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Potentiometric titrations have been used as an effective means for determining surface reactivity and protonation constants for microbial surface ligands. The data gathered from the experiments have been applied to remediation projects with the aim of determining accurate metal immobilization estimates. These data have been generated in a laboratory setting using cultured microbes. We have attempted to investigate the implications of carrying out these titrations in situ at the sampling location, to attempt to determine reactivities more representative of the natural conditions. We hoped to distinguish the differences in titrating microbial mats in the field versus in the laboratory. Unfortunately, authigenic carbonate minerals complicated this process. We have highlighted the difficulties in titrating microbial mats in the field, and have made suggestions for future endeavors. It is clear that conditions under which surface reactivities are determined are much more complex in the field than in the laboratory. This preliminary study highlights the significant differences between field and laboratory surface reactivities, and the need for further in situ field investigations.

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Marine facies of carbonate and siliciclastic sediments deposited on top of the upper Devonian Alamo Breccia Member identify the shape and size of the Alamo impact crater in south-central Nevada (western USA). There are 13 measured sections that record peritidal to deep-subtidal deposition across the impacted platform, and these are correlated to three regional depositional sequences above the Alamo Breccia Member. Facies and accommodation patterns identify a concave seafl oor that we interpret as the post-impact legacy of the Alamo crater. Together with isopach and lithostratigraphic trends in the underlying Alamo Breccia Member, a new map of the Alamo crater is presented showing the eastern outer rim fault and the annular trough. Size estimates were made using the newly defi ned crater features and linear scaling relationships from other marinetarget complex craters. Revised dimensions of the Alamo crater place its transient diameter between 37 and 65 km, and its apparent diameter between 111 and 150 km. These estimates are more than double previous estimates based on the biostratigraphy of the Alamo Breccia Member. If correct, these new estimates place the Alamo crater as one of the largest marine impacts of the Phanerozoic, and conservatively larger than the wellstudied Eocene Chesapeake Bay crater.