Lithium isotopic composition of benthic foraminifera: A new proxy for paleo-pH reconstruction

Roberts, J. and Kaczmarek, K. and Langer, G. and Skinner, L. C. and Bijma, J. and Bradbury, H. and Turchyn, A. V. and Lamy, F. and Misra, S. (2018) Lithium isotopic composition of benthic foraminifera: A new proxy for paleo-pH reconstruction. Geochimica Et Cosmochimica Acta. ISSN 0016-7037 DOI https://doi.org/10.1016/j.gca.2018.02.038

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Official URL: http://doi.org/10.1016/j.gca.2018.02.038

Abstract

The lithium isotopic composition of foraminifera is an established tracer of long-term changes in the global silicate weathering cycle, following the assumption that foraminifera faithfully record the lithium isotopic composition (δ7Li) of seawater. In this study, we demonstrate by utilizing benthic foraminifera (Amphistegina lessonii) that were cultured under decoupled pH-[CO32-] conditions, that foraminifera δ7Li is strongly dependent on pH. This is reinforced with δ7Li data from globally distributed core-top samples of Cibicidoides mundulus and Cibicidoides wuellerstorfi, which show the same negative correlation with pH. The dependency of δ7Li on pH is perhaps a surprising result given that lithium speciation in seawater is independent of both pH and carbonate ion speciation. The dependence of lithium incorporation on growth rate was assessed by measuring the calcium isotopic composition; no growth rate dependent incorporation was observed. Instead, we propose that the strength of the 6Li and 7Li hydration spheres (and hence their respective desolvation energy) is pH-dependent, resulting in a significant isotopic fractionation during the incorporation of lithium into foraminifer calcite. The core-top derived δ7Li-pH calibration is used to demonstrate the applicability of this δ11B-independent pH proxy in reconstructing deglacial variations in pH in the South Pacific. The use of foraminifera δ7Li to compliment δ11B-based pH reconstructions has the potential to provide insight into time-dependent variations in porewater/seawater δ11B, temperature and salinity, which were previously unresolvable.

Item Type: Article
Uncontrolled Keywords: 2018AREP; IA74
Subjects: 01 - Climate Change and Earth-Ocean Atmosphere Systems
Divisions: 01 - Climate Change and Earth-Ocean Atmosphere Systems
08 - Green Open Access
12 - PhD
Journal or Publication Title: Geochimica Et Cosmochimica Acta
Identification Number: https://doi.org/10.1016/j.gca.2018.02.038
Depositing User: Sarah Humbert
Date Deposited: 22 Mar 2018 14:14
Last Modified: 08 Jan 2019 11:53
URI: http://eprints.esc.cam.ac.uk/id/eprint/4134

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