Barium isotope fractionation in the global barium cycle: First evidence from barium minerals and precipitation experiments

Abstract

In this study, we present first results from an ongoing investigation on the stable barium (Ba) isotope fractionation in the natural barium cycle. Stable Ba isotope signatures of international IAEA reference materials (synthetic barium sulfate, IAEA-SO-5, -6, and barium carbonate, IAEA-CO-9), natural Ba minerals and experimental Ba precipitates have been analyzed as a first approach to evaluate potential discriminating processes in the global geochemical barium cycle. Ba = − 0.5‰ was found in a diagenetic barite sample from ODP Leg 207. The observed natural discriminations are clearly larger than the analytical uncertainty of the stable isotope measurements, indicating significant isotope discrimination in the natural barium cycle. Precipitation experiments from aqueous barium chloride solutions at temperatures of~21°C and 80°C indicate that the light Ba isotope is enriched in pure barium carbonate and barium sulfate compared to the aqueous solution. A maximum isotope fractionation of − 0.3‰ is observed for both barium carbonate and sulfate, that -in the case of BaCO 3 -seems to be influenced by precipitation rate and/or the aqueous speciation, but less by temperature.

Barium isotopes reveal role of ocean circulation on barium cycling in the Atlantic

“…More negative Ba * in STUW are also associated with +0.1 ‰ increases in dissolved Ba isotope compositions relative to surface waters, consistent with the removal of isotopically-light Ba into BaSO 4 (e.g. Von Allmen et al, 2010). Such a pattern of more negative Ba * associated with heavier Ba isotope compositions in shallow subsurface waters is only expected if two conditions are met: if both the removal of Si and Ba are vertically segregated (e.g.…”

“…The direction of the d 138 Ba anomaly is opposite to that expected if the signature was a result of sinking barite, which would be expected to add Ba with an isotopic composition of~+0.3 ‰ (assuming that BaSO 4 precipitates are approximately À0.2 to À0.4 ‰ lighter than mesopelagic waters; e.g. Von Allmen et al, 2010;Horner et al, 2015;Cao et al, 2016). Instead, this Ba may be sourced from seafloor sediments, which would indicate possible Ba isotopic fractionation effects during sediment diagenesis.…”

“…Barium stable isotope analysis provides a new and powerful approach for investigating Ba cycling in seawater (Horner et al, 2015;Cao et al, 2016), as isotopic tracers are sensitive to ocean mixing and biogeochemistry: the precipitation of barite -a non-conservative process -preferentially incorporates the lighter isotopes of Ba (Von Allmen et al, 2010;Bö ttcher et al, 2012;Miyazaki et al, 2014;Nan et al, 2015), rendering residual waters depleted in Ba and isotopically 'heavier' than before precipitation occurred. In contrast, ocean mixing -a conservative process -does not fractionate isotopic distributions; resultant concentrations and isotopic patterns follow predictable isotopic mixing schemes (Hoefs, 2015).…”

Barium-isotopic fractionation in seawater mediated by barite cycling and oceanic circulation

“…A handful of recent studies have begun examining Ba-isotopic variability in nature, and have identified significant Ba-isotopic fractionation during mineral precipitation experiments (Von Allmen et al, 2010;Böttcher et al, 2012), between various CaCO 3 minerals (Pretet, 2014), and most recently in igneous rocks (Miyazaki et al, 2014). Theoretical predictions support the direction, though not the magnitude, of Ba-isotopic fractionation observed during low-temperature mineral precipitation, which may be related to Ba 2+ ion desolvation (Hofmann et al, 2012).…”

The control of carbonate mineral Mg isotope composition by aqueous speciation: Theoretical and experimental modeling

“…This is observed for calcium and magnesium isotopes in calcite and magnesite, respectively (i.e. Tang et al, 2008;Pearce et al, 2012) and for Sr and Ba isotopes in calcite (Boehm et al, 2012;von Allmen et al, 2010). In contrast, increasing calcite precipitation rates produces a less negative Δ 26 Mg calcite-fluid fractionation, as it reduces the preferential incorporation of 24 Mg into the lattice.…”