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.