Organic matter preservation in sediments of the Peru and Oman continental margin

Detailed petrographical and bulk geochemical investigations of organic matter (OM) have been performed on sediments deposited below or close to upwelling areas offshore Peru (ODP-Leg 112; Sites 679, 681, 688) and Oman (ODP-Leg 117; Sites 720, 723, 724) in order to obtain a quantitative understanding of its accumulation and degradation. Microscopical as well as nanoscopical investigations reveal that the OM in sediments affected by upwelling mechanisms mainly (up to 98%) consists of unstructured (amorphous) organic aggregates without any apparent biological structures. In sediments which are not or to a lesser extent affected by upwelling (Site 720) terrestrial OM predominates. Organic carbon (TOC) contents are highly variable and range between 9.8% in sediments deposited below upwelling cells and 0.2% in sediments outside the upwelling zone. The TOC/sulphur ratios of the sediments scatter widely. The samples from the deep-water locations (Sites 688 and 720), show C/S-ratios of "normal" marine sediments, whereas at the other locations no correlation or even a negative correlation between sulphur and TOC concentration exists. In most of the upwelling-influenced sediments OM contains a significant amount of sulphur. The incorporation of sulphur into the OM followed microbial sulphate reduction and occurred in the upper meters of the sedimentary column. Below, OM is still present in vast amounts and relatively hydrogen-rich, but is nevertheless non-metabolizable and becomes the limiting factor for bacterial sulphate reduction. According to mass balance calculations 90-99% of the OM produced in the photic zone was remineralized and 1-3% was consumed by microbial sulphate reduction. The aerobic and anaerobic processes have greatly affected degradation and conservation of OM.

Radioisotope stratigraphy, sedimentology and geochemistry of Late Quaternary sediments from the Eastern Arctic Ocean

To reconstruct Recent and past sedimentary environments, marine sediments of Upper Pleistocene and Holocene ages from the eastern Arctic Ocean and especially from the Nansen-Gakkel Ridge (NGR) were investigated by means of radioisotopic, geochemical and sedimentological methods. In combination with mass physical property data and lithological analysis these investigations allow clearly to characterize the depositional environments. Age dating by using the radioisotope 230Th gives evidence that the investigated sediments from the NGR are younger than 250,000 years. Identical lithological sediment sequences within and between sediment cores from the NGR can be related to sedimentary processes which are clearly controlled by palaeoclimate. The sediments consist predominantly of siliciclastic, terrigenous ice-rafted detritus (IRD) deriving from assorted and redeposited sediments from the Siberian shelfs. By their geochemical composition the sediments are similar to mudstone, graywacke and arcose. Sea-ice as well as icebergs play a major roll in marine arctic sedimentation. In the NGR area rapid change in sedimentary conditions can be detected 128,000 years ago. This was due to drastic change in the kind of ice cover, resulting from rapid climatic change within only hundreds of years. So icebergs, deriving mostly from Siberian shelfs, vanished and sea-ice became dominant in the eastern Arctic Ocean. At least three short-period retreats of the shelf ice between 186,000 and 128,000 years are responsible for the change of coarse to fine-grained sediments in the NGR area. These warmer stages lasted between 1,000 and 3,000 years. By monitoring and comparing the distribution patterns of sedimentologic, mass physical and geochemical properties with 230Th ex activity distribution patterns in the sediment cores from the NGR, there is clear evidence that sediment dilution is responsible for high 230Th ex activity variations. Thus sedimentation rate is the controlling factor of 230Th ex activity variations. The 230Th flux density in sediments from the NGR seems to be highly dependent On topographic Position. The distribution patterns of chemical elements in sediment cores are in general governed by lithology. The derivation of a method for dry bulk density determination gave the opportunity to establish a high resolution stratigraphy on sediment cores from the eastern Arctic Ocean, based on 230Thex activity analyses. For the first time sedimentation and accumulation rates were determined for recent sediments in the eastern Arctic Ocean by 230Th ex analyses. Bulk accumulation rates are highly variable in space and time, ranging between 0.2 and 30 g/cm**2/ka. In the sediments from the NGR highly variable accumulation rates are related to the kind of ice cover. There is evidence for hydrothermal input into the sediments of the NGR. Hydrothermal activity probably also influences surficial sediments in the Sofia Basin. High contents of As are typical for surficial sediments from the NGR. In particular SL 370-20 from the bottom of the rift valley has As contents exceeding in parts 300 ppm. Hydrothermal activity can be traced back to at least 130,000 years. Recent to subrecent tectonic activity is documented by the rock debris in KAL 370 from the NGR. In four other sediment cores from the NGR rift valley area tectonically induced movements can be dated to about 130,000 years ago, related most probably to the rapid climate change. Processes of early diagenesis in sediments from the NGR caused the aobilization and redeposition of Fe, Mn and Mo. These diagenetic processes probably took place during the last 130,000 years. In sediment cores from the NGR high amounts of kaolinite are related to coarse grained siliciclastic material, probably indicating reworking and redeposition of siberian sandstones with kaolinitic binding material. In contrast to kaolinite, illite is correlated to total clay and 232Th contents. Aragonite, associated with serpentinites in the rift valley area of the NGR, was precipitated under cold bottom-water conditions. Preliminary data result in a time of formation about 60 - 80 ka ago. Manganese precipitates with high Ni contents, which can be related to the ultrabasic rocks, are of similar age.

Geochemistry of recent sediments from the Indian Ocean

During the International Indian Ocean Expedition (1964/65) sediment cores were taken on six profiles off the western coast of the Indian Subcontinent. These profiles run approximately perpendicular to the coast, from the deep-sea over the continental slope to the continental shelf. Additional samples and cores were taken in a dense pattern in front of the delta of the Indus River. This pattern of sampling covered not only marine sediments, but also river and beach sediments in Pakistan. The marine samples were obtained with piston, gravity and box corers and by a Van Veen grab sampler. The longest piston core is about 5 meters long. 1. Distribution of the elements on the sediment surface The area of maximal carbonate values (aprox. 80-100% CaCO3) essentially coincides with the continental shelf. The highest Sr values were observed largely within this area, but only in the vicinity of the Gulf of Cambay. Mainly the aragonitic coprolites are responsible for those high Sr contents. The Mg contents of the carbonates are comparatively low; surprisingly enough the highest Mg concentrations were also measured in the coprolites. The maximum contents of organic matter (Core) were found along the upper part of the continental slope. They coincide with the highest porosity and water content of the sediments. Frequently the decomposition of organic matter by oxydation is responsible for the measured Corg contents. On the other side the quantity of originally deposited organic material is less important in most cases. The enrichment of the "bauxitophile" elements Fe, Ti, Cr and V in the carbonate- and quartz-free portions of the sediments is essentially due to the influence of coarse terrigenous detritus. For the elements Mn, Ni and Cu (in per cent of the carbonateand quartz-free sediment) a strong enrichment was observed in the deep-sea realm. The strong increase in Mn toward the deep-sea is explained by authigenesis of Mn-Fe-concretions. Mn-nodules form only under oxydizing conditions which obviously are possible only at very low rates of deposition. The Mg, B and, probably also Mn contents in the clay minerals increase with increasing distance from the continent. This can be explained by the higher adsorption of those elements from sea water because of increasing duration of the clay mineral transport. The comparison of median contents of some elements in our deep-sea samples with deep-sea sediments described by TUREKIAN & WEDEPOHL (1961) shows that clear differences in concentration exist only in the case of "bauxitophile" elements Cr and Be. The Cr and Be contents show a clear increase in the Indian Ocean deep-sea samples compared to those described by TUREKIAn & WEDEPOHL (1961) which can obviously be attributed to the enrichment in the lateritic and bauxitic parent rocks. The different behaviour of the elements Fe, Ti and Mn during decomposition of the source rocks, transport to the sea and during oxydizing and reducing conditions in the marine environment can be illustrated by Ti02/Fe and MnO/Fe ratios. The different compositions of the sediments off the Indus Delta and those of the remaining part of the area investigated are characterized by a different distribution of the elements Mn and Ti. 2. Chemical inhomogenities in the sediments Most longer cores show 3 intervals defined by chemical and sedimentological differences. The top-most interval is coarse-grained, the intermedial interval is fine grained and the lower one again somewhat coarser. At the same time it is possible to observe differences from interval to interval in the organogenic and detrital constituents. During the formation of the middle interval different conditions of sedimentation from those active during the previous and subsequent periods have obviously prevailed. Looking more closely at the organogenic constituents it is remarkable that during the formation of the finer interval conditions of a more intensive oxydation have prevailed that was the case before and after: Core decreases, whereas P shows a relative increase. This may be explained by slower sedimentation rate or by a vertical migration of the oxygen rich zone of the sea-water. The modifications of the elements from minerals in detrital portion of the sediments support an explanation ascribing this fact to modifications of the conditions of denudation and transportation which can come about through a climatic change or through tectonic causes. The paleontological investigations have shown (ZOBEL, in press) that in some of the cores the middle stratum of fine sedimentation represents optimal conditions for organic life. This fact suggests also oxydizing conditions during the sedimentation of this interval. In addition to the depositional stratification an oxydation zone characterized by Mn-enrichment can be recognized. The thickness of the oxidation zone decreases towards the coast and thins out along the middle part of the continental slope. At those places, where the oxydation zone is extremely thin, enrichment of Mn has its maximum. This phenomenon can probably be attributed to the migration of Mn taking place in its dissociated form within the sediment under reducing conditions. On the other side this Mn-migration in the sediment does not take place in the deep-sea, where oxydizing conditions prevail. 3. Interstitial waters in the sediments Already at very small core depths, the interstitial waters have undergone a distinct modification compared with the overlying sea water. This distinct modification applies both to total salinity and to the individual ions. As to the beginning of diagenesis the following conclusions can be drawn: a) A strong K-increase occurs already at an early stage. It may be attributable to a diffusion barrier or to an exchange of Mg-ions on the clays. Part of this increase may also originate from the decomposition of K-containing silicates (mica and feldspars). A K-decrease owing to the formation of illite (WEAVER 1967), however, occurs only at much greater sediment depth. b) Because of an organic protective coating, the dissolution of carbonate is delayed in recent organogenic carbonates. At the same time some Ca is probably being adsorbed on clay minerals. Consequently the Ca-content of the interstitial water drops below the Ca-content of the sea water. c) Already at an early stage the Mg adsorption on the clays is completed. The adsorbed Mg is later available for diagenetic mineral formations and transformations.

Pb isotopes in Cenozoic sediments of ODP Site 177-1090

Nd and Pb isotopic compositions extracted from bulk deep sea sediments have been shown to be robust proxies for deep water circulation as well as weathering provenance and intensity over geologically young time scales. In this study we evaluated ten deep sea samples from Ocean Drilling Program (ODP) site 1090 ranging in age from mid Eocene to early-Miocene to test whether Pb isotopic compositions extracted from geologically older sediments record reliable seawater isotopic ratios and to evaluate the source of the extracted Pb. The sequential extraction protocol used in this study is similar to protocols reported for previous studies and produces acetic acid, hydroxylamine hydrochloride (HH) and residue fractions. Each extracted fraction was analyzed for Pb isotopes, rare earth elements (REEs), and a suite of major elements. Similar 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios are recorded from the acetic acid and HH fractions for ~70-80% of the samples, suggesting that either the acetic acid dissolves Fe-Mn oxides or multiple phases are recording the same seawater isotopic value. Several indirect tests, such as Al mass balance, comparison of Sr isotopes in HH extracts to contemporaneous seawater Sr isotopes, and comparison of Nd isotopic compositions in HH extracts to published fossil fish teeth values, provide evidence that Pb isotopic compositions measured in our bulk HH extracts record bottom water values. The relationship between Pb, Mn and Ca concentrations in HH fractions indicates that Fe-Mn oxides and a Mn-bearing carbonate are the dominant phases contributing seawater Pb. Comparison of REE patterns derived from the HH fraction and total digestions of Fe-Mn nodule standards reveals that the trivalent REEs exhibit patterns consistent with the parent archive, but Ce can be fractionated during extraction. Ratios of REEs also produce unique fields for each fraction and can be used to test the purity of the seawater signal of the extraction protocol. Finally, an initial evaluation of Pb isotopic compositions in fossil fish indicates that this archive is not suitable for bottom water Pb isotope studies.

Organic matter composition of Cretaceous sediments of the central Pacific Ocean

Complete records of organic-carbon-rich Cretaceous strata were continuouslycored on the flanks of the Mid-Pacific Mountains and southern Hess Rise in the central North Pacific Ocean during DSDP Leg 62. Organic-carbon-rich laminated silicified limestones were deposited in the western Mid-Pacific Mountains during the early Aptian, a time when that region was south of the equator and considerably shallower than at present. Organic-carbon-rich, laminated limestone on southern Hess Rise overlies volcanic basement and includes 136 m of stratigraphic section of late Albian to early Cenomanian age. This limestone unit was deposited rapidly as Hess Rise was passing under the equatorial high-productivity zone and was subsiding from shallow to intermediate depths. The association of volcanogenic components with organic-carbon-rich strata on Hess Rise in the Mid-Pacific Mountains is striking and suggests that there was a coincidence of mid-plate volcanic activity and the production and accumulation of organic matter at intermediate water depths in the tropical Pacific Ocean during the middle Cretaceous. Pyrolysis assays and analyses of extractable hydrocarbons indicate that the organic matter in the limestone on Hess Rise is composed mainly of lipid-rich kerogen derived from aquatic marine organisms and bacteria. Limestones from the Mid-Pacific Mountains generally contain low ratios of pyrolytic hydrocarbons to organic carbon and low hydrogen indices, suggesting that the organic matter may contain a significant proportion of land-derived material, possibly derived from numerous volcanic islands that must have existed before the area subsided. The organic carbon in all samples analyzed is isotopically light (d13C -24 to -29 per mil) relative to most modern rine organic carbon, and the lightest carbon is also the most lipid-rich. There is a positive linear correlation between sulfur and organic carbon in samples from Hess Rise and from the Mid-Pacific Mountains. The slopes and intercepts of C-S regression lines however, are different for each site and all are different from regression lines for samples from modern anoxic marine sediments and from Black Sea cores. The organic-carbon-rich limestones on Hess Rise, the Mid-Pacific Mountains, and other plateaus and seamounts in the Pacific Ocean are not synchronous but do occur within the same general middle Cretaceous time period as organic-carbon-rich lithofacies elsewhere in the world ocean, particularly in the Atlantic Ocean. Strata of equivalent age in the deep basins of the Pacific Ocean are not rich in organic carbon, and were deposited in oxygenated environments. This observation, together with the evidence that the plateau sites were considerably shallower and closse to the equator during the middle Creataceous suggests that local tectonic and hydrographic conditions may have resulted in high surface-water productivity and the preservation of organic matter in an oxygen-deficient environment where an expanded mid-water oxygen minimum developed and impinged on elevated platforms and seamounts.

Carbon and Lead chemistry of sediments from the Middle Atlantic Bight

A mass budget was constructed for organic carbon on the upper slope of the Middle Atlantic Bight, a region thought to serve as a depocenter for fine-grained material exported from the adjacent shelf. Various components of the budget are internally consistent, and observed differences can be attributed to natural spatial variability or to the different time scales over which measurements were made. The flux of organic carbon to the sediments in the core of the depocenter zone, at a water depth of 1000 m, was measured with sediment traps to be 65 mg C m**-2 day**-1, of which 6-24 mg C m**-2 day**-1 is buried. Oxygen fluxes into the sediments, measured with incubation chambers attached to a free vehicle lander, correspond to total carbon remineralization rates of 49-70 mg C m**-2 day**-1. Carbon remineralization rates estimated from gradients of Corg within the mixed layer, and from gradients of dissolved ammonia and phosphate in pore waters, sum to only 4-6 mg C m**-2 day**-1. Most of the Corg remineralization in slope sediments is mediated by bacteria and takes place within a few mm of the sediment-water interface. Most of the Corg deposited on the upper slope sediments is supplied by lateral transport from other regions, but even if all of this material were derived from the adjacent shelf, it represents <2% of the mean annual shelf productivity. This value is further lowered by recognizing that as much as half of the Corg deposited on the slope is refractory, having originated by reworking from older deposits. Refractory Corg arrives at the sea bed with an average 14C age 600-900 years older than the pre-bomb 14C age of DIC in seawater, and has a mean life in the sediments with respect to biological remineralization of at least 1000 years. Labile carbon supplied to the slope, on the other hand, is rapidly and (virtually) completely remineralized, with a mean life of < 1 year. Carbon-14 ages of fine-grained carbonate and organic carbon present within the interstices of shelf sands are consistent with this material acting as a source for the old carbon supplied to the slope. Winnowing and export of reworked carbon may contribute to the often-described relationship between organic carbon preservation and accumulation rate of marine sediments.

Clay mineraology and grain size composition of surface sediments from South Pacific Ocean

The reconstruction of low-latitude ocean-atmosphere interactions is one of the major issues of (paleo-)environmental studies. The trade winds, extending over 20° to 30° of latitude in both hemispheres, between the subtropical highs and the intertropical convergence zone, are major components of the atmospheric circulation and little is known about their long-term variability on geological time-scales, in particular in the Pacific sector. We present the modern spatial pattern of eolian-derived marine sediments in the eastern equatorial and subtropical Pacific (10°N to 25°S) as a reference data set for the interpretation of SE Pacific paleo-dust records. The terrigenous silt and clay fractions of 75 surface sediment samples have been investigated for their grain-size distribution and clay-mineral compositions, respectively, to identify their provenances and transport agents. Dust delivered to the southeast Pacific from the semi- to hyper-arid areas of Peru and Chile is rather fine-grained (4-8 µm) due to low-level transport within the southeast trade winds. Nevertheless, wind is the dominant transport agent and eolian material is the dominant terrigenous component west of the Peru-Chile Trench south of ~ 5°S. Grain-size distributions alone are insufficient to identify the eolian signal in marine sediments due to authigenic particle formation on the sub-oceanic ridges and abundant volcanic glass around the Galapagos Islands. Together with the clay-mineral compositions of the clay fraction, we have identified the dust lobe extending from the coasts of Peru and Chile onto Galapagos Rise as well as across the equator into the doldrums. Illite is a very useful parameter to identify source areas of dust in this smectite-dominated study area.

Physical properties of deep sea sediments

Erstmals quantitativ bearbeitete Rutschungen aus dem Tiefseebereich des äquatorialen Ostatlantiks liegen auf Hängen von 0,4 Grad und 0,7 Grad - das ist sehr viel flacher als die für statische Rutschungsauslösung benötigte kritische Hangneigung (14 Grad bis 16 Grad ). Im Gegensatz zu Flachwassergebieten kann bei Wassertiefen von über 4000 m natürlich der Einfluß von Wellenwirkung und Tidenhub auf die Hangstabilität vernachlässigt werden. Die Sedimentationsraten sind in diesem Bereich zur Bildung eines Porenwasserüberdruckes vielfach zu niedrig. Nach den Hangstabilitätsanalysen bilden hier Erdbeben den wirksamsten Auslösemechanismus für die Rutschungen. Dies gilt auch für Rutschungen an den Kontinentalrändern von Nordwest- und Westafrika sowie für das europäische Nordmeer und für Rutschungen im nördlichen Fidji-Becken. Das Alter der besonders gut datierten Rutschungen vom nordwestafrikanischen Kontinentalrand und der Tiefsee des äquatorialen Ostatlantiks schwankt zwischen 16,000 J.v.h. und 18,000 J.v.h. sowie etwa 130,000 J.v.h.. Es handelt sich dabei um Phasen des Beginns besonders starker Meeresspiegelschwankungen. Ein Vergleich der Meeresspiegelkurve mit dem Alter älterer Rutschungen zeigt ebenfalls eine Parallele mit Zeiten von Regressionen und Transgressionen. Durch die Meeresspiegelschwankungen werden isostatische Vertikalbewegungen des Tiefseebodens von bis zu 30 m bewirkt, die Spannungen in den Lithosphärenplatten erzeugen. Sie allein sind jedoch nicht groß genug, um Brüche in intakten Plattenbereichen zu verursachen. Entlang alter, ehemals aktiver Transform-Bruchzonen (Fracture Zones) können jedoch die aufgebauten Spannungen eher wieder abgebaut werden. Dabei entstehen kleinere Erdstösse und führen zur Auslösung von Rutschungen. Ein Vergleich der Verbreitung von Transform-Störungen und Rutschungen vor Norwegen, Nordwest- und Westafrika sowie vor dem südlichen Afrika zeigt, daß in diesen Gebieten Rutschmassen tatsächlich besonders häufig entlang und in der Verlängerung von Fracture Zones auftreten. Modellrechnungen, die mit typischen Werten für Hangwinkel (0,5 Grad bis 3 Grad) von Tiefseeböschungen und passive Kontinentalränder sowie für häufig ermittelte Scherfestigkeitsgradienten im Sediment (0,5 kPa/m bis 1,7 kPa/m) durchgeführt wurden, ergaben, daß in Gebieten mit normal konsolidierten Sedimenten (ohne Porenwasserüberdruck) nur Erdbeben Rutschungen ausgelöst haben können.

Calcium carbonate content and concentration of phosphorus components in sediments of ODP Leg 171B sites

Quantifying phosphorus (P) concentrations in marine sediments is necessary for constraining the oceanic record of phosphorus burial and helps to constrain P sedimentary geochemistry. To understand P geochemistry in the sediments, we must determine the geochemical forms of P as well as the transformations occurring between these P components with depth and age. Although several records now exist of P geochemistry in the western and eastern equatorial Pacific (Filippelli and Delaney, 1995, doi:10.2973/odp.proc.sr.138.144.1995; 1996, doi:10.1016/0016-7037(96)00042-7), the western equatorial Atlantic (Delaney and Anderson, 1997, doi:10.2973/odp.proc.sr.154.124.1997), the California Current (Delaney and Anderson, in press), and the Benguela Current (Anderson et al., 2001, doi:10.1029/2000GB001270), most of these are Neogene records. Relatively little data exist from sediments of the Paleogene or Cretaceous, time periods when carbon isotope records indicate major carbon shifts and when the nature of P geochemistry has not been well constrained. Samples from several sites at various water depths, oceanographic regions, and ages are needed to understand how P geochemistry and burial in sediments reflect ocean history. We determined P geochemistry and reactive P concentrations in Atlantic sediments of Eocene to Cretaceous age. These are the first records of P geochemistry with good age control from this period. Blake Nose sites are ideal for investigating P geochemistry, as the sediments are shallowly buried at a range of water depths and sedimentation rates. We determined P concentrations and geochemistry, along with calcium carbonate contents, in mid-Cretaceous to upper Eocene sediments drilled on Blake Nose (Ocean Drilling Program Leg 171B) in a depth transect of four sites (Sites 1052, 1051, 1050, and 1049; water depths: 1345, 1983, 2300, and 2656 m, respectively).

Solid phase phosphorus geochemistry in surface sediments of sediment cores GeoB3718-4, GeoB3702-2 and GeoB3707-4

In this study we investigate benthic phosphorus cycling in recent continental margin sediments at three sites off the Namibian coastal upwelling area. Examination of the sediments reveals that organic and biogenic phosphorus are the major P-containing phases preserved. High Corg/Porg ratios just at the sediment surface suggest that the preferential regeneration of phosphorus relative to that of organic carbon has either already occurred on the suspension load or that the organic matter deposited at these sites is already rather refractory. Release of phosphate in the course of benthic microbial organic matter degradation cannot be identified as the dominating process within the observed internal benthic phosphorus cycle. Dissolved phosphate and iron in the pore water are closely coupled, showing high concentrations below the oxygenated surface layer of the sediments and low concentrations at the sediment-water interface. The abundant presence of Fe(III)-bound phosphorus in the sediments document the co-precipitation of both constituents as P-containing iron (oxyhydr)oxides. However, highly dissolved phosphate concentrations in pore waters cannot be explained, neither by simple mass balance calculations nor by the application of an established computer model. Under the assumption of steady state conditions, phosphate release rates are too high as to be balanced with a solid phase reservoir. This discrepancy points to an apparent lack of solid phase phosphorus at sediment depth were suboxic conditions prevail. We assume that the known, active, fast and episodic particle mixing by burrowing macrobenthic organisms could repeatedly provide the microbially catalyzed processes of iron reduction with authigenic iron (oxyhydro)oxides from the oxic surface sediments. Accordingly, a multiple internal cycling of phosphate and iron would result before both elements are buried below the iron reduction zone.

Preliminary lipid analyses of two Quaternary sediments from DSDP Holes 66-487 and 66-491

We investigated the solvent-extractable hydrocarbons, ketones, alcohols, and carboxylic acids of two Quaternary sediments from the Middle America Trench (Sections 487-2-3 and 491-1-5). These lipids are derived from a mixed input of autochthonous and allochthonous materials, with minor contributions from thermally mature sources. Their compositions are typical of those of immature Quaternary marine sediments, and their lipid distributions show many similarities to those of Japan Trench sediments.

C1-C8 hydrocarbons in DSDP Leg 64 Holes sediments

Sediments from the Baja California Continental Margin Transect - Sites 474 and 476 - showed small amounts of C2-C8 hydrocarbons and functionalized compounds (alkenes) typical of organic-rich, Recent, cold (<30°C) marine sediments. In contrast, some samples from Sites 477, 478, 479, and Hole 481A in the Guaymas Basin, an active spreading center, showed the characteristics of thermally generated hydrocarbons. These include an increase (sometimes exponential) in amount and diversity of C2-C8 hydrocarbons and a decrease in alkenes in more thermally mature sediments. The results indicate that the injection of basaltic sills has minimal effect on C2-C8 hydrocarbon generation except in the immediate vicinity of the sill. The absence of light hydrocarbons close to the hottest sills suggests that the compounds distill away as they are formed in these areas of very active hydrothermal circulation. A sample of young sediment exposed to very high temperatures (>300°C) from deeper thermal sources at the hottest site, 477, showed a very limited hydrocarbon distribution, including primarily ethane, benzene, and toluene, together with smaller amounts of propane and butane.

Major element oxides in ODP Site 164-997 sediments

Since being first discovered in the Blake-Bahama region of the west Atlantic in the 1970s (Hollister, Ewing, et al., 1972, doi:10.2973/dsdp.proc.11.1972), submarine gas hydrates have been identified in the continental margin worldwide. Ocean Drilling Program (ODP) Leg 164 was the first drilling designated to study the occurrence and distribution of natural gas hydrates in Blake Ridge where a well developed, distinct BSR (Bottom Simulating Reflector) has been identified (Paull, Matsumoto, Wallace, et al., 1996, doi:10.2973/odp.proc.ir.164.1996). It has been reported there is a prominent discrepancy between the BSR and the base of gas hydrate stability (Paull, Matsumoto, Wallace, et al., 1996, doi:10.2973/odp.proc.ir.164.1996; Ruppel, 1997, doi:10.1130/0091-7613(1997)025<0699:ACTOAT>2.3.CO;2), though theoretically they should be at the same depth. Natural gas hydrate in marine sediments coexists with sediment particles, so detailed delineation of sediment geochemistry will be of benefit to solve this apparent discrepancy. The main objectives of this study are to supply background data of the major chemical compositions of sediments from a hydrated sediment section.

Phospholipids in sediments of the deep Arabian Sea

Eight different sites from 2300 to 4420 m water depth in the Arabian Sea were sampled for a biochemical quantification of phospholipid concentrations in the sediments. This method serves as a measure of microbial biomass in marine sediments comprising all small-sized organisms, including bacteria, fungi, protozoa and metazoa. Phospholipid concentrations can be converted to carbon units as an estimate of total microbial biomass in the sediments. The average phospholipid concentrations in the surface sediments (0-1 cm) of the 4 abyssal sites ranged from 7 nmol cm?3 at the southern site (SAST, 10°N 65°E, 4425 m) to 29 nmol/cm**3 at the western site (WAST, 16°N 60°E, 4045 m). The high values detected at the abyssal station WAST exceeded those in the literature for other abyssal sites and were comparable to values from the upper continental slope of the NE-Atlantic and the Arctic. At the four continental slope sites in the Arabian Sea, average phospholipid concentrations ranged from 9 to 53 nmol/cm**3 with the maximum values at stations A (2314 m) and D (3142 m) close to the Omani coast. Records of particulate organic carbon flux to the deep sea are available for four of the investigated locations, allowing a test of the hypothesis that the standing stock of benthic microorganisms in the deep sea is controlled by substrate availability, i.e. particle sedimentation. Total microbial biomass in the surface sediments of the Arabian Sea was positively correlated with sedimentation rates, consistent with previous studies of other oceans. The use of the measurement of phospholipid concentrations as a proxy for input of particulate organic matter is discussed.