Paleozoic Anoxic event

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.