Blue holes: Definition and genesis

Abstract

Blue holes are karst features that were initially described from Bahamian islands and banks, which have been documented for over 100 years. They are water-filled vertical openings in the carbonate rock that exhibit complex morphologies, ecologies, and water chemistries. Their deep blue color, for which theyare named, is the resultof theirgreat depth, and they may lead to cave systems belowsea level. Blue holes are polygenetic in origin, having formed: by drowning of cHssolutional sinkholes and shaftsdeveloped in the vadose zone; by phreatic dissolution along an ascending halocline; by progradational collapse upward from deep dissolution voids produced in the phreatic zone; or by fracture of the bank margin. Blue holes are the cumulative result of carbonate deposition and cHssolution cycles which have been controlled by Quaternary glacioeustatic fluctuations of sea-level.Blue holes have been widely studied during the past 30 years, and they have providedinformation regarding karst processes, global climate change, marine ecology, and carbonate geochemistry. The literature contains a wealth of references regarding blue holes that are at times misleading, and often confusing. To standaIdize use of the term bluehole,and to familiarize the scientific community with their nature, we herein define them as follows: "Blue holes are subsurface voids that are developed in carbonate banks and islands; are open to the earth's surface; contain tidally-influenced waters of fresh, marine, or mixedchemistry; extend below sea level for a majority of their depth; and may provide access to submerged cavepassages." Blue holes are foundin two settings: ocean holes open directly into the present marine environment and usually contain marinewater with tidal flow; inland blue holes are isolated by present topography from surface marine conditions, and open directly onto the land surface or into an isolated pond or lake, and contain tidally-influenced water of a variety of chemistries from fresh to marine.

Exceptionally well preserved late Quaternary plant and vertebrate fossils from a blue hole on Abaco, The Bahamas

“…Sawmill Sink is an inland ''blue hole,'' a subsurface void in carbonate bedrock that is open to the Earth's surface. Inland blue holes contain tidally influenced water of fresh, marine, and mixed chemistry, extend below sea level for most of their depth, and commonly provide access to submerged cave passages (1). The nearly circular opening of Sawmill Sink has a diameter up to 15.5 m and a circumference of 50 m. Dissolutional undercutting and subsequent collapse of the carbonate rock has created a bell-shaped profile dominated by a well developed talus cone from a depth of 9 to 34 m (Fig.…”

Baseline physicochemical investigations on waters from three blue holes, San Salvador Island, Bahamas

“…by Mylroie et al (1995): "Blue holes are subsurface voids that are developed in carbonate banks and islands; are open fresh, marine or mixed chemistry; extend below sea level for a majority of their depth, and may provide access to cave passages". For an in-depth look into blue hole morphology, Studia UBB Geologia, 2013, 58 (1), 11 -19 Sampson & Guilbeault salinity values that range from fresh to saline, with the fresh Hudson, 2001).…”

Diffuse Versus Conduit Flow in Coastal Karst Aquifers: The Consequences of Island Area and Perimeter Relationships

“…Blue holes are deep ponds and flooded shafts ( Figure 9) that are found on land, called inland blue holes, and in near-shore shallow lagoons, called ocean holes [25]. They take their name from the deep blue color that distinguishes them from the browns and greens of the land surface for inland blue holes and the deep blue color that contrasts them from the turquoise color of shallow lagoons for ocean holes.…”