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Published March 23, 2021 | Supplemental Material + Published
Journal Article Open

Uranium Distribution and Incorporation Mechanism in Deep-Sea Corals: Implications for Seawater [CO₃²⁻] Proxies

Chen, Sang ORCID icon
Littley, Eloise F. M.
Rae, James W. B. ORCID icon
Charles, Christopher D.
Adkins, Jess F. ORCID icon

Abstract

A conservative element in seawater, uranium is readily incorporated into the aragonitic skeletons of scleractinian corals, making them an important paleoclimate archive that can be absolutely dated with U-Th techniques. In addition, uranium concentrations (U/Ca ratios) in corals have been suggested to be influenced by the temperature and/or carbonate ion concentration of the ambient seawater based on empirical calibrations. Microsampling techniques have revealed strong heterogeneities in U/Ca within individual specimens in both surface and deep-sea corals, suggesting a biological control on the U incorporation into the skeletons. Here we further explore the mechanism of uranium incorporation in coral skeletons with the deep-sea species Desmophyllum dianthus, an ideal test organism for the biomineralization processes due to its relatively constant growth environment. We find a negative correlation between bulk coral U/Ca and temperature as well as ambient pH and [CO₃²⁻] that is consistent with previous studies. By sampling the growth bands of individual corals, we also find a twofold change in U/Ca within individual corals that is strongly correlated with the δ¹⁸O, δ¹³C, and other Me/Ca ratios of the bands. A similar correlation between U/Ca and stable isotopes as well as other Me/Ca ratios are observed in bulk deep-sea coral samples. With a numerical coral calcification model, we interpret the U/Ca-stable isotope correlation as a result of changes in uranium speciation in response to internal pH elevations in the extracellular calcifying fluid (ECF) of the corals, and suggest that the Ca₂UO₂ (CO₃)₃ (aq) complex, the dominant U species in seawater, may be the major species incorporated into the coral skeleton. Therefore, the correlation between U/Ca and ambient [CO₃²⁻] is likely a result of the response of the biomineralization process, especially the magnitude of internal pH elevation, to the growth environment of the corals. Our data suggest overall lower alkalinity pump rates in corals from low saturation seawater compared to those from high saturation seawater, and possible increases in Ca²⁺ supply from active pumping relative to seawater transport in response to the environmental stress of low saturation.

Additional Information

© 2021 Chen, Littley, Rae, Charles and Adkins. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. Received: 14 December 2020; Accepted: 22 February 2021; Published: 23 March 2021. We would like to thank Jessica Crumpton-Banks, Jared Marske, and Grecia Ames for assistance in the lab. We would also like to thank Joe Stewart and Russell Day for providing standards for the geochemical analyses. We are indebted to co-editors Guillaume Paris, AG, Claire Rollion-Bard, and Oscar Branson for consideration of our work in this special issue of biomineralization. The manuscript benefited from comments by two reviewers. Data Availability Statement: The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s. Author Contributions: SC, JR, and JA designed the research. SC prepared the coral samples. SC, EL, JR, and CC performed the geochemical analyses. SC, EL, JR, and JA analyzed the data. All authors contributed to the writing of the manuscript. This research received funding from NSF grant OCE-1737404 awarded to JA and China Scholarship Council Ph.D. Scholarship 201508020007 awarded to SC. JR was supported by ERC Starting Grant 805246 OldCO2NewArchives. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The handling editor declared a past co-authorship with JA and JR.

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Supplemental Material - Table_1_Uranium_Distribution_and_Incorporation_Mechanism_in_Deep-Sea_Corals__Implications_for_Seawater__CO32–__Proxies.XLSX

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