Everything about Stromatolites totally explained
Stromatolites (from Greek
strōma, mattress, bed, stratum, and
lithos, rock) are layered accretionary
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).
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.
Background
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 .
Prior to 2.4
billion years ago, the earth's atmosphere was rich in
carbon dioxide. However, the Precambrian air lacked the
oxygen that sustains the complex
multicellular life that has
evolved since the "
Cambrian explosion". 540
Ma. Stromatolites in the
fossil record decline sharply in both diversity and number during the late
Proterozoic eon, although they're present, but not common, in
Paleozoic era strata. Today, stromatolites are quite uncommon in marine environments, and thus are called "
living fossils."
Their former abundance may be because there were no burrowing or grazing animals back during the Precambrian to destabilize sediments and consume growing
microbial mats, thereby favoring the preservation of these
microbialites. Also, changing chemical conditions in the ocean during this time could be responsible for the precipitation of non-biological stromatolites through the growth of tiny
crystals.
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 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. Fresh-water stromatolites can be found in
Cuatro Ciénegas, a unique
ecosystem in the Mexican desert.
Layered spherical growth structures similar to stromatolites, named
oncolites, are also known from the
fossil record.
Further Information
Get more info on 'Stromatolites'.
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