by Gerald R. Dickens
Department of Earth Sciences, Rice University, Houston, TX, USA
Prominent negative 13C excursions characterize several past intervals of abrupt (6100 kyr) environmental change. These anomalies, best exemplified by the greater than 2.5x 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 13C-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. 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 13C records and the operation of the global carbon cycle throughout time.
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