EGU2020-11141
https://doi.org/10.5194/egusphere-egu2020-11141
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Long-term stability of large-scale hydroclimate processes in the North American Great Plains revealed by a Neogene stable isotope study

Livia Manser1, Tyler Kukla2, and Jeremy K. Caves Rugenstein3
Livia Manser et al.
  • 1Departement of Earth Sciences, ETH Zürich, Zürich, Switzerland (lmanser@student.ethz.ch)
  • 2Department of Geological Sciences, Stanford University, Stanford, CA, USA
  • 3Max Planck Institute for Meteorology, Hamburg, Germany

The North American Great Plains are characterized by a sharp aridity gradient at around the 100th meridian with a more humid climate to the east and a more arid climate to the west. This aridity gradient shapes the region's agriculture and economy, and recent work suggests that arid conditions on the Great Plains may expand eastward with global warming. The abundant Neogene sediments of the Ogallala Formation in the Great Plains present an opportunity to reconstruct regional hydroclimate conditions at a time when pCO2 and global temperatures were much higher than today, providing insight into the aridity and ecosystem response to warming. We present new paleosol carbonate δ13C and δ18O data (n=366) across 37 sites spanning the Great Plains and compile previously published measurements (n=381) to evaluate the long-term hydroclimatic and ecosystem changes in the region during the late Neogene. This study combines a spatial and temporal analysis of carbon and oxygen isotope data with reactive-transport modeling of oxygen isotopes constrained by climate model output, providing critical constraints on the paleoenvironmental and paleoclimatological evolution of the Neogene Great Plains. Carbonate δ18O demonstrate remarkable similarity between the spatial pattern of paleo-precipitation δ18O and modern precipitation δ18O. Today, modern precipitation δ18O over the Great Plains is set by the mixing between moist, high-δ18O moisture delivered by the Great Plains Low-Level Jet and drier, low-δ18O westerly air masses. Thus, in the absence of countervailing processes, we interpret this similarity between paleo and modern δ18O to indicate that the proportional mixing between these two air masses has been minimally influenced by changes in global climate and that any changes in the position of the 100th meridian aridity gradient has not been forced by dynamical changes in these two synoptic systems. In contrast, prior to the widespread appearance of C4 plants in the landscape of the Great Plains, paleosol carbonate δ13C show a robust east-to-west gradient, with higher values to the west. We interpret this gradient as reflective of lower primary productivity and hence soil respiration to the west. Close comparison with modern primary productivity data indicates that primary productivity has declined and shifted eastward since the late Neogene, likely reflecting declining precipitation and/or a reduction in CO2 fertilization during the late Neogene. Finally, δ13C increases across the Miocene-Pliocene boundary, which, consistent with previous studies, we interpret as a shift from a C3 to a C4 dominated landscape. We conclude that, to first order, the modern aridity gradient and the hydrologic processes that drive it are not strongly sensitive to changes in global climate and any shifts in this aridity gradient in response to rising CO2 will be towards the west, rather than towards the east.

How to cite: Manser, L., Kukla, T., and Caves Rugenstein, J. K.: Long-term stability of large-scale hydroclimate processes in the North American Great Plains revealed by a Neogene stable isotope study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11141, https://doi.org/10.5194/egusphere-egu2020-11141, 2020

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Presentation version 1 – uploaded on 03 May 2020
  • CC1: Comment on EGU2020-11141, Lydian Boschman, 04 May 2020

    Hi Livia, you mentioned in the chat the your modeling results show that the result is largely driven by topography. Does that mean changes in topography? If so, what can you say about paleo-topography or paleo-elevation based on your study?

    Thanks,

    Lydian Boschman

    • AC1: Reply to CC1, Livia Manser, 05 May 2020

      Hi Lydian, thanks for your question.

      So the long-lasting invariability in our oxygen isotope record suggests that mountain ranges west of the Great Plains, over which westerly moisture is transported, were already at high elevations, comparable to today, and did not undergo substantial changes in height during the Neogene. Paleoaltimetry studies rely on the depletion of d18O in precipitation with increasing elevation due to orographic rainout. However, the spatial pattern of d18O over the Great Plains does not record orographic rainout of a southerly airmass, but airmass mixing of the low-level jet and the midlatitude westerlies; therefore it is difficult to directly relate our results to topographic height. If there have been changes in height, it is probably small and not detectable in our results (or canceled out by the coincidental effects of climate change during the Neogene). More generally, likely this portion of the North American Cordillera has been elevated since the Eocene (Mix et al., 2011; Chamberlain et al., 2012).

       

      References:

      Chamberlain, C. P., Mix, H. T., Mulch, A., Hren, M. T., Kent-Corson, M. L., Davis, S. J., ... & Graham, S. A. (2012). The Cenozoic climatic and topographic evolution of the western North American Cordillera. American Journal of Science, 312(2), 213-262.

      Mix, H. T., Mulch, A., Kent-Corson, M. L., & Chamberlain, C. P. (2011). Cenozoic migration of topography in the North American Cordillera. Geology, 39(1), 87-90.