On the interpretation of the oceanic variations in Sr-87/Sr-86 as recordedin marine limestones

The present published inventory of fluvial Sr and Sr-87/Sr-86 data, combine d with new information from the big rivers of Eastern Siberia (a combined t otal of similar to 1,000 measurements), is used to investigate the probable origin of the large rise in the marine isotopic ratio, recorded in limesto nes, over the last similar to 20 million years. With the exception of the d ata from the Ganga-Brahmaputra all measurements fall on what is proposed to be called the "Wickman trend", essentially a mixing line between the Limes tone sink for Sr, with the integrated marine ratio, and the flux from the w eathering of average continental crust. However, time-variations along this trend, i.e. changes in relative weathering intensity, cannot explain the o bservations from Limestones. They can only be caused by very high and radio genic fluxes of Sr as are occurring from the present Himalayan orogeny, lyi ng far above the Wickman trend and caused by metamorphic remobilization of radiogenic Sr during underthrusting and subsequent unroofing associated wit h the collision of India with Eurasia. In general the variations in the rat io are therefore caused by specific tectonic events, not by general climati c variations in the intensity of aluminosilicate weathering.