HYDROLOGICAL CHANGES DURING THE MIDDLE MIOCENE CLIMATIC OPTIMUM IN SOUTHWEST MONTANA USING MOLECULAR BIOMARKERS OF LEAF WAXES

The Middle Miocene Climatic Optimum (MMCO) occurred ~17-15 Ma and is characterized by global temperature ~3-6ºC warmer than modern. While there is debate over the mechanism behind climate change during this interval, a growing body of data suggests that increased warmth resulted from increased pCO₂. Global temperature during the MMCO is equivalent to several model predictions for modern climate over the next 100 years. An unresolved issue of modern climate change is how hydrological patterns will change with increased warmth and pCO₂. Samples were collected from the Miocene Railroad Canyon Sequence (RCS) in Southwest Montana to develop a record of isotope hydrology during the MMCO using hydrogen and carbon isotopes of terrestrial plant waxes. The RCS was deposited within an intermontane basin of the Rocky Mountains along the border of Montana and Idaho. Sediments are composed of siltstones, sandstones and intermittent ashes that span 17.8-12 Ma and show gradual aridification and a transition from lacustrine to fluvial depositional environments.

The isotopic compositions of n-alkanes synthesized in plant leaf waxes preserve a record of paleoenvironmental conditions at the time of wax synthesis, including isotopes of precipitation and plant water use efficiency. The δD and δ¹³C values of n-alkanes show a general decrease over the study interval with two brief periods of enrichment occurring ~15.8 and ~17.6 Ma. These data indicate long-timescale changes in regional conditions and a highly variable hydrologic system during the MMCO. Short intervals of enrichment suggest the presence of a drier climate for brief periods of time within a broader trend of reduced plant water stress during an episode of increased global warmth.