Reconstructing deglacial Atlantic deep water circulation using the Nd isotopic composition of Fe-Mn oxide coatings from planktonic foraminfera

During the last few decades, neodymium isotopes have been increasingly used as a paleoceanographic proxy, to reconstruct past changes in deep ocean circulation [1,2] and river outputs to the surface ocean [3]. The widespread use of the Nd isotope proxy depends on whether the Nd isotopic composition of past seawater can be reliably extracted from authigenic sediment phases. Here we show that the Fe-Mn oxide coatings which are removed from planktonic foraminifera during reductive cleaning can be used to reconstruct past deep water Nd isotopic composition. In effect, we are using the planktonic foraminiferal calcite as a low-Nd carrier phase to leach authigenic Fe-Mn oxides, which have higher Nd concentration and record the Nd isotopic composition of bottom water. Leaching experiments on northeastern North Atlantic core BOFS 8K (52N, 22W, 4045mbsl) show methodological artifacts during bulk sediment leaching, but that the Fe-Mn oxide coatings from the planktonic foraminifera have the same Nd isotopic composition as bottom water. Interestingly, the Nd isotopic composition of planktonic foraminifera which have not been cleaned of Fe-Mn oxide coatings, and planktonic foraminifera which have been reductively cleaned by the Boyle and Kiegwin (1987) [4] method are within error of each other throughout the deglacial records, suggesting incomplete removal or significant re-scavenging of Nd from Fe-Mn oxides to cleaned foraminiferal calcite during reductive cleaning. There is no indication that we are incorporating a significant surface-water Nd isotopic signal. Instead, these records and laboratory tests may suggest that some published “cleaned” planktonic foraminiferal records are preserving a deep water signal, rather than a surface ocean signal. The downcore BOFS 8K record of planktonic foraminferal Fe-Mn coating Nd isotopes shows a large deglacial change consistent with benthic carbon isotopes, supporting more southern-sourced water reaching the site during the last glacial maximum. A shift to unradiogenic Nd isotopes from 20 to 10 ka is indicative of an increasing presence of North Atlantic Deep Water reaching the site as the deglaciation progressed. Some millennial-scale variations may correspond to deglacial climate events. We have also applied this method downcore on South Atlantic Cape Basin core TNO57-21 (40S, 7E, 4981mbsl). The Nd isotopic composition of planktonic foraminiferal Fe-Mn oxide coatings matches those on the decarbonated bulk-sediment leaches [1] through time, confirming previous Nd isotope records from this site as reflecting bottom water compositions. Taken as a whole, these records provide a coherent reconstruction of glacial Atlantic deep circulation which is consistent with benthic carbon isotope reconstructions. [1] Piotrowski et al. (2005) Science 307 1933-1938. [2] Gutjahr et al. (2008) EPSL 266 61-77. [3] Vance and Burton, (1999) EPSL 173 365-379. [4] Boyle and Keigwin, (1987) Nature 330, 35.