Balancing the oceans alkalinity budget by physical weathering of carbonate rocks

Abstract

Alkalinity, the excess of bases over acids, is a frequently used conservative variable in Biogeochemistry and Oceanography to calculate carbonate system parameters and estimate the oceans susceptibility to acidification. On timescales smaller than the residence time of the bicarbonate ion, the major contributor to ocean alkalinity, this budget should be at steady state, implying input and output to be balanced. Despite its importance and incorporation into earth system models, the classic ocean alkalinity budget leaves an imbalance of 27 Tmol(eq)/a, that is close to the whole input term (32 Tmol(eq)/a of riverine dissolved inorganic carbon, DIC) and almost half of the output by carbonate burial (60 Tmol(eq)/a). Marine processes and transformations narrow the budget by contributing 12 Tmol(eq)/a. The residual imbalance can potentially be closed by including the so far unaccounted delivery of riverine detrital carbonates (PIC) to the ocean, as these carbonate minerals will either dissolve and contribute to the input term, or should be subtracted from the burial term. PIC seems a largely overlooked variable of riverine systems, and global estimates range from 0.04 – 1 wt% (1.0 – 25.1 Tmol(eq)/a) relying on small datasets or regionally restricted estimates. Preliminary results from a new global database of riverine suspended matter geochemistry, as well as estimates using global soil composition, suggest, that the global average PIC concentration rather lies between 0.3 – 0.7 wt% (7.5 – 17.6 Tmol(eq)/a). Detrital carbonate delivery by melting icebergs and atmospheric (dry) deposition could possibly represent further sources of alkalinity, but still have to be constrained.