Paper No. 2
Presentation Time: 1:35 PM
BURROWING CRABS FROM THE TROPIC OF CANCER: GEORADAR IMAGING OF BIOGENIC STRUCTURES IN COASTAL CARBONATES, BAHAMAS
High-resolution geophysical imaging methods, such as ground-penetrating radar (GPR), are increasingly being employed in neoichnological research. In one of the first applications to carbonate coastal ichnocoenoses, >50 images of crustacean bioturbation structures were collected using 800 MHz georadar along tropical shorelines of Eleuthera and Exuma islands, Bahamas. Across the berm and adjacent dune sites, numerous active and partially filled burrows of ghost crabs (Ocypode quadrata) generate characteristic ground-wave gap (tunnel opening) and hyperbolic diffraction patterns. In unconsolidated aeolian sand (foredune ridges and backdune swales), dwelling tunnels of blackback land crabs (Gecarcinus lateralis) produce gently dipping reflection or en-echelon diffraction patterns, with their 2D geometry dependent on burrow spacing and orientation relative to survey lines. At Rock Sound, Eleuthera, a dense network of burrows (3-6/m2) by blue land crabs (Cardisoma guanhumi) dominates the landward margin of a recent overwash fan. Openings for inclined tunnels range from 4 to >20 cm in diameter and terminate at the saline groundwater table (depth: 40-60 cm). Intense bioturbation occasionally transports pre-historic (Lucayan) artifacts to the surface, thereby constraining their spatial distribution and depth range. Survey grids with closely spaced GPR profiles allowed pseudo-3D reconstruction of subsurface burrow segments, with 4-5 cm vertical resolution achievable in unsaturated carbonate substrates (better in saturated media). Lateral continuity and bottom “pull up” (hollow segments) are emerging as diagnostic features that distinguish bioturbation structures at different taphonomic stages from point-source diffraction patterns induced by buried clasts and tree roots. Future burrow casting will allow direct comparison to radargram patterns, with the ultimate goal of employing non-invasive geophysical methods at sensitive sites, for targeted excavation, and for recognizing large crustacean burrows in the rock record. Our research builds on previous findings that bioturbation structures in ancient tropical beach and aeolianite sequences may serve as sea-level indicators (e.g., ghost crab trace Psilonichnus upsilon) and act as megaroporosity anomalies in carbonate reservoirs.