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The Li isotope composition of marine biogenic carbonates: Patterns and Mechanisms

Dellinger, Mathieu and West, A. Joshua and Paris, Guillaume and Adkins, Jess F. and von Strandmann, Philip-Pogge and Ullmann, Clemens V. and Eagle, Robert A. and Freitas, Pedro and Bagard, Marie-Laure and Ries, Justin B. and Corsetti, Frank A. and Pérez-Huerta, Alberto and Kampf, Anthony R. (2018) The Li isotope composition of marine biogenic carbonates: Patterns and Mechanisms. Geochimica et Cosmochimica Acta, 236 . pp. 315-335. ISSN 0016-7037. https://resolver.caltech.edu/CaltechAUTHORS:20180320-152210071

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Abstract

Little is known about the fractionation of Li isotopes during formation of biogenic carbonates, which form the most promising geological archives of past seawater composition. Here we investigated the Li isotope composition (δ^7Li) and Li/Ca ratios of organisms that are abundant in the Phanerozoic record: mollusks (mostly bivalves), echinoderms, and brachiopods. The measured samples include (i) modern calcite and aragonite shells from various species and natural environments (13 mollusk samples, 5 brachiopods and 3 echinoderms), and (ii) shells from mollusks grown under controlled conditions at various temperatures. When possible, the mollusk shell ultrastructure was micro-sampled in order to assess intra-shell heterogeneity. In this paper, we systematically characterize the influence of mineralogy, temperature, and biological processes on the δ^7Li and Li/Ca of these shells and compare with published data for other taxa (foraminifera and corals). Aragonitic mollusks have the lowest δ^7Li, ranging from +16 to +22‰, echinoderms have constant δ^7Li of about +24‰, brachiopods have δ^7Li of +25 to +28‰, and finally calcitic mollusks have the largest range and highest δ^7Li values, ranging from +25‰ to +40‰. Measured brachiopods have similar δ7Li compared to inorganic calcite precipitated from seawater (δ^7Li of +27 to +29‰), indicating minimum influence of vital effects, as also observed for other isotope systems and making them a potentially viable proxy of past seawater composition. Calcitic mollusks, on the contrary, are not a good archive for seawater paleo–δ^7Li because many samples have significantly higher δ^7Li values than inorganic calcite and display large inter-species variability, which suggests large vital effects. In addition, we observe very large intra-shell variability, in particular for mixed calcite-aragonite shells (over 20‰ variability), but also in mono-mineralic shells (up to 12‰ variability). Aragonitic bivalves have less variable δ^7Li (7‰ variability) compared to calcitic mollusks, but with significantly lower δ^7Li compared to inorganic aragonite, indicating the existence of vital effects. Bivalves grown at various temperatures show that temperature has only a minor influence on fractionation of Li isotopes during shell precipitation. Interestingly, we observe a strong correlation (R^2 = 0.83) between the Li/Mg ratio in bivalve Mytilus edulisand temperature, with potential implications for paleo-temperature reconstructions. Finally, we observe a negative correlation between the δ^7Li and both the Li/Ca and Mg/Ca ratio of calcite mollusks, which we relate to biomineralization processes. To explain this correlation, we propose preferential removal of ^6Li from the calcification site of calcite mollusks by physiological processes corresponding to the regulation of the amount of Mg in the calcifying medium. We calculate that up to 80% of the initial Li within the calcification site is removed by this process, leading to high δ^7Li and low Li/Ca in some calcite mollusk specimens. Collectively, these results suggest that Mg (and thus [Li]) is strongly biologically controlled within the calcifying medium of calcite mollusks. Overall, the results of this study show that brachiopods are likely to be suitable targets for future work on the determination of paleo-seawater Li isotope composition—an emerging proxy for past weathering and hydrothermal processes.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1016/j.gca.2018.03.014DOIArticle
ORCID:
AuthorORCID
West, A. Joshua0000-0001-6909-1471
Paris, Guillaume0000-0001-8368-1224
Adkins, Jess F.0000-0002-3174-5190
Kampf, Anthony R.0000-0001-8084-2563
Additional Information:© 2018 Elsevier Ltd. Received 30 August 2017, Accepted 10 March 2018, Available online 17 March 2018. This work was primarily supported by the American Chemical Society Petroleum Research Fund (award 53418-DNI2 to AJW). We thank the Natural History Museum of Los Angeles County for providing bivalve samples. We thank Jonathan Erez and an anonymous reviewer for their constructive comments on the manuscript. MD acknowledges financial support from Durham University and a Marie Curie COFUND International Junior research Fellowship held at Durham University. RAE and JBR acknowledge support from NSF grants OCE #1437166 and 1437371. PPvS and CVU analyzed C. gigas from the List Basin with support from NERC Advanced Fellowship NE/I020571/2 and ERC Consolidator grant 682760 - CONTROLPASTCO2.
Funders:
Funding AgencyGrant Number
American Chemical Society Petroleum Research Fund53418-DNI2
NSFOCE-1437166
NSFOCE-1437371
Natural Environment Research Council (NERC)NE/I020571/2
European Research Council (ERC)682760
Subject Keywords:lithium isotopes; biogenic carbonates; brachiopods; mollusks; biomineralization; Li/Mg ratio
Record Number:CaltechAUTHORS:20180320-152210071
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20180320-152210071
Official Citation:Mathieu Dellinger, A. Joshua West, Guillaume Paris, Jess F. Adkins, Philip A.E. Pogge von Strandmann, Clemens V. Ullmann, Robert A. Eagle, Pedro Freitas, Marie-Laure Bagard, Justin B. Ries, Frank A. Corsetti, Alberto Perez-Huerta, Anthony R. Kampf, The Li isotope composition of marine biogenic carbonates: Patterns and mechanisms, Geochimica et Cosmochimica Acta, Volume 236, 2018, Pages 315-335, ISSN 0016-7037, https://doi.org/10.1016/j.gca.2018.03.014.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:85385
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:26 Mar 2018 22:10
Last Modified:18 Nov 2020 22:57

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