Thallophyte boring and micritization within skeletal sands from Connemara, western Ireland

Carbonate grains from the skeletal sands off the western Irish coast of Connemara (53 degrees N latitude) are extensively bored by thallophytes, yet the borings, unlike those in warm-sea sediments, are not associated with cryptocrystalline carbonate (micrite). On the other hand, living Lithothamnium colonies and Lithothamnium fragments frequently have their cell cavities filled by high Mg-calcite micrite and, less commonly, parts of the algal skeleton may be micritized. In living colonies, the micrite is often confined to certain growth lamellae of the red alga. The presence of micrite as a void filler or as a replacement of preexisting carbonates is indicative of environments capable of precipitating metastable cements. The environments of precipitation may be depositional or post-depositional. All available evidence favors a biochemical precipitation mechanism in the micritization of the red algal skeletons, brought about essentially by the life activities of the organism. In the light of the present study, the significance of micritized grains within carbonate sediments is re-examined.