The retrieval of marine weathering records preserved by strontium stable isotopes in foraminifera

Abstract

The strontium (88Sr/86Sr) stable isotope composition of seawater reflects input from continental weathering and hydrothermal exchange at mid-ocean ridges, and output in carbonate sediments. Variations in the stable isotope composition of seawater over time reflect either changes in the balance of carbonate to silicate weathering or changes in carbonate productivity and sedimentation in the oceans. However, temperature and species dependent fractionation of Sr stable isotopes during incorporation into marine carbonate has to be quantified in order to reconstruct past seawater compositions accurately. This study presents high-precision δ88Sr data, obtained using double-spike TIMs technique, for seawater, modern foraminifera from core-top samples and a record through the last glacial maximum (LGM). Present-day seawater yields a δ88Sr composition of 0.356±0.007 (2σm) with no resolvable difference between Pacific, Atlantic and Indian Oceans. Globigerinoides sacculifer (in the 350-450μm size range) from sites in the South Atlantic, covering a mixed layer temperature range of ~10°C, show no systematic variation with temperature, and have an average δ88Sr value of 0.22±0.07. Both G. sacculifer and G. menardii show systematic variations with growth rate (shell size) with heavier compositions in the larger size fractions. By contrast, G. aequilateralis and G. ruber yield δ88Sr values of -0.056±0.067 and -0.023±0.008 respectivley, with no systematic variation with shell size. These observations indicate that for G. sacculifer, at least, there is no effect of temperature on Sr stable isotope uptake, but the species specific, and/or shell size effects need to be considered in order to retrieve seawater δ88Sr values from foraminiferal tests. Preliminary δ88Sr data for Pulleniatina obliquiloculata covering the last 43 kyr indicate that there was no resolvable change in the δ88Sr composition of seawater across the LGM and deglaciation. In this case the postulated enhanced weathering of shelf carbonates during glacial intervals, delivering light Sr isotopes to the ocean may not have been as significant as predicted or else was offset by increased production and preservation of carbonates, driving seawater to heavier δ88Sr values. Alternatively the very long residence time of Sr in the oceans may simply buffer the changes in input or output such that no changes are resolved at the level of precision of this study