Late Devonian Mass Extinction event

What is it?

The Devonian period can be divided into Early, Middle and Late epochs and they are then subdivided into several stages. The Late Devonian Mass Extinction (LDME) happened 359 million years ago around the Frasnian and Famennian (F-F) stages of the Late Devonian.  As one of the “Big Five” event, the Late Devonian time devastated most marine and terrestrial life on Earth. It was considered to be a prolongation for the marine biotic crisis extending for 20-25 million years from the late Middle Devonian to the Devonian/Carboniferous boundary when taken place of 8-10 extinction events. Two events among these are considered to be the most serious. The first occurred at the Frasnian-Famennian boundary is known as the Kellwasser event which is considered as the severest event during that period.The other, severe as 70% as the Kellwasser one, happened at the boundary with the Carboniferous, is known as the Hangenbery event. 

What happened in the event?

This event made a severe impact on floral and fauna on Earth. At the end of the Frasnian, 60% of existing taxa suffered the extinction. 13% to 38% families are estimated to disappear with further lowering the taxonomic hierarchy with losses of 55% to 60% genus and 70% to 82% species (Goehring, 2001). Besides, 43% to 50% plant species of terrestrial life were also lost during the LDME.

(Figure of the Late Devonian period)

Why did it happen?

The suggested possible causes of the LDME are mostly summarized into those four parts:

climate changes, tectonics, sea level fluctuations and asteroid impacts. None of them can solely cause the extinction, many believed that the LDME was caused by the combination of those factors. 

Climate changes:

One possible factor causing the Devonian extinction is global cooling as it can lead to the disruption of marine environments. The cooling was suggested to be caused by the paleogeography. This was theorized that the ocean between Laurussia and Gondwana closed at the F-F boundary which may disrupt the low-latitute warm water circumequatorial flow. Then restricted circulation and anoxic conditions in warmer water basins on the easter margin would be created as the high-latitute colder water would flow into equatorial areas on the west margins of the joined continents. This caused the subtropical reef and perireef life suffering higher extinction rate. Besides, global cooling can cause a lower water temperature which would accompany with the glaciation event.

Another speculation for the LDME proposed by Thompson and Newton is a lethal increase in temperature. Isotopic oxygen and carbon studies have proved this point as they presented an increase of temperature to over 60EC at the F-F boundary, while the general range was 36EC to 54EC over the earlier Devonian period. As many marine life live closer to their physiologic tolerance upper temperature limits, a small change could take them into intolerable temperatures. Besides, warmer water could hold less oxygen which leads to an anoxic condition. 

Tectonics:

Many theories for the LDME have been derived from tectonic mechanisms. Fisher and Arthur made a climate model according to tectonic megacycles, changes related to phases of accelerated plate tectonic activity. By the increase of magma upwelling and spreading at mid-ocean ridges, ocean would change to continents. Then, with increased global volcanism, carbon dioxide would be added into the atmosphere. On the other side, less carbon dioxide in the atmosphere may be caused by the slowing of plate tectonics which can lead to an icehouse cycle. There rapid condition changes could result in a global ecosystem collapse.

Sea level fluctuations:

Although less evidences can show the LDME was related to the sea level fluctuations as these periods suffered transgressions and regressions without an associated extinction event, some reasons can still prove this point. For instance, regression would be caused by glaciation. This would result in loss of habitat for sessile shallow marine organisms or deadly hyper-saline conditions in shallow water. On the other hand, the transgression, can lead to the "drowning" of perched reef ecosystem together with an influx of anoxic deeper water into shallow areas. This can kill sessile sea floor marine organisms. 

Asteroid impacts:

This aspect has become a popular theory that caused extinction events. A theory suggested the LDME was the first bolide-induced event. The asteroid would need a diameter greater than 10 kilometers. If the asteroid that impacted earth satisfying this condition, it would destroy life in the target area, result in earthquakes, tsunamis, wildfires and ballistic molten debris. The consequences for those events can listed as following:

1. Affect shallow marine ecosystems (Tsunamis)

2. The atmosphere would be heated sufficiently leading a combination of nitrogen and oxygen to generate nitric oxide and nitric acid. Rain then falling in high concentrations could pollute upper surface waters and kill phytoplanktonic life (Blast).  

3. The environment could be poisoned by dioxins and aromatic hydrocarbons produced by wildfires.

4. The sunlight could be blocked disrupting the photosynthesis (Global dust clouds).

5. Icehouse effect that decrease the global temperature.

Reference:

S. Goehring (2001) The Late Devonian Mass Extinction Event. Elizabeth City, NC.

http://www.bbc.co.uk/nature/extinction_events/Late_Devonian_extinction