Reassessment of the Pliocene hydroclimate proxy network in western North America yields new insights into spatial patterns and temporal variability
Authors: White Sarah , Ibarra Daniel , Wahl David 
Published: July 2022

&lt;p&gt;Hydroclimate proxy data from lake sediments, vegetation, and stable isotope measurements suggest western North America was wetter in the Pliocene than today. This is unexpected because CO&lt;sub&gt;2&lt;/sub&gt; was higher than preindustrial levels and models predict future drying in western North America. Pliocene proxy data also disagree with most PlioMIP2 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 PlioMIP2 orography accurate? Furthermore, recent work in southeastern California [Knott et al., 2018] raises a new question: 4) Is apparent Pliocene wetness actually a time-localized feature of the M2 glaciation at 3.3 Ma, and not representative of the KM5 interglacial (targeted by PlioMIP2) or the warm Pliocene as a whole?&lt;/p&gt;
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&lt;p&gt;Here we address 1) and 4) and touch on 2) by reassessing and expanding past syntheses [e.g. Pound et al., 2014; Ibarra et al., 2018; Fu et al., 2021] of Pliocene hydroclimate data. We improve on previous syntheses in two key ways: age control and characterizing uncertainty. Past compilations 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 datums to the most recent published timescales. Individual sites are assessed as wetter, drier, similar, or unclear relative to today. To illustrate uncertainty, we create &amp;#8220;conservative&amp;#8221; and &amp;#8220;generous&amp;#8221; maps. For a given time period, the conservative map has only well-dated perennial lakes, vegetation, and stable isotope data. The generous map shows the conservative map plus ephemeral lakes, &amp;#8220;potential&amp;#8221; lakes (for which geophysical data indicate basin fill of probable Pliocene age), and poorly dated perennial lakes, vegetation, and stable isotope data.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;We find the data still support a qualitatively wetter western North America in the Pliocene than today, in agreement with recent quantitative results by Menemenlis et al. (revised) showing wetter conditions in southeastern California. Our conservative map/generous map approach elucidates temporal variability that cannot explicitly be resolved, including changes between M2 and KM5. Despite uncertainties in the proxy data, even the most conservative reconstruction implies wetter conditions than most models simulate for KM5 [Menemenlis et al., revised; this study]. Distinguishing between mechanisms of moisture transport, including El Ni&amp;#241;o-like teleconnections from the tropical Pacific [Goldner et al 2011], weaker Hadley circulation [Burls and Fedorov 2017], or wetting from higher coastal SSTs [Fu et al 2022], awaits further compilation work (ongoing). However, it is clear that the &amp;#8220;wet gets wetter, dry gets drier&amp;#8221; spatial pattern of change in most PlioMIP2 models is not supported by proxy data.&lt;/p&gt;