Carbon isotope analyses reveal relatively high abundance of C4 grasses during early-middle Miocene in southwestern Europe

Before their rapid expansion during the late Miocene, C4 plants were generally uncommon or absent from most grasslands. Recent studies have furthered our understanding of shifts in C4 abundance prior to their late-Miocene expansion; however, the early history of C4 grasses remains poorly constrained. Distinguishing C4 grasses from C3 taxa (i.e., most trees, shrubs, herbs, and about half of all grasses) using isotopic data in the paleorecord is challenging, especially when C4 plants are in relatively low abundance. We used SPIRAL (Single Pollen Isotope Ratio AnaLysis) to measure δ13C values of individual grass-pollen grains in early-middle Miocene sediments from three sites in Spain where grass pollen represents 5-20% of the pollen assemblages, including 25 samples from Rubielos de Mora, (20.3-15.5 Ma), seven from Gor (15.2-13.7 Ma) and seven from Andalucía A1 (13.7-10.1 Ma). Results indicate that C4-grass abundance varied between 21 and 72% of total grasses during the early-middle Miocene. C4 grasses are uncommon (10-20% of the grass flora) in this region today, and thus C4-grass abundance likely decreased after the early-middle Miocene as the region became cooler and precipitation shifted to the winter months. This pattern contrasts with that of a late-Miocene expansion of C4 grasses in many other regions of the world. Our results thus help elucidate the history of grasslands in Europe and the spatiotemporal dynamics of C4 grasses worldwide.