Abstract

Uncalcified and calcified algae grow on and color walls of cavities that occur in karst breccia, which fills pockets in the Tertiary Bluff Formation of Grand Cayman Island. The calcified algae are important because the calcification has been achieved through the formation of dendritic calcite crystals. Although common in metals, such branching crystals have not been widely recognized in naturally occurring calcite. The dendritic calcite crystals are exactly like most dendritic crystals in that they have a primary needle or stem with secondary and tertiary branches. Most dendritic calcite crystals have an open, treelike form and are easy to recognize. As calcification proceeds, the dendritic crystals become progressively filled in and are increasingly difficult to recognize as dendritic crystals. Filling of the dendritic crystals can result in two-dimensional sheets or three-dimensional columns. Assuming the three-dimensional columnar dendritic crystals developed planar and/or curving smooth surfaces, then, lacking other evidence, they would be indistinguishable from normal columnar calcite crystals, and their dendritic origin would be hidden. Variables thought to control dendrite crystal formation include a) creation of localized, highly supersaturated solutions leading to rapid rates of nucleation and crystal growth, and b) "impurities" in the form of organic molecules and other substances. Algae are excellent candidates for controlling a via their metabolism and contributing to b via their decay. Combining these ideas with the results of this study and with the known common occurrence of algae in many speleothemic environments provides an impetus for a reexamination of the origin of subaerially formed calcite crystals.

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