MODELLING THE GLOBAL CARBON CYCLE WITH A GAS HYDRATE CAPACITOR

MODELLING THE GLOBAL CARBON CYCLE WITH A GAS HYDRATE CAPACITOR: SIGNIFICANCE FOR THE LATEST PALEOCENE

Investigators:	Gerald Dickens
Funding Sources:	Australian Research Council JCU Merit Grant NSF

Figure 1. A Paleogene exogenic carbon cycle at steady state conditions just prior to the LPTM with a gas hydrate capacitor.

Abstract

The "Latest Paleocene Thermal Maximum" (or LPTM) ca. 55 Ma was characterized by a 4 to 6 °C rise in deep ocean water temperature and an extraordinary injection of 12C-rich carbon into the exogenic carbon cycle. The best explanation for the carbon cycle perturbation is that the bottom water warming converted massive amounts of marine gas hydrate to free CH₄ gas, and this CH₄ was added to the ocean. If correct, basic models for the global carbon cycle must be reconstructed with a gas hydrate "capacitor" so that large quantities of 12C-rich carbon can be stored and released over time. Although recent work has discussed CH₄ release from gas hydrate, the first operational model of the global carbon cycle including CH₄ storage is presented here both conceptually and mathematically (Fig. 1). Using the Blake Ridge gas hydrate deposit as an example, the capacitor contains three internal reservoirs: dissolved gas, gas hydrate and free gas. Carbon enters dissolved gas through methanogenesis of organic matter. Upon saturation of pore waters, carbon is transferred to gas hydrate and then free gas at appropriate temperature and pressure conditions. Carbon leaves free gas to the exogenic carbon cycle through anaerobic CH₄ oxidation or, if there is no overlying gas hydrate, direct injection to the water column. Because the amount and location of free gas depends on temperature, the capacitor can suddenly release massive amounts of 12C-rich carbon with abrupt bottom water warming. When the "gas hydrate" capacitor is connected to a Paleogene exogenic carbon cycle and subjected to a 5°C warming, the response is similar to that observed for the LPTM (Fig. 2).

Figure 2. The predicted change in the mass of the gas hydrate capacitor and isotopic composition of exogenic carbon cycle as in Figure 13 compared to oxygen and carbon isotope records of single species benthic foraminifera at ODP Site 1051 [Katz et al., 1999]. Note the presence of a mud clast interval precisely at the onset of the LPTM _13C excursion.