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On the origin of the marine zinc–silicon correlation

On the origin of the marine zinc–silicon correlation
On the origin of the marine zinc–silicon correlation

The close linear correlation between the distributions of dissolved zinc (Zn) and silicon (Si) in seawater has puzzled chemical oceanographers since its discovery almost forty years ago, due to the apparent lack of a mechanism for coupling these two nutrient elements. Recent research has shown that such a correlation can be produced in an ocean model without any explicit coupling between Zn and Si, via the export of Zn-rich biogenic particles in the Southern Ocean, consistent with the observation of elevated Zn quotas in Southern Ocean diatoms. Here, we investigate the physical and biological mechanisms by which Southern Ocean uptake and export control the large-scale marine Zn distribution, using suites of sensitivity simulations in an ocean general circulation model (OGCM) and a box-model ensemble. These simulations focus on the sensitivity of the Zn distribution to the stoichiometry of Zn uptake relative to phosphate (PO4), drawing directly on observations in culture. Our analysis reveals that OGCM model variants that produce a well-defined step between relatively constant, high Zn:PO4 uptake ratios in the Southern Ocean and low Zn:PO4 ratios at lower latitudes fare best in reproducing the marine Zn–Si correlation at both the global and the regional Southern Ocean scale, suggesting the presence of distinct Zn-biogeochemical regimes in the high- and low-latitude oceans that may relate to differences in physiology, ecology or (micro-)nutrient status. Furthermore, a study of the systematics of both the box model and the OGCM reveals that regional Southern Ocean Zn uptake exerts control over the global Zn distribution via its modulation of the biogeochemical characteristics of the surface Southern Ocean. Specifically, model variants with elevated Southern Ocean Zn:PO4 uptake ratios produce near-complete Zn depletion in the Si-poor surface Subantarctic Zone, where upper-ocean water masses with key roles in the global oceanic circulation are formed. By setting the main preformed covariation trend within the ocean interior, the subduction of these Zn- and Si-poor water masses produces a close correlation between the Zn and Si distributions that is barely altered by their differential remineralisation during low-latitude cycling. We speculate that analogous processes in the high-latitude oceans may operate for other trace metal micronutrients as well, splitting the ocean into two fundamentally different biogeochemical, and thus biogeographic, regimes.

diatoms, GEOTRACES, ocean biogeochemistry, Southern Ocean
0012-821X
22-34
de Souza, Gregory F.
7eb06867-ccc4-49d7-81ba-a23f85e82c79
Khatiwala, Samar P.
18c48c52-226d-4451-857f-f6118ed317d0
Hain, Mathis P.
d31486bc-c473-4c34-a814-c0834640876c
Little, Susan H.
e85f87f2-ac3f-461c-bcc7-034a684b93b9
Vance, Derek
9c0575d3-caf4-4d57-b08b-b7a81f6c107c
de Souza, Gregory F.
7eb06867-ccc4-49d7-81ba-a23f85e82c79
Khatiwala, Samar P.
18c48c52-226d-4451-857f-f6118ed317d0
Hain, Mathis P.
d31486bc-c473-4c34-a814-c0834640876c
Little, Susan H.
e85f87f2-ac3f-461c-bcc7-034a684b93b9
Vance, Derek
9c0575d3-caf4-4d57-b08b-b7a81f6c107c

de Souza, Gregory F., Khatiwala, Samar P., Hain, Mathis P., Little, Susan H. and Vance, Derek (2018) On the origin of the marine zinc–silicon correlation. Earth and Planetary Science Letters, 492, 22-34. (doi:10.1016/j.epsl.2018.03.050).

Record type: Article

Abstract

The close linear correlation between the distributions of dissolved zinc (Zn) and silicon (Si) in seawater has puzzled chemical oceanographers since its discovery almost forty years ago, due to the apparent lack of a mechanism for coupling these two nutrient elements. Recent research has shown that such a correlation can be produced in an ocean model without any explicit coupling between Zn and Si, via the export of Zn-rich biogenic particles in the Southern Ocean, consistent with the observation of elevated Zn quotas in Southern Ocean diatoms. Here, we investigate the physical and biological mechanisms by which Southern Ocean uptake and export control the large-scale marine Zn distribution, using suites of sensitivity simulations in an ocean general circulation model (OGCM) and a box-model ensemble. These simulations focus on the sensitivity of the Zn distribution to the stoichiometry of Zn uptake relative to phosphate (PO4), drawing directly on observations in culture. Our analysis reveals that OGCM model variants that produce a well-defined step between relatively constant, high Zn:PO4 uptake ratios in the Southern Ocean and low Zn:PO4 ratios at lower latitudes fare best in reproducing the marine Zn–Si correlation at both the global and the regional Southern Ocean scale, suggesting the presence of distinct Zn-biogeochemical regimes in the high- and low-latitude oceans that may relate to differences in physiology, ecology or (micro-)nutrient status. Furthermore, a study of the systematics of both the box model and the OGCM reveals that regional Southern Ocean Zn uptake exerts control over the global Zn distribution via its modulation of the biogeochemical characteristics of the surface Southern Ocean. Specifically, model variants with elevated Southern Ocean Zn:PO4 uptake ratios produce near-complete Zn depletion in the Si-poor surface Subantarctic Zone, where upper-ocean water masses with key roles in the global oceanic circulation are formed. By setting the main preformed covariation trend within the ocean interior, the subduction of these Zn- and Si-poor water masses produces a close correlation between the Zn and Si distributions that is barely altered by their differential remineralisation during low-latitude cycling. We speculate that analogous processes in the high-latitude oceans may operate for other trace metal micronutrients as well, splitting the ocean into two fundamentally different biogeochemical, and thus biogeographic, regimes.

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More information

Accepted/In Press date: 25 March 2018
e-pub ahead of print date: 10 April 2018
Published date: 15 June 2018
Keywords: diatoms, GEOTRACES, ocean biogeochemistry, Southern Ocean

Identifiers

Local EPrints ID: 422422
URI: http://eprints.soton.ac.uk/id/eprint/422422
ISSN: 0012-821X
PURE UUID: 2d17d862-a21a-4f5b-858b-ca642f7029d8

Catalogue record

Date deposited: 23 Jul 2018 16:31
Last modified: 07 Oct 2020 00:56

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