HIGHER RESOLUTION O ISOTOPE RECORDS FROM PEDOGENIC CARBONATE LAMINATIONS, WIND RIVER BASIN, WYOMING

Oxygen and carbon isotopes in pedogenic carbonates provide informative records of continental climate and vegetation patterns, though such records are sometimes discontinuous or characterized by uncertain or low temporal resolutions. We propose that dense, laminated carbonate clast-coatings a few mm thick can provide late Quaternary paleo-environmental records with resolutions of a few ka when laser ablation ICP-MS U-series dating is combined with O isotope measurements by SIMS. Such coatings, when sampled for U-series and stable isotope measurements, respectively, at resolutions of ~90 and ~30 microns, reveal the preservation of long (>100 ka), quasi-continuous isotopic records. U-series age sequences confirm that selected laminations preserve closed U-Th systems (and by inference, primary stable isotopic compositions), and indicate that peak coating growth rates in the sampled laminations coincide with local glacial advances. Co-variation of U concentrations with initial U isotope ratios in the carbonates may reflect changes in the amounts or sources of dust delivered to Wind River terrace surfaces. Stable O isotopes oscillate between ~18 and 22 ‰ (SMOW), with changes roughly paralleling growth rate patterns. Interpreting such pedogenic carbonate O isotope records requires considering: (1) potential interference from O in interleaved pedogenic opal, as identified by SEM, (2) fluctuations in past soil temperatures, and (3) isotopic fractionation caused by soil-water evaporation. Nevertheless, appropriate sampling of laminated pedogenic carbonates can provide stable isotope records with resolutions of a few ka in arid to semi-arid continental settings.