Ordovician–Silurian extinction events

The Ordovician–Silurian extinction events, also known as the Ordovician extinction, were, combined, the second-largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct and second largest overall in the overall loss of life.[1] Between about 447 Ma to 443 Ma (million years ago), two pulses of extinction, separated by four million years, appear to have happened.[2] They were the second biggest extinction of marine life, ranking below only the Permian–Triassic extinction event, and as they occurred, all known life was confined to the seas and oceans.[3] More than 60% of marine invertebrates died[4][5] including two-thirds of all brachiopod and bryozoan families.[3] Brachiopods, bivalves, echinoderms, bryozoans and corals were particularly affected.[2] The immediate cause of extinction[which?] appears to have been the movement of Gondwana into the south polar region. This led to global cooling, glaciation and consequent sea level fall. The falling sea level disrupted or eliminated habitats along the continental shelves.[2][6] Evidence for the glaciation was found through deposits in the Sahara Desert. A combination of lowering of sea level and glacially driven cooling are likely driving agents for the Ordovician mass extinction.

The fifth mass extinction
The extinction occurred 443.8 million years ago, during the Great Ordovician Biodiversification Event.[7] It marks the boundary between the Ordovician and following Silurian period. During this extinction event there were several marked changes in biologically responsive carbon and oxygen isotopes. This complexity may indicate several distinct closely spaced events, or particular phases within one event.

At the time, most complex multicellular organisms lived in the sea, and around 100 marine families became extinct, covering about 49%[8] of faunal genera (a more reliable estimate than species). The brachiopods and bryozoans were decimated, along with many of the trilobite, conodont and graptolite families.

Statistical analysis of marine losses at this time suggests that the decrease in diversity was mainly caused by a sharp increase in extinctions, rather than a decrease in speciation.

Possible causes
These extinctions are currently being intensively studied. The pulses appear to correspond to the beginning and end of the most severe ice age of the Phanerozoic, which marked the end of a longer cooling trend in the Hirnantian faunal stage towards the end of the Ordovician,[7] which had more typically experienced greenhouse conditions.

The late Ordovician glaciation event was preceded by a fall in atmospheric carbon dioxide (from 7000 ppm to 4400 ppm).[10][11] The dip is correlated with a burst of volcanic activity that deposited new silicate rocks, which draw CO2 out of the air as they erode.

As the southern supercontinent Gondwana drifted over the South Pole, ice caps formed on it. The strata have been detected in late Ordovician rock strata of North Africa and then-adjacent northeastern South America, which were south-polar locations at the time. Glaciation locks up water from the world-ocean, and the interglacials free it, causing sea levels repeatedly to drop and rise; the vast shallow intra-continental Ordovician seas withdrew, which eliminated many ecological niches, then returned, carrying diminished founder populations lacking many whole families of organisms. Then they withdrew again with the next pulse of glaciation, eliminating biological diversity at each change (Emiliani 1992 p. 491). In the North African strata, Julien Moreau reported five pulses of glaciation from seismic sections.[12]

This incurred a shift in the location of bottom-water formation, shifting from low latitudes, characteristic of greenhouse conditions, to high latitudes, characteristic of icehouse conditions, which was accompanied by increased deep-ocean currents and oxygenation of the bottom-water. An opportunistic fauna briefly thrived there, before anoxic conditions returned. The breakdown in the oceanic circulation patterns brought up nutrients from the abyssal waters. Surviving species were those that coped with the changed conditions and filled the ecological niches left by the extinctions.

Gamma ray burst hypothesis
A small minority of scientists have suggested that the initial extinctions could have been caused by a gamma ray burst originating from a hypernova within 6,000 light years of Earth (in a nearby arm of the Milky Way Galaxy). A ten-second burst would have stripped the Earth's atmosphere of half of its ozone almost immediately, exposing surface-dwelling organisms, including those responsible for planetary photosynthesis, to high levels of ultraviolet radiation.[13][14][15][16] Although the hypothesis is consistent with patterns at the onset of extinction, there is no unambiguous evidence that such a nearby gamma ray burst ever happened.

Volcanism and weathering
A major role of CO2 is implied by a 2009 paper.[17] Through the Late Ordovician outgassing from major volcanism was balanced by heavy weathering of the uplifting Appalachian Mountains, which sequestered CO2. In the Hirnantian Stage the volcanism ceased, and the continued weathering caused a significant and rapid draw down of CO2.[11] This coincides with the rapid and short ice age.

Metal Poisoning
Toxic metals, on the ocean floor, may have dissolved into the water when the ocean's oxygen was depleted caused by an increase in nutrients in the oceans. The toxic metals may have harmed or killed life forms in the food chain, causing starvation on the dependent higher feeding life forms in the chain.