The effect of soil temperature seasonality on climate reconstructions from paleosols

Abstract

Paleosols comprise an important archive of continental paleoclimate information throughout geologic history, but resolving temperature seasonality poses a challenge to the application of paleosol-derived temperature proxies. Not only does seasonality exert a strong control on the biosphere, but it can also obfuscate accurate interpretation of proxy records of paleotemperature. In order to examine the effect temperature seasonality has on paleosol temperature proxies, soil temperature data were compiled from over 200 stations that comprise the NRCS Soil Climate Analysis Network. Observed and modeled seasonal soil temperature variations were then compared to quantify system behavior. Greater than predicted soil temperature seasonality is observed at nearly half of the sites, driven primarily by direct heating of the soil surface by solar radiation. The ground-heating effect becomes more pronounced at sites receiving less than 600 mm of annual precipitation, with an average effect > 4 °C when mean annual precipitation falls below 300 mm. Large ground heating effects cannot be presumed for all carbonate-bearing paleosols, but the effect can be approximated when combined with paleo-precipitation estimates. Approximately two-fifths of sites record less temperature variation than predicted. This reduction in soil temperature seasonality is a result of warmer cold-season soil temperatures, driven by processes such as snow cover insulation and explains why pedotransfer functions break down below mean annual air temperatures (MAAT) of 6 to 8 °C. Clumped isotope measurements of pedogenic carbonate from a stacked series of late Eocene paleosols from Northeastern Spain are also examined to demonstrate how the documented seasonal trends in modern soils can inform paleo-applications, by considering carbonate formation depth and predicted ground heating effect. These paleosol results are best explained by a MAAT of ∼27 °C with annual temperature seasonality of 25 °C.