For years, it has been customary to interpret Precambrian
stromatolite structures as the fossilised remains of communities
of cyanobacteria and single-celled algae. Modern analogues were
found in a few places (such as Shark Bay in Australia) and the
consensus was that their origin was biogenic. Earlier and
earlier stromatolites were found - going back to 3.5 billion
years. This raised some questions in people's minds as to
whether they are genuine fossils. It emerged that there are no
Archaean stromatolites which contain evidence of the
microorganisms which are supposed to have constructed them.
Also, that there is ample evidence of carbonate precipitation
during the Precambrian - and very stromatolite-like structures
resulted.
A test of the biotic/abiotic models has recently been made,
utilising stromatolites from sediments considered to be 1.9
billion years old. This summary is taken from Walter (1996):
"Using photographs taken in the field, and polished slabs
of rock in the laboratory, [the researchers] traced and digitised
the layers of stromatolites to allow the calculation of
mathematical power spectra. They show that, over three orders
of magnitude in size, the stromatolite laminae as seen in the
field and on the slabs obey the same power law. From this they
obtain the fractal dimension of the stromatolites, and use it to
deduce the processes of stromatolite growth.
"The upshot, they propose, is that the morphogenesis of all
stromatolites can be accounted for by four abiotic processes:
fallout of suspended sediment, downslope movement of that
sediment (or the mathematical equivalent surface-tension effects
in chemical precipitation), surface-normal precipitation, and
random effects. They conclude that, theoretically, abiotic
processes can generate stromatolites, and in the absence of
microfossils within them it may be impossible to prove their
biological origins."
My reading of the primary paper (Grotzinger and Rothman, 1996)
suggests that they hold a stronger position regarding the
microfossil evidence:
"However, inasmuch as microorganisms exist in virtually all
shallow marine environments today, it would be misleading to
assume that they did not inhabit the surfaces of most, if not
all, Precambrian stromatolites. But if they did, then it is no
longer clear what role they played in morphogenesis."
These two articles have further discussion which is worth
reading. I would like to comment on a few aspects which strike
me as relevant.
(a) The existence of stromatolite fossils has provided the
evolutionary scenario of origins with quite a bit to talk about
in the Precambrian. If their origin is abiogenic, the story gets
a bit thin.
(b) Abiotic stromatolites do not appear to be forming today.
This suggests that we ought to be much more open to the "unusual"
when interpreting the Precambrian.
(c) There has been an influential group of geologists who have
used stromatolites as indicators of past environments. If the
biogenic explanation is inappropriate, then it opens the door for
some radical ideas about the Precambrian palaeoenvironments.
It is probably to early to say anything definite - but it seems
to me that the door is wide open for some fresh thinking about
the Precambrian and its significance for origins.
References:
Grotzinger, J.P. & Rothman, D.H. 1996. An abiotic model for
stromatolite morphogenesis, Nature, 383(3 October), 423-425.
Walter, M. 1996. Old fossils could be fractal frauds, Nature,
383(3 October), 385-386.
Best wishes,
*** From David J. Tyler, CDT Department, Hollings Faculty,
Manchester Metropolitan University, UK.
Telephone: 0161-247-2636 ***