Organic matter composition and sulfate reduction intensity in Oman Margin sediments

Petrographical and geochemical studies of Neogene marine sediments from the Oman Sea (Leg 117, Sites 720, 724, 726 and 730), show a close relationship between the nature and amount of the organic matter, and the degree of degradation of organic matter by sulfate reduction, i.e. pyritization. Petrographically, three major pyritization types were observed: (1) Finely dispersed pyrite framboids in sediments from Oman Margin and Indus Fan, enriched in autochthonous marine organic matter. (2) Infilling of pores by massive pyrite crystals in Oman Margin sediments with a low TOC and a high microfossil content. (3) Pyrite mineralization of lignaceous fragments in organic-depleted sediments from the Indus Fan leading to more massive pyrite. Geochemically, we can define a sulfate reduction index (SRI) as the percentage of initial organic carbon versus that of residual organic carbon. Finely laminated Pliocene-Pleistocene sediments from the Oman Margin exclusively contain organic matter deriving from organic phytoplankton, for which the quantity (TOC) positively correlates with the geochemical quality (Hydrogen Index). We think that the occurrence of this residual organic matter is linked mainly to a high primary paleo-productivity. The intensity of sulfate reduction is constant for sediments with TOC up to 2% and becomes more important when organic input decreases. This degradation process can destroy up to 50% of the initial organic matter, but is not sufficient to explain some of the encountered very low TOC values. It can be seen that sharp increases of certain plankton species (with mineral skeletons) are responsible for a pronounced degradation of organic matter, due to increased sulfate reduction. In that case, the organic matter may be strongly degraded (high SRI), although deposited in an oxygen-depleted environment. Conversely, Miocene-Pliocene sediments contain an autochthonous organic matter that is typical of both low productivity and oxic processes; their very low sulfate reduction index indicates that very little metabolizable organic matter was initially present.

Geochemical and optical study of organic matter in some Pleistocene and Pliocene sediments at DSDP Leg 64 Holes

Pleisto-Pliocene hemipelagic and diatomaceous mud was recovered from Deep Sea Drilling Project (DSDP) Sites 474 through 481 in the Gulf of California. The organic matter is mostly marine and mainly derived from diatomaceous protoplasm. We found some continental organic matter in sediments near the bottom basalts or near dolerites (Holes 474A and 478). The organic matter in most of the samples is in an early stage of evolution.

Chemical and mineral compositions of bottom sediments and ferromanganese crusts and nodules near Iceland

Results of study of bottom sediments near Iceland and on the Jan Mayen Island are reported. It was found that in recent sediments chemical elements are mainly associated with pyro- and volcanoclastics. In some areas adjusted to deep-seated faults ancient iron-manganese crusts and sediments occur. They are rich in Ni, Co, V, Cu, Mo, Cd and other elements associated with endogenic matter.

Facies and diagenesis of organic matter in sediments at DSDP Leg 87

A series of upper Pliocene to Pleistocene sediment samples from DSDP Sites 582 and 583 (Nankai Trough, active margin off Japan) were investigated by organic geochemical methods including organic carbon determination, Rock- Eval pyrolysis, gas chromatography of extractable hydrocarbons, and kerogen microscopy. The organic carbon content is fairly uniform and moderately low (0.35 to 0.77%) at both sites, although accompanied by high sedimentation rates. The low organic matter concentrations are the result of the combined effect of several factors: low bioproductivity, oxic depositional environment, and dilution with lithogenic material. Organic petrography revealed a mixture of three maceral types: (1) fresh, green fluorescent alginites of aquatic origin probably transported by turbidites from the shelf edge, (2) gelified huminites and paniculate liptinites derived from the erosion of unconsolidated peat, and (3) highly reflecting inertinites derived from continental erosion. By a combination of organic petrography and Rock-Eval pyrolysis results, the organic matter is characterized as mainly type III kerogen with a slight tendency to a mixed type II-III. During Rock-Eval pyrolysis, a mineral matrix effect on the generated hydrocarbons was observed. The organic matter in all sediments has a low level of maturity (below 0.45% Rm) and has not yet reached the onset of thermal hydrocarbon generation according to several geochemical maturation parameters. This low maturity is in contrast to anomalously high extract yields at both sites and large hydrocarbon proportions in the extracts at Site 583. This contrast may be due to early generation of polar compounds and perhaps redistribution of hydrocarbons caused by subduction tectonics. Carbon isotope data of the interstitial hydrocarbon gases indicate their origin from bacterial degradation of organic matter, although only very few bacterially degraded maceral components were detected.

(Appendix 1-4) Geochemistry, desorption methods, TOC, TC, and mineral content of different settings

Sorption of volatile hydrocarbon gases (VHCs) to marine sediments is a recognized phenomenon that has been investigated in the context of petroleum exploration. However, little is known about the biogeochemistry of sorbed methane and higher VHCs in environments that are not influenced by thermogenic processes. This study evaluated two different extraction protocols for sorbed VHCs, used high pressure equipment to investigate the sorption of methane to pure clay mineral phases, and conducted a geochemical and mineralogical survey of sediment samples from different oceanographic settings and geochemical regimes that are not significantly influenced by thermogenic gas. Extraction of sediments under alkaline conditions yielded higher concentrations of sorbed methane than the established protocol for acidic extraction. Application of alkaline extraction in the environmental survey revealed the presence of substantial amounts of sorbed methane in 374 out of 411 samples (91%). Particularly high amounts, up to 2.1 mmol kg**-1 dry sediment, were recovered from methanogenic sediments. Carbon isotopic compositions of sorbed methane suggested substantial contributions from biogenic sources, both in sulfate-depleted and sulfate-reducing sediments. Carbon isotopic relationships between sorbed and dissolved methane indicate a coupling of the two pools. While our sorption experiments and extraction conditions point to an important role for clay minerals as sorbents, mineralogical analyses of marine sediments suggest that variations in mineral composition are not controlling variations in quantities of sorbed methane. We conclude that the distribution of sorbed methane in sediments is strongly influenced by in situ production.

(Table T1) Mineral composition of turbidite sands and sandstones from ODP Leg 190 sites

During Ocean Drilling Program Leg 190 several turbidite successions in the Nankai Trough were drilled through including Pleistocene trench fill (Sites 1173 and 1174), Pleistocene-Pliocene slope basin deposits and underlying trench fill (Sites 1175 and 1176), Miocene Shikoku Basin deposits (Site 1177), and upper Miocene trench fill (Site 1178). Sands from the Pleistocene trench-fill succession of the Nankai Trough are of mixed derivation with significant monomineralic components (quartz and feldspar) and mafic to intermediate volcanic rock fragments, in addition to sedimentary and less abundant metamorphic detritus. They have a source in the Izu collision zone in central Honshu. Sands from the slope and accreted trench fill at Sites 1175 and 1176 are dominated by quartz with less abundant feldspar, sedimentary rock fragments, and only minor volcanic and metamorphic rock fragments. In contrast to the trench turbidites of Sites 1173 and 1174, these sands are very quartzose with characteristic radiolarian chert fragments. Volcanic rock fragments are mainly of silicic composition. Potential sources of these sands are uplifted subduction complexes of southwest Japan. Sands from the accreted trench turbidites at Site 1178 have clast types similar to those at Sites 1175 and 1176. In contrast, however, framework detrital modes are distinctive, with Site 1178 sands having substantially lower total quartz contents and more abundant fine-grained sedimentary rock fragments. These sands were also probably derived from the island of Shikoku, but their composition indicates that sedimentary rocks were abundant in the source area and these may have been Miocene forearc basin successions that were largely removed by erosion. Erosional remnants of Miocene forearc basin deposits are present on the Kii Peninsula east-northeast of Shikoku. Erosion followed a phase of exhumation of the Shimanto Belt indicated by apatite fission track ages at ~10 Ma. Sand in the lower-upper Miocene turbidites of the lower Shikoku Basin section at Site 1177 is more varied in composition, with the upper part of the unit similar to Site 1178 (i.e., rich in sedimentary rock fragments) and the lower part similar to those at Sites 1175 and 1176 (i.e., rich in quartz with some silicic volcanic rock fragments). Sands from the lower part of the Miocene turbidite unit were derived from a continental source with plutonic and volcanic rocks, possibly the inner zone of southwest Japan.

Grain size, mineral and chemical compositions of modern deposits fron the northern Caspian Sea and the Volga River delta

Comprehensive investigations revealed that modern deposits in the northern Caspian Sea involve terrigenous sands and aleurites with admixture of detritus and intact bivalve shells, including coquina. Generally, these deposits overlay dark grayish viscous clays. Similar geological situation occurs in the Volga River delta; however, local deposits are much poorer in biogenic constituents. Illite prevails among clay minerals. In coarse aleurite fraction (0.100-0.050 mm) heavy transparent minerals are represented mostly by epidotes, while light minerals - mostly by quartz and feldspars. Sedimentary material in the Volga River delta is far from completely differentiated into fractions due to abundant terrigenous inflows. Comparatively better grading of sediments from the northern Caspian Sea is due to additional factors such as bottom currents and storms. When passing from the Volga River delta to the northern Caspian Sea, sediments are enriched in rare earth elements (except Eu), Ca, Au, Ni, Se, Ag, As, and Sr, but depleted in Na, Rb, Cs, K, Ba, Fe, Cr, Co, Sc, Br, Zr, ??, U, and Th. Concentrations of Zn remain almost unchanged. Sedimentation rates and types of recent deposits in the northern Caspian Sea are governed mainly by abundant runoff of the Volga River.

Chemical and mineral compositions of recent and ancient sediments and sedimentary rocks from the Mariana Trench

On the bed and on the ocean slope of the southern latitudinal part of the Mariana Trench ancient sediments, as well as sedimentary and igneous rocks are exposed. In the lower part of the sampled part of the studied section Late Oligocene to Early Miocene chalk-like limestones and marls occur. Upward marly tuffites and tuffs (apparently alternating with carbonate rocks) occur. These rocks are overlain by Early Miocene tuffaceous clays and siliceous-clayey muds. In the upper part of the section there are Pleistocene pelagic clays and ethmodiscus oozes.

Composition of organic matter from black shales drilled in the Cape Verde Basin in DSDP Hole 41-367

Results of geochemical studies of organic matter in black shales from the Cape Verde Basin are reported. Based on these results, in combination with data of petrographic analysis, conclusions are made about sapropelic nature of their organic matter and low degree of its coalification. It corresponds to the proto-catagenetic substage of sedimentary rocks. Black shales of the Cape Verde Basin are classified as potential oil source strata.

Chemical and mineral compositions of Northwest Pacific bottom sediments and their grain size fractions

A detailed study of chemical composition of bottom sediments along a profile through the Northwest Pacific Basin has allowed to identify and describe four lithofacies types of bottom sediments. Distinguished types of sediments form a genetic series reflecting changing conditions of sedimentation from near-shore to central regions of the ocean. Along the strike of pelagic clays a gradual transition from ash containing clays to zeolite containing clays is established. Ash particles and zeolites have similar forms of occurrence. Together with other data it suggests that zeolites have been formed by diagenetic transformation of rhyolitic glass. Regular changes of CaCO3, amorphous SiO2, Fe and Mn contents in bottom sediments from the coast to the pelagic zone are shown.

Mineral and chemical composition of surface bottom sediments from the Onega Bay, White Sea

The distribution of temperature and salinity, current velocities, suspended particulate matter, bottom sediments, bottom morphology, and planktonic and benthic organisms during the summer period are studied in the estuary of the large Onega River and coastal areas of the Onega Bay (White Sea) influenced by interacting marine and riverine factors.