Vegetation response during the lead-up to the middle Miocene warming event in the Northern Rocky Mountains, USA

Abstract

The mid-Miocene climatic optimum (MMCO; initial warming beginning ca. 18Ma and with peak warming ca. 17–14.75Ma) constitutes the Earth's most recent greenhouse episode, characterized by a transient shift to higher global atmospheric CO2 levels and warmer, possibly wetter, climatic conditions. Combined with the spread of grass-dominated habitats, it has been suggested that the MMCO lead to significant turnover and modernization of fauna and flora. However, records documenting local vegetation change during the MMCO are rare, hence this hypothesis remains largely untested. Herein, we combine phytolith assemblages and δ13C records from paleosol organic matter from the Railroad Canyon section (RCS), eastern Idaho, to provide the first direct, detailed, and continuous, long-term record of vegetation composition and structure in a single basin during the lead-up to the MMCO (22.9–15.2Ma; late Arikareean–early Barstovian). Phytolith assemblage analysis indicates that grasses, primarily C3 pooids, dominated early–middle Miocene vegetation. Potential C4 PACMAD grasses were present by ca. 21.6–20.7Ma (late Arikareean) and vary in abundance (0.4–17.2% of diagnostic phytoliths) through the section. Although relatively rare overall, PACMAD grasses decrease significantly through time, in parallel with declining diatom abundances and an increase in pooid grasses. In contrast, paleosol δ13Corg indicates 0–29.1% C4 vegetation but no consistent temporal change. Where both phytolith- and isotope-based estimates are made from the same paleosol, they are within error most (82%) of the time; thus, the apparent discrepancy in temporal trends could either be due to differences in the resolution of the two records, or could be explained if many PACMADs were not C4 grasses, but instead water-loving C3 species. Overall, these data suggest that open-habitat mosaic vegetation with grass-dominated grasslands and open woodlands occurred in the North Rocky Mountains, USA, by at least the early Miocene, more comparable to the timing for the spread of grass-dominated habitats in the Great Plains than previously thought. Additionally, phytolith data suggest that overall vegetation structure remained relatively stable throughout the RCS, inconsistent with published paleosol morphology data. This points to regionally unique floral patterns decoupled from global climate change leading into the MMCO.