Data from: Stable isotope evidence for long-term stability of large-scale hydroclimate in the Neogene North American Great Plains

The Great Plains of North America host a stark climatic gradient,
separating the humid and well-watered eastern US from the semi-arid and
arid western US, and this gradient shapes the region’s water availability,
its ecosystems, and its economies. This climatic boundary is largely set
by the influence of two competing atmospheric circulation systems that
meet over the Great Plains—the wintertime westerlies bring dominantly dry
air that gives way to moist, southerly air transported by the Great Plains
Low-Level Jet in the warmer months. Climate model simulations suggest
that, as CO2 rises, this low- level jet will strengthen, leading to
greater precipitation in the spring, but less in the summer and, thus, no
change in mean annual precipitation. Combined with rising temperatures
that will increase potential evapotranspiration, semi-arid conditions will
shift eastward, with potentially large consequences for the ecosystems and
inhabitants of the Great Plains. We examine how hydroclimate in the Great
Plains varied in the past in response to warmer global climate by studying
the paleoclimate record within the Ogallala Formation, which underlies
nearly the entire Great Plains and provides a spatially resolved record of
hydroclimate during the globally warmer late Miocene. We use the stable
isotopes of oxygen (δ18O) as preserved in authigenic carbonates hosted
within the abundant paleosol and fluvial successions that comprise the
Ogallala Formation as a record of past hydroclimate. Today, and coincident
with the modern aridity gradient, there is a sharp meteoric water δ18O
gradient with high (−6 to 0‰) δ18O in the southern Great Plains and low
(−12 to −18‰) δ18O in the northern Plains. We find that the spatial
pattern of reconstructed late Miocene precipitation δ18O is
indistinguishable from the spatial pattern of modern meteoric water δ18O.
We use a recently developed vapor transport model to demonstrate that this
δ18O spatial pattern requires air mass mixing over the Great Plains
between dry westerly and moist southerly air masses in the late
Miocene—consistent with today. Our results suggest that the spatial extent
of these two atmospheric circulation systems have been largely unchanged
since the late Miocene and any strengthening of the Great Plains Low-Level
Jet in response to warming has been isotopically masked by proportional
increases in westerly moisture delivery. Our results hold implications for
the sensitivity of Great Plains climate to changes in global temperature
and CO2 and also for our understanding of the processes that drove
Ogallala Formation deposition in the late Miocene.