Maldives: the Blue Hole

Scientific research at the Maldives with Albatros Top Boat
THE FIRST BLUE HOLE OF THE INDIAN OCEAN

The Blue Hole recently discovered by Massimo Sandrini at the Maldives is still the object of  much exploration. During the 2001 cruise a topographic and bathymetric survey was carried out, the first physical-chemical parameters were measured and samples of water and sediment were taken. According to those studies, the Blue Hole would be of karstic origin, although this is further complicated by a subsequent corrosion of the stony coral by hydrogen sulphide, the entity and origin of which still have to be ascertained.

Blue Holes are deep submerged caves of karstic origin and have been known to exist along the coral reefs of the Caribbean and the Bahamas for a long time, having been made famous by the pioneering explorations of the unforgettable Captain Jacques-Yves Cousteau. Up to recently nobody was aware of their existence in the Indian Ocean, so much so that their absence was in fact given for granted. On this subject the effects of the great climatic changes occurred in the Ice Age were thought to have been felt more in the Atlantic Ocean than in the Indian Ocean; this would have explained the fact that no Blue Holes had formed there. A first exploration of the Blue Hole was organised during the scientific cruise of April 2000 by Albatros Top Boat in co-operation with the International School for Scientific Diving (ISSD), an organisation recognised by the EU and by UNESCO comprising professional scientists affiliated with Universities and other Research institutes. Exactly one year later, in April 2001, a new Albatros-ISSD scientific cruise set among its main objectives further exploration and survey of the Blue Hole. This was achieved after four days of hard work by a large team of researchers and divers, organised into three groups: Ali Abdul Gadir, Giovanna Bernardini, Sara Mazzanti and Sandro Riccio carried out geomorphic, sedimentological and biologic research on the edge of the Blue Hole and on the environment just outside the hole; Paolo Colantoni and Carla Morri studied the geology and the biological element of its walls; Giuseppe Baldelli, Carlo Nike Bianchi and Massimo Sandrini took samples and surveyed the seabed. Hydrologic measurements and water samples were also taken at various depths and the outlines of the seabed were traced by using both a surface echo-sounder and a portable sonar during dives. Even though there is much work that still remains to be done to complete the exploration and surveying of the blue hole, the research carried out during the 2001 cruise allows us to have a more precise idea of the cave’s morphology. The Blue Hole opens out at a depth of about 32 metres through a circular 70-m mouth, which widens gradually reaching, at the bottom, about eighty metres.

The shape of the cave resembles a slightly eccentric truncated cone, with a wider base on the western and southern sides. This Maldivian Blue Hole is characterised by a peculiar stratification of waters. Up to the depth of 45 m there is normal sea water, at times rather murky because of the presence of white floccules in suspension. At this depth there is a slight thermocline, the temperature rapidly losing 2 or 3 degrees Centigrade, and a marked chemocline, that is a sudden change in the water’s chemical composition. From normal well-oxygenated sea water, the water becomes anoxic, that is lacking in oxygen and rich in hydrogen sulphide (H2S), which has a characteristic pungent smell. This change is evidenced by a sharp reduction in the pH values, which indicates a relative increase in acidity, and in strongly negative values of Eh, which typifies the conditions of environments lacking in oxygen. Below the chemocline there is complete darkness, although the water seems to get slightly clearer, thanks to a reduction of particles in suspension.

For most living organism hydrogen sulphide is toxic and its presence excludes the presence of dissolved oxygen. Below the chemocline there is therefore no trace of animal life. The walls are virtually bare and are scattered with numerous small recesses and corrosion ridges. Among the few organisms which are able to live in the presence of hydrogen sulphide are some types of bacteria. The reddish or greenish patinas which can be observed just below the chemocline and the long mucilaginous filaments forming uninterrupted fringes on the walls at high depths are probably due to different species of bacteria.

Life reappears in the oxygenated water above the chemocline. Initially there are only a few encrusted organisms, sponges in particular. In the sinuous recesses which characterise the entrance to the cavern there are schools of various fish. Immediately outside the blue hole, there are only a few specimen of coral, mostly soft coral, showing signs of past episodes of mortality of sessile fauna; occasional leakages of hydrogen sulphide from the cavern may cause the immediate death of all those fixed forms of fauna which cannot escape. Temperatures, pH and Eh of the Blue Hole’s water: at a depth of 45 metres the water is separated by the chemocline into normal sea water and dark waters lacking in oxygen and rich in hydrogen sulphide.

In geological terms the most interesting discovery of this new stage of exploration has been numerous speleothems at a depth of 50 – 60 m., under the vaults of the western and southern part of the Blue Hole. Several stalactites and some stalagmites have been observed, which have formed through the union and fusion of a stalagmite and a stalactite. This morphology proves the karstic past of this cave, at a time when the sea level was several tens of metres lower that is it now and the stony coral which forms its walls was completely emerged and penetrated by the rain. Because of its morphology and its structure, the Blue Hole is reminiscent of dolines or ponors. The fact that in the past the subsoil in the area where the Blue Hole currently is was penetrated by a large amount of water rich in CO2 coming from a large collection basin still remains to be demonstrated. For the time being however no trace of such basin has been found in the surrounding area. Further research is needed to support this theory on the Blue Hole’s origin. Another peculiarity deserves further study. The presence of hydrogen sulphide in concentrations which increase with the water’s depth leads to hypothesize that the origins of the Blue Hole are much more complex. Besides having originated from the corrosion of the stony coral following the typical pattern of karstic phenomena, according to which gravity causes rain water to penetrate the rock from top to bottom, the blue hole could be the result of corrosion due to the rise of hydrogen sulphide, which would have corroded the roof of the cavern, causing it to collapse, thus creating the characteristic circular opening. This phenomenon is called hyperkarstification.

The activity of hyperkarstification occurs in an oxygenated environment, where hydrogen sulphide (H2S) reacts with oxygen and becomes sulphuric acid (H2S04). The latter reacts with limestone, promoting the release of carbon dioxide which in turn increases corrosion. In the case of the Maldives’ Blue Hole therefore the H2S originating at great depths climbs back up and comes into contact with sea water, thus reacting and dissolving the calcium carbonate of which the reef is made and forming caves which, in exceptional circumstances, reach the surface. Nobody had ever described this type of surface structure before and, above all, before our discovery relative to the presence of hydrogen sulphide, nobody had thought of the mechanism of hyperkarstification as an explanation for the formation of a Blue Hole.

What could however be the origin of the hydrogen sulphide present at the Maldives? On the one hand it could come from sulphates present in deep strata, although our current knowledge does not seem to support this hypothesis. According to a second possibility hydrocarbons would be present in the rocky mass. It is certainly possible to suppose the presence, at the Maldives, of flows of hydrocarbons which can climb up through faults and gaps of the stony coral although, even though hydrocarbons rich in sulphur compounds were present, this would not be enough. The last remaining possibility concerns the reduction of sulphates, which abound in sea water, occurring in a non oxygenated environment thanks to sulphur reducing bacteria; this is perhaps the most plausible theory. But it is only through further studies, specific research, measurements and sampling that in the future the origin and the nature of the Maldives’ first Blue Hole can be explained.

Carlo Nike Bianchi, Paolo Colantoni and Carla Morri
International School for Scientific Diving