Comparison of paleoprecipitation proxies in modern Texas floras and applications for early cretaceous fossil locations
Climate change has occurred often in Earth's history, the details of which may have bearing on preparing for our future greenhouse climate. Some of these details are available through careful analysis of sedimentary rocks. Various climate proxies such as foliar physiognomy and stable carbon isotope analysis used together can constrain climate parameters of the past such as mean annual precipitation to build a clearer picture of ancient ecosystems and their responses to changing climate. In this study, leaf litter from modern floras were collected across a precipitation gradient in Texas using a map of δ 13 C developed through application of published δ 13 C- precipitation models in a geographic information systems (GIS) environment to identify collection sites. Measured for δ13 C, the samples evaluated the effectiveness of recently published models of δ 13 C to predict precipitation in a ground-truthing of this tool for paleoclimate reconstructions. Both the Kohn (2010) algorithm and the Diefendorf et al. (2010) global dataset were accurate in estimating the precipitation of west Texas sites such as the Franklin Mountains near El Paso and the Davis Mountains north of Big Bend. However, these algorithms lost accuracy with mid-to east Texas sites. In these wetter environments, foliar physiognomic proxies were shown to be more accurate in their estimates of precipitation. Fossil charcoal separated from Early to md-Cretaceous sediments such as those collected in Yujingzi Basin, China and Arlington, Texas provide a test for the combination of these proxies in producing paleoclimate estimates under estimated conditions of δ 13 Catm and with offsets of ±2%. In these cases, the Kohn (2010) algorithm produced some paleoprecipitation estimates in the unreasonably dry end of the spectrum possibly due to its use of an offset from regression statistics. The Diefendorf et al. (2010) models also produced dry estimates for the White Pagoda samples, agreeing with the less dry Kohn estimates. The White Pagoda area is interpreted as a palustrine environment with very wet and very dry monsoonal seasons. The Arlington Archosaur Site estimates were wetter in both the Kohn and the Diefendorf models agreeing with the Mississippi-delta like interpretation of that environment for the given estimate of δ 13 Catm . Accuracy in assigning ages of strata, paleolatitude and paleoaltitude is necessary to an accurate estimate of paleoprecipitation.