HR: 1330h
AN: PP23A-05    [Abstracts]
TI: Effects of Continental Weathering and Sedimentary Sorting on the Hf, Nd, and Pb Isotopic Composition of Sediments and Implications for Dissolved Input to the Ocean
AU: * van de Flierdt, T
EM: tina@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AU: Hemming, S R
EM: sidney@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AU: Goldstein, S L
EM: steveg@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AB: The application of radiogenic isotopes of dissolved trace metals (Hf, Pb, Nd) in the ocean to study climate induced weathering changes on the continents is a relatively new and exciting field of research. Most studies on the radiogenic isotope composition of past seawater are based on chemical sediments such as ferromanganese crusts, and suggest a strong linkage in changes in the style of weathering (chemical vs. physical/glacial) on the continents and the isotopic composition of seawater for elements such as Hf and Pb. This is evidenced by records from the northwest Atlantic, which show significant changes in the Hf and Pb isotopic composition of seawater associated with the onset of Northern Hemisphere Glaciation (NHG) over the past 3 Myr. On the other hand, the impact of glacial weathering on the Nd isotopic composition of seawater is generally thought to be negligible. In order to better understand the systematics and mechanisms of weathering, erosion, and run-off, as well as their bearing on the isotopic composition of the ocean we choose the following strategy: sediments from the Amerilik and Gothabs Fjords near Nuuk (Greenland) have been analyzed for their bulk rock and grain size specific Hf, Nd, and Pb isotopic compositions (sand, silt, mud). Both Fjords are surrounded by early Archean gneisses, among the oldest rocks on Earth, and are filled with glacially floured sediments. Precambrian rocks surround a large fraction of the circum-North Atlantic area that was covered by ice sheets during the NHG, and the source rocks in this region are the most extreme examples. Current knowledge about the impact of weathering processes on isotope systematics implies that the largest changes in the Hf (and Pb) isotopic composition of seawater are expected to arise from glacial weathering of old continental sources. Our data reveal large differences in grain size specific isotope ratios, e.g. between the mud and sand fractions, for sediments with an old provenance. Hafnium and Pb show the lowest isotopic ratios in sands and the highest ratios in clays, while Nd isotopes display the opposite trend. This grain size specific fractionation of the isotopic ratios is in line with known partitioning of Lu-Hf, Sm-Nd, and U-Th-Pb in main and trace minerals (e.g. apatite, zircon, monazite, garnet, feldspar). In Nd vs. Hf isotope space the new data describe a trend vertical to the terrestrial array with a 20 ε unit range in Hf isotopes and a 8 ε unit range in Nd isotopes, proving that sedimentary sorting could be responsible for deflection of points from the terrestrial array. This suggests that the combined effect of sedimentary sorting and incongruent weathering may be of major importance for understanding and interpreting the glacial-interglacial radiogenic isotope composition of North Atlantic deep water.
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 4885 Weathering
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2005 Joint Assembly