Characteristics and Anthropogenic Indications of Blue Hole Five, San Salvador, Bahamas

Blue Hole Five on San Salvador Island, Bahamas was examined with a focus on its hydrogeology, geologic record, and surrounding flora. Although located in a failed housing development site in the southwestern portion of the island, Blue Hole Five has no historical record of human use. This lake is ~0.1 ha and the shoreline consists of epikarsted limestone bedrock covered by surface vegetation. Algae, liver gel, mollusks, ostracodes and minnows are prevalent in the lake. The keyhole shape of Blue Hole Five can be divided into two regions: a shallow bench and a deeper hole (~6 m) with steep sides and a conduit connecting it with marine water and subjecting it to tidal fluctuations. Saline water entering the conduit explains the higher salinity (~17 ppt) and total dissolved solids (TDS) of approximately 17.8 g/L, along with the anoxic bottom conditions. Lake conditions in Blue Hole Five are brackish with a pH that varies between 7-8, and a halocline at approximately 4 m.

Lake sediment cores from three different sites in Blue Hole Five were collected in June 2012. One location was a deeper region near the conduit, another was an area of medium-depth, and the last was on the shallow bench. At each site, a dark green flocculent layer comprised the uppermost sediment, likely representing very recent human impacts such as vegetation clearance for the housing development. Sediments were composed almost exclusively of mollusk shells in some intervals, with little to no fine grained material, except towards deeper layers. Loss on ignition (LOI) results shows that the sediment is largely comprised of carbonate, with small amounts of organic material and even less inorganic material, probably deposited dust from a remote area. XRF data confirmed high levels of carbonate and strontium. SEM and smear slide observations showed an abundance of various mollusk shells and ostracodes. C. costata and A. auberiana mollusks were the most consistently dominant species throughout the first and last sites, although several other bivalve and gastropod species were present. Calcite, pyrite, diatoms, foraminifera, and iron oxides were discovered under microscopic examination. Evidence of charcoal, likely serving as a fire proxy, was also found in sediment layers. With a geologic record involving anthropogenic activity, Blue Hole Five will provide vital contrasting results with neighboring Bahamian blue holes in reconstructing climatic conditions.