Hot or not? Impact of seasonally variable soil carbonate formation on paleotemperature and O-isotope records from clumped isotope thermometry

Quantifying the timing and temperature of soil carbonate formation is important for interpreting isotopic records of Earth-surface temperature and soil water composition from paleosols. Pedogenic carbonates typically have been assumed to form at mean annual temperature, but recent work suggests warm-season bias in carbonate formation could impact the interpretation of δ¹⁸O values and carbonate formation temperatures for paleosol carbonates. We investigate the relationship between seasonally variable soil temperatures and carbonate formation temperatures estimated using clumped isotope thermometry (T(Δ₄₇)). Holocene soil carbonates were collected along the eastern flank of the Andes (33°S) over 2km of relief and a nearly 15°C range of mean annual air temperature (MAAT). This area receives both westerly, winter season precipitation and easterly, summer season precipitation, leading to a reversal of soil moisture regimes that occurs at ~2km elevation. In instrumented pits sampled to 1m depth, we do not observe systematic variation in T(Δ₄₇) with depth or with elevation-dependent MAAT. Average T(Δ₄₇) values for soil carbonates collected above 2km elevation reflect summer soil temperatures. In contrast, T(Δ₄₇) values below ~2km track mean annual soil temperature. These results may reflect the dominance of summer precipitation below ~2km, which likely delays soil drying and carbonate growth until fall. This seasonal variability in carbonate formation greatly impacts carbonate formation temperature, with important implications for estimating soil water δ¹⁸O values. Calculating soil water δ¹⁸O values using T(Δ₄₇) produces a soil water δ¹⁸O lapse rate of -4.6%/km, remarkably similar to that of modern surface waters (-4.8%/km), and significantly improving on previous soil water estimates assuming MAAT for carbonate formation temperatures. Although seasonal bias can prevent the straightforward translation of soil carbonate T(Δ₄₇) into meaningful surface air temperatures, direct constraints on temperature from clumped isotopes can both provide a window into soil processes and aid in the interpretation of conventional stable isotopic data to reconstruct surface conditions in the past.