ENDOLITHIC MICROBORINGS IN BASALT GLASS FRAGMENTS IN HYALOCLASTITES: EXTENDING THE ICHNOFABRIC INDEX TO MICROBIOEROSION

Endolithic microborings found in sideromelane fragments in hyaloclastites from the Hawaii Scientific Drilling Project #2 phase one core provide evidence for microbe-rock interactions causing dissolution of glass. The samples are from the drilling site located near Hilo, Hawaii, and range in age from 413 to approximately 636 kya. Such evidence as the presence of DNA along with vital elements suggests that the tubes are microborings of organic origin. The conclusion that the microendolithic borings are in fact biotic allows for their description using standard ichnological methods.

The overall shape of the borings is best described as tubular. The majority of the borings are linear or curvilinear. The borings are 1 µm to over 100 µm long. The borings are approximately 0.5 µm to 2 µm in diameter. The borings originate from the edges of vesicles, margins of shards, or along fractures. The point of origin does not appear to affect the shape of the borings. The complexity of the borings increases with the core depth from which they were taken from about 1080 m to 1600 m. Near the top of the core, the borings are simple. Increased complexity in borings includes branching at different points along the length of the boring and flaring at the termination. The tortuosity of the borings ranges from 1.1 to 8.9. The standard categories of the ichnofabric index were modified from Droser and Bottjer (1986) to suit more appropriately the size, distribution, and medium of the microborings. The modified ichnofabric index is based on dissolution of glass rather than bedding disruption. Preliminary research suggests the use of five categories ranging from no dissolution of glass to complete dissolution of glass. Data suggests that the optimal field of view for analysis of microbial ichnofabric indices is 30 µm by 30 µm. The morphologic characterization of the microendolithic borings aids in the understanding of interactions between microorganisms and basaltic glass that results in the dissolution of the glass. The modification of the ichnofabric index extends the measuring scale of bioerosion to what is near the minimum size range for trace fossils.