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The impact of circulation and dust deposition in controlling the distributions of dissolved Fe and Al in the south Indian subtropical gyre

The impact of circulation and dust deposition in controlling the distributions of dissolved Fe and Al in the south Indian subtropical gyre
The impact of circulation and dust deposition in controlling the distributions of dissolved Fe and Al in the south Indian subtropical gyre
The South Indian Subtropical Gyre (SISG) is one of the least studied gyre systems of the world ocean with respect to trace elements. Here we report dissolved (< 0.45 ?m) Fe and Al measurements collected during two high-resolution US-CLIVAR CO2 Repeat Hydrography sections, which transected the upper 1000 m of the gyre zonally along ~ 32°S (I05) and meridionally along ~ 30°E (I06S). Particulate Fe and Al concentrations in waters influenced by the Agulhas Current are also presented. The distributions of dissolved Fe and Al in the gyre are primarily impacted by mineral dust deposition at the surface and the large-scale circulation patterns of the gyre at depth. Using mean mixed layer dissolved Al concentrations, we estimate that the deposition and partial dissolution of mineral dust emanating from South Africa and Australia vary from 60 to 685 mg (dust) m? 2 yr? 1 across the 32°S transect. This translates into a dust source of dissolved Fe ranging from 1.7 to 20 ?mol Fe m? 2 y? 1. The zonal patterns of aeolian deposition and those of N*, an indirect geochemical tracer for nitrogen fixation, show remarkable similarities along the I05 transect, suggesting that aeolian delivery of Fe may regulate nitrogen fixation rates in the SISG. In the western SISG (west of 60°E), which receives some of the highest aeolian Fe fluxes of the 32°S section, the Fe:AOU ratio in Indian Central Water was elevated relative to that observed in the Indian Central Water occupying the eastern Indian Ocean. These elevated Fe:AOU ratios may reflect the remineralization of Fe-rich organic material from nitrogen fixing organisms at the western end of the basin. Below the mixed layer, the distribution of dissolved Al appears to trace the principal features of the large-scale circulation of the SISG. Elevated subsurface concentrations of dissolved Al (> 4 nM) in the southwest Indian Ocean west of 45–50°E are most likely sustained by leakage of Al-rich waters from the Agulhas Return Current. Along the southeast African margin, the elevated particulate Fe (up to 230 nM) and Al (up to 690 nM) concentrations reflect the resuspension and transport of shelf sediments by the highly energetic Agulhas Current. However, while the particulate inputs at the margin are massive and appear to supply modest amounts of dissolved Fe, the distribution of dissolved Al is decoupled from the particulate phase. This observation suggests that the elevated subsurface dissolved Al concentrations observed near the African shelf are not the result of sediment resuspension processes occurring in situ along I05 but are more likely an advected signal originating from the upper reaches of the Agulhas Current.
Fe iron, Al aluminium, Indian Ocean, CLIVAR
0304-4203
110-125
Grand, Maxime M.
659acbde-d639-42b7-9fab-52fa1b3655ff
Measures, Christopher I.
bee0ab0a-cbec-491e-bcfa-e0cec9df67bb
Hatta, Mariko
b8c210ca-90aa-4660-b1b7-3538b120e31d
Morton, Peter L.
b759bd86-5942-4389-8962-ac9b84981e45
Barrett, Pamela
bf1a971d-d1b7-4363-ab48-b4780b42f9a6
Milne, Angela
39b44fbd-8f5e-40ea-80c9-244c20998020
Resing, Joseph A.
10aa7ef6-6c66-4be8-85c6-7a28db61e200
Landing, William M.
26eb4b62-d05c-42bb-a757-f840915f6576
Grand, Maxime M.
659acbde-d639-42b7-9fab-52fa1b3655ff
Measures, Christopher I.
bee0ab0a-cbec-491e-bcfa-e0cec9df67bb
Hatta, Mariko
b8c210ca-90aa-4660-b1b7-3538b120e31d
Morton, Peter L.
b759bd86-5942-4389-8962-ac9b84981e45
Barrett, Pamela
bf1a971d-d1b7-4363-ab48-b4780b42f9a6
Milne, Angela
39b44fbd-8f5e-40ea-80c9-244c20998020
Resing, Joseph A.
10aa7ef6-6c66-4be8-85c6-7a28db61e200
Landing, William M.
26eb4b62-d05c-42bb-a757-f840915f6576

Grand, Maxime M., Measures, Christopher I., Hatta, Mariko, Morton, Peter L., Barrett, Pamela, Milne, Angela, Resing, Joseph A. and Landing, William M. (2015) The impact of circulation and dust deposition in controlling the distributions of dissolved Fe and Al in the south Indian subtropical gyre. Marine Chemistry, 176, 110-125. (doi:10.1016/j.marchem.2015.08.002).

Record type: Article

Abstract

The South Indian Subtropical Gyre (SISG) is one of the least studied gyre systems of the world ocean with respect to trace elements. Here we report dissolved (< 0.45 ?m) Fe and Al measurements collected during two high-resolution US-CLIVAR CO2 Repeat Hydrography sections, which transected the upper 1000 m of the gyre zonally along ~ 32°S (I05) and meridionally along ~ 30°E (I06S). Particulate Fe and Al concentrations in waters influenced by the Agulhas Current are also presented. The distributions of dissolved Fe and Al in the gyre are primarily impacted by mineral dust deposition at the surface and the large-scale circulation patterns of the gyre at depth. Using mean mixed layer dissolved Al concentrations, we estimate that the deposition and partial dissolution of mineral dust emanating from South Africa and Australia vary from 60 to 685 mg (dust) m? 2 yr? 1 across the 32°S transect. This translates into a dust source of dissolved Fe ranging from 1.7 to 20 ?mol Fe m? 2 y? 1. The zonal patterns of aeolian deposition and those of N*, an indirect geochemical tracer for nitrogen fixation, show remarkable similarities along the I05 transect, suggesting that aeolian delivery of Fe may regulate nitrogen fixation rates in the SISG. In the western SISG (west of 60°E), which receives some of the highest aeolian Fe fluxes of the 32°S section, the Fe:AOU ratio in Indian Central Water was elevated relative to that observed in the Indian Central Water occupying the eastern Indian Ocean. These elevated Fe:AOU ratios may reflect the remineralization of Fe-rich organic material from nitrogen fixing organisms at the western end of the basin. Below the mixed layer, the distribution of dissolved Al appears to trace the principal features of the large-scale circulation of the SISG. Elevated subsurface concentrations of dissolved Al (> 4 nM) in the southwest Indian Ocean west of 45–50°E are most likely sustained by leakage of Al-rich waters from the Agulhas Return Current. Along the southeast African margin, the elevated particulate Fe (up to 230 nM) and Al (up to 690 nM) concentrations reflect the resuspension and transport of shelf sediments by the highly energetic Agulhas Current. However, while the particulate inputs at the margin are massive and appear to supply modest amounts of dissolved Fe, the distribution of dissolved Al is decoupled from the particulate phase. This observation suggests that the elevated subsurface dissolved Al concentrations observed near the African shelf are not the result of sediment resuspension processes occurring in situ along I05 but are more likely an advected signal originating from the upper reaches of the Agulhas Current.

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More information

Published date: 20 November 2015
Keywords: Fe iron, Al aluminium, Indian Ocean, CLIVAR
Organisations: Geochemistry

Identifiers

Local EPrints ID: 380937
URI: http://eprints.soton.ac.uk/id/eprint/380937
ISSN: 0304-4203
PURE UUID: d8e5a07e-e566-42e2-affb-f4f7ae95d0b3
ORCID for Maxime M. Grand: ORCID iD orcid.org/0000-0001-9338-694X

Catalogue record

Date deposited: 15 Sep 2015 09:10
Last modified: 15 Jul 2019 21:07

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