SIGNIFICANCE OF THE CARBON SINK PRODUCED BY COUPLED INTERACTION OF CARBONATE WEATHERING AND AQUATIC PHOTOSYNTHESIS: REGULATION BY GLOBAL WARMING AND LAND USE/COVER CHANGES

One of the most important questions in the science of global change is how to balance the atmospheric CO₂ budget. There is a large terrestrial missing carbon sink amounting to about one billion tonnes of carbon per annum. The locations, magnitudes, variations and mechanisms responsible for this terrestrial missing carbon sink are uncertain and the focus of much continuing debate. Although the positive feedback between global change and silicate weathering is used in geochemical models of atmospheric CO₂, this feedback is believed to operate over a long timescale and is therefore generally left out of the current discussion of human impact upon the carbon budget. Here we show, by synthesizing recent findings in carbonate weathering research and studies into biological carbon pump effects in surface aquatic ecosystems, that the carbon sink produced by coupled interaction of carbonate weathering and aquatic photosynthesis not only totals half a billion tonnes per annum, but also displays a significant increasing trend under the influence of global warming (thus with intensified river runoff) and land use change (thus with increased concentrations of DIC and/or AOC); thus it needs to be included in the global carbon budget. The carbon sink flux (CSF) in a karst catchment is written as:

CSF=F_DIC+F_AOC+F_SAOC= 0.5×Q×([DIC]+[AOC])/A+F_SAOC

where F_DIC and F_AOC are the dissolved inorganic carbon (DIC) flux and the autochthonous organic carbon (AOC) flux via karst surface runoff (Q), respectively; F_SAOC is the sedimentary flux of AOC in karst surface water system(s); Factor 0.5 indicates that only one half of the HCO₃^- generated by carbonate dissolution is of atmospheric origin; A is the catchment area; and [DIC] and [AOC] are the concentrations of the DIC and AOC at river mouths, respectively.