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Distinguishing base-level change and climate signals in a Cretaceous alluvial sequence

¹ Earth and Environmental Systems Institute, Pennsylvania State University, University Park, Pennsylvania 16802, USA
² Iowa Geological Survey, Iowa City, Iowa 52242, USA
³ Geoscience Department, University of Iowa, Iowa City, Iowa 52240, USA

We present the results of oxygen isotope and electron-microprobe analyses of sphaerosiderites obtained from Cretaceous paleosols in Iowa. The sphaerosiderite ¹⁸O values record Cretaceous meteoric groundwater chemistry and an overall waning of brackish groundwater inundation during alluvial-plain aggradation and soil genesis. We focus on horizons that precipitated from freshwater, in which ¹⁸O values ranging from –3.3 to –6.8 relative to the Peedee belemnite standard are interpreted to record variations in the Cretaceous atmospheric hydrologic cycle. During relative sea-level highstands, moisture was derived from the Cretaceous Western Interior Seaway, whereas during lowstands, when the seaway narrowed and occasionally withdrew from the Midcontinent, the dominance of hemispheric-scale atmospheric moisture transport initiated in the tropical Tethys Ocean led to decreased precipitation rates. These processes did not operate like a switch, but rather as a continuum of competing moisture sources and mechanisms of transport between the nearby epicontinental sea and the distant tropics. The sphaerosiderite data demonstrate (1) temporal variation in the intensity of hemispheric-scale atmospheric moisture transport and (2) long-term amplification of the global hydrologic cycle marked by extreme ¹⁸O depletion at the Albian-Cenomanian boundary.

Key Words: Cretaceous • Dakota Formation • paleosols • sphaerosiderite • oxygen isotopes

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