| Summary: | The last glacial period was punctuated by a number of transient events during which massive iceberg discharges profoundly perturbed the deep ocean circulation dynamics, causing widespread climate and environmental changes. Detailed analysis of polar ice cores demonstrated millennial-scale atmospheric CO2 variations characterized by rapid shifts between relatively cold (stadial) and warm (interstadial) stages and in particular, a gradual rising of CO2 during Heinrich Stadials (HS). To date, the observed CO2 increase during HS has been attributed to ventilation changes in Southern Ocean, and/or reduced biological uptake. However, understanding the respective contribution of ocean ventilation and changes in the efficiency of the biological carbon pump is challenging because of difficulties in estimating global biosphere productivity based on local reconstructions as they are often based on indirect geochemical tracers and exhibit spatial heterogeneities. To address this, we measured the triple isotopic composition of air oxygen in trapped air in NEEM ice core samples and reconstructed the global biosphere productivity over the ~42 to ~37 ka interval. Our preliminary results indicate no clear evidence of significant reduction of global biosphere productivity during HS4, inconsistent with some paleoproductivity indicators such as European pollen assemblages, Antarctic ice-core non-sea-salt Na and Ca, or marine sediment core opal flux records from sub-Antarctic zone of Southern Ocean. In order to disentangle current disagreement between local- to global estimate of biological productivity, we will present the first results from novel simulations using the IPSL-CM5A2 coupled climate model forced by 40 ka climate configurations with new ice-sheet reconstructions.
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