The role of microorganisms in phytokarst development on dolostones and limestones, Grand Cayman, British West Indies

Phytokarst is spectacularly developed on the hard, finely crystalline dolostones of the Oligocene–Miocene Bluff Formation and to a lesser extent on the soft limestones of the Pleistocene Ironshore Formation on Grand Cayman. The black weathered surfaces, which are a reflection of an organic coating, contrast sharply with the white colour of the unaltered host rocks, which lack an organic coating. On the dolostones this organic coating is formed principally of fungal(?) sporangia and spores associated with mucilage along with lesser numbers of algae and bacteria. Endolithic and epilithic filamentous microorganisms are rare. On the limestones the organic coating is formed principally of endolithic and epilithic filaments and mucilage. Fungal(?) spores are rare, and no sporangia were found. The limestones of the Ironshore Formation are characterized by an intensely bored zone beneath the weathering surface. In contrast, the dolostones of the Bluff Formation have relatively few endoliths penetrating from the weathering surface.Dolomite rhombs on the weathered surfaces of the Bluff Formation are severely etched with well-developed rhomb-shaped etch pits, have holes through them, or have one or more of their faces removed and their cores partly leached. The dissolution of these crystals appears to be related to surface reaction-controlled kinetic processes mediated by the organic coatings. In contrast, the surfaces of the limestones do not display any of the delicate etch topography and appear to be dissolved at a much faster rate.Although both the limestones and dolostones are undergoing similar surface-weathering processes there are important, but subtle, differences in which those processes are operating. A critical factor in this contrast is the hardness of the substrate. The hard dolostones have a rich, diverse epilithic flora but poor endolithic flora, whereas the softer limestones have a relatively poor, low-diversity epilithic flora but abundant endoliths.