Quantifying the Strontium Budget of the Oceans, past and present, using coupled Radiogenic and Stable Strontium Isotopes

Lead Research Organisation: University of Oxford
Department Name: Earth Sciences

Abstract

The chemical evolution of the oceans is controlled by a range of biological and sedimentary processes, many of which are influenced by tectonic and climatic change. Of these, consumption of atmospheric CO2 through chemical weathering of the continents is thought to play a fundamental role in regulating the Earth's temperature. Therefore, records of ancient seawater chemistry potentially provide a means of determining the importance of weathering on the global carbon cycle and its affect on the Earth's climate. Many natural radiogenic isotopes in seawater are sensitive to changes in the balance of input from continental weathering, and sedimentary archives preserve a record of changes in chemical weathering through geological time. For over twenty five years the rubidium-strontium (Rb-Sr) radiogenic isotope system has been amongst the most commonly used and variations in seawater 87Sr/86Sr ratios on both long and short timescales reflect changing continental input through time. However, changes in the 87Sr/86Sr record cannot be used to reconstruct past CO2 consumption rates because it is unlikely that the composition of the continental source (in particular that delivered by rivers) remains constant through time. Indeed, coupled Sr/Ca and 87Sr/86Sr records indicate significant changes in both the chemical weathering flux and composition of weathered material delivered to seawater over the Cenozoic. Remarkably the published 87Sr/86Sr seawater record may itself be compromised because recent data suggests that there are significant variations in the stable isotopes of Sr. This is because the stable 88Sr/86Sr ratio is traditionally considered to be a constant value and used to correct instrumental mass fractionation during measurement of the radiogenic ratio. Consequently, variations in the stable isotope composition may dramatically alter the measured 87Sr/86Sr ratios in marine records relative to their true value. Our own preliminary data for diverse continental rock types and rivers indicates a total variation in the 88Sr/86Sr ratio of at least 0.9 per mil. While such a variation might be considered small compared to other lighter or redox sensitive elements, it results in a variation of 0.45 per mil (450 ppm) in the 87Sr/86Sr ratio (some 50-100 times greater than current analytical uncertainties). If this continental 88Sr/86Sr variation is imparted to seawater through chemical weathering then this may, in turn, significantly alter the 87Sr/86Sr seawater record. For example, some 20% of the change in Cenozoic seawater 87Sr/86Sr could simply be due to variations in 88Sr/86Sr rather than any actual variation in 87Sr/86Sr. Our own preliminary stable Sr isotope data for a 2.3 Ma record obtained from planktonic foraminifera from the Labrador Sea indicates a significant shift (>100 ppm) in the 88Sr/86Sr composition of seawater over this interval, altering both the pattern and magnitude of change seen in the marine 87Sr/86Sr record. Despite the complexities introduced by variations in 88Sr/86Sr, the coupled measurement of both radiogenic and stable isotope ratios offers a means of determining the true 87Sr/86Sr value of seawater. The corrected the corrected 87Sr/86Sr record can then be combined with Sr/Ca data to deconvolve changes in the global average continental flux from changes in global average composition. Where concomitant variations in 88Sr/86Sr may themselves reveal information on the nature of those changes in composition, for example, whether they result from changes in the continental source, hydrothermal exchange or the precipitation of marine carbonate. Overall these results will thus serve to provide a better understanding of the relationship between chemical weathering, its regulation of the atmospheric partial pressure of carbon dioxide, and thus influence on the greenhouse effect and global climate.
 
Description (1) Quantifying the influence of temperature, species and shell size on the incorporation of strontium stable isotopes into foraminiferal calcite: This part of the project quantified the effect of temperature, species and growth rate (shell size) on Sr stable isotope incorporation into foraminiferal calcite. Analyses of present-day seawater yield a _88Sr composition of 0.356±0.007 (2_m) with no resolvable variation between the Pacific, Atlantic and Indian Oceans. G. sacculifer (in the 350-450 _m 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 _88Sr value of 0.089±0.085‰. Both G. sacculifer and G. menardii show systematic variations with shell size with heavier compositions in the larger size fractions. By contrast, G. aequilateralis and G. ruber yield _88Sr values of -0.023±0.008 and -0.056±0.067 respectively, with no systematic variation with growth rate. These observations indicate that for G. sacculifer, at least, there is no effect of temperature on Sr stable isotope uptake, but both species and shell size need to be accounted for in order to retrieve seawater _88Sr values.



(2) Experimental determination of the temperature dependence of _88Sr in coccolith and inorganic calcite: Three species of coccolithophores, Emiliana huxleyi, Coccolithus pelagicus (ssp. Braruudi) and Gephyrocapsa oceanica were individually cultured and grown in different temperature environments (10, 15, 20 and 25°C) as well as two different nutrient media, one replete (K) and one deplete (K/6, i.e. one sixth of the nutrients in the K media). The results of the experiments indicate that as temperature increases _88Sr ratios shift to lighter values i.e. away from seawater this is the opposite trend to the incorporation of Sr/Ca ratios which decrease as temperature increases. Taken together, when coccolithophores have a higher concentration of Sr in their CaCO3 liths the stable strontium isotopes are closer to those of seawater (i.e. less fractionated). In contrast, results from inorganic precipitation experiments suggest that there is little resolvable stable strontium isotope fractionation during calcite precipitation.



(3) The strontium stable isotope composition of seawater during recent glacial intervals: A seawater _88Sr record has been produced for Globigerinoides ruber (white) for the last 145 ka, at ODP site 758, in the N E Indian Ocean. These data do not show any covaraition with temperature, as might be expected if there were a temperature dependence of _88Sr into foraminiferal calcite or changes in the balance of carbonate to silicate weathering during glacial intervals. However, they do show a 75 ppm variation which may correlate with local changes in seawater hydrography or global changes in carbonate burial in the deep ocean.
Exploitation Route Determination of the temperature dependence of _88Sr in inorganic calcite. (1) Quantifying the influence of temperature, species and shell size on the incorporation of strontium stable isotopes into foraminiferal calcite.

(2) Determination of the temperature dependence of _88Sr in coccolith and inorganic calcite.

(3) Reconstructing change in the balance of strontium input to the oceans accompanying climate change
Sectors Environment
 
Description Summary of Impact At the stage the main users of the research will be other academics, particularly those involved in the study of climate change and ocean chemistry, it is possible that this research will ultimately inform policymakers and regulators if the techniques developed here are applied elsewhere
First Year Of Impact 2012
Sector Environment