Trace element concentrations and Li isotope composition in DSDP and ODP sediments from island arcs

We have studied the sedimentary and basaltic inputs of lithium to subduction zones. Various sediments from DSDP and ODP drill cores in front of the Mariana, South Sandwich, Banda, East Sunda and Lesser Antilles island arcs have been analysed and show highly variable Li contents and d7Li values. The sediment piles in front of the Mariana and South Sandwich arcs largely consist of pelagic sediments (clays and oozes). The pelagic clays have high Li contents (up to 57.3 ppm) and Li isotope compositions ranging from +1.3? to +4.1?. The oozes have lower Li contents (7.3-16 ppm) with d7Li values of the diatom oozes from the South Sandwich lower (+2.8? to +3.2?) than those of the radiolarian oozes from the Mariana arc (+8.1? to +14.5?). Mariana sediment also contains a significant portion of volcanogenic material, which is characterised by a moderate Li content (14 ppm) and a relatively heavy isotope composition (+6.4?). Sediments from the Banda and Lesser Antilles contain considerable amounts of continental detritus, and have high Li contents (up to 74.3 ppm) and low d7Li values (around 0?), caused by weathering of continental bedrock. East Sunda sediments largely consist of calcareous oozes. These carbonate sediments display intermediate to high Li contents (2.4-41.9 ppm) and highly variable d7Li values (-1.6? to +12.8?). Basaltic oceanic crust samples from worldwide DSDP and ODP drill cores are characterised by enrichment of Li compared to fresh MORB (6.6-33.1 vs. 3.6-7.5 ppm, respectively), and show a large range in Li isotope compositions (+1.7? to +11.8?). The elemental and isotopic enrichment of Li in altered basalts is due to the uptake of isotopically heavy seawater Li during weathering. However, old oceanic crust samples from Sites 417/418 exhibit lighter Li isotope compositions compared to young basaltic crust samples from Sites 332B and 504B. This lighter Li isotope signature in old crust is unexpected and further research is needed to explore this issue.

Site characteristics, and major and trace element composition of sediment cores sampled around the Windmill Islands in 1998

To establish a natural background and its temporal and spatial variability for the area around Casey Station in the Windmill Islands, East Antarctica, the authors studied major and trace element concentrations and the distribution of organic matter in marine and lacustrine sediments. A wide range of natural variability in trace metal concentrations was identified between sites and within a time scale of 9 ka (e.g., Ni 5-37 mg/kg, Cu 20-190 mg/kg, Zn 50-300 mg/kg, Pb 4.5- 34 mg/kg). TOC concentrations are as high as 3 wt.% at the marine sites and 20 wt.% at the lacustrine sites, and indicate highly productive ecosystems. These data provide a background upon which the extent of human impact can be established, and existing data indicate negligible levels of disturbance. Geochemical and lithological data for a lacustrine sediment core from Beall Lake confirm earlier interpretation of recent climatic changes based on diatom distribution, and the onset of deglaciation in the northern part of the Windmill Islands between 8.6 and 8.0 ka BP. The results demonstrate that geochemical and lithological data can not only be used to define natural background values, but also to assess long-term climatic changes of a specific environment. Other sites, however, preserve a completely different sedimentary record. Therefore, inferred climatic record, and differences between sites, can be ascribed to differences in elevation, distance from the shore, water depth, and local catchment features. The extreme level of spatial variability seems to be a feature of Antarctic coastal areas, and demonstrates that results obtained from a specific site cannot be easily generalized to a larger area.

Composition of aerosols collected in the Western Russian Arctic in August-September 1991

In August-September 1991 during the SPASIBA expedition (Scientific Program on the Arctic and Siberian Aquatorium) aboard R/V Yakov Smirnitzky in the Laptev Sea ten samples of aerosols were collected by nylon nets. A combined approach including various analytical techniques, such as single-particle analysis, instrumental neutron activation analysis, and atomic absorption spectrophotometry, was used to study composition of the samples. Mass concentration of coarse-grained (>0.001 mm) insoluble fraction of aerosols ranged from 80 to 460 ng/m**3. In all the samples remains of land vegetation were found to be the dominant component. Organic carbon content of the aerosols ranged from 23 to 49%. Inorganic part of the samples was represented mainly by alumosilicates and quartz. Anthropogenic ''fly ash'' particles were observed in all the samples. Temporal variations of element concentrations resulted from differences in air masses entering the studied area.

Holocene sedimentation in the Strait of Otranto, Mediterranean Sea

An extensive radiograph study of 24 undisturbed, up to 206-cm long box and gravity cores from the western part of the Strait of Otranto revealed a great variety of primary bedding structures and secondary burrowing features. The regional distribution of the sediments according to their structural, textural, and compositional properties reflects the major morphologic subdivisions of the strait into shelf, slope, and trough bottom (e.g., the bottom of the northern end of the Corfu-Kephallinia Trough, which extends from the northeastern Ionian Sea into the Strait of Otranto): (1) The Apulian shelf (0 to -170m) is only partly covered by very poorly sorted, muddy sands without layering. These relict(?) sands are rich in organic carbonate debris and contain glauconite and reworked (?Pleistocene) ooids. (2) The slope sediments (-170 to -1,000 m) are poorly sorted, sandy muds with a high degree of burrowing. One core (OT 5) is laminated and shows slump structures. An origin of these slumped sediment masses from older deposits higher on the slope was inferred from their abnormal compaction, color, texture, organic content, and mineral composition. (3) Cores from the northern end of the Corfu-Kephallinia Trough (-980 to -1,060 m) display a few graded sand layers, 2-5 cm (maximum 30 cm) thick with parallel and ripple-cross-laminations, deposited by oceanic bottom or small-scale turbidity currents. They are intercalated with homogeneous lutite. (4) Hemipelagic sediments prevail in the more southerly part of the Corfu-Kephallinia Trough and on the "Apulian-Ionian Ridge", the southern submarine extension of the Apulian Peninsula. Below a core depth of 160 cm, these cores have a laminated ("varved") zone, representing an Early Holocene (Boreal-Atlanticum) "stagnation layer" (14C age approximately 9,000 years). The terrigenous components of the surface sediments as well as those of the deeper sand layers can be derived from the Apulian shelf and the Italian mainland (Cretaceous Apulian Plateau and Gargano Mountains, southern Apennines, volcanic province of the Monte Vulture). Indicated by the heavy mineral glaucophane, a minor proportion of the sedimentary material is probably of Alpine origin. If this portion is considered to be first-cycle clastic material it reaches the Strait of Otranto after a longitudinal transport of 700 km via the Adriatic Sea. The lack of phyllosilicates in the coarse- to medium-grained shelf samples might be explained by the activity of the "Apulian Current" (surface velocities up to 4 knots) which in the past possibly has affected the bottom almost down to depths of the shelf edge. The percentage of planktonic organisms, and also the plankton: benthos ratio in the sediments is a useful indicator for bathymetry (depth zonation).

Trace element concentrations in zircons from ODP Holes 147-894G and 153-923A

The trace element compositions of Hadean zircons have been used in two ways to argue for the existence of Hadean continental crust. One argument is based on low crystallization temperatures of Hadean zircons that have been determined using a novel geothermometer based on the Ti content of zircons in equilibrium with rutile. The second argument is based on using the trace element abundances in zircons to calculate their parental melt compositions, especially the rare earth elements. Here we demonstrate that zircons that grow from a melt formed by basalt differentiation at modern mid-ocean ridges cannot be unambiguously distinguished from Hadean zircons on either of these grounds. Thus, we conclude that the trace element compositions of Hadean zircons are permissive of models that do not include the generation of continental crust in the Hadean.

Zooplankton and seston in the White Sea and its chemical composition

A technique of zooplankton net sampling at night in the Kandalaksha and Dvinskii Bays and during the full tide in the Onezhskii Bay of the White Sea allowed us to obtain "clean" samples without considerable admixtures of terrigenous particulates. Absence of elements-indicators of the terrigenous particulates (Al, Ti, and Zr) in the EDX spectra allows to conclude that ash composition of tested samples is defined by constitutional elements comprising organic matter and integument (chitin, shells) of plankton organisms. A quantitative assessment of accumulation of ca. 40 chemical elements by zooplankton based on a complex of modern physical methods of analysis is presented. Values of the coefficient of the biological accumulation of the elements (Kb) calculated for organic matter and the enrichment factors (EF) relative to Clarke concentrations in shale are in general determined by mobility of the chemical elements in aqueous solution, which is confirmed by calculated chemical speciation of the elements in the inorganic subsystem of surface waters of Onezhskii Bay.

Geochemistry and mineralogy of interstitial waters and clays in sediments of ODP Leg 180 sites

Oxygen and strontium isotopes and Rb and Ba were determined in interstitial water (IW) collected from Sites 1109, 1115, and 1118 drilled on the Woodlark Rise during Ocean Drilling Program Leg 180. The trace element and mineralogical composition of the clay fraction of sediments isolated from the squeeze cakes corresponding to IW samples from Site 1109 was also determined.

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.

Measurements of trace metal abundances and isotopes in carbonate sediments, Exuma, Bahamas

In order to validate the use of 238U/235U as a paleoredox proxy in carbonates, we examined the incorporation and early diagenetic evolution of U isotopes in shallow Bahamian carbonate sediments. Our sample set consists of a variety of primary precipitates that represent a range of carbonate producing organisms and components that were important in the past (scleractinian corals, calcareous green and red algae, ooids, and mollusks). In addition, four short push cores were taken in different depositional environments to assess the impact of early diagenesis and pore water chemistry on the U isotopic composition of bulk carbonates. We find that U concentrations are much higher in bulk carbonate sediments (avg. 4.1 ppm) than in primary precipitates (avg. 1.5 ppm). In almost all cases, the lowest bulk sediment U concentrations were as high as or higher than the highest concentrations found in primary precipitates. This is consistent with authigenic accumulation of reduced U(IV) during early diagenesis. The extent of this process appears sensitive to pore water H2S, and thus indirectly to organic matter content. d238/235U values were very close to seawater values in all of the primary precipitates, suggesting that these carbonate components could be used to reconstruct changes in seawater U geochemistry. However, d238/235U of bulk sediments from the push cores was 0.2-0.4 per mil heavier than seawater (and primary precipitates). These results indicate that authigenic accumulation of U under open-system sulfidic pore water conditions commonly found in carbonate sediments strongly affects the bulk U concentrations and 238U/235U ratios. We also report the occurrence of dolomite in a tidal pond core which contains low 234U/238U and 238U/235U ratios and discuss the possibility that the dolomitization process may result in sediments depleted in 238U. From this initial exploration, it is clear that 238U/235U variations in ancient carbonate sediments could be driven by changes in global average seawater, by spatial and temporal variations in the local deposition environment, or subsequent diagenesis. To cope with such effects, proxies for syndepositional pore water redox conditions (e.g., organic matter content, iron speciation, and trace metal distributions) and careful consideration of possible post-deposition alteration will be required to avoid spurious interpretation of 238U/235U data from ancient carbonate sediments.

Mineralogy and geochemistry of alteration products in DSDP Leg 81 Holes

Alteration products of basalts from the four holes drilled during Leg 81 were studied and found to be characterized by the widespread occurrence of trioctahedral clay minerals (Mg smectite to chlorite). In some cases zeolites (analcite, chabazite) are associated with the saponite. A more oxidizing stage is marked by a saponite-celadonite association, presenting the geochemical characteristics of hydrothermal processes. Later stages of alteration are represented by palagonitization and subaerial weathering at two sites. These different alteration processes of basalts from Leg 81 record the paleoenvironment during the first opening stages of the Northeast Atlantic Ocean in the Paleocene-Eocene periods.