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Global climate and the concentration of atmospheric carbon dioxide (CO2) are correlated over recent glacial cycles. The combination of processes responsible for a rise in atmospheric CO2 at the last glacial termination (23,000 to 9,000 years ago), however, remains uncertain. Establishing the timing and rate of CO2 changes in the past provides critical insight into the mechanisms that influence the carbon cycle and helps put present and future anthropogenic emissions in context. Here we present CO2 and methane (CH4) records of the last deglaciation from a new high-accumulation West Antarctic ice core with unprecedented temporal resolution and precise chronology. We show that although low-frequency CO2 variations parallel changes in Antarctic temperature, abrupt CO2 changes occur that have a clear relationship with abrupt climate changes in the Northern Hemisphere. A significant proportion of the direct radiative forcing associated with the rise in atmospheric CO2 occurred in three sudden steps, each of 10 to 15 parts per million. Every step took place in less than two centuries and was followed by no notable change in atmospheric CO2 for about 1,000 to 1,500 years. Slow, millennial-scale ventilation of Southern Ocean CO2-rich, deep-ocean water masses is thought to have been fundamental to the rise in atmospheric CO2 associated with the glacial termination, given the strong covariance of CO2 levels and Antarctic temperatures. Our data establish a contribution from an abrupt, centennial-scale mode of CO2 variability that is not directly related to Antarctic temperature. We suggest that processes operating on centennial timescales, probably involving the Atlantic meridional overturning circulation, seem to be influencing global carbon-cycle dynamics and are at present not widely considered in Earth system models.

Our work was motivated by discoveries of prokaryotic communities that survive with little nutrient in ice and permafrost, with implications for past or present microbial life in Martian permafrost and Europan ice. We compared the temperature dependence of metabolic rates of microbial communities in permafrost, ice, snow, clouds, oceans, lakes, marine and freshwater sediments, and subsurface aquifer sediments. Metabolic rates per cell fall into three groupings: (i) a rate, g (T), for growth, measured in the laboratory at in situ temperatures with minimal disturbance of the medium; (ii) a rate, m(T), sufficient for maintenance of functions but for a nutrient level too low for growth; and (iii) a rate, s(T), for survival of communities imprisoned in deep glacial ice, subsurface sediment, or ocean sediment, in which they can repair macromolecular damage but are probably largely dormant. The three groups have metabolic rates consistent with a single activation energy of Ϸ110 kJ and that scale as g(T): m(T): s(T) Ϸ 10 6 :10 3 :1. There is no evidence of a minimum temperature for metabolism. The rate at ؊40°C in ice corresponds to Ϸ10 turnovers of cellular carbon per billion years. Microbes in ice and permafrost have metabolic rates similar to those in water, soil, and sediment at the same temperature. This finding supports the view that, far below the freezing point, liquid water inside ice and permafrost is available for metabolism. The rate s(T) for repairing molecular damage by means of DNA-repair enzymes and proteinrepair enzymes such as methyltransferase is found to be comparable to the rate of spontaneous molecular damage.T here is now both direct and indirect evidence that microorganisms live inside glacial and sea ice and permafrost at temperatures well below the freezing point of pure water (1-11). The thermodynamic stability of ion-rich liquid veins at triple grain-boundaries in polycrystalline ice (12) and of thin films of unfrozen water on microbial surfaces in permafrost (13,14) enables transport of nutrients to and waste from them. Certain impurities such as mineral acids or salts can reduce the freezing point of water in narrow intergranular veins to as low as Ϫ90°C. Acidophilic psychrophiles in a Greenland mine (15) survive the cold winters at Ϫ30°C, probably by taking advantage of such a habitat. It has been conjectured that microbial life may have arisen under similar conditions in subsurface Martian permafrost or in warm diapirs in Europa's ice.After some brief remarks about metabolism in a natural microbial community, we cite several examples of microbes in ice or permafrost at subfreezing temperatures that we cannot analyze quantitatively, either because information on both microbial concentration and metabolic rate in the same locations was not provided or because only isolates in pure cultures were studied. We then discuss methods and present quantitative data from the literature and from our unpublished work that enable us to calculate metabolic rates per cell. We display the results in Fig. 1 ...

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