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A possible sequence of events for the generalized glacial-interglacial cycle

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  • There is not yet widespread agreement as to the underlying cause of the 80– 100 ppmv roughly 100-kyr-duration glacial-interglacial cycles in atmospheric pCO2. Most of the mechanisms which have been proposed to account for the observed pCO2 variations appear to in some way violate interpretations of paleo proxy data. The inability of a single mechanism to explain the observed cycles in atmospheric CO2 (which show amazing similarity over the past 430,000 years) is perplexing, and leads us to consider whether a combination of mechanisms might be consistent with available evidence. Consistent with previous work, we find that physical changes (ocean circulation, temperature, mixing) can only explain part of the observed atmospheric pCO2 variability; changes in ocean chemistry are invoked to explain the remainder. In order to account for the initial pCO2 drawdown (from ‘‘interglacial’’ to ‘‘intermediate’’ levels), we invoke physical changes in the ocean (mixing, temperature). The transition from intermediate atmospheric pCO2 levels to full glacial conditions involves a small increase in mean ocean nutrient levels and mean ocean alkalinity, accomplished by falling sea level and subsequent erosion of organic-rich shelf sediments. The first part of the transition out of full glacial conditions is achieved through increased temperature and increased mixing in the Southern Ocean. The final part of the atmospheric pCO2 rise up to full interglacial conditions is accomplished through rising sea level and the subsequent change in mean ocean alkalinity and phosphate, and a rise in the Northern Hemisphere temperature and ocean mixing. The proposed sequence of events is consistent with most existing proxy evidence for paleo-nutrient levels and changes in export production over the last glacial-interglacial cycle. Furthermore, it is consistent with evidence for a whole-ocean shift in d13C toward significantly more negative values in the late glacial. The proposed scenario is also consistent with ice core-based timing constraints, as summarized by Broecker and Henderson (1998). We show that we are able to explain the full magnitude of the glacial-interglacial cycle in atmospheric pCO2 without the need to invoke iron-fertilization in the Southern Ocean.
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