Major cation gradients in DSDP interstitial waters: the role of diffusive exchange between seawater and upper oceanic crust
Interstitial water analyses from DSDP Legs 1-53 were surveyed to identify sites at which the dominant process supporting gradients of major seawater cations is alteration of the underlying oceanic crust. Sites with thin sediment cover are generally without gradients: sites with appreciable sediment accumulation are generally with gradients. The transition is probably the result of cessation of convective circulation within the sediments. A model is developed for the diffusive flux through an accumulating, unreactive sedimentary layer supported by continued reaction within the crust. The model is consistent with the observations provided that convection within the crust extends to depths > 1 km and that convective flow within the sediments ends once 100-150 m of sediment accumulates. The model suggests a steady state gradient inversely proportional to the sedimentation rate: this dependence is apparent in the data, however, some variation in the reaction rate is also required. Fluxes of Mg and Na are systematically related to the flux of Ca which because of its mobility serves as a tracer of hydrolysis of the basalt. When the Ca flux is low, the Mg flux serves as the principal sink for alkalinity produced during basalt hydrolysis. When the Ca flux is high, the Mg flux becomes diffusion-limited and Nabearing alteration phases must form. The fluxes resulting from these alteration reactions exert little influence on either the chemistry of seawater or that of the upper crust.