LATE MIOCENE ATMOSPHERIC CIRCULATION AND HIGH TOPOGRAPHY IN THE CENTRAL ROCKY MOUNTAINS: CONSTRAINTS FROM INTEGRATED CARBONATE CLUMPED ISOTOPE AND VOLCANIC GLASS HYDROGEN ISOTOPE STUDIES

Precipitation isotope values reflect the complex interaction of atmospheric circulation, topography, and climate change. We examine late Miocene (~9 ± 2 Ma) atmosphere circulation pattern and paleotopography in the central Rocky Mountain area by integrating the reconstructed mean annual precipitation δD values from volcanic glass and summer precipitation δ¹⁸O values from clumped isotope thermometry of lacustrine and soil carbonate. Our samples were collected from a longitudinal transect of decreasing elevation from the central Rocky Mountains to the western Great Plains. Our results show that late Miocene soil temperature and precipitation isotope values increase eastward, with similar temperature and greater isotopic gradient relative to present, suggesting that a similar to modern elevation gradient along the transect had been established by ~9 ± 2 Ma. Our results also indicate that the late Miocene summer precipitation δ¹⁸O values are 2 - 4 ‰ lower, and the mean annual precipitation δD values are 0 - 5 ‰ higher than modern precipitation isotope values in the central Rocky Mountains, whereas in the western Great Plains the late Miocene summer precipitation δ¹⁸O values are 0 - 1‰ lower, and the mean annual precipitation δD values are 10 - 20 ‰ higher than modern values. Late Miocene winter precipitation must be abundant and isotope-enriched to balance the isotope-depleted summer precipitation in order to increase mean annual precipitation isotope values. We propose different-from-modern atmospheric circulation patterns and climate to explain the results. We suggest that the late Miocene summers were cool and moist with intense monsoonal precipitation originated from the equatorial Pacific Ocean and Gulf of Mexico. The equatorial Pacific air masses experienced rainout as they passed the high Sevier hinterland plateau before its latest Miocene collapse. The late Miocene winters were warm and moist, with El-Niño-like atmospheric circulation bringing water vapor from the warm eastern Pacific Ocean. Cool summer and warm winter air temperatures favor isotope-depleted summer precipitation, and isotope-enriched winter precipitation, suggesting that the late Miocene climate in the central Rocky Mountain area were less seasonal than today.