Methane-induced early diagenesis of foraminiferal tests in the Southwestern Greenland Sea

Planktic and epibenthic foraminiferal delta C-13 records at Site PS62/015-3 (southwestem Greenland Sea) reveal a series of transient events of extreme 13 C depletion down to -6 parts per thousand during the period 90-40 ka. Scanning electron microscope studies of the ultrastructures of foraminiferal tests suggest that C-13 depleted specimens are affected by some 10-20% overgrowth by authigenic calcite contributing to the light delta C-13 signal. Incremental-leaching experiments and census counts of pristine versus overgrowth-affected specimens show that the C-13 depleted foraminiferal tests incorporate a primary delta C-13 signature most likely ranging from +1 parts per thousand to -1.7 parts per thousand and a post-depositional delta C-13 signature around -17 parts per thousand to -19.5 parts per thousand. Extremely low values of productivity and organic carbon in Late Quaternary sediments along the east Greenland margin preclude organic matter as potential source of the isotopically light carbon. In contrast, thermal instability of clathrates and subsequent aerobic oxidation of (highly C-12 enriched) methane in pore and ocean water provide a compelling mechanism to account for the negative delta C-13 excursions of both primary and post-depositional carbonates. Here, pore water methane may have led to a supersaturation of C-13 depleted bicarbonate and precipitation of isotopically light authigenic calcite on and in foraminiferal tests, a feature that may serve as a tracer to former sites of clathrate destabilization. (c) 2005 Elsevier B.V. All rights reserved.