Elsevier Encyclopedia of Geology

Подождите немного. Документ загружается.

microbial activity during formation of the rock; and

(3) unique sedimentary structures, interpreted as

stromatolites (laminated sedimentary structures

formed by the activities of microbes) (Figure 3).

Arguably, these are the oldest traces of life on

Earth. However, owing to the significance of deter-

mining when, where, and how life on our planet

arose, a great deal of controversy surrounds the inter-

pretation of the claimed fossils. As Carl Sagan once

said, ‘‘extraordinary claims require extraordinary

proof’’. Consequently, rigorous criteria have been de-

veloped for authenticating Early Archaean fossils.

Moreover, early life on Earth is now recognized as

an analogue for ancient microbial life on Mars, stimu-

lating considerable research into the recognition of

ancient microbial life using techniques that can be

carried out by a rover or, eventually, a geologist on

the surface of Mars.

Life is thought to have arisen on Earth sometime

early during the 1.3 billion year period from 3.8 to

2.5 Ga, although the exact timing is still debated (see

Origin of Life). In order to understand how we might

interpret Archaean fossils, we will examine the way in

which microbes interact with sediments in modern

settings. Not only is this critical for interpreting the

very oldest traces of life, but it is also important to

hone our techniques in the familiar geological envir-

onments of Earth to aid us in the search for signs of

life elsewhere.

Nature of Modern and Ancient

Biosedimentary Systems

Microbial Sediments: Significance and Distribution

Today, almost no sedimentary environments are

devoid of life. Through sheer numbers, microbes

have played a fundamental role in shaping surface

environments through time by their metabolic contri-

butions to the global cycling of important elements,

such as carbon, oxygen, sulphur, nitrogen, phos-

phorus, and iron. Microbes have also shaped the

geological record of sedimentary environments,

particularly through the activities of microbial

communities living at the sediment–water interface.

Although microbes live both as free-floating or swim-

ming individuals (plankton), as well as structured,

surface-adhered communities (biofilms), here we are

particularly interested in the latter. This is because

some of the activities undertaken by these film or

mat-like communities can influence the way in

which sediment is deposited and impart unique phys-

ical and chemical signatures that may be preserved in

the geological record.

p0030Although microbes are ubiquitous in all watery

surface environments today, formation of microbial

sedimentary structures, such as stromatolites, that

could be recognized in the geological record is less

common. Today, communities that form such struc-

tures occur mostly in environments that are inhospit-

able to higher, grazing organisms that feed upon the

microbes. Such environments include those with ex-

tremes of temperature, pH, oxygen depletion (eg., hot

springs), salinity (stranded lakes) or where the organ-

isms are repeatedly exposed to drying conditions

(intertidal zones). Sediment influx must also be min-

imal to prevent rapid burial of the mats. A famous

modern example occurs in a hypersaline restricted

coastal embayment at Hamelin Pool, Shark Bay,

Western Australia (Figure 4). Salinity levels in Shark

Bay are up to twice those of normal seawater, creating

an inhospitable environment for most marine animals

but hospitable for microorganisms associated with

the stromatolites.

Although Shark Bay may not provide a perfect ana-

logue for the first ecosystems, its lack of predators and

Figure 3 (A) Filamentous microfossils from the 1.9 Ga Gunflint Formation, Ontario, Canada, arguably the oldest widely accepted

microfossils, although older ones have been reported. Photographs supplied by J. W. Schopf. (B) Modern filamentous microbes. Part A

and B based on data from: Hoffman H (2000) Archean Stromatolites as Microbial Archives. In: Riding R and Awramik S (eds.)

Microbial

Sediments

, pp. 315–327. Berlin: Springer-Verlag.

282 BIOSEDIMENTS AND BIOFILMS

consequent free reign of microbial activity certainly

mimic one important aspect of the early biosphere:

the dominance of micro-organisms. Microbes were

the only form of life on Earth during the Archaean

(3.8–2.5 Ga) and much of the Proterozoic (2.5–

0.5 Ga) aeons (Figure 2C). Indeed, it was the rise of

the first grazing organisms at the end of the Protero-

zoic (0.5 Ga) that is thought to have triggered the

near disappearance of stromatolite-forming microbial

activity from that point onwards.

Nature of Mats and Biofilms

Biofilms are structured communities of micro-organisms

adhering to a submersed surface. The species within a

biofilm have planktonic equivalents (individual cells

suspended in the water column), but only organisms

in a biofilm are structured in a way that can signifi-

cantly affect sedimentation. Several microbial species

may be present within a biofilm, arranged in tiny

mushroom- and pillar-type structures within an extra-

cellular matrix of organic material (Figure 5). The

Figure 4 (A) Modern stromatolites from the subtidal realm of Hamelin Pool, Shark Bay, Western Australia. A range of stromatolitic

microbial communities also occur in the intertidal zone, forming different structures, including tabular stromatolites and microbially

bound ripples. (B) Shark Bay stromatolite in cross-section, showing a clotted, slightly laminated texture.

Figure 5 Schematic cross-section of a biofilm. Cells are grouped together in mushroom- and pillar-type structures surrounded by a

matrix of extracellular material. The intervening matrix provides a pathway for the diffusion of nutrients and wastes. Adapted from an

illustration by Dirckx P (1999) Not Just Slime. In: Ben Ari E (ed.)

Bioscience V. 49, no. 9. p. 691.

BIOSEDIMENTS AND BIOFILMS 283

matrix provides a stable micro-environment that

offers several advantages for cells, including:

1. Protection from harmful environmental fluxes

and agents (e.g., temperature change, radiation

damage, toxic materials).

2. Adhesive properties to anchor the community to

the surface.

3. Efficient nutrient uptake from the surrounding

environment.

4. Pretreatment of assimilated molecules, such as the

breakdown of large organic molecules to smaller,

more manageable components, which are then

able to diffuse through the matrix and be taken

up by the cells.

5. Conduits for the diffusion of metabolic by-products

away from the cells.

Thus, cells in the biofilm are able to efficiently

assimilate a range of organic and inorganic molecules

from the environment through the matrix for use in

reactions that provide energy and building blocks for

cells. Moreover, the by-products from one species

may provide the nutrients for another species within

the biofilm. These processes are a significant compon-

ent of the interaction between microbes and their

environment.

A ‘microbial mat’ is a thicker and more complex

type of biofilm, comprising many different species

divided into specific layers. Different layers are

favoured by different species because of their pre-

ference for certain light conditions, oxygen levels,

sulphide concentrations, and so forth (Figure 6).

Effects of Microbial Activity upon Sedimentation

Mineral precipitation Mineral precipitation is

commonly associated with microbial mat growth.

However, precipitation may or may not be actively

caused by the metabolic activities of the organisms in

the mat. The difference between active and passive

precipitation processes is described in Table 1.

Biotic (active) precipitation occurs when the by-

products of microbial reactions alter the chemical

balance within the micro-environment and cause the

precipitation of compounds such as carbonate, iron

oxide, iron sulphide, or silica. Microbial photosyn-

thesis, for example, is one of the most important

mechanisms for biological calcium carbonate (lime-

stone) precipitation on a global scale. In microbial

mats, this process involves the photosynthetic re-

moval of CO

from a bicarbonate-bearing solution,

which increases the carbonate (CO

) concentration

in the residual fluid. The carbonate may then bond

with a cation, such as calcium, and precipitate out

of the fluid as calcium carbonate. Photosynthesis is

not the only mechanism for CO

removal and carbon-

ate precipitation; the same can be achieved through

certain biological oxidation, reduction, or hydrolysis

reactions. These reactions could have provided a

mechanism for biological carbonate precipitation

prior to the rise of photosynthetic organisms. Mineral

Figure 6 A piece of modern microbial mat from the shallow

subaqueous sediments at Shark Bay (cross-sectional view). The

tufted, dark green layers at the top (photosynthetic cyanobac-

teria) are underlain by a light brown layer, a thin red–brown

layer, and then a grey–black layer. The different colours in each

layer reflect pigments, mineral matter, and organic material

associated with different microbial species.

Table 1 Comparison of non-biological and biological precipitation processes

Passive (non-biological) precipitation Active (biological) precipitation

Mechanism

Fluid saturation levels are sufficiently high such that mineral

precipitation would occur without biological influence

Fluid saturation levels are lower than those necessary for

spontaneous precipitation, yet precipitation continues due to

microbial mediation of chemical reactions in the environment,

i.e., microbes are responsible for maintaining

physicochemical parameters, such as minimum levels of

carbonate concentration (via photosynthesis) necessary for

precipitation

Microbes simply act as nucleation sites, much as a twig or

fallen piece of rock

Microbially influenced crystal fabrics may form and be

preserved within such deposits, but precipitation is not

caused by microbial activity

Example environment

Common in hot spring environments where supersaturated

fluids emanate from springs or vents

May occur in marine environments where minerals will not

spontaneously precipitate from seawater

284 BIOSEDIMENTS AND BIOFILMS

deposition can also occur when elements, such as

iron, become concentrated by complexing (bonding)

with microbially produced organic molecules.

Abiotic precipitation may occur in certain envir-

onments, such as hot springs, where fluids may be

sufficiently saturated to spontaneously precipitate

minerals even without microbial influence. These en-

vironments are commonly inhabited by microbes,

perhaps simply because they are the only organisms

that are able to survive under the prevailing condi-

tions. Where fluid saturation approaches a level high

enough for spontaneous precipitation, the microbes

may simply act as templates for precipitation, a con-

cept that is supported by the observation that fallen

branches, insects, and other detritus in the same

hot springs provide equally efficient substrates for

mineral precipitation.

Trapping and binding In addition to causing

the in situ precipitation of new minerals, thereby

forming new, localized sediment deposits, microbial

communities can also trap and bind detritus washed in

from elsewhere. As material is trapped, the microbial

community moves up through the sediment layer to

form a new organic layer at the surface. Upward move-

ment may be driven by, for example, a need to reach

sunlight. The rate of sediment influx must be such so

as to avoid either deeply burying the microbial mat

or starving the mat of substrate material for growth.

Ancient Microbial Sediments

Microbes of the Early Archaean left traces of their

passing in much the same way as microbes do today,

forming distinctive fabrics, structures, and chemical

signatures in the sediments that fell and precipitated

around them. There are four basic kinds of fossil

evidence, or biosignatures, that we can study in the

geological record to unravel the nature of the oldest

biosedimentary systems. These are listed in Table 2.

The interpretation of ancient biosedimentary

systems is aided by the study of younger analogues,

but this is limited by the fact that surface processes

and environments on the early Earth were very differ-

ent to modern systems. Thus, direct comparisons

cannot always be made between the Archaean Earth

and the Phanerozoic or modern Earth in order to

understand the types of processes that were respon-

sible for the formation of certain fossil-like structures.

Nonetheless, careful and selective study of modern

and younger analogues provides an extremely useful

starting point for the interpretation of ancient sys-

tems. A discussion of the formation, occurrence, and

interpretation of different microbial sedimentary

products and their fossils follows.

Stromatolites

Stromatolites constitute by far the most abundant and

important component of the early fossil record and

thus provide the main body of evidence regarding the

origins of life. The term ‘stromatolite’ has been widely

used to refer to a diverse range of organic and/or

laminated sedimentary structures. However, most re-

searchers have used the definition by Awramik and

Margulis, which defines a stromatolite as: ‘‘an orga-

nosedimentary structure produced by sediment trap-

ping, binding, and/or precipitation resulting from the

growth and metabolic activity of microorganisms’’.

Stromatolites accrete through the trapping and

binding of sediment and/or by mineral precipita-

tion that is influenced or caused by microbes at the

sediment–water interface. Growth occurs because,

as sediment accumulates on a stromatolite, either

by deposition or precipitation, the microbes must

move upwards to remain at the sediment–water inter-

face. Thus, in living stromatolites, the constructing

microbes generally live only in the top 1–10 mm of

the structure. The physical shape of stromatolites

may be columnar, domed, branching, tabular, or

Table 2 Summary of the main types of fossil evidence, or biosignatures, that may be sought in the geological record

Fossil type

Type of evidence

provided Description

Microfossils Direct,

morphological

Occurs where mineral precipitation has entombed and preserved actual microbes. Should be

made of biological organic matter, but fossil casts occur where organic matter has been

replaced by other minerals

Stromatolites Indirect,

morphological

Sedimentary structure formed by the combination of sedimentation and the activities of

microbes. Classed as trace fossils (similar to a dinosaur footprint where the remains of the

organism itself may be absent)

Chemical

fossils

Indirect,

chemical

Remnant environmental effects and by-products of biochemical reactions that take place during

microbial metabolism. Perhaps the subtlest of all fossils are the chemical fossils

Biomarkers Indirect,

biochemical

Remnant biological marker molecules that survive alteration processes that degrade

morphological fossils and turn the original organic material to kerogen

BIOSEDIMENTS AND BIOFILMS 285

mound-shaped (Figure 7). The combined attributes of

a stromatolite reflect the interaction of physical,

chemical, and biological processes (Figure 8) in the

environment. For example, currents may cause

elongation of the stromatolite along the current dir-

ection, a feature observed in modern stromatolites in

Shark Bay and in Archaean stromatolites of Western

Australia. When interpreting the morphology of stro-

matolites, it is difficult to determine what proportion

of the physical attributes of a structure can be attrib-

uted to environmental influences and how much to

purely biological factors.

The hard, laminated interior of the stromatolite re-

cords the depositional history of the stromatolite-

building process. We can look at the nature of the

laminations in order to understand the behaviour and

character of a stromatolite-building community.

Lamination within any sedimentary deposit reflects

some past cyclicity in the sedimentation processes. In

the case of stromatolites, lamination reflects cyclic

variations in non-biological sedimentation processes,

coupled with the variation in microbial activity.

Lamination may accrete rapidly or slowly, depending

on the causes of accretion and cyclicity. Table 3

describes four generalized mechanisms observed in

modern stromatolites.

Interpretation of stromatolite-like structures

Stromatolite-like structures can form by a variety of

abiological mechanisms. In order to verify a biologic

origin for purported stromatolites, all possible non-

biological origins ought to be considered, including:

1. Mechanical sedimentary processes such as those

caused by waves or currents.

2. Post-burial deformation of laminar deposits by

dewatering processes.

3. Tectonic deformation, i.e., folding.

4. Soft sediment deformation or slumping, i.e., cohe-

sive downhill movement of a portion of sediment-

ary layers.

5. Chemical precipitation, e.g., from saturated fluids,

such as those associated with hot spring environ-

ments.

Figure 7 Physical attributes of Archaean stromatolites. Different attributes can be combined; for example, one ‘community’ may

comprise spaced domes with higher order curvature and later deformation. These basic characteristics can be used to describe and

classify stromatolites observed in the field. Adapted from Hoffman HJ (2000) Archaean stromatolites as microbial archives. In: Riding

R and Awramik S (eds.)

Microbial Sediments, pp. 315–327. Berlin, Heidelberg: Springer-Verlag.

286 BIOSEDIMENTS AND BIOFILMS

Ideally, microfossils, chemical fossils, and microscale

fabrics should be used in the interpretation of stro-

matolite-like structures, as the macroscale physical

shapes created by biological and non-biological pro-

cesses may be the same for a given set of environmental

parameters. Unfortunately, microfossils and microfab-

rics are commonly destroyed by diagenesis (pressure-

and temperature-related alteration during burial) (see

Diagenesis, Overview). This is the case in all known

Early Archaean stromatolites.

Archaean stromatolites Except for the latest Ar-

chaean Transvaal (Africa), Carawine and Fortescue

(Australia) sequences, Archaean stromatolites occur

mainly in thin, discontinuous, localized carbonate

units formed at times of transient stability between

tectonism and volcanism. These stromatolitic de-

posits are now preserved in the volcano-sedimentary

sequences (greenstone belts) of Archaean cratons

(Figure 2). A summary of the attributes of Archaean

stromatolites is presented in Table 4 .

Figure 8 (A) Schematic representation of the processes affecting stromatolite formation. Although stromatolites form as a result of

interactions between microbes and sediments, the final characteristics of the stromatolite are also affected by environmental

parameters, such as waves and currents, sediment influx, water chemistry, temperature, and pH. All of these parameters can affect

the structure and development of the microbial communities and their interaction with the materials around them. (B) Stromatolites

are influenced by physical, chemical, and microbial processes, which can be represented on a triangular diagram as shown.

Stromatolites in the geological record are clustered in the shaded area. The apices represent end members; for example, in a system

in which only physical and/or chemical processes dominate, microbial communities, and therefore stromatolites, will not form.

BIOSEDIMENTS AND BIOFILMS 287

The geographical distribution of Early Archaean

stromatolites today reflects the occurrence of pre-

served Early Archaean sedimentary rocks, which are

restricted to narrow zones of sedimentary rocks,

known as greenstone belts, of the Barberton (South

Africa) and East Pilbara (Western Australia) regions.

The oldest reputed stromatolites, from the 3.48 Ga

Dresser Formation (Warrawoona Group, Pilbara),

are domical laminated structures made of chert,

barite, and iron oxide-rich laminae with minor car-

bonate (Figure 9; Dresser Formation stromatolites).

The biological origin of these structures has been

contested, with claims made that they could be arte-

facts of structural deformation (folding) and other

post-burial processes. Moreover, the fact that these

stromatolites occur in isolated clusters within struc-

turally deformed rocks makes it difficult to acquire

supporting contextual information.

Also in the Warrawoona Group lies the 3.46 Ga

Strelley Pool Chert, a formation that contains abun-

dant, well-preserved stromatolites (Figure 10; Strelley

Pool Chert stromatolites). The stromatolites in the

Strelley Pool Chert display a range of conical mor-

phologies that grade into small ripple structures, sug-

gesting that the stromatolites formed in a shallow

environment intermittently washed by gentle waves.

However, several abiological hypotheses have been

proposed for the formation of these stromatolites,

including non-biological chemical precipitation and/

or wave/current deposition. Nonetheless, the Strelley

Pool Chert stromatolites offer good prospects in the

search for early evidence of life. Even better prospects

are offered by the younger, but more complex, stro-

matolites in the 2.7 Ga Fortescue Group. These

examples may provide a useful benchmark for the

study of other Archaean stromatolites (Figure 11;

Fortescue stromatolites).

Microfossils

Fossilization processes Microbial fossilization

occurs only under special circumstances, typically

where mineral precipitation entombs the microbes

as or soon after they die. This may occur in highly

precipitative environments, such as hot springs, in

which fluids laden with dissolved chemicals emanate

at the Earth’s surface. The mineral deposits associ-

ated with these environments are called sinters (silica

precipitates) or travertine (carbonate precipitates).

Modern examples occur at Yellowstone Park (USA),

Taupo Volcanic Zone (New Zealand), and Lake

Bogoria (Kenya). However, even in these environ-

ments, microbial fossilization is never complete and

perfect. In multispecies systems, some species may be

more readily fossilized, whereas others are destroyed.

Furthermore, studies of modern sinters have shown

that all cellular level information of colonies is lost on

geologically short time-scales as opaline silica recrys-

tallizes to a more stable quartz phase. In many sedi-

mentary environments, microbial remains degrade

or are completely obliterated almost immediately

(in geological terms) through physical and chemical

processes, including oxidation and thermal degrad-

ation (in which organic material breaks down to

kerogen as sediments are heated through burial, igne-

ous intrusion, or circulation of hydrothermal waters).

Under most circumstances, the remains of microbes

will not survive intact.

Interpreting microfossil-like structures Some re-

searchers have suggested that, for the important

question of the oldest evidence of life, the ‘null hy-

pothesis’ should be adopted, which stipulates that a

fossil cannot be authenticated until all abiological

explanations (e.g., abiotic sedimentation, post-burial

alteration, sample contamination) can be refuted. The

Table 3 Comparison of four generalized mechanisms for the formation of stromatolite lamination

Cause of accretion cycles Microbial response Example

Daily solar cycle.

Independent of

sedimentation

Phototactic (light-responsive). Filamentous cells lie flat at night,

but stand upright during the day in order to reach sunlight

Phormidium hendersonii, Conophyton

(Yellowstone National Park)

Episodic sediment influx.

Independent of light

conditions

Normally flat-lying filamentous organisms; glide vertically after

new sediment influx in order to escape burial. Lamination is

dependent on sediment supply

Schizothrix (subtidal stromatolites,

Bahamas),

Microcoleus

Seasonal changes in water

chemistry

Low rainfall and warmer temperatures during the summer

cause a change in organism dominance and behaviour

Lacustrine stromatolites,

south-western Australia

Extended pauses in

sedimentation

Profound changes in species composition in the mat. Laminae

construction styles vary due to different behaviours of the

new community

Multispecies mats at Great

Sippewissett Salt Marsh,

Massachusetts

Based on data from Seong-Joo

et al. (2000) On Stromatolite Lamination. In: Riding R and Awramik S (eds.) Microbial Sediments, pp. 16–24.

Berlin: Springer-Verlag.

288 BIOSEDIMENTS AND BIOFILMS

Table 4 Summary of selected data on Archaean stromatolites. Entries in italics are questionably stromalites or questionably

Archaean. Colours correspond to date in Figure 2

Continued

BIOSEDIMENTS AND BIOFILMS 289

identification of microfossils is a topic of particular

scrutiny in view of recent work demonstrating

that microfossil-like objects can be reproduced abio-

tically in the laboratory. ‘Biomorphs’ made of silica

and witherite (minerals that could occur in hydro-

thermal systems) were produced in the laboratory

(Figure 12), displaying many features previously

thought to be indicative of a biological origin, in-

cluding spherical, segmented (septate), filamentous

structures reminiscent of bacterial cells, and car-

bonaceous composition. Whereas kerogen is bio-

logical in origin, it should not be confused with

The references in the fourth column correlate with the map in Figure 2.

Depositional association refers to the type of overall geological setting in that the stromatolites occur in. Note that most

stromatolites are found in volcanic settings; the actual deposits are typically hot spring-type sediments such as exist today at

Yellowstone National Park (USA), Lake Bogoria (Kenya), and Taupo Volcanic Zone (New Zealand).

Size ranges: XS ¼ micrometres, S ¼ millimetres, M ¼ centimetres, L ¼ tens of centimetres, XL ¼ metres, XXL ¼ tens of metres,

XXXL ¼ hundreds of metres.

Morphology codes refer to: b ¼ branching, c ¼ conical, co ¼ columnar, d ¼ domed, m ¼ ministromatolite, o ¼ oncoids,

p ¼ Pseudocolumnar, r ¼ rolled up, s ¼ stratiform, w ¼ walled.

Based on data from Hoffman H (2000) Archean Stromatolites as Microbial Archives. In: Riding R and Awramik S (eds.)

Microbial

Sediments

, pp. 315–327. Berlin: Springer-Verlag.

Table 4 Continued

Figure 9 Domical stromatolite from the Dresser Formation, Pilbara region, Western Australia.

290 BIOSEDIMENTS AND BIOFILMS

non-biological organic carbonaceous material, which

can form by a number of different processes, and

has been shown experimentally to adhere to abiolo-

gical fossil-like structures, lending them additional

biological character.

The oldest microfossils The microfossils that were,

for a long time, cited as the earliest evidence for life

come from the Apex Chert in the Warrawoona Group

(Pilbara region, Western Australia). This discovery

comprised septate, filamentous structures (Figure 13)

in what was thought to be a sedimentary chert

layer. However, this chert layer was later shown to

be a black chert dyke, which led to the contention

that the putative microfossils could be hydrothermal

artefacts or biomorphs (see above). Other carbon-

aceous structures interpreted as microfossils have

been found in hydrothermal black chert deposits

in the 3.49 Ga Dresser Formation (Warrawoona

Group). Spheroidal structures of similar age, but

more poorly preserved, have been reported from the

Onverwacht Group in the Swaziland Sequence of

South Africa. Although a biological origin for these

discoveries is plausible, every report of microfossils

from rocks of Early Archaean age has been contested

and alternative abiotic explanations have been offered

for their formation. Less contentious filamentous

structures, interpreted as fossilized bacteria, have

been found in the Late Archaean (2.7 Ga) Tumbiana

Formation (Fortescue Group, Pilbara), but the oldest

microfossils of widely accepted microbial origin come

Figure 10 Conical stromatolite from the 3.42 Ga Strelley Pool

Chert, Pilbara region, Western Australia. The layering comprises

alternating fine chert and carbonate laminations. These are rela-

tively flat, except in the stromatolite itself, where the laminae

form a distinct cone. This kind of structure may be attributed to

the activity of microbes.

Figure 11 Columnar stromatolites, exposed in cross-section, from the 2.7 Ga Tumbiana Formation (Fortescue Group) in the Pilbara

region, Western Australia. Hammer is approximately 40 cm in length.

BIOSEDIMENTS AND BIOFILMS 291

Elsevier Encyclopedia of Geology - vol I A-E

Подождите немного. Документ загружается.