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Constraining the evolution of Neogene ocean carbonate chemistry using the boron isotope pH proxy

Constraining the evolution of Neogene ocean carbonate chemistry using the boron isotope pH proxy
Constraining the evolution of Neogene ocean carbonate chemistry using the boron isotope pH proxy
Over the course of the Neogene, the Earth underwent profound climatic shifts from the sustained warmth of the middle Miocene to the development of Plio-Pleistocene glacial–interglacial cycles. Major perturbations in the global carbon cycle have occurred alongside these shifts, however the lack of long-term carbonate system reconstructions currently limits our understanding of the link between changes in CO2, carbon cycling, and climate over this time interval. Here we reconstruct continuous surface ocean pH, CO2, and surface ocean aragonite saturation state using boron isotopes from the planktonic foraminifer Trilobatus trilobus and we perform a sensitivity analysis of the key variables in our calculations (e.g. Bsw, [Ca]sw, CCD). We show that the choice of sw influences both seawater pH and CO2 while [Ca]sw reconstructed dissolved inorganic carbon exerts a significant influence only on CO2. Over the last 22 Myr, the lowest pH levels occurred in the Middle Miocene Climate Optimum (MMCO; 17–14 Myr ago) reaching units in all our scenarios. The extended warmth of the MMCO corresponds to mean CO2 and aragonite saturation state levels of 470–630 ppm and 2.7–3.5, respectively. Despite a general correspondence between our CO2 record and climate, all CO2 scenarios show a peak at ∼9 Ma not matched by corresponding changes in climate reconstructions. This may suggest decoupling (i.e. significant CO2 change without a discernible climate response) for a limited interval in the Late Miocene (11.6–8.5 Ma), although further refinement of our understanding of the temporal evolution of the boron isotopic composition of seawater is necessary to fully evaluate the nature of the relationship between CO2 and climate. Nonetheless, from our long-term view it is clear that low-latitude open ocean marine ecosystems are unlikely to have experienced sustained surface pH and saturation levels below 7.7 and 1.7, respectively, during the past 14 million
0012-821X
362-376
Sosdian, S.M.
718c374a-7460-4c7e-a616-f93b1f0a46a5
Greenop, R.
9a08d945-03bb-41b9-b8f2-f6e84731057e
Hain, M.P.
d31486bc-c473-4c34-a814-c0834640876c
Foster, G.L.
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Pearson, P.N.
46e74b33-1228-4c0a-adc3-231052d60036
Lear, C.H.
c0caf74d-ed6c-4b04-b5c8-de8fc794943c
Sosdian, S.M.
718c374a-7460-4c7e-a616-f93b1f0a46a5
Greenop, R.
9a08d945-03bb-41b9-b8f2-f6e84731057e
Hain, M.P.
d31486bc-c473-4c34-a814-c0834640876c
Foster, G.L.
fbaa7255-7267-4443-a55e-e2a791213022
Pearson, P.N.
46e74b33-1228-4c0a-adc3-231052d60036
Lear, C.H.
c0caf74d-ed6c-4b04-b5c8-de8fc794943c

Sosdian, S.M., Greenop, R., Hain, M.P., Foster, G.L., Pearson, P.N. and Lear, C.H. (2018) Constraining the evolution of Neogene ocean carbonate chemistry using the boron isotope pH proxy. Earth and Planetary Science Letters, 498, 362-376. (doi:10.1016/j.epsl.2018.06.017).

Record type: Article

Abstract

Over the course of the Neogene, the Earth underwent profound climatic shifts from the sustained warmth of the middle Miocene to the development of Plio-Pleistocene glacial–interglacial cycles. Major perturbations in the global carbon cycle have occurred alongside these shifts, however the lack of long-term carbonate system reconstructions currently limits our understanding of the link between changes in CO2, carbon cycling, and climate over this time interval. Here we reconstruct continuous surface ocean pH, CO2, and surface ocean aragonite saturation state using boron isotopes from the planktonic foraminifer Trilobatus trilobus and we perform a sensitivity analysis of the key variables in our calculations (e.g. Bsw, [Ca]sw, CCD). We show that the choice of sw influences both seawater pH and CO2 while [Ca]sw reconstructed dissolved inorganic carbon exerts a significant influence only on CO2. Over the last 22 Myr, the lowest pH levels occurred in the Middle Miocene Climate Optimum (MMCO; 17–14 Myr ago) reaching units in all our scenarios. The extended warmth of the MMCO corresponds to mean CO2 and aragonite saturation state levels of 470–630 ppm and 2.7–3.5, respectively. Despite a general correspondence between our CO2 record and climate, all CO2 scenarios show a peak at ∼9 Ma not matched by corresponding changes in climate reconstructions. This may suggest decoupling (i.e. significant CO2 change without a discernible climate response) for a limited interval in the Late Miocene (11.6–8.5 Ma), although further refinement of our understanding of the temporal evolution of the boron isotopic composition of seawater is necessary to fully evaluate the nature of the relationship between CO2 and climate. Nonetheless, from our long-term view it is clear that low-latitude open ocean marine ecosystems are unlikely to have experienced sustained surface pH and saturation levels below 7.7 and 1.7, respectively, during the past 14 million

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Accepted/In Press date: 13 June 2018
e-pub ahead of print date: 20 July 2018
Published date: 15 September 2018

Identifiers

Local EPrints ID: 423120
URI: http://eprints.soton.ac.uk/id/eprint/423120
ISSN: 0012-821X
PURE UUID: 0cd9f4c2-cc72-431c-88db-e40b9040b4f0
ORCID for G.L. Foster: ORCID iD orcid.org/0000-0003-3688-9668

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Date deposited: 14 Aug 2018 16:30
Last modified: 26 Nov 2021 02:55

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Contributors

Author: S.M. Sosdian
Author: R. Greenop
Author: M.P. Hain
Author: G.L. Foster ORCID iD
Author: P.N. Pearson
Author: C.H. Lear

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