Deep time foraminifera Mg/Ca paleothermometry: nonlinear correction for secular change in seawater Mg/Ca
Deep time foraminifera Mg/Ca paleothermometry: nonlinear correction for secular change in seawater Mg/Ca
[1] The Mg/Ca ratio of foraminifera tests is increasingly being utilized as a paleotemperature proxy. Deep time (pre-Pleistocene) Mg/Ca paleothermometry is complicated by the fact that the Mg/Ca ratio of seawater (Mg/Casw) has undergone considerable secular variation over the Cenozoic. Previous studies have corrected for this by assuming an invariant Mg distribution coefficient (DMg) with Mg/Casw. More recent laboratory culturing has shown that this is not the case, demonstrating that a power relationship best describes the variation in test Mg/Ca (Mg/Catest) with Mg/Casw. Therefore, previous corrections are likely to have led to inaccurate temperature reconstructions. Here, we show how the systematics of such a correction should be applied and demonstrate why this provides good evidence that the Mg/Ca ratio of Paleogene seawater was lower than previously implied by foraminiferal constraints, in agreement with the majority of the proxy Mg/Casw evidence. We also demonstrate how it is indirectly possible to constrain the value of H, the power component of a Mg/Catest–Mg/Casw calibration, potentially enabling the appropriate correction of results derived from species where this relationship has not been calibrated. However, this technique should not be treated as a substitute for culturing. The previous erroneous assumptions regarding both (1) the relationship between Mg/Catest and Mg/Caswand (2) the Mg/Ca ratio of seawater at a given time in the past may counteract each other to differing extents. As a result, previous absolute pre-Pleistocene paleotemperature estimates derived from Mg/Ca ratios in foraminifera should be treated with caution, although relative temperature changes over short (<1 Ma timescales) are likely to be reliable.
Evans, David
878c65c7-eab9-4362-896b-166e165eb94b
Mller, Wolfgang
b45cba6e-2a6d-4d96-bf25-dd4ce89ecd8e
10 November 2012
Evans, David
878c65c7-eab9-4362-896b-166e165eb94b
Mller, Wolfgang
b45cba6e-2a6d-4d96-bf25-dd4ce89ecd8e
Evans, David and Mller, Wolfgang
(2012)
Deep time foraminifera Mg/Ca paleothermometry: nonlinear correction for secular change in seawater Mg/Ca.
Paleoceanography, 27 (4).
(doi:10.1029/2012PA002315).
Abstract
[1] The Mg/Ca ratio of foraminifera tests is increasingly being utilized as a paleotemperature proxy. Deep time (pre-Pleistocene) Mg/Ca paleothermometry is complicated by the fact that the Mg/Ca ratio of seawater (Mg/Casw) has undergone considerable secular variation over the Cenozoic. Previous studies have corrected for this by assuming an invariant Mg distribution coefficient (DMg) with Mg/Casw. More recent laboratory culturing has shown that this is not the case, demonstrating that a power relationship best describes the variation in test Mg/Ca (Mg/Catest) with Mg/Casw. Therefore, previous corrections are likely to have led to inaccurate temperature reconstructions. Here, we show how the systematics of such a correction should be applied and demonstrate why this provides good evidence that the Mg/Ca ratio of Paleogene seawater was lower than previously implied by foraminiferal constraints, in agreement with the majority of the proxy Mg/Casw evidence. We also demonstrate how it is indirectly possible to constrain the value of H, the power component of a Mg/Catest–Mg/Casw calibration, potentially enabling the appropriate correction of results derived from species where this relationship has not been calibrated. However, this technique should not be treated as a substitute for culturing. The previous erroneous assumptions regarding both (1) the relationship between Mg/Catest and Mg/Caswand (2) the Mg/Ca ratio of seawater at a given time in the past may counteract each other to differing extents. As a result, previous absolute pre-Pleistocene paleotemperature estimates derived from Mg/Ca ratios in foraminifera should be treated with caution, although relative temperature changes over short (<1 Ma timescales) are likely to be reliable.
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Accepted/In Press date: 21 September 2012
Published date: 10 November 2012
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Local EPrints ID: 502323
URI: http://eprints.soton.ac.uk/id/eprint/502323
ISSN: 0883-8305
PURE UUID: 375ffd84-99e7-43c3-a88b-871c7f8bde88
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Date deposited: 23 Jun 2025 16:34
Last modified: 24 Jun 2025 02:11
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Author:
David Evans
Author:
Wolfgang Mller
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