Reassessing the stable (δ^88/86Sr) and radiogenic (⁸⁷Sr/⁸⁶Sr) strontium isotopic composition of marine inputs

Abstract

The stable strontium isotope system (δ^88/86Sr) has recently been suggested to be a suitable proxy for determining variations in the strength of the marine carbonate system, the principal output flux of oceanic Sr. However, in order to be able to interpret carbonate-driven variations in δ^88/86Sr_seawater a robust understanding of δ^88/86Sr_input is required. Surprisingly only a limited amount of δ^88/86Sr data currently exists for rivers and hydrothermal fluids, thus this study assesses the variability of δ^88/86Sr and ⁸⁷Sr/⁸⁶Sr in global rivers, hydrothermal fluids and porewaters, as well as minor marine Sr sources such as continental dust, rainwater and glacial ice. Our analyses broadly confirm the findings of Krabbenhöft et al. (2010), and reveal flux-weighted δ^88/86Sr_riverine and ⁸⁷Sr/⁸⁶Sr_riverine compositions of 0.32‰ and 0.71299 respectively. The hydrothermal fluids analysed in this study are consistent with an end-member δ^88/86Sr_hydrothermal composition that is the same as the oceanic crust at ∼0.24‰, although three samples that display δ^88/86Sr compositions offset from the seawater-hydrothermal mixing trend suggest that the precipitation of alteration phases such as anhydrite may drive δ^88/86Sr_hydrothermal to higher values. Porewater fluids obtained from sediment cores in the Atlantic and Pacific Oceans have δ^88/86Sr compositions within error of seawater (0.39‰), implying that the diagenetic flux of Sr may not significantly affect the δ^88/86Sr composition of seawater. Continental loess samples have δ^88/86Sr compositions that are consistently lighter than, or equal to, terrestrial silicates, with their tendency to lower values thought to reflect the preferential removal of heavier Sr isotopes into solution during weathering. Finally, rainwater and glacial ice samples have δ^88/86Sr compositions that are also isotopically lighter than their associated water sources, a factor that may be attributed to interaction with isotopically light loess and additional Sr contributions from the bedrock. Together, the principal marine inputs define flux-weighted oceanic δ^88/86Sr_input and ⁸⁷Sr/⁸⁶Sr_input compositions of 0.32‰ and 0.71161. These values are consistent with an elevated supply of riverine Sr to the oceans due to increased post-glacial weathering, but require the enhanced weathering of exposed carbonate shelves during glacial periods or significant changes in the rate of carbonate burial to match observed changes in the ⁸⁷Sr/⁸⁶Sr ratio of seawater. Our results confirm that, providing a diagenetically robust proxy can be found, the δ^88/86Sr and ⁸⁷Sr/⁸⁶Sr isotope systems should provide a useful proxy for investigating changes in the marine carbonate system through time.

Introduction

Strontium is readily transferred to solution during continental weathering processes and is stable in dissolved form within the oceans. Relative to the marine input and output fluxes, this high solubility results in a uniform seawater Sr concentration of 89 μmol/kg and residence time of ∼2.5 Ma (Broecker and Peng, 1982). Continental discharge (riverine transport) is the dominant source of Sr to the oceans, with the remainder primarily entering the oceans via hydrothermal fluids (Palmer and Edmond, 1989, Davis et al., 2003, Vance et al., 2009, Allègre et al., 2010, Beck et al., 2013). Changes in the relative importance of these principal marine Sr fluxes throughout Earth’s history can be resolved using the radiogenic Sr isotope system (⁸⁷Sr/⁸⁶Sr); the decay of ⁸⁷Rb to ⁸⁷Sr over geological timescales causes the Rb-rich silicate continental crust to have an elevated (more radiogenic) ⁸⁷Sr/⁸⁶Sr ratio of ∼0.7136, whereas mantle-derived Sr sources such as hydrothermal fluids and exposed oceanic basalts have lower Rb contents and correspondingly low ⁸⁷Sr/⁸⁶Sr ratios of ∼0.703 (Albarède et al., 1981, Goldstein and Jacobsen, 1987, Palmer and Edmond, 1989, Palmer and Edmond, 1992, Gaillardet et al., 1999, Allègre et al., 2010, Peucker-Ehrenbrink et al., 2010). Because marine processes do not affect the proportion of radiogenic ⁸⁷Sr in solution, the ⁸⁷Sr/⁸⁶Sr composition of seawater directly reflects the relative mixing of these inputs, which results in a modern ocean composition of 0.70917 (e.g., Palmer and Edmond, 1989, Allègre et al., 2010). The normalisation procedure used to account for instrumental mass-dependent fractionation during radiogenic Sr isotopic analysis (Nier, 1938) means that the ⁸⁷Sr/⁸⁶Sr composition of marine carbonate is identical to the seawater in which it formed, enabling carbonate ⁸⁷Sr/⁸⁶Sr records to be used to identify changes in both the rate and nature of global continental weathering, as well as variations in the hydrothermal flux driven by changes in the rate of mid ocean ridge spreading (e.g., Hodell et al., 1990, McArthur, 1994, McArthur et al., 2001).

Whilst this two end-member source scenario is useful for assessing changes in Earth system processes, it is now well established that the thermally plausible flux of unradiogenic hydrothermal Sr is insufficient to balance the amount of radiogenic Sr transported to the oceans via rivers (e.g., Davis et al., 2003, Vance et al., 2009, Allègre et al., 2010, Jones et al., 2012a, Jones et al., 2012b, Beck et al., 2013). This imbalance means that the ⁸⁷Sr/⁸⁶Sr composition of seawater should actually be increasing faster than the current rate of 0.000054 Myr⁻¹ (Vance et al., 2009). Possible explanations for this discrepancy include missing sources of unradiogenic Sr, such as off-axis hydrothermal fluids (Davis et al., 2003), particulate weathering (Jones et al., 2012a, Jones et al., 2012b) and volcanic islands and groundwater sources (Allègre et al., 2010, Beck et al., 2013), as well as variable rates of glacial–interglacial continental weathering (Vance et al., 2009, Krabbenhöft et al., 2010). Although each of these fluxes/processes are likely to play a significant role in controlling the ⁸⁷Sr/⁸⁶Sr composition of the oceans, their impact on the total Sr content of seawater is harder to establish as this also depends on the output flux of Sr into marine carbonates. In order to asses how this output flux (and hence Sr abundance) varied in the past it is necessary to use the stable Sr isotope system.

The stable Sr isotopic composition of seawater reflects the relative proportion of naturally occurring Sr isotopes (i.e., those not produced by the radiogenic decay of ⁸⁷Rb); ⁸⁴Sr, ⁸⁶Sr, ⁸⁷Sr and ⁸⁸Sr. As with other stable isotope systems, variations in the relative proportion of the stable Sr isotopes are commonly reported in delta notation, with the accepted convention being: δ^88/86Sr = ((⁸⁸Sr/⁸⁶Sr)_sample/(⁸⁸Sr/⁸⁶Sr)_standard − 1) × 1000. The international standard used for normalising all δ^88/86Sr measurements is the National Institute of Standards and Technology SrCO₃ standard reference material 987 (hereafter NIST987; Fietzke and Eisenhauer, 2006, Krabbenhöft et al., 2009, Ma et al., 2013, Shalev et al., 2013, Neymark et al., 2014).

Unlike internally normalised ⁸⁷Sr/⁸⁶Sr isotope ratios, the sensitivity of δ^88/86Sr to mass fractionation processes means that the composition of marine carbonates does not match that of seawater. In fact, carbonates have been shown to have significantly lower δ^88/86Sr compositions than the fluid in which they formed, with Δ^88/86Sr_{carbonate-fluid} offsets ranging between −0.02‰ and −0.37‰ (Fietzke and Eisenhauer, 2006, Halicz et al., 2008, Ohno et al., 2008, Rüggeberg et al., 2008, Krabbenhöft et al., 2010, Böhm et al., 2012, Raddatz et al., 2013, Stevenson et al., 2014, Vollstaedt et al., 2014). This preferential incorporation of lighter Sr isotopes into the carbonate lattice is equivalent to trends observed in the Ca and Mg isotope systems (e.g., Fantle and Tipper, 2014, Saenger and Wang, 2014 and references therein), and has been suggested to correlate with the rate of precipitation (Böhm et al., 2012, Stevenson et al., 2014). This sensitivity of δ^88/86Sr to carbonate formation indicates that variations in the δ^88/86Sr composition of seawater may help constrain past changes in the marine carbonate system if used in conjunction with other carbonate system parameters (e.g., Vollstaedt et al., 2014). However, before δ^88/86Sr can be used to assess how this marine Sr output flux has varied through time, it is essential to fully quantify potential variability in the δ^88/86Sr composition of the dominant marine inputs. Surprisingly, only a limited amount of δ^88/86Sr data currently exists for rivers and hydrothermal fluids (de Souza et al., 2010, Krabbenhöft et al., 2010, Wei et al., 2013). In their initial assessment of the δ^88/86Sr composition of marine fluxes, Krabbenhöft et al. (2010) concluded that, as with the radiogenic Sr system, the modern stable Sr isotopic composition of riverine input is in disequilibrium as a result of non steady state weathering processes. This study assesses their interpretation by expanding the available river water dataset to account for ∼50% of global riverine discharge, and by providing a more robust assessment of δ^88/86Sr variability in the other principal marine sources such as hydrothermal fluids and porewaters, as well as minor fluxes such as dust, ice and rainwater. This new δ^88/86Sr data is interpreted in conjunction with ⁸⁷Sr/⁸⁶Sr ratios determined from the same samples and is used to reconsider our current understanding of the relative balance between marine Sr sources.