GEORADAR-AIDED ASSESSMENT OF BIOGENIC STRUCTURES IN BAHAMIAN COASTAL DUNES: IMPLICATIONS FOR HETEROGENEITY OF SHORELINE RETREAT

Ground-penetrating radar (GPR) has emerged as an effective tool for imaging biogenic structures in Quaternary carbonate aeolianites of the Bahamas, like the burrows of the blackback land crab (Gecarcinus lateralis). This study uses georadar surveys to assess, for the first time, the shore-normal burrow distribution and density (per linear meter of strandplain) as a means of forecasting relative bulk hydrological pathways and vulnerability to erosion. High density of open burrows and decayed roots was used as an indicator of lower structural integrity of an exposed dune scarp. Alternatively, areas indicating low bioturbation and uniform primary bedding would result in a more stable slope. This study focused on a seaward 100-m segment of a 300-m-long 800 MHz GPR profile across a strandplain at a classic Hanna Bay locality, San Salvador Island. Attribute analysis of the 2D radargram reveals hollow burrow sections as reverse-polarity diffractions, which contrast with buried vegetation manifested by normal polarity. Segments with high burrow density (≥50%) were diagnosed as prone to accelerated erosion and retreat. There were three areas with low bioturbation that are hypothesized to experience greater slope stability and resistance to erosion. The average depths (d) and fraction (%) of three types of anomalies are: roots (d = 0.33 m; 8%), open and partially filled discrete burrow segments having openings proximal to survey line (d = 0.22 m; 24%), and inclined voids interpreted as burrows oriented longitudinally to the imaged trend (d = 0.21 m; 10%). These parameters are consistent with recent analyses of radargrams at several sites on the island. Detailed analysis of burrow distribution can have implications not only for assessing biomantle (“white soil”) extent and for simulating erosion vulnerability, but also for potential of abandoned burrow networks to be utilized by other organisms or to serve as temporary sites for breeding insects. Therefore, our findings show widespread implications of biogenically induced reworking, which is often overlooked in traditional coastal geomorphic studies.