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A boundary exchange influence on deglacial neodymium isotope records from the deep western Indian Ocean

Wilson, DJ; Piotrowski, AM; Galy, A; McCave, IN; (2012) A boundary exchange influence on deglacial neodymium isotope records from the deep western Indian Ocean. Earth and Planetary Science Letters , 341-4 pp. 35-47. 10.1016/j.epsl.2012.06.009. Green open access

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Abstract

The use of neodymium (Nd) isotopes to reconstruct past water mass mixing relies upon the quasi-conservative behaviour of this tracer, whereas recent studies in the modern oceans have suggested that boundary exchange, involving the addition of Nd from ocean margin sediments, may be an important process in the Nd cycle. Here we suggest that the relative importance of water mass advection versus boundary exchange can be assessed where the deep western boundary current in the Indian Ocean flows past the Madagascan continental margin; a potential source of highly unradiogenic Nd. Foraminiferal coatings and bulk sediment reductive leachates are used to reconstruct bottom water Nd isotopic composition (εNd) in 8 Holocene age coretops, with excellent agreement between the two methods. These data record spatial variability of ∼4 εNd units along the flow path of Circumpolar Deep Water; εNd≈−8.8 in the deep southern inflow upstream of Madagascar, which evolves towards εNd≈−11.5 offshore northern Madagascar, whereas εNd≈−7.3 where deep water re-circulates in the eastern Mascarene Basin. This variability is attributed to boundary exchange and, together with measurements of detrital sediment εNd, an isotope mass balance suggests a deep water residence time for Nd of ≤400 yr along the Madagascan margin. Considering deglacial changes, a core in the deep inflow upstream of Madagascar records εNd changes that agree with previous reconstructions of the Circumpolar Deep Water composition in the Southern Ocean, consistent with a control by water mass advection and perhaps indicating a longer residence time for Nd in the open ocean away from local sediment inputs. In contrast, sites along the Madagascan margin record offset εNd values and reduced glacial–interglacial variability, underlining the importance of detecting boundary exchange before inferring water mass source changes from Nd isotope records. The extent of Madagascan boundary exchange appears to be unchanged between the Holocene and Late Glacial periods, while a consistent shift towards more radiogenic εNd values at all sites in the Late Glacial compared to the Holocene may represent a muted signal of a change in water mass source or composition.

Type: Article
Title: A boundary exchange influence on deglacial neodymium isotope records from the deep western Indian Ocean
Open access status: An open access version is available from UCL Discovery
DOI: 10.1016/j.epsl.2012.06.009
Publisher version: https://doi.org/10.1016/j.epsl.2012.06.009
Language: English
Additional information: This version is the author accepted manuscript. For information on re-use, please refer to the publisher’s terms and conditions.
Keywords: Indian Ocean, Circumpolar Deep Water, neodymium isotopes, ocean circulation, boundary exchange, deglaciation
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Earth Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/10059420
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