DEPTH AND TEMPORAL TRENDS IN MICROBIAL BORINGS ON MOLLUSK SHELL

Boring traces in carbonate skeletal material left by fungus, cyanobacteria, algae, and sponges show important trends by depth and their patterns of infestation change with exposure time. These observations have use in determining paleoenvironment from fossil carbonate skeletal material back at least as far as the Cambrian. Therefore, it is worthwhile to gain a better understanding of microboring habitats, infestation rates, and activities in modern carbonate shell. The Shelf and Slope Experimental Taphonomy Initiative (SSETI) placed bivalve shells along a depth transect from 15m to 300m in the Bahamas in 1993. Samples were collected after two-, six-, and thirteen years so that temporal trends in taphofacies could be documented. This study of trends in endoskeletobiont traces reports on prevalence of boring in shell interiors (to avoid any borings that might have accumulated during life). This is not the first study to examine microborings along a depth gradient, but it adds information on the rates of boring into various bivalve shell mineralogies and documents changes in infestation patterns with time of exposure.

Results show that larger-diameter borings are most prevalent between 15m and 72m. Deeper samples are dominated by smaller-diameter borings most likely corresponding to fungal filaments. Depth of penetration into the shell surface decreased with depth and angle of penetration became more horizontal with depth. Some fine borings (about 1 micron diameter) at 15 and 33m were found to penetrate the full thickness of some shells. The depth into the shell at which boring density dropped below 50% drops from about 15 microns at 15m to <2 microns deeper than 70m water depth. This may correspond to the thickness of the micrite envelope in fossil shells viewed in thin section. Rates of infestation were greatest within the first two years of the experiment and intensity leveled off after that. Data show an important correlation between water depth and depth of penetration by borers as well as angle of boring traces that will prove useful in microtaphofacies analyses. Rate of infestation appears to be non-linear. But if levels of infestation are attained very quickly (within a year or two) and remain diagnostic with respect to depth, endoskeletobionts have great potential for paleoenvironmental analyses.