Western Interior Seaway anoxia

Western Interior Seaway Anoxia

Anoxic events within the Western Interior Seaway(Cretaceous time) were brought about by sea level highstand proximal tectonic activity from the Sevier Orogeny and Caribbean large igneous province. While these anoxic events are expressed globally in deep marine strata by a positive shift in the ¹³C_organic curve, the shallow Western Interior Seaway exhibited a unique paleonenvironment compared to other basins. Anoxic events of note in the Western Interior Seaway are Oceanic Anoxic Events I, II, and III from the Aptian-Albian, Cenomanian-Turonian, and Coniacian-Santonian stage boundaries, respectively.

Tectonic Setting

On the western margin of the Western Interior Seaway there was active volcanism, mountain building and subsidence from the Sevier Orogeny. The Sevier Orogeny formed by continent-continent convergence of the Farallon and Kula plates with the North American plate. Most of the source of bentonite in Western Interior Seaway strata comes from volcanism that occurred in the north and south portions of the western margin.

Formation of the Caribbean Plate in the Tethys Sea near the southern extent of the Western Interior Seaway created submarine large igneous province that actively erupted from 95-87 Ma.

Anoxic Events

A look down the Western Interior Seaway at OAE II time (~93 Ma). Created using Inkscape on 2012-10-10 and uploaded to Wikimedia Commons.

Initiation

The Western Interior Seaway anoxic (WIS) events are the product of warm Cretaceous climate and plate tectonics. For example, Oceanic Anoxic Event II (OAE II) was brought about by one of the greatest sea level highstands of the Cretaceous due to high global temperatures caused climate-driven eustasy and thermal expansion of seawater that led to continuous inundation of the epicontinental seaway. At maximum transgression, the WIS stretched from the Boreal Sea (proto-Arctic Sea) to the Tethys Sea (proto-Gulf of Mexico) making it 6000 km long and 2000 km wide. The deepest portions were around 500 m deep.

Nutrient Sourcing

The Caribbean Plateau Large Igneous Province sourced hydrothermal fluids containing trace metals and sulfides. Increased sourcing of the trace metals and sulfides increased primary production of microorganisms, which used up much of the oxygen during metabolism that in turn increased CO₂ production. Additionally, dissolved oxygen bound to metals and sulfides further depleting the oxygen in the water column. More proximal to the Western Interior Seaway, active volcanism, as indicated by thick bentonite layers, increased nutrient input and in turn decreased dissolved oxygen in the water column.

Environmental Perturbation/Stratification

A significant loss of oxygen leads to environmental perturbations. Water column stratification can occur when the redox boundary moves up above the sediment-water interface and into the water column. This is predicted to occur during anoxic settings in shallow basins. The extinctions of benthic macro and microfauna at the Cenomanian-Turonian Boundary Event can be explained by ocean stratification causing low-oxygen conditions in the benthic zone. Further, the ocean acidifies due to the influx of CO­­₂ and other metabolic pathway byproducts mixing with water. Eventually the ocean can become so acidified that calcite cannot be biomineralization. Oceanic anoxic event II is interpreted as exhibiting the longest duration and most potent water column stratification in Western Interior Seaway history.

Anoxic conditions allow for burial of organic matter from bodies of without much degradation. Therefore, it has been argued that the global positive δ¹³C_organic excursion is due to excellent preservation of organic detritus. This also indicates that the bottom-waters had to be anoxic for the organic content to be preserved with little degradation.

Alternate Theories to Anoxic Events

Even though there have been many stratigraphic, sedimentological, paleontological, paleoceanographic, paleoenvironmental, and geochemical analyses performed on WIS sediments, the impact of OAE II on the oxygen content of the benthic zone is still contested. Some relatively recent researchers agree that WIS benthic waters during OAE II were dysoxic (2.0 - 0.2 ml of O₂/l of H₂O) rather than anoxic (< 0.2 ml of O₂/l of H₂O), but the type of dysoxia present at the time is debated. Dysoxic waters might contain a moderate amount of oxygen, or they might experience a temporal variation between oxic and anoxic, oxic and dysoxic, or dysoxic and anoxic conditions. If the benthic oxygen was variable, the rates of change in the oxygen will affect hydrocarbon preservation.

While WIS strata do preserve the +δ¹³C_organic excursion, there are a few studies indicating that the bottom-waters were not anoxic, but rather dysoxic. One of these studies showed lack of molybdenum in many of the WIS strata during OAE II. Mo is a redox-sensitive trace metal, meaning it will be present in low oxygen/reducing conditions. Other studies demonstrated the prevalence and persistence of benthic macroorganisms. For example, fossil inoceramid bivalves are present in every layer throughout the WIS where the+δ¹³C_organic excursion is found. These bivalves would not have been able to live in an environment without any oxygen.

Further, models of water circulation in the earliest Turonian WIS indicate that waters were homogenously mixed and not stratified. The WIS can be modeled as a large estuary with a very broad gyre formed by moving warm saline-rich water from the Tethys northward along the eastern shore, and cool Boreal waters southward along the western shore. While waters of differing salinity and temperatures could become stratified and cause bottom-water anoxia, models predict that waters were homogenously mixed due to the circulation gyre.

There is a disparity in interpretations of the relationship between benthic oxygen conditions of the benthic environment and the δ¹³C_organic excursion because anoxia in the WIS is still an enigma.