Glacial Erosion and the Variable Isotopic Composition of Strontium in Sea Water

Abstract

Peterman, Hedge and Turtelot1 have carefully documented the variations in isotopic composition of sea water strontium during Phanerozoic time (Fig. 1). Because sea water strontium represents an erosion-averaged sample of rocks exposed above sea level, Peterman et al. put forward two causes for the observed variation : periods of intense volcanism which would provide easily eroded material depleted in radiogenic 87Sr relative to sea water, and widespread uplift (epeirogenic movements) of the continents resulting in accelerated erosion of ancient rocks enriched in radiogenic 87Sr. I would like to point out a further cause, perhaps the most influential one. The two periods of highest 87Sr/86Sr ratio are in the late Cenozoic and Pennsylvanian (upper Carboniferous). These correspond to the two times of most extensive glaciation2 during the Phanerozoic (Fig. 1). Erosion of Pre-Cambrian shields by continental glaciers would be an effective means of adding radiogenic 87Sr to the dissolved load input to the oceans. Lowering of the sea level as a result of the trapping of water in continental ice sheets would act in concert with the glaciers in accelerating erosion of shield areas.