Low-temperature alteration of the upper oceanic crust and the alkalinity budget of seawater

Abstract

While the input of river-alkalinity into seawater is relatively well known, 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 with the low-temperature weathering of the upper oceanic crust. The acidity flux is calculated based on major-element fluxes and charge-balance considerations. The relevant chemical fluxes from seawater to the upper 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 are minor for Na, Mg and acidity. Thus, riverine fluxes of alkalinity are not significantly balanced by acidity fluxes from low-temperature upper ocean crust alteration.