BURROW CHARACTERISTICS AND SEDIMENT REWORKING BY LAND CRABS IN COASTAL ENVIRONMENTS OF THE BAHAMAS AS REVEALED BY CASTING AND GEORADAR IMAGING

The land crabs Gecarcinus lateralis (blackback crab) and Cardisoma guanhumi (blue crab) are common throughout the Bahamas, and their burrowing activity can generate significant levels of bioturbation. Plaster-of-Paris casting and ground-penetrating radar imaging were used to characterize their active burrows and to assess their potential importance as paleoenvironmental indicators. G. lateralis is most common on vegetated coastal dunes and other inland sandy substrates proximal to the coast. On San Salvador Island, burrow casts revealed horizontally elongate, somewhat meandering and normally unbranched burrow geometries, with laterally compressed 3-6 cm-wide cross-sections and lengths sometimes exceeding 1 m. G. lateralis is known to bring plant matter into its burrow and to deposit fecal material there, so bioturbation by this crab likely is an important process in generating soils on vegetated dune-sand substrates. Therefore, their fossil burrows may serve as indicators of “white paleosols.” In contrast, C. guanhumi typically occupies muddy sand substrates of the supratidal zone along semi-protected coastal areas fringed by mangroves or adjacent wetlands. Its burrows are generally circular in cross section, larger in diameter (some >20 cm), and with much steeper inclination than those of G. lateralis. In addition, blue crab burrows typically intersect the water table, with saline to fresh water filling the burrow terminus, making them potential paleo-water table indicators. In areas where direct casting is not feasible due to burrow fill or lithification, 800 MHz georadar surveys provide rapid high-resolution visualization of gross burrow geometry and extent. Recent imaging of bioturbation features produced by the two crab species on Eleuthera and Exuma islands captured the differences in their diameter and inclination. With potential resolution of 4-5 cm and ~2 m penetration in unconsolidated and lithified supratidal sequences, near-surface geophysics can be used to assess the depth of bioturbated paleosols, with the ultimate goal of characterizing the ichnofabric index in ancient coastal and aeolian carbonates.