HR: 09:45h
AN: V41H-08    [Abstracts]
TI: No change in the neodymium isotope composition of deep water exported from the North Atlantic on glacial-interglacial timescales
AU: Vance, D
EM: D.Vance@bristol.ac.uk
AF: Department of Earth Sciences, University of Bristol, Bristol, BS8 1RJ
AU: * Foster, G
EM: g.l.foster@bristol.ac.uk
AF: Department of Earth Sciences, University of Bristol, Bristol, BS8 1RJ
AB: Ocean circulation controls climate in two key ways: it transports heat and it determines the rate at which the carbon-laden deep ocean equilibrates with the atmosphere. In the Atlantic the meridional overturning circulation is a particularly important mechanism for the inter-hemispheric transport of heat. Recent studies¹ have used neodymium (Nd) isotopes in the Southern Ocean to confirm a weakening of southward deep water export from the North Atlantic, and by implication a diminution in northward heat transport at the surface, during glacial periods. The timing of the Nd shift also suggests that such weakening post-dates both ice-sheet growth and major re-organisation of carbon reservoirs at glaical-interglacial boundaries. The use of Nd isotopes in the Southern Ocean to track changes in deep water export from the North Atlantic requires a knowledge of the secular evolution in the North Atlantic source itself. Here we report results of un-precedently high resolution (20 kyr) Nd isotope analyses of Fe-Mn crusts, obtained by laser ablation MC-ICPMS. The crusts we have analysed come from 1800-2000m in western North Atlantic and are well-placed to sample North Atlantic Deep Water (NADW). The data show that NADW has maintained an epsilon Nd of around -12.5 to -13.5 for the past 500 kyr, through 5 glacial-interglacial (G-IG) cycles. There is a hint of a slight shift towards more radiogenic values during glacials, but these variations are within the uncertainties on the analyses (about 0.5 epsilon units). The first conclusion from these data is that attempts to use the Nd isotope composition of the deep Southern Ocean as a record of the strength of NADW¹ are not complicated by changes in the isotopic composition of the northern source itself. However, the constancy of the Nd isotopic composition of NADW through G-IG climate change is in itself a surprising result. The unradiogenic character of Nd in NADW at the present day is determined by the contribution from Labrador Sea Water. Our data require that the NADW mix during glacial periods resulted in a co-incidentally similar Nd isotope composition despite the changes in the sites of convection that models of the circulation seem to demand². ¹ Piotrowski, A. et al. 2005, Science 307, 1933 ² Rahmstorf, S. 2002, Nature 419, 207
DE: 1040 Radiogenic isotope geochemistry
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 4962 Thermohaline
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005