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\begin{document}

\title{Drier winters drove Cenozoic open habitat expansion in North America}



\author[1,1]{Tyler Kukla}%
\author[2,2]{Jeremy Caves Rugenstein}%
\author[3,3]{Daniel Enrique Ibarra}%
\author[4,4]{Matthew Winnick}%
\author[5,5]{Caroline AE Strömberg}%
\author[1,1]{Page Chamberlain}%
\affil[1]{Stanford University}%
\affil[2]{Colorado State University}%
\affil[3]{Brown University}%
\affil[4]{University of Massachusetts Amherst}%
\affil[5]{Department of Biology and Burke Museum of Natural History and Culture, University of Washington}%


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  \date{June 17, 2024}


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\begin{abstract}
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 transition (OHT) is considered by some to be evidence for drier
summers, more seasonal precipitation, or a cooler climate, but others
have proposed that wetter conditions and/or warming initiated the OHT.
Here, we use published (n=2065) and new (n=173) oxygen isotope
measurements (\selectlanguage{greek}δ\selectlanguage{english}18O) in authigenic clays and soil carbonates to test the
hypothesis that the OHT is linked to increasing wintertime aridity.
Oxygen isotope ratios in meteoric water (\selectlanguage{greek}δ\selectlanguage{english}18Op) vary seasonally, and
clays and carbonates often form at different times of the year.
Therefore, a change in precipitation seasonality can be recorded
differently in each mineral. We find that oxygen isotope ratios of clay
minerals increase across the OHT while carbonate oxygen isotope ratios
show no change or decrease. This result cannot be explained solely by
changes in global temperature or a shift to drier summers. Instead, it
is consistent with a decrease in winter precipitation that increases
annual mean \selectlanguage{greek}δ\selectlanguage{english}18Op (and clay \selectlanguage{greek}δ\selectlanguage{english}18O) but has a smaller or negligible effect
on soil carbonates that primarily form in warmer months. We suggest that
forest communities in west-central North America were adapted to a
wet-winter precipitation regime for most of the Cenozoic, and they
subsequently struggled to meet water demands when winters became drier,
resulting in the observed open habitat expansion.%
\end{abstract}%



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