Drier Winters Drove Cenozoic Open Habitat Expansion in North America

The shift from denser forests to open, grass-dominated vegetation in west-central North America between 26 and 15 million years ago is a major ecological transition with no clear driving force. This open habitat (OHT) considered by some be evidence for drier summers, more seasonal precipitation, or cooler climate, but others have proposed that wetter conditions and/or warming initiated the OHT. Here, we use published (n = 2,065) new 173) oxygen isotope measurements (δ18O) authigenic clays soil carbonates test hypothesis OHT linked increasing wintertime aridity. Oxygen ratios meteoric water (δ18Op) vary seasonally, often form at different times of year. Therefore, change precipitation seasonality can recorded differently each mineral. We find clay minerals increase across while carbonate show decrease. result cannot explained solely changes global temperature summers. Instead, it consistent decrease winter increases annual mean δ18Op (and δ18O) has smaller negligible effect on primarily warmer months. suggest forest communities were adapted wet-winter regime most Cenozoic, they subsequently struggled meet demands when winters became drier, resulting observed expansion.