DIAGNOSTIC GEORADAR SIGNATURES OF BRACHYURAN BURROWS, SAN SALVADOR ISLAND, THE BAHAMAS: IMPLICATIONS FOR CARBONATE NEOICHNOLOGY AND BEYOND

Bioturbation in supratidal carbonate substrates has only recently received attention from neoichnologists, relying primarily on traditional ground-based techniques. Our study is the first high-resolution geophysical investigation of three decapod species inhabiting adjacent coastal biotopes on San Salvador Island. Extensive (>1.5 km) 800 MHz georadar surveys captured a diverse suite of >120 burrows of ghost (Ocypode quadrata), blackback (Gecarcinus lateralis) and blue (Cardisoma guanhumi) land crabs. Site-specific post-processing protocols were used to establish characteristic attributes that distinguish unfilled burrows from other discordant anomalies based on signal polarity structure. Variably oriented 2D and quasi-3D (~0.5λ) images allow the diagnostic signature of G. lateralis burrows to be distinguished from those of O. quadrata based on: 1) higher density of point-source diffractions (0.9-1.9/m vs 0.01-0.06/m) and 2) shallower inclination angle (21-46^◦ vs >60^◦). C. guanhumi burrows are differentiated by: 1) larger diameter (>10 cm); 2) presence of end chamber (where not attenuated by saline groundwater), and 3) low electromagnetic signal velocity (9 cm/ns vs 11-12 cm/ns) in organic-rich muddy substrate. Our study establishes GPR as a viable non-invasive subsurface imaging technique, with potential implications for: 1) identification of shifting ichnocoenoses, as exemplified by intense hurricane impacts (2015/2016); 2) recognition and mapping of similar biogenic structures in buried or lithified carbonates and, 3) quantification of a near-surface ichnofabric index and the dual porosity/permeability structure of prospective hydrocarbon reservoirs. Furthermore, the basic attributes of subsurface visualization can be readily extended to other mesoscale biogenic structures in evaporite and siliciclastic media.