GEORADAR CHARACTERIZATION OF A SUPRATIDAL ESTUARINE TURTLE NESTING SITE: ICHNOLOGICAL AND GEOFORENSIC IMPLICATIONS

In coastal environments, several species of turtles produce a suite of diagnostic biogenic structures, including trackways and nests. Whereas the geological significance of large sea turtle nests has been the focus of recent research, few studies have addressed the neoichnology of smaller estuarine chelonians. We examined a nest of a diamondback terrapin (Malaclemys terrapin) on a back-barrier island in New Jersey immediately following oviposition and backfilling. For noninvasive imaging of the resulting structure, a grid of 800 MHz georadar (GPR) profiles was collected in a sandy nesting ground. In contrast to sea turtle nests, the unobstructed terrapin excavation was shallow and less complex, lacking body and covering pits, as well as deep entrance and exit trackways. GPR images captured a concave-up structure within the upper 20-30 cm, accentuated by the truncation of subhorizontal reflections from the surrounding sediment. In 3D visualization and sequential depth slices, a high-amplitude anomaly corresponds to the new nest. Other anomalies likely represent previous nesting sites, with surface amplitude variations due to moisture retention differences between vegetated and exposed sand. Whereas terrapin hibernacula in muddy substrates limits GPR use, nesting within unsaturated sandy part of the island (water table ~70 cm during spring high tide) makes it a viable imaging tool and demonstrates the use of nests as maximum paleo-water table (~sea-level) indicators. Paleoichnological efforts must consider recognition of similar biogenic structures as part of marginal marine ichnofacies in the fossil record, especially in the absence of associated repichnia. Our findings also have geoforensic applications due to similarity of turtle nests with shallow graves. The dielectric aspects of fluid-filled eggs in a shallow cavity are analogous to recently buried remains, whereas eggshells or buried hatchlings are more akin to decomposed remains. In natural settings, nest depredation by mammals and birds will likely mask the original structure, but will also produce a larger compound biogenic feature. Surveying nest distribution, as well as recognizing evidence of egg poaching or disturbance of threatened species nests, highlights the importance of near-surface geophysics in conservation efforts.