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. In: GEOCHIMICA ET COSMOCHIMICA ACTA, 236 (SI). pp. 336-350.

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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 (delta Li-7) of seawater. In this study, we demonstrate by utilising benthic foraminifera (Amphistegina lessonii) that were cultured under decoupled pH-CO32-] conditions, that foraminifera delta Li-7 is strongly dependent on pH. This is reinforced with delta Li-7 data from globally distributed core-top samples of Cibicidoides mundulus and Cibicidoides wuellerstorfi, which show the same negative correlation with pH. The dependency of delta Li-7 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 Li-6 and Li-7 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 delta Li-7-pH calibration is used to demonstrate the applicability of this delta B-11-independent pH proxy in reconstructing deglacial variations in pH in the South Pacific. The use of foraminifera delta Li-7 to compliment delta B-11-based pH reconstructions has the potential to provide insight into time-dependent variations in porewater/seawater delta B-11, temperature and salinity, which were previously unresolvable. (C) 2018 Elsevier Ltd. All rights reserved.

Item Type:	Journal Article
Publication:	GEOCHIMICA ET COSMOCHIMICA ACTA
Publisher:	PERGAMON-ELSEVIER SCIENCE LTD, THE BOULEVARD, LANGFORD LANE, KIDLINGTON, OXFORD OX5 1GB, ENGLAND
Additional Information:	Copy right for this article belong to PERGAMON-ELSEVIER SCIENCE LTD, THE BOULEVARD, LANGFORD LANE, KIDLINGTON, OXFORD OX5 1GB, ENGLAND
Department/Centre:	Division of Mechanical Sciences > Centre for Earth Sciences
Date Deposited:	04 Sep 2018 15:49
Last Modified:	04 Sep 2018 15:49
URI:	http://eprints.iisc.ac.in/id/eprint/60574

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