Carbon cycle variations on the Earth's surface during the last two glacial cycles

Abstract

Data concerning carbon cycle variations on the earth's surface during the past 200,000 years are reviewed. The variations of the surface temperature ( T) and concentration of carbon dioxide (CO 2) in the atmosphere of Antarctica are compared to those of the isotopic ratios of oxygen 18O/ 16O ( δ 18O) and of carbon 13C/ 12C (° 13C) of waters in the deep oceans for the two last glacial cycles. This comparison shows that the decrease of the atmospheric CO 2 concentration is accompanied by a carbon transferase from the continental biosphere to the oceanic deep waters. At the glacial maximum this transfer is estimated to be about 500 GtC (1 GtC = 10 15g of carbon) equivalent to 25% of the carbon storage of the biosphere. It occurs mainly in the high latitudes of the Southern Hemisphere by incorporation of CO 2 into particulate matter during photosynthesis. It is shown that the mean oceanic productivity does not increase with a supplementary supply of ions such as phosphate (PO 4 3−) or nitrate (NO 3 −) but that the intensity of the thermohaline circulation is certainly reduced. As the warming up of the oceans and the melting of the ice-sheet begin carbon transfer takes place to restore the continental biosphere. Another carbon transfer of a much more important intensity is also at work in the sea shore environment. Its intensity could be sufficient to renew the entire carbon of the continental biospheric, atmospheric and oceanic reservoirs in a length of time comparable to a glacial cycle. This fact shows the importance of studying the deposition of carbon in oceanic zones which are uncovered with drops in sea level. At the present time data on the coastal environment in relation to the global carbon cycle are very scarce and warrants more research in this area.