Anoxic events in Earth's history
Cretaceous Anoxic Event
Sulphidic (or euxinic) conditions, which exist today in many water bodies from ponds to various land-surrounded mediterranean seas such as the Black Sea, were particularly prevalent in the Cretaceous Atlantic but also characterised other parts of the world ocean. In an ice-free sea of these supposed super-greenhouse worlds, oceanic waters were as much as 200 meters higher, in some eras. During the time spans in question, the continental plates are believed to have been well separated, and the mountains we know today were (mostly) future tectonic events meaning the overall landscapes were generally much lower and even the half super-greenhouse climates would have been eras of highly expedited water erosion carrying massive amounts of nutrients into the world oceans fuelling an overall explosive population of microorganisms and their predator species in the oxygenated upper layers.
Detailed stratigraphic studies of Cretaceous black shales from many parts of the world have indicated that two oceanic anoxic events (OAEs) were particularly significant in terms of their impact on the chemistry of the oceans, one in the early Aptian (~120 Ma), sometimes called the Selli Event (or OAE 1a) after the Italian geologist, Raimondo Selli (1916–1983), and another at the Cenomanian-Turonian boundary (~93 Ma), sometimes called the Bonarelli Event (or OAE 2) after the Italian geologist, Guido Bonarelli (1871–1951). OAE1a lasted for ~1.0 to 1.3 Myr. The duration of OAE2 is estimated to be ~820 kyr based on a high-resolution study of the significantly expanded OAE2 interval in southern Tibet, China.
- Insofar as the Cretaceous OAEs can be represented by type localities, it is the striking outcrops of laminated black shale within the various coloured clay-stones and pink and white limestone near the town of Gubbio in the Italian Apennines that are the best candidates.
- The 1-meter thick black shale at the Cenomanian-Turonian boundary that crops out near Gubbio is termed the ‘Livello Bonarelli’ after the man who first described it in 1891.
More minor oceanic anoxic events have been proposed for other intervals in the Cretaceous (in the Valanginian, Hauterivian, Albian and Coniacian–Santonian stages), but their sedimentary record, as represented by organic-rich black shales, appears more parochial, being dominantly represented in the Atlantic and neighbouring areas, and some researchers relate them to particular local conditions rather than being forced by global change.
Jurassic
The only oceanic anoxic event documented from the Jurassic took place during the early Toarcian (~183 Ma). Because no DSDP (Deep Sea Drilling Project) or ODP (Ocean Drilling Program) cores have recovered black shales of this age there being little or no Toarcian ocean crust remaining in the world ocean the samples of black shale primarily come from outcrops on land. These outcrops, together with material from some commercial oil wells, are found on all major continents and this event seems similar in kind to the two major Cretaceous examples.
Paleozoic
The boundary between the Ordovician and Silurian periods is marked by repetitive periods of anoxia, interspersed with normal, oxic conditions. In addition, anoxic periods are found during the Silurian. These anoxic periods occurred at a time of low global temperatures (although CO2 levels were high), in the midst of a glaciation.
Jeppsson (1990) proposes a mechanism whereby the temperature of polar waters determines the site of formation of downwelling water. If the high latitude waters are below 5 °C (41 °F), they will be dense enough to sink; as they are cool, oxygen is highly soluble in their waters, and the deep ocean will be oxygenated. If high latitude waters are warmer than 5 °C (41 °F), their density is too low for them to sink below the cooler deep waters. Therefore, thermohaline circulation can only be driven by salt-increased density, which tends to form in warm waters where evaporation is high. This warm water can dissolve less oxygen, and is produced in smaller quantities, producing a sluggish circulation with little deep water oxygen. The effect of this warm water propagates through the ocean, and reduces the amount of CO2 that the oceans can hold in solution, which makes the oceans release large quantities of CO2 into the atmosphere in a geologically short time (tens or thousands of years). The warm waters also initiate the release of clathrates, which further increases atmospheric temperature and basin anoxia. Similar positive feedback operate during cold-pole episodes, amplifying their cooling effects.
The periods with cold poles are termed "P-episodes" (short for primo), and are characterised by bioturbated deep oceans, a humid equator and higher weathering rates, and terminated by extinction events for example, the Ireviken and Lau events. The inverse is true for the warmer, oxic "S-episodes" (secundo), where deep ocean sediments are typically graptolitic black shales. A typical cycle of secundo-primo episodes and ensuing event typically lasts around 3 Ma.
The duration of events is so long compared to their onset because the positive feedback must be overwhelmed. Carbon content in the ocean-atmosphere system is affected by changes in weathering rates, which in turn is dominantly controlled by rainfall. Because this is inversely related to temperature in Silurian times, carbon is gradually drawn down during warm (high CO2) S-episodes, while the reverse is true during P-episodes. On top of this gradual trend is overprinted the signal of Milankovic cycles, which ultimately trigger the switch between P- and S- episodes.
These events become longer during the Devonian; the enlarging land plant biota probably acted as a large buffer to carbon dioxide concentrations.
The end-Ordovician Hirnantian event may alternatively be a result of algal blooms, caused by sudden supply of nutrients through wind-driven upwelling or an influx of nutrient-rich meltwater from melting glaciers, which by virtue of its fresh nature would also slow down oceanic circulation.
Archean and Proterozoic
Throughout most of Earth's history, it was thought that oceans were largely oxygen-deficient. During the Archean, euxinia was largely absent because of low availability of sulphate in the oceans, but during the Proterozoic, it would become more common.
Consequences of Oceanic Anoxic Event
Oceanic anoxic events have had many important consequences. It is believed that they have been responsible for mass extinctions of marine organisms both in the Paleozoic and Mesozoic. The early Toarcian and Cenomanian-Turonian anoxic events correlate with the Toarcian and Cenomanian-Turonian extinction events of mostly marine life forms. Apart from possible atmospheric effects, many deeper-dwelling marine organisms could not adapt to an ocean where oxygen penetrated only the surface layers.
An economically significant consequence of oceanic anoxic events is the fact that the prevailing conditions in so many Mesozoic oceans has helped produce most of the world's petroleum and natural gas reserves. During an oceanic anoxic event, the accumulation and preservation of organic matter was much greater than normal, allowing the generation of potential petroleum source rocks in many environments across the globe. Consequently, some 70 percent of oil source rocks are Mesozoic in age, and another 15 percent date from the warm Paleogene: only rarely in colder periods were conditions favourable for the production of source rocks on anything other than a local scale.
Atmospheric effects
A model put forward by Lee Kump, Alexander Pavlov and Michael Arthur in 2005 suggests that oceanic anoxic events may have been characterised by up-welling of water rich in highly toxic hydrogen sulphide gas, which was then released into the atmosphere. This phenomenon would probably have poisoned plants and animals and caused mass extinctions. Furthermore, it has been proposed that the hydrogen sulphide rose to the upper atmosphere and attacked the ozone layer, which normally blocks the deadly ultraviolet radiation of the Sun. The increased UV radiation caused by this ozone depletion would have amplified the destruction of plant and animal life. Fossil spores from strata recording the Permian-Triassic extinction event show deformities consistent with UV radiation. This evidence, combined with fossil biomarkers of green sulphur bacteria, indicates that this process could have played a role in that mass extinction event, and possibly other extinction events. The trigger for these mass extinctions appears to be a warming of the ocean caused by a rise of carbon dioxide levels to about 1000 parts per million.
Ocean chemistry effects
Reduced oxygen levels are expected to lead to increased seawater concentrations of redox-sensitive metals. The reductive dissolution of iron-manganese oxyhydroxides in seafloor sediments under low-oxygen conditions would release those metals and associated trace metals. Sulphate reduction in such sediments could release other metals such as barium. When heavy-metal-rich anoxic deep water entered continental shelves and encountered increased O2 levels, precipitation of some of the metals, as well as poisoning of the local biota, would have occurred. In the late Silurian mid-Pridoli event, increases are seen in the Fe, Cu, As, Al, Pb, Ba, Mo and Mn levels in shallow-water sediment and microplankton; this is associated with a marked increase in the malformation rate in chitinozoans and other microplankton types, likely due to metal toxicity. Similar metal enrichment has been reported in sediments from the mid-Silurian Ireviken event.
