Proposing a mechanistic understanding of atmospheric CO2 during the last 740,000 years

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Köhler, P. and Fischer, H. (2005): Proposing a mechanistic understanding of atmospheric CO2 during the last 740,000 years , DEKLIM/PAGES conference "Climate change at the very end of a warm stage" 7-10 March 2005, Mainz, Germany. .
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Paleo-climate records in ice cores revealed high variability in temperature, atmospheric dust content and carbon dioxide. The longest CO2 record from the Antarctic ice core of the Vostok station (Petit et al., 1999) went back in time as far as about 410 kyr BP showing a switch of glacials and interglacials in all those parameters approximately every 100 kyr during the last four glacial cycles with CO2 varying between 180- 280 ppmv. New measurements of dust and the isotopic temperature proxy deuterium of the EPICA Dome C (EDC) ice core covered the last 740 kyr, however, revealed glacial cycles of reduced temperature amplitude (EPICA, 2004). These new archives offer the possibility to propose atmospheric CO2 for the pre-Vostok time span as called for in the EPICA challenge (Wolff et al., 2004). Here, we contribute to this challenge using a box model of the isotopic carbon cycle (Köhler et al., submitted) based on process understanding previously derived for Termination I. Our Box model of the Isotopic Carbon cYCLE BICYCLE (Fischer et al., 2004; Köhler and Fischer, 2004; Köhler et al.,submitted) consists of ten ocean resvervoir in three high layers distinguishing Atlantic, Indo-Pacific, and Southern Ocean, a seven compartment terrestrial biosphere and considers also fluxes of dissolved inorganic carbon and alkalinity between ocean and sediments. BICYCLE is forced by various ice core and marine sediment records to depict observed changes in temperature, sea level, lysocline dynamics, and aeolian iron input into the Southern Ocean.Our results show that major features of the Vostok period are reproduced while prior to Vostok our model predicts significantly smaller amplitudes in CO2 variations. The main contributions (in decreasing order) to the variations in pCO2 were given by changes in Southern Ocean vertical mixing, exchange fluxes between ocean and sediment, sea surface temperature, North Atlantic deep water formation, iron fertilisation, and Heinrich events. While most processes were reduced in their magnitude during the terminations of the pre-Vostok period, the absolute contribution of iron fertilisation changed only slightly. Thus, the relative importance of biological and biogeochemical processes is enhanced (approximately doubling their relative share) in the pre-Vostok period. The contribution of physical processes (ocean temperature, sea level, sea ice) to the pCO2 rise during terminations stayed always below 25%, while ocean circulation contributed up to 75% during the Vostok era but less than 50% before.

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