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

Abstract

Modern physical and chemical soil properties can favor or exclude C 3 and C 4 plants, yet little is known regarding these relationships from deep-time records that track the evolution and expansion of C 4 vegetation. In this study, we used a multi-proxy approach to reconstruct vegetation (C 3 vs. C 4 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 C 4 herbivory in the Great Plains of North America. Local proportion of C 4 biomass was assessed using stable carbon isotope ratios of calcium carbonates (δ 13 C cc ) and organic matter (δ 13 C 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 δ 13 C cc (range = −8.5 to −5.2‰ VPDB) and δ 13 C om values (range = −25.9 to −24.2‰ VPDB) to be consistent with C 4 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 δ 13 C values, contrary to some modern mixed C 3 -C 4 biomes. Using late Miocene CO 2 and paleoclimate model reconstructions, we argue that conditions were at or near crossover thresholds favoring C 4 over C 3 photosynthesis in the Great Plains despite the low abundance of C 4 vegetation across the paleolandscapes. This study presents evidence that abiotic factors that select for C 4 plants in modern systems—high growing season temperature, low atmospheric CO 2 , salinity-sodicity, and soil texture—were less influential in the late Miocene than biotic factors (i.e., ecological feedbacks) that suppressed C 4 plants prior to their increase in abundance in the Great Plains in the Pliocene. • Paleosols were studied from the late Miocene Coffee Ranch fossil site. • Two paleosol profiles show possible evidence of saline-sodic conditions. • C 4 biomass was estimated using δ 13 C analyses on carbonates and bulk organic matter. • A Monte Carlo analysis was used to assess uncertainty in C 4 biomass estimates. • Low abundance of C4 plants suggests biotic control on plant community composition.