Constraining the oceanic barium cycle with stable barium isotopes

Cao, Zhimian, Siebert, Christopher, Hathorne, Ed C., Dai, Minhan and Frank, Martin (2016) Constraining the oceanic barium cycle with stable barium isotopes Earth and Planetary Science Letters, 434 . pp. 1-9. DOI 10.1016/j.epsl.2015.11.017.

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Abstract

Highlights

• We present a Ba isotope data set of seawater, river waters and biogenic particles.
• Ba isotope signatures of upper ocean waters are heavier than river and deep waters.
• Adsorption of lighter Ba isotopes on biogenic particles induces the fractionation.
• Ba isotopes trace land–sea interactions and ocean mixing processes.
• Decoupling of Ba from major nutrients confirms Ba to be a biointermediate element.

Abstract

The distribution of barium (Ba) concentrations in seawater resembles that of nutrients and Ba has been widely used as a proxy of paleoproductivity. However, the exact mechanisms controlling the nutrient-like behavior, and thus the fundamentals of Ba chemistry in the ocean, have not been fully resolved. Here we present a set of full water column dissolved Ba (DBa) isotope (δ137BaDBa) profiles from the South China Sea and the East China Sea that receives large freshwater inputs from the Changjiang (Yangtze River). We find pronounced and systematic horizontal and depth dependent δ137BaDBa gradients. Beyond the river influence characterized by generally light signatures (0.0 to +0.3‰+0.3‰), the δ137BaDBa values in the upper water column are significantly higher (+0.9‰+0.9‰) than those in the deep waters (+0.5‰+0.5‰). Moreover, δ137BaDBa signatures are essentially constant in the entire upper 100 m, in which dissolved silicon isotopes are fractionated during diatom growth resulting in the heaviest isotopic compositions in the very surface waters. Combined with the decoupling of DBa concentrations and δ137BaDBa from the concentrations of nitrate and phosphate this implies that the apparent nutrient-like fractionation of Ba isotopes in seawater is primarily induced by preferential adsorption of the lighter isotopes onto biogenic particles rather than by biological utilization. The subsurface δ137BaDBa distribution is dominated by water mass mixing. The application of stable Ba isotopes as a proxy for nutrient cycling should therefore be considered with caution and both biological and physical processes need to be considered. Clearly, however, Ba isotopes show great potential as a new tracer for land–sea interactions and ocean mixing processes.

Document Type: Article
Additional Information: Corrigendum in: Earth and Planetary Science Letters, Volume 530, 15 January 2020, Page 116003 Abstract: After publication of our paper (Cao et al., 2016), we recently detected an unaccounted for matrix effect in the seawater Ba isotope measurements. Because our data were among the first seawater Ba isotope data published, the lack of other published data at that time did not allow us to compare our results and this analytical offset was only discovered when more seawater data became available in the literature. We have now re-analyzed all seawater and river water samples published in Cao et al. (2016) on a Neptune MC-ICP-MS at GEOMAR, Kiel. Unfortunately, the suspended particle samples used for our original paper have been completely consumed. We therefore have analyzed filter samples collected from two additional depths in the upper 150 m at the same station A0. Note that we now present our data in the [Formula presented]Ba notation relative to Ba standard NIST 3104a (Tables 1–3). The re-analyzed seawater data (Table 1) are consistent with published seawater [Formula presented]Ba numbers and their range observed in the oceanic water column (Horner et al., 2015; Bates et al., 2017; Hsieh and Henderson, 2017; Bridgestock et al., 2018; Hemsing et al., 2018; Geyman et al., 2019). We also analyzed the SAFe seawater reference material (GEOTRACES), and our data are indistinguishable within analytical uncertainty from the measurements by two other groups (Hsieh and Henderson, 2017; Geyman et al., 2019). Our re-analysis of river waters and filters are within error consistent with those reported in Cao et al. (2016), with an exception of the Changjiang River which is slightly heavier than the original measurement (Tables 2 and 3). In this corrigendum we present all corrected data in Tables 1–3 and in Figs. 2–5 in accordance with those in Cao et al. (2016). We also point out that the variations and patterns of the data and correspondingly the interpretations and implications of our corrected results remain exactly the same as in our original paper. We acknowledge the invaluable effort and time that Yang Yu (GEOMAR, Kiel) invested into correcting the matrix problem and re-measuring all the samples including the SAFe samples and the new suspended particle samples. We would like to sincerely apologize for any inconvenience caused.
Keywords: seawater Ba isotopes; nutrient-like fractionation; water mass mixing; proxy; oceanic Ba dynamics; RV Dongfanghong II
Research affiliation: OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-P-OZ Paleo-Oceanography
Kiel University
Refereed: Yes
DOI etc.: 10.1016/j.epsl.2015.11.017
ISSN: 0012-821X
Related URLs:
Projects: National Key Scientific Research Project
Date Deposited: 04 Jan 2016 08:19
Last Modified: 07 Jan 2020 13:32
URI: http://eprints.uni-kiel.de/id/eprint/30757

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