Carbon cycle constraints during the last glacial/interglacial cycle derived from [CO2] and δ13Catm measurements from ice cores

Abstract:

Antarctic ice cores represent an invaluable source for understanding the climate of the past. Climatically important gases, including CO₂, can be extracted from the ice and measured, thus providing atmospheric records for the past 800 kyr. Research has shown that atmospheric CO₂ has varied naturally in conjunction with Antarctic air temperature. Additional knowledge of the variations of the stable carbon isotope of CO₂, δ¹³C_atm, can help us better understand the processes involved in these fluctuations. Here, we present a complete δ¹³C_atm record extending from 160 kyrBP to the present. The present record, measured primarily on ice from the EPICA Dome C and Talos Dome ice cores, demonstrates a complex interplay of the ocean, terrestrial biosphere and atmosphere carbon reservoirs. For instance, a long-term increasing trend in δ¹³C_atm, starting at the penultimate glacial maximum, extends well into MIS 4, while CO₂ shows major drops already between 115 kyrBP and the MIS 5/4 boundary. In contrast, δ¹³C_atm evolved roughly in antiphase during the MIS 4/3 transition with the atmospheric carbon storage increasing by 50 Gt while δ¹³C_atm decreased by 0.5‰, a signal larger than that observed during the last glacial/interglacial termination. This antiphase relationship suggests that a single natural process or multiple processes acting on similar timescales may be responsible for this change in CO₂. Modelling studies have shown that the ocean has the greatest impact on the concentration and stable isotope abundance of this greenhouse gas, in particular due to changes in stratification, upwelling, or marine productivity in the Southern Ocean. Similar to the situation at the onset of the last glacial/interglacial termination (Schmitt et al., 2012), the MIS 4/3 transition is characterized by the occurrence of a Heinrich event in the North Atlantic, presumably related to changes in the Atlantic Meridional Overturning Circulation and the upwelling of old carbon enriched waters in the Southern Ocean, which may explain the antiphase relationship of CO₂ and δ¹³C_atm at that time.