MODELING THE CONTRIBUTION OF GLACIAL EROSION TO THE CENOZOIC SR ISOTOPE EVOLUTION OF SEAWATER

The increase in the oceanic Sr isotopic ratio during the Cenozoic reflects some combination of increased glaciation and continental collision of India with Asia. Here, we use a Sr model built into an existing coupled climate-ice sheet-sediment model to examine the contribution of Antarctic glaciation to the flux of radiogenic Sr to the ocean. An asynchronous coupling method is used, which enables the ice model spanning the whole Antarctic continent to be integrated for several million years through the observed major transition to large ice volumes around the Eocene-Oligocene boundary. The sediment component predicts the thickness of unconsolidated sediment in response to deformation under the ice sheet, generation by quarrying of exposed bedrock, and physical erosion and fluvial transport to the ocean in ice-free areas. The fractions of the major minerals within the sediment that are relevant to Sr are added as tracers. Their fractions within the evolving sediment layer are predicted in the model, depending on (i) advection by subglacial deformation, (ii) input of quarried bedrock, and (iii) chemical weathering in a 1-m thick soil layer in ice-free areas, at a rate depending on time of exposure modified by a species-dependent factor. Separate fractions are predicted for the soil and sub-soil layers in ice-free regions, mixed together when overrun by ice. The rate of chemical weathering for each species is calculated at each timestep, then summed over the entire Antarctic continent, and used to deduce the contribution to the change of ⁸⁷Sr/⁸⁶Sr of global ocean water throughout the simulation.

Preliminary results for a 5 Myr run of the coupled model across the E-O transition predict a rate of increase of ocean ⁸⁷Sr/⁸⁶Sr of only ~.00006 per Myr, which is smaller than the observed oceanic rate for this time. Whereas this might indicate the greater contribution from ongoing Asia-India collision, the low rate from Antarctic glaciation might be due to the suddenness of the transition in the ice model, which exhibits a very rapid jump from minor ice caps to a large ice sheet. A more gradual transition over several million years, with more extensive and repeated fluctuations in ice area over more of the continent, would convert more bedrock to till and thus increase flux of radiogenic Sr to the ocean.