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.