The University of Southampton
University of Southampton Institutional Repository

The effects of secular calcium and magnesium concentration changes on the thermodynamics of seawater acid/base chemistry: Implications for Eocene and Cretaceous ocean carbon chemistry and buffering

The effects of secular calcium and magnesium concentration changes on the thermodynamics of seawater acid/base chemistry: Implications for Eocene and Cretaceous ocean carbon chemistry and buffering
The effects of secular calcium and magnesium concentration changes on the thermodynamics of seawater acid/base chemistry: Implications for Eocene and Cretaceous ocean carbon chemistry and buffering
Reconstructed changes in seawater calcium and magnesium concentration ([Ca2+], [Mg2+]) predictably affect the ocean’s acid/base and carbon chemistry. Yet inaccurate formulations of chemical equilibrium “constants” are currently in use to account for these changes. Here we develop an efficient implementation of the MIAMI Ionic InteractionModel to predict all chemical equilibrium constants required for carbon chemistry calculations under variable [Ca2+] and [Mg2+]. We investigate the impact of [Ca2+] and [Mg2+] on the relationships among the ocean’s pH,CO2, dissolved inorganic carbon (DIC), saturation state of CaCO3 (?), and buffer capacity. Increasing [Ca2+] and/or [Mg2+] enhances “ion pairing,” which increases seawater buffering by increasing the concentration ratio of total to “free” (uncomplexed) carbonate ion. An increase in [Ca2+], however, also causes a decline in carbonate ion to maintain a given ?, thereby overwhelming the ion pairing effect and decreasing seawater buffering. Given the reconstructions of Eocene [Ca2+] and [Mg2+] ([Ca2+] ~20mM; [Mg2+]~30mM), Eocene seawater would have required essentially the same DIC as today to simultaneously explain a similar-to-modern ? and the estimated Eocene atmospheric CO2 of ~1000 ppm. During the Cretaceous, at ~4 times modern [Ca2+], ocean buffering would have been at a minimum. Overall, during times of high seawater [Ca2+], CaCO3 saturation, pH, and atmospheric CO2 were more susceptible to perturbations of the global carbon cycle. For example, given both Eocene and Cretaceous seawater [Ca2+] and [Mg2+], a doubling of atmospheric CO2 would require less carbon addition to the ocean/atmosphere system than under modern seawater composition. Moreover, increasing seawater buffering since the Cretaceous may have been a driver of evolution by raising energetic demands of biologically controlled calcification and CO2 concentration mechanisms that aid photosynthesis.
ion pairing, carbon chemistry, seawater calcium, seawater buffering, carbon dioxide, acidification
0886-6236
517-533
Hain, Mathis P.
d31486bc-c473-4c34-a814-c0834640876c
Sigman, D.M.
84ca0cf1-decc-4133-b104-4fc3ee751721
Higgins, J.A.
8303e3de-3881-4432-bbb2-27cd383cd360
Haug, G.H.
39dd0949-c65e-4f26-ad5f-41ddb5bafadb
Hain, Mathis P.
d31486bc-c473-4c34-a814-c0834640876c
Sigman, D.M.
84ca0cf1-decc-4133-b104-4fc3ee751721
Higgins, J.A.
8303e3de-3881-4432-bbb2-27cd383cd360
Haug, G.H.
39dd0949-c65e-4f26-ad5f-41ddb5bafadb

Hain, Mathis P., Sigman, D.M., Higgins, J.A. and Haug, G.H. (2015) The effects of secular calcium and magnesium concentration changes on the thermodynamics of seawater acid/base chemistry: Implications for Eocene and Cretaceous ocean carbon chemistry and buffering. Global Biogeochemical Cycles, 29 (5), 517-533. (doi:10.1002/2014GB004986).

Record type: Article

Abstract

Reconstructed changes in seawater calcium and magnesium concentration ([Ca2+], [Mg2+]) predictably affect the ocean’s acid/base and carbon chemistry. Yet inaccurate formulations of chemical equilibrium “constants” are currently in use to account for these changes. Here we develop an efficient implementation of the MIAMI Ionic InteractionModel to predict all chemical equilibrium constants required for carbon chemistry calculations under variable [Ca2+] and [Mg2+]. We investigate the impact of [Ca2+] and [Mg2+] on the relationships among the ocean’s pH,CO2, dissolved inorganic carbon (DIC), saturation state of CaCO3 (?), and buffer capacity. Increasing [Ca2+] and/or [Mg2+] enhances “ion pairing,” which increases seawater buffering by increasing the concentration ratio of total to “free” (uncomplexed) carbonate ion. An increase in [Ca2+], however, also causes a decline in carbonate ion to maintain a given ?, thereby overwhelming the ion pairing effect and decreasing seawater buffering. Given the reconstructions of Eocene [Ca2+] and [Mg2+] ([Ca2+] ~20mM; [Mg2+]~30mM), Eocene seawater would have required essentially the same DIC as today to simultaneously explain a similar-to-modern ? and the estimated Eocene atmospheric CO2 of ~1000 ppm. During the Cretaceous, at ~4 times modern [Ca2+], ocean buffering would have been at a minimum. Overall, during times of high seawater [Ca2+], CaCO3 saturation, pH, and atmospheric CO2 were more susceptible to perturbations of the global carbon cycle. For example, given both Eocene and Cretaceous seawater [Ca2+] and [Mg2+], a doubling of atmospheric CO2 would require less carbon addition to the ocean/atmosphere system than under modern seawater composition. Moreover, increasing seawater buffering since the Cretaceous may have been a driver of evolution by raising energetic demands of biologically controlled calcification and CO2 concentration mechanisms that aid photosynthesis.

Text
Hain_et_al_2015_GBC.pdf - Version of Record
Download (1MB)

More information

Accepted/In Press date: 24 March 2015
e-pub ahead of print date: 4 May 2015
Published date: 4 May 2015
Keywords: ion pairing, carbon chemistry, seawater calcium, seawater buffering, carbon dioxide, acidification
Organisations: Ocean and Earth Science

Identifiers

Local EPrints ID: 376746
URI: http://eprints.soton.ac.uk/id/eprint/376746
ISSN: 0886-6236
PURE UUID: a70838ca-779d-4127-bff7-7ba6fab2ed9d

Catalogue record

Date deposited: 06 May 2015 08:56
Last modified: 08 Jan 2022 03:17

Export record

Altmetrics

Contributors

Author: Mathis P. Hain
Author: D.M. Sigman
Author: J.A. Higgins
Author: G.H. Haug

Download statistics

Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.

View more statistics

Atom RSS 1.0 RSS 2.0

Contact ePrints Soton: eprints@soton.ac.uk

ePrints Soton supports OAI 2.0 with a base URL of http://eprints.soton.ac.uk/cgi/oai2

This repository has been built using EPrints software, developed at the University of Southampton, but available to everyone to use.

We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we will assume that you are happy to receive cookies on the University of Southampton website.

×