QUESTIONS POSED AND QUESTIONS ANSWERED BY PLIOCENE LAKES IN DEATH VALLEY AND ACROSS WESTERN NORTH AMERICA

Hydroclimate proxy data from lakes, vegetation, and stable isotopes suggest western North America was wetter in the Pliocene than today. This is unexpected because CO₂ was higher than preindustrial levels and models predict future drying in western North America. Pliocene proxy data also disagree with most Pliocene climate models, which simulate extensive drying. These discrepancies raise questions: 1) Are proxy data misinterpreted or biased? 2) Do models not accurately simulate moisture delivery to western North America in warm climates? 3) Is the Pliocene not analogous to the future? In Death Valley, superbly dated lacustrine sediments [Knott et al., 2018] raise a new question: 4) Is apparent Pliocene wetness actually a time-localized feature of the M2 glaciation at 3.3 Ma, and not broadly representative of warm Pliocene conditions?

Here we address 1) and 4) by reassessing and expanding past syntheses using lake-based Pliocene hydroclimate data. Past efforts were hindered by poor age control; data were often diachronous and reflected differing orbital forcing across sites. We reassess age control at all sites and update paleomagnetic dates, North American Land Mammal Ages, and correlated tephras to the most recent published ages. We then compare Pliocene and modern environments and assign each site “wetter,” “drier,” or “unclear” using lake presence/absence, type (perennial/ephemeral), lake extent, and salinity. To illustrate uncertainty, we create “wet” and “dry” maps. The dry map has only perennial lakes with unambiguous ages. The wet map adds perennial lakes with large age uncertainty, ephemeral lakes, and “potential” lakes, for which geophysical data indicate basin fill of probable Pliocene age.

We find the data still support a wetter Pliocene than today, as most clearly shown in the southwest Great Basin. There is a substantial difference between the wet and dry maps throughout the Pliocene, illustrating both natural hydroclimate variability and persistent large uncertainties in age control. Furthermore, the question of whether the M2 glacial was wetter than Pliocene interglacials outside of Death Valley remains difficult to resolve, due to limitations of existing age models. However, even the most conservative reconstruction implies wetter conditions than most models simulate for the mid-Pliocene.