| Abstract: | The burial of organic carbon in sediments constitutes a
transfer of carbon from the oxidized atmospheric
reservoir (CO2) to the reduced lithospheric reservoir
(CH2O), and functions as an important control on
atmospheric carbon dioxide and oxygen levels over
geologic time. Increase in primary production of
organic matter and decrease in its decomposition
(preservation) have been hypothesized as the main
mechanisms responsible for this transfer of carbon
from atmosphere to lithosphere. However, the relative
contribution of each process continues to be a source
of debate. In the Cretaceous period, there are several
well-studied intervals of organic-rich sedimentation
(e.g., "oceanic anoxic events") during which basinal
sediments likely became temporary sinks for
atmospheric CO2 with concomitant oceanic and
climatic perturbations. Such events offer an ideal
opportunity to study the controls on, and consequences
of enhanced carbon burial. Although significant
progress has been made through the use of many
proxy methods, the lack of detailed time scales has
made flux estimates difficult, thus placing an inherent
limitation of the quantification of Cretaceous productivity
and preservation.
In this study, Evolutive Harmonic
Analysis has been applied to the rhythmically bedded
deposits of the Cenomanian/Turonian Bridge Creek
Limestone Member (Western Interior Basin) to develop
a high-resolution orbital time scale during and
following Oceanic Anoxic Event II. This time-scale
allows calculation of accumulation rates for selected
geochemical constituents that represent proxies for
important processes in the depositional environment.
The analysis identifies two distinct intervals of elevated
organic carbon accumulation (during OAE II, and just
following OAE II), and permits evaluation of the
productivity and preservation hypotheses within them.
The results indicate that the causes of organic matter
burial are more complex than the classic end-member
production and preservation models suggest. Based
on these results we present a geochemical model that
illustrates the controls on organic matter burial by
assessing the linkage between organic carbon and
molybdenum accumulation in sediments. The model
explains our main findings that: 1) the first order control
on organic carbon accumulation in Western Interior
fine-grained deposits is the rate of export of organic
matter to the sulfate reduction zone (SRZ), which is
controlled by primary production and export, bulk
sedimentation rate, and location of the SRZ in the
sediment/water column; and 2) a threshold level of
preservational conditions (combination of the last two
factors in #1) dictates whether or not changes in
production will impact organic carbon burial. In
addition to the Bridge Creek data, a spatial data set
from the Hartland Shale Member will be used to further
illustrate the model. |