ESTABLISHING CONSTRAINTS ON TAPHONOMIC INERTIA IN SHALLOW MARINE SEDIMENTS: ST. CROIX, USVI

Significant ecological changes in marine environments can result in measurable shifts in the biodiversity of flora and fauna in these ecosystems. If taxa involved in the shift are preservable, they may leave a record of the biotic transition in the subfossil (and eventually fossil) record, allowing researchers to determine the type and extent of change over time. Application of live/dead fidelity techniques is an effective method of determining the extent to which shallow marine biota have deviated from their so-called “baseline” states. One challenge with this method, however, lies in the fact that death (subfossil) assemblages eventually equilibrate to such changes in biodiversity, eventually adjusting to a new “baseline” state; in other words, there is an upper limit on the amount of time a death assemblage preserves. The delay between the time when changes occur in the life assemblage, and when they are reflected in the death assemblage is referred to as taphonomic inertia; if death assemblages are to be used to track ecological change, then constraining this phenomenon has important implications for conservation ecology.

A primary aim of this work was to quantify the strength of taphonomic inertia by constructing a geochronological framework using a set of death assemblages collected from St. Croix, USVI. Since taphonomic inertia is impacted by the degree of time averaging – the extent to which individuals from multiple generations are mixed in the same layer – in a system, determining the degree of time-averaging was the first step. Utilizing a unique set of serially collected death assemblages from Smuggler’s Cove, we obtained 30 radiocarbon ages, which we obtained from the shells of Cerithium litteratum using the Mini Carbon Dating System ( MICADAS) at Northern Arizona University.¹⁴C ages were calibrated to years BP (before present) using Calib 8.20 and CALIBomb protocols. Calibrated ¹⁴C ages were then used to estimate the time averaging of the death assemblage by constructing age distribution histograms. By comparing the known variation in live/dead fidelity across each of the historical sampling periods (1980, 2002, 2011, and 2012) with the calculated degree of time-averaging, we hope to establish estimates of the strength of taphonomic inertia in shallow carbonate settings.