DEVELOPING THE MICRITE ENVELOPE: PATTERNS OF MICROBORING INTENSITY ACROSS DEPTH AND GEOGRAPHIC LOCATION

Microboring organisms create a network that penetrates the outer surface of carbonate grains and results in a micrite envelope as grains/shells go through diagenesis. Borings are created by photosynthetic microbes (e.g., algae and bacteria) or non-photosynthetic microbes (e.g., fungus) and normally extend less than 1mm into a carbonate surface. The intensity of boring, as well as the community of boring organisms, has been shown to decline with water depth, which is primarily a light-intensity signature. The character of the micrite envelope on fossils and carbonate grains is therefore a useful paleoenvironmental indicator. However, more nuanced information about environments may be available from this signature.

This study examines the timing of trace development and compares boring intensity between experimentally placed bivalves in the Bahamas and Gulf of Mexico. Four species of bivalve were sewn into mesh bags and deployed on the sea floor in the Bahamas and GOM in 1993; they were collected 2yr, 8yr, and 12yr after deployment. Recovered shells were epoxy impregnated under vacuum followed by high pressure to fill the microbial traces. The shells were then cut perpendicular to the surface and etched to reveal depth and density of boring into the shells. Replicate photomicrographs were taken under SEM along the interior surface of each shell. Intensity and depth of boring were tallied by point-counting five micron panels from the shell surface to a depth of 50 microns. The results show that boring intensity within the first 5-10 microns is high in the first two years and that both intensity and depth of boring into the shell increase over the course of the following 6 to 10 years at most water depths in the Bahamas. In addition, for shells deployed in the Bahamas, the zone of heavily bored shell surface (percent of edge bored at or near 100%) was reached in less time than in shells from the Gulf of Mexico at similar water depths. In the Bahamas, shells developed bored surfaces even to depths of 300m. However, Gulf of Mexico shells were rarely bored at water depths below 100m despite their presumed availability to fungal borers. Therefore micrite envelopes may be good depth indicators in tropical carbonate environments, but the lack of microbial borings in offshore shelf environments with terrigenous sedimentation may be less reliable.