LOW-TEMPERATURE ALTERATION OF THE UPPER OCEANIC-CRUST AND THE ALKALINITY BUDGET OF SEAWATER

While the input of river-alkalinity into seawater is relatively well k nown, the complementary acidity production is poorly understood. Using the major-element budget of seafloor alteration of the upper 500 m of 120-Ma-old oceanic crust at DSDP/ODP Sites 417A, 417D and 418A in the central western Atlantic, we estimate the acidity flux associated wit h the low-temperature weathering of the upper oceanic crust. The acidi ty flux is calculated based on major-element fluxes and charge-balance considerations. The relevant chemical fluxes from seawater to the upp er crust are 4.1 +/- 0.1; 1.4 +/- 1.4; 2.2 +/- 0.6 and -12 +/- 2 10(11 ) mol yr-1 for K, Mg, Na and silicate-Ca, respectively. The associated acidity flux is (3.5 +/- 3) . 10(11) eq yr-1. Relative to continental weathering, these fluxes are significant for K and silicate-Ca, but a re minor for Na, Mg and acidity. Thus, riverine fluxes of alkalinity a re not significantly balanced by acidity fluxes from low-temperature u pper ocean crust alteration.