376 RESEARCH REPORT Copyright  2005, SEPM (Society for Sedimentary Geology) 0883-1351/05/0020-0376/$3.00 Lacustrine Fossil Preservation in Acidic Environments: Implications of Experimental and Field Studies for the Cretaceous–Paleogene Boundary Acid Rain Trauma JAKE V. BAILEY* and ANDREW S. COHEN Department of Geosciences, University of Arizona, Tucson, AZ 85721-0077; E-mail: jvbailey@usc.edu DAVID A. KRING Department of Planetary Sciences, Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721 PALAIOS, 2005, V. 20, p. 376–389 DOI 10.2110/palo.2003.p03–88 The effects of acidification on the preservation of buried la- custrine microfossils were evaluated using experimental tanks to simulate pH, depth, duration of acidification, and buffering conditions below the sediment/water interface of a hypothetical acidified lake. The taphonomic data provid- ed from these experiments suggest that buffering by the host catchment (i.e., the CaCO 3 content of the soils and outcrops that underlie the lake and drainage basin) is the primary factor promoting calcareous-fossil preservation under con- ditions of moderate and severe acidification. Global acid-rain fallout was a likely environmental con- sequence of the Chicxulub impact event at the Cretaceous– Paleogene boundary, and may have been important at other times in Earth history. Fossil preservation at the K/Pg boundary may have been affected by acidic groundwater leaching. Whereas the duration and intensity of the acid- rain fallout is poorly constrained, acid rain would have had varying effects on the acidity of lacustrine and fluvial envi- ronments with different acid-buffering capacities. Varia- tions in acid-buffering capacities of lacustrine and fluvial catchments also may be a factor in the apparent extinction selectivity of non-marine aquatic fauna at the K/Pg bound- ary. Last appearances of taxa can result from poor preser- vation conditions or extinction—both of which may result from acidification. Last appearances observed at the species level, but not in higher taxa, may be the result of regional heterogeneities in catchment geology. Understanding local buffering conditions may be important for interpreting the continental fossil record at the K/Pg boundary. INTRODUCTION From a paleontological perspective, water-body acidifi- cation is an environmental condition with demonstrated biological effects and the potential to cause ecosystem stresses (e.g., Wright et al., 1975; Schindler et al., 1985; Rosseland, 1986; Schindler, 1988). Acidification also can be viewed as a destructive taphonomic mechanism where- by calcareous shells and skeletons are selectively dis- solved by acidic waters or sediments (e.g., Linse, 1992; * Current address: Department of Earth Sciences, University of South- ern California, Los Angeles, CA 90089-0740 Clayburn et al., 2004). Surface-water acidification can oc- cur through the deposition of strong acids from the atmo- sphere (e.g., Driscoll et al., 2001). It also can result from the oxidation of sulfide minerals, such as pyrite (FeS 2 ), or by the activity of organic acids in humid climates (e.g., Likens et al., 1972). Various biological effects, such as fish kills, have been attributed to modern water-body acidifi- cation in lakes of Canada, Europe, and the eastern United States (Beamish, 1976; Muniz et al., 1978; Baker and Schofield, 1982; Battarbee, 1984; Charles, 1985; Freda, 1986; Hartmann and Steinberg, 1986; Rhenberg et al., 1990). Acidification also results in effects such as metal leaching in lakes with low alkalinity (poorly buffered lakes) and calcium carbonate whitings in lakes with high alkalinities (well-buffered lakes) (Lajewski et al., 2003). Whereas local acidification can occur from sulfide weathering and organic-acid accumulation, regional or global acidification requires a substantial atmospheric perturbation of acid-forming chemical species. Large vol- canoes (Pinto et al., 1989) and impacting asteroids or com- ets (Lewis et al., 1982; Prinn and Fegley, 1982; Brett, 1992; Kring et al., 1996) are capable of producing such per- turbations and were likely primary sources of acid-form- ing aerosols and gasses capable of producing regional or global acid-rain fallout and lake acidification in the geo- logic past. Perhaps the best-known and paleontologically signifi- cant acid-forming event is the Chicxulub impact, which co- occurred with the K/Pg mass extinction (Alvarez et al., 1980; Bohor et al., 1984; Kring et al., 1991; Hildebrand et al., 1991). The Chicxulub impact is widely thought to have produced both nitric-acid and sulfuric-acid rain. Acid was produced from vaporized material excavated from Earth’s crust, the obliterated asteroid or comet, and interactions of this material with the atmosphere (Lewis et al., 1982; Prinn and Fegley, 1982; Brett, 1992). The Chicxulub impact event occurred on a shallow-ma- rine shelf that included a 3-km-thick sequence of carbon- ates and evaporites (primarily anhydrite). Vaporized an- hydrite (CaSO 4 ) is thought to have introduced sulfate aerosols into the stratosphere (Brett, 1992; Sigurdsson et al., 1992; Pope et al., 1994; Ivanov et al., 1996; Pierazzo et al., 1998; Yang and Ahrens, 1998). Estimates of the mass of S vary (Table 1), ranging from 4  10 16 to 4.3  10 18 g, based on simple scaling models (Brett, 1992; Sigurdsson et al., 1992; Kring, 1993). However, recent computer simu- lations of the impact event suggest values of 7.5  10 16 to