HOW AND WHY LAKES FORM ON CARBONATE PLATFORMS: A NEW MODEL FOR LAKE FORMATION IN THE BAHAMIAN ARCHIPELAGO

Saline lakes in the Caribbean occur as blue holes, karsted depressions, and cutoff lagoons. Compared to continental saline lakes, little is known about the sedimentological dynamics of these lakes and ponds. Because they are at or very near sea level, their water balance and chemistry often responds quickly to even subtle changes in sea level or local climate. They are typically isolated from surficial marine contact but often are connected via underground conduits. Their salinity is controlled not only by seepage of seawater through the porous carbonate bedrock, but also seasonal rainfall and evaporation. Lake formation is influenced by local hydrologic balance which is in turn controlled by climatic conditions that drive both karst dissolution and the deposition of aolian dune ridges that isolate basins and cut off lagoons and tidal creeks.

Over 80 lakes were sampled from the Bahamian islands of San Salvador, Eleuthera, Rum Cay and Exuma for their water chemistry and other physical characteristics. We propose a process-driven model and classification scheme for these lakes that is based upon lake hydrology as well as a variety of physical and chemical variables, such as maximum and average depth, basin morphology, presence/absence of conduits, salinity, pH, alkalinity, and concentrations of major and minor ions. Our model suggests that there are 2 major categories of lakes—constructional and destructional. Constructional lakes occur in response to sea level fluctuations and are comprised of pre-highstand depressions and highstand depressions. Destructional lakes occur at any time and are dissolution and collapse features. A non-metric, multi-dimensional scaling model, on measured physical and chemical parameters of lakes, supports these groupings and provides a quantitative means by which to verify the classification scheme and group new lakes. By understanding how these lakes were formed and by utilizing the classification scheme, it is possible to predict faunal composition and diversity, lake chemistry and history within a local, regional and global paleoclimatic framework. Integrating the archival records of these lakes with the lithologic history of these islands will allow for a better understanding of climate and sea level change in the Caribbean.