One potential proxy for the injection of significant volumes of methane into the ocean waters is the changes in carbon-isotopic composition of the carbon reservoir. The 513C of biogenic methane in hydrate averages about -60%o
(Kvenvolden, 1993), perhaps the lightest (most enriched in 612C) carbon anywhere in the Earth system. It has been suggested that massive release of methane from gas-hydrate sources is the most likely mechanism for a pronounced enrichment of 612C seen in late Paleocene marine sediments which was also a period of rapid bottom-water warming (Dickens et al., 1995). The dissolution of methane (and its oxidative byproduct, CO2) in the seawater also led to increased dissolution of carbonate on the seafloor. Thus, major negative shifts in 613C that occur together with an increase in benthic temperature (bottom-water warming, see the Santa Barbara example discussed earlier) or a sea-level fall event (reduction of hydrostatic pressure) may provide clues to past behavior of gas hydrates.
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