DERIVING QUANTITATIVE LOWER LIMITS ON PALEO-PRECIPITATION BASED ON THE PRESENCE OF COAL AND LIGNITE: A MULTIPROXY APPROACH

Paleo-precipitation reconstructions are a key component for characterizing the climate system in deep time. Such reconstructions are an essential source of information regarding Earth’s hydrologic system under greenhouse conditions, which are thought to be characterized by enhanced cycling. The presence of coal and lignite in sedimentary successions has historically been taken as a qualitative indicator of humid conditions; however, quantitative paleo-precipitation estimates based on the presence of coal and lignite have not been possible because the correlation between coal/lignite accumulation and local precipitation is also influenced by mean annual temperature (MAT). In this study, we develop and test the accuracy of a new multi-proxy method for estimating lower limits of paleo-precipitation based on the presence of coal and lignite using local estimates of MAT. Employing modern datasets of global peat localities, MAT and mean annual precipitation (MAP), we show that the lower 10^th percentile of MAP in peat-bearing areas increases with increasing MAT. Based on this observation, we hypothesize that increasing MAT leads to higher evapotranspiration rates, which in turn require higher MAP values to sustain peat accumulation. Based on our results, we present a regression equation that estimates MAP lower limits at coal/lignite localities. To constrain the accuracy of this new climofunction we perform a Monte Carlo analysis comparing the predicted MAP lower limits to actual MAP values at modern peat localities. Our results provide a new tool for using occurrences of coal and lignite in sedimentary successions as a quantitative “censored” paleo-precipitation proxy, which should allow for more robust characterizations of ancient hydrologic systems during greenhouse periods where lithologic indicators like these are common.