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Distal and proximal controls on the silicon stable isotope signature of North Atlantic Deep Water

Distal and proximal controls on the silicon stable isotope signature of North Atlantic Deep Water
Distal and proximal controls on the silicon stable isotope signature of North Atlantic Deep Water
It has been suggested that the uniquely high ?30Si signature of North Atlantic Deep Water (NADW) results from the contribution of isotopically fractionated silicic acid by mode and intermediate waters that are formed in the Southern Ocean and transported to the North Atlantic within the upper limb of the meridional overturning circulation (MOC). Here, we test this hypothesis in a suite of ocean general circulation models (OGCMs) with widely varying MOCs and related pathways of nutrient supply to the upper ocean. Despite their differing MOC pathways, all models reproduce the observation of a high ?30Si signature in NADW, as well showing a major or dominant (46–62%) contribution from Southern Ocean mode/intermediate waters to its Si inventory. These models thus confirm that the ?30Si signature of NADW does indeed owe its existence primarily to the large-scale transport of a distal fractionation signal created in the surface Southern Ocean. However, we also find that more proximal fractionation of Si upwelled to the surface within the Atlantic Ocean must also play some role, contributing 20–46% of the deep Atlantic ?30Si gradient. Finally, the model suite reveals compensatory effects in the mechanisms contributing to the high ?30Si signature of NADW, whereby less export of high-?30Si mode/intermediate waters to the North Atlantic is compensated by production of a high-?30Si signal during transport to the NADW formation region. This trade-off decouples the ?30Si signature of NADW from the pathways of deep water upwelling associated with the MOC. Thus, whilst our study affirms the importance of cross-equatorial transport of Southern Ocean-sourced Si in producing the unique ?30Si signature of NADW, it also shows that the presence of a deep Atlantic ?30Si gradient does not uniquely constrain the pathways by which deep waters are returned to the upper ocean.
biogeochemical cycles, silicon isotopes, meridional overturning circulation
0012-821X
342-353
de Souza, G.F.
2e71dac9-7d84-4c2c-9f62-d46786854e36
Slater, R.D.
02cdf450-d76f-468f-9eee-9afd70d9e05c
Hain, M.P.
d31486bc-c473-4c34-a814-c0834640876c
Brzezinski, M.A.
323e7e81-b47b-421a-a0c8-9987b20a5b25
Sarmiento, J.L.
5887047e-92ac-47f7-a504-fb1699dd8d17
de Souza, G.F.
2e71dac9-7d84-4c2c-9f62-d46786854e36
Slater, R.D.
02cdf450-d76f-468f-9eee-9afd70d9e05c
Hain, M.P.
d31486bc-c473-4c34-a814-c0834640876c
Brzezinski, M.A.
323e7e81-b47b-421a-a0c8-9987b20a5b25
Sarmiento, J.L.
5887047e-92ac-47f7-a504-fb1699dd8d17

de Souza, G.F., Slater, R.D., Hain, M.P., Brzezinski, M.A. and Sarmiento, J.L. (2015) Distal and proximal controls on the silicon stable isotope signature of North Atlantic Deep Water. Earth and Planetary Science Letters, 432, 342-353. (doi:10.1016/j.epsl.2015.10.025).

Record type: Article

Abstract

It has been suggested that the uniquely high ?30Si signature of North Atlantic Deep Water (NADW) results from the contribution of isotopically fractionated silicic acid by mode and intermediate waters that are formed in the Southern Ocean and transported to the North Atlantic within the upper limb of the meridional overturning circulation (MOC). Here, we test this hypothesis in a suite of ocean general circulation models (OGCMs) with widely varying MOCs and related pathways of nutrient supply to the upper ocean. Despite their differing MOC pathways, all models reproduce the observation of a high ?30Si signature in NADW, as well showing a major or dominant (46–62%) contribution from Southern Ocean mode/intermediate waters to its Si inventory. These models thus confirm that the ?30Si signature of NADW does indeed owe its existence primarily to the large-scale transport of a distal fractionation signal created in the surface Southern Ocean. However, we also find that more proximal fractionation of Si upwelled to the surface within the Atlantic Ocean must also play some role, contributing 20–46% of the deep Atlantic ?30Si gradient. Finally, the model suite reveals compensatory effects in the mechanisms contributing to the high ?30Si signature of NADW, whereby less export of high-?30Si mode/intermediate waters to the North Atlantic is compensated by production of a high-?30Si signal during transport to the NADW formation region. This trade-off decouples the ?30Si signature of NADW from the pathways of deep water upwelling associated with the MOC. Thus, whilst our study affirms the importance of cross-equatorial transport of Southern Ocean-sourced Si in producing the unique ?30Si signature of NADW, it also shows that the presence of a deep Atlantic ?30Si gradient does not uniquely constrain the pathways by which deep waters are returned to the upper ocean.

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Accepted/In Press date: 18 October 2015
e-pub ahead of print date: 3 November 2015
Published date: 15 December 2015
Keywords: biogeochemical cycles, silicon isotopes, meridional overturning circulation
Organisations: Ocean and Earth Science

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Local EPrints ID: 383624
URI: http://eprints.soton.ac.uk/id/eprint/383624
ISSN: 0012-821X
PURE UUID: 7112b4a5-da5a-41a4-aa80-b52171bfe549

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Date deposited: 06 Nov 2015 14:10
Last modified: 08 Jan 2022 06:22

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Contributors

Author: G.F. de Souza
Author: R.D. Slater
Author: M.P. Hain
Author: M.A. Brzezinski
Author: J.L. Sarmiento

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