A SILICATE WEATHERING MECHANISM LINKING INCREASES IN MARINE SR-87/SR-86 WITH GLOBAL GLACIATION

THE Sr-87/Sr-86 ratio of sea water is generally regarded as a proxy for the rate of chemical weathering of the continents. Interpretation of the Sr-87/Sr-86 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 Sr-87/Sr-86 by changing global riverine Sr fluxes (that is, rock weathering rates) and/or Sr-87/Sr-86 ratios (that is, relative mineral weathering rates). Here We investigate how the Sr-87/Sr-86 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 Sr-87/Sr-86 ratio of exchangeable Sr in soils and the soil age, indicating that the Sr-87/Sr-86 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 Sr-87/Sr-86 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 Sr-87/Sr-86 and the intensity of glaciation over at least the past 10 Myr.