A silicate weathering mechanism linking increases in marine 87Sr/ 86Sr with global glaciation

THE ⁸⁷Sr/ ⁸⁶Sr ratio of sea water is generally regarded as a proxy for the rate of chemical weathering of the continents. Interpretation of the ⁸⁷Sr/⁸⁶Sr record in marine limestones requires an understanding of how changes in this quantity are linked to climate and tectonic events. A connection between the strontium isotope record and the extent of global glaciation has been proposed^{1 -8}, but without a well established mechanism. Glaciation may influence marine ⁸⁷Sr/ ⁸⁶Sr by changing global riverine Sr fluxes (that is, rock weathering rates) and/or ⁸⁷Sr/⁸⁶Sr ratios (that is, relative mineral weathering rates). Here we investigate how the ⁸⁷Sr/⁸⁶Sr release from silicate weathering is affected by glaciation, using the Sr isotope systematics of granitoid soils on alpine glacial moraines in the Wind River Range, Wyoming. We observe a negative correlation between the ⁸⁷Sr/⁸⁶Sr ratio of exchangeable Sr in soils and the soil age, indicating that the ⁸⁷Sr/⁸⁶Sr ratio of Sr released in the early stages of weathering is significantly higher than in later stages. We estimate that this mechanism can increase global riverine ⁸⁷Sr/⁸⁶Sr by an average of 0.0002 during periods of glacial-interglacial cycling. When superimposed on long-term trends controlled by tectonic processes^2-7,9,10, this shift can account for the correlation between the rate of change of marine ⁸⁷Sr/⁸⁶Sr and the intensity of glaciation over at least the past 10 Myr.