The Nd and Sr isotopic systematics of river-water dissolved material: Implications for the sources of Nd and Sr in seawater

Abstract

We report the isotopic composition and concentration of Nd and Sr in the dissolved load of rivers from North America, Australia, Japan, the Philippines and Venezuela. The data are used to: (1) characterize the behavior of Nd and Sr during weathering and river transport; (2) estimate the concentration and isotopic composition of Nd and Sr in river runoff to the oceans; and (3) construct an isotopic mass balance for Nd and Sr in seawater. In general, Nd is mobilized in rivers of low pH, whereas Sr concentrations in rivers are determined by the degree of chemical weathering of source rocks. The isotopic composition of Nd and Sr in river waters is strongly influenced by the age of materials in a drainage basin, and this results in a rough overall inverse correlation between 87 Sr 86 Sr and ϵ Nd. Preferential weathering of marine precipitates, such as carbonates and phosphates, significantly affects the 87 Sr 86 Sr , and to a lesser degree, the ϵ Nd of river waters. Established geochemical relationships and data in the literature are used to estimate the weighted average ϵ Nd of the primary river runoff to each of the ocean basins. New Sr isotope data for rivers (including new island arc data) lower the estimated mean 87 Sr 86 Sr of river runoff and reduces the calculated seawater-basalt Sr exchange near mid-ocean ridges. This yields a hydrothermal water flux of (2.9 ± 2.5) · 10 16 g yr. −1. It is shown that river inputs are the dominant factor in determining the Nd and Sr isotopic signature of modern seawater.