Interpreting Nd isotope and 231Pa/230Th records in the deep Western North Atlantic

Advances in the our knowledge of past changes in ocean circulation have been made using Nd isotopes, as a proxy for changes in water mass structure [1], and 231Pa/230Th ratios as a proxy for the rate of ocean circulation [2]. Despite the potential power of these proxies when applied together, no records have yet been published of both proxies measured at the same location. Based on previous records of ocean circulation [2, 3], we know the central deep Western North Atlantic is sensitive to changes in water mass structure and advection rates on a glacial-interglacial timescale, making the Bermuda Rise hydrographically one of the best places to test the application of both proxies together. Understanding changes in deep ocean circulation and water mass structure during the last deglaciation is crucial to unravelling the dynamics of glacial-interglacial and millenial climate shifts. Here we will present Nd isotopes measured on multiple sediment phases from the core OCE326-GGC6 (33°41.443’N; 57°34.559’W, 4541m), which is a sister core to OCE326-GGC5 on which the McManus et al 231Pa/230Th record was measured. Both cores were taken at the same depth and location, allowing accurate comparison of both proxies along with benthic δ13C and Δ14C records from the same site, spanning the last 20,000 years. In order to better constrain the best archive phase for bottom water Nd isotopes we compare bulk sediment leachate data with Fe-Mn oxide coatings removed from planktonic foraminifera and underpinned with fish debris data where possible. The planktonic foraminifera data are all within error of the fish debris and have a stronger correlation with the three other proxies than bulk sediment leachates indicating the coatings on planktonic foraminifera are recording bottom water Nd isotopes more accurately at the Bermuda Rise than bulk sediment leachates and, since readily available, are the optimal phase. All four proxies shift rapidly and synchronously during the last deglacial transition, but do not suggest changes in the vertical water mass stucture across Heinrich event 1, with a constant proportion of Northern and Southern source water reaching this site between 20 Ka and 14.6 Ka. This implies that under the glacial mode of ocean circulation perturbations in advection rates effect only intermediate depth water masses, without changes in vertical water mass structure. In comparison, glacial-interglacial shifts are accompanied by whole water column reorganisation coincident with changes in advection rates. [1] Piotrowski et al (2004) EPSL 225, 205-220. [2] McManus et al (2004) Nature 428, 834-837. [3] Keigwin et al (1991) JGR 96,16811-16826.