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Sources and input mechanisms of hafnium and neodymium in surface waters of the Atlantic sector of the Southern Ocean

Sources and input mechanisms of hafnium and neodymium in surface waters of the Atlantic sector of the Southern Ocean
Sources and input mechanisms of hafnium and neodymium in surface waters of the Atlantic sector of the Southern Ocean
Radiogenic isotopes of hafnium (Hf) and neodymium (Nd) are powerful tracers for water mass transport and trace metal cycling in the present and past oceans. However, due to the scarcity of available data the processes governing their distribution are not well understood. Here we present the first combined dissolved Hf and Nd isotope and concentration data from surface waters of the Atlantic sector of the Southern Ocean. The samples were collected along the Zero Meridian, in the Weddell Sea and in the Drake Passage during RV Polarstern expeditions ANTXXIV/3 and ANTXXIII/3 in the frame of the International Polar Year (IPY) and the GEOTRACES program. The general distribution of Hf and Nd concentrations in the region is similar. However, at the northernmost station located 200 km southwest of Cape Town a pronounced increase of the Nd concentration is observed, whereas the Hf concentration is minimal, suggesting much less Hf than Nd is released by the weathering of the South African Archean cratonic rocks. From the southern part of the Subtropical Front (STF) to the Polar Front (PF) Hf and Nd show the lowest concentrations (<0.12 pmol/kg and 10 pmol/kg, respectively), most probably due to the low terrigenous flux in this area and efficient scavenging of Hf and Nd by biogenic opal. In the vicinity of landmasses the dissolved Hf and Nd isotope compositions are clearly labeled by terrigenous inputs. Near South Africa Nd isotope values as low as ?Nd = ?18.9 indicate unradiogenic inputs supplied via the Agulhas Current. Further south the isotopic data show significant increases to ?Hf = 6.1 and ?Nd = ?4.0 documenting exchange of seawater Nd and Hf with the Antarctic Peninsula. In the open Southern Ocean the Nd isotope compositions are relatively homogeneous (?Nd ? ?8 to ?8.5) towards the STF, within the Antarctic Circumpolar Current, in the Weddell Gyre, and the Drake Passage. The Hf isotope compositions in the entire study area only show a small range between ?Hf = + 6.1 and +2.8 support Hf to be more readily released from young mafic rocks compared to old continental ones. The Nd isotope composition ranges from ?Nd = ?18.9 to ?4.0 showing Nd isotopes to be a sensitive tracer for the provenance of weathering inputs into surface waters of the Southern Ocean.
0016-7037
22-37
Stichel, Torben
c04e16ca-782d-47af-b955-7bbdd9e7f91d
Frank, Martin
09ec65ac-f62d-41da-86b2-81908973b8a1
Rickli, Jörg
c9b959cc-aa18-4eb7-9fa8-0a59299f74fe
Hathorne, Ed C.
e171d49c-e8ce-47d1-a316-8924886afef0
Haley, Brian A.
d6375324-0988-4da9-bc69-391a48d4a91c
Jeandel, Catherine
d2a06452-d4f2-4ea5-9205-845b645067c1
Pradoux, Catherine
aca2a234-b4c2-4949-9eeb-7d2118b15ba0
Stichel, Torben
c04e16ca-782d-47af-b955-7bbdd9e7f91d
Frank, Martin
09ec65ac-f62d-41da-86b2-81908973b8a1
Rickli, Jörg
c9b959cc-aa18-4eb7-9fa8-0a59299f74fe
Hathorne, Ed C.
e171d49c-e8ce-47d1-a316-8924886afef0
Haley, Brian A.
d6375324-0988-4da9-bc69-391a48d4a91c
Jeandel, Catherine
d2a06452-d4f2-4ea5-9205-845b645067c1
Pradoux, Catherine
aca2a234-b4c2-4949-9eeb-7d2118b15ba0

Stichel, Torben, Frank, Martin, Rickli, Jörg, Hathorne, Ed C., Haley, Brian A., Jeandel, Catherine and Pradoux, Catherine (2012) Sources and input mechanisms of hafnium and neodymium in surface waters of the Atlantic sector of the Southern Ocean. Geochimica et Cosmochimica Acta, 94, 22-37. (doi:10.1016/j.gca.2012.07.005).

Record type: Article

Abstract

Radiogenic isotopes of hafnium (Hf) and neodymium (Nd) are powerful tracers for water mass transport and trace metal cycling in the present and past oceans. However, due to the scarcity of available data the processes governing their distribution are not well understood. Here we present the first combined dissolved Hf and Nd isotope and concentration data from surface waters of the Atlantic sector of the Southern Ocean. The samples were collected along the Zero Meridian, in the Weddell Sea and in the Drake Passage during RV Polarstern expeditions ANTXXIV/3 and ANTXXIII/3 in the frame of the International Polar Year (IPY) and the GEOTRACES program. The general distribution of Hf and Nd concentrations in the region is similar. However, at the northernmost station located 200 km southwest of Cape Town a pronounced increase of the Nd concentration is observed, whereas the Hf concentration is minimal, suggesting much less Hf than Nd is released by the weathering of the South African Archean cratonic rocks. From the southern part of the Subtropical Front (STF) to the Polar Front (PF) Hf and Nd show the lowest concentrations (<0.12 pmol/kg and 10 pmol/kg, respectively), most probably due to the low terrigenous flux in this area and efficient scavenging of Hf and Nd by biogenic opal. In the vicinity of landmasses the dissolved Hf and Nd isotope compositions are clearly labeled by terrigenous inputs. Near South Africa Nd isotope values as low as ?Nd = ?18.9 indicate unradiogenic inputs supplied via the Agulhas Current. Further south the isotopic data show significant increases to ?Hf = 6.1 and ?Nd = ?4.0 documenting exchange of seawater Nd and Hf with the Antarctic Peninsula. In the open Southern Ocean the Nd isotope compositions are relatively homogeneous (?Nd ? ?8 to ?8.5) towards the STF, within the Antarctic Circumpolar Current, in the Weddell Gyre, and the Drake Passage. The Hf isotope compositions in the entire study area only show a small range between ?Hf = + 6.1 and +2.8 support Hf to be more readily released from young mafic rocks compared to old continental ones. The Nd isotope composition ranges from ?Nd = ?18.9 to ?4.0 showing Nd isotopes to be a sensitive tracer for the provenance of weathering inputs into surface waters of the Southern Ocean.

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Published date: 1 October 2012
Organisations: Geochemistry

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Local EPrints ID: 368786
URI: http://eprints.soton.ac.uk/id/eprint/368786
ISSN: 0016-7037
PURE UUID: 1c098e39-87c2-4252-8a04-c9f9645a9da1

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Date deposited: 16 Sep 2014 09:36
Last modified: 15 Jul 2019 21:45

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Contributors

Author: Torben Stichel
Author: Martin Frank
Author: Jörg Rickli
Author: Ed C. Hathorne
Author: Brian A. Haley
Author: Catherine Jeandel
Author: Catherine Pradoux

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