Global cooling forced increase in marine strontium isotopic ratios: Importance of mica weathering and a kinetic approach

摘要

The knowledge of how and why marine Sr-87/Sr-86 ratios changed helps understand the impacts of many processes on the global biogeochemical cycle in the geological past. Here we examine the possible influence of global cooling on the evolution of marine Sr-87/Sr-86 curve by a kinetic approach. The importance of mica weathering is emphasized due to its high content of radiogenic strontium and low activation energy in weathering reaction. Since the activation energy determines the sensitivity of weathering rate in response to temperature changes, global cooling will increase the Sr-87/Sr-86 ratio of continental flux by relatively preferential weathering of mica. Based on the average strontium contents and Sr-87/Sr-86 ratios of the mica minerals in the exposed upper continental crust, a kinetic model is established to survey the response of riverine Sr-87/Sr-86 to the changes of global temperatures. The model result indicates that the increase of riverine Sr-87/Sr-86 caused by global cooling can solely account for most of the increase in seawater Sr-87/Sr-86 since 3.4 Myr B.P. This finding emphasizes that the weathering kinetics is an important mechanism relating the global climate change to marine Sr-87/Sr-86 curve; and the marine Sr-87/Sr-86 curve should be used with great care in the climate-weathering-tectonic connections. (c) 2006 Elsevier B.V All rights reserved.