Paper No. 209-5
Presentation Time: 9:15 AM
EQUILIBRIUM FORMATION AND KINETIC ISOTOPE EFFECTS IN SOIL CARBONATES FROM COLD ENVIRONMENTS: THE EFFECTS OF SOIL MOISTURE AND SOIL SEDIMENT SIZE
The clumped and stable isotope (Δ47, δ18O, and δ13C) composition of pedogenic (soil) carbonates from cold, arid environments may be a valuable paleoclimate archive for climate change-sensitive areas at high latitudes or elevations. However, previous work suggests that the isotopic values of cold-climate soil carbonates can be modified by kinetic isotope effects (KIE). To evaluate the conditions under which KIE occur in cold-climate soil carbonates, we analyze the Δ47, δ18O, and δ13C composition of soil carbonate pendants from Antarctica (Dry Valleys, 77°S), the Arctic (Svalbard 79°N), the Chilean Andes (4700 m elevation), and the Tibetan plateau (3800-4800 m), and compare the results to local climate and water δ18O records. At each site we calculate the expected equilibrium soil carbonate Δ47 and δ18O values and estimate carbonate Δ47 and δ18O anomalies (observed Δ47 or δ18O minus the expected equilibrium Δ47 or δ18O). To provide context for interpreting the anomalies, the soil carbonate results are compared to results for sub-glacial carbonates, which exhibit large Δ47 anomalies (≤ -0.29 ‰). The Antarctic and Andean samples have negative Δ47 anomalies and positive δ18O anomalies consistent with KIE due to rapid bicarbonate dehydration during cryogenic carbonate formation. In contrast, the Arctic and Tibetan results are consistent with equilibrium, summer carbonate formation. We attribute the differences in Δ47 and δ18O anomalies to variations in soil moisture and inter-cobble matrix grain size. The Antarctic and Andean soils are hyper-arid and have coarse-grained matrices that facilitate rapid bicarbonate dehydration. In contrast, the Arctic and Tibetan soils have finer-grained matrices, as well as slightly higher soil moisture levels, that decrease effective soil pore space and limit bicarbonate dehydration rates, leading to equilibrium carbonate formation. Our findings suggest that carbonates from soils with coarse-grained matrices may exhibit KIE in cold climates, making them poor paleoclimate proxies. Soils with fine-grained matrices should yield equilibrium carbonates suitable for paleoclimate reconstructions regardless of climate. Paleosol matrix grain size should therefore be taken into account in the evaluation of carbonate stable and clumped isotope values in paleoclimate studies.