Constraining the marine strontium budget with natural strontium isotope fractionations (87Sr/86Sr∗, δ88/86Sr) of carbonates, hydrothermal solutions and river waters

Abstract

We present strontium (Sr) isotope ratios that, unlike traditional 87 Sr/ 86 Sr data, are not normalized to a fixed 88 Sr/ 86 Sr ratio of 8.375209 (defined as d 88/86 Sr = 0 relative to NIST SRM 987). Instead, we correct for isotope fractionation during mass spectrometry with a 87 Sr-84 Sr double spike. This technique yields two independent ratios for 87 Sr/ 86 Sr and 88 Sr/ 86 Sr that are reported as ( 87 Sr/ 86 Sr*) and (d 88/86 Sr), respectively. The difference between the traditional radiogenic ( 87 Sr/ 86 Sr normalized to 88 Sr/ 86 Sr = 8.375209) and the new 87 Sr/ 86 Sr* values reflect natural mass-dependent isotope fractionation. In order to constrain glacial/interglacial changes in the marine Sr budget we compare the isotope composition of modern seawater (( 87 Sr/ 86 Sr*, d 88/86 Sr) Seawater ) and modern marine biogenic carbonates (( 87 Sr/ 86 Sr*, d 88/86 Sr) Carbonates ) with the corresponding values of river waters (( 87 Sr/ 86 Sr*, d 88/86 Sr) River ) and hydrothermal solutions (( 87 Sr/ 86 Sr*, d 88/86 Sr) HydEnd ) in a triple isotope plot. The measured ( 87 Sr/ 86 Sr*, d 88/86 Sr) River values of selected rivers that together account for $18% of the global Sr discharge yield a Sr flux-weighted mean of (0.7114 (8), 0.315(8)&). The average ( 87 Sr/ 86 Sr*, d 88/86 Sr) HydEnd values for hydrothermal solutions from the Atlantic Ocean are (0.7045(5), 0.27(3)&). In contrast, the ( 87 Sr/ 86 Sr*, d 88/86 Sr) Carbonates values representing the marine Sr output are (0.70926(2), 0.21(2)&). We estimate the modern Sr isotope composition of the sources at (0.7106(8), 0.310(8)&).The difference between the estimated ( 87 Sr/ 86 Sr*, d 88/86 Sr) input and ( 87 Sr/ 86 Sr*, d 88/86 Sr) output values reflects isotope disequilibrium with respect to Sr inputs and outputs. In contrast to the modern ocean, isotope equilibrium between inputs and outputs during the last glacial maximum (10-30 ka before present) can be explained by invoking three times higher Sr inputs from a uniquely "glacial" source: weathering of shelf carbonates exposed at low sea levels. Our data are also consistent with the "weathering peak" hypothesis that invokes enhanced Sr inputs resulting from weathering of postglacial exposure of abundant fine-grained material.

The Phanerozoic δ88/86Sr record of seawater: New constraints on past changes in oceanic carbonate fluxes

“…δ 88/86 Sr sw range between 0.25‰ and 0.60‰, and has a mean value of 0.37 ± 0.12‰ (2 s.d.). Phanerozoic δ 88/86 Sr sw , like δ 44/40 Ca sw , were always isotopically heavier than the mantle sources (δ 88/86 Sr hyd-in = 0.27‰ (Charlier et al, 2012;Krabbenhöft et al, 2010); δ 44/40 Ca hyd-in = 1.1‰ ), the latter defining the lower boundary conditions for isotope trends (Fig. 3) (Böhm et al, 2012)) is also considered to be constant through time.…”

“…3) (Böhm et al, 2012)) is also considered to be constant through time. In contrast, changes in F(Sr) wc , F(Sr) carb and the related isotope fractionation factor Δ 88/86 Sr carb-sw , as well as imbalances between Sr input and output fluxes may have a large impact on δ 88/86 Sr sw (Krabbenhöft et al, 2010 Fig. 3) which is also controlled mostly by carbonates as the main output flux (Blättler et al, 2012;Farkaš et al, 2007a).…”

“…The 88 Sr/ 86 Sr in modern marine carbonates were found to be distinctly lower than present day seawater (Böhm et al, 2012;Krabbenhöft et al, 2010;Krabbenhöft et al, 2009 The remaining samples are from the Phanerozoic carbonate database published in Veizer et al, (1999). Their assignment to biostratigraphic zones has a resolution of about 1-2Myrs.…”

“…Enhancement of weathering rates after deglaciations and tectonic uplifts (Hodell et al, 1989;Krabbenhöft et al, 2010;Porder et al, 2007;Stoll and Schrag, 1998;Taylor and Blum, 1995;Vance et al, 2009) and associated incongruent weathering of silicates (Blum and Erel, 1997), the ease of weathering of island arcs (Allègre et al, 2010), the release of Sr from riverine particulate matter (Jones et al, 2013), and uncertain estimates of the lowtemperature alteration of the oceanic crust (Butterfield et al, 2001;Derry, 2009) and groundwater discharge (Basu et al, 2001) all contribute to the divergence of modeling and observations. Furthermore, while the radiogenic Sr isotope system can provide information about the Sr input fluxes to the ocean it is not suitable for quantification of Sr output fluxes.…”

“…The application of the DS-technique in combination with an iterative spike correction algorithm that uses an exponential law for the mass fractionation correction (Krabbenhöft et al, 2009) (Krabbenhöft et al, 2010;Krabbenhöft et al, 2009;Ohno and Hirata, 2007). The conventional radiogenic 87 Sr/ 86 Sr-ratio on JCp-1 is 0.709172±0.000022 (n=32; 2 s.d.…”