Terrigenous input and marine productivity for the Pacific sector of the Southern Ocean based on lipid biomarkers

In this study, we present a new multiproxy data set of terrigenous input, marine productivity and sea surface temperature (SST) from 52 surface sediment samples collected along E-W transects in the Pacific sector of the Southern Ocean. Allochtonous terrigenous input was characterized by the distribution of plant wax n-alkanes and soil-derived branched glycerol dialkyl glycerol tetraethers (brGDGTs). 230Th-normalized burial rates of both compound groups were highest close to the potential sources in Australia and New Zealand and are strongly related to lithogenic contents, indicating common sources and transport. Detection of both long-chain n-alkanes and brGDGTs at the most remote sites in the open ocean strongly suggests a primarily eolian transport mechanism to at least 110°W, i.e. by prevailing westerly winds. Two independent organic SST proxies were used, the UK'37 based on long-chain alkenones, and the TEX86 based on isoprenoid GDGTs. Both, UK'37 and TEX86 indices show robust relationships with temperature over a temperature range between 0.5 and 20°C, likely implying different seasonal and regional imprints on the temperature signal. While alkenone-based temperature estimates reliably reflect modern SST even at the low temperature end, large temperature residuals are observed for the polar ocean using the TEX86 index. 230Th-normalized burial rates of alkenones are highest close to the Subtropical Front and are positively related to lithogenic fluxes throughout the study area. In contrast, highest isoGDGT burial south of the Antarctic Polar Front is not related with dust flux but may be largely controlled by diatom blooms, and thus high opal fluxes during austral summer.

(Table 1) Lipid biomarkers in surface sediments from the Gulf of Genoa

A series of molecular organic markers were determined in surface sediments from the Gulf of Genoa (Ligurian Sea) in order to evaluate their potential for palaeo-environmental reconstructions. Allochthonous input can be characterized by the distributions of n-C29 and n-C31 alkanes, n-C26 and n-C28 alkanols and branched glycerol dialkyl glycerol tetraethers (GDGTs), whose concentrations are generally highest near the river mouths. In the open basin however, terrestrial n-alkanes and n-alkanols may have an additional, aeolian source. Autochthonous input is represented by crenarchaeol and isoprenoid GDGTs. Their concentrations are highest in the open basin showing the preference of Thaumarchaeota for oligotrophic waters. Indications of a significant degradation of sterols and C37 alkenones exclude these lipids as reliable productivity proxies. Using terrestrial and aquatic lipids as end-members allows estimating the percentage of terrestrial organic matter between 20% and 58% in the coastal area decreasing to 1 to 30% in the deep basin. The spatial distribution of sea surface temperature (SST) estimates using the alkenone-based UK'37 index is very similar to the autumnal (November) mean satellite-based SST distribution. Conversely, TEXH86-derived SST estimates are close to winter SSTs in the coastal area and summer SSTs in the open basin. This pattern reflects presumably a shift in the main production of Thaumarchaeota from the coastal area in winter to the open basin in summer. This study represents a major prerequisite for the future application of lipid biomarkers on sediment cores from the Gulf of Genoa.

(Table 1) Rates of biological activity in deep-sea sediments

Global maps of sulfate and methane in marine sediments reveal two provinces of subsurface metabolic activity: a sulfate-rich open-ocean province, and an ocean-margin province where sulfate is limited to shallow sediments. Methane is produced in both regions but is abundant only in sulfate-depleted sediments. Metabolic activity is greatest in narrow zones of sulfate-reducing methane oxidation along ocean margins. The metabolic rates of subseafloor life are orders of magnitude lower than those of life on Earth's surface. Most microorganisms in subseafloor sediments are either inactive or adapted for extraordinarily low metabolic activity.

Magnetic susceptibility and ice-rafted debris in surface sediments of the Nordic Sea

Magnetic susceptibility and ice-rafted debris of surface sediments in the Nordic Seas were investigated to reconstruct source areas and recent transport pathways of magnetic minerals. From the distribution of magnetic susceptibility and ice-rafted debris and published data on petrographic tracers for iceberg drift, we reconstructed a counter-clockwise iceberg drift pattern during cooler phases in the Holocene, which is similar to conceptual and numerical models for Weichselian iceberg drift. The release of basaltic debris at Scoresby Sund played a significant role for the magnetic signature of stadial/interstadial events during isotope stage 3 recorded in sediment cores of the Nordic Seas.

Biogeochemical parameters and processes in waters and bottom sediments of the Chukchi Sea

Study of biogeochemical processes in waters and sediments of the Chukchi Sea in August 2004 revealed atypical maxima of biogenic element (N, P, and Si) concentrations and rate of microbial sulfate reduction in the surface layer (0-3 cm) of marine sediments. The C/N/P ratio in organic matter (OM) of this layer does not fit the Redfield-Richards stoichiometric model. Specific features of biogeochemical processes in the sea are likely related to the complex dynamics of water, high primary produc¬tivity (110-1400 mg C/m**2/day), low depth of the basin (<50 m for 60% of the water area), reduced food chain due to low population of zooplankton, high density of zoobenthos (up to 4230 g/m**2), and high activity of microbial processes. Drastic decrease in concentrations of biogenic elements, iodine, total alkalinity, and population of microorganisms beneath the 0-3 cm layer testify to large-scale OM decay at the water-seafloor barrier. Our original experimental data support high annual rate of OM mineralization at the bottom of the Chukchi Sea.

(Table T1) Density, porosity, void ratio, diffusivity and permeability of ODP Site 185-1149 sediments

The subduction of oceanic plates regulates crustal growth, influences arc volcanism, and refertilizes the mantle. Continental growth occurs by subduction of crustal material (seawater components, marine sediments, and basaltic crust). The geochemical and physical evolution of the Earth's crust depends, in large part, on the fate of subducted material at convergent margins (Armstrong, 1968, doi:10.1029/RG006i002p00175; Karig and Kay, 1981, 10.1098/rsta.1981.0108). The crustal material on the downgoing plate is recycled to various levels in the subduction zone. The recycling process that takes place in the "Subduction Factory" is difficult to observe directly but is clearly illuminated using chemical tracers. Von Huene and Scholl (1991, doi:10.1029/91RG00969) and Plank and Langmuir (1993, doi:10.1038/362739a0) preliminarily calculated a large flux of subducted materials. By mass balancing the chemical tracers and measuring the fractionations that occur between them, the Subduction Factory work and the effect on the Earth's evolution can be estimated. In order to elucidate this mass balance, Ocean Drilling Program Leg 185 drilled two deepwater shales into the oceanic crust situated in the Mariana-Izu Trenches and recovered core samples of incoming oceanic crust. The calculations of mass circulation in the subduction zone, however, did not take into account the mass transfer properties within subducted oceanic crust, although the dewatering fluid and diffused ions may play an important role in various activities such as seismogeneity, serpentine diapiring, and arc volcanism. Thus, this paper focuses on the quantitative measurements of the physical and mass transfer properties of subducted oceanic crust.

Geochemistry of ODP Hole 108-658C sediments

During the African Humid Period (AHP), much of the modern hyperarid Saharan desert was vegetated and covered with numerous lakes. In marine sediments off northwestern Africa, the AHP is represented by markedly reduced siliciclastic sediment flux between ~ 12.3 and 5.5 ka. Changes in the origin of this terrigenous sediment fraction can be constrained by sediment chemistry and radiogenic isotope tracers. At Ocean Drilling Program (ODP) Site 658, Hole C (20°44.95'N, 18°34.85'W, 2263 mbsl), the neodymium (Nd) isotope composition of terrigenous detritus shows little variability throughout the last 25 kyr, indicating that the contributing geological terranes have not changed appreciably since the last glacial period. In contrast, there were large and abrupt changes in strontium (Sr) isotope ratios and chemical compositions associated with the AHP, during which 87Sr/86Sr ratios were markedly less radiogenic, and sediments show higher chemical indices of alteration. We show that sediment geochemical changes during the AHP cannot be attributed to changes in the source terranes, physical sorting, or intensity of chemical weathering. The low 87Sr/86Sr and high Sr concentrations of AHP-age samples also conflict with the interpretation of increased fine-grained, fluvially derived sediments. We propose that the most significant compositional changes at ODP 658C are due to the addition of an aluminosilicate component that has a highly altered major element signature but is enriched in soluble elements like Sr and magnesium (Mg) compared to aluminum (Al) and has low 87Sr/86Sr relative to local terrigenous source areas. We interpret these characteristics to reflect authigenic sediment supply from extensive North African paleolake basins that were prevalent during the AHP.

Fossilization and degradation of archaeal intact polar tetraether lipids in ODP Site 201-1229 sediments

Glycerol dibiphytanyl glycerol tetraether (GDGT) lipids are part of the cellular membranes of Thaumarchaeota, an archaeal phylum composed of aerobic ammonia oxidizers, and are used in the paleotemperature proxy TEX86. GDGTs in live cells possess polar head groups and are called intact polar lipids (IPL-GDGTs). Their transformation to core lipids (CL) by cleavage of the head group was assumed to proceed rapidly after cell death but it has been suggested that some of these IPL-GDGTs can, just like the CL-GDGTs, be preserved over geological timescales. Here, we examined IPL-GDGTs in deeply buried (0.2-186 mbsf, ~2.5 Myr) sediments from the Peru Margin. Direct measurements of the most abundant IPL-GDGT, IPL-crenarchaeol, specific for Thaumarchaeota, revealed depth profiles which differed per head group. Shallow sediments (<1 mbsf) contained IPL-crenarchaeol with both glycosidic- and phosphate headgroups, as also observed in thaumarchaeal enrichment cultures, marine suspended particulate matter and marine surface sediments. However, hexose, phosphohexose-crenarchaeol is not detected anymore below 6 mbsf (~7 kyr), suggesting a high lability. In contrast, IPL-crenarchaeol with glycosidic head groups is preserved over time scales of Myr. This agrees with previous analyses of deeply buried (>1 m) marine sediments, which only reported glycosidic and no phosphate-containing IPL-GDGTs. TEX86 values of CL-GDGTs did not markedly change with depth, and the TEX86 of IPL-derived GDGTs decreased only when the proportions of monohexose- to dihexose-GDGTs changed, likely due to the enhanced preservation of the monohexose GDGTs. Our results support the hypothesis that in situ GDGT production and differential IPL degradation in sediments is not substantially affecting TEX86 paleotemperature estimations based on CL GDGTs and indicate that likely only a small amount of IPL-GDGTs present in deeply buried sediments is part of cell membranes of active Archaea. The amount of archaeal biomass in the deep biosphere based on these IPLs may have been substantially overestimated.

(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.

Organic carbon concentrations and isotope ratios of sediments from the Laptev Sea

Stable carbon isotope ratios in the organic fraction of surface sediments from the Laptev Sea shelf were analyzed in order to study the modern distribution pattern of terrestrial organic matter. The delta13Corg signature of the surface sediments range from -26.6? near the coastal margin to -22.8? in the north towards the outer shelf. Characterizing the possible sources of organic matter by their delta13Corg signature reveals that the terrestrial influence reaches further north in the eastern than in the western Laptev Sea. Downcore records of the delta13Corg, measured on three AMS 14C-dated cores from water depths between 46 and 77 m, specify the spatial and temporal changes in the deposition of terrestrial organic matter on the Laptev Sea shelf during the past 12.7 ka. The major depositional changes of terrestrial organic matter occurred between 11 and 7 ka and comprised the main phase of the southward retreat of the coastline and of the river depocenters due to the postglacial sea level rise.

n-Alkane content and compound-specific d13C values of soil samples, river samples, and marine surface samples

Southwestern Africa's coastal marine mudbelt, a prominent Holocene sediment package, provides a valuable archive for reconstructing terrestrial palaeoclimates on the adjacent continent. While the origin of terrestrial inorganic material has been intensively studied, the sources of terrigenous organic material deposited in the mudbelt are yet unclear. In this study, plant wax derived n-alkanes and their compound-specific d13C in soils, flood deposits and suspension loads from regional fluvial systems and marine sediments are analysed to characterize the origin of terrestrial organic material in the southwest African mudbelt. Soils from different biomes in the catchments of the Orange River and small west coast rivers show on average distinct n-alkane distributions and compound-specific d13C values reflecting biome-specific vegetation types, most notably the winter rainfall associated Fynbos Biome of the southwestern Cape. In the fluvial sediment samples from the Orange River, changes in the n-alkane distributions and compound-specific d13C compositions reveal an overprint by local vegetation along the river's course. The smaller west coast rivers show distinct signals, reflecting their small catchment areas and particular vegetation communities. Marine surface sediments spanning a transect from the northern mudbelt (29°S) to St. Helena Bay (33°S) reveal subtle, but spatially coherent, changes in n-alkane distributions and compound-specific d13C, indicating the influence of Orange River sediments in the northern mudbelt, the increasing importance of terrigenous input from the adjacent western coastal biomes in the central mudbelt, and contributions from the Fynbos Biome to the southern mudbelt. These findings indicate the different sources of terrestrial organic material deposited in the mudbelt, and highlight the potential the mudbelt has to preserve evidence of environmental change from the adjacent continent.

Carbon analysis of surface sediments from the Laptev Se

During the past six years organic geochemical, micropaleontological, and sedimentological investigations were carried out within the framework of the multidisciplinary bilateral German-Russian research project ''System Laptev Sea'' and detailed biological investigations within the project ''German-Russian Investigations of the Marginal Seas of the Eurasian Arctic'', In order to understand the Laptev Sea ecosystem and to obtain information about sources and fate of organic carbon, the distribution of phyto- and zooplankton, diatoms, chlorophyll a benthic macrofauna, palynomorphs, grain size, total organic carbon, d13Corg and biomarkers (n-alkanes, fatty acids) were determined. In general, the influence of the major rivers draining into the Laptev Sea, is reflected in the water column as well as in the surface sediments. In both habitats three ecological provinces can be distinguished, i.e., the southeastern Laptev Sea, the central Laptev Sea, and the northern Laptev Sea. Additionally, clear differences between the western and the eastern Laptev Sea occur. The comparison of the different data sets of the water column and the surface sediments provide information about organic carbon sources and pathways in the Laptev Sea shelf and continental slope area.

Phytoplanktonic microfossil groups of sediments from the South and Equatorial Atlantic

Individual planktonic microfossil species, or assemblage groups of different species, are often used to, qualitatively and/or quantitatively, reconstruct past (sub)surface-water conditions of the world's oceans and seas. Until now, little information has been available on the surface sediment distribution patterns and paleoenvironmental reconstruction potential of coccolith, calcareous dinoflagellate cyst and organic-walled dinoflagellate cyst assemblages of the South and equatorial Atlantic, especially at the species level. This paper (i) summarizes the distributions of these three phytoplanktonic microfossil groups in numerous Atlantic surface sediments from 20°N-50°S and 30°E-65°W and determines their relationship with the physicochemical and trophic conditions of the overlying (sub)surface-waters, and (ii) determines the synecology of the three phytoplankton groups by carrying out statistical analyses (i.e. detrended and canonical correspondence analyses) on all groups simultaneously. Ecological relationships are additionally strengthened by statistically comparing the distribution patterns of the phytoplankton groups with those of planktonic foraminifera (Pflaumann et al. 1996; Niebler et al. 1998), as the ecological preferences of the latter are much better known. Many of the analyzed phytoplanktonic microfossil species or groups of species in the surface sediments do show restricted distributions which primarily reflect the environmental conditions of the upper water masses above them (e.g. sea-surface temperature, productivity, stratification). The acquired 'reference' data sets are large and diverse enough to allow future development of transfer functions for the reconstruction of past surface-water conditions, and show that there is still an enormous paleoenvironmental reconstruction potential concealed in many fossil coccolith and dinoflagellate cyst assemblages.

Clay mineralogy of South Atlantic surface sediments

Surface samples, mostly from abyssal sediments of the South Atlantic, from parts of the equatorial Atlantic, and of the Antarctic Ocean, were investigated for clay content and clay mineral composition. Maps of relative clay mineral content were compiled, which improve previous maps by showing more details, especially at high latitudes. Large-scale relations regarding the origin and transport paths of detrital clay are revealed. High smectite concentrations are observed in abyssal regions, primarily derived from southernmost South America and from minor sources in Southwest Africa. Near submarine volcanoes of the Antarctic Ocean (South Sandwich, Bouvet Island) smectite contents exhibit distinct maxima, which is ascribed to the weathering of altered basalts and volcanic glasses. The illite distribution can be subdivided into five major zones including two maxima revealing both South African and Antarctic sources. A particularly high amount of Mg- and Fe-rich illites are observed close to East Antarctica. They are derived from biotite-bearing crystalline rocks and transported to the west by the East Antarctic Coastal Current. Chiorite and well-crystallized dioctaedral illite are typical minerals enriched within the Subantarctic and Polarfrontal-Zone but of minor importance off East Antarctica. Kaolinite dominates the clay mineral assemblage at low latitudes, where the continental source rocks (West Africa, Brazil) are mainly affected by intensive chemical weathering. Surprisingly, a slight increase of kaolinite is observed in the Enderby Basin and near the Filchner-Ronne Ice shelf. The investigated area can be subdivided into ten, large-scale clay facies zones with characteristic possible source regions and transport paths. Clay mineral assemblages of the largest part of the South Atlantic, especially of the western basins are dominated by chlorite and illite derived from the Antarctic Peninsula and southernmost South America and supported by advection within the Circumantarctic Deep Water flow. In contrast, the East Antarctic provinces are relatively small. Assemblages of the eastern basins north of 30°S are strongly influenced by African sources, controlled by weathering regimes on land and by a complex interaction of wind, river and deep ocean transport. The strong gradient in clay mineral composition at the Brazilian slope indicate a relatively low contribution of tropically derived assemblages to the western basins.