HIGH-RESOLUTION GEOPHYSICAL INVESTIGATION OF A SEA TURTLE NEST COMPLEX, CAYO COSTA STATE PARK, FLORIDA: POTENTIAL FOR IDENTIFICATION OF ANCIENT NEST STRUCTURES

This study is contribution to recent efforts aimed at neoichnological characterization of sea turtle crawlways and nesting structures and their use for improving signal-noise ratios in ground-penetrating radar (GPR) images. For endangered species, such as the green turtle (Chelonia mydas), traditional methods of nest identification can be time consuming and invasive, so a rapid means of clutch location with minimal sediment disturbance is crucial. At Cayo Costa State Park, a microtidal barrier island along southwestern Florida, prominent entrance and exit crawlways with symmetrical flipper marks and plastron drag were used to locate a ~5-m-long cover pit on the upper berm. For comparative purposes, GPR surveys of the undisturbed beach and foredune were collected adjacent to the nesting area. Without the surface expression of the body pit, three 800 MHz georadar traverses across the area were required to locate the deepest anomaly. Within a broad near-surface depression, a 1.2-m-wide, downward-tapering cut-and-fill structure was groundtruthed to terminate at the top of the egg chamber ~0.5 m below sediment surface. Truncation of surrounding dune stratification, uncompacted nature of the backfill (water retention contrast), and collective dielectric properties of the egg clutch all contributed to a diagnostic subsurface signature. Intradune nests may be harder to locate due to chaotic background signal return, whereas lithological anomalies may accentuate the egg chamber discontinuity. Our findings corroborate recent claims that high-frequency GPR is a viable tool for coastal nest characterization above the extent of saline water. Following egg hatching and partial collapse (fugichnia), a modified structure may be preserved in a regime of net progradation or aggradation. Whereas traditional shore-normal geophysical transects are unlikely to intersect turtle nests, shore-parallel surveys along paleo-berm sections of coastal strandplains, such as many prograded Late Holocene barrier islands, may capture such structures even if their surface expression has been masked. Ancient sea turtle repichnia may lead to nest sites, which are challenging to identify in cross-section and especially on bedding planes. Conversely, upper layers of nesting grounds may be used to constrain tracking paleo-surfaces.