THE EARTH HATH BUBBLES, AS THE WATER HAS, AND THESE ARE OF THEM: THE STORMY ORIGINS OF SUPRATIDAL FENESTRAE IN THE BAHAMAS DURING THE LAST INTERGLACIAL

Beach fenestrae (“beach bubbles” or “keystone fenestrae”) are small voids that form as sheets of water wash over dry sand, trapping air between the percolating water above and interstitial water below (Dunham, 1970; Shinn, 1983). In the Bahamas, rapid carbonate cementation preserves and lithifies these structures. Originally thought to be exclusive to the intertidal, beach fenestrae were initially used to reconstruct sea levels in the Bahamas (e.g., Garrett and Gould, 1984). However, in last interglacial (MIS 5e) supratidal deposits, fenestrae have been reported at high elevations across the archipelago.

On Providenciales, Wanless and Dravis (1989) described fenestral beds in MIS 5e eolian ridges (up to +30m), implicating storm waves in their formation. Fenestral porosity in coeval eolianites on Eleuthera (+43m; Kindler, 1991) and San Salvador (+12m; Bain and Kindler, 1994), led to the suggestion of torrential rainfall as a genetic mechanism. Neumann and Hearty (1996) subsequently linked eolian fenestrae to intensified storms, coupled with deteriorating climate at the end of MIS 5e. A storm-wave origin was further substantiated by observations of fenestrae-rich beds in chevron-shaped storm-beach ridges on multiple islands (Hearty et al., 1998).

In several detailed outcrop descriptions from Eleuthera, Tormey (1999) noted distinct differences in the abundance and character of fenestral bedding within MIS 5e beach, storm-beach, and eolian facies. Further work on Providenciales and San Salvador led Tormey and Neumann (2007) to conclude that fenestral bedding character varied as storm waves ran over, ran up, and ran out on coastal dunes. Additional MIS 5e outcrops also revealed fenestrae associated with scour, rip-ups, and onlap (Duncan et al., 2007; Donovan and Tormey, 2015; Hearty and Tormey, 2017).

Fenestral porosity in MIS 5e chevron and eolian ridges has now been documented in dozens of localities on multiple islands, from Abaco to Providenciales. Several lines of outcrop evidence, consistent with patterns of wave attenuation, indicate repeated inundation by waves from intense storms during MIS 5e. Conrad Neumann was among the first to warn of the climatic “madhouse” at the end of the last interglacial, serving as an omen for our present interglacial, and motivation for those who carry on his work.