400 ng/g of dryweight sediment. Highest yields are measured in the two black shale samples of Core 603B-34 (hydrogen index of 360 and 320 mg/g Corg, respectively). In organic-carbon-normalized units these samples have hydrocarbon contents of 12,700 and 21,500 ng/g Corg, respectively, indicating the immaturity of their kerogens. Unusually high organic-carbonnormalized yields are associated with samples that are extremely lean in organic carbon. It is most likely that they are enriched by small amounts of migrated light hydrocarbons. This applies even to those samples with high organic-carbon contents (1.3-2.2%) of Sections 603B-28-4, 603B-29-1, 603B-49-2, and 603B-49-3, because they have an extremely low hydrocarbon potential (hydrogen index between 40 and 60 mg/g Corg). Nearly all samples were found to be contaminated by varying amounts of acetone that is used routinely in large quantities on board ship during core-cutting procedures. Therefore, 48 samples from the original set of 75 collected had to be excluded from the present study.

(Table 1) Summary of low-molecular-weight hydrocarbon concentrations, organic carbon contents, Rock-Eval pyrolysis and lithology at DSDP Leg 79 Holes

A series of core samples taken during Cruise 79 of Glomar Challenger, drilling offshore Morocco (Mazagan Plateau), is analyzed for their low-molecular-weight hydrocarbon contents. Fifty-four samples from DSDP Holes 544A, 545, 547A, and 547B, deep frozen on board immediately after recovery, are studied by a hydrogen-stripping/thermovaporization technique combined with capillary gas chromatography. Thirty-eight compounds in the C2-C8 molecular range, including saturated, olefinic, and aromatic hydrocarbons, are identified. Because of large differences in organic carbon contents, the total C2-C8 hydrocarbon concentrations vary from about 20 to 1500 ng/g dry sediment weight in the whole sample series. Organic-carbon normalized values are about 3.2 x 10**4 ng/g Corg for Lithologic Subunits IIIA and IIIB at Site 545 (Cenomanian to Aptian) and 1.0 x 10**5 ng/g Corg for Unit V at Site 547 (Cenomanian to Albian) reflecting the slightly more advanced maturity stage at the latter site. Values exceeding 10**5 ng/g Corg (Site 545) and 2 x 10**5 ng/g Corg (Site 547) are associated with samples that are very lean in organic carbon and are generally rich in carbonate. These samples are enriched by small amounts of gaseous hydrocarbons. A detailed study of individual hydrocarbon concentrations, plotted against depth, reveal additional indications for migration phenomena. At Site 547, for instance, the most mobile hydrocarbons studied (e.g., ethane) appear to migrate by diffusion or a related process from more than 700 m depth toward the surface.

Temporal variation of temperature and related proxies from the penultimate glacial towards the Eemian in the Black Sea

The last glacial-interglacial transition or Termination I (T I) is well documented in the Black Sea, whereas little is known about climate and environmental dynamics during the penultimate Termination (T II). Here we present a multi-proxy study based on a sediment core from the SE Black Sea covering the penultimate glacial and almost the entire Eemian interglacial (133.5 ±0.7-122.5 ±1.7 ka BP). Proxies comprise ice-rafted debris (IRD), O and Sr isotopes as well as Sr/Ca, Mg/Ca, and U/Ca ratios of benthic ostracods, organic and inorganic sediment geochemistry, as well as TEX86 and UK'37derived water temperatures. The ending penultimate glacial (MIS 6, 133.5 to 129.9 ±0.7 ka BP) is characterised by mean annual lake surface temperatures of about 9°C as estimated from the TEX86 palaeothermometer. This period is impacted by two Black Sea melt water pulses (BSWP-II-1 and 2) as indicated by very low Sr/Ca ostracods but high sedimentary K/Al values. Anomalously high radiogenic 87Sr/86Sr ostracod values (max. 0.70945) during BSWP-II-2 suggest a potential Himalayan source communicated via the Caspian Sea. The T II warming started at 129.9 ±0.7 ka BP, witnessed by abrupt disappearance of IRD, increasing d18O ostracod values, and a first TEX86 derived temperature rise of about 2.5°C. A second, abrupt warming step to ca. 15.5°C as the prelude of the Eemian warm period is documented at 128.3 ka BP. The Mediterranean-Black Sea reconnection most likely occurred at 128.1 ±0.7 ka BP as demonstrated by increasing Sr/Ca ostracods and U/Ca ostracods values. The disappearance of ostracods and TOC contents >2% document the onset of Eemian sapropel formation at 127.6 ka BP. During sapropel formation, TEX86 temperatures dropped and stabilised at around 9°C, while UK'37 temperatures remain on average 17°C. This difference is possibly caused by a habitat shift of Thaumarchaeota communities from surface towards nutrient-rich deeper and colder waters located above the gradually establishing halo-and redoxcline.

Geochemical analyses from the Caravaca K/T boundary section, Spain

Thirty-six different geochemical and foraminiferal analyses were conducted on samples collected at closely spaced intervals across the Cretaceous/Tertiary (K/T) boundary exposed at Caravaca, Spain. A rapid reduction in the gradient between d13C values in fine fraction carbonate and benthic foraminiferal calcite and a decrease in the abundance of phosphorus (a proxy for organic carbon) and calcium were recorded in sediments 0-0.5 cm above the K/T boundary. These trends imply that an abrupt mass mortality occurred among pelagic organisms, leading to a significant reduction in the flux of organic carbon to the seafloor. In addition, variations in sulfur isotope ratios, the hydrocarbon-generating potential of kerogen (measured as the hydrogen index), and foraminiferal indices of dissolved oxygen level all imply that a rapid decrease in dissolved oxygen was coincident with the d13C event. Evidence of the low oxygen event has also been recognized in Japan and New Zealand, suggesting that intermediate water oxygen minima were widely developed during earliest Danian time. A threefold increase in the kaolinite/illite ratio and a 1.2 per mill decrease in d18O (carbonate fine fraction) were recorded in the basal 0.1-2 cm of Danian age sediments. These trends suggest that atmospheric warming and an increase in surface water temperature occurred 0-3 kyr after the d13C event. Recovery in the difference between d13C values in the carbonate fine fraction and in benthic foraminiferal calcite as well as increases in phosphorus and calcium contents occur at the base of planktonic foraminiferal Zone Pla, implying that an increase in primary productivity commenced some 13 kyr after the K/T boundary.

Investigations on two sediment cores from the Arctic Ocean

During the "RV Polarstem"-Expedition ARK VIII/2 sediment samples were obtained at the continental slope of NW-Spitsbergen. Detailed sedimentological and geochemical analysis were carried out at two undisturbed box cores (PS2122-1GKG, PS2123-2GKG) as well as two gravity cores (PS2122-1SL, PS2123-2SL). The following parameters were deterrnined: Organic carbon, nitrogen and carbonate contents, hydrogen index, stable isotopes, ice rafted debris, grain-size distribution and biogenic opal. The main objective of this study was the reconstruction of paleoenvironmental changes off the northwest coast of Spitsbergen during the last glacial/interglacial-cycle, i.e., during the last about 128.000 years. The results of the investigations can be summarized as follows: - During isotope stage 1 (Holocene) and 5.5 (Eemian Interglacial), light stable isotopes (d180: 3.4-2 %o; d13C: 0.26-0.5 %o), increased bioturbation, high content of planktonic foraminifera and biogenic opal and low quantity of ice-rafted material, indicate seasonally ice-free conditions along the northwest coast due to the intfluence of the Westspitsbergen Current. - Additionally, the sediment characteristics of the middle of isotope stage 2 (Last Glacial Maximum) and at the end of stage 3 confirms an inflow of warmer Atlantic water. The highest production of planktonic and benthic foraminifera (N. pachyderma sin., Cassidulina teretis) (CaC03: 10 %) may reflect the expansion of the 'Whalers Bay'-Polynya as a result of the influence of the Westspitsbergen Current. Presumably, occasionally open-ice conditions provide sufficient precipitation to buildup the Svalbard/Barents Ice Sheet. - The time intervals for the glacier advances on Svalbard given by Mangerud et al. (1992), can be correlated with increased accumulation of ice-rafted material in the sediments at the northwest coast of Spitsbergen. Especially during isotope stage 4 and at the beginning of the Last Glacial Maximum (isotope stage 2), a drastically increased supply of coarse terrigenous material occurs. The high accumulation rate (0.18-0.21 g/cm**2/ka) of terrigenous organic carbon is indicated by high C/N ratios (until 16) and low hydrogen index (50 mg HC/gC). In constrast to deep sea sediments in the Fram-Strait (Hebbeln 1992), the glacier advance between 118.000 and 108.000 years B.P. ist documented in the continental slope sediments. - At the end of the Weichselian ice age, the deglaciation at the northwest coast starts with a typical melt-water signal in the stables isotope record (d18O: 3.5 %o; d13C: -0.16 %o) and high contents of gravel (6-13 %). The signal can be assigned to an event at the westcoast of Spitsbergen (core NP90-39), dated to 14.500 years B.P. (Andersen et al. 1993).

(Table) Content of organic compounds in the bottom sediments of the Volga River estuary

Materials from different spheres of the Earth are ultimately delivered to bottom sediments, which serve as a natural recorder of the functioning of other spheres and originate as a result of the accumulation of their substances. Sedimentary material and species of river-transported elements are subjected to dramatic reworking in marginal filters, where river and sea waters are mixed. These processes are most important for the Caspian Sea, where runoffs of rivers (especially the Volga River) and the intense development and transportation of hydrocarbon fuel by tankers and pipelines (related to the coastal petroleum industry in the Sumgait and Baku ports, Apsheron Peninsula) are potential sources of hydrocarbon pollution. Previously obtained data showed that the total content of hydrocarbon fraction (i.e., the sum of aliphatic hydrocarbons (AHC) and polycyclic aromatic hydrocarbons (PAH)) in bottom sediments varied within 29-1820 µg/g. The content of petroleum hydrocarbons in the northeastern Caspian region varied from 0.052 to 34.09 µg/g with the maximum content in the Tengiz field. The content of six polyarenes in the Volga delta sediments was no more than 40 ng/g. To determine the recent HC pollution of bottom sediments and trends in the functioning of the Volga marginal filter, in summer of 2003 and 2004 we analyzed bottom sediments (58 samples) in the river waterway; Kirovsk channel; Bakhtemir and Ikryanoe branches; tributaries of the Kizan, Chagan, and other rivers; and the Caspian seashore.

(Table 1) Organic and inorganic geochemical analysis of Late Jurassic to Early Cretaceous black shale sediments from the Norwegian-Greenland Seaway

The Late Jurassic to Early Cretaceous (Volgian-Ryazanian) was a period of a second-order sea-level low stand, and it provided excellent conditions for the formation of shallow marine black shales in the Norwegian-Greenland Seaway (NGS). IKU Petroleum Research drilling cores taken offshore along the Norwegian shelf were investigated with geochemical and microscopic approaches to (1) determine the composition of the organic matter, (2) characterize the depositional environments, and (3) discuss the mechanisms which may have controlled production, accumulation, and preservation of the organic matter. The black shale sequences show a wide range of organic carbon contents (0.5-7.0 wt %) and consist of thermally immature organic matter of type II to II/III kerogen. Rock-Eval pyrolysis revealed fair to very good petroleum source rock potential, suggesting a deposition in restricted shallow marine basins. Well-developed lamination and the formation of autochthonous pyrite framboids further indicate suboxic to anoxic bottom water conditions. In combination with very low sedimentation rates it seems likely that preservation was the principal control on organic matter accumulation. However, a decrease of organic carbon preservation and an increase of refractory organic matter from the Volgian to the Hauterivian are superimposed on short-term variations (probably reflecting Milankovitch cycles). Various parameters indicate that black shale formation in the NGS was gradually terminated by increased oxidative conditions in the course of a sea-level rise.

Bulk geochemical and Rock-Eval data of sediment core IKU-7430/10-U-01 (Table 1)

The late Volgian (early "Boreal" Berriasian) sapropels of the Hekkingen Formation of the central Barents Sea show total organic carbon (TOC) contents from 3 to 36 wt%. The relationship between TOC content and sedimentation rate (SR), and the high Mo/Al ratios indicate deposition under oxygen-free bottom-water conditions, and suggest that preservation under anoxic conditions has largely contributed to the high accumulation of organic carbon. Hydrogen index values obtained from Rock-Eval pyrolysis are exceptionally high, and the organic matter is characterized by well-preserved type II kerogen. However, the occurrence of spores, freshwater algae, coal fragments, and charred land-plant remains strongly suggests proximity to land. Short-term oscillations, probably reflecting Milankovitch-type cyclicity, are superimposed on the long-term trend of constantly changing depositional conditions during most of the late Volgian. Progressively smaller amounts of terrestrial organic matter and larger amounts of marine organic matter upwards in the core section may have been caused by a continuous sea-level rise.