DEFINITION OF STROMATOLITE

Columnar or mound-like structures produced

by a combination of biological secretions from cyanobacteria

and sediment trapping by filamentous organisms including

bacteria or algae. There have been various definitions proposed

for stromatolites based on a variety of models for their

origin, morphology, age, and distribution. One of the most

useful definitions was proposed by Stanley Awramik and

Lynn Margulis, and modified by M. Walter as “organosedimentary

structures produced by sediment trapping, binding,

and/or precipitation as a result of the growth and metabolic

activity of micro-organisms, principally cyanophytes.”

Stromatolites are among the oldest known organic structures

on Earth, found in rocks as old as 3.5 billion years.

They are the most common form of life preserved in all of the

Precambrian rock record, peaking in abundance in the Proterozoic,

with a few active stromatolite colonies in the recent

record. Stromatolites nearly completely disappeared from the

rock record at the beginning of the Phanerozoic with the first

appearance of grazing Metazoans, perhaps indicating that

they were low on the food chain and largely devoured by

these higher organisms. However it is also possible that stromatolites

declined because of competition for ecological niches

by other organisms such as green and red algae and other

marine organisms.

There are a bewildering number of morphological variations

in different types of stromatolites which some stratigraphers,

notably those in the Russian and Chinese schools,

attempted to use as index fossils for certain periods. However,

these attempts failed and current thinking now attributes

variation in stromatolite morphology to local environmental

influences and bacterial processes.

Some stromatolites form mats, crinkled mats, or gently

domed layers. These grade into bulbous, nodular, and hemispherical

forms that may be isolated or laterally linked. Other

stromatolites form columns that may have conical or subspherical

laminations and may form singular columns,

branching columns, or dendritic structures. The columns are

typically erect but may also be inclined, wavy, sinusoidal, or

irregular. Sizes of stromatolites range from a few millimeters

to several meters in height.

Most stromatolites grow by photosynthetic processes

where sunlight reaches mats of filamentous microorganisms

that initiate growth on some feature, such as a pebble or carbonate

grain. The microorganisms secrete layers of calcium

carbonate, perhaps daily, and the filamentous organisms then

grow or move upward though the layer to the surface.

Repeated growth over many days and years leads to a layered

mound, column, or branching structure. Occasionally the filaments

may trap or be buried by sedimentary particles that

move over the mound, many probably moved by storms. In

these cases the sticky mats may trap the particles forming a

detrital layer. The microorganisms must then be able to move

through this layer to the surface in order for the organic community

to survive.

See also HELIOTROPISM; LIFE’S ORIGINS AND EARLY EVOLUTION;

PRECAMBRIAN; PROTEROZOIC.

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