PEDOGENESIS OF SUB-AERIALLY EXPOSED CONTINENTAL SHELVES DURING GLACIAL PERIODS AND THE PRODUCTION OF ATMOSPHERIC GREENHOUSE GASES

The pedogenesis of sub-aerially exposed continental shelves is suggested to have played an important role in the production of atmospheric greenhouse gases causing a change in the earth’s climate. During glacial periods including the Last Glacial Maximum (LGM), coral reefs contracted towards the equator. Both fossilized coral reefs and carbonate platforms became sites of karstification resulting in the release of carbon dioxide into the atmosphere through weathering. In muddy inner continental shelves such as the northern South China Sea off Hong Kong, the sub-aerial exposure of seafloor muds of marine isotope stage (MIS) 5, 7, 9 and 11 ages resulted in the release of greenhouse gases. Carbon dioxide was produced by acid-sulphate soil development through sulphuric acid reaction with biogenic carbonates while methane was generated through the bacterial decomposition of fossilized plant matter. The release of these greenhouse gases may be the triggering mechanism needed for switching the earth from a full glacial mode back into an interglacial mode (Yim et al., 2002). Such a mechanism is supported by the dramatic increase in carbon dioxide and methane content in the Vostok ice core following the glacial periods of MIS 2, 6, 8, 10 and 12 (Petit et al., 1999). The presence of methane trapped within pre-Holocene shelf sediments is supported by seismic profiles showing areas of the seabed affected by acoustic turbidity while microfabric studies of palaeo-desiccated crust developed in the upper part of MIS 5 muds (Tovey and Yim, 2002) also supported the conclusions drawn.

References

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