Barium isotopes in cold-water corals

Abstract

Recent studies have introduced stable Ba isotopes (δ 138/134 Ba) as a novel tracer for ocean processes. Ba isotopes could potentially provide insight into the oceanic Ba cycle, the ocean's biological pump, watermass provenance in the deep ocean, changes in activity of hydrothermal vents, and land-sea interactions including tracing riverine inputs. Here, we show that aragonite skeletons of various colonial and solitary cold-water coral (CWC) taxa record the seawater (SW) Ba isotope composition. Thirty-six corals of eight different taxa from three oceanic regions were analysed and compared to δ 138/134 Ba measurements of co-located seawater samples. Sites were chosen to cover a wide range of temperature, salinity, Ba concentrations and Ba isotope compositions. Seawater samples at the three sites exhibit the wellestablished anti-correlation between Ba concentration and δ 138/134 Ba. Furthermore, our data set suggests that Ba/Ca values in CWCs are linearly correlated with dissolved [Ba] in ambient seawater, with an average partition coefficient of DCWC/SW = 1.8 ± 0.4 (2SD). The mean isotope fractionation of Ba between seawater and CWCs Δ 138/134 BaCWC-SW is-0.21 ± 0.08‰ (2SD), indicating that CWC aragonite preferentially incorporates the lighter isotopes. This fractionation likely does not depend on temperature or other environmental variables, suggesting that aragonite CWCs could be used to trace the Ba isotope composition in ambient seawater. Coupled [Ba] and δ 138/134 Ba analysis on fossil CWCs has the potential to provide new information about past changes in the local and global relationship between [Ba] and δ 138/134 Ba and hence about the operation of the past global oceanic Ba cycle in different climate regimes.

High‐Precision Barium Isotope Measurements of Carbonates by MC‐ICP‐MS

“…() and Hemsing et al . () were 0.29 ± 0.03‰ and 0.25 ± 0.03‰, respectively. Considering the mass‐dependent fractionation, the calculated δ 137/134 Ba values for these two studies were 0.22 ± 0.02‰ (2 s ) and 0.19 ± 0.02‰ (2 s ), respectively, which were consistent with our results within uncertainty.…”

“…, Hemsing et al . ). This stalagmite may have inherited the Ba isotope signature of the bedrock, Triassic limestone (T 1 J), deposited in the shallow sea of the Palaeo‐Tethys Ocean.…”

“…The average d 137/134 Ba of JCp-1 is 0.21 ± 0.03‰ (2s, n = 16, Table 3). The d 138/134 Ba values for JCp-1 reported in Horner et al (2015) and Hemsing et al (2018) were 0.29 ± 0.03‰ and 0.25 ± 0.03‰, respectively. Considering the mass-dependent fractionation, the calculated d 137/ 134 Ba values for these two studies were 0.22 ± 0.02‰ (2s) and 0.19 ± 0.02‰ (2s), respectively, which were consistent with our results within uncertainty.…”

“…Hemsing et al . () measured Ba isotopes in eight different species of cold‐water coral and found a constant Δ 137/134 Ba coral‐seawater of −0.16 ± 0.06‰, regardless of changes in the growth environment. These two studies demonstrated that Ba isotopes of natural carbonates (e.g., corals) could be used to record the Ba isotope signatures of ambient seawater and to distinguish the different source regions and further help in understanding palaeo‐ocean circulation patterns and palaeoclimate changes.…”

Barium Isotopic Compositions in Thirty‐Four Geological Reference Materials Analysed by MC‐ICP‐MS

“…GBW07104 is a hornblende‐phyric andesite with partial secondary carbonatisation, indicative of at least weak forms of alteration. Secondary carbonatisation seems to have little effect on the Ba and Fe isotope systems of andesite, but a demonstrable effect on the Mg isotopic composition due to extremely low δ 26 Mg of carbonates (low to −5.57‰) (Wombacher et al ) and high δ 138/134 Ba of carbonates (0.11–0.40‰) (Hemsing et al , Zeng et al ).…”