Enigmatic Coral Rock Pillars -Another Look Into Reef Dynamics

Stone capped calcareous pillars, rising 10-70 cm above the surroundingreef surface are to be found at Aqaba (Jordan) and on several otherfringing reefs of the Northern Red Sea from intertidal down to 3 mdepth. Terrigenous (non-calcareous) cobbles and boulders are fixed ontop of coral limestone. It is assumed that the stones were onceintroduced by fishermen or by exceptional floods and came to rest indepressions of the reef. Afterwards the surrounding reef limestone waseroded so that only the substrate underlying the granite stones has beenleft as singular towers. 14C-dating of a column sample from Aqabaprovided an age of 560-345 years. Several interpretations are possible:the respective reef part did not grow since then, or younger layers wereremoved by bioerosion during which time? Some suggestions areprovided based on erosion data of sea urchins (Diadema setosum) andfish, gathered at that site. More (subtidal and intertidal) examples ofpartial reef decline are presented to stir discussion about circumstancesand modes of bioerosional reef shaping.