FORAMINIFERAL ECOLOGY IN AN UNDERWATER CAVE: TOWARDS A NEW METHOD FOR OBTAINING CAVE-BASED PALEOCLIMATE RECORDS

Fossil and living foraminifera have been recovered from caves, yet foraminiferal ecology in underwater (phreatic) caves has been largely speculative. Seventy-five surface sediment samples were collected on SCUBA throughout Green Bay Cave System, Bermuda. This cave is ideal for studying cave habitats because an anchialine cave (terrestrially-dominated, sinkhole entrance) is interconnected to a submarine cave (marine-dominated, lagoon entrance). Five foraminiferal assemblages were distributed throughout the cave, which are primarily controlled by groundwater salinity and the dominant source of organic matter influencing the sample site (terrestrial vs. marine). The anchialine cave environment was located near a sinkhole, and consisted of two foraminiferal assemblages: (a) Meteoric Lens, dominated by Helenina anderseni, Discorinopsis aguayoi, and marsh taxa in mean salinity of 24.6, and (b) Anchialine Cave, dominated by Bolivina striatula, Rosalina globularis in the saline groundwater. The submarine cave environment was located throughout most of the cave in the saline ground water, consisting of three foraminiferal assemblages: (a) Entrance (or cavern), dominated by Quinqueloculina spp., (b) Circulated Submarine Cave, dominated by Spirillina vivapara, Triloculina oblonga, and (c) Isolated Submarine Cave, dominated by Spirophthalmidium emaciatum, Patellina corrugata, T. oblonga. The limit of terrestrial impact on the cave (sediment and organic flux) was the limit of the anchialine cave environment, and attenuating marine nutrients partitioned the submarine cave habitats. So, if foraminifera live in underwater coastal caves, can cave sediments preserve foraminiferal-based records of climate change? A 1.7 m (compacted length) sediment core was extracted from 18 m below sea level in Walsingham Cave, Bermuda, a site flooded by saline groundwater and tidally-circulated with the ocean. Detailed core sampling produced a multi-decadal to sub-decadal record of North Atlantic climate change, spanning the last 3.1 ka. Maximum thermal cooling (proxy: δ¹⁸O_foram) and storminess (proxy: allogenic terrigenous organic matter) was identified at the onset of the Little Ice Age (0.7 ka). These results indicate that underwater cave sediments are a promising new source of coastal paleoclimate records.