Microbial alteration of 0–30-Ma seafloor and sub-seafloor basaltic glasses from the Australian Antarctic Discordance

Abstract

Scanning electron microscopic observations of alteration rims in basaltic glasses dredged from 0–2.5-Ma seafloor and drilled from 18–28-Ma ocean crust (Ocean Drilling Program (ODP) Leg 187) in the Australian Antarctic Discordance (AAD) document the presence of endolithic microbes in altered basalt glass. In very young AAD lavas, ∼10-μm-thick alteration rims are developed along intersecting fractures and cracks, and the altered glass contains numerous spherical, rod- and star-shaped, partially fossilised microbial cells, similar to those from the Arctic ridges [1,2]. In 2.5-Ma basalt glasses, altered rims are up to 250 μm thick, and zeolite (phillipsite) is present within many fractures. Spherical cells occur in porous outer zones of alteration rims and on zeolite crystal surfaces within fractures, indicating that microbial activity persists in the region for at least 2.5 Ma. Mn-rich microbial cell-encrustations associated with zeolite suggest that Mn is used in an energy yielding metabolic process. Combined with recent results from the Arctic ridges the results from this study demonstrate that endolithic microbial growth is a persistent feature of mid-ocean spreading ridges. In glasses from ODP cores, ∼1-mm-thick alteration rims are developed along the widest fractures lined with Mn(Fe)-oxyhydroxides and/or clay and filled by zeolite and calcite. Most common, however, are <10–200-μm-thick rims developed along zeolite-filled, more narrow fractures and cracks. Zeolite-filled fractures with only minor to no alteration indicate several episodes of fracturing followed by relatively fast sealing. There is no age progression in alteration thickness along fractures or other characteristics, suggesting that alteration is essentially completed between 2.5 and 18 Ma. A comparison of alteration in the 2.5-Ma glass with that in the ODP samples indicates that a significant proportion of the glass alteration in the drilled samples developed prior to burial, although one type of diffuse, highly irregular front that is only observed in the ODP samples most likely developed after burial. These diffuse alteration fronts are caused by dissolution and alteration of the glass into minute globules, 0.05–0.2 μm in diameter, with no associated microbial morphologies. Fossilised, Mn-rich cells do occur within zeolite-filled fractures, possibly indicating that microbial activity continued in the fractures for as long as circulation continued. The apparent non-biological origin of diffuse, irregular alteration fronts in buried AAD glasses indicates that these textural features are not reliable as diagnostic criteria for the existence of a deep biosphere in the volcanic ocean crust.