Foraminifera and testate amoebae (thecamoebians) in an anchialine cave: Surface distributions from Aktun Ha (Carwash) cave system, Mexico

Surface sediment samples from Aktun Ha (Carwash) cave system, Mexico, were analyzed for microfossils, carbonate isotopes, and organic matter content. Saline water flooded the cave during the Sangamon Interglacial highstand (124–119 kyr), hosting a marine assemblage dominated by saline foraminifera Bolivina sp. (73%) and Elphidium sp. (11‰) with a mean δ13C = 25.5‰ and a mean δ18O = 22.7‰. This assemblage was found distal to sinkholes (>75 m upstream, >150 m downstream) and in yellow‐orange sediment (mean total organic carbon [TOC] = 3.6‰). Late Pleistocene (95‐15 kyr) sea level fall reestablished vadose conditions in the cave, allowing for localized speleothem (flowstone) to seal in the marine assemblage. Holocene sea level rise completely reflooded the cave with freshwater by 6.5 kyr, providing a suitable habitat for a freshwater assemblage of living testate amoebae Centropyxis aculeata (~38‰), Arcella vulgaris (~10‰), and the foraminifer Ammonia tepida var. juvenile (~35‰), with a mean δ13C = 210.8‰ and a mean δ18O = 24.9‰ on the ostracod Cytheridella ilosvayi. This assemblage was found proximal to sinkholes (<75 m upstream, <150 m downstream) and in black to brown sediment (mean TOC = 17.5‰). Foraminifera and testate amoebae colonize aquatic cave environments, respond to physicochemical conditions in the cave similarly to other coastal settings, and thereby provide promise for the application of protists as proxies in flooded caves.