Mechanisms of microtunneling in rock substrates:distinguishing endolithic biosignatures from abioticmicrotunnelsN. MCLOUGHLIN,1 H. STAUDIGEL,2 H. FURNES,1 B. EICKMANN1 AND M. IVARSSON3

1Department of Earth Science, Centre for Geobiology, University of Bergen, Bergen, Norway2Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California SanDiego, La Jolla,CA, USA

3Department of Palaeozoology, Swedish Museum of Natural History, Stockholm, Sweden

ABSTRACT

Rock-dwelling, endolithic micro-organisms can create tubular microcavities (TMCs) by the dissolution of rock

substrates. Microtunnels can also conceivably be formed by abiotic processes, and collectively, these structures

are here termed tubular microcavities. A textural record of life in subseafloor environments is provided by biolog-

ical TMCs, and it is imperative to distinguish these from abiological tunnels. To this end, the morphologies and

petrographic context of tunnels formed by chemical solution, physical abrasion, and biological processes are here

described. Biological TMCs in volcanic glass are restricted to sites that were connected to early fluid circulation.

Their shapes, distribution, and the absence of intersections exclude an origin by chemical dissolution of

pre-existing heterogeneities such as, radiation damage trails, gas-escape structures, or fluid inclusion trails.

Rather their characteristics are best explained by microbial dissolution, involving perhaps, cellular extensions that

provide a mechanism of localizing and directing microtunnel formation as observed in terrestrial soils. Biological

TMCs are contrasted with ambient inclusion trails (AITs) found in cherts and authigenic minerals. These differ in

exhibiting longitudinal striae, a constant diameter, and polygonal cross-section, sometimes with terminal

inclusions. The origin(s) of AITs remain unclear but they are hypothesized to form by migration of crystalline or

organic inclusions in sealed substrates, in contrast to biotic TMCs that form in open systems.We present diagnos-

tic morphological and petrographic criteria for distinguishing these different types of TMCs. Moreover, we argue

that AIT-type processes are not viable in volcanic glass because of the absence of crystalline millstones, localized

chemical solution agents, and elevated fluid pressures, necessary to drive this process.

Received 14 December 2009; accepted 30March 2010

Corresponding author: NicolaMcLoughlin. Tel.: +47 55583440; fax: +47 55583660; e-mail: nicola.mcloughlin@

geo.uib.no

INTRODUCTION

Tubular microcavities (TMCs) can be found in a range of

geological settings, and here we address two major classes:

ambient inclusion trails (AITs) most often found in low-grade

meta-sedimentary rocks; and tubular alteration textures found

in fresh andmetamorphosed volcanic glass. The interpretation

of their origins is dramatically different with AITs being widelyregarded as abiotic (e.g. Lepot et al., 2009), whereas tubularglass alteration is inferred to be biotic (e.g. Thorseth et al.,1992; Furnes et al., 1996). The ability to distinguish these two

phenomena is critical to investigations of life in the subseafloor

volcanic glass with far-reaching implications for our under-

standingofglobal biomass and theoriginof life onEarth.

Morphological evidence alone can sometimes be ambiva-lent in constraining the mechanism of TMC formation, and

we summarize this evidence here. We argue, however, that

textural and petrographic evidence has not yet been used to its

full potential in the investigation of TMCs. For this reason,

we review these two most-discussed types of TMCs, glass bio-

alteration and AITs, then describe their textural and petro-

graphic characteristics, and the mechanistic inferences that

! 2010 Blackwell Publishing Ltd 245

Geobiology (2010), 8, 245–255 DOI: 10.1111/j.1472-4669.2010.00243.x

can be made. In doing so, we provide diagnostic criteria for

separating these classes of TMCs, and highlight the remaining

questions that surround processes of microtunneling in glass

and mineral substrates.

Tubular microcavities are formed through the excavationor mining of a substrate, and can remain hollow, or be pre-

served by the later growth of infilling minerals. Thus, TMCs

form by dissolution, demineralization, and removal of a sub-

strate, exactly the opposite mechanism to the more widely

studied phenomenon of precipitation, (bio)-mineralization,

and the growth of filamentous structures by accretion. These

mechanistic differences underpin the criteria presented here

to recognize the different classes of TMC, and demand thatwe investigate TMCs as a unique class of biosignatures (cf.

McLoughlin et al., 2007).

A brief introduction to the bioalteration of glass

Volcanic glass contains reduced elemental species such as

iron, sulfur, and manganese that can conceivably be oxidizedby chemolithoautotrophic micro-organisms using, for exam-

ple, Fe2+, Mn4+, SO4)2, and CO2 in the glass, or circulating

seawater as the electron acceptor (e.g. Bach & Edwards,

2003). Micro-organisms are associated with etch pits on

man-made glasses (e.g. Krumbein et al., 1991), and are

inferred to etch natural volcanic glass, creating granular, and

tubular microcavities that are preserved in marine and sub-

glacial environments (e.g. Thorseth et al., 1991; Furneset al., 1996; Fisk et al., 1998; Cockell et al. 2009). The

TMCs produced are typically a few micrometerss across, up

to tens to hundreds of micrometers long, and may be annu-

lated, branched, or spiral shaped (e.g. McLoughlin et al.,2009). Several lines of evidence have come together over the

last c. 15 years to support the biogenicity of these microtex-

tures, and are extensively reviewed elsewhere (e.g. Furnes

et al., 2008; Staudigel et al., 2008). In brief, these include:morphological characteristics, sequencing and staining of

associated microbial DNA, elemental mapping, C-isotope

patterns, and comparison with well-documented microbor-

ings in carbonate and silicate substrates (e.g. Torsvik et al.,1998; Banerjee & Muehlenbachs, 2003; Fisk et al., 2003;Walton & Schiffman, 2003;). The morphological and textural

characteristics of TMCs in volcanic glass are summarized in

column 1 of Table 1; further information on their distribu-tion in the modern seafloor, and putative metabolisms is avail-

able in Staudigel et al. (2008), and references therein. The

geological record of glass bioalteration extends beyond the

in situ oceanic crust to include well-preserved Precambrian

rocks, given early mineralization of the TMCs, together with

little or no deformation, and greenschist facies metamor-

phism or below (e.g. Furnes et al., 2004, 2007). Thus, TMCs

have provided a new tool for investigating the ancient, deepbiosphere, and we aim here to elucidate these discussions by

clarifying the mechanisms of biological microtunneling.

A brief introduction to AITs

Ambient inclusion trails were first identified in cherts, and are

filamentous structures a few micrometers wide, up to tens to

hundreds of micrometers long, polygonal in cross-section,

with longitudinal striae, and sometimes with terminal inclu-

sions (Tyler & Barghoorn, 1963). These structures are

hypothesized to form by locally elevated fluid pressures that

propel a mineral grain through a relatively ‘soft’ substrate,

leaving a TMC in its wake (Tyler & Barghoorn, 1963; Knoll& Barghoorn, 1974). This crystal migration is believed to

create a microtunnel through a poorly understood combina-

tion of: mechanical abrasion, pressure solution, and chemical

dissolution. A comprehensive recent review of AITs can be

found in Wacey et al. (2008a). AITs are most frequently

reported from cherts (e.g. Tyler & Barghoorn, 1963) and

phosphorites (e.g. Zhang, 1984), with one example known

from phosphatic fish scales (e.g. Wacey et al., 2008a). Theymay remain hollow, or be secondarily infilled by minerals

including quartz, carbonate, or, less commonly, chlorite in

their appendages. Owing to their similarity in size, AITs have

previously been confused for biogenic microtextures includ-

ing microborings (e.g. Conway Morris & Bengston, 1994;

re-interpreted as AITs by Xiao & Knoll, 1999) and filamen-

tous microfossils (e.g. Awramik et al., 1983; re-interpreted as

AITs by Buick, 1990). Several authors have previously com-mented upon the resemblance of AITs to bioalteration

textures in glass but refuted the feasibility of this process in

volcanic systems (e.g. Banerjee et al., 2006; Furnes et al.,2007; Staudigel et al., 2008; Walton, 2008). Below we will

develop and expand on these discussions. The only report to

date of AIT-like structures in a volcanic sequence is from a

quartz–chalcedony-filled vug in late Archean basaltic lavas

(Lepot et al., 2009). These structures occur in radial arrays,have concentration of carbon at their tips, and have constant

diameters. The authors hypothesize that these TMCs formed

by chemical dissolution of quartz or silica gel in the vug by

migrating carbonaceous particles. Below we review the viabil-

ity of this and other possible mechanisms for AIT formation in

volcanic glass, and provide diagnostic morphological and

textural criteria for identifying this type of TMC. These are

summarized in column 2 of Table 1.

THE GEOLOGIC–HYDRAULIC CONTEXT FORTMC FORMATION

The geologic history of the host material offers key constraints

for the origin of TMCs. For example, volcanic glass is the

quench product of a silicate magma that formed at tempera-tures that preclude a biological precursor for TMCs. There-

fore, all cavities in volcanic glass must represent either abiotic

cavities formed during quenching, or cavities that have been

subsequently excavated. TMCs in sediments and mineral-

filled vugs and veins, on the other hand, are created by the

246 N. MCLOUGHLIN et al .

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dissolution of minerals precipitated from relatively low-temperature fluids. Insights to the geological history of such

TMC-bearing lithologies can be gained, for example, by fluid

inclusion analysis (e.g. Ivarsson et al., 2009), Mg ⁄Ca ratios ofcarbonates (e.g. Paul et al., 2006) or radiocarbon dating of

soils (e.g. Smits et al., 2004).

The hydrological regime of TMC formation is also impor-tant because it provides the context for dissolution processes.

Specifically, fluid flow determines the ability of a system to

transport chemical components that are required for tunnel-

ing, such as, dissolution agents, or nutrients in biotic systems

(see fig. 14 in Staudigel et al., 2008). Fluid flow also controls

Table 1 A summary of themorphological and petrographic characteristics of TMCs showing hypothesizedmechanism of formation

Euendolithic

microborings

Ambient

inclusion trails

Dissolution of

Heterogeneities

Cryptoendolithic

filaments

Chasmoendolithic

filaments

Timing of

Formation

Pre-metamorphic,

dissolution of glass

Diagenetic to syn-

metamorphic, form by

migration in a solid

substrate

Diagenetic Pre-metamorphic growth

into a fluid-filled cavity

Pre-metamorphic growth

into a fluid-filled vein or

fracture

Morphology Tubes with circular

cross-section and variable

diameters.Maybe

curvilinear, branched,

annulated, helical, or show

terminal swellings. Also

granular clusters

Tubes with polygonal

cross-section and constant

diameters. Show

longitudinal straie, are

curvilinear, may be

branched, helical, and

some retain terminal

crystals

Show irregular

diameters,

may intersect

Filaments with circular

cross-section, of a non-

uniform diameter are

curvilinear, or branched,

andmay show swellings

along their lengths

Filaments with circular

cross-section, of non-

uniform diameter, may be

curvilinear, branched, show

internal septae or terminal

swellings

Infilling

mineralogy

Clays, iron-oxyhydroxide,

titanite with or without

traces of carbon

Silica, carbonate, sulfide,

chlorite, garnet, sometimes

traces of carbon

Hollow, fluid-filled, or

secondary minerals

Clays with or without

organics

Iron-oxides or clays

Host matrix Volcanic glass ormeta-

volcanic glass assemblage

of: chlorite, quartz,

epidote, and ⁄ or carbonate

Chert, i.e. microcrystalline

silica, or phosphorite, fish

scales, or chlorite

Sedimentary quartz

grains or

crystalline

silicate minerals

Carbonate-filled vesicles Carbonate or zeolite-filled

veins

Distribution Grow into glass are

rooted at external

surfaces such as

pillow rims or the

surface of volcanic

glass fragment

Radial ‘starburst’ propagate

outwards, concentrated in

the vicinity of organic

material and ⁄ or pyritegrains. Or isolated

examples

Show no preference for

external

surfaces andmay

intersect

one another

Grow into cavities Grow into fluid-filled vein

or fracture, nucleate on

walls

Mechanical

Abrasion

No Very unlikely No No No

Dissolution Biologically mediated Chemical and ⁄ or pressuredissolution

Chemical and ⁄ orpressure solution

Possibility of secondary

chemical dissolution

Possibility of secondary

chemical dissolution

The columns compare endolithic microborings (column 1) in volcanic glass; with ambient inclusion trails (AITs) (column 2) in sedimentary rocks and authigenic

minerals; with the dissolution of pre-existing heterogeneities (column 3); with the remains of cryptoendolithic filaments (column 4) and chasmoendolithic filaments

(column 5). The distinguishing criteria are gathered from a review of the literature, and diagnostic criteria are given in italics. In the sketches: fresh glass (brown);

meta-volcanic glass (green); carbonate minerals or quartz (blue); microtubular cavities (black). Scale: vesicle approximately 500 lm across.

Mechanisms of microtunneling in rock substrates 247

! 2010 Blackwell Publishing Ltd

the removal of dissolution products helping to minimizere-precipitation that would inhibit tunnel growth (Staudigel

et al., 2008). In other words, fluid flow acts to maintain the

chemical conditions in the tunnel microenvironment. Well-

preserved exposures of seafloor volcanics clearly show that

tunneling in glass proceeds from external surfaces into the

glass, i.e. toward pillow cores. During subseafloor alteration

and metamorphism, the growth of authigenic minerals in void

spaces within the pillow lava pile reduces the connectivitybetween TMCs and the fracture network (e.g. Walton &

Schiffman, 2003). Low-permeability alteration products are

not a barrier to microbial tunneling. But when fluid circula-

tion ceases, the window for tunnel formation ends, because

the microbial activity is sustained by fluid circulation. This

contrasts with the hydrological context of AITs that are

reported from systems with low fluid–rock ratios and with

little or no exposure to fluid circulation. AITs are often foundisolated from external surfaces, sealed within clasts, or massive

microcrystalline layers (e.g. Fig. 1). Thus, the hydraulic

setting for AIT formation is that of burial diagenesis. In con-

trast, TMCs in volcanic glass are formed in open systems that

are more comparable with that found, for example, in soils

where fungal microtunneling is observed (e.g. Jongmanns

et al., 1997).

MECHANISMS OF TMC FORMATION

Here, we describe the end-member mechanisms that can

form microtunnels before comparing these in subsequent

sections with our two classes of TMCs. The shapes and

dimensions of TMCs set important boundary conditions for

the processes that create them, as summarized in row 2 of

A

C D

E F

B

Fig. 1 Transmitted light images of examples of tubular microcavities (TMCs). (A) A fracture in the lower part of the image with a dense zone of simple TMCs termed

Tubulohyalichnus simplus propagating into the fresh glass. (B) Diagonal fracture with granular alteration on both sides and the TMC Tubulohyalichnus annularis

(enlarged on right), note the unevenly spaced annulations along its length and the presence of small phenocrysts in the host glass. (C) A formerly fluid-filled vesicle sur-

rounded by radiating examples of Tubulohyalichnus simplus, and a fracture (lower left) with granular alteration. (D) Concentric cooling fractures around an olivine

phenocryst in volcanic glass that have been exploited by Tubulohyalichnus simplus (arrowed). (E) An ambient inclusion trail in a chert matrix containing a terminal

iron-oxide crystal (arrowed). (F) Automontaged image of a helical ambient inclusion trail with a terminal jarosite crystal (circled). Sample (A) and (C) are from hole

418A on the Bermuda Rise; (B) CY-1-30 from the Akaki River section of the Troodos ophiolite, Cyprus see McLoughlin et al. (2009); (D) from hole 396B in the

Mid-Atlantic; (E) and (F) from the Strelley Pool Sandstone ofWestern Australia seeWacey et al. (2008a).

248 N. MCLOUGHLIN et al .

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Table 1. First, the consistent shape of a TMC requires

guidance for excavation. This may come from pre-existing

heterogeneities in the substrate, or be controlled by bio-

logical activity. Without such guidance and localization, a

tubular cavity is unlikely because dissolution will be limitedby the build-up of solutes in the fluid. A tunnel, if formed

at all, is more likely the site of precipitation, unless there

is a transport mechanism for removing the products of

dissolution. In the absence of a guidance or localization

mechanism, dissolution produces planar interfaces between

the fresh glass and alteration products, rounding-off corners

and protrusions, see for example, fig. 13 in Furnes et al.(2007).

Chemical and pressure dissolution

The chemical dissolution of rock substrates is strongly tem-

perature and pH dependent. At low temperatures, volcanic

glass is altered to palagonite, a poorly crystalline assemblage of

clays and iron-oxyhydroxides (Stroncik & Schmincke, 2001).A change in pH from neutral to a pH of 10 can increase the

dissolution rate of glass by an order of magnitude (Casey &

Bunker, 1990). Under alkaline conditions, experiments have

shown incongruent dissolution of the glass to produce a

leached layer (Berger et al., 1987), whereas acidic experi-

mental conditions appear to result in congruent dissolution

(Crovisier et al., 1987). Organic material within rocks may

provide the dissolution agent necessary to form tunnels. Tosubstantiate this hypothesis, traces of organic material should

be sought in close association with the TMCs. Alternatively,

the dissolution agent could be sourced from external fluids

that infiltrated the rock, and if so, the TMCs should be con-

centrated on the outer surfaces of grains, and along fractures.

In addition, however, this requires a means for localizing the

dissolution so as to produce tunnels, and a pre-existing weak-

ness in the rock may provide this (see below). Tunneling mayalso result from corrosive substances produced in situ by

living organisms. For example, fungi exude organic com-

pounds like oxalic acid at the tip of their hyphae, and in

sub-recent, or exceptionally well-preserved material, these

may be found (Gadd, 1999).

Pressure solution may act together with, or independently

of, chemical dissolution, and results from locally elevated

lithostatic stresses produced by compaction or deformation.For instance, in clastic rocks pressure solution occurs at grain

contacts during compaction; in carbonate rocks along stylo-

lites; and sometimes, in sheet silicates at the tips of fungal

hyphae (e.g. Bonneville et al., 2009). Finally, both chemical

and pressure dissolution require a mechanism of inhibiting

immediate re-precipitation, and for transporting the dissolved

components away from the locus of solution at the head of

the tunnel. This requirement has previously been highlightedas especially important in carbonate minerals that are bored by

microbial phototrophs, given that the expected consequence

of oxygenic photosynthesis is carbonate precipitation, rather

than dissolution (Garcia-Pichel, 2006).

Mechanical abrasion

Mechanical abrasion of a rock requires a hardness contrast

between a millstone and the host matrix. For example,

potholes form in river channels where pebbles driven by water

flow erode the underlying bedrock. This process is accelerated

by rotation of the millstone producing a smooth rounded

cavity, or by a hammering action like that employed by

pneumatic drills, involving repetitive changes in the fluidpressure. Mechanical abrasion is not an exclusively abiotic

phenomenon, as some organisms like mollusks have evolved

radula or teeth that can mechanically erode the substrate (e.g.

Bromley, 2004).

The role of material heterogeneities as precursors for TMCformation

Linear, planar, or three-dimensional features in rocks may

provide pathways for enhanced dissolution as reviewed in

column 3 of Table 1. In feldspar crystals, for example, disso-

lution along perthitic exsolution lamellae creates planar

solution cavities (e.g. fig. 2b in Lee et al., 2007). In quartz

grains, dissolution along crystal dislocations produces trian-gular and arcuate etch pits in laboratory and natural environ-

ments (e.g. Brantley et al., 1986). Here, we describe several

different types of heterogeneities that may be found in glass,

how these are formed, and what this predicts about the shape

and distribution of TMCs produced by their subsequent

dissolution.

First, tunnels can be formed by the exploitation of fluid

inclusion trails in single crystals or sedimentary grains, andthese need not be rooted at external surfaces. Tunnels might

also conceivably occur along gas vesicles in extrusive rocks,

and this type of tunnel would be concentrated in areas of high

vesicularity. Such TMCs are also likely to be associated with

strings or clouds of fluid inclusions that are yet to be exploited

by the tunneling. Their diameters would be variable as they

‘join up’ adjacent inclusions of different sizes, and perhaps

show narrow constrictions along their lengths but lack regularannulations. The observation of endolithic cyanobacteria

within aqueous inclusion trails in sedimentary quartz grains

supports the feasibility of this process (e.g. Parnell et al.,2005). Note, however, that because glass is a non-crystalline

quench product of a silicate melt, it does not trap fluid inclu-

sions.

Second, tunnels might also form along radiation damage

trails, and these would occur with random orientations in thevicinity of a source of radioactive particles, and may intersect

one another like fission track trails in, for example, apatite

(fig. 2 in Gallagher et al., 1998). Such tunnels would not

show any directionality or preference for external surfaces, and

Mechanisms of microtunneling in rock substrates 249

! 2010 Blackwell Publishing Ltd

should occur throughout a pillow, unless fluids circulating

between the pillows or authigenic minerals in interpillow cavi-

ties were the source of the radioactivity. It is also conceivable

that fluids infiltrating pillow rims may enhance such radiation

damage trails by chemical dissolution. But still, the diagnosticfeature of such tunnels is their probability of intersecting and

this contrast with biogenic TMCs.

Last, TMCs can be secondarily produced by the dissolution

of mineralized or encrusted filaments that could be either

biological or abiological in origin, leaving a hollow (sub)-

cylindrical cavity. This type of TMC is, therefore, the mold

of a precipitated filament. Examples include: the dissolution of

sponge spicules in rock substrates; the decay or dissolution ofcryptoendolithic filaments encrusted by cavity filling cements;

and collapse or dissolution of the tubular often twisted sheaths

produced by Fe-oxidizing bacteria such asGallionella.

MATCHING TMC FORMATION MECHANISMSTO THE TEXTURAL EVIDENCE

Textural evidence can provide powerful constraints upon

TMC formation mechanisms. In particular, their distribution

with respect to external surfaces, fluid pathways, and one

another can be used to assess their timing of formation, and

potential excavation mechanisms. When considering the

early Earth, the assessment of the antiquity of the TMCs is

especially important, to ensure that recent, crypto- and

chasmo-endolithic traces are not confused for ancientstructures (e.g. Westall & Folk, 2003).

TMCs in volcanic glass

Microtubular and granular cavities found in volcanic glass

propagate inwards from the rims of pillows, from fractures,

from vesicles, and from the outer surfaces of glass fragments.In recent volcanic glass, microtubular cavities are hollow, or

partially filled with phyllosilicates as well as Fe-oxyhydroxides

(e.g. Benzerara et al., 2007), and in metamorphosed

sequences by microcrystalline titanite (see review in Furnes

et al., 2008). The tunnels are subcircular in cross-section, and

are curvilinear, spiral shaped (e.g. fig. 5 in McLoughlin et al.,2009), or branched (e.g. Box 2 in Cockell & Herrera, 2008).

They can show annulations and terminal swellings (e.g. fig. 1fin Walton, 2008). We will focus now on these morphological

characteristics, and especially their petrographic context, to

test the various mechanisms of TMC formation in volcanic

glass outlined above. These arguments are also summarized in

column 1 of Table 1.

First, there are several observations that lead us to reject

mechanical abrasion by a millstone as the origin for TMCs in

volcanic glass. This is because a single impacting grain wouldbe expected to produce a tube of uniform diameter, or

perhaps a tapering tube, if the millstone was abraded during

tunneling. This is not observed. Rather tubes in volcanic glass

show variable diameters, only sometimes taper, and can exhi-

bit complex ornamentation that exclude a migrating crystal as

the source. In addition, remnants of terminal crystals are not

observed in volcanic TMCs, and neither is there a source of

crystalline inclusions that could be employed as millstones,nor a means of generating and sustaining unidirectional fluid

pressure to drive such crystals, if present. Sometimes, igneous

phenocrysts are found in pillow rims, although most exam-

ined glasses containing bioalteration textures are aphyric. We

have not found examples of phenocrysts that have migrated in

volcanic glass. In summary, volcanic glasses lack both suitable

millstones and a propulsion agent necessary for creating

TMCs bymechanical abrasion.The abiotic dissolution of pre-existing structural or compo-

sitional heterogeneities found in volcanic glass can also be dis-

counted as the source of TMCs, because their distributions

and geometries are markedly different. Specifically, an origin

as some type of pipe-vesicle-related phenomenon can easily be

discounted because these are restricted to the base of lava

flows, and grow inwards of the glassy rims of pillows (cf. Phil-

potts & Lewis, 1987). Thus, a pipe-vesicle process cannotaccount for TMCs that are restricted to glassy pillow rims and

interpillow hyaloclastites. In addition, deep-water pillow mar-

gins that lack vesicles (Jones, 1969) can contain TMCs. In

other words, the distribution of TMCs in volcanic glass is

independent of gas-escape structures, with one exception. It

has been observed that in some vesicular pillow lavas, TMCs

can be found radiating from vesicles (Fig. 1E), and these are

interpreted to be sites where fluids in the vesicles introducedmicrobes into the glass. This type of relationship show some

resemblance to mechanically produced microchannels

observed around collapsed bubbles in tektites (fig. 1 in Barnes

& Russell, 1966). These structures, however, were formed by

heating of the glass to 825"C in the lab, causing fluid to escape

through the heat-softened glass. This re-heating process can

be discounted as unviable in seafloor pillow lavas sequences

however, and moreover, cannot explain bands of TMCs prop-agating from fractures into the glass. Some early reports of

biogenic grooving on volcanic glass shards were contrasted to

U-shaped grooves observed on natural microtektites, these

are argued to form by chemical solution of thermal contrac-

tion cracks, but intersect and can therefore be distinguished

from bioalteration textures (Glass, 1987; Ross & Fisher,

1987; and references therein).

Another type of pre-existing weakness to be consideredare radiation damage trails. This can be rejected in volcanic

glass however, because TMCs do not intersect one another

as is commonly observed in radiation trails, and such features

would be expected throughout the pillow. Specifically, alpha

recoil trails have been proposed as a source of tunnels in

volcanic glass (K. Muehlenbachs, pers. commun.) but inves-

tigation of mineral substrates have shown these to be orders

of magnitude smaller than biotic tunnels in volcanic glass(cf. Jonckheere & Gogen, 2001), and would need to be

250 N. MCLOUGHLIN et al .

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enlarged by dissolution processes to become micrometer

scale features. Moreover, if radioactive decay was the deter-

mining factor in creating TMCs in volcanic glass, then there

should be a logarithmic relationship between the age of the

pillow lava and the density of TMCs matching the half-life ofthe parent nucleus; yet no such relationship has been

reported.

A special case worthy of further discussion here is the

secondary dissolution of biogenic filaments in volcanic rocks

as summarized in columns 4 and 5 of Table 1. Crypto- and

chasmo-endolithic micro-organisms nucleate on the margins

of fractures and pore-spaces where they grow in fluid-filled

cavities, and then become preserved in the rock record by crys-tallization of authigenic minerals, especially carbonates, clays,

and zeolites (e.g. Ivarsson et al., 2008a,b; Peckmann et al.,2008; Eickmann et al., 2009). These traces can be distin-

guished from AITs because their filamentous sheaths are sub-

circular in cross-section, and may become encrusted in iron-

oxides, clays, carbonate, or zeolites. If the filamentous micro-

organisms are not dissolved, they can contain traces of carbon

and ToF-SIMS analyses have detected the presence of C2H4,PO3, and lipid-like molecules (e.g. Ivarsson et al., 2008b).Such crypto- and chasmo-endolithic traces can also branch,

show annulations, or swellings along their length that reflect

cellular structures (e.g. Ivarsson et al., 2008a,b), or internalseptae (e.g. Schumann et al., 2004; Reitner et al., 2006).

They do not exhibit longitudinal striae, or polygonal cross-

sections like AITs. Subsequent decay or secondary dissolution

of these organic filaments can conceivably produce TMCswithin the vein and fracture-filling phases (see columns 4 and

5 of Table 1).

Chemical dissolution, thus, remains the only plausible

mechanism for TMC formation in volcanic glass. Dissolution

experiments in the laboratory have produced microcracks at

the glass-hydration interface (e.g. fig. 5 in Crovisier et al.,1987). These structures were not illustrated in detail, but

would be expected to be curviplanar features that can intersectin contrast to biotic TMCs that avoid intersecting. In the

open-system found in pillow lavas with high volumes of fluid

circulation, dissolution to form tunnels requires a mechanism

of localizing the chemical solution agent. The only known

mechanism is in the vicinity of micro-organisms. Early work

by Thorseth et al. (1991) suggested individual cells as the

locus for dissolution, and later work has hypothesized chains

of cells tunneling into the glass (e.g. fig. 1 in Furnes et al.,2008) This cannot, however, fully explain the production of

extended TMCs, because the micro-organism would find it

difficult to access circulating fluids, and excrete waste products

through a tunnel with such a narrow diameter, and extended

length relative to their size. This would seem to require some

kind of biological pump, and thus cellular extensions similar

to fungal hyphae have been suggested by Staudigel et al.(2008) as a mechanism for localizing chemical dissolution in atunnel. The prevalence of fungi in marine environments is

only now becoming apparent, and includes species from

subseafloor volcanic environments that are functionally

capable of Fe(II) and Mn(II) oxidation (Connell et al.,2009). Occasionally, filamentous structures have been

observed in tubular bioalteration textures, for example, figs5–9 in Banerjee &Muehlenbachs (2003). Such cellular exten-

sions are not exclusive to eukaryotes. Prokaryotes, including

the actinomyces, which are abundant in deep-sea sediments,

can create hypha-like extensions. Bacterial nanowires might

also conceivably play a role but their study is at a relatively

early stage and their function is largely unstudied, except for

measurements of their conductivity (Gorby et al., 2006).

Cellular extensions have the advantage of being able to trans-port chemical agents through the tunnel, as well as being able

to communicate with other members of the microbial

community located in the fracture. These cellular extensions

can also later be withdrawn, leaving a hollow cavity, when

tunneling is no longer advantageous.

Finally, we explore the suggestion that microbes may

dissolve pre-existing heterogeneities in the glass because these

are less-metabolically expensive to bioerode. For example, ithas been observed that TMCs exploit concentric stress

fractures around phenocrysts in volcanic glass (Fig. 1D).

Typically, TMCs develop at high angles to fractures in the

glass, and are not symmetric on either side of fractures. In

some atypical examples, however, the TMCs do ‘match up’

on either side of fractures, and occur with orientations that are

independent of the fracture geometry. This mismatch has

been argued to reflect the TMCs exploiting structural or com-positional anisotropy in the glass (e.g. fig. 4 in Furnes et al.2001), but this type of behavior seems to be the exception

rather than the rule. Perhaps a related observation is that of

spiral microtubes that ‘wrap on and off’ one another, tenta-

tively interpreted as evidence of tubes providing support to

one another (fig. 5 in McLoughlin et al., 2009). Lastly, it hasalso been reported that the higher the density of microtubes,

the lower the tortuosity of their paths, and this has been inter-preted to reflect the micro-organisms maximizing their

utilization of resources in the glass (Walton, 2008). Taken

together, these observations have been interpreted as evidence

of biological behavior recorded by TMCs exploiting structural

weaknesses and compositional heterogeneities in volcanic

glass.

Ambient inclusion trails

The hallmark features of AITs are a polygonal cross-section

with a constant width and cross-sectional shape, also longitu-

dinal striae that correspond to the corners of the terminal crys-

tal (Brasier et al., 2006; Wacey et al., 2008b; and references

therein). AITs may be hollow, or infilled with quartz, carbon-

ate, garnet, or chlorite (see column 2 of Table 1); their cross-sectional shape and longitudinal striae are best observed by

scanning electron microscopy, especially if the tubes are

Mechanisms of microtunneling in rock substrates 251

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hollow. The most diagnostic feature of AITs is the presence of

a terminal crystal, although this may not always be seen in a

thin section, if the tube migrates out of the plane of section.

AITs occur either as isolated examples or in ‘starburst’ arrays

of radially orientated filaments that propagate outwards fromlocalized clots of organic matter (e.g. Knoll & Barghoorn,

1974; Buick, 1990; Lepot et al., 2009). AITs may sometimes

spiral, especially toward their ends, and occasionally branch

(e.g. fig. 4 in Tyler & Barghoorn, 1963).

The concept of AITs rose to some prominence again after

the first reports of bioalteration textures in Archean metavol-

canic glass: Kerr (2004) quoting M.D. Brasier wrote ‘That

decomposition – possibly of organic matter – produced fluidsthat drove a mineral grain into the glass as chemical reactions

at the grain ate into the glass, sort of like a corrosive-tipped

pile driver’. We now use the textural and petrographic charac-

teristics of AITs, to asses this and other mechanisms that have

been proposed for AIT formation. In doing so, we highlight

the questions that remain concerning exactly how this

hypothesized phenomenon operates, and evaluate whether

this is a viable process in volcanic glasses.The early reports of AITs proposed that pressure solution

on the leading surface of the terminal sulfide grains enabled

crystal migration and the formation of microtunnels (Tyler &

Barghoorn, 1963). This requires the presence of a thin film of

solvent at the sulfide–chert interface and the driving force for

this dissolution creep process is a stress-induced gradient in

the chemical potential of the solid at the chert–fluid boundary

(e.g. Lehner, 1990). In response to an applied stress, the silicagrains become preferentially dissolved along grain boundaries

oriented at high angles to the stress. This results in a chemical

concentration gradient in the fluid, which control the diffu-

sion of the dissolved silica away from sites of dissolution to

those of lower stresses. In addition, it has been suggested that

this process may be aided by a ‘push’ produced by ‘the force

of crystallization of either quartz or carbonate in the append-

age’ (e.g. fig. 2 from Tyler & Barghoorn, 1963). Many AITshave hollow appendages, however, suggesting that propulsion

bymineral growth is not the major driving force.

Growth by pressure solution requires locally elevated fluid

pressures, yet it is unclear how to generate and, moreover,

sustain this. Specifically, to generate a tunnel-shaped cavity

requires sustained fluid pressure at the head of the tunnel,

concomitant with removal of the solution products to stop

these from precipitating and sealing the cavity. However,these processes act in opposite directions, and to overcome

this would require compartmentalization of the cavity. One

proposed source of elevated fluid pressures is the thermal

degradation of organic matter trapped within impermeable

substrate (Knoll & Barghoorn, 1974; Wacey et al., 2008a,b).This suggestion is consistent with the radial arrays of AITs

sometimes seen around clots of organic matter (e.g. fig. 2 in

Knoll & Barghoorn, 1974) that have been suggested to formby an ‘explosive’ maturation process. Although, we note that

the kinetics of kerogen decomposition under burial diagenesis

means that this is a long-timescale process. These elevated

fluid pressures acts radially outwards from the organic source

and are focused, and directed, by the crystal inclusions. Given

the long timescales required for kerogen maturation duringburial diagenesis, it is essential that the enclosing substrate is

impermeable so the fluid pressure is not dissipated.

The formation of AITs by a millstone that creates a micro-

tunnel by mechanical abrasion can be readily discounted in

cherts. This is because crystalline silica has a hardness of 7 on

theMohs scale, which exceeds the hardness of terminal sulfide

or iron-oxide grains that have a hardness of 6 or less. In addi-

tion, the terminal crystals seen in AITs are euhedral and showno signs or abrasion or rounding of their corners, as would be

expected, if they were grinding away like pebbles forming pot-

holes. Furthermore, the effectiveness of mechanical abrasion

can be increased by rotation of the impacting grain but this is

excluded by the observation of linear striae along the walls of

AITs. Neither is a mechanism by hammering of the millstone

forthcoming, as repeated fluctuations in fluid pressure cannot

easily be envisaged. Mechanical abrasion can, therefore, bediscounted as a realistic mechanism for AIT formation in lithi-

fied substrates.

Purely chemical dissolution should also be explored as an

origin for AITs, and this has most recently been favored by

Lepot et al. (2009), who hypothesize that low-grade meta-

morphism of carbonaceous matter can generate organic com-

pounds capable of dissolving a crystalline quartz–chalcedony

or silica gel matrix thereby producing TMCs. These authorscould not identify terminal millstones in their structures, and

rather argued that these formed by the migration of corrosive

organic droplets through a gel or crystalline silica matrix.

However, it is unclear what prevents these tunnels closing up

behind the migrating carbonaceous inclusions? Sometimes,

necking behind the terminal particle in AITs is observed

(N. McLoughlin, unpubl. data), supporting the suggestion

that this process occurred in the gel phase, with early mineralgrowth or sustained fluid pressure stopping the tube from

collapsing. But it is unknown if a silica gel can survive under

burial diagenesis and the low-grade metamorphic conditions

necessary to generate the organic compounds required by this

hypothesized mechanism. We also highlight that these struc-

tures were found in a silica-filled vug within a lava flow, and

not within volcanic glass, thus they are not directly compara-

ble with argued bioalteration textures found in glassy pillowlava rims.

After reviewing the various hypothesized mechanisms of

AIT formation above, we conclude this section by asking: is

this a viable process in volcanic glasses? In short, there are

several chemical and rheological requirements for the

hypothesized mechanisms of AIT formation that we are

not met in volcanic systems, notwithstanding the uncer-

tainties that surround these hypothesized mechanisms. First, asource of crystalline inclusions is absent. Second, generating

252 N. MCLOUGHLIN et al .

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unidirectional and sustained fluid pressure is not possible inthe open-fluid system found in heavily fractured pillow lava

piles. Third, localized sources of chemical dissolution agents

are absent, especially given the low organic carbon content of

pillow lava sequences. Lastly, we agree with previous authors

who have highlighted the need for experimental work to

investigate the hypothesized mechanisms of AIT formation

summarized in Fig. 2, in particular whether the pressure, tem-

perature time widow for generating sustained fluid pressureby kerogen maturation overlaps with the stability field of silica

gel (cf. Wacey et al., 2008a; Lepot et al., 2009).

CONCLUDING REMARKS

Microbes have evolved the capacity for targeted dissolution of

rock, mineral, and biological substrates as driven by the need

to gain metabolites, to shelter from harmful radiation, and toescape mineral encrustation and predation (e.g. Cockell &

Herrera, 2008). The challenge when seeking evidence of

microbial bioerosion in the rock record is to distinguish

biologically mediated rock tunneling, from purely abiotic

physical, chemical, or physio-chemical mechanisms. Investiga-

tions of the mechanisms of microboring have to some

degree been hampered by the difficulties of culturing micro-

organisms that can create extended tunnels in laboratoryexperiments (Einen et al., 2006 and Garcia-Pichel, 2006);

nonetheless, there are abundant textural and petrographic

features that can be used to confidently identify microbial

microborings in natural systems. In Table 1, we present

criteria for recognizing TMCs formed by endolithic micro-

organisms in volcanic glass, and to distinguish these from

abiotic microcavities, especially structures known as AITs.

A

B

C

Fig. 2 Thin-section images of ambient inclusion trails (left-hand side) and possible mechanistic interpretations (right-hand side). (A) Euhedral pyrite crystal with a

quartz-filled appendage from a chert of the Gunflint Iron Fm, Ontario. (B) Cluster of pyrite and organic carbon (inset) with radiating tubular structures that show con-

stant diameters, and terminal pyrite grains, from the North Pole Chert of Western Australia. (C) Pyrite crystal with a hollow trail of constant cross-sectional geometry,

from the Strelley Pool Chert, ofWestern Australia. (A) From plate 2 of Tyler & Barghoorn (1963), (B) from fig. 6 of Buick (1990), and (C) fromWacey et al. (2008a).

Mechanisms of microtunneling in rock substrates 253

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Moreover, we provide a critique of the hypothesized

mechanisms of AIT formation, and explain why this migration

trail-type process is unlikely in pillow lava systems because of

the absence of several key prerequisites. In conclusion, this

study reminds us to be on-the-look-out for abiotic micro-textures when investigating tunneling in the rock record. We

argue, however, that migration trail-type textures are not

easily mistaken for endolithic biosignatures in ancient pillow

lavas, and furthermore, we elucidate criteria that can be used

to distinguish these two classes of tubular microtextures in the

rock record.

ACKNOWLEDGMENTS

NM and HF were supported by funding from the Norwegian

Research Council. We thank I. Thorseth and E. Grosch for

very constructive discussions. B.E. acknowledges funding

from the European Science Foundation.

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