We present strontium (Sr) isotope ratios that, unlike traditional 87 Sr/ 86 Sr data, are not normalized to a fixed 88 Sr/ 86 Sr ratio of 8.375209 (defined as δ 88/86 Sr=0 relative to NIST SRM 987). Instead, we correct for isotope fractionation during mass spectrometry with a 87 Sr– 84 Sr double spike. This technique yields two independent ratios for 87 Sr/ 86 Sr and 88 Sr/ 86 Sr that are reported as ( 87 Sr/ 86 Sr∗) and (δ 88/86 Sr), respectively. The difference between the traditional radiogenic ( 87 Sr/ 86 Sr normalized to 88 Sr/ 86 Sr=8.375209) and the new 87 Sr/ 86 Sr∗ 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 (( 87 Sr/ 86 Sr∗, δ 88/86 Sr) Seawater ) and modern marine biogenic carbonates (( 87 Sr/ 86 Sr∗, δ 88/86 Sr) Carbonates ) with the corresponding values of river waters (( 87 Sr/ 86 Sr∗, δ 88/86 Sr) River ) and hydrothermal solutions (( 87 Sr/ 86 Sr∗, δ 88/86 Sr) HydEnd ) in a triple isotope plot. The measured ( 87 Sr/ 86 Sr∗, δ 88/86 Sr) 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 ( 87 Sr/ 86 Sr∗, δ 88/86 Sr) HydEnd values for hydrothermal solutions from the Atlantic Ocean are (0.7045(5), 0.27(3)‰). In contrast, the ( 87 Sr/ 86 Sr∗, δ 88/86 Sr) 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 ( 87 Sr/ 86 Sr∗, δ 88/86 Sr) input and ( 87 Sr/ 86 Sr∗, δ 88/86 Sr) 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 (10–30ka 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 post-glacial exposure of abundant fine-grained material.