Variation in the strontium isotopic composition of seawater (8 Ma to present) : Implications for chemical weathering rates and dissolved fluxes to the oceans

Abstract

Measurements of 87Sr/ 86Sr on samples of planktonic foraminifers were used to reconstruct changes in the Sr isotopic composition of seawater for the past 8 Ma. The late Neogene was marked by a general, but not regular, increase in 87S/ 86Sr with two breaks in slope at 5.5 and 2.5 Ma. These times mark the beginning of two periods of steep increase in 87Sr/ 86Sr values, relative to preceding periods characterized by essentially constant values. During the last 2.5 Ma, 87Sr/ 86Sr values increased at an average rate of 54° 10 −6 Ma −1. This steep increase suggests that the modem ocean is not in Sr isotopic equilibrium relative to its major input fluxes. A non-equilibrium model for the modern Sr budget suggests that the residence time of Sr is ∼ 2.5 Ma, which is significantly less than previously accepted estimates of 4–5 Ma. Modelling results suggest that the increase in 87Sr/ 86Sr over the past 8 Ma could have resulted from a 25% increase in the riverine flux of Sr or an increase in the average 87Sr/ 86Sr of this flux by 0.0006. The dominant cause of increasing 87Sr/ 86Sr values of seawater during the late Neogene is believed to be increased rates of uplift and chemical weathering of mountainous regions. Calculations suggest that uplift and weathering of the Himalayan-Tibetan region alone can account for the majority of the observed 87Sr/ 86Sr increase since the early Late Miocene. Exhumation of Precambrian shield areas by continental ice-sheets may have contributed secondarily to accelerated mechanical and chemical weathering of old crustal silicates with high 87Sr/ 86Sr values. In fact, the upturn in 87Sr/ 86Sr at 2.5 Ma coincides with increased glacial activity in the Northern Hemisphere. A variety of geochemical ( 87Sr/ 86Sr, Ge/Si, δ 13C, CCD, etc.) and sedimentologic data (accumulation rates) from the marine sedimentary record are compatible with a progressive increase in the chemical weathering rate of continents and dissolved riverine fluxes during the late Cenozoic. We hypothesize that chemical weathering of the continents and dissolved riverine fluxes to the oceans reached a maximum during the late Pleistocene because of repeated glaciations, increased continental exposure by lowered sea level, and increased continental relief resulting from high rates of tectonism.