HOW AND WHY DOES LITHIFICATION BIAS BIODIVERSITY? A TEST BASED ON EXPERIMENTAL LITHIFICATION OF UNCONSOLIDATED FOSSILIFEROUS SEDIMENTS

Understanding biodiversity change in deep time requires data from the fossil record, which is biased by taphonomic and diagenetic processes that destroy or alter potential fossils based on their resistance to chemical and physical alteration. Untangling the effects of these processes will aid in the further interpretation of biodiversity. We present preliminary results of an experiment in which sample-level diversity was compared between replicate samples from a single locality under two treatments: sieved fossils from unconsolidated sediment and fossils from artificially cemented samples representing “rocks”. The same fossil assemblage was present in both sets of samples, but they were processed differently.

Bulk samples of unconsolidated sediment and fossils were collected from a single outcrop of the Fort Thompson and Bermont Formations (Pleistocene) at the former Caloosa Shell Quarry, Florida. Three replicates were taken from each of 24 sampling holes in the quarry wall. One replicate sample from each set was sieved, and bivalve and gastropod fossils were counted. We created artificially lithified rocks by adding industrial hydraulic cement (normally used to create anchors for bolts and posts) to additional replicates. We will compare sample-level biodiversity derived from loose, sieved shells versus shells embedded in a solid matrix.

In nature, lithification can be a complex process involving not just cementation, but also processes like fossil dissolution. The synthetic rocks described here do not replicate all processes involved in lithification and are not perfect analogs for many natural fossiliferous rocks. However, our intent is not to simulate all natural processes simultaneously, but rather to examine them separately in order to isolate the effects of each on biodiversity. Our preliminary synthetic rocks are similar to beach rock in that they were rapidly cemented by precipitation at Earth-surface pressures and temperatures, and they do not simulate dissolution of fossils. Preliminary results indicate that physical processes are not the only important factors that determine systematic biases in biodiversity data; rather, one must consider how the physical condition of a sample affects how fossils are processed and counted.