HR: 16:15h
AN: PP34A-02 [Abstracts]
TI: Resolving Variations in Continental Weathering Flux From Changes in Continental Source Using Marine
Radiogenic Isotope Records
AU: * Burton, K W
EM: k.w.burton@open.ac.uk
AF: Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA
United Kingdom
AU: Gannoun, A
EM: a.gannoun@open.ac.uk
AF: Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA
United Kingdom
AU: Allegre, C J
EM: allegre@ipgp.jussieu.fr
AF: Lab. de Gochimie et Cosmochimie, CNRS, IPG-Paris, 4 Place Jussieu, Paris, 75252
France
AU: Christensen, J N
EM: jnchristensen@lbl.gov
AF: LBNL, Centre for Isotope Geochemistry, MS 70A-4418, 1 Cyclotron Rd, Bldg 70A4418, Berkeley, CA 94720
United States
AU: Hein, J R
EM: jhein@usgs.gov
AF: U.S.G.S, MS 999, 345 Middlefield Road, Menlo Park, CA 94025
United States
AB:
Determining the past record of chemical weathering is essential for understanding changes in climate and atmospheric CO2,
such as those that occur throughout the Cenozoic (the last 65 Million years) (e.g. 1). Many natural radiogenic isotopes in
seawater are sensitive to variations in chemical weathering (e.g. 2), but none alone can distinguish such changes from those
caused by variations in erosional source. Comparison of isotope systems with different sources and different behaviour in
seawater can however resolve such effects.
This study presents multi-isotope records for both long (Cenozoic) and short (glacial-interglacial) timescales, and these are
used to deconvolve changes in the balance of continental weathering from those related to changes in source. Comparison of a
high-resolution marine 187Os/188Os record for the Cenozoic with Pb, Hf and Nd isotopes (from the same sample [3-5]) strongly
suggests that for much of the Cenozoic isotope variations reflect a simple change in the balance of continental versus
hydrothermal input, with little change in continental source. Changes in this relationship at the middle Miocene climate
transition, at least to some extent, reflect a major reorganisation of ocean circulation [6] resulting in a shift of the
geographical sources of weathered continental material. Comparison of Sr, Os and Nd records for the past 140 ka from the Bay
of Bengal suggest that changes in the Nd isotope composition of seawater are climatically driven by local changes in the
composition of riverine input, whereas variations in Os likely reflect a change in the balance of continental weathering.
These studies highlight the difficulties of using a single radiogenic isotope system as a proxy for continental weathering or
source, but show that by using a combination of radiogenic isotopes with different sources to the oceans, different
behaviour during weathering, and different residence times in the oceans it may sometimes be possible to resolve such
effects.
[1] J.C.G. Walker, P.B. Hays, J.F. Kasting. JGR. 86, 9776-9782 (1981). [2] M. Frank. Rev. Geophys. 40(1), 1001, 1-38
(2002).[3] H.-F. Ling, K.W. Burton, R.K. O'Nions, B.S. Kamber, F. von Blanckenburg. EPSL. 146, 1-12 (1997). [4] J.N.
Christensen, A.N. Halliday, L.VC. Godfrey, J.R.Hein, D.K. Rea. Science 277, 913-918 (1997).
[5] D.-C. Lee, A.N. Halliday, J.R. Hein, K.W. Burton, J.N. Christensen, D. Gnther. Science 285, 1052-1054 (1999). [6] J.
Zachos, M. Pagani, L. Sloan, E. Thomas, K. Billups. Science 292, 686-693 (2001).
DE: 0330 Geochemical cycles (1030)
DE: 1040 Radiogenic isotope geochemistry
DE: 1615 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 4805, 4912)
DE: 4914 Continental climate records
DE: 4924 Geochemical tracers
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005