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Project Progress

Effects of climate on mineral weathering rates. We continue work with Associate Member O.A. Chadwick in applying radiogenic isotopes to understand regional and global patterns of soil weathering and nutrient cycling. Weathering models developed from an isotopic investigation of a basaltic soil climosequence in Hawaii indicate that a major acceleration in mineral weathering rates occurs above mean annual precipitation values of 120-160 cm/yr, suggesting a non-linear “threshold” for weathering on a volcanic substrate.

Sources of solutes to the oceans. Marine carbonates provide a global record of solutes introduced into the oceans by terrestrial weathering and hydrothermal exchange. In a study of major streams of the ~8500 km² Owens Lake, California drainage basin, we showed that hydrothermal exchange and acid production by sulfide reduction may be the most important factors in the basin-averaged ⁸⁷Sr/⁸⁶Sr, even during a major climate shift. Neodymium isotope variations in clastic and authigenic sediments in Owens Lake sediments indicate a systematic shift in the nature of the clastic sediment source related to a global climate shift from glacial to interglacial conditions. These studies help to clarify the relative importance of climate and solute sources to the ocean, and provide guidance for understanding the geochemical record preserved in ancient marine carbonates.

Stratification in Precambrian Oceans. Our neodymium isotope results from the Hamersley Basin (2.5-2.6 Ga) of Western Australia (in collaboration with M. Bau, Pennsylvania State Astrobiology Research Center (PSARC)) indicate that the shallow marine carbonates have an isotopic signature that is surprisingly close to the deeper banded iron formation deposits; these results argue against a Precambrian ocean that was stratified with respect to REE and Fe. Neodymium isotope data from the deeper marine carbonates define an isochron with an age of ~2.1 Ga, which is consistent with a previously inferred metamorphic event.

Planetary Geochronology. Extending our understanding of the Earth’s evolution to the terrestrial planets requires strong geochronologic constraints for planetary surface features. As an outgrowth of our efforts to determine ages of rock units at the Earth’s surface, we are continuing to make progress in our work with G. Cardell (Jet Propulsion Laboratory (JPL)) and Associate Member D.A. Crown (Planetary Science Institute) to develop an instrument capable of in situ geochronology of surface igneous rocks on Mars.