CARBONATE PRECIPITATION DURING VERTEBRATE DECOMPOSITION IN A HYPERALKALINE AQUATIC SETTING: A CASE STUDY IN EXPERIMENTAL TAPHONOMY

The role of environmental chemistry on early-stage vertebrate fossil preservation is not fully understood. Specifically, few studies, either with field-collected materials or through taphonomic experimentation, have been conducted that investigate the role of high pH on early taphonomic processes. Here, we present preliminary results of such an experiment in which extant theropod remains of varying biostratinomic states were placed in a “natural,” high pH setting in order to qualitatively assess early diagenetic alteration. Bone samples of domestic chickens (Gallus gallus domesticus) were placed in Big Marsh in South Deering, Chicago, IL, which has a unique chemical setting (pH of ~12) owing to industrial waste. Remains included fleshed and de-fleshed samples as the two primary categories of biostratinomic variables, with several other pre-burial states (whole bones, crushed bones, and articulated pairs). Samples were placed in PVC canisters permitting water flow, and submerged in the study area for one-month, five-month, and seven-month intervals. Upon removal, samples were photographed and sectioned into shafts and articular ends for SEM-EDS analyses in order to characterize mineral precipitation. Though results are preliminary, certain trends are present. De-fleshed bone samples show little variation in gross morphology; whole-bones show virtually no alteration; and components of crushed and articulated pairs remained intact. However, all samples possessing flesh show increased degradation of soft-tissues, including gelatinization of skin, muscle, and cartilage, and an increase in mineral precipitation. SEM-EDS analyses of whole-bone samples reveals a dense layer of euhedral calcium carbonate crystals, likely due to hyperalkaline conditions of the water column. In addition to this layer of carbonate, cartilaginous and articular surfaces of de-fleshed samples also possess abundant spheroids of calcium carbonate, suggesting a microbial origin. Biodeposition and diagenetic alteration has previously been hypothesized for the presence of calcite and barite nodules on articular surfaces of Mesozoic vertebrate remains. Further investigation into the role of microbial degradation on early-stage taphonomy in extreme chemical settings is ongoing.