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Changes in North Atlantic nitrogen fixation controlled by ocean circulation

Changes in North Atlantic nitrogen fixation controlled by ocean circulation
Changes in North Atlantic nitrogen fixation controlled by ocean circulation
In the ocean, the chemical forms of nitrogen that are readily available for biological use (known collectively as ‘fixed’ nitrogen) fuel the global phytoplankton productivity that exports carbon to the deep ocean1, 2, 3. Accordingly, variation in the oceanic fixed nitrogen reservoir has been proposed as a cause of glacial–interglacial changes in atmospheric carbon dioxide concentration2, 3. Marine nitrogen fixation, which produces most of the ocean’s fixed nitrogen, is thought to be affected by multiple factors, including ocean temperature4 and the availability of iron2, 3, 5 and phosphorus6. Here we reconstruct changes in North Atlantic nitrogen fixation over the past 160,000?years from the shell-bound nitrogen isotope ratio (15N/14N) of planktonic foraminifera in Caribbean Sea sediments. The observed changes cannot be explained by reconstructed changes in temperature, the supply of (iron-bearing) dust or water column denitrification. We identify a strong, roughly 23,000-year cycle in nitrogen fixation and suggest that it is a response to orbitally driven changes in equatorial Atlantic upwelling7, which imports ‘excess’ phosphorus (phosphorus in stoichiometric excess of fixed nitrogen) into the tropical North Atlantic surface5, 6. In addition, we find that nitrogen fixation was reduced during glacial stages 6 and 4, when North Atlantic Deep Water had shoaled to become glacial North Atlantic intermediate water8, which isolated the Atlantic thermocline from excess phosphorus-rich mid-depth waters that today enter from the Southern Ocean. Although modern studies have yielded diverse views of the controls on nitrogen fixation1, 2, 4, 5, our palaeobiogeochemical data suggest that excess phosphorus is the master variable in the North Atlantic Ocean and indicate that the variations in its supply over the most recent glacial cycle were dominated by the response of regional ocean circulation to the orbital cycles.
0028-0836
200-203
Straub, Marietta
a2bf4c04-8ea4-4d93-94c7-f6dbe16ad5fc
Sigman, Daniel M.
b7945f7b-3945-4082-9204-feb1eb8cfed7
Ren, Haojia
8bda3a52-50de-4e34-9a67-d8fee0994c98
Martínez-García, Alfredo
29c2896b-3a8c-4457-b640-5c3ac1b9d272
Meckler, A. Nele
66ca3c7a-5177-4418-9d60-b647428957ca
Hain, Mathis P.
d31486bc-c473-4c34-a814-c0834640876c
Haug, Gerald H.
b6d161a0-4bdc-44d3-8a88-8de0e3517ca8
Straub, Marietta
a2bf4c04-8ea4-4d93-94c7-f6dbe16ad5fc
Sigman, Daniel M.
b7945f7b-3945-4082-9204-feb1eb8cfed7
Ren, Haojia
8bda3a52-50de-4e34-9a67-d8fee0994c98
Martínez-García, Alfredo
29c2896b-3a8c-4457-b640-5c3ac1b9d272
Meckler, A. Nele
66ca3c7a-5177-4418-9d60-b647428957ca
Hain, Mathis P.
d31486bc-c473-4c34-a814-c0834640876c
Haug, Gerald H.
b6d161a0-4bdc-44d3-8a88-8de0e3517ca8

Straub, Marietta, Sigman, Daniel M., Ren, Haojia, Martínez-García, Alfredo, Meckler, A. Nele, Hain, Mathis P. and Haug, Gerald H. (2013) Changes in North Atlantic nitrogen fixation controlled by ocean circulation. Nature, 501 (7466), 200-203. (doi:10.1038/nature12397).

Record type: Article

Abstract

In the ocean, the chemical forms of nitrogen that are readily available for biological use (known collectively as ‘fixed’ nitrogen) fuel the global phytoplankton productivity that exports carbon to the deep ocean1, 2, 3. Accordingly, variation in the oceanic fixed nitrogen reservoir has been proposed as a cause of glacial–interglacial changes in atmospheric carbon dioxide concentration2, 3. Marine nitrogen fixation, which produces most of the ocean’s fixed nitrogen, is thought to be affected by multiple factors, including ocean temperature4 and the availability of iron2, 3, 5 and phosphorus6. Here we reconstruct changes in North Atlantic nitrogen fixation over the past 160,000?years from the shell-bound nitrogen isotope ratio (15N/14N) of planktonic foraminifera in Caribbean Sea sediments. The observed changes cannot be explained by reconstructed changes in temperature, the supply of (iron-bearing) dust or water column denitrification. We identify a strong, roughly 23,000-year cycle in nitrogen fixation and suggest that it is a response to orbitally driven changes in equatorial Atlantic upwelling7, which imports ‘excess’ phosphorus (phosphorus in stoichiometric excess of fixed nitrogen) into the tropical North Atlantic surface5, 6. In addition, we find that nitrogen fixation was reduced during glacial stages 6 and 4, when North Atlantic Deep Water had shoaled to become glacial North Atlantic intermediate water8, which isolated the Atlantic thermocline from excess phosphorus-rich mid-depth waters that today enter from the Southern Ocean. Although modern studies have yielded diverse views of the controls on nitrogen fixation1, 2, 4, 5, our palaeobiogeochemical data suggest that excess phosphorus is the master variable in the North Atlantic Ocean and indicate that the variations in its supply over the most recent glacial cycle were dominated by the response of regional ocean circulation to the orbital cycles.

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More information

Published date: 12 September 2013
Organisations: Paleooceanography & Palaeoclimate

Identifiers

Local EPrints ID: 358632
URI: https://eprints.soton.ac.uk/id/eprint/358632
ISSN: 0028-0836
PURE UUID: 72e429b4-0088-4bc9-bf44-a0c632b7c669

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Date deposited: 09 Oct 2013 13:56
Last modified: 18 Jul 2017 03:28

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