Preprints
https://doi.org/10.5194/cp-2021-149
https://doi.org/10.5194/cp-2021-149

  19 Nov 2021

19 Nov 2021

Review status: this preprint is currently under review for the journal CP.

Expression of the “4.2 ka event” drought in the southern Rocky Mountains, USA

David Thomas Liefert and Bryan Nolan Shuman David Thomas Liefert and Bryan Nolan Shuman
  • Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82070, USA

Abstract. The use of the climatic anomaly known as the “4.2 ka event” as the stratigraphic division between the mid- and late Holocene has prompted debate over its impact, geographic pattern, and significance. The anomaly has primarily been described as abrupt drying, but evidence of hydroclimate change at ca. 4 ka is inconsistent among sites globally, and few sites in North America document a major drought. Climate records from the southern Rocky Mountains demonstrate the challenge with diagnosing the extent and severity of the anomaly. Dune-field chronologies and a pollen record in southeast Wyoming reveal several centuries of low moisture at around 4.2 ka and prominent low stands in lakes in Colorado suggest the drought was unique amid Holocene variability, but detailed carbonate oxygen isotope (δ18Ocarb) records from Colorado do not record it. We find new evidence from δ18Ocarb in a small mountain lake in southeast Wyoming of an abrupt reduction in effective moisture or snowpack from approximately 4.2–4 ka that coincides in time with the other evidence from the southern Rocky Mountains and the western Great Plains of regional drying at around 4.2 ka. We find that the δ18Ocarb in our record may reflect cool-season inputs into the lake, which do not appear to track the strong enrichment of heavy oxygen by evaporation during summer months today. The modern relationship differs from some widely applied conceptual models of lake-isotope systems and may indicate reduced winter precipitation rather than enhanced evaporation at ca. 4.2 ka. Inconsistencies among the North American records, particularly in δ18Ocarb trends, thus show that site-specific factors can prevent identification of the patterns of multi-century drought. However, the prominence of the drought at ca. 4 ka among a growing number of sites in the North American interior suggests it was a regionally substantial climate event amid other Holocene variability.

David Thomas Liefert and Bryan Nolan Shuman

Status: open (until 14 Jan 2022)

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David Thomas Liefert and Bryan Nolan Shuman

David Thomas Liefert and Bryan Nolan Shuman

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Short summary
A large drought potentially occurred roughly 4,200 years ago, but its impacts and significance are unclear. We find new evidence in carbonate oxygen isotopes from a mountain lake in southeast Wyoming, southern Rocky Mountains, of an abrupt reduction in effective moisture (precipitation–evaporation) or snowpack from approximately 4,200–4,000 years ago. The drought’s prominence among a growing number of sites in the North American interior suggests it was a regionally substantial climate event.