Stromatolites (from Greek στρώμα,
strōma,
mattress, bed, stratum, and λιθος,
lithos, rock) are
layered
accretion structures formed in shallow water by the
trapping, binding and cementation of sedimentary grains by
biofilms of
microorganisms, especially
cyanobacteria (commonly known as blue-green
algae). They include some of the most ancient
records of life on Earth.
Morphology
A variety of stromatolite morphologies exist including conical,
stratiform, branching, domal, and columnar types. Stromatolites
occur widely in the fossil record of the
Precambrian, but are rare today. Very few
ancient stromatolites contain
fossilized
microbes. While features of some
stromatolites are suggestive of
biological
activity, others possess features that are more consistent with
abiotic (non-organic) precipitation. Finding
reliable ways to distinguish between biologically-formed and
abiotic (non-biological) stromatolites is an active area of
research in
geology.
Fossil record
Stromatolites (Pika Formation, Middle
Cambrian) near Helen Lake, Banff National Park, Canada.
Stromatolites were much more abundant on the planet in Precambrian
times. While older,
Archean fossil remains are presumed to be
colonies of
single-celled blue-green bacteria, younger (that is,
Proterozoic) fossils may be
primordial forms of the
eukaryote chlorophytes
(that is,
green algae). One
genus of stromatolite very common in the
geologic record is
Collenia. The earliest stromatolite of
confirmed microbial origin dates to .A recent discovery provides
strong evidence of microbial stromatolites extending as far back as
.
Stromatolites are a major constituent of the fossil record for
about the first 3.5 billion years of life on earth, with their
abundance peaking about 1,250 million years ago. They subsequently
declined in abundance and diversity, which by the start of the
Cambrian had fallen to 20% of their peak. The most widely-supported
explanation is that stromatolite builders fell victims to grazing
creatures (the
Cambrian
substrate revolution), implying that sufficiently complex
organisms were common over 1 billion years ago.
The connection between grazer and stromatolite abundance is well
documented in the younger
Ordovician
evolutionary radiation;
stromatolite abundance also increased after the
end-Ordovician and
end-Permian extinctions decimated
marine animals, falling back to earlier levels as marine animals
recovered.
While
prokaryotic cyanobacteria
themselves reproduce asexually through cell division, they were
instrumental in priming the environment for the
evolutionary development of more
complex
eukaryotic organisms.
Cyanobacteria are thought to be largely responsible for increasing
the amount of
oxygen in the primeval earth's
atmosphere through their continuing
photosynthesis.
Cyanobacteria use
water,
carbon dioxide, and
sunlight to create their food. The byproducts of
this process are
oxygen and
calcium carbonate (
lime). A layer of
mucus
often forms over mats of cyanobacterial cells. In modern microbial
mats, debris from the surrounding habitat can become trapped within
the mucus, which can be cemented together by the calcium carbonate
to grow thin laminations of
limestone.
These laminations can accrete over time, resulting in the banded
pattern common to stromatolites. The domal morphology of biological
stromatolites is the result of the vertical growth necessary for
the continued infiltration of sunlight to the organisms for
photosynthesis.
Modern occurrence
Modern stromatolites are mostly found in
hypersaline lakes and marine
lagoons where extreme conditions due to high saline
levels exclude animal grazing.
One such location is Hamelin Pool
Marine Nature Reserve
, Shark
Bay
in Western Australia
where excellent specimens are observed today, and
another is Lagoa Salgada, state of
Rio de
Janeiro
, Brazil
, where
modern stromatolites can be observed as bioherm (domal type) and
beds. Inland stromatolites can also be found in
saline waters in Cuatro Ciénegas
, a unique ecosystem in the
Mexican desert, and in Lake
Alchichica, a maar lake in Mexico's
Oriental Basin. Modern
stromatolites are only known to prosper in an open marine
environment in the Exuma Cays in the Bahamas (for excellent
underwater photos, see # 197-209:
http://strata.geol.sc.edu/Bahamas/BahamasGalleryIndex_3.html).
Freshwater
stromatolites are found in Lake Salda
in southern Turkey
. The
waters are rich in
magnesium and the
stromatolite structures are made of
hydromagnesite.
Layered
spherical growth structures named
oncolites are similar to stromatolites, and are
also known from the
fossil
record.
See also
Notes
- Cited
- An unusually complete domed stromatolite with a well-preserved
upper surface, over 5 feet in diameter, from the Boxley Blue Ridge
Quarry near Roanoke, Virginia, was donated to the Virginia Museum
of Natural History ( "Two-ton, 500 Million-year-old Fossil Of
Stromatolite Discovered In Virginia, U.S."), accessed 4 July
2008.
- Earlier start to life on Earth - Science - Specials
- smh.com.au
- Braithwaite, C. and Veysel Zedef, Living hydromagnesite
stromatolites from Turkey, Sedimentary Geology, Volume 106,
Issues 3-4, November 1996, Page 309, DOI:
10.1016/S0037-0738(96)00073-5
- Uncited
Stromatolite diversity through time:
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