PALEOSALINITY HISTORY OF A HOLOCENE LAGOON IN THE ENRIQUILLO VALLEY, DOMINICAN REPUBLIC: PORE MORPHOMETRICS OF OSTRACODA
The southwestern region of the Dominican Republic (Enriquillo Valley) contains exceptionally well-preserved deposits of early-to middle Holocene coral crest, patch, and back reef beds. Before 4.3 ka, the valley was an extension of the Caribbean Sea where a healthy fringing coral reef thrived until flood plain and alluvium deposits choked the mouth of the valley and separated the lagoon from the sea. We recognize 3 primary biofacies associations: 1) Monastraea annularis calcarenite with stenohaline Trachyleberidae and Bairdidae ostracoda; 2) Acropora cervicornis calcarenite with the mesohaline ostracode Loxoconcha levis; 3) Tagelus sp. calcilutite with the euryhaline ostracodes Cyprideis salebrosa, C. mexicana, C. similis, and C. edentata.
Conventional morphometric methods performed on the ostracode C. torosa indicate an inverse relationship between the abundance of circular pores and the salinity of the environment. Sieve pores were examined on uncoated carapaces at low magnification using backscatter mode and low gas pressures of 50 pascals on a variable pressure scanning electron microscope (VPSEM). Digital images were captured and the area and circularity of the sieve pores were determined using image analysis software (ImageJ 1.30v: NIH). We recognize three pore circularity trends: 1) regular pore shapes indicating hyposalinity; 2) moderately irregular pores indicating stenohaline to slightly hypersaline conditions; 3) highly irregular pores indicating hypersalinity.
Cyprideis exhibit alternations between highly irregular pores and slightly irregular pores in the 0.5 m of strata that overlie the Acropora cervicornis calcarentite. Following the hypersaline deposit, Cyprideis exhibit regular pores throughout the remaining 2.2 m of strata. When the lagoon was first separated from the sea, arid conditions contributed to evaporative hypersaline waters in the restricted coastal lake. By middle Holocene time, increased precipitation in the valley resulted in a transgressive shoreline that is marked by tiered serpulid worm mounds and coeval hyposaline calcilutite deposits in the back reef.
This refined pore morphometric study of ostracodes using the VPSEM highlights the potential to determine paleosalinity histories in ancient marginal marine environments.