Effect of temperature‐dependent organic carbon decay on atmospheric pCO2

Abstract

Extendingy an almost universal observation that the rate of microbial activity increases with temperature, we propose that marine microbial activity was suppressed during previous glacial periods and allowed proportionally more organic carbon to be exported out of the surface ocean. A stronger organic carbon pump and therefore lower rain ratios of CaCO3 to organic carbon may have contributed to the low atmospheric CO2 content during the Last Glacial Maximum. Previous study of temperature‐dependent export production (Laws et al., 2000) and our map of data‐based, global distribution of the rain ratios lend support to today's rain ratios being controlled at least partly by temperature. A close examination with a high‐resolution regional ocean ecosystem model indicates that the correlation between rain ratio and temperature is caused indeed by preferential remineralization of organic matter, but a part of the correlation is also driven by temperature‐dependent community composition. An extrapolation of these results to the globe using a global carbon cycle box model with a module for sediments indicates that the drawdown of atmospheric CO2 by the proposed mechanism is approximately 30 ppm. While this estimate is subject to uncertainty, the fact that it represents nearly one third of the glacial‐interglacial variation in atmosphere pCO2 suggests the potential importance of the new mechanism. Given the historical difficulty in explaining the full CO2 amplitude with a single cause, we suggest that a set of multiple mechanisms were responsible and that the temperature‐dependent POC degradation rate is one of them. We discuss two possible difficulties with our proposal that have to do with the potentially important role that ballasts play in organic carbon export and the possibility that enhanced biological pump is self limiting.