Geochemistry of continental slope and deep sea basin sediments off Uruguay

Pore water and solid phase from surface sediments of the continental slope off Uruguay and from the Argentine Basin (southwestern Atlantic) were investigated geochemically to ascribe characteristic early diagenetic reactions of iron and manganese. Solid-phase iron speciation was determined by extractions as well as by Mössbauer spectroscopy. Both methods showed good agreement (<6% deviation) for total-Fe speciation. The proportion of easy reducible iron oxyhydroxide relative to total-Fe oxides decreased from the continental slope to the deep sea which is attributed to an increase in crystallinity during transport as well as to a general decrease of iron mobilization. The product of iron reoxidation is Fe oxyhydroxide which made up less than 5% of total Fe. In addition to this fraction, a proportion of smectite bound iron was found to be redox reactive. This fraction made up to 10% of total Fe in sediments of the Argentine Basin and was quantitatively extracted by 1 N HCl. The redox reactive Fe(+II) fraction of smectite was almost completely reoxidized within 24 h under air atmosphere and may therefore considerably contribute to iron redox cycling if bioturbation occurs. In the case of the slope sediments we found concurrent iron and manganese release to pore water. It is not clear whether this is caused by dissimilatory iron and manganese reduction at the same depth or dissimilatory iron reduction alone inducing Mn(+IV) reduction by (abiotic) reaction with released Fe2+. The Argentine Basin sediment showed a significant manganese solid-phase enrichment above the denitrification depth despite the absence of a distinct pore-water gradient of Mn. This implies a recent termination of manganese mobilization and thus a non-steady-state situation with respect to sedimentation or to organic carbon burial rate.

Barium concentrations und stable isotope ratios of late Paleocene deep-sea sediments

Deep-sea sediment Ba* (Ba/Al2O3(sample) * 15% - Ba(aluminosilicate) records show increasing values synchronous with the evolution of the late Paleocene global d13C maximum, reflecting an increase in marine surface primary production and biogenic barite formation at this time. At two oligotrophic locations, Deep Sea Drilling Project (DSDP) Sites 384 and 527 in the North and South Atlantic, respectively, Ba* increases from 160-360 ppm in the early Paleocene to 1100-3000 ppm during the d13C maximum. At equatorial DSDP Site 577, positioned within or near the high-productivity zone, Ba* increases from ~15,500 ppm in the early Paleocene to ~25,400 ppm in conjunction with late Paleocene maximum d13C values. Linear fitted correlation plots of sediment Ba* content versus surface water d13C in all three regions support barite originating in the euphotic zone. The early to late Paleocene relative increase in Ba* illustrates how burial rates of Corg (relative to Al2O3) accelerated by a factor of ~1.8 and ~6.0 in the eutrophic and oligotrophic areas, respectively. A tentative estimate, weighing our result for the entire ocean, suggests that accumulation rates of organic carbon increased by a factor of 2 during the late Paleocene d13C maximum.

Eocene siliceous and calcareous phytoplankton, Deep Sea Drilling Project Holes 95-612 and 95-613

Eocene siliceous and calcareous phytoplankton, with emphasis on silicoflagellates, were studied in 62 samples from DSDP Sites 612 and 613 on the continental slope and rise off New Jersey. The mid-latitude assemblages correlate well with assemblages from California, Peru, and offshore of southern Brazil, but are distinctly different from high-latitude cold-water assemblages of the Falkland Plateau off southern Argentina. Coccoliths and silicoflagellates provide evidence for the presence of a fairly complete middle and upper Eocene sequence, represented by a composite of Sites 612 and 613. A major unconformity occurs at the middle Eocene to upper Eocene contact at Site 612. The genus Bachmannocena Locker is emended and proposed as a replacement for genus Mesocena Ehrenberg for ring silicoflagellates. Six new silicoflagellates and one new diatom are described: Bachmannocena apiculata monolineata Bukry, n. subsp., Corbisema amicula Bukry, n. sp., C. bimucronata elegans Bukry, n. subsp., C. hastata incohata Bukry, n. subsp., C. jerseyensis Bukry, n. sp., Dictyocha acuta Bukry, n. sp., and Coscinodiscus eomonoculus Bukry, n. sp. Also, one new replacement name, B. paulschulzn Bukry, nom. nov., and 24 new combinations are proposed for genus Bachmannocena.

Organic geochemistry of deep sea sediments from the Nankai Trough and the Southwest Japan forearc region

Earth's largest reactive carbon pool, marine sedimentary organic matter, becomes increasingly recalcitrant during burial, making it almost inaccessible as a substrate for microorganisms, and thereby limiting metabolic activity in the deep biosphere. Because elevated temperature acting over geological time leads to the massive thermal breakdown of the organic matter into volatiles, including petroleum, the question arises whether microorganisms can directly utilize these maturation products as a substrate. While migrated thermogenic fluids are known to sustain microbial consortia in shallow sediments, an in situ coupling of abiotic generation and microbial utilization has not been demonstrated. Here we show, using a combination of basin modelling, kinetic modelling, geomicrobiology and biogeochemistry, that microorganisms inhabit the active generation zone in the Nankai Trough, offshore Japan. Three sites from ODP Leg 190 have been evaluated, namely 1173, 1174 and 1177, drilled in nearly undeformed Quaternary and Tertiary sedimentary sequences seaward of the Nankai Trough itself. Paleotemperatures were reconstructed based on subsidence profiles, compaction modelling, present-day heat flow, downhole temperature measurements and organic maturity parameters. Today's heat flow distribution can be considered mainly conductive, and is extremely high in places, reaching 180 mW/m**2. The kinetic parameters describing total hydrocarbon generation, determined by laboratory pyrolysis experiments, were utilized by the model in order to predict the timing of generation in time and space. The model predicts that the onset of present day generation lies between 300 and 500 m below sea floor (5100-5300 m below mean sea level), depending on well location. In the case of Site 1174, 5-10% conversion has taken place by a present day temperature of ca. 85 °C. Predictions were largely validated by on-site hydrocarbon gas measurements. Viable organisms in the same depth range have been proven using 14C-radiolabelled substrates for methanogenesis, bacterial cell counts and intact phospholipids. Altogether, these results point to an overlap of abiotic thermal degradation reactions going on in the same part of the sedimentary column as where a deep biosphere exists. The organic matter preserved in Nankai Trough sediments is of the type that generates putative feedstocks for microbial activity, namely oxygenated compounds and hydrocarbons. Furthermore, the rates of thermal degradation calculated from the kinetic model closely resemble rates of respiration and electron donor consumption independently measured in other deep biosphere environments. We deduce that abiotically driven degradation reactions have provided substrates for microbial activity in deep sediments at this convergent continental margin.

Geochemical analyses of chert nodules and beds from deep-sea sediments (Table 1)

Concentrations of Fe, Mg, Ca, Sr, Mn, Zn, and other heavy metals were analyzed by atomic absorption spectrometry in 27 chert samples from the Pacific deep sea, 17 chert samples from land, and 4 associated sediments from the Pacific Ocean. Among the elements, Fe and Mg concentrations are highly correlatable as are the relationships between Ca and Sr, or between Ca and CO2. The correlation between Fe and Mg is particularly high for Pacific deep-sea flints and cherts, and for cherts of deep-sea origin from outcrops on land. Enrichments in heavy metals were recognized in some deep-sea cherts; volcanogenic cherts are enriched in Fe, a chert nodule containing basaltic fragments is enriched in Zn and Cr, and biogenically enclosed carbonates in flint nodules are enriched in Mn. The correlation of Fe and Mg and their constant ratio [Mg(%)/Fe(%)] of around 0.33 might be characteristic features in the pelagic clays contained in deep-sea flints and cherts, and the concentrations of heavy metals in them would be controlled by the concentrations of Fe-Mg correlated clays. Although the mineralogical nature of the Fe-Mg clay in deep-sea cherts was not clarified by dissolution experiments on opaline minerals in chert, the high concentrations of Fe-montmorillonite and fine-grained olivine or other ferromagnesian silicate minerals in the clay may result in the high correlations between Fe and Mg.

3He/4He and 4He/20Ne ratios in pore water and gas in Pacific deep-sea sediments

We have measured the 3He/3He and 3He/20 Ne ratios of thirty-nine pore water and gas samples in deep-sea sediments collected at twelve sites on the Pacific Ocean bottom during the cruises of Deep Sea Drilling Project Legs 87, 89, 90 and 92. The 3He/4He and 4He/20Ne ratios vary from 0.000000215 to 0.00000165 and from 0.29 to 20, respectively. He in the sample is composed of four components: (1) atmospheric He dissolved in seawater; (2) atmospheric He with mantle-derived He in Pacific bottom water; (3) in situ radiogenic He in the sediment; and (4) crustal He in the basement rock. Assuming that the 20Ne contents are constant with the value of seawater, the depth variations in the 4He/20Ne ratios at five Sites, 583D, 594, 597A, 598A and 504B, may provide useful information on 4He flux at the ocean bottom. The estimated 4He fluxes vary from 2000 to 40000 atoms cm**-2 s**-1 and are one to three orders of magnitude less than those calculated from the excess He in deep ocean water. An overall similarity between the geographical distribution of the 3He/4He ratios and heat flow data is found in the study area, between the East Pacific Rise across the Pacific Ocean and the Japanese Islands. The tendency is well explained by a conventional sea-floor spreading model.

Bacterial activity in sediments of the deep Arabian Sea

In the Arabian Sea, productivity in the surface waters and particle flux to the deep sea are controlled by monsoonal winds. The flux maxima during the South-West (June-September) and the North-East Monsoon (December-March) are some of the highest particle fluxes recorded with deep-sea sediment traps in the open ocean. Benthic microbial biomass and activities in surface sediments were measured for the first time in March 1995 subsequent to the NE-monsoon and in October 1995 subsequent to the SW-monsoon. These measurements were repeated in April/May 1997 and February/March 1998, at a total of six stations from 1920 to 4420 m water depth. This paper presents a summary on the regional and temporal variability of microbial biomass, production, enzyme activity, degradation of 14C-labeled Synechococcus material as well as sulfate reduction in the northern, western, eastern, central and southern Arabian deep sea. We found a substantial regional variation in microbial biomass and activity, with highest values in the western Arabian Sea (station WAST), decreasing approximately threefold to the south (station SAST). Benthic microbial biomass and activity during the NE-monsoon was as high or higher than subsequent to the SW-monsoon, indicating a very rapid turnover of POC in the surface sediments. This variation in the biomass and activity of the microbial assemblages in the Arabian deep sea can largely be explained by the regional and temporal variation in POC flux. Compared to other abyssal regions, the substantially higher benthic microbial biomasses and activities in the Arabian Sea reflect the extremely high productivity of this tropical basin.

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.