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Western Interior Seaway anoxia

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File:Paleogeography of North America during the late Campanian Stage of the Late Cretaceous.png
Western Interior Seaway during the late Campanian Stage, about 75 mya

Three Western Interior Seaway anoxic events occurred during the Cretaceous in the shallow epicontinental seaway. During these anoxic events much of the water column was depleted in dissolved oxygen. Western Interior Seaway (WIS) anoxic events exhibit a unique paleonenvironment compared to other basins. Cretaceous anoxic events of note in the WIS are Oceanic Anoxic Events I, II, and III at the Aptian-Albian, Cenomanian-Turonian, and Coniacian-Santonian stage boundaries, respectively. The anoxia events came about due to very high sea level and the nearby Sevier Orogeny and Caribbean large igneous province, which delivered nutrients and oxygen-adsorbing compounds into the water column.

Most anoxic events are recognized using the 13C isotope as a proxy to indicate total organic carbon preserved in sedimentary rocks. If there is very little oxygen, then organic material that settles to the bottom of the water column will not be degraded as readily compared to normal oxygen settings and can be incorporated into the rock. 13Corganic curves are used to track changes in organic carbon content through rocks over time, which serves as a benthic oxygen curve.

The excellent organic carbon preservation brought about by these successive anoxic events makes Western Interior Seaway strata some of the richest plays for oil and gas.

Farralon, Kula, and North American Plate distribution between 64 and 74 million years ago.

Western Interior Seaway Tectonics and Geography

During the Cretaceous Period, along the western shore of the Western Interior Seaway there was active volcanism and foreland subsidence brought about by the Sevier Orogeny. The Sevier Orogeny formed by the convergence of the oceanic Farallon and Kula Plates with the North American Plate.[1] Active volcanism during the Sevier Orogeny was the product of partial melting of the subducting Farralon and Kula Plates, and that molten rock travelling up through the overlying North American Plate. Most active volcanism occurred in the northern and southern portions along the western shoreline of the seaway. [1]


To the east of the orogeny, a back-arc basin formed due to the warping of the North American plate in response to the horizontal stress of the subducting oceanic plates. The low-lying area was underwater throughout the Cretaceous due to warm climate, causing seawater to expand and flood the continent's interior. Sea level during Oceanic Anoxic Event II was at its highest of the Cretaceous due to high global temperatures, and the Western Interior Seaway stretched from the Boreal Sea (present Arctic Sea) to the Tethys Sea (present Gulf of Mexico) making it 6000 km long and 2000 km wide.[2][3][4] The deepest portions were around 500 m deep.[3]

Formation of the Caribbean Plate in the Tethys Sea near the southern region of the Western Interior Seaway created a large igneous province (called the Caribbean Plateau) that produced underwater lava flows from 95-87 million years ago.[5]

Anoxic Events

Nutrient Sourcing

Ash and dissolved trace metals from Sevier and Caribbean eruptions provided nutrients to the water column, which was the driving mechanism for anoxia in the Western Interior Seaway. Ash from volcanic eruptions is the source of thick bentonite layers in Western Interior Seaway strata. Ash contains trace metals that, while in low concentration, provide nutrients to microorganisms that live in the water column. Caribbean Plateau lavas sourced hydrothermal fluids containing trace metals and sulfides. Together both events changed the chemistry of the water column by fertilizing microorganisms, which in turn increased production of primary producers. Increases in primary production will affect the rest of the water column by increasing the biomass (the amount of organisms in a certain volume), which will use up much of the available oxygen during metabolism. Additionally, dissolved oxygen passively binds to metals and sulfides, further depleting the oxygen in the water column. [6]

Stratification

A significant loss of oxygen leads to environmental perturbations. Water column stratification can occur when the zone below the sediment-water interface that is normally devoid of oxygen moves up above the sediment and into the water column. While this is a common phenomenon in deep water, this is interpreted to have occurred during anoxic settings in the shallow Western Interior Seaway, as evidenced by extinctions of benthic fauna at the Cenomanian-Turonian Boundary Event brought about by Oceanic Anoxic Event II. The extinction can be explained by ocean stratification causing low-oxygen conditions in the benthic zone. Further, increasing primary production causes an excess of metabolism waste products like CO2. When CO2 combines with water the water can become slightly acidic. Eventually the ocean can become so acidified that calcite cannot be incorporated into the hard parts of shelly organisms (biomineralization) and therefore toxic to live in.


A look down the Western Interior Seaway during Oceanic Anoxic Event II. The structure of the North American plate and sea level (blue line) projected over the United States of America (red line) with nutrient sourcing from volcanoes along the convergent margin, and the resulting water column stratification (green).

Alternate Theories to Anoxic Events in the Western Interior Seaway

Oceanic Anoxic Event II

Western Interior Seaway strata preserve the positive13Corganic excursion during Oceanic Anoxic Event II, meaning there was excellent preservation of organic carbon. However, other evidence is conflicting. Molybdenum is a trace metal that will be present in strata only if there is stratification, making it an oxygen-sensitive trace metal. One study showed lack of molybdenum in Oceanic Anoxic Event II strata.[7] Other studies demonstrated the persistence of benthic organisms that could not live in anoxic settings throughout the entirety of Oceanic Anoxic Event II.[8]

There is a difference in opinion of the relationship between benthic oxygen conditions and what a positive shift of the 13Corganic curve represents. Anoxia in the Western Interior Seaway during Oceanic Anoxic Event II is still an enigma.

Anoxic vs. Dysoxic Hypothesis

Oceanic Anoxic Event II is believed to have caused the longest duration and most potent water and water column stratification in Western Interior Seaway history.[7] Although there has been much research devoted to Western Interior Seaway strata, the impact of Oceanic Anoxic Event II on the oxygen content of the benthic zone is still contested.[6][9][10] Some relatively recent research suggests that Western Interior Seaway waters during Oceanic Anoxic Event II were dysoxic (2.0 - 0.2 mL of O2/L of H2O[with oxic being > 2.0 mL of O2/L]) rather than anoxic (< 0.2 mL of O2/L of H2O).[11] Dysoxic water can be interpreted as having a moderate amount of oxygen, or oxygen varying through time 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 organic carbon preservation, benthic fossil abundance and diversity, and oxygen-sensitive trace metal concentrations.

Circulation Models

It has been argued that the Western Interior Seaway could have had patches of anoxia, or places where water is stratified. This would be represented by variations in 13Corganic levels in rocks deposited at the same time in different parts of the seaway.[8]

Some models of Western Interior Seaway water circulation indicate that waters were homogenously mixed and not stratified.[12] The seaway, when modeled as a large bay, can have a very broad gyre formed from moving warm salt-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, models predict that the seaway was well-mixed due to the circulation gyre.


Multiple working hypotheses and evidence that would indicate the affect Oceanic Anoxic Event II had on the Western Interior Seaway:

1. Anoxic, stratified
Evidence: High organic carbon content preserved, low benthic fossil abundance and diversity, high concentration of oxygen-sensitive trace metals
2. Anoxic, patchy
Evidence: High organic carbon content preserved, low benthic fossil abundance and diversity, high concentration of oxygen-sensitive trace metals in only certain areas of the basin
3. Dysoxic, moderate (constant level between 2.0 - 0.2 mL of of O2/L of H2O)
Evidence: High to moderate organic carbon content preserved, moderate benthic fossil abundance and diversity, moderate to low concentrations of oxygen-sensitive trace metals
4. Dysoxic, variable
Evidence: Change in organic carbon content preserved, benthic fossil abundance and diversity, and concentration of oxygen-sensitive trace metals in a bed or group of beds; vary depending on three possible conditions the oxygen content can change between
A. Anoxic-dysoxic: High to moderate organic carbon preserved, low to moderate benthic fossil abundance and diversity, high to moderate concentrations of oxygen-sensitive trace metals
B. Anoxic-oxic: Low to moderate organic carbon preserved, high to moderate benthic fossil abundance and diversity, low to moderate concentrations of oxygen-sensitive trace metals
C. Dysoxic-oxic: Low levels of organic carbon preserved, high to moderate benthic fossil abundance and diversity, low concentrations to no oxygen-sensitive trace metals

Note: The same criteria can be applied to other anoxic events in the Western Interior Seaway and other shallow seas.

References

  1. ^ a b Shurr, G.W., Ludvigson, G.A., Hammond, R.H. 1994. Perspectives on the Eastern Margin of the Cretaceous Western Interior Basin. Geological Society of America, Boulder: Special Paper #287, 264 p.
  2. ^ Slingerland , R.L., Kump, L.R, Arthur, M.A., Fawcett, P.J., Sageman, B.B., and Barron, E.J. 1996. Geological Society of America Bulletin, 108, 941-952.
  3. ^ a b Bowman, A.R. and Gale, A.S., Hardenbol, J., Hathaway, B., Kennedy, W.J., Young, J.R., and Phansalkar, V. 2002. Global correlation of Cenomanian (Upper Cretaceous) sequences: Evidence for Milankovitch control on sea level. Geology, 30, 291-294.
  4. ^ Bralower, T.J. 2005. Paleoceanographic significance of high-resolution carbon isotope records across the Cenomanian-Turonian boundary in the Western Interior and New Jersey coastal plain, USA. Marine Geology, 217, 305-321.
  5. ^ Bralower, T.J. 2008. Volcanic cause of catastrophe. Nature, 454, 285-287.
  6. ^ a b Sageman, B.B., Meyers, S.R., and Arthur, M.A. 2006. Orbital time scale and new C-isotope record for Cenomanian-Turonian boundary stratotype. Geology, 34, 125-128.
  7. ^ a b Meyers, S.R., Sageman, B.B., and Lyons, T.W. 2005. Organic carbon burial rate and the molybdenum proxy: Theoretical framework and application to Cenomanian-Turonian oceanic anoxic event 2. Paleoceanography, 20, PA2002. doi:10.1029/2004PA001068
  8. ^ a b Henderson, R.A. 2004. A Mid-Cretaceous association of shell beds and organic rich shale:bivalve exploitation of nutrient-rich, anoxic sea-floor environment. Palaios, 19, 156-169.
  9. ^ Keller, G., Berner, Z., Adatte, T., and Stueben, D. 2004. Cenomanian-Turonian and δ13C, and δ18O, sea level and salinity variations at Pueblo, Colorado. Palaeogeography,Palaeoclimatology, Palaeoecology, 211, 19-43.
  10. ^ Kennedy, W.J., Walaszczyk, I., and Cobban, W.A. 2005. The Global Boundary Stratotype Section and Point for the base of the Turonian Stage of the Cretaceous: Pueblo, Colorado, U.S.A. Episodes: Journal of International Geoscience, 28, 93-104.
  11. ^ Tyson, R.V. and Pearson, T.H. 1991. Modern and ancient continental shelf anoxia: an overview.Geological Society, London, Special Publications, 58, 1-24. doi:10.1144/GSL.SP.1991.058.01.01
  12. ^ Slingerland , R.L., Kump, L.R, Arthur, M.A., Fawcett, P.J., Sageman, B.B., and Barron, E.J. 1996. Geological Society of America Bulletin, 108, 941-952.
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