SPATIAL ARAGONITE BIAS IN EPICONTINENTAL SEAS

Ancient shelly marine faunas are potentially susceptible to misrepresentation due to preferential dissolution of the biogenic carbonate polymorph aragonite. Epicontinental seas, predisposed to water column stratification and seasonally anoxic/dysoxic conditions, may be more prone to preferential aragonite loss than modern oceans, with regions of seafloor where the oxycline coincides with the sediment water interface being the most taphonomically destructive. A ramp like basin floor topography combined with temporal variation of oxycline position in the water column might result in spatially extensive zones with conditions predisposed for aragonite dissolution. However, how these zones could influence long term patterns in the distribution and preservation of aragontic organisms remains to be examined. Here we present a multifaceted spatial investigation of aragonite dissolution within the Western Interior Seaway. Occurrence data of lower Campanian molluscs were gathered from the literature, USGS databases and the Paleobiology Database (6986 specimens total) and plotted on a new high resolution paleogeographic/paleobathymetric reconstruction of this time slice in order to assess controls on aragonite distribution within the seaway. The distribution of shelly fauna was compared to areas likely to experience enhanced dissolution, defined by either paleobathymetry or stratification data generated from GCM climate models and Fluidity-ICOM tidal modelling. Calcitic faunas showed equivalent fossil densities per unit area for normal and increased dissolution zones (12% of occurrences in 12% of the total area). Occurrences of aragonitic faunas are reduced compared to expected fossil densities, with 6% of total occurrences occurring in the enhanced dissolution region; however, chi squared tests produced no statistically significant results. Aragonite occurrences from enhanced dissolution zones are predominantly from concretions. Thus, whilst aragonite dissolution is likely to have affected the distribution of fauna on a local scale, time averaging effects and instantaneous preservation events allow for potentially lost biodiversity to be captured. As such, our results suggest paleo-epicontinental seas provide as accurate a snapshot of deep time biodiversity as ancient oceans.