Lead isotopic ratios of Barremian-Aptian marine sediments

We present initial isotopic ratios of lead for Early Cretaceous (Barremian-Aptian) sections from Shatsky Rise (Pacific) and Gorgo a Cerbara (Italy). Our Pb isotopic data track an interval representing Oceanic Anoxic Event (OAE)-1a, which is characterized by quasi-global deposition of organic carbon-rich black shale. Pb isotopic compositions of sediments from Shatsky Rise decrease at the end of Barremian time, from radiogenic continental values to unradiogenic values, and subsequently remained less radiogenic until the end of early Aptian time. We explain the isotopic shift by a significant increase in supply rate of unradiogenic Pb, most likely due to massive volcanism. In contrast, the Pb isotopic compositions from the Italian section, which was situated at the western end of Tethys, are mostly identical to those of upper continental crust, showing no significant change in supply rate of unradiogenic Pb. The discrepancy between two sites is attributed to quiescent deep-submarine eruptions of Pacific large igneous provinces (LIPs) such as the Ontong Java Plateau (OJP), which severely limited dispersion of Pb-carrying particles out of the Pacific Ocean. Published Os isotopic data from the Italian section indicate two episodes of massive eruptions of OJP or contemporaneous Manihiki and Hikurangi plateaus starting from earliest Aptian time, slightly later than that indicated by the sedimentary Pb isotopic record from Shatsky Rise. Differences in isotopic variations between Pb and Os likely reflect differences in their chemical behaviors in the oceans, i.e., Pb isotopic compositions would have varied in response to local or regional changes in sediment provenances, whereas large-scale changes in Os inputs are required to explain variations in seawater Os isotopic compositions. Our Pb isotopic data, together with the published Os isotopic record, provide new evidence for the eruptive history of OJP together with contemporaneous Pacific plateaus and its environmental consequences, starting from end-Barremian time and extending through early Aptian time.

Pore water isotopic ratios of CO2 and SO4, and sediment CH4 content of ODP Leg 164 sites

A unique set of geochemical pore-water data, characterizing the sulfate reduction and uppermost methanogenic zones, has been collected at the Blake Ridge (offshore southeastern North America) from Ocean Drilling Program (ODP) Leg 164 cores and piston cores. The d13C values of dissolved CO2 (sum CO2) are as 13C-depleted as -37.7 per mil PDB (Site 995) at the sulfate-methane interface, reflecting a substantial contribution of isotopically light carbon from methane. Although the geochemical system is complex and difficult to fully quantify, we use two methods to constrain and illustrate the intensity of anaerobic methane oxidation in Blake Ridge sediments. An estimate using a two-component mixing model suggests that ~24% of the carbon residing in the sum CO2 pool is derived from biogenic methane. Independent diagenetic modeling of a methane concentration profile (Site 995) indicates that peak methane oxidation rates approach 0.005 µmol/cm**3/yr, and that anaerobic methane oxidation is responsible for consuming ~35% of the total sulfate flux into the sediments. Thus, anaerobic methane oxidation is a significant biogeochemical sink for sulfate, and must affect interstitial sulfate concentrations and sulfate gradients. Such high proportions of sulfate depletion because of anaerobic methane oxidation are largely undocumented in continental rise sediments with overlying oxic bottom waters. We infer that the additional amount of sulfate depleted through anaerobic methane oxidation, fueled by methane flux from below, causes steeper sulfate gradients above methane-rich sediments. Similar pore water chemistries should occur at other methane-rich, continental-rise settings associated with gas hydrates.

Benthic foraminiferal Mg/Ca ratios

Core-top samples from different ocean basins have been analyzed to refine our current understanding of the sensitivity of benthic foraminiferal calcite magnesium/calcium (Mg/Ca) to bottom water temperatures (BWT). Benthic foraminifera collected from Hawaii, Little Bahama Bank, Sea of Okhotsk, Gulf of California, NE Atlantic, Ceara Rise, Sierra Leone Rise, the Ontong Java Plateau, and the Southern Ocean covering a temperature range of 0.8 to 18°C were used to revise the Cibicidoides Mg/Ca-temperature calibration. The Mg/Ca-BWT relationship of three common Cibicidoides species is described by an exponential equation: Mg/Ca = 0.867 ± 0.049 exp (0.109 ± 0.007 * BWT) (stated errors are 95% CI). The temperature sensitivity is very similar to a previously published calibration. However, the revised calibration has a significantly different preexponential constant, resulting in different predicted absolute temperatures. We attribute this difference in the preexponential constant to an analytical issue of accuracy. Some genera, notably Uvigerina, show apparently lower temperature sensitivity than others, suggesting that the use of constant offsets to account for vital effects in Mg/Ca may not be appropriate. Downcore Mg/Ca reproducibility, as determined on replicate foraminiferal samples, is typically better than 0.1 mmol/mol (2 S.E.). Thus, considering the errors associated with the Cibicidoides calibration and the downcore reproducibility, BWT may be estimated to within ±1°C. Application of the revised core-top Mg/Ca-BWT data to Cenozoic foraminiferal Mg/Ca suggests that seawater Mg/Ca was not more than 35% lower than today in the ice-free ocean at 50 Ma.