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Barium-isotopic fractionation in seawater mediated by barite cycling and oceanic circulation (2015)

Horner, Tristan J. ; Kinsley, Christopher W. ; Nielsen, Sune G.

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Publication Date: 2022-05-25

Description: Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters, 430 (2015): 511-522, doi:10.1016/j.epsl.2015.07.027.

Description: The marine biogeochemical cycle of Ba is thought to be controlled by particulate BaSO4 (barite) precipitation associated with the microbial oxidation of organic carbon and its subsequent dissolution in the BaSO4-undersaturated water column. Despite many of these processes being largely unique to Ba cycling, concentrations of Ba and Si in seawater exhibit a strong linear correlation. The reasons for this correlation are ambiguous, as are the depth ranges corresponding to the most active BaSO4 cycling and the intermediate sources of Ba to particulate BaSO4. Stable isotopic analyses of dissolved Ba in seawater should help address these issues, as Ba-isotopic compositions are predicted to be sensitive to the physical and biogeochemical process that cycle Ba. We report a new methodology for the determination of dissolved Ba-isotopic compositions in seawater and results from a 4, 500 m depth profile in the South Atlantic at 39.99 S, 0.92 E that exhibit oceanographically-consistent variation with depth. These data reveal that water masses obtain their [Ba] and Ba-isotopic signatures when at or near the surface, which relates to the cycling of marine BaSO4. The shallow origin of these signatures requires that the substantial Ba-isotopic variations in the bathypelagic zone were inherited from when those deep waters were last ventilated. Indeed, the water column below 600 m is well explained by conservative mixing of water masses with distinct [Ba] and Ba-isotopic compositions. This leads us to conclude that large scale oceanic circulation is important for sustaining the similar oceanographic distributions of Ba and Si in the South Atlantic, and possibly elsewhere. These data demonstrate that the processes of organic carbon oxidation, BaSO4 cycling, and Ba-isotopic fractionation in seawater are closely coupled, such that Ba-isotopic analyses harbor great potential as a tracer of the carbon cycle in the modern and paleo-oceans.

Description: T.J.H. acknowledges support from Makoto A. Saito (Gordon and Betty Moore Foundation; Project # 3782) and the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution, with funding provided by the Doherty Foundation. Development of Ba-isotopic protocols at NIRVANA was made possible with funding from The Andrew W. Mellon Foundation Endowed Fund for Innovative Research (T.J.H. and S.G.N.).

Keywords: Barium ; Isotopic fractionation ; Barite ; Seawater ; Biogeochemistry

Repository Name: Woods Hole Open Access Server

Type: Preprint

Format: application/pdf

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Barite precipitation on suspended organic matter in the mesopelagic zone (2020)

Martinez-Ruiz, Francisca ; Paytan, Adina ; Gonzalez-Muñoz, Maria Teresa ; [et al.]

Frontiers Media

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Publication Date: 2022-10-26

Description: © The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Martinez-Ruiz, F., Paytan, A., Gonzalez-Munoz, M. T., Jroundi, F., Abad, M. M., Lam, P. J., Horner, T. J., & Kastner, M. Barite precipitation on suspended organic matter in the mesopelagic zone. Frontiers in Earth Science, 8, (2020): 567714, doi:10.3389/feart.2020.567714.

Description: Mechanisms underlying barite precipitation in seawater and the precise depths of barite precipitation in the water column have been debated for decades. Here we present a detailed study of water column barite distribution in the mesopelagic zone at diverse stations in the open ocean by analyzing samples collected using multiple unit large volume in-situ filtration systems in the Pacific, Atlantic and Indian oceans. Our results demonstrate that barite is an organo-mineral particularly abundant at intermediate depths throughout the world’s ocean regardless of saturation state with respect to barite. This is confirming the notion of precipitation at depths of intense organic matter mineralization. Our observations further support the link between barite formation and microbial activity, demonstrated by the association of barite particles with organic matter aggregates and with extracellular polymeric substances. Evidence for microbial mediation is consistent with previous experimental work showing that in bacterial biofilms Ba binds to phosphate groups on cell surfaces and within extracellular polymeric substances. This organo-accumulation promotes high concentrations of Ba leading to saturated microenvironments and nucleation sites favoring precipitation. The distribution of Ba isotopes in the water column and in particulate matter is also consistent with the proposed precipitation mechanism.

Description: This study was supported by the European Regional Development Fund (ERDF) co-financed grants CGL2017-92600-EXP and PID2019-104624RB-I00 (Agencia Estatal de Investigación, Ministerio de Ciencia e Innovación, Spain), Research Groups RNM-179 and BIO 103, and Excellence Projects P18-RT-3804 and P18-RT-4074 (Junta de Andalucía), Unidad Científica de Excelencia UCE-PP2016-05 (University of Granada) and grant OCE-1443577.

Keywords: Pelagic barite ; Organo-mineralization ; Barite saturation state ; Extracellular polymeric substances ; Bioaccumulation