Rethinking the global carbon cycle with a large, dynamic and microbially mediated gas hydrate capacitor

Prominent negative delta(13)C excursions characterize several past intervals of abrupt ( < 100 kyr) environmental change. These anomalies, best exemplified by the > 2.5parts per thousand drop across the Paleocene/Eocene thermal maximum (PETM) ca. 55.5 Ma, command our attention because they lack explanation with conventional models for global carbon cycling. Increasingly, Earth scientists have argued that they signify massive release Of CH4 from marine gas hydrates, although typically without considering the underlying process or the ensuing ramifications of such an interpretation. At the most basic level, a large, dynamic 'gas hydrate capacitor' stores and releases C-13-depleted carbon at rates linked to external conditions such as deep ocean temperature. The capacitor contains three internal reservoirs: dissolved gas, gas hydrate, and free gas. Carbon enters and leaves these reservoirs through microbial decomposition of organic matter, anaerobic oxidation of CH4 in shallow sediment, and seafloor gas venting; carbon cycles between these reservoirs through several processes, including fluid flow, precipitation and dissolution of gas hydrate, and burial. Numerical simulations show that simple gas hydrate capacitors driven by inferred changes in bottom water warming during the PETM can generate a global VC excursion that mimics observations. The same modeling extended over longer time demonstrates that variable CH4 fluxes to and from gas hydrates can partly explain other delta(13)C excursions, rapid and slow, large and small, negative and positive. Although such modeling is rudimentary (because processes and variables in modern and ancient gas hydrate systems remain poorly constrained), acceptance of a vast, externally regulated gas hydrate capacitor forces us to rethink VC records and the operation of the global carbon cycle throughout time. (C) 2003 Elsevier B.V. All rights reserved.