Microbes and volcanoes: A tale from the oceans, ophiolites, and greenstone belts Hubert Staudigel, Scripps Institution of Oceanography, textural, and geochemical evidence for life has been found in University of California, La Jolla, California 92093-0225, USA; oceanic crust of almost any age and all ocean basins and in a Harald Furnes, Department of Earth Science, University of large number of ophiolites and greenstone belts. As for all tex- Bergen, Allegt.41, 5007, Bergen, Norway; Neil R. Banerjee, tural evidence for life, the biogenicity of alteration textures has Department of Earth Sciences, University of Western Ontario, to be argued carefully, in the context of geochemical data and London, Ontario N6A 5B7, Canada; Yildirim Dilek, Department geology (Furnes et al., 2002; Staudigel and Furnes, 2004). of Geology, 116 Shideler Hall, Miami University, Oxford, Ohio 45056, USA; and Karlis Muehlenbachs, Department of Earth MICROBIAL ALTERATION OF VOLCANIC GLASS and Atmospheric Sciences, University of Alberta, Edmonton, Volcanic glass is a common quench product of lavas in sub- Alberta T6G 2E3, Canada marine volcanic oceanic crust. It breaks down easily in the presence of seawater. For these reasons, glass alteration con- tributes more to the chemical mass balance of seafloor altera- tion than any other igneous phase in the extrusive oceanic ABSTRACT crust (Staudigel and Hart, 1983). Submarine volcanic glass alteration displays two easily dis- cernable types of textures, one that is best interpreted as the Microscopic Textures result of an abiotic diffusive exchange process and another that Bioalteration of basaltic glass was first described by Ross and involves microbial activity. Glass bioalteration textures domi- Fisher (1986) and then explained by localized dissolution of nate in the upper 300 m of the oceanic crust and have been glass from metabolic waste products of colonizing microbes found in nearly all ocean basins and in many ophiolites and (Thorseth et al., 1992). Subsequently, it was recognized that greenstone belts back to 3.5 Ga. Bioalteration may involve a bioalteration is very common in submarine glass from any globally significant biomass and may influence geochemical tectonic setting and geological age (see GSA Data Repository fluxes from seafloor alteration. Glass bioalteration creates an Table DR11). entirely new discipline of research that involves microbiolo- It is important to contrast abiotic from biotic alteration of gists and volcanologists working in active volcanic systems and glass, which display unique textural characteristics (Fig. 1). in the geologic record. Submarine volcanoes exposed on the Abiotic alteration of basaltic volcanic glass in a hydrous envi- ocean floor are studied along with ophiolites and greenstone ronment can be recognized by the darkening of the originally belts to understand Earth not only as a physical and chemical light yellow to colorless isotropic and noncrystalline glass (Fig. heat engine but also as a bioreactor. 1A). Glass transformation into yellow or tan palagonite or into slightly birefringent fibropalagonite always proceeds from the INTRODUCTION external surfaces toward an unaltered core in progressive alter- Studies of modern and ancient volcanoes on the ocean floor ation fronts (Fig. 1A). Palagonite defines concentric fronts that as well as in ophiolites and greenstone belts tell important migrate inward toward the fresh interior of the glassy frag- parts of the story of how Earth works as a “heat engine,” in ments, progressively smoothing or rounding off sharp edges which planetary heat loss drives mantle convection and plate of individual grains. Extensive petrographic and geochemical motions. Recently, these studies have shown that submarine observations have led to a broad consensus about key pro- volcanoes may host substantial biological communities that cesses that define abiotic alteration of glass (e.g., Stroncik and create characteristic bioalteration textures in volcanic glass Schmincke, 2001): It is largely a diffusively controlled chemi- (Fisk et al., 1998; Furnes et al., 2001a). These processes could cal exchange process, in which hydration progresses inward, play a globally significant role in terms of the distribution of removing various fractions of the mobile chemical inventory biomass or mediating basalt alteration and the chemical fluxes of the glass, adding some seawater components, and form- between the oceanic crust and seawater (Furnes and Staudigel, ing an array of alteration products, many of them resembling 1999). These observations add an exciting new angle to the clays. These range in grain size from barely visible with the study of submarine volcanoes on the ocean floor, in ophiolites, transmission electron microscope to clearly birefringent in a and in greenstone belts. petrographic microscope. There is very little evidence in nature In the spirit of interdisciplinary integration, we offer a critical for wholesale (congruent) dissolution of glass under abiotic review and some new data for what we consider to be some of conditions except for the very earliest phases, before an immo- the most intriguing evidence for microbial life inside submarine bile product layer has been established on exterior surfaces. volcanoes: the bioalteration of basaltic glass. This geological, Elements removed from glass typically crystallize as authigenic GSA Today: v. 16, no. 10, doi: 10.1130/GSAT01610A.1 1GSA Data Repository Item 2006215, Table DR1: Locations and ages of submarine extrusives with bioalteration, is available on the Web at www. geosociety.org/pubs/ft2006.htm. You can also obtain a copy of this item by writing to [email protected]. 4 OCTOBER 2006, GSA TODAY Alteration Mode Model dissolution then forms these two types of bioalteration (Figs. 1B, 1C, and Fig. 2). Progressive alteration A biological origin for these features A Abiotic is supported by a range of textural palagonite Time0 Time1 Time2 observations: 50 μm Fragments of Fresh glass and palagonite (P) rinds • Bioalteration is never found com- fresh glass (FG) pletely enclosed in glass; it is always rooted on surfaces that are t0 FG t1 t2 P exposed to external water. P • Tubular and granular alteration FG P P locations on conjugate sides of a P P FG crack do not line up with one FG another (Fig. 1C), eliminating a pre- FG P existing weakness of the glass as a P P FG FG FG cause. • Tube and granule diameters are of Biotic Fresh glass (FG) Progressive colonization of micron to submicron scale, like B granular texture (GT) with open fracture microorganisms and contemparaneous microbes. Tubes tend to be larger along which dissolution of the glass adjacent to than granules, yet both display log- microorganisms (M) each individual attach normal size distributions, a com- mon attribute in biological systems t0 t1 t2 (e.g., van Dover et al., 2003). 50 μm FG • Tubular alteration does not show FG FG flaring at the entry point or nar- rowing deeper inside the glass, as M would be expected from abiotic GT dissolution. water flow water flow water flow • Some tubes show segmentation, in 5 μm FG FG FG which the diameter of tubes varies regularly. This is highly suggestive of Biotic pulsed growth and/or the presence C tubular texture (TT) of several cells (Figs. 1C and 2C). 20 μm TT • Some tubes bifurcate, which can t0 t1 t2 be explained satisfactorily by cell FG FG FG division. • Some tubes show spirals (Fig. 2D) M that are extremely hard to generate w o l f abiotically with the regularity r e TT t a observed. Spirals are common in TT FG FG w FG biology and biologically produced water flow water flow materials (e.g., twisted stalks of the Fe-oxidizing bacterium Gallionella). Figure 1. Thin section photomicrographs from seafloor basalts and a schematic two-step model for HF the development of different types of glass alteration. Unaltered fresh glass (FG) without or with • Granular alteration often forms hemispherical agglomerations of microbes (green, “M”) is labeled t0; t1 and t2 are two successive stages of alteration. (A) Abiotic alteration of glass to palagonite, with fine grained grains completely altered and big grains containing cavities, radiating out from a single some fresh cores with rounded corners. Examples for granular and tubular bioalteration are given in point at a crack surface and pro- (B) and (C), respectively. Note the common asymmetry of tubes with respect to opposing sides. The ducing the texture of a sponge. photomicrograph in model C also contains a crack with granular alteration. These agglomerations closely resemble the growth of microbial cultures on an agar dish, except phases in the interstices between the in the formation of cavities that enter the that they are three-dimensional glass fragments using dissolved compo- glass from exterior surfaces in granu- and the medium is basaltic glass. nents from the glass and from seawater lar-appearing agglomerations (Fig. 1B) None of these textures can be rec- (Hay and Iijima, 1968; Staudigel and Hart, or tubular (tunnel-like) morphologies onciled with the diffusive model of 1983; Stroncik and Schmincke, 2001). (Figs. 1C and 2A–2D). In both cases, it is abiotic glass alteration; a microbially The microscopic appearance of micro- inferred that microbes colonize exterior mediated congruent dissolution process bially mediated glass alteration textures surfaces or surfaces of cracks and begin (Figs. 1B and 1C) is a more plausible is quite distinct from the abiotic expres- to dissolve the rock through changes in explanation. While many details of this sion, in that alteration is reflected largely pH at their contact area. The localized process remain areas of active research, GSA TODAY, OCTOBER 2006 5 300 Mm 25 Mm Figure 2. Tubular bioalteration textures in A B volcanic glass from pillow lavas and inter- pillow hyaloclastite. (A) Photomicrograph of thin section from interpillow sample from the Euro Basalt of the Warrawoona Group, Pilbara Craton. The tubular structures (filled B with titanite) occur along fragment boundaries. The other minerals are chlorite—light green; quartz—white; calcite—brownish. (B) Detail of the titanite-filled bioalteration tubular structures.