Boundary processes and neodymium cycling along the Pacific margin of West Antarctica
Boundary processes and neodymium cycling along the Pacific margin of West Antarctica
Neodymium (Nd) isotopes have been utilized as a tracer of water mass source in the modern ocean and in palaeoceanographic studies, though the oceanic cycling of Nd is not yet fully constrained. Recent studies have highlighted the importance of processes that occur near the seawater – sediment interface in altering the Nd isotopic composition of bottom waters. The two major observed processes “boundary exchange” and “benthic flux” have been suggested as playing an important role in setting water mass compositions, however, more studies are needed to constrain their chemical mechanism and the extent to which these processes set the composition of deep waters. The Antarctic continental margin is an important place to study these processes because Antarctic-sourced waters dominate the Southern Ocean and ventilate the global deep ocean. This study is the first to measure and compare seawater, porewater and sediment data from along the margin of Antarctica to examine the nature of potential boundary processes. We show that a process similar to boundary exchange seems to be occurring within porewaters, modifying porewater chemistry by shifting its Nd isotopic ratios to more radiogenic values without significantly increasing the concentration of dissolved Nd. We hypothesize that this shift results from partial dissolution of radiogenic detrital particles, such as smectite, amphibole and/or volcanic glass, while re-scavenging maintains low Nd concentrations. We infer the existence of benthic flux of porewaters to deep waters by examining chemical gradients in porewaters and show that it is much lower on the Antarctic margin compared to other studies. Benthic flux appears to be slightly higher along the Antarctic Peninsula than in the Bellingshausen Sea due to partial degradation of organic matter and associated dissolution of Fe-Mn oxyhydroxides. Taken together, boundary processes do not significantly change the Nd isotopic composition of Antarctic margin seawater because while the porewaters have an altered Nd isotopic composition the Nd concentration of these porewaters is low compared to other settings.
Antarctic margin, Benthic flux, Boundary exchange, Neodymium cycling, Neodymium isotopes
1-20
Wang, Ruixue
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Williams, Thomas J.
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Hillenbrand, Claus-Dieter
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Ehrmann, Werner
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Larkin, Christina S.
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Hutchings, Alec M.
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Piotrowski, Alexander M.
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15 June 2022
Wang, Ruixue
0baf61f1-82c7-45d9-b8d6-c4483d7733ba
Williams, Thomas J.
ea3414d2-e467-4449-aa74-3153c80e536d
Hillenbrand, Claus-Dieter
8dc0c76e-e79b-4c8f-90ed-07035aabab98
Ehrmann, Werner
7b06f4a1-14a9-4d24-b85d-3238f2bb221c
Larkin, Christina S.
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Hutchings, Alec M.
870cc803-3af4-47fc-8063-c6adeee22f49
Piotrowski, Alexander M.
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Wang, Ruixue, Williams, Thomas J., Hillenbrand, Claus-Dieter, Ehrmann, Werner, Larkin, Christina S., Hutchings, Alec M. and Piotrowski, Alexander M.
(2022)
Boundary processes and neodymium cycling along the Pacific margin of West Antarctica.
Geochimica et Cosmochimica Acta, 327, .
(doi:10.1016/j.gca.2022.04.012).
Abstract
Neodymium (Nd) isotopes have been utilized as a tracer of water mass source in the modern ocean and in palaeoceanographic studies, though the oceanic cycling of Nd is not yet fully constrained. Recent studies have highlighted the importance of processes that occur near the seawater – sediment interface in altering the Nd isotopic composition of bottom waters. The two major observed processes “boundary exchange” and “benthic flux” have been suggested as playing an important role in setting water mass compositions, however, more studies are needed to constrain their chemical mechanism and the extent to which these processes set the composition of deep waters. The Antarctic continental margin is an important place to study these processes because Antarctic-sourced waters dominate the Southern Ocean and ventilate the global deep ocean. This study is the first to measure and compare seawater, porewater and sediment data from along the margin of Antarctica to examine the nature of potential boundary processes. We show that a process similar to boundary exchange seems to be occurring within porewaters, modifying porewater chemistry by shifting its Nd isotopic ratios to more radiogenic values without significantly increasing the concentration of dissolved Nd. We hypothesize that this shift results from partial dissolution of radiogenic detrital particles, such as smectite, amphibole and/or volcanic glass, while re-scavenging maintains low Nd concentrations. We infer the existence of benthic flux of porewaters to deep waters by examining chemical gradients in porewaters and show that it is much lower on the Antarctic margin compared to other studies. Benthic flux appears to be slightly higher along the Antarctic Peninsula than in the Bellingshausen Sea due to partial degradation of organic matter and associated dissolution of Fe-Mn oxyhydroxides. Taken together, boundary processes do not significantly change the Nd isotopic composition of Antarctic margin seawater because while the porewaters have an altered Nd isotopic composition the Nd concentration of these porewaters is low compared to other settings.
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More information
Accepted/In Press date: 12 April 2022
e-pub ahead of print date: 20 April 2022
Published date: 15 June 2022
Additional Information:
Funding Information:
This study forms part of the British Antarctic Survey’s Polar Science for Planet Earth Programme and was made possible by NERC UK-IODP grants NE/J006513/1 and NE/J006548/1, a NERC funded PhD project to T. Williams, and a NERC collaborative gearing scheme grant to support A. Piotrowski and T. Williams for participating in cruise JR298. Ruixue Wang is supported by CSC Cambridge International Scholarship jointly funded by Cambridge TRUST and China Scholarship Council .
Publisher Copyright:
© 2022 The Authors
Keywords:
Antarctic margin, Benthic flux, Boundary exchange, Neodymium cycling, Neodymium isotopes
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Local EPrints ID: 456967
URI: http://eprints.soton.ac.uk/id/eprint/456967
ISSN: 0016-7037
PURE UUID: f2124001-fe56-40ff-ad15-e1a90d46ad1b
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Date deposited: 18 May 2022 17:03
Last modified: 17 Mar 2024 04:05
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Contributors
Author:
Ruixue Wang
Author:
Thomas J. Williams
Author:
Claus-Dieter Hillenbrand
Author:
Werner Ehrmann
Author:
Christina S. Larkin
Author:
Alec M. Hutchings
Author:
Alexander M. Piotrowski
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