Analysis of global surface ocean alkalinity to determine controlling processes

Abstract

The export of calcium carbonate (CaCO3) from the surface ocean is poorly constrained. A better understanding of the magnitude and spatial distribution of this flux would improve our knowledge of the ocean carbon cycle and marine biogeochemistry. Here, we investigate controls over the spatial distribution of total alkalinity in the surface global ocean and produce a tracer for CaCO3 cycling. We took surface ocean bottle data for total alkalinity from global databases (GLODAP, CARINA, PACIFICA) and subtracted the effects of several processes: evaporation and precipitation, river discharge, and nutrient uptake and remineralization. The remaining variation in alkalinity exhibits a robust and coherent pattern including features of large amplitude and spatial extent. Most notably, the residual variation in alkalinity is more or less constant across low latitudes of the global ocean but shows a strong poleward increase. There are differences of ~110μmolkg−1 and ~85μmolkg−1 between low latitudes and the Southern Ocean and the subarctic North Pacific, respectively, but, in contrast, little increase in the high-latitude North Atlantic. This global pattern is most likely due to production and export of CaCO3 and to physical resupply of alkalinity from deep waters. The use of river corrections highlights the large errors that are produced, particularly in the Bay of Bengal and the North Atlantic, if alkalinity normalization assumes all low salinities to be caused by rainfall. The residual alkalinity data can be used as a tracer to indicate where in the world's ocean most CaCO3 export from the surface layer takes place, and of future changes in calcification, for instance due to ocean acidification.