HR: 17:30h
AN: PP34A-07    [Abstracts]
TI: The Behaviour of Uranium and Lithium Isotopes During Basalt Weathering and Erosion
AU: * Pogge von Strandmann, P A
EM: p.a.e.pogge-von-strandmann@open.ac.uk
AF: Department of Earth Sciences The Open Univeristy, Walton Hall , Milton Keynes, MK7 6AA United Kingdom
AU: Burton, K W
EM: k.w.burton@open.ac.uk
AF: Department of Earth Sciences The Open Univeristy, Walton Hall , Milton Keynes, MK7 6AA United Kingdom
AU: James, R H
PP34A-07 AF: Department of Earth Sciences The Open Univeristy, Walton Hall , Milton Keynes, MK7 6AA United Kingdom
AU: van Calsteren, P
PP34A-07 AF: Department of Earth Sciences The Open Univeristy, Walton Hall , Milton Keynes, MK7 6AA United Kingdom
AU: Gislason, S
PP34A-07 AF: Institute of Earth Sciences Univeristy of Iceland, Hjardarhaga 2-6, Reykjavik, 107 Iceland
AB: Basalt weathering rates are higher than for many other silicate rocks, and thus exert a strong control on atmospheric CO₂ [1]. Moreover, variations in weathering in response to climatic or tectonic change will inevitably affect chemical fluxes to the oceans. Li isotopes potentially provide key information on weathering rates, but their behaviour in the riverine and estuarine environment remains poorly constrained. While U-series nuclides can give information on both the weathering process and the timescale over which it occurs, comparison of both U and Li provides some insight into the processes which govern the geochemical cycles of each element. This study presents new U and Li isotope data for rivers draining basalt terrains in Iceland and Sao Miguel (Azores). Both of these islands are essentially monolithologic, but have different climatic regimes, and variations in river chemistry are linked to differences in the weathering environment, rather than to rock type. River waters from Iceland were taken from both a large catchment in the west of the island, an associated estuary, and from glacial rivers in the south. The U concentration in the rivers ranges from 0.2 to 20 ng/l, and rises from 3 ng/l to almost 3 μg/l in the estuary, showing conservative behaviour in the mixing zone. Li concentrations range from 0.02 to 1.05 μg/l in the rivers, and rise to 160 μg/l in the estuary. Li also behaves conservatively in the mixing zone. For the Azores U and Li river concentrations range from 3 to 212 ng/l and from 0.09 to 3.5 μg/l, respectively. Highest concentrations are linked to hydrothermal input. Uranium activity ratios (²³⁴U/²³⁸U) range from 1.13 to 2.14 in Iceland, and from 1.02 to 1.92 in the Azores, showing that silicate weathering can yield a large range of values (down to 1, which is secular equilibrium). Lowest U concentrations typically correspond to highest activity ratios, especially in glacial rivers. The behaviour of U in the catchment shows mixing between the inputs of glaciers, groundwater and hydrothermal water. The ²³⁴U/²³⁸U ratios in the colloidal fractions are closer to secular equilibrium than their host waters, indicating that these isotopes do not partition equally between the dissolved and colloidal phases. Li isotope values for rivers in Iceland range from 10 to 33 ‰, with low values due to hydrothermal input and high values stemming from superficial weathering in glacial rivers, indicating significant fractionation during weathering of the source basalts. These results indicate that although both elements behave conservatively in the mixing zone, the isotope signal to the oceans may be significantly modified by the weathering environment, in particular the balance of physical/chemical weathering and for U the presence of a colloidal phase. [1] Dessert, C. et al., 2001. Erosion of Deccan Traps determined by river geochemistry: impact on the global climate and the Sr-87/Sr-86 ratio of seawater. EPSL, 188(3-4): 459-474. [2] Riotte, J. et al., 2003. Uranium colloidal transport and origin of the U-234-U-238 fractionation in surface waters: new insights from Mount Cameroon. Chem Geol, 202(3-4): 365-381.
DE: 1039 Alteration and weathering processes (3617)
DE: 1040 Radiogenic isotope geochemistry
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005