24The stable carbon isotope ratio of atmospheric CO 2 (! 13 C atm ) is a key parameter to decipher 25 past carbon cycle changes. Here we present ! 13 C atm data for the last 24,000 years derived 26 from three Antarctic ice cores. We conclude that a pronounced 0.3‰ decrease in ! 13 C atm 27 during the early deglaciation can be best explained by upwelling of old, carbon-enriched 28 waters in the Southern Ocean. Later in the deglaciation, regrowth of the terrestrial 29 biosphere, changes in sea surface temperature, and ocean circulation governed the ! 13 C atm 30 evolution. During the Last Glacial Maximum, ! 13 C atm and CO 2 were essentially constant, 31suggesting that the carbon cycle was in dynamic equilibrium and that the net transfer of 32 carbon to the deep ocean had occurred before then. showing pronounced differences in atmospheric CO 2 rates of change in the course of the 47 last glacial/interglacial transition (3). Many processes have been involved in attempts to 48 explain these CO 2 variations, but it has become evident that none of these mechanisms 49 alone can account for the 90 ppmv increase in atmospheric CO 2 . A combination of 50 processes must have been operating (4, 5), with their exact timing being crucial. However, 51 a unique solution to the deglacial carbon cycle changes has not been yet found. 52 53

During each of the late Pleistocene glacial-interglacial transitions, atmospheric carbon dioxide concentrations rose by almost 100 ppm. The sources of this carbon are unclear, and efforts to identify them are hampered by uncertainties in the magnitude of carbon reservoirs and fluxes under glacial conditions. Here we use oxygen isotope measurements from air trapped in ice cores and ocean carbon-cycle modelling to estimate terrestrial and oceanic gross primary productivity during the Last Glacial Maximum. We find that the rate of gross terrestrial primary production during the Last Glacial Maximum was about 40 +/- 10 Pg C yr(-1), half that of the pre-industrial Holocene. Despite the low levels of photosynthesis, we estimate that the late glacial terrestrial biosphere contained only 330 Pg less carbon than pre-industrial levels. We infer that the area covered by carbon-rich but unproductive biomes such as tundra and cold steppes was significantly larger during the Last Glacial Maximum, consistent with palaeoecological data. Our data also indicate the presence of an inert carbon pool of 2,300 Pg C, about 700 Pg larger than the inert carbon locked in permafrost today. We suggest that the disappearance of this carbon pool at the end of the Last Glacial Maximum may have contributed to the deglacial rise in atmospheric carbon dioxide concentrations