Paleovegetation and paleo-pedogenic properties from the upper Miocene Coffee Ranch fossil site in the North American Great Plains

Modern physical and chemical soil properties can favor or exclude C₃ and C₄ plants, yet little is known regarding these relationships from deep-time records that track the evolution and expansion of C₄ vegetation. In this study, we used a multi-proxy approach to reconstruct vegetation (C₃ vs. C₄ biomass) and pedogenic properties (soil alkalinity, salinity, sodicity, and texture) from paleolandscapes at Coffee Ranch, Texas, a site from which fossil horses provide the earliest evidence for C₄ herbivory in the Great Plains of North America. Local proportion of C₄ biomass was assessed using stable carbon isotope ratios of calcium carbonates (δ¹³C_cc) and organic matter (δ¹³C_om) analyzed on four different paleosol types, freshwater tufa, and reworked carbonate nodules in fluvial channel lags. Using a Monte Carlo uncertainty analysis, we interpret δ¹³C_cc (range = −8.5 to −5.2‰ VPDB) and δ¹³C_om values (range = −25.9 to −24.2‰ VPDB) to be consistent with C₄ biomass low in abundance and variability at the study site, but with large uncertainties that would be overlooked using simple linear mixing model approaches. Paleo-pedogenic properties were reconstructed using pedotransfer functions and provide evidence of possible salinity and sodicity in two of five paleosol profiles. However, saline-sodic conditions and soil texture were not correlated with δ¹³C values, contrary to some modern mixed C₃-C₄ biomes. Using late Miocene CO₂ and paleoclimate model reconstructions, we argue that conditions were at or near crossover thresholds favoring C₄ over C₃ photosynthesis in the Great Plains despite the low abundance of C₄ vegetation across the paleolandscapes. This study presents evidence that abiotic factors that select for C₄ plants in modern systems—high growing season temperature, low atmospheric CO₂, salinity-sodicity, and soil texture—were less influential in the late Miocene than biotic factors (i.e., ecological feedbacks) that suppressed C₄ plants prior to their increase in abundance in the Great Plains in the Pliocene.