Constraining the marine strontium budget with natural strontium isotope fractionations (87Sr/86Sr*, δ88/86Sr) of carbonates, hydrothermal solutions and river waters

We present strontium (Sr) isotope ratios that, unlike traditional 87Sr/86Sr data, are not normalized to a fixed 88Sr/86Sr ratio of 8.375209 (defined as d88/86Sr = 0 relative to NIST SRM 987). Instead, we correct for isotope fractionation during mass spectrometry with a 87Sr-84Sr double spike. This technique yields two independent ratios for 87Sr/86Sr and 88Sr/86Sr that are reported as (87Sr/86Sr*) and (d88/86Sr), respectively. The difference between the traditional radiogenic (87Sr/86Sr normalized to 88Sr/86Sr = 8.375209) and the new 87Sr/86Sr* values reflect natural mass-dependent isotope fractionation. In order to constrain glacial/interglacial changes in the marine Sr budget we compare the isotope composition of modern seawater ((87Sr/86Sr*, d88/86Sr)Seawater) and modern marine biogenic carbonates ((87Sr/86Sr*, d88/86Sr)Carbonates) with the corresponding values of river waters ((87Sr/86Sr*, d88/86Sr)River) and hydrothermal solutions ((87Sr/86Sr*, d88/86Sr)HydEnd) in a triple isotope plot. The measured (87Sr/86Sr*, d88/86Sr)River values of selected rivers that together account for ~18% of the global Sr discharge yield a Sr flux-weighted mean of (0.7114(8), 0.315(8)&). The average (87Sr/86Sr*, d88/86Sr)HydEnd values for hydrothermal solutions from the Atlantic Ocean are (0.7045(5), 0.27(3)&). In contrast, the (87Sr/86Sr*, d88/86Sr)Carbonates values representing the marine Sr output are (0.70926(2), 0.21(2)&). We estimate the modern Sr isotope composition of the sources at (0.7106(8), 0.310(8)&). The difference between the estimated (87Sr/86Sr*, d88/86Sr)input and (87Sr/86Sr*, d88/86Sr)output values reflects isotope disequilibrium with respect to Sr inputs and outputs. In contrast to the modern ocean, isotope equilibrium between inputs and outputs during the last glacial maximum (10-30 ka before present) can be explained by invoking three times higher Sr inputs from a uniquely "glacial" source: weathering of shelf carbonates exposed at low sea levels. Our data are also consistent with the "weathering peak" hypothesis that invokes enhanced Sr inputs resulting from weathering of postglacial exposure of abundant fine-grained material.