A SYNTHESIS OF EXISTING CLUMPED ISOTOPE DATA REVEALS VARIED SEASONAL BIASES IN SOIL CARBONATE ACCUMULATION

Despite the importance of soil carbonates as paleoclimate archives, interpretations of their isotopic compositions (ẟ¹⁸O, ẟ¹³C, and ∆₄₇) is hampered by uncertainty in when and how soil carbonates accumulate. This uncertainty is critical to address because seasonal variability in surface temperatures is typically much larger than the secular variations through geologic time that we wish to infer from soil carbonates. Several studies have paired temperatures measured directly in the modern air/soil with temperatures estimated from the clumped isotopic composition (T∆₄₇) of Holocene soil carbonates in order to infer the timing of soil carbonate accumulation. Here, we synthesize these previous studies in order to identify which environmental conditions are critical to controlling the seasonal bias of carbonate formation, and the extent to which those conditions are preserved and interpretable in paleosol records. Considering recent improvements in the standardization of clumped isotope methods, we recalculate ∆₄₇ values where possible and only examine ∆₄₇ data that adheres to modern standards. Most soil carbonates record T∆47 values higher than mean annual air temperature, but the exceedance varies from 1-20°C. Similarly, the ẟ¹⁸O values of soil waters calculated from soil carbonate ẟ¹⁸O and T∆₄₇ values largely correlate with the ẟ¹⁸O values of modern rainfall, but disparities up to 10‰ exist. Disparities in the seasonal bias persist even when considering soil carbonates from similar localities (i.e., the eastern flank of the Andes). We hypothesize that the character and timing of precipitation (rain or snow) interacts with the texture of the soil matrix (fine or coarse) to modulate what time of year carbonates primarily accumulate, and thus which temperature and meteoric waters they record. While additional data is required to test this hypothesis, if true, descriptions of paleosol texture could be used to inform reconstructions of paleoclimates and paleoaltimetry.