Constraining hydroclimate interpretations of the North American monsoon since the Last Glacial Maximum

Despite its importance for sustaining human populations and ecosystems in southwestern North America, little is understood regarding the response of the North American monsoon (NAM) to orbital-scale climate forcings since the Last Glacial Maximum (LGM). While southwestern North American proxy reconstructions of late Pleistocene and Holocene d18O of speleothem calcite (d18OC) and dD of leaf wax n-alkanes (dDWAX) differ from reconstructions from other Northern Hemisphere monsoonal regions, their signal has been difficult to interpret due to the complex influence of seasonality and moisture source location on stable water isotope values reconstructed from a small number of continuous isotopic records from this region. Here, we investigate the orbital-scale changes in both summer and winter hydroclimate in southwestern North America from the LGM to the Preindustrial era (PI) to better understand the evolution of the NAM and its isotopic signature. We show that the North American monsoon generally increases in strength from the LGM to the PI and introduce the hypothesis that the greening of the African Sahara in the early to mid-Holocene may suppress NAM rainfall, especially at the northern fringe of the monsoon, via modifications to the local Walker circulation. This study provides a first-ever comparison between water isotope-enabled Earth system models and proxy reconstructions in southwestern North America to investigate NAM hydroclimate in the late Pleistocene and Holocene. Thus, this work contributes to disentangling the influences of summer and winter hydroclimate on reconstructions of d18OC and dDWAX in order to better understand the NAM evolution since the LGM.