Foraminifera in elevated Bermudian caves provide further evidence for +21 m eustatic sea level during Marine Isotope Stage 11

Two hypotheses have been proposed to explain the origin of marine isotope stage (MIS) 11 deposits in
small Bermudian caves at þ21 m above modern sea level: (1) a þ21 m MIS 11 eustatic sea-level highstand,
and (2) a MIS 11 mega-tsunami event. Importantly, the foraminifera reported in these caves have
yet to be critically evaluated within a framework of coastal cave environments. After statistically
comparing foraminifera in modern Bermudian littoral caves and the MIS 11 Calonectris Pocket A (þ21 m
cave) to the largest available database of Bermudian coastal foraminifera, the assemblages found in
modern littoral caves – and Calonectris Pocket A – cannot be statistically differentiated from lagoons.
This observation is expected considering littoral caves are simply sheltered extensions of a lagoon
environment in the littoral zone, where typical coastal processes (waves, storms) homogenize and
rework lagoonal, reefal, and occasional planktic taxa. Fossil protoconchs of the Bermudian cave stygobite
Caecum caverna were also associated with the foraminifera. These results indicate that the MIS 11
Bermudian caves are fossil littoral caves (breached flank margin caves), where the total MIS 11 microfossil
assemblage is preserving a signature of coeval sea level at þ21 m. Brackish foraminifera
(Polysaccammina, Pseudothurammina) and anchialine gastropods (w95%, >300 individuals) indicate
a brackish anchialine habitat developed in the elevated caves after the prolonged littoral environmental
phase. The onset of sea-level regression following the þ21 m highstand would first lower the ancient
brackish Ghyben-Herzberg lens (<0.5 m) and flood the cave with brackish water, followed by drainage of
the cave to create a permanent vadose environment. These interpretations of the MIS 11 microfossils
(considering both taphonomy and paleoecology) are congruent with the micropaleontological, hydrogeological
and physical mechanisms influencing modern Bermudian coastal cave environments. In
conclusion, we reject the mega-tsunami hypothesis, concur with the þ21 m MIS 11 eustatic sea-level
hypothesis, and reiterate the need to resolve the disparity between global marine isotopic records and
the physical geologic evidence for sea level during MIS 11.