Geoscience Reference
In-Depth Information
into other chemical constituents. James found that he was able generate
mass-independent sulfur isotope fractionations this way, and these frac-
tionations were similar to those observed in sedimentary rocks from the
Archean Eon.
Therefore, a reasonable case can be made that the mass-independent
fractionations observed by James in Archean rocks were produced by
the interaction between UV light and volcano-derived SO 2 gas. Now,
here's the really cool part. Most of the UV light causing the mass-
independent fractionation of SO 2 is absorbed today by Earth's ozone
layer. Ozone is made from atmospheric oxygen. If you take away oxy-
gen, you take away ozone, and you allow the mass-independent sulfur
fractionations observed by James.
However, even if you produce a mass-independent sulfur isotope sig-
nal, it needs to be preserved. Enter again Jim Kasting, whom we met
inĀ  chapter 1, and his then postdoc Alex Pavlov. They approached this
problem with a complex atmospheric photochemical model and con-
cluded that the preservation of the mass-independent sulfur isotope
effect requires atmospheric oxygen levels 100,000 times less than those
prevailing today (that's <0.001% of today's levels!). 9 Therefore, James's
results, combined with atmospheric modeling, give us an important
ancient oxygen barometer, and it says that oxygen levels on early Earth
must have been really, really low. This is completely consistent with Dick
Holland's original view.
One would be tempted to end the story here, but fortunately scien-
tists are an inquisitive bunch. It's hard to tell them what to think, and
they keep poking, probing, and questioning, trying to find something
new. In two parts of the world, two different groups were looking
closely at rocks deposited in the late Archean Eon. One group, headed
by Martin Wille from the Australian National University was looking
at rocks from South Africa in the age range of 2.65- to 2.5-billion-years-
old. The other group, headed by Ariel Anbar from Arizona State Uni-
versity, was looking at 2.5-billion-year-old rocks from Western Australia
(not far from the Hamersley Gorges we visited above). Let's see what
they did.
These two teams both asked the same question. Could they identify
some oxygen-sensitive mineral phases generating a product that could
somehow be observed in the geologic record? This approach is poten-
 
Search WWH ::




Custom Search