High-precision radiogenic strontium isotope measurements of the modern and glacial ocean: Limits on glacial-interglacial variations in continental weathering

摘要

Existing strontium radiogenic isotope (Sr-87/Sr-86) measurements for foraminifera over Quaternary glacial-interglacial climate cycles provide no evidence for variations in the isotope composition of seawater at the +/- 9-13 ppm level of precision. However, modelling suggests that even within this level of uncertainty significant (up to 30%) variations in chemical weathering of the continents are permitted, accounting for the longer-term rise in Sr-87/Sr-86 over the Quaternary, and the apparent imbalance of Sr in the oceans at the present-day. This study presents very high-precision Sr-87/Sr-86 isotope data for modern seawater from each of the major oceans, and a glacial-interglacial seawater record preserved by planktic foraminifera from Ocean Drilling Program (ODP) Site 758 in the north-east Indian ocean. Strontium isotope Sr-87/Sr-86 measurements for modern seawater from the Atlantic, Pacific and Indian Oceans are indistinguishable from one another (Sr-87/Sr-86 = 0.7091792 +/- 0.0000021, n = 17) at the level of precision obtained in this study (+/- 4.9 ppm 2 sigma). This observation is consistent with the very long residence time of Sr in seawater, and underpins the utility of this element for high precision isotope stratigraphy. The Sr-87/Sr-86 seawater record preserved by planktic foraminifera shows no resolvable glacial-interglacial variation (Sr-87/Sr-86 = 0.7091784 +/- 0.0000035, n = 10), and limits the response of seawater to variations in the chemical weathering flux and/or composition to +/- 4.9 ppm or less. Calculations suggest that a variation of +/- 12% around the steady-state weathering flux can be accommodated by the uncertainties obtained here. The new data cannot accommodate a short-term weathering pulse during de-glaciation, although a more a diffuse weathering pulse accompanying protracted ice retreat is permissible. However, these results still indicate that modern weathering fluxes are potentially higher than average over the Quaternary, and such variations through glacial cycles can also account for the longer-term rise in Sr-87/Sr-86 over this time interval. The very high-precision measurements made for the marine Sr-87/Sr-86 record in this study place clear limits on the magnitude and timing of changes in the chemical weathering flux during glacial-interglacial cycles. Further, constraints must be sought from even higher precision measurement or elements with shorter residence times in the ocean, such as osmium (Os), that have the capacity to respond to short-term variations in input. Crown Copyright (C) 2015 Published by Elsevier B.V. All rights reserved.