The authors find that the oil was emplaced before metamorphism sealed
the voids, making some of it of Archaean age. Some of the interesting
implications of this are pointed out in the article, and I supply
quotations below.
"The discovery of oil preserved in Archaean fluid inclusions ·
confirms the contention that hydrocarbon generation by thermal
maturation of biogenic kerogen was extensive in Archaean sedimentary
basins. This is somewhat at odds with conventional wisdom about
petroleum distribution, which regards Archaean terrains as essentially
unpetroliferous. This view is based on the presumption that no
suitably organic-rich source rocks occurred in the Archaean, and that
even if they did, subsequent metamorphism and deformation would have
destroyed any petroleum they produced. However, surveys of the
organic carbon contents in Archaean sedimentary rocks clearly show
that after accounting for metamorphism decarbonation, kerogen levels
in many Archaean mudrocks were high enough to have yielded significant
quantities of hydrocarbon on thermal maturation. Moreover, the
survival of oil for about 3 billion years, as documented here, shows
that post-depositional trauma did not always destroy all traces of
petroleum fluid."
"As hydrocarbon generation and migration were evidently widespread
phenomena in Archaean and early Palaeoproterozoic basins, global
burial of organic carbon must have been substantial during these early
stages of the Earth's history. Indeed, isotopic mass balances
indicate that at least 12% of all buried Archaean carbon was organic.
·. Indeed, as nutrient levels are unlikely to have been strongly
limiting in Achaean water bodies, with exposed continents supplying
phosphate and nitrogen-fixing cyanobacteria probably present, the
Archaean aquatic biota may well have been as productive as its modern
counterpart. So despite marked growth of the crustal organic carbon
reservoir during the Proterozoic, the hydrocarbon record implies that
soon after life appeared on Earth it proliferated in aquatic
environments. The diversity of this primordial biosphere may also be
constrained by the oil in the fluid inclusions, should it prove to
contain molecular fossils of the organisms from which it was derived
(biomarkers)."
Reference: Dutkiewicz, A., Rasmussen, B and Buick, R. 1998, Oil
preserved in fluid inclusions in Archaean sandstones, Nature, 395(29
October), 885-887.
I have found this extremely interesting, as it was not that long ago
that the Archaean was portrayed as a world unsuitable for life as we
know it. Now we find evidence that the aqueous Archaean was teeming
with life - with the possibility of using biomarkers to find out more
as to what kinds of life were represented.
It might be worth drawing attention again to an article that I wrote
18 months ago.
http://www.pages.org/bcs/Bcs074.html
This was a discussion of Ohmoto's significant study of Precambrian
paleosols. My concluding paragraph reads as follows:
"An atmosphere with free oxygen points to the contemporaneity of
plants which photosynthesise. However, to date, studies of organic
life in the Archaean have suggested the existence of only bacteria and
single-celled algae. But this evidence is not plentiful. Even the
growth mounds, the Precambrian stromatolites, now appears to be better
explained as having an abiotic origin (Grotzinger & Rothman, 1996).
This meagre fossil evidence has allowed the earlier postulate of a
neutral atmosphere during the later Archaean. With the new evidence of
a significantly oxygenated atmosphere, it may be inferred that more
plant life was around than we have direct evidence for. Ohmoto says:
"Terrestrial biomass on the early continents may have been more
extensive than previously recognised". Following through this thought
leads to a number of interesting possibilities for further research:
(i) with more rigorous investigation, will body fossils of this more
extensive biomass be found?
(ii) were post-deposition processes less conducive to fossil
formation in the PreCambrian than in the Phanerozoic?
(iii) were sedimentary processes less conducive to fossil
formation in the PreCambrian than in the Phanerozoic?"
It appears to me that the Archaean oil finds are supportive of the
concept of a more extensive biomass and a search for biomarkers may
allow Ohmoto's comment (above) to be elaborated on.
Best wishes,
David J. Tyler.