A continental perspective of the seawater 87Sr/86Sr record: A review

Before submarine hydrothermal vents were discovered, the Sr-87/Sr-86 record of seawater was interpreted as a mixture of unradiogenic basaltic and radiogenic granitic continental runoff, strongly buffered by contributions from carbonate weathering. Since the discovery of submarine hydrothermal vents this record has generally been viewed as a mixture of multiple components of which radiogenic continental runoff and unradiogenic submarine hydrothermal fluids are the most significant. A critical review of sources of Sr to seawater indicates that the Sr budget of the modern ocean requires similar to 1.4 x 10(10) mol yr(-1) of submarine hydrothermal Sr to balance the present-day Sr-87/Sr-86 of freshwater inputs. This flux is about twice what observations of ocean crust alteration and hydrothermal inputs seem to support. The globally averaged, bias-corrected, and Sr flux-weighted Sr-87/Sr-86 of rivers and submarine groundwater discharge is 0.7103-0.7108, and thus significantly less radiogenic than the average Sr-87/Sr-86 of the upper continental crust (> 0.72). Global analyses of runoff from large-scale drainage regions as well as 76 individual river basins with large dissolved Sr fluxes to the ocean reveal positive correlations between the dissolved riverine Sr-87/Sr-86 and the average bedrock age of drainage regions and individual river basins. The current flux of continental Sr to the ocean is biased towards contributions from younger bedrock (331 Myr) compared to the average bedrock age of the exorheic land area (445 Myr). This correlation supports the notion that variations in the marine Sr-87/Sr-86 are influenced by variations in the composition and age structure of exorheic continental bedrock. Quantitative evaluation of the average age of exorheic bedrock, weighted according to the dissolved Sr flux, indicates that the temporal variations of this parameter that are required to account for the observed temporal variations in the Sr-87/Sr-86 of seawater depend primarily on the slope of the correlations between dissolved Sr-87/Sr-86 and bedrock age. The required variations in average exorheic bedrock age are consistent with changes predicted by the paleogeologic reconstructions of the Phanerozoic and appear to be geologically sensible. The data imply that improvements in our understanding of the driving forces of changes in Sr-87/Sr-86 and other radiogenic isotope records of seawater critically depend on improved reconstructions of paleogeology, paleogeography and paleohydrology.