Neodymium and hafnium boundary contributions to seawater along the West Antarctic continental margin

  • Rickli J
  • Gutjahr M
  • Vance D
 et al. 
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Abstract

Neodymium and hafnium isotopes and elemental concentrations (Sm, Nd, Hf, Zr) have been measured in three water column profiles south of the Antarctic Circumpolar Current in, and to the east of the Ross Sea, in conjunction with five bottom water samples from the Amundsen Sea Embayment.Neodymium and hafnium both appear to be released from sediments in the Embayment. In the case of Nd, this is reflected in radiogenic isotope compositions (εNdup to -5.4) and highly elevated concentrations (up to 41 pmol/kg). Hafnium isotopes, on the other hand, are only very slightly altered relative to the open ocean sites, and boundary release is most prominently indicated by elevated concentrations (>1 pmol/kg versus ~0.7 pmol/kg). There is also a local input of both Hf and Nd at the Marie Byrd Seamounts, which leads to Nd isotope compositions as radiogenic as -3.1, and hafnium shifted to less radiogenic compositions in local bottom water.A compilation of the new data with literature data reveals a consistent view of the influence of Antarctica on the Nd isotope composition in Lower Circumpolar Deep Water (LCDW) and Antarctic Bottom Water (AABW). Sector specific Nd addition shifts AABW formed in the Atlantic sector to less radiogenic isotope compositions (averageεNd= - 9) relative to LCDW (averageεNd= - 8.4), whereas AABW in the Pacific sector is shifted to more radiogenic values (averageεNd= - 7). The evolution towards more radiogenicεNdwith depth in LCDW in the Pacific sector is likely to reflect admixture of AABW but, in addition, is also controlled by boundary exchange with the slope as observed at the Marie Byrd Seamounts.Hafnium isotopes are relatively homogeneous in the data set, ranging betweenεHf= + 2 and +3.8 for most samples, excluding less radiogenic compositions in deep waters close to the Marie Byrd Seamounts. The Hf isotope composition in the Pacific sector is, however, slightly less radiogenic than in the Atlantic, corresponding to an average of +3 relative to an average of +3.8. This probably reflects unradiogenic Hf inputs from Antarctica to the Pacific sector, which are vertically homogenized by reversible scavenging. The Hf isotope heterogeneity in LCDW between both sectors is likely to indicate a shorter seawater residence time for Hf than for Nd, which is consistent with the dissolved - particulate phase partitioning of both elements. © 2014 Elsevier B.V.

Author-supplied keywords

  • Hafnium
  • Neodymium
  • Radiogenic isotopes
  • Seawater
  • Southern Ocean

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