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Influence of the Amazon River on the Nd isotope composition of deep water in the western equatorial Atlantic during the Oligocene-Miocene transition

Influence of the Amazon River on the Nd isotope composition of deep water in the western equatorial Atlantic during the Oligocene-Miocene transition
Influence of the Amazon River on the Nd isotope composition of deep water in the western equatorial Atlantic during the Oligocene-Miocene transition
Dissolved and particulate neodymium (Nd) are mainly supplied to the oceans via rivers, dust, and release from marine sediments along continental margins. This process, together with the short oceanic residence time of Nd, gives rise to pronounced spatial gradients in oceanic 143Nd/144Nd ratios (?Nd). However, we do not yet have a good understanding of the extent to which the influence of riverine point source Nd supply can be distinguished from changes in mixing between different water masses in the marine geological record. This gap in knowledge is important to fill because there is growing awareness that major global climate transitions may be associated not only with changes in large-scale ocean water mass mixing, but also with important changes in continental hydroclimate and weathering. Here we present ?Nd data for fossilised fish teeth, planktonic foraminifera, and the Fe-Mn oxyhydroxide and detrital fractions of sediments recovered from Ocean Drilling Project (ODP) Site 926 on Ceara Rise, situated approximately 800 km from the mouth of the River Amazon. Our records span the Mi-1 glaciation event during the Oligocene-Miocene transition (OMT; ~23 Ma). We compare our ?Nd records with data for ambient deep Atlantic northern and southern component waters to assess the influence of particulate input from the Amazon River on Nd in ancient deep waters at this site. ?Nd values for all of our fish teeth, foraminifera, and Fe-Mn oxyhydroxide samples are extremely unradiogenic (?Nd ? ?15); much lower than the ?Nd for deep waters of modern or Oligocene-Miocene age from the North Atlantic (?Nd ? ?10) and South Atlantic (?Nd ? ?8). This finding suggests that partial dissolution of detrital particulate material from the Amazon (?Nd ? ?18) strongly influences the ?Nd values of deep waters at Ceara Rise across the OMT. We conclude that terrestrially derived inputs of Nd can affect ?Nd values of deep water many hundreds of kilometres from source. Our results both underscore the need for care in reconstructing changes in large-scale oceanic water-mass mixing using sites proximal to major rivers, and highlight the potential of these marine archives for tracing changes in continental hydroclimate and weathering.
neodymium isotopes, fish teeth, foraminifera, Amazon, Oligocene–Miocene, ODP Site 926
0012-821X
132-141
Stewart, Joseph
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Gutjahr, Marcus
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James, Rachael
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Anand, Pallavi
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Wilson, Paul
f940a9f0-fa5a-4a64-9061-f0794bfbf7c6
Stewart, Joseph
3d7f8398-d59b-458b-a3c9-e6a54d87e9b1
Gutjahr, Marcus
5babbbc4-2a1a-48df-a2e3-d87b2483ea9c
James, Rachael
79aa1d5c-675d-4ba3-85be-fb20798c02f4
Anand, Pallavi
c9c35583-fcdd-440b-ab99-575c5839178d
Wilson, Paul
f940a9f0-fa5a-4a64-9061-f0794bfbf7c6

Stewart, Joseph, Gutjahr, Marcus, James, Rachael, Anand, Pallavi and Wilson, Paul (2016) Influence of the Amazon River on the Nd isotope composition of deep water in the western equatorial Atlantic during the Oligocene-Miocene transition. Earth and Planetary Science Letters, 454, 132-141. (doi:10.1016/j.epsl.2016.08.037).

Record type: Article

Abstract

Dissolved and particulate neodymium (Nd) are mainly supplied to the oceans via rivers, dust, and release from marine sediments along continental margins. This process, together with the short oceanic residence time of Nd, gives rise to pronounced spatial gradients in oceanic 143Nd/144Nd ratios (?Nd). However, we do not yet have a good understanding of the extent to which the influence of riverine point source Nd supply can be distinguished from changes in mixing between different water masses in the marine geological record. This gap in knowledge is important to fill because there is growing awareness that major global climate transitions may be associated not only with changes in large-scale ocean water mass mixing, but also with important changes in continental hydroclimate and weathering. Here we present ?Nd data for fossilised fish teeth, planktonic foraminifera, and the Fe-Mn oxyhydroxide and detrital fractions of sediments recovered from Ocean Drilling Project (ODP) Site 926 on Ceara Rise, situated approximately 800 km from the mouth of the River Amazon. Our records span the Mi-1 glaciation event during the Oligocene-Miocene transition (OMT; ~23 Ma). We compare our ?Nd records with data for ambient deep Atlantic northern and southern component waters to assess the influence of particulate input from the Amazon River on Nd in ancient deep waters at this site. ?Nd values for all of our fish teeth, foraminifera, and Fe-Mn oxyhydroxide samples are extremely unradiogenic (?Nd ? ?15); much lower than the ?Nd for deep waters of modern or Oligocene-Miocene age from the North Atlantic (?Nd ? ?10) and South Atlantic (?Nd ? ?8). This finding suggests that partial dissolution of detrital particulate material from the Amazon (?Nd ? ?18) strongly influences the ?Nd values of deep waters at Ceara Rise across the OMT. We conclude that terrestrially derived inputs of Nd can affect ?Nd values of deep water many hundreds of kilometres from source. Our results both underscore the need for care in reconstructing changes in large-scale oceanic water-mass mixing using sites proximal to major rivers, and highlight the potential of these marine archives for tracing changes in continental hydroclimate and weathering.

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More information

Accepted/In Press date: 6 August 2016
Published date: 15 November 2016
Keywords: neodymium isotopes, fish teeth, foraminifera, Amazon, Oligocene–Miocene, ODP Site 926
Organisations: Geochemistry, Paleooceanography & Palaeoclimate

Identifiers

Local EPrints ID: 400091
URI: https://eprints.soton.ac.uk/id/eprint/400091
ISSN: 0012-821X
PURE UUID: b16e3a0f-6201-4c67-9062-1cc8b48569c2

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Date deposited: 13 Sep 2016 08:32
Last modified: 15 Aug 2019 05:20

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