Neoproterozoic Oxygen and The Rise of Animals - Oxygen: A Four Billion Year History - page 140

more of modern values during the late Neoproterozoic Era, but still at
levels considerably less than those of today. This is about the best I can
offer at present with the evidence at hand, but in the next chapter we
may be able to check this estimate with other approaches.
If there was a rise in atmospheric oxygen levels during the late Neo-
proterozoic Era, there must have been a cause. Some years ago, the an-
swer seemed clear. In a major geochemical breakthrough, Andy Knoll,
in 1986, provided the first detailed carbonate carbon isotope results
from Neoproterozoic rocks. These rocks displayed a preponderance
of 13 C-enriched values, and if you remember from chapter 7, this means
high rates of organic carbon burial. That also means high rates of oxygen
release to the atmosphere. Supercontinent Rodinia was undergoing
breakup into smaller continental units during the late Neoproterozoic,
and Andy reasoned that with the ensuing increase in coastal area, 11 there
would be more sediment deposition, giving higher rates of organic car-
bon burial. Refining this argument, Lou Derry from Cornell, but a Har-
vard postdoc at the time, introduced other isotope systems that helped
to isolate the peak of organic carbon burial to around 580 million years
ago. 12 This was perfect. Problem solved.
But as often happens in science, an interesting idea is subject to test-
ing and the accumulation of more data, and in this case, lots more data.
hereas Lou had perhaps one hundred carbon isotope measurements to
consider for his modeling, there now exist thousands ( ig. 10.4 ). hese
new data have preserved many of the broad features of the carbon iso-
tope record that Lou modeled, but the dating has changed and new
features have emerged that considerably confound the picture. One
of these is a big excursion to very 13 C-depleted values known as the
Shuram-Wonoka carbon isotope anomaly ( ig. 10.4 ). The dating on this
anomaly and its duration are uncertain, but most agree that it comes
sometime after the Gaskiers glaciation at 580 million years ago, and
before 551 million years ago. It has now been identified in many places
from around the world, and it seems to be a robust feature of the late
Neoproterozoic carbon cycle. 13
The problem with this anomaly is that it's almost impossible to un-
derstand how it originates given our normal picture of how the carbon
cycle works. The issue is where to get all of the 13 C- depleted carbonate
that winds up in the limestone. There are a variety of possible solutions