Paleoenvironmental evolution and geomorphic controls on a microbialite-bearing coastal carbonate–evaporite system: Laguna Providencia, Puerto Rico

Coastal lagoons preserve sedimentary records of environmental change and, under restricted hydrology, may develop organic-rich, evaporite, and carbonate facies such as microbialites. Laguna Providencia, a coastal lagoon in Guánica, southwestern Puerto Rico, preserves a ca. 3700-year late Holocene record and hosts the only documented microbialite buildup in Puerto Rico. This study reconstructs lagoon evolution and evaluates how that history influenced microbialite distribution. Sedimentology, geochronology, and geochemistry identify four phases: (1) a mangrove-dominated estuary (ca. 3700–2500 cal yr BP); (2) abrupt lagoonal reorganization and carbonate influx at ca. 2500 cal yr BP; (3) progressive restriction culminating in hypersaline conditions and gypsum deposition by ca. 1100 cal yr BP; and (4) late Holocene hydrologic and salinity variability, followed by later anthropogenic modification. Alternation between organic-rich sediment and evaporites during the hypersaline phase reflects repeated wet–dry forcing in parts of the record. Remote sensing and modern geomorphic observations show that the present microbialite buildup aligns with inherited geomorphic features formed during earlier lagoonal development, indicating that geomorphic inheritance exerts a primary control on microbialite distribution, with localized groundwater and structural influences as secondary factors. Facies distributions also reflect variable hydrologic restriction and seawater connectivity across the lagoon, producing a laterally heterogeneous carbonate–evaporite system. Recent opening of the artificial canal has increased seawater exchange and turbidity, highlighting the sensitivity of the system to anthropogenic hydrologic modification and its conservation relevance. Laguna Providencia provides a process-based analog for microbialite-bearing evaporative coastal systems in tropical seasonal settings, where such systems remain underrepresented.