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Iron distribution in the subtropical North Atlantic: the pivotal role of colloidal iron

Iron distribution in the subtropical North Atlantic: the pivotal role of colloidal iron
Iron distribution in the subtropical North Atlantic: the pivotal role of colloidal iron
The low availability of the essential micronutrient iron (Fe) in the ocean impacts the efficiency of the biological carbon pump, and hence, it is vital to elucidate its sources, sinks, and internal cycling. We present size‐fractionated dissolved Fe (dFe, <0.2 μm) measurements from 130 surface samples and 7 full‐depth profiles from the subtropical North Atlantic during summer 2017 and demonstrate the pivotal role of colloidal (cFe, 0.02 to 0.2 μm) over soluble (sFe, <0.02 μm) Fe in controlling the dFe distribution. In the surface (<5 m), a strong west‐to‐east decrease in dFe (1.53 to 0.26 nM) was driven by a dust gradient, which retained dFe predominantly as cFe (61% to 85% of dFe), while sFe remained largely constant at 0.19 ± 0.05 nM. In the euphotic zone, the attenuation of dFe resulted from the depletion of cFe (0% to 30% of dFe), with scavenging as an important driver. In the mesopelagic, cFe was released from sinking biogenic and lithogenic particles, creating a zone of elevated dFe (0.7 to 1.0 nM) between 400 to 1100 m depth. While the ocean interior, below the mesopelagic and above the seafloor boundary, exhibited a narrow range of cFe (40% to 60% of dFe), the abyssal cFe fraction varied in range from 26% to 76% due to interactions with seafloor sediments and a hydrothermal source with almost 100% cFe. Overall, our results produced an hourglass shape for the vertical cFe‐to‐dFe fraction and highlight the primary control of cFe on the dFe distribution.
Plain Language Summary

Phytoplankton require nutrients such as phosphorus, nitrogen, and iron. Of these, iron is particularly interesting due to the paradox of its requirement for life‐supporting mechanisms on the one hand and its low oceanic concentrations on the other. Iron is >1000‐fold lower than the “traditional” nutrients. Hence, it is important to know how much iron is introduced to the ocean (sources), how much is removed (sinks), and how it is processed during its residence in the water. This study addressed these questions by measuring the iron concentrations in the subtropical North Atlantic. Our samples were filtered through two filter sizes to investigate the distributions of iron's different size fractions, a popular tool to gain a detailed understanding of the overall iron cycle. We found that the smallest size fraction, “soluble iron,” does not vary much throughout the water column, but the slightly larger “colloidal iron” varies a lot, especially in the upper ocean and close to the seafloor, where dynamic supply and removal processes occur. The unequal behaviour of these fractions is an important finding that will improve the accuracy of biogeochemical models for iron, which in turn can improve the prediction of phytoplankton growth in the present and future ocean.

0886-6236
1532-1547
Kunde, K.
71069d95-19b5-45eb-a571-81103caf515d
Wyatt, N.J.
258d214b-9dae-4a5f-acc9-c0a55fb66efd
González‐Santana, D.
e603ade7-49a1-4969-aa28-503cd4e9fdf9
Tagliabue, A.
23ecb1dd-3cf4-46eb-b059-637a04f2439b
Mahaffey, C.
d92065c5-3a1f-4bbe-b708-447448967c88
Lohan, M.C.
6ca10597-2d0f-40e8-8e4f-7619dfac5088
Kunde, K.
71069d95-19b5-45eb-a571-81103caf515d
Wyatt, N.J.
258d214b-9dae-4a5f-acc9-c0a55fb66efd
González‐Santana, D.
e603ade7-49a1-4969-aa28-503cd4e9fdf9
Tagliabue, A.
23ecb1dd-3cf4-46eb-b059-637a04f2439b
Mahaffey, C.
d92065c5-3a1f-4bbe-b708-447448967c88
Lohan, M.C.
6ca10597-2d0f-40e8-8e4f-7619dfac5088

Kunde, K., Wyatt, N.J., González‐Santana, D., Tagliabue, A., Mahaffey, C. and Lohan, M.C. (2019) Iron distribution in the subtropical North Atlantic: the pivotal role of colloidal iron. Global Biogeochemical Cycles, 33 (12), 1532-1547. (doi:10.1029/2019GB006326).

Record type: Article

Abstract

The low availability of the essential micronutrient iron (Fe) in the ocean impacts the efficiency of the biological carbon pump, and hence, it is vital to elucidate its sources, sinks, and internal cycling. We present size‐fractionated dissolved Fe (dFe, <0.2 μm) measurements from 130 surface samples and 7 full‐depth profiles from the subtropical North Atlantic during summer 2017 and demonstrate the pivotal role of colloidal (cFe, 0.02 to 0.2 μm) over soluble (sFe, <0.02 μm) Fe in controlling the dFe distribution. In the surface (<5 m), a strong west‐to‐east decrease in dFe (1.53 to 0.26 nM) was driven by a dust gradient, which retained dFe predominantly as cFe (61% to 85% of dFe), while sFe remained largely constant at 0.19 ± 0.05 nM. In the euphotic zone, the attenuation of dFe resulted from the depletion of cFe (0% to 30% of dFe), with scavenging as an important driver. In the mesopelagic, cFe was released from sinking biogenic and lithogenic particles, creating a zone of elevated dFe (0.7 to 1.0 nM) between 400 to 1100 m depth. While the ocean interior, below the mesopelagic and above the seafloor boundary, exhibited a narrow range of cFe (40% to 60% of dFe), the abyssal cFe fraction varied in range from 26% to 76% due to interactions with seafloor sediments and a hydrothermal source with almost 100% cFe. Overall, our results produced an hourglass shape for the vertical cFe‐to‐dFe fraction and highlight the primary control of cFe on the dFe distribution.
Plain Language Summary

Phytoplankton require nutrients such as phosphorus, nitrogen, and iron. Of these, iron is particularly interesting due to the paradox of its requirement for life‐supporting mechanisms on the one hand and its low oceanic concentrations on the other. Iron is >1000‐fold lower than the “traditional” nutrients. Hence, it is important to know how much iron is introduced to the ocean (sources), how much is removed (sinks), and how it is processed during its residence in the water. This study addressed these questions by measuring the iron concentrations in the subtropical North Atlantic. Our samples were filtered through two filter sizes to investigate the distributions of iron's different size fractions, a popular tool to gain a detailed understanding of the overall iron cycle. We found that the smallest size fraction, “soluble iron,” does not vary much throughout the water column, but the slightly larger “colloidal iron” varies a lot, especially in the upper ocean and close to the seafloor, where dynamic supply and removal processes occur. The unequal behaviour of these fractions is an important finding that will improve the accuracy of biogeochemical models for iron, which in turn can improve the prediction of phytoplankton growth in the present and future ocean.

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More information

Accepted/In Press date: 6 November 2019
e-pub ahead of print date: 12 November 2019
Published date: December 2019

Identifiers

Local EPrints ID: 437772
URI: http://eprints.soton.ac.uk/id/eprint/437772
ISSN: 0886-6236
PURE UUID: abb18d61-4893-4b2f-809f-788d41b0c692
ORCID for N.J. Wyatt: ORCID iD orcid.org/0000-0002-1080-7778
ORCID for M.C. Lohan: ORCID iD orcid.org/0000-0002-5340-3108

Catalogue record

Date deposited: 14 Feb 2020 17:33
Last modified: 22 Nov 2021 08:01

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Contributors

Author: K. Kunde
Author: N.J. Wyatt ORCID iD
Author: D. González‐Santana
Author: A. Tagliabue
Author: C. Mahaffey
Author: M.C. Lohan ORCID iD

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