Global environmental effects of large volcanic eruptions on ocean chemistry: Evidence from “hydrothermal” sediments (ODP Leg 185, Site 1149B)

摘要

A Sr-Nd-Pb isotope investigation has been carried out on sediments overlying the Cretaceous oceanic crust at ODP Leg 185 Site 1149B. The sediments (biogenic carbonate and siliceous, clays) contain two horizons with high “excess” concentrations of Fe, Zn, Cu, and Pb. These horizons are both characterized by rare Earth element patterns that are concave-upwards and exhibit both negative Ce and positive Eu anomalies; comparable to those from modern hydrothermal plume particles and sediments. However, Pb isotope signatures differ significantly between the two metal-rich horizons. Metal-rich sediments in the lower part of Unit IV have Pb isotopes that lie on a mixing line between seawater and local oceanic crust end-members, which is consistent with a hydrothermal fall-out originating from a local oceanic spreading centre. In contrast, sediments from the upper part of Unit IV have much more radiogenic Pb isotope ratios, that cannot be related readily to local end-members (off-axis hydrothermal vent, clays, oceanic crust or large igneous provinces (LIP)). Their age-corrected Pb isotope compositions do, however, overlap with basalts from the Parana-Etendeka LIP. Evidence for related environmental effects include a drastic biotic change, higher oceanic δ 13C values and more radiogenic seawater 87Sr/86Sr. We estimate that the Parana-Etendeka LIP released ～1 × 1019 g of carbon into the atmosphere as CO2, corresponding to Pb and S degassing fluxes of ～1 × 109 g a1 and ～15 × 1012 g a1, respectively. The study demonstrates that Pb isotopes when combined with other geochemical parameters are ideal tools for detecting and tracing LIP volcanism in the marine geological records.