HR: 0800h
AN: H31D-1331    [Abstracts]
TI: A Sequential Leach Method and Pb Isotope Approach to Studying Apatite Weathering in Granitoid Soils at Hubbard Brook Experimental Forest, NH, USA
AU: * Nezat, C A
EM: cnezat@umich.edu
AF: University of Michigan, Geological Sciences 1100 North University Avenue, Ann Arbor, MI 48109-1005 United States
AU: Blum, J D
EM: jdblum@umich.edu
AF: University of Michigan, Geological Sciences 1100 North University Avenue, Ann Arbor, MI 48109-1005 United States
AB: Easily dissolved minerals such as calcite and apatite can be important in controlling stream and ground water chemistry even though these minerals are only present in trace amounts in granitoid rocks. Because of its solubility, apatite, a calcium phosphate mineral, may be a significant source of essential nutrients (especially phosphorous) for vegetation, and has been shown to strongly influence stream and soil water composition (e.g, calcium, strontium and rare earth elements). There are additional sources of Ca (e.g., feldspars, hornblende) and P (e.g., organic matter or bound to Fe and Al oxides) in granitoid soils. In order to distinguish the chemical constituents of apatite from other pools in the bulk soil, we selectively dissolved apatite with a dilute acid leach, and measured Pb isotopic ratios of apatite, feldspar, and leachates. We tested the leaching procedure on mineral separates and verified that a dilute nitric solution primarily dissolves apatite. Silicates were dissolved in subsequent steps by successively stronger acids. We then applied this method to bulk soils collected from several soil pits across a small watershed at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA, to determine the spatial distribution of Ca and P pools, and determine the depth of apatite depletion in the soil. We also measured Pb isotope ratios in the soil leachates to distinguish among the various sources of Pb (e.g., apatite, feldspars and anthropogenic sources). We found that Pb in the dilute nitric leach of the HBEF organic soils is dominated by anthropogenic sources and that Pb from apatite becomes increasingly important with depth.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 1030 Geochemical cycles (0330)
DE: 1065 Major and trace element geochemistry
DE: 1865 Soils (0486)
DE: 1886 Weathering (0790, 1625)
SC: Hydrology [H]
MN: Fall Meeting 2005