Holocene ooids from Joulters Ooid Shoal (Bahamas) are bored in various ways by blue-green algae that groove along the grain surface, reside just beneath the grain surface, and tunnel extensively a few tens of microns within the grain. The microborings, morphologically distinctive, are documented with scanning electron micrographs of open borings and resin casts. Gentle dissolution of ooid aragonite permits identification of several algal genera by light microscopy and enables comparison with the microboring casts.

Pleistocene ooids from the Miami Limestone (Florida) contain natural casts of microborings, some of which are similar in form to Holocene examples. Significantly, these aragonite casts are more resistant to solution than surrounding ooid aragonite. They remain after most of the ooid is leached away and survive replacement of the ooid by low-Mg calcite. Dissolution or precipitation may occur along the walls of microborings, causing morphological alteration during their preservation. This points out a difficulty in the specific identification of endoliths on the basis of fossilized microborings in ancient rocks composed of original aragonite grains.

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