Microbial alteration of Bahamian deep-sea carbonates

An analysis of microborings within sediment and hardground samples collected from the Northwest Providence Channel and the western margin of the Little Bahama Bank was conducted to characterize the endolithic assemblage present, to examine the role of microboring organisms in the alteration of deep-sea carbonates, and to evaluate the palaeoecologic potential of the aphotic microboring assemblage found. Samples examined in this study were collected at depths ranging from 210 to 1450 m. The microboring assemblage was found to contain: (a) filamentous fungal borings of five distinct types, (b) a 1.0 × 2.5 μm vermicular form of fungal or bacterial origin, (c) an 8-12 μm tubular, branching form of probably fungal origin, (d) a subapically branched form considered to be fungal, (e) a spinate form of uncertain affinity and (f) sponge borings. Three of these forms are known only from the deep-marine environment; the remainder are also known from shallow-marine sediments found well within the photic zone. Both carbonate sediments and lithified hardgrounds are highly altered through the activity of endolithic organisms. Infestation of individual skeletal fragments by microborers may be so extensive as to produce heavily bored envelopes resembling those previously reported to occur only under shallow-marine conditions. Although the geological ranges of these microborings remain to be established, the presence of such an aphotic assemblage, coupled with the absence of photosynthetic algal borings, could provide a valuable tool in palaeoecological studies. Other potential applications include the determination of turbidite sediment sources and the establishment of relative water depths for the formation of hardground surfaces.