B. Schnetger

Geochemical characteristics of deep-sea sediments from the Arabian Sea: a high-resolution study

Abstract Five deep-sea cores from the Arabian Sea, covering a time interval of 170 ky, were sampled at high resolution and analysed for major and trace elements. The stratigraphy of the cores was obtained by comparing the Ba/Al ratio with the SPECMAP data. This method is based on the close correlation between Ba concentrations (or Ba/Al ratios) and oxygen isotope ratios (δ18O) of foraminifera in Arabian Sea sediments. Ba/Al ratios, used as a productivity proxy, are variable but high in all deep-sea cores of the Arabian Sea, indicating a basin-wide influence of nutrient-rich water masses. Compared to glacial intervals the warmer periods are characterised by higher element/Al ratios of proxies, which are directly (Ba, Ca, Sr, P, i.e. hard parts of organisms) or indirectly (U, 230Thex) related to biological productivity. 230Thex provides evidence for intense boundary scavenging caused by high productivity due to enhanced upwelling and terrigenous input from the Indus fan. The Mn distribution in a core from the western Arabian Sea shows enrichments during interglacial periods and may indicate Mn export owing to the presence of a stronger oxygen-minimum zone. Several processes limit the applicability of proxies. Intercalated turbidites in the deep-sea cores can be identified by means of Si/Al, Ti/Al and Zr/Al ratios versus depth due to the enrichment of quartz, Ti-minerals and zircon in the basal layer. These elements are also important for the reconstruction of the history and extent of the eolian dust input. Diagenetic redistribution of redox sensitive elements occurs at the boundaries between turbidites and “normal” pelagic sedimentation and may mask or destroy primary signals.

Distribution of organic carbon in surface sediments along the eastern Arabian Sea: a revisit

Abstract One hundred twelve surface sediment samples along the western continental margin of India were analyzed for organic carbon (OC). Elevated OC concentrations are associated with sediments deposited in areas where productivity is moderate to high. Very high OC concentrations, as reported in earlier studies even for regions where productivity is comparatively low, could not be confirmed. The re-analysis of selected samples by coulometry shows that OC concentrations determined previously by the wet oxidation method may be too high by up to more than a factor of two. A broad similarity in the distribution of OC and bioproductivity is observed, which is in accordance with other productivity proxies, such as the abundance of planktonic foraminiferal species G. bulloides and hydrogen indices (HI). Our results indicate the importance of bioproductivity for OC accumulation along the western continental margin of India, even though low dissolved oxygen concentrations in the water column, sedimentation rate, sediment texture, clay mineral content, dilution by quartz and carbonate, as well as topography are considered responsible for the general OC distribution in this area.

Barium as a productivity proxy in continental margin sediments: a study from the eastern Arabian Sea

Abstract Barium, aluminum, cadmium and uranium were analyzed in 112 surface sediment samples from the western continental margin of India. Excess Ba (Baxs) concentrations are high at the SW margin of India, an area of high productivity and lower at the central western continental margin, an area of lower productivity. High organic carbon export coupled with high sedimentation rates and a relatively oxygenated sediment sea water interface appear to result in the accumulation of high concentrations of Baxs at the SW Indian margin. At the NW Indian margin, where the productivity is moderate to high, suboxic diagenesis is in accord with relatively low Baxs values and high concentrations of Cd and U, two elements accumulating under oxygen-deficient and/or sulfidic conditions. Our results indicate that although Ba tends to be partially lost during suboxic diagenesis, the productivity trend along the western continental margin of India still appears to be reflected by this element.

Solar forcing of Nile discharge and sapropel S1 formation in the early to middle Holocene eastern Mediterranean

We present high-resolution records for oxygen isotopes of the planktic foraminifer Globigerinoides ruber (δ18Oruber) and bulk sediment inorganic geochemistry for Holocene-age sediments from the southeast Mediterranean. Our δ18Oruber record appears to be dominated by Nile discharge rather than basin-scale salinity/temperature changes. Nile discharge was enhanced in the early to middle Holocene relative to today. The timing of the long-term maximum in Nile discharge during the early Holocene corresponds to the timing of maximum intensity of the Indian Ocean-influenced Southwest Indian summer monsoon (SIM). This coincidence suggests a major influence of an Indian Ocean moisture source on Nile discharge in the early to middle Holocene, while, presently, the Atlantic Ocean is the main moisture source. Nile discharge was highly variable on multicentennial time scale during the early to middle Holocene, being strongly influenced by variable solar activity. This solar-driven variability is also recorded in contemporaneous SIM records, however, not observed in an Atlantic Ocean-derived West African summer monsoon record from the Holocene. This supports the hypothesis that the Indian Ocean moisture source predominantly controlled Nile discharge at that time. Solar-driven variability in Nile discharge also influenced paleoenvironmental conditions in the eastern Mediterranean. Bulk sediment Ba/Al and V/Al, used as indicators for (export) productivity and redox conditions, respectively, varied both in response to solar forcing on multicentennial time scales. We suggest that changes in Nile discharge on these time scales have been concordant with nutrient inputs to, and shallow ventilation of, the eastern Mediterranean.

Paleoenvironmental implications of Cenozoic sediments from the Central Arctic Ocean (IODP Expedition 302) using inorganic geochemistry

[1]xa0Integrated Ocean Drilling Program (IODP) Expedition 302 (Arctic Coring Expedition, ACEX) recovered a unique sediment record from the central Arctic Ocean, revealing that this region underwent major environmental fluctuations since the Late Cretaceous. Major and trace element composition of 1,300 samples were determined using X-ray fluorescence (XRF). The results show significant compositional variability of the sediments with depth that can be attributed to changes in (a) provenance and pathways of detrital material, (b) paleoenvironmental conditions and depositional processes, and (c) diagenetic overprint of the primary record. In addition to existing lithological units, we introduce new geochemical units for a more process-related approach interpreting the ACEX record. In detail, via the geochemical signature of Siberian flood basalts we are able to reconstruct the discontinuous rifting and deepening of the central Lomonosov Ridge during the Paleogene, accompanied by changing current regimes and the onset of sea ice. Eocene biosiliceous sedimentation took place in a relatively shallow setting under predominantly anoxic bottom water conditions, causing a positive anoxia-productivity feedback, although water column stratification was repeatedly interrupted by ventilation events. Anoxic to sulfidic conditions were even more extreme after biosilica production ceased, and significant amounts of pyrite were deposited on the Lomonosov Ridge. Especially in organic matter-rich Paleogene deposits, diagenetic processes obscured the paleoenvironmental signals. Fundamental environmental changes occurred in the Middle Eocene, but geochemical and micropaleontological proxies point not to the identical sediment depth. After approximately 26 Ma of non-deposition or erosion, the Middle Miocene record shows the transition to dominantly oxic bottom water conditions, although suboxic diagenesis seemingly affected these deposits.

Repeated enrichment of trace metals and organic carbon on an Eocene high-energy shelf caused by anoxia and reworking

Petroleum source rocks are strongly enriched in organic carbon (OC), and their trace metal (TM) contents often reach low-grade ore levels. The mechanisms leading to these coenrichments are important for understanding how extreme environmental conditions support the formation of natural resources. We therefore studied organic-rich Eocene marls and limestones (oil shale) from the central Jordan Amzaq-Hazra subbasin, part of a Cretaceous–Paleogene shelf system along the southern Neo-Tethys margin. Geochemical analyses on two cores show highly dynamic depositional conditions, consistent with sedimentological and micropaleontological observations. Maximum and average contents, respectively, in OC (~26 and ~10 wt%), sulfur (~7 and ~2.4 wt%), phosphorus (~10 and ~2 wt%), molybdenum (>400 and ~130 ppm), chromium (>500 and ~350 ppm), vanadium (>1600 and ~550 ppm) and zinc (>3800 and ~900 ppm) are exceptional, in particular without any indication of hydrothermal or epigenetic processes. We propose a combination of two processes: physical reworking of OC- and metal-rich 30 material from locally exposed Cretaceous–Paleogene sediments (as supported by reworked nannofossils), and high marine productivity fueled by chemical remobilization of nutrients and metals on land that sustained anoxic-sulfidic conditions. Burial of high-quality organic matter (hydrogen index 600–700 mgHC/gOC) was related to strongly reducing conditions, punctuated by only short-lived oxygenation events, and to excess H2S, promoting organic matter sulfurization. These processes likely caused the OC and TM coenrichments in a high-energy shallow-marine setting that contradicts common models for black shale formation, but may explain similar geochemical patterns in other black shales.

Determination of major and minor ions in brines by x‐ray fluorescence spectrometry: comparison with other common analytical methods

A method is presented for the determination of major and some minor ions (Cl−, SO42−, Br−; Na+, K+, Ca2+, Mg2+, Sr2+) in marine pore waters. X-ray fluorescence spectrometry provides the possibility of analysing large numbers of samples within a short time interval (5 min per sample) using a simple sample preparation. Calibration graphs were obtained from diluted salt or standard solutions. The precision and accuracy were found to be comparable to those of common titrimetric, colorimetric, spectrometric and ion chromatographic methods which are generally used for such matrices. Copyright