|  This
is for all practical purposes, modern stromatolite, coming from the famous selenite
collecting beds of Lake Marion, Australia. In fact, the specimen contains crystals
of selenite. The specimen also exhibits extremely fine layering. The material
is difficult to obtain. Coming from a lake/lagoon environment with brackish and
hypersaline water, this stromatolite is an example of how modern stromatolite
has retreated to less hospital habitats where competition from other forms is
reduced.
Being
more modern, this domal section of stromatolite has apparently retained its form
from when the colony was alive and growing. Unlike most ancient stromatolite,
it has not been metamorphosized. We
will likely have no more than a sketchy understanding of the paleoenvironments
in which stromatolite was formed in the deep time Precambrian, and only an incomplete
understanding of the environments in the Paleozoic. Sound conjecture is possible
if we examine the now rare environments that support extant stromatolitic growth
during modern times. Cyanobacteria are found to be a primary organism in the formation
of microbial carbonates. These prokaryotic bacteria (slang name is blue-green
algae owning to pigmentation involved in photosynthesis) are now only found in
areas where there is reduced grazing and burrowing by other organisms, and a low
occurrence of macro-algae and plants. Environments where modern stromatolites
are found typically are hypersaline, but also include areas of high alkalinity,
low nutrients, high or low temperatures, and strong wave or current actions. The
obvious pattern emerges that modern stromatolites tend to exist in areas that
most other life forms consider less desirable or possibly intolerable. Thus, organisms
producing modern stromatolite are generally limited to areas where organisms with
which they have to compete and/or organisms that might use them for nutrients
are not prevalent. |
