| Abstract: | With ammonite/inoceramid species loss of up to 85%
in the Western Interior basin, the
Cenomanian-Turonian boundary event represents a
significant biotic crisis. Hypotheses proposed for the
driving mechanism of species turnover include mainly
changes in benthic and water column oxygen levels
associated with Oceanic Anoxic Event II, and changes
in substrate consistency associated with
limestone-marlstone alternation. Although the critical
agent of biotic deterioration under oxygen deficient
conditions is likely to be elevated pore water and
bottom water sulfide concentrations, evidence for
extensive and prolonged euxinia during OAE II in the
Western Interior is lacking. In this study the main
hypotheses for biotic turnover are reviewed in light of
new sediment and geochemical accumulation rates,
as well as recalculated evolutionary rates across the
stage boundary. These new rates are based on a high
resolution time scale developed through
cyclostratigraphic analysis of the
Cenomanian-Turonian Bridge Creek Limestone
Member.
Evolutive harmonic analysis of an optical densitometry
record from the central basin #1 Portland core
(Colorado) permits detailed reconstruction of changes
in sedimentation rate, including the quantification of
hiatuses, thus allowing construction of an unbiased
time-scale. The new high-resolution time scale
facilitates an independent quantitative assessment of
the rates of accumulation of environmentally sensitive
geochemical proxies, and calculation of rates of
evolutionary change (FA/kyr, LA/kyr) in the C-T boundary
interval. We employ the new proxy flux data, and
published paleobiologic, stratigraphic, and
sedimentologic data, to assess secular
paleoenvironmental change in the Western Interior sea
(e.g., redox state of sediments/water column,
productivity of pelagic autotrophs, changes in
terrigenous silicilastic flux). Based on this, a series of
major modal switches in sedimentation are identified.
These modal switches may represent fundamental
changes in the ocean-climate-sediment transport
system, and include: (1) a shift from clay-dominated
black shale (Hartland Shale Member) with evidence of
sulfidic pore/bottom waters to carbonate dominated
limestone-marlstone (lower Bridge Creek Limestone)
lacking such evidence, (2) a shift to increased organic
carbon/carbonate flux in the later portion of OAE II
(lower Bridge Creek Limestone) that lacks evidence of
extensive pore/bottom water sulfide, and (3) a shift to
highest organic matter accumulation following OAE II
(upper Bridge Creek Limestone) that shows evidence
of sulfidic pore/bottom waters. Interestingly, the highest
rates of extinction do not correspond to intervals with
the greatest indication of sulfidic conditions. Further
comparisons of paleoenvironmental data with
molluscan evolutionary rate allow evaluation of
alternate biotic controls, such as substrate consistency,
turbidity, and nature/frequency of environmental
disturbance. |