Pb concentrations, Pb isotopic composition, and amount of metalliferous components in pelagic sediments from the Pacific Ocean

The amount of lead annually transferred from oceanic crust to metalliferous sediments was estimated in order to test the hypothesis that a non-magmatic flux of lead causes the Pb surplus in the continental crust. A Pb surplus has been inferred from global crust-mantle lead mass balances derived from lead concentration correlations with other trace elements and from lead isotope systematics in oceanic basalts. DSDP/ODP data on the amount of metalliferous sediments in the Pacific Ocean and along a South Atlantic traverse are used to calculate the mean worldwide thickness of 3 (+/-1) m for purely metalliferous sediment componens. Lead isotope ratios of 39 metalliferous sediments from the Pacific define mixing lines between continent-derived (seawater) and mantle-derived (basaltic) lead, with the most metal-rich sediments usually having the most mantle-like Pb isotope composition. We used this isotope correlation and the Pb content of the 39 metalliferous sediments to derive an estimate of 130 (+/-70) µg/g for the concentration of mantle-derived lead in the purely metalliferous end-member. Mass balance calculations show that at least 12 (+/-8)% of the lead, annually transferred from upper mantle to oceanic crust at the ocean ridges, is leached out by hydrothermal processes and re-deposited in marine sediments. If all of the metalliferous lead is ultimately transferred to the continental crust during subduction, the annual flux of this lead from mantle to continental crust is 2.6 (+/-2.0) * 10**6 kg. Assuming this transfer rate to be proportional to the rate of oceanic plate production, one can fit the lead transfer to models of plate production rate variations through time. Integrating over 4 Ga, hydrothermal lead transfer to the continental crust accounts for a significant portion of the Pb surplus in the continental crust. It therefore appears to be one of the main reasons for the anomalous behavior of lead in the global crust-mantle system.

isotopic composition of gases and interstitial fluides of ODP Site 104-644

Pleistocene- to middle Miocene-age sediment was drilled at Site 341 (67? 20.1'N, 6? 06.6'E) on the inner Voring Plateau during Leg 38 of the Deep Sea Drilling Project (DSDP). In 1985, the Ocean Drilling Program (ODP) returned to the inner Wring Plateau near Site 341 and drilled a new hole at Site 644 (66° 40.7'N, 4° 34.6'E) as part of a transect to study Norwegian Sea paleoenvironments. In Hole 341, gas expansion pockets formed in cores which were recovered from depths below 50 m. This gas was characterized as predominantly methane with delta13C values in the range of -87 to -77 per mil (Morris, 1976, doi:10.2973/dsdp.proc.38.124.1976). At Site 644, sediment gas and pore-water samples were obtained to study the geochemistry of methanogenesis. Of particular interest is the possibility that methane hydrate might be present in these sediments.

Carbon and oxygen isotopic composition of benthic foraminifers from ODP Leg 167 sites

Three sites, drilled during Ocean Drilling Program (ODP) Leg 167, were chosen for detailed late Pleistocene paleoceanographic studies of intermediate water along the California margin. These sites are Site 1011 (Animal Basin, 31°17'N, 117°38'W, 2033 m water depth, 1600 m sill depth), Site 1012 (East Cortez Basin, 32°17'N, 118°23'W, 1783 m water depth, 1415 m sill depth), and Site 1018 (Guide Seamount, 36°59'N, 123°17'W, 2476 m water depth). Here we present carbon and oxygen isotopic measurements of benthic foraminifers from these three sites. We made 135 measurements from Site 1011, 387 measurements from Site 1012, and 231 measurements from Site 1018. This data report includes an explanation of the methods used to generate these isotopic records and the age models for each site. Detailed paleoceanographic interpretations of the isotopic records are currently under way.

Oxygen and carbon isotopic composition in Eocene and Oligocene planktonic and benthic foraminifera from DSDP sediments

Oxygen and carbon isotope ratios in Eocene and Oligocene planktonic and benthic foraminifera have been investigated from Atlantic, Indian, and Pacific Ocean locations. The major changes in Eocene-Oligocene benthic foraminiferal oxygen isotopes were enrichment of up to 1 per mil in 18O associated with the middle/late Eocene boundary and the Eocene/Oligocene boundary at locations which range from 1- to 4-km paleodepth. Although the synchronous Eocene-Oligocene 18O enrichment began in the latest Eocene, most of the change occurred in the earliest Oligocene. The earliest Oligocene enrichment in 18O is always larger in benthic foraminifera than in surface-dwelling planktonic foraminifera, a condition that indicates a combination of deep-water cooling and increased ice volume. Planktonic foraminiferal d18O does not increase across the middle/late Eocene boundary at our one site with the most complete record (Deep Sea Drilling Project Site 363, Walvis Ridge). This pattern suggests that benthic foraminiferal d18O increased 40 m.y. ago because of increased density of deep waters, probably as a result of cooling, although glaciation cannot be ruled out without more data. Stable isotope data are averaged for late Eocene and earliest Oligocene time intervals to evaluate paleoceanographic change. Average d18O of benthic foraminifera increased by 0.64 per mil from the late Eocene to the early Oligocene d18O maximum, whereas the average increase for planktonic foraminifera was 0.52 per mil. This similarity suggests that the Eocene/Oligocene boundary d18O increase was caused primarily by increased continental glaciation, coupled with deep sea cooling by as much as 2°C at some sites. Average d18O of surface-dwelling planktonic foraminifera from 14 upper Eocene and 17 lower Oligocene locations, when plotted versus paleo-latitude, reveals no change in the latitudinal d18O gradient. The Oligocene data are offset by ~0.45 per mil, also believed to reflect increased continental glaciation. At present, there are too few deep sea sequences from high latitude locations to resolve an increase in the oceanic temperature gradient from Eocene to Oligocene time using oxygen isotopes.

Occurrence and isotopic composition (d18O, d13C) of benthic foraminifera in sediments from DSDP Leg 72

Changes in the vertical water mass structure of the Vema Channel during the Pliocene have been inferred from benthic foraminiferal assemblages and stable isotopic analyses from three sites of DSDP Leg 72 (South Atlantic). Faunal and isotopic results from Sites 516A and 518 suggest that a major change occurred in deep-water circulation patterns in the late Pliocene near 3.2 Ma. Benthic oxygen isotopic records from Sites 516A and 518 show a characteristic increase in d18O values near 3.2 Ma. This has been documented in numerous Pliocene isotopic records. The magnitude of the oxygen isotopic enrichment near 3.2 Ma appears to increase with water depth from an average enrichment of 0.34 per mil in Site 516A (1313 m) to an average enrichment of 0.58 per mil in Site 518 (3944 m). We suggest that this enrichment resulted partly from a change in deep-water circulation patterns which included a decrease in bottom-water temperatures. Planktonic d18O values near 3.2 Ma show no evidence of an enrichment which would be indicative of an increase in global ice volume. On the contrary, d18O values in Sites 517 and 518 become more depleted near 3.2 Ma, indicating a surface-water warming perhaps due to a change in the strength and/or position of the Brazil Current. An increase in the relative abundance of the benthic foraminifer Nuttalides umbonifera, which is associated with Antarctic Bottom Water (AABW) in the modern ocean, coincides with the benthic 18O enrichment in Site 518. At 3.2 Ma, oxygen and carbon isotopic gradients between Sites 518 (3944 m) and 516A (1313 m) show a marked increase such that Site 518 becomes enriched in 18O and depleted in 13C relative to Site 516A. This enrichment in d18O is interpreted as partly representing a temperature decrease at Site 518; the depletion in d13C indicates a corrosive water mass which is high in metabolic CO2. We suggest that benthic foraminiferal and stable isotopic changes in Site 518 resulted from a pulse-like increase in the formation of AABW near 3.2 Ma. The cause of this circulation event may have been linked to global cooling and/or the final closure of the Central American Seaway.

Mercury content and isotopic composition of teeth, and sex and age of skulls of polar bears collected in Svalbard

We examined the use of mercury (Hg) and nitrogen and carbon stable isotopes in teeth of polar bear (Ursus maritimus) from Svalbard as biotracers of temporal changes in Hg pollution exposure between 1964 and 2003. Teeth were regarded as a good matrix of the Hg exposure, and in total 87 teeth of polar bears were analysed. Dental Hg levels ranged from 0.6 to 72.3 ng/g dry weight and increased with age during the first 10 years of life. A decreasing time trend in Hg concentrations was observed over the recent four decades while no temporal changes were found in the stable isotope ratios of nitrogen (d15N) and carbon (d13C). This suggests that the decrease of Hg concentrations over time was more likely due to a lower environmental Hg exposure in this region rather than a shift in the feeding habits of Svalbard polar bears.

Petrology and carbon and oxygen stable isotopic composition of macrofossils and sediments from the Blake-Bahama Formation, DSDP Hole 93-603B

Strata that record the evolutionary history of the North American continental margin in a region that serves as the basin margin interface between allochthonous sedimentation from the continent and pelagic sedimentation from the oceanic realm were recovered at Deep Sea Drilling Project Site 603, on the lower continental rise. The lowermost unit recovered at this site is composed of upper Berriasian-Aptian interbedded laminated limestone and bioturbated limestone with sandstone to claystone turbidites. This unit can be correlated with the Blake-Bahama Formation in the western North Atlantic. Studies of the laminated and bioturbated limestones were used to determine the depositional environment. Geochemical and petrographic studies suggest that the laminated limestones were deposited from the suspended particulate loads of the nepheloid layer associated with weak bottom-current activity as well as moderate to poorly oxygenated bottom-water conditions. Fragments of macrofossils are also found in the Blake-Bahama Formation drilled at Site 603. Twelve specimens and their host sediment were analyzed for their carbon and oxygen isotopic composition. The macrofossil samples chosen for analysis consist of nine samples of Inoceramus, two ammonite aptychi, and one belemnite sample. Depletion in 18O is observed in recrystallized specimens. The ammonite aptychi have been diagenetically altered and/or exhibit evidence of isotopic fractionation by the organism. Oxygen isotope paleotemperatures obtained from five well-preserved specimens - four of Inoceramus and one of a belemnite - suggest that bottom-water temperatures in the North Atlantic Basin during the Early Cretaceous were very warm, at least 11°C.