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A large amount of calcite pseudomorphs after gypsum were discovered in a volcanic ash bed and its underlying sand/ siltstone in the Gering Formation (late Oligocene), northwestern NE, USA. The highly positive 17 0 anomalies in the residual sulfate indicate that the sulfate could only have come from the oxidation of sulfur gases by ozone in the atmosphere. We previously proposed that it was most likely the result of an extreme tropospheric sulfate haze (dry fog) event that was associated with volcanic eruptions in the west. To test the dry fog hypothesis, we examine here two alternative scenarios that also involve atmospheric sulfate deposition but do not imply a dramatic volcanic and dry fog source: (1) is it possible that some playas in and or semiarid regions accumulate the type of atmospheric sulfate deposits as we see in the mid-Gering bed by the inflow of surface water or groundwater? (2) Could a period of hyperaridity and stability during the late Oligocene have caused a significant accumulation of atmospheric background sulfate onto the mid-Gering ash bed surface?

Sulfates from a variety of playa settings were collected and measured for their O-17 anomalies. The sulfate isotope (both O and S) and concentration profiles of two volcanic ash beds that are currently exposed in the central Atacama Desert (Chile) and in Taylor Valley of the Antarctica Dry Valleys were studied along with their sedimentary features. These two ash beds were chosen for comparison because of their significant levels of atmospheric sulfate accumulated during prolonged and hyperarid climatic conditions. Results showed no or minimal O-17 anomalies in any of the playa sulfate samples from around the world, and none of the parameters in the mid-Gering ash profile matches those in either of the two volcanic ash profiles in modem hyperarid settings. Conclusion: The observed O-17-anomalous gypsum deposit in the mid-Gering ash bed is most likely the result of a series of extreme dry fogs that resulted in an intense pulse of atmospheric sulfate deposition to a semiarid playa setting. (C) 2004 Elsevier B.V. All rights reserved.