Geoscience Reference
In-Depth Information
to you, but over geologic time scales this is short, and small imbalances
between the liberation and consumption rates of oxygen are quickly
channeled into rapid changes in atmospheric oxygen concentration.
In addition to rapid recycling, Bob Berner had another great idea.
He reasoned that there may be more direct ways than the isotope re-
cords to obtain a history of carbon and sulfur burial. To see how this
works, we start with Aleksandr Borisovich Ronov, a famous Russian
geologist.
4
He spent much of his career carefully compiling geologic
maps of a group of rocks known as the Russian platform and from other
rocks around the world. Ronov used these maps to assess the volumes
of the different types of rocks preserved through geologic time. Thus,
Ronov determined, among other things, the volume of sediments pre-
served from marine environments, from continental environments,
and the amount preserved from coal deposits. This data set (and much
of the other data that Ronov compiled) is a veritable gold mine of infor-
mation, and a graph showing the distribution of different rock types
through the Phanerozoic Eon is shown in
igure 11.1.
Bob Berner noted that these different types of sediment each carried,
on average, different quantities of organic carbon and pyrite sulfur. For
example, coal deposits are rich in organic carbon and generally poor in
sulfur,
5
while other sediments deposited on the continents (frequently
comprised of sands and gravel) are poor in both organic carbon and
pyrite sulfur. Marine sediments have intermediate organic carbon levels
and elevated pyrite sulfur contents compared to terrestrial deposits. All
this means that if the average sulfur and organic carbon contents of
these rock types are known, as well as their deposition rates through
time, then rates of oxygen liberation to the atmosphere can be directly
determined. Bob assumed that the relative distribution of the preserved
rock types through time, as revealed by Ronov's compilations, reflected
their relative distribution at the time they were deposited. Bob also used
a variety of scenarios to calculate total rates of sediment deposition.
These scenarios ranged from constant rates of total sediment deposition
through time to variable rates,
6
but in the end, variability in total sedi-
ment deposition rate had little influence on the model outcome.
7
Thus, with estimates of carbon and sulfur contents in the different
sediment types, and with a constant rate of sediment deposition, Bob
calculated rates of organic carbon and pyrite sulfur burial through time.
