Microtubules in hyaloclasts from the Hawaii Scientific Drilling Project #2 phase 1 core, Hilo, Hawaii: evidence of microbe-rock interactions

Abstract

Minute tubules etched into basalt glass in hyaloclastites from the Hawaii Scientific Drilling Project #2 (HSDP) phase 1 borehole are interpreted as trace fossils formed by microbes, i.e. microendolithic borings. Such borings are one to a few micrometers in diameter and up to >100 µm long; they extend into glass shards from free surfaces (broken shards, vesicles, fractures). Morphologic characterization of microendolithic borings quantitatively describes them for comparison with other occurrences and aids in understanding the interactions between microorganisms and basaltic glass that result in the dissolution of the glass. The first step in working with these features as trace fossils was to modify the ichnofabric index of Droser and Bottjer (1986) for use with minute features that extend into homogeneous material. The modification includes six semiquantitative classes of disruption and is scale-independent, applicable to any size feature. The second step was to apply the new microendolithic ichnofabric index (MII) to the HSDP samples. Analysis of the HSDP samples using the MII showed that the abundance of bioerosion varied throughout the core. Assigned MII values ranged from 1 to 3, average MII values ranged from 1 to 2.44, while the mean MII value of 1.2. Areas with the most bioerosion were located between 1,365.9 and 1,478.8 mbsl and a section of the core centered around 2,117.0 mbsl. The MII values of these locations ranged from 2 to 2.5. Areas with low bioerosion (all samples 100 µm long; they extend into glass shards from free surfaces (broken shards, vesicles, fractures). Morphologic characterization of microendolithic borings quantitatively describes them for comparison with other occurrences and aids in understanding the interactions between microorganisms and basaltic glass that result in the dissolution of the glass. The first step in working with these features as trace fossils was to modify the ichnofabric index of Droser and Bottjer (1986) for use with minute features that extend into homogeneous material. The modification includes six semiquantitative classes of disruption and is scale-independent, applicable to any size feature. The second step was to apply the new microendolithic ichnofabric index (MII) to the HSDP samples. Analysis of the HSDP samples using the MII showed that the abundance of bioerosion varied throughout the core. Assigned MII values ranged from 1 to 3, average MII values ranged from 1 to 2.44, while the mean MII value of 1.2. Areas with the most bioerosion were located between 1,365.9 and 1,478.8 mbsl and a section of the core centered around 2,117.0 mbsl. The MII values of these locations ranged from 2 to 2.5. Areas with low bioerosion (all samples <2) were located between 1,079.0 and 1,320.0 mbsl, 1,799.0 and 1,900.0 mbsl, and all depths below 2,500.0 mbsl. Lastly, such features as length, diameter, ornamentation, density, and complexity and tortuosity were measured to better describe the interactions between microorganisms and basaltic hyaloclastite media. The shortest measured 0.907 µm and the longest measured 129.22 µm. Lengths were approximately log-normally distributed with a geometric mean of 18.9 µm. The tortuosity of borings had a median of 1.29 with a range of 1.227 to 1.37. The least tortuous measured 1.22 and the most tortuous measured 16.46. This was one of the first attempts to quantify the range of morphology and density, of euendolithic microborings in basalt glass. This study extends the sampling scale for ichnological study to what is near the minimum size range of trace fossils. It demonstrates that trace fossil abundance does not simply decrease with depth in ocean islands, unlike basalts of oceanic crust, but varies, probably as a result of variation of the rate of accumulation of suitable substrates.