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to the sea bottom when compared to the smaller, slowly settling micro-
bial biomass, which previously dominated the oceans.
You can think of it like this. Imagine you have a bucket with a small
hole near the bottom letting water out. If you stood on a high balcony
and you lowered the bucket slowly, you'd lose lots of water as it ap-
proached the ground, and if you lowered slowly enough, you might
lose all the water while the bucket was still in the air. But, lower quickly
and much less water will be lost from the bucket on its trip to ground.
Now, the water loss from our bucket is analogous to the oxygen used
during the decomposition of organic matter as it settles from the upper
ocean to the ocean depths. If the organic matter settles quickly, less
oxygen is used through its decomposition in the upper portions of the
ocean (by analogy, less water is lost from the bucket) as compared to the
situation where the organic matter settles slowly. In this second case,
lots more oxygen will be used in the upper reaches of the ocean as the
organic matter gradually settles and decomposes.
One could thus imagine that with an increase in the settling rate of
organic matter, less oxygen would be used during its decomposition in
the upper depths of the ocean (meaning the upper several hundred me-
ters). This would result in an increase in the oxygenation of these waters
compared to when organic matter settled slowly. Indeed, an increase in
the oxygenation of the oceans is what our chemical proxies tell us, but
if we follow the logic outlined just above, such an increase could maybe
occur without any change in atmospheric oxygen levels. If this was true,
it might help to explain our difficulty in recognizing the driving forces
for a late Neoproterozoic increase in atmospheric oxygen. Indeed, maybe
there wasn't an increase.
Graham Logan and his colleagues also offered some support for their
idea that animal evolution might have changed the carbon dynamics
of the oceans. They looked at organic biomarkers and found very little
evidence for the preservation of algae-produced organic matter from the
upper water column photosynthetic organisms in sediments older than
about 590 million years. However, in sediments younger than about
530 million years old, such biomarker remains were found. There is a
big and important gap in the data between 590 and 530 million years
ago, but Logan and colleagues nonetheless argued that before 590
million years ago photosynthetic organisms largely decomposed during
 
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