Multi-Proxy Ostracode Analyses of Paleosalinity in Two Bays, Vieques, Puerto Rico

Puerto Mosquito and Puerto Ferro are south-facing bays on the Puerto-Rican island of Vieques. Puerto Mosquito is one of the world's brightest bioluminescence bays (Carpenter and Seliger, 1968). Its bioluminescence is caused by high year round populations of Pyrodinium bahamense var. bahamense (up to 150,000 organisms/liter) (Seliger, 2001). Adjacent bays, e.g. Puerto Ferro, have lower concentrations (150 to 400 organisms/liter) (Gasparich, 2007).

Several hypotheses have been proposed to explain why Puerto Mosquito is able to sustain dinoflagellate blooms. These include differences in available nutrients, water residence times, and environmental stresses. Our study examines the paleoenvironmental and paleohydrologic histories of Puerto Mosquito and Puerto Ferro, specifically focusing on salinity and paleosalinity, using ostracodes.

Ostracodes are carbonate microfossils, small bivalved Crustaceans, that occur as both benthic and planktic forms. Research groups have shown that ostracodes are effective proxies for paleosalinity reconstruction. Three different methods are used: 1) species assemblages, 2) pore morphometrics, and 3) Mg/Ca ratios. Assemblage analyses of ostracodes have provided paleosalinity records in shallow marine settings, such as Gulf of Carpentaria (Reeves et al., 2006) and Kuwait Bay (Al-Zamel et al., 2006). Rosenfeld and Vesper (1975) demonstrate an inverse relationship between salinity and the abundance of round, sieve-type pores in the ostracode genus Cyprideis. In Florida Bay, Mg/Ca ratios of the ostracode genus Loxoconcha were used to estimate paleosalinity (Dwyer and Cronin, 2002).

Ostracode assemblages from core tops and bottoms show different environmental histories for these two bays (Robertson, 2007). During the approximate 3000 years of record, Puerto Mosquito core bottom sediments contain open marine ostracode assemblages while core tops contain hypersaline lagoon assemblages. In contrast, Puerto Ferro core tops and bottoms contain open marine ostracode assemblages. We will present data comparing the consistency of the three paleosalinity methods using samples from different sedimentological facies in each bay.