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

We present the results of oxygen isotope and electronmicroprobe analyses of sphaerosiderites obtained from Cretaceous paleosols in Iowa. The sphaerosiderite [[delta].sup.18]O 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 [[delta].sup.18]O values ranging from -3.3[per thousand] to -6.8[per thousand] 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 [sup.18]O depletion at the AlbianCenomanian boundary. Keywords: Cretaceous, Dakota Formation, paleosols, sphaerosiderite, oxygen isotopes.