SLOW BORING LIFE IN HAWAIIAN HYALOCLASTITE: A COMMENT ON THE DEEP BIOSPHERE

Endolithic microborings in hyaloclastites from Hawaii allow some comment on one manifestation of the deep biosphere. The Hawaii Scientific Drilling Project #2 Phase 1 (HSDP) core, drilled at the Hilo Airport, contains submarine hyaloclastite and glass-rich re-sedimented beds interbedded with pillow lavas and intrusions from a depth of 1080 meters below sea level (mbsl) to total depth at 3109 m. Endolithic microborings are common in hyaloclastites in from 1080 to 2100 m, but largely absent below that depth. Hyaloclastite and sediment samples dredged at depths of 2716 to 4037 m from the Hilina Slope contain similar structures.

At least in initial stages, these borings closely resemble those described from pillow margins in oceanic crust and ophiolites. Published chemical and isotopic anomalies demonstrate that the tubular structures, the microborings, in other occurrences are of organic origin. Some HSDP samples contain DNA of Archaea. The HSDP core permits observation of the changes of abundance, development, and alteration with depth.

Microborings are absent or modestly developed in the shallowest samples from the submarine portion of the HSDP core. Microborings increase in abundance and degree of development from 1079 to 2100 mbsl in the core, suggesting that they grew slowly and could have been active until sampled. Most rocks that contain microborings below 1573 m in the HSDP core are palagonitized with very low porosity. Microborings in them have undergone secondary alteration and do not show the pre-alteration morphology, suggesting that the microbes are not active. Development of microborings postdates formation of palagonite in Hilina Slope samples and pre-dates that stage in alteration of HSDP samples.

Microorganisms do not begin to form endolithic microborings immediately upon cooling of glass to seawater temperature. Microborings reach lengths of up to 200 micrometers over the period of time between deposition and development of palagonite, ~6*10⁴ years. Microbial activity slows very greatly or stops when porosity, and therefore permeability, drops to very low levels. This pattern suggests that in basalt glass and in this boring niche, life occurs where pore waters circulate easily, is slow, and begins after burial.