Anoxic waters

Anoxic waters are areas of sea water, fresh water, or groundwater that are depleted of dissolved oxygen and are a more severe condition of hypoxia. The US Geological Survey defines anoxic groundwater as those with dissolved oxygen concentration of less than 0.5 milligrams per litre.[1] This condition is generally found in areas that have restricted water exchange.

In most cases, oxygen is prevented from reaching the deeper levels by a physical barrier[2] as well as by a pronounced density stratification, in which, for instance, heavier hypersaline waters rest at the bottom of a basin. Anoxic conditions will occur if the rate of oxidation of organic matter by bacteria is greater than the supply of dissolved oxygen.

Anoxic waters are a natural phenomenon,[3] and have occurred throughout geological history. In fact, some postulate that the Permian–Triassic extinction event, a mass extinction of species from world's oceans, resulted from widespread anoxic conditions. At present anoxic basins exist, for example, in the Baltic Sea,[4] and elsewhere (see below). Recently, there have been some indications that eutrophication has increased the extent of the anoxic zones in areas including the Baltic Sea, the Gulf of Mexico,[5] and Hood Canal in Washington State.[6]

Causes and effects

Anoxic conditions result from several factors; for example, stagnation conditions, density stratification,[7] inputs of organic material, and strong thermoclines. Examples of which are fjords (where shallow sills at their entrance prevent circulation) and deep ocean western boundaries where circulation is especially low while production at upper levels is exceptionally high. In wastewater treatment, the absence of oxygen alone is indicated anoxic while the term anaerobic is used to indicate the absence of any common electron acceptor such as nitrate, sulfate or oxygen.

When oxygen is depleted in a basin, bacteria first turn to the second-best electron acceptor, which in sea water, is nitrate. Denitrification occurs, and the nitrate will be consumed rather rapidly. After reducing some other minor elements, the bacteria will turn to reducing sulfate. This results in the byproduct of hydrogen sulfide (H2S), a chemical toxic to most biota and responsible for the characteristic "rotten egg" smell and dark black sediment color.[8]

SO4−2 + H+1 → H2S +H2O + chemical energy

If anoxic sea water becomes reoxygenized, sulfides will be oxidized to sulfate according to the chemical equation:

HS + 2 O2 → HSO4

or, more precisely:

(CH2O)106(NH3)16H3PO4 + 53 SO42− → 53 CO2 + 53 HCO3 + 53 HS +16 NH3 + 53 H2O + H3PO4

Anoxia is quite common in muddy ocean bottoms where there are both high amounts of organic matter and low levels of inflow of oxygenated water through the sediment. Below a few centimeters from the surface the interstitial water (water between sediment) is oxygen free.

Anoxia is further influenced by biochemical oxygen demand (BOD), which is the amount of oxygen used by marine organisms in the process of breaking down organic matter. BOD is influenced by the type of organisms present, the pH of the water, temperature, and the type of organic matter present in the area. BOD is directly related to the amount of dissolved oxygen available, especially in smaller bodies of water such as rivers and streams. As BOD increases, available oxygen decreases. This causes stress on larger organisms. BOD comes from natural and anthropogenic sources, including: dead organisms, manure, wastewater, and urban runoff.[9]

In the Baltic Sea, the slowed rate of decomposition under anoxic conditions has left remarkably preserved fossils retaining impressions of soft body parts, in Lagerstätten.[10]

Human caused anoxic conditions

Eutrophication, an influx of nutrients (phosphate/nitrate), often a byproduct of agricultural run-off and sewage discharge, can result in large but short-lived algae blooms. Upon a bloom’s conclusion, the dead algae sink to the bottom and are broken down until all oxygen is expended. Such a case is the Gulf of Mexico where a seasonal dead zone occurs, which can be disturbed by weather patterns such as hurricanes and tropical convection. Sewage discharge, specifically that of nutrient concentrated “sludge”, can be especially damaging to ecosystem diversity. Species sensitive to anoxic conditions are replaced by fewer hardier species, reducing the overall variability of the affected area.[8]

Gradual environmental changes through eutrophication or global warming can cause major oxic-anoxic regime shifts. Based on model studies this can occur abruptly, with a transition between an oxic state dominated by cyanobacteria, and an anoxic state with sulfate-reducing bacteria and phototrophic sulfur bacteria.[11]

Daily and seasonal cycles

The temperature of a body of water directly affects the amount of dissolved oxygen it can hold. Following Henry's law, as water becomes warmer, oxygen becomes less soluble in it. This property leads to daily anoxic cycles on small geographic scales and seasonal cycles of anoxia on larger scales. Thus, bodies of water are more vulnerable to anoxic conditions during the warmest period of the day and during summer months. This problem can be further exacerbated in the vicinity of industrial discharge where warm water used to cool machinery is less able to hold oxygen than the basin to which it is released.

Daily cycles are also influenced by the activity of photosynthetic organisms. The lack of photosynthesis during nighttime hours in the absence of light can result in anoxic conditions intensifying throughout the night with a maximum shortly after sunrise.[12]

Biological adaptation

Organisms have adapted a variety of mechanisms to live within anoxic sediment. While some are able to pump oxygen from higher water levels down into the sediment, other adaptations include specific hemoglobins for low oxygen environments, slow movement to reduce rate of metabolism, and symbiotic relationships with anaerobic bacteria. In all cases, the prevalence of toxic H2S results in low levels of biologic activity and a lower level of species diversity if the area is not normally anoxic.[8]

Anoxic basins

See also


  1. ^ "Volatile Organic Compounds in the Nation's Ground Water and Drinking-Water Supply Wells: Supporting Information: Glossary". US Geological Survey. Retrieved 3 December 2013.
  2. ^ Bjork, Mats; Short, Fred; McLeod, Elizabeth; Beer, Sven (2008). Managing Sea-grasses for Resilience to Climate Change. Volume 3 of IUCN Resilience Science Group Working Papers. Gland, Switzerland: International Union for Conservation of Nature (IUCN). p. 24. ISBN 978-2-8317-1089-1.
  3. ^ Richards, 1965; Sarmiento 1988-B
  4. ^ Jerbo, 1972;Hallberg, 1974
  5. ^ "Streamflow and Nutrient Delivery to the Gulf of Mexico for October 2009 to May 2010 (Preliminary)". Retrieved 2011-02-09.
  6. ^ "Archived copy" (PDF). Archived from the original (PDF) on 2011-09-27. Retrieved 2013-03-05.CS1 maint: Archived copy as title (link)
  7. ^ Gerlach, 1994
  8. ^ a b c Castro, Peter; Huber, Michael E. (2005). Marine Biology 5th ed. McGraw Hill. ISBN 978-0-07-250934-2.
  9. ^ "5.2 Dissolved Oxygen and Biochemical Oxygen Demand". Water: Monitoring & Assessment. US Environmental Protection Agency. Retrieved 3 December 2013.
  10. ^ Nudds, John; Selden, Paul (2008-07-01). "Fossil–Lagerstätten". Geology Today. 24 (4): 153–158. doi:10.1111/j.1365-2451.2008.00679.x. ISSN 1365-2451.
  11. ^ Bush; et al. (2017). "Oxic-anoxic regime shifts mediated by feedbacks between biogeochemical processes and microbial community dynamics". Nature Communications. 8 (1): 789. Bibcode:2017NatCo...8..789B. doi:10.1038/s41467-017-00912-x. PMC 5630580. PMID 28986518.
  12. ^ "Dissolved Oxygen Depletion in Lake Erie". Great Lakes Monitoring. US Environmental Protection Agency. Retrieved 3 December 2013.
  • Gerlach, S. (1994). "Oxygen conditions improve when the salinity in the Baltic Sea decreases". Marine Pollution Bulletin. 28 (7): 413–416. doi:10.1016/0025-326X(94)90126-0.
  • Hallberg, R.O. (1974) "Paleoredox conditions in the Eastern Gotland Basin during the recent centuries". Merentutkimuslait. Julk./Havsforskningsinstitutets Skrift, 238: 3-16.
  • Jerbo, A (1972). "Är Östersjöbottnens syreunderskott en modern företeelse?". Vatten. 28: 404–408.
  • Fenchel, Tom & Finlay, Bland J. (1995) Ecology and Evolution in Anoxic Worlds (Oxford Series in Ecology and Evolution) Oxford University Press. ISBN 0-19-854838-9
  • Richards, F.A. (1965) "Anoxic basins and fjords", in Riley, J.P., and Skirrow, G. (eds) Chemical Oceanography, London, Academic Press, 611-643.
  • Sarmiento, J. L.; Herbert, T. D.; Toggweiler, J. R. (1988). "Causes of anoxia in the world ocean". Global Biogeochemical Cycles. 2 (2): 115. Bibcode:1988GBioC...2..115S. doi:10.1029/GB002i002p00115.
  • Sarmiento, J.A. et al. (1988-B) "Ocean Carbon-Cycle Dynamics and Atmospheric pCO2". Philosophical Transactions of the Royal Society of London, Series A, Mathematical and Physical Sciences, Vol. 325, No. 1583, Tracers in the Ocean (May 25, 1988), pp. 3–21.
  • Van Der Wielen, P. W. J. J.; Bolhuis, H.; Borin, S.; Daffonchio, D.; Corselli, C.; Giuliano, L.; d'Auria, G.; De Lange, G. J.; Huebner, A.; Varnavas, S. P.; Thomson, J.; Tamburini, C.; Marty, D.; McGenity, T. J.; Timmis, K. N.; Biodeep Scientific, P. (2005). "The Enigma of Prokaryotic Life in Deep Hypersaline Anoxic Basins". Science. 307 (5706): 121–123. Bibcode:2005Sci...307..121V. doi:10.1126/science.1103569. PMID 15637281..
Ancient Black Sea shipwrecks

Ancient Black Sea shipwrecks found in the Black Sea date to Antiquity. In 1976, Willard Bascom suggested that the deep, anoxic waters of the Black Sea might have preserved ships from antiquity because typical wood-devouring organisms could not survive there. At a depth of 150 m, the Black Sea contains insufficient oxygen to support most familiar biological life forms.


The term anoxia means a total depletion in the level of oxygen, an extreme form of hypoxia or "low oxygen". The terms anoxia and hypoxia are used in various contexts:

Anoxic waters, sea water, fresh water or groundwater that are depleted of dissolved oxygen

Anoxic event, when the Earth's oceans become completely depleted of oxygen below the surface levels

Euxinic, anoxic conditions in the presence of hydrogen sulfide

Hypoxia (environmental), low oxygen conditions

Hypoxia (medical), when the body or a region of the body is deprived of adequate oxygen supply

Cerebral anoxia, when the brain is completely deprived of oxygen, an extreme form of cerebral hypoxia


Benthos is the community of organisms that live on, in, or near the seabed, river, lake, or stream bottom, also known as the benthic zone. This community lives in or near marine or freshwater sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.

Many organisms adapted to deep-water pressure cannot survive in the upperparts of the water column. The pressure difference can be very significant (approximately one atmosphere for each 10 metres of water depth).Because light is absorbed before it can reach deep ocean-water, the energy source for deep benthic ecosystems is often organic matter from higher up in the water column that drifts down to the depths. This dead and decaying matter sustains the benthic food chain; most organisms in the benthic zone are scavengers or detritivores.

The term benthos, coined by Haeckel in 1891, comes from the Greek noun βένθος "depth of the sea". Benthos is used in freshwater biology to refer to organisms at the bottom of freshwater bodies of water, such as lakes, rivers, and streams. There is also a redundant synonym, benthon.

Black Sea

The Black Sea is a body of water and marginal sea of the Atlantic Ocean between the Balkans, Eastern Europe, the Caucasus, and Western Asia. It is supplied by a number of major rivers, such as the Danube, Dnieper, Southern Bug, Dniester, Don, and the Rioni. Many countries drain into the Black Sea, including Austria, Belarus, Bosnia and Herzegovina, Bulgaria, Croatia, Czech Republic, Georgia, Germany, Hungary, Moldova, Poland, Romania, Russia, Serbia, Slovakia, Slovenia, Turkey and Ukraine.

The Black Sea has an area of 436,400 km2 (168,500 sq mi) (not including the Sea of Azov), a maximum depth of 2,212 m (7,257 ft), and a volume of 547,000 km3 (131,000 cu mi). It is constrained by the Pontic Mountains to the south, Caucasus Mountains to the east, Crimean Mountains to the north, Strandzha to the southwest, Dobrogea Plateau to the northwest, and features a wide shelf to the northwest.

The longest east–west extent is about 1,175 km (730 mi).Important cities along the coast include Batumi, Burgas, Constanța, Giresun, Istanbul, Kerch, Novorossiysk, Odessa, Ordu, Poti, Rize, Samsun, Sevastopol, Sochi, Sukhumi, Trabzon, Varna, Yalta, and Zonguldak.

The black sea shares border with Ukraine,Romania,Bulgaria,Turkey, Georgia and Russia.The Black Sea has a positive water balance; that is, a net outflow of water 300 km3 (72 cu mi) per year through the Bosphorus and the Dardanelles into the Aegean Sea. There is a two-way hydrological exchange: the more saline and therefore denser, but warmer, Mediterranean water flows into the Black Sea under its less saline outflow. This creates a significant anoxic layer well below the surface waters. The Black Sea drains into the Mediterranean Sea, via the Aegean Sea and various straits, and is navigable to the Atlantic Ocean. The Bosphorus Strait connects it to the Sea of Marmara, and the Strait of the Dardanelles connects that sea to the Aegean Sea region of the Mediterranean. These waters separate Eastern Europe, the Caucasus and Western Asia. The Black Sea is also connected, to the North, to the Sea of Azov by the Strait of Kerch.

The water level has varied significantly. Due to these variations in the water level in the basin, the surrounding shelf and associated aprons have sometimes been land. At certain critical water levels it is possible for connections with surrounding water bodies to become established. It is through the most active of these connective routes, the Turkish Straits, that the Black Sea joins the world ocean. When this hydrological link is not present, the Black Sea is an endorheic basin, operating independently of the global ocean system, like the Caspian Sea for example. Currently, the Black Sea water level is relatively high; thus, water is being exchanged with the Mediterranean. The Turkish Straits connect the Black Sea with the Aegean Sea, and comprise the Bosphorus, the Sea of Marmara and the Dardanelles.

Cariaco Basin

The Cariaco Basin lies off the north central coast of Venezuela and forms the Gulf of Cariaco. It is bounded on the east by Margarita Island, Cubagua Island, and the Araya Peninsula; on the north by Tortuga Island and the Tortuga Banks; on the west by Cape Codera and the rocks known as Farallón Centinela; and on the south by the coast of Venezuela.


Claraia is an extinct genus of scallop-like bivalve molluscs that lived from the Capitanian stage of the Late Permian to the Anisian stage of the Middle Triassic, 266-237 million years ago. Fossils have been found worldwide in North America, Europe, Asia, Africa, and Australia. These are common fossils subsequent to the Permian-Triassic boundary, suggesting that the genus experienced rapid diversification during and after the Permian–Triassic extinction event, around 251.4 million years ago.

Dusty Rose Lake

Dusty Rose Lake is an anoxic lake located in north region of Tweedsmuir Park in British Columbia, Canada. It gets its name from its pink coloring and is unique among other lakes which have a similar coloring as it is not a salt water lake, but rather gets its coloring from pigmentation from the surrounding environment.

Gotland Basin

The Gotland Basin is the large central basin in the Baltic Sea between Sweden and the Baltic countries. It is subdivided into the Gdansk Deep (or Gdansk Basin), the Western Gotland Basin, and the Eastern Gotland Basin. Within the Eastern Gotland Basin is the Gotland Deep (249 metres deep) which is an anoxic basin. The Western Gotland Basin contains Landsort Deep, which is the deepest spot of the Baltic sea (459 metres deep).

The sediments in the Gotland Basin are important for studying the climate changes in northern Europe over the past 5,000 years.

L'Atalante basin

L'Atalante basin is a hypersaline brine lake at the bottom of the Mediterranean Sea about 192 km (119 mi) west of the island of Crete. It is named for the French L'Atalante, one of the oceanographic research vessels involved in its discovery in 1993. L'Atalante and its neighbors the Urania and Discovery deep hyper saline anoxic basins (DHABs) are at most 35,000 years old. They were formed by Messinian evaporite salt deposits dissolving out of the Mediterranean Ridge and collecting in abyssal depressions about 3,000 m (9,800 ft) deep. L'Atalante is the smallest of the three; its surface begins at about 3,500 m (11,500 ft) below sea level.


Limnology ( lim-NOL-ə-jee; from Greek λίμνη, limne, "lake" and λόγος, logos, "knowledge"), is the study of inland aquatic ecosystems.

The study of limnology includes aspects of the biological, chemical, physical, and geological characteristics and functions of inland waters (running and standing waters, fresh and saline, natural or man-made). This includes the study of lakes, reservoirs, ponds, rivers, springs, streams, wetlands, and groundwater. A more recent sub-discipline of limnology, termed landscape limnology, studies, manages, and seeks to conserve these ecosystems using a landscape perspective, by explicitly examining connections between an aquatic ecosystem and its watershed. Recently, the need to understand global inland waters as part of the Earth System created a sub-discipline called global limnology. This approach considers processes in inland waters on a global scale, like the role of inland aquatic ecosystems in global biogeochemical cycles.Limnology is closely related to aquatic ecology and hydrobiology, which study aquatic organisms and their interactions with the abiotic (non-living) environment. While limnology has substantial overlap with freshwater-focused disciplines (e.g., freshwater biology), it also includes the study of inland salt lakes.

List of environmental issues

This is an alphabetical list of environmental issues, harmful aspects of human activity on the biophysical environment. They are loosely divided into causes, effects and mitigation, noting that effects are interconnected and can cause new effects.


A mortichnia is the "death march", or last walk, of a living creature. These are sometimes preserved as fossil footprints.

Orca Basin

The Orca Basin is a mid-slope, silled, mini-basin in the northern Gulf of Mexico some 300 km southwest of the Mississippi River mouth on the Louisiana continental slope. It is unique amongst the mini-basins in this area, in containing a large brine pool of anoxic salt brine. The pool is approximately 123 km2 (47 sq mi) in area and up to 220 m (720 ft) deep under 2,400 m (7,900 ft) depth of Gulf water and is derived from dissolution of underlying Jurassic age Louann Salt. With a volume of 13.3 km3 (3.2 cu mi) the pool results from the dissolution of about 3.62 billion tonnes of the Louann Salt bed into seawater. The basin owes its shape to ongoing salt tectonics and is surrounded by salt diapirs.Gas hydrates were detected in a number of cores collected in the Orca basin during Leg 96 of the Deep Sea Drilling Program (DSDP). The cores were recovered from a water depth of 2,412 m (7,913 ft) at Holes 618 and 618A, with first evidence of gas hydrate occurring in Hole 618. Hydrates were observed in the top section of Core 618-4 at 85 fbsf (26 mbsf) in gray mud and consisted of a few white crystals of a few millimeters in diameter. At Hole 618A, gas hydrates were observed in both Cores 618A-2 and 618-3 in the 62-121 fbsf (19-37 mbsf) range, with hydrates distributed throughout Core 618A-3. The hydrates ranged in size from a few millimeters to possibly a centimeter in diameter and were white.Based on light δ13C values, the origin of the hydrate gas is biogenic. Researchers also noted that some of the hydrates appeared to occur in the sandy layers of the cores. In contrast to other gas hydrate occurrences in the Gulf of Mexico, the gas hydrate was found within a mini-basin instead of on the fractured and faulted rim of the mini-basin. It was also noted that the depth of gas hydrate occurrence coincides with the presence of black organic and/or pyrite-rich mud.

Orcadian Basin

The Orcadian Basin is a sedimentary basin of Devonian age that formed mainly as a result of extensional tectonics in northeastern Scotland after the end of the Caledonian orogeny. During part of its history, the basin was filled by a lake now known as Lake Orcadie. In that lacustrine environment, a sequence of finely bedded sedimentary rocks was deposited, containing well-preserved fish fossils, with alternating layers of mudstone and coarse siltstone to very fine sandstone. These flagstones split easily along the bedding and have been used as building material for thousands of years. The deposits of the Orcadian Basin form part of the Old Red Sandstone (ORS). The lithostratigraphic terms lower, middle and upper ORS, however, do not necessarily match exactly with sediments of lower, middle and upper Devonian age, as the base of the ORS is now known to be in the Silurian and the top in the Carboniferous.

Particle (ecology)

In marine and freshwater ecology, a particle is a small object. Particles can remain in suspension in the ocean or freshwater. However, they eventually settle (rate determined by Stokes' law) and accumulate as sediment. Some can enter the atmosphere through wave action where they can act as cloud condensation nuclei (CCN). Many organisms filter particles out of the water with unique filtration mechanisms (filter feeders). Particles are often associated with high loads of toxins which attach to the surface. As these toxins are passed up the food chain they accumulate in fatty tissue and become increasingly concentrated in predators (see bioaccumulation). Very little is known about the dynamics of particles, especially when they are re-suspended by dredging. They can remain floating in the water and drift over long distances. The decomposition of some particles by bacteria consumes a lot of oxygen and can cause the water to become hypoxic.

Robert Ballard

Robert Duane Ballard (born June 30, 1942) is a retired United States Navy officer and a professor of oceanography at the University of Rhode Island who is most noted for his work in underwater archaeology: maritime archaeology and archaeology of shipwrecks. He is most known for the discoveries of the wrecks of the RMS Titanic in 1985, the battleship Bismarck in 1989, and the aircraft carrier USS Yorktown in 1998. He discovered the wreck of John F. Kennedy's PT-109 in 2002 and visited Biuku Gasa and Eroni Kumana, who saved its crew. He leads ocean exploration on E/V Nautilus.

Saanich Inlet

Saanich Inlet (also Saanich Arm) is a body of salt water that lies between the Saanich Peninsula and the Malahat highlands of Vancouver Island, British Columbia, Canada. Located just northwest of Victoria, the inlet is 25 km (16 mi) long, has a surface area of 67 km2 (26 sq mi), and its maximum depth is 226 m (741 ft). It extends from Satellite Channel in the north (separating Salt Spring Island from the Saanich Peninsula) to Squally Reach and Finlayson Arm in the south. The only major tributary feeding the inlet is the Goldstream River.

The inlet has been of importance as a fishery to the Malahat and Saanich First Nations for centuries, and many First Nations reserves are situated on the shoreline. Since the arrival of Europeans, the inlet has also provided a recreational and commercial fishery. It has also been popular with SCUBA divers. For several years, a port existed on the western shore at Bamberton, servicing a cement works.

For most of the year the deep waters are anoxic, and hydrogen sulphide (H2S) is often detected near the bottom. In the late summer and early fall, oxygenated waters from the Haro Strait and the Satellite Channel spill over into the deep basin of Saanich Inlet.

Notable bays and inlets adjacent to Saanich Inlet include:

Brentwood Bay, a calm, sheltered bay on the east side of the Inlet, with a small ferry service to Mill Bay.

Tod Inlet, a smaller inlet off the southwest corner of Brentwood Bay with water access to the northeast corner of Gowlland Tod Provincial Park and also a private dock at the adjacent world-famous Butchart Gardens.

Coles Bay

Deep Cove

Finlayson Arm, a long narrow arm on the south of the Inlet draining Goldstream River at Goldstream Provincial Park. The scenic and steep Malahat Drive is on the west side of Finlayson Arm, while Gowlland Tod Provincial Park is on its east.

Mill Bay, a calm bay and community centre on the west side of the Inlet. The ferry to Brentwood Bay allows a way of travelling to Greater Victoria while avoiding the steep Malahat Drive.

Patricia Bay, a very deep bay and home to the Institute of Ocean Sciences research facility.

Squally Reach, an east-west channel connecting the main Saanich Inlet to the north and Finlayson Arm to the south.

McKenzie Bight, an easy to sail bight on the east side of Saanich Inlet where its meets Squally Reach.Other notable features around the inlet are:

Butchart Gardens

Goldstream Provincial Park

Gowlland Tod Provincial Park

Mount Finlayson


Sinop D

Sinop D is an ancient Black Sea shipwreck located to the east of Sinop, Turkey. The ship was discovered by a team led by Robert Ballard in 2000. The team discovered the well-preserved wreck at a 320 m depth, in the Black Sea's deep anoxic waters. The vessel's entire hull and cargo are intact, buried in sediments. Its deck structures are also intact, including a mast rising some 11 m into the water column. Radiocarbon dating of wood from the wreck provides a date of 410-520 CE.

Western Interior Seaway anoxia

Three Western Interior Seaway anoxic events occurred during the Cretaceous in the shallow inland seaway that divided North America in two island continents, Appalachia and Laramidia (see map). During these anoxic events much of the water column was depleted in dissolved oxygen. While anoxic events impact the world's oceans, Western Interior Seaway anoxic events exhibit a unique paleoenvironment compared to other basins. The notable Cretaceous anoxic events in the Western Interior Seaway mark the boundaries at the Aptian-Albian, Cenomanian-Turonian, and Coniacian-Santonian stages, and are identified as Oceanic Anoxic Events I, II, and III respectively. The episodes of anoxia came about at times when very high sea levels coincided with the nearby Sevier orogeny that affected Laramidia to the west and Caribbean large igneous province to the south, 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 is calculated by comparing the amount of 13C to a carbon isotope standard, and using multiple samples can track changes (δ) in organic carbon content through rocks over time, forming a δ13Corganic curve. The δ13Corganic, as a result, 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 source rocks for oil and gas.

Aquatic ecosystems

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