Plant wax isotope records of spatial and temporal change to climate and hydrology in western North America during the Miocene and Middle Miocene Climatic Optimum

Abstract:

The Miocene Epoch marks a prolonged period of terrestrial cooling and aridification in the late Cenozoic in concert with the long-term development of the Antarctic Ice sheet. Within the Miocene, the Middle Miocene Climatic Optimum (MMCO) represents a period of overall global warmth between 15-17 Ma, with mid-latitude temperatures of up to ~3-6°C hotter than modern. This interval of global warmth and possible decrease in Antarctic ice volume is argued to have occurred during a period with relatively modest, but highly variable atmospheric CO₂ (300-500 ppm) on a roughly 100-kyr timescale. Effects of prolonged Middle Miocene and peak MMCO warmth are readily observed in marine and terrestrial sediments, however there is considerable debate over the impact of globally-warm and peak MMCO conditions on spatial patterns of precipitation and temperature on land. Here we present hydrogen and carbon isotope data from Miocene-aged higher plant waxes across western North America to evaluate large-scale spatial patterns of isotope hydrology and climate during the Middle Miocene period. These data are coupled with modern regional biomarker C and H isotopic data and records through the interval including the Middle Miocene Climatic Optimum (~17-13) to assess temporal changes in isotope hydrology and climate during high-frequency atmospheric CO₂ fluctuations. Regional and high-resolution terrestrial wax data show generally wetter and warmer conditions than today in western North America during the Middle Miocene (~15 Ma) with short-timescale perturbations to regional hydrology during the MMCO. These terrestrial changes are coincident with the pacing of change observed in the marine realm and show two periods of hydrogen and carbon isotopic enrichment. Spatial patterns of middle Miocene leaf wax δD and temperature and short timescale fluctuations in δD and δ¹³C during the MMCO indicate close coupling of the continental system to short-duration variations observed in the marine/atmosphere system.The Miocene Epoch marks a prolonged period of terrestrial cooling and aridification in the late Cenozoic in concert with the long-term development of the Antarctic Ice sheet. Within the Miocene, the Middle Miocene Climatic Optimum (MMCO) represents a period of overall global warmth, with mid-latitude temperatures of up to ~3-6°C hotter than modern during peak conditions at ~15-17 Ma. This period of global warmth and relative decrease in Antarctic ice volume is argued to have occurred during a period with relatively modest atmospheric CO₂ marked by high frequency fluctuations (300-500 ppm) on a roughly 100-kyr timescale. Effects of prolonged Middle Miocene and peak MMCO warmth are readily observed in marine and terrestrial sediments, however there is considerable debate over the impact of globally-warm and peak MMCO conditions on spatial patterns of precipitation and temperature on land. Here we present hydrogen and carbon isotope data from Miocene-aged higher plant waxes across western North America to evaluate large-scale spatial patterns of isotope hydrology and climate during the Middle Miocene period. These data are coupled with biomarker C and H isotopic records through the interval including the Middle Miocene Climatic Optimum (~17-13) from sites in Montana and Idaho to assess temporal changes in isotope hydrology and climate during high-frequency atmospheric CO₂ fluctuations. Regional and high-resolution terrestrial wax data show generally wetter and warmer conditions in western North America during the Middle Miocene (~15 Ma) with short-timescale perturbations to regional hydrology during the MMCO. These terrestrial changes are coincident with pacing of change observed in the marine realm and show two periods of hydrogen and carbon isotopic enrichment, consistent with marine records of increased CO₂. Spatial patterns of middle Miocene leaf wax δD and temperature and short timescale fluctuations in δD and δ¹³C during the MMCO indicate close coupling of the continental system to short-duration variations observed in the marine/atmosphere system.