Lacustrine fossil preservation in acidic environments; implications of experimental and field studies for the Cretaceous-Paleogene boundary acid rain trauma

The effects of acidification on the preservation of buried lacustrine 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 provided from these experiments suggest that buffering by the host catchment (i.e., the CaCO (sub 3) content of the soils and outcrops that underlie the lake and drainage basin) is the primary factor promoting calcareous-fossil preservation under conditions of moderate and severe acidification. Global acid-rain fallout was a likely environmental consequence 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 environments with different acid-buffering capacities. Variations 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 boundary. Last appearances of taxa can result from poor preservation 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.