The Foraminifera, Sediment, and Radiocarbon Signal in Shallow Water (<4 m) from Two Bermudian Anchialine Caves: Implications for Paleoenvironmental Reconstructions

Abstract

With a high sensitivity to local environmental changes and high preservation potential in coastal sediments, benthic foraminifera are useful proxies of long-term environmental change. However, their ecology and distributions in modern coastal underwater caves remains poorly understood, which hampers their applicability in these environments. It is also unknown if the microfossil processing approach (≥45 µm vs ≥63 µm mesh sieve size) influences resultant foraminiferal assemblages, and if current assumptions regarding calibration of cave-based radiocarbon dates are correct. Shallow water pools (<4 m) in two Bermudian anchialine caves (Deep Blue, Cow Cave) provide a modern analog for the potential benthic foraminifera that may colonize coastal underwater caves during initial inundation by groundwater-level rise. With the presence of a brackish meteoric lens overlaying saline groundwater, the faunal distribution of foraminifera is influenced by changing environmental and hydrographic parameters. However, the role of these environmental parameters on benthic foraminifera in such unique environments is not well studied. In this study we show that: 1) future work in anchialine cave environments can be limited to the ≥63 µm sediment size fraction, 2) the marine reservoir correction cannot be assumed as required for calibrating radiocarbon results from all carbonate samples from anchialine caves, 3) initial inundation of anchialine cave systems produces an assemblage of foraminifera most likely tolerant of brackish-water conditions, assuming a sufficient sediment supply to promote a stratigraphic record.