Anoxic events in Earth.27s history Anoxic event

1 anoxic events in earth s history

1.1 cretaceous
1.2 jurassic
1.3 paleozoic
1.4 archean , proterozoic

anoxic events in earth s history
cretaceous

sulfidic (or euxinic) conditions, exist today in many water bodies ponds various land-surrounded mediterranean seas such black sea, particularly prevalent in cretaceous atlantic characterized other parts of world ocean. in ice-free sea of these supposed super-greenhouse worlds, oceanic waters as 200 meters higher, in eras. during time spans in question, continental plates believed have been separated, , mountains know today (mostly) future tectonic events—meaning overall landscapes lower— , half super-greenhouse climates have been eras of highly expedited water erosion carrying massive amounts of nutrients world oceans fueling overall explosive population of microorganisms , predator species in oxygenated upper layers.

detailed stratigraphic studies of cretaceous black shales many parts of world have indicated 2 oceanic anoxic events (oaes) particularly significant in terms of impact on chemistry of oceans, 1 in aptian (~120 ma), called selli event (or oae 1a) after italian geologist, raimondo selli (1916–1983), , @ cenomanian–turonian boundary (~93 ma), called bonarelli event (or oae 2) after italian geologist, guido bonarelli (1871–1951). oae1a lasted ~1.0 1.3 myr. duration of oae2 estimated ~820 kyr based on high-resolution study of expanded oae2 interval in southern tibet, china.

insofar cretaceous oaes can represented type localities, striking outcrops of laminated black shales within vari-colored claystones , pink , white limestones near town of gubbio in italian apennines best candidates.
the 1-meter thick black shale @ cenomanian–turonian boundary crops out near gubbio termed ‘livello bonarelli’ after man first described in 1891.

more minor oceanic anoxic events have been proposed other intervals in cretaceous (in valanginian, hauterivian, albian , coniacian–santonian stages), sedimentary record, represented organic-rich black shales, appears more parochial, being dominantly represented in atlantic , neighboring areas, , researchers relate them particular local conditions rather being forced global change.

jurassic

the oceanic anoxic event documented jurassic took place during toarcian (~183 ma). because no dsdp (deep sea drilling project) or odp (ocean drilling program) cores have recovered black shales of age – there being little or no toarcian ocean crust remaining in world ocean – samples of black shale come outcrops on land. these outcrops, material commercial oil wells, found on major continents , event seems similar in kind 2 major cretaceous examples.

paleozoic

the boundary between ordovician , silurian periods marked repetitive periods of anoxia, interspersed normal, oxic conditions. in addition, anoxic periods found during silurian. these anoxic periods occurred @ time of low global temperatures (although co2 levels high), in midst of glaciation.

jeppsson (1990) proposes mechanism whereby temperature of polar waters determines site of formation of downwelling water. if high latitude waters below 5 °c (41 °f), dense enough sink; cool, oxygen highly soluble in waters, , deep ocean oxygenated. if high latitude waters warmer 5 °c (41 °f), density low them sink below cooler deep waters. therefore, thermohaline circulation can driven salt-increased density, tends form in warm waters evaporation high. warm water can dissolve less oxygen, , produced in smaller quantities, producing sluggish circulation little deep water oxygen. effect of warm water propagates through ocean, , reduces amount of co2 oceans can hold in solution, makes oceans release large quantities of co2 atmosphere in geologically short time (tens or thousands of years). warm waters initiate release of clathrates, further increases atmospheric temperature , basin anoxia. similar positive feedbacks operate during cold-pole episodes, amplifying cooling effects.

the periods cold poles termed p-episodes (short primo), , characterised bioturbated deep oceans, humid equator , higher weathering rates, , terminated extinction events - example, ireviken , lau events. inverse true warmer, oxic s-episodes (secundo), deep ocean sediments typically graptolitic black shales. typical cycle of secundo-primo episodes , ensuing event typically lasts around 3 ma.

the duration of events long compared onset because positive feedbacks must overwhelmed. carbon content in ocean-atmosphere system affected changes in weathering rates, in turn dominantly controlled rainfall. because inversely related temperature in silurian times, carbon gradually drawn down during warm (high co2) s-episodes, while reverse true during p-episodes. on top of gradual trend overprinted signal of milankovic cycles, trigger switch between p- , s- episodes.

these events become longer during devonian; enlarging land plant biota acted large buffer carbon dioxide concentrations.

the end-ordovician hirnantian event may alternatively result of algal blooms, caused sudden supply of nutrients through wind-driven upwelling or influx of nutrient-rich meltwater melting glaciers, virtue of fresh nature slow down oceanic circulation.

archean , proterozoic

it has been thought through of earth s history, oceans largely oxygen-deficient. during archean, euxinia largely absent because of low availability of sulfate in oceans, during proterozoic, become more common.