HR: 09:30h
AN: V41H-07    [Abstracts]
TI: Secular Variation of Oceanic Pb Isotopes as a Monitor of Glacial-Interglacial Weathering Rate Variability
AU: * Foster, G
EM: g.l.foster@bristol.ac.uk
AF: Department of Earth Sciences, University of Bristol, Bristol, BS8 1RJ United Kingdom
AU: Vance, D
EM: D.vance@bristol.ac.uk
AF: Department of Earth Sciences, University of Bristol, Bristol, BS8 1RJ United Kingdom
AB: The rate of continental weathering plays a central role in regulating atmospheric carbon dioxide concentration, and hence global climate, on both long and short time scales. Yet the role played by weathering in generating and maintaining the Earths present ice-house conditions is uncertain. We propose that Pb isotopes in ferromanganese crusts offer a fresh perspective on this problem and here we use a new high-resolution technique to resolve changes in the Pb isotope composition of the oceans on a 5-10 kyr timescale. Ferromanganese crusts are faithful recorders of the isotopic composition of oceanic water from which they precipitate. Their slow growth rate (1-10 mm/Myr), however, typically prohibits sampling at an age resolution of less than 200 kyr. Consequently, most studies are concerned with long-term secular changes in isotopic composition. With the advent of laser ablation multi-collector mass spectrometry, however, <10 kyr resolution is readily achievable, with no loss in analytical precision. When placed within high resolution age models constructed from solution U-series and orbital tuning of compositional variability, our data show that over the last 500 kyr there have been large glacial-interglacial swings (over 1%) in the Pb isotopic composition of North Atlantic Deep Water (NADW). In contrast, we find that the Nd isotopic composition of NADW has changed very little over the same time period (see Vance and Foster, this volume). This observation suggests the swings in Pb isotopic ratio are best explained in terms of weathering intensity in the source areas of the Pb through an established relationship between soil age, soil Pb isotopic composition, and cationic weathering flux. We find that during the glacial periods of the last 500 ka, relatively unradiogenic Pb predominates, reflecting a drop in weathering intensity in the circum-North Atlantic of 1.5-2.5 times, depending on ice volume. When scaled to land area, this decrease offsets the increase in weathering rate thought to occur as a consequence of sea level fall due to ice sheet growth. We suggest that the interplay between these two processes (sea level fall and ice sheet size) ensures that global weathering rates remain constant over short to intermediate (10³-10⁶ years) time scales despite large, glacially induced, changes in regional weathering rates.
DE: 0790 Weathering (1625, 1886)
DE: 1039 Alteration and weathering processes (3617)
DE: 1040 Radiogenic isotope geochemistry
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005