Under the direction of Dr. Trish Beddows, the hydrogeology program has been investigating the interplay of fresh and saltwater in the present and the past. The data on the present hydrogeologic regime has been collected through a combination of a water sampling strategy in the wet (December) and dry (May) season as well as a longitudinal study of shifting hydrological regimes through autonomous data loggers designed by Ed Mallon. These data are then combined with the δ18O of carbonate sediments from the cores. The δ18O isotopes serve as a paleo-evaporation record. Evaporation results in the depletion of 16O in the remaining water. Meteoric precipitation is consequently isotopically lighter than seawater. By extension, the fresh groundwater recharged by meteoric water will discharge isotopically light waters into the coastal seawater. By comparing the δ18O value of the carbonate fraction to the δ18O values of fresh water and seawater today, a mixing curve can be constructed showing the proportions of these two water sources through time at a specific location. In the case of Vista Alegre, retreat of freshwater correlates with the base of each of the shell beds in Core 9 (taken in 2011), suggesting there were transgressive cycles of sea level rise. Within each core, shell beds correlate with high salinity and are interpreted as sea level highstands. Fine grain intervals in between the shell beds have lower carbonate content and are interpreted as sea level lowstands. Overall, the chemical proxies, lithology, and paleosalinity model reconstructed to date reveal four onlapping parasequences representing an overall transgression of the coastline with strong seasonality of water chemistry that has been changing under the control of rising sea levels over the past 3000 years.
With the understanding that freshwater was more abundant during certain periods of time in the past, we have tried to identify possible relic springs that may have been “drowned-out” by rising sea-levels. To do this we undertook novel, drone-based thermal imagery mapping. The freshwater, aside from being isotopically lighter than seawater, is also about 2 degrees C cooler. The mapping did reveal cool water seeps around Vista Alegre but none of the water was potable today. That might not have been the case centuries ago, but investigation on that front continues.
