Geochemistry and Nd-Pb isotope ratios of sediments from the Lesser Antilles arc

The Lesser Antilles arc is a particularly interesting island arc because it is presently very active, it is located perpendicular to the South American continent and its chemical and isotopic compositions display a strong north-south gradient. While the presence in the south of a thick pile of sedimentary material coming from the old South American continent has long been suspected to explain the geochemical gradient, previous studies failed to demonstrate unambiguously a direct link between the arc lava compositions and the subducted sediment compositions. Here, we present new Nd, Sm, Th, U and Pb concentrations and Nd-Pb isotopic data for over 60 sediments from three sites located in the fore arc region of the Lesser Antilles arc. New data for DSDP Site 543 drill core located east of Dominica Island complement the data published by White et al. (1985, doi:10.1016/0016-7037(85)90082-1) and confirm their relatively uniform isotopic compositions (i.e., 206Pb/204Pb between 19.13 and 19.53). In contrast, data obtained on DSDP Site 144 located further south, on the edge of the South American Rise and on sediments from Barbados Island are much more variable (206Pb/204Pb ranges from 18.81 to 27.69). The very radiogenic Pb isotopic compositions are found in a 60 m thick black shale unit, which has no age equivalent in the Site 543 drill core. We interpret the peculiar composition of the southern sediments as being due to two factors, (a) the proximity of the South American craton, which contributes coarse grain old detrital material that does not travel far from the continental shelf, and (b) the presence of older sediments including the thick black shale unit formed during Oceanic Anoxic events 2 and 3. The north-south isotopic change known along the Lesser Antilles arc can be explained by the observed geographical changes in the composition of the subducted sediments. About 1% contamination of the mantle wedge by Site 543 sediments explains the composition of the northern islands while up to 10% sediments like those of Site 144 is required in the source of the southern island lavas. The presence of black shales in the subducted pile provides a satisfactory explanation for the very low Delta8/4 values that characterize the Lesser Antilles arc.

Gotland Deep (Baltic Sea) independent age model based on Palaeomagnetic Secular Variation (PSV) and lead (Pb) deposition profiles

Dating of sediment cores from the Baltic Sea has proven to be difficult due to uncertainties surrounding the 14C reservoir age and a scarcity of macrofossils suitable for dating. Here we present the results of multiple dating methods carried out on cores in the Gotland Deep area of the Baltic Sea. Particular emphasis is placed on the Littorina stage (8 ka ago to the present) of the Baltic Sea and possible changes in the 14C reservoir age of our dated samples. Three geochronological methods are used. Firstly, palaeomagnetic secular variations (PSV) are reconstructed, whereby ages are transferred to PSV features through comparison with varved lake sediment based PSV records. Secondly, lead (Pb) content and stable isotope analysis are used to identify past peaks in anthropogenic atmospheric Pb pollution. Lastly, 14C determinations were carried out on benthic foraminifera (Elphidium spec.) samples from the brackish Littorina stage of the Baltic Sea. Determinations carried out on smaller samples (as low as 4 µg C) employed an experimental, state-of-the-art method involving the direct measurement of CO2 from samples by a gas ion source without the need for a graphitisation step - the first time this method has been performed on foraminifera in an applied study. The PSV chronology, based on the uppermost Littorina stage sediments, produced ten age constraints between 6.29 and 1.29 cal ka BP, and the Pb depositional analysis produced two age constraints associated with the Medieval pollution peak. Analysis of PSV data shows that adequate directional data can be derived from both the present Littorina saline phase muds and Baltic Ice Lake stage varved glacial sediments. Ferrimagnetic iron sulphides, most likely authigenic greigite (Fe3S4), present in the intermediate Ancylus Lake freshwater stage sediments acquire a gyroremanent magnetisation during static alternating field (AF) demagnetisation, preventing the identification of a primary natural remanent magnetisation for these sediments. An inferred marine reservoir age offset (deltaR) is calculated by comparing the foraminifera 14C determinations to a PSV & Pb age model. This deltaR is found to trend towards younger values upwards in the core, possibly due to a gradual change in hydrographic conditions brought about by a reduction in marine water exchange from the open sea due to continued isostatic rebound.

(Table 1) Lead isotopic composition and concentration at DSDP Leg 82 Holes

The aim of DSDP Leg 82 was to decipher the temporal and spatial evolution of Azores Plume. The Pb-isotopic results of this leg are rather complex, and can be summarized as follows: 1. At a given site (561, 558), variations of Pb isotopic compositions are generally accompanied by major changes in trace-element ratios, indicating significant heterogeneity of the source region. There is a correlation between Pb isotopes and trace elements. 2. In contrast, if all the data (i.e., all studied sites) of Leg 82 are considered together, no correlation can be discerned between Pb isotopes and trace elements. Site 556, especially, shows abnormal behavior. 3. Leg 82 samples not only cover the entire range of Pb isotopic composition previously established for the Atlantic Ocean, but extend this field to more radiogenic values. 4. The data are compatible with the hot-spot model proposed by Schilling (1975), if one considers that the Azores Plume itself is isotopically heterogeneous, and that it has been progressively contaminated to various degrees by upper mantle material.

(Table 1) Color, Fe-C-S chemistry and carbonate content from ODP Sites 172-1062 and 172-1063

Reflectance spectra collected during ODP Leg 172 were used in concert with solid phase iron chemistry, carbonate content, and organic carbon content measurements to evaluate the agents responsible for setting the color in sediments. Factor analysis has proved a valuable and rapid technique to detect the local and regional primary factors that influence sediment color. On the western North Atlantic drifts, sediment color is the result of primary mineralogy as well as diagenetic changes. Sediment lightness is controlled by the carbonate content while the hue is primarily due to the presence of hematite and Fe2+/Fe3+ changes in clay minerals. Hematite, most likely derived from the Permo-Carboniferous red beds of the Canadian Maritimes, is differentially preserved at various sites due to differences in reductive diagenesis and dilution by other sedimentary components. Various intensities for diagenesis result from changes in organic carbon content, sedimentation rates, and H2S production via anaerobic methane oxidation. Iron monosulfides occur extensively at all high sedimentation sites especially in glacial periods suggesting increased high terrigenous flux and/or increased reactive iron flux in glacials.

Calcareous nannofossil assemblages of Ocean Anoxic Event 2 in ODP Sites 207-1258 and 207-1260

The calcareous nannofossils of the Cenomanian/Turonian boundary interval of Sites 1258 and 1260 (Ocean Drilling Program Leg 207) have been studied in order to understand the depositional environment during Oceanic Anoxic Event 2 (OAE2) in the equatorial Atlantic. Nannofossil assemblages show a significant change in relative abundances during the positive d13Corg excursion interval. The strong increase of the high productivity indicator Zeugrhabdotus erectus and the simultaneous decrease of the oligotrophic taxa Watznaueria barnesiae and Watznaueria fossacincta are indicative of enhanced fertility. The decrease of Eprolithus floralis may be attributed to the surface-water temperature increase during OAE2, which is, however, not very significant (~2-3 °C), as suggested by published TEX86 data. It seems more likely that the decrease of E. floralis during OAE2 was evoked by the breakdown of water-column stratification, indicating it as a deep-dwelling species, which prefers stratified waters with a deep nutricline. Prediscosphaera spp. and Retecapsa ficula, which show a significant increase in relative abundances during OAE2, seem to prefer eutrophic environments, while Amphizygus brooksii and Zeugrhabdotus noeliae lower surface-water fertility. Gartnerago segmentatum, Broinsonia spp., Watznaueria biporta, and Seribiscutum gaultensis decrease in abundances during OAE2. It is not clear if they preferred an oligotrophic environment, cooler surface-waters, or if they were inhabitants of the lower photic zone. Published geochemical data suggest that enhanced fertility and higher temperatures during OAE2 may have been caused by submarine volcanic activity through the release of biolimiting micronutrients into the ocean and carbon dioxide into the atmosphere. The breakdown of water-column stratification may have increased further nutrient availability.