Intertidal and subtidal benthic foraminifera in flooded caves: Implications for reconstructing coastal karst aquifers and cave paleoenvironments

Coastal karst aquifers have an upper meteoric water mass stratified from a saline groundwater mass below that increasingly mix and discharge towards the ocean. Preserved sediment in caves and sinkholes provide an opportunity to evaluate the long-term hydrodynamics in the local coastal karst aquifer (e.g., salinity, oxygenation, vertical displacement in response to sea-level forcing). Here we evaluate the response of shallow (< 2 m water depth) benthic foraminifera to water level variability from tidal forcing in Bermudian caves. Intertidal habitats had a higher proportion of brackish-tolerant agglutinated and hyaline taxa (e.g., Trochammina infiata, Entzia macrescens, Miliammina fusca, Helenina anderseni, and Trichohyalus aguayoi). Subtidal habitats proximal to cave entrances with higher proportion of terrestrially-derived organic carbon had an increased abundance of hyaline and infaunal taxa (e.g., Bolivina spp., Melonis barleeanum). In contrast, porcelaneous (e.g., Spirophthalmidium, Sigmoilina) and hyaline genera (e.g., Spirillina, Patellina, Mychostomina) dominated subtidal habitats deeper into the cave, where marine organic carbon sources were dominant. Lastly, similar benthic foraminiferal assemblages were determined with both > 45 mu m versus > 63 mu m mesh sieve fractions. These modern analog results can be used to interpret subfossil benthic foraminiferal records from coastal karst aquifers, especially during early cave inundation by concomitant groundwater and sea-level rise.