Australia: The Land Where Time Began

A biography of the Australian continent 

Flood Basalts -  Trigger for End-Triassic Extinction

A new spreading centre began forming beneath Pangaea, that was at the equator at the time, with a very large network of fissures and vents opening and pouring 3 million cu km of lava and volatile greenhouse gases. Pangaea was gradually split apart as the flood basalt eruptions continued for the next 600,000 years and the proto-Atlantic Ocean filled the expanding gap, and at this time about half the species of Earth died out. It is now claimed by researchers that the flood basalts were the cause of the mass extinction event of the end-Triassic.

At the present the remnants of the massive eruptions are present on 4 continents in what is known as the Central Atlantic Magmatic Province (CAMP), which is one of most extensive igneous provinces at the present. There have been 5 major extinction events over the past 500 My, and all 5 have been associated with large igneous provinces that have resulted from flood basalt eruptions, which led scientists to suspect rapid environmental changes, that included substantial warming of the atmosphere, acidification of the oceans, or "volcanic winters" that resulted from atmospheric sulphur particles that block sunlight, had all resulted from the massive amount of flood basalt eruptions.

It has been difficult to reconcile the timelines of the palaeontological record of extinctions in sedimentary structures, that are often great distances from the remnants of the flood eruptions. The exact chronology of events occurring at the end of the Triassic has been unclear as a result of the lack of precision of the dating techniques that have been used previously, such as argon-argon, that had margins of error that were too wide.

According to Terrence Blackburn, writing in a recent issue of Science2, that has been described as the most precise dates so far for the CAMP, evidence of a causal relationship between flood basalt eruptions and extinction events has been lacking in previous studies, in particular evidence of flood basalt eruptions that occurred prior to, or synchronous with, the extinction event.  

Uranium-lead dating, used by Blackburn et al., of zircon crystals obtained from basalts and gabbros from sills in the Argana Basin, Morocco, as well as in sites from North America, from North Carolina to Nova Scotia, indicate the extinction event could have been triggered by the first pulse of flood basalt eruptions.

It has been calculated by the Blackburn et al. that the initial pulse of flood basalt eruption produced a high enough volume of magma, and the volume of volatile gases were large enough, to trigger the global impacts that led to the mass extinction event. Following the extinction event later pulses of flood basalt eruptions, in places such as North America, continued as life had already began to recover.

Blackburn et al. used U-Pb dating to constrain more tightly the timing of the pulses to 20,000-30,000 years. According to Blackburn their high-precision geochronological constraints on the flood basalts allowed the correlation of the timing and relative position of the extinction event that is observed within the North American stratigraphy to Morocco, the place where the oldest flood basalts can be placed directly above or coincident with the extinction event.

According to Paul Wignall, a geologist at the University of Leeds, a researcher on the environmental impacts of large igneous provinces and continental flood basalts, and not involved in the study, the dates are tightly constrained by the new study, though the connection of the CAMP to the extinction event has been generally well accepted. Paul Renne, a geochronologist at the University of California, Berkeley, who wasn't involved in the study, claims this is only an incremental improvement over the constraints that existed previously. Renne has previously published research that linked the CAMP to the extinction event at the end of the Triassic.

It was noted by Wignall that a new aspect reveled by the study is the beginning of the recovery by life as the final stages of the eruption of flood basalts were still in progress. He  notes that the same phenomenon is beginning to become apparent in the case of other giant basalt flows, saying it is apparently the peak volume of the eruptions, usually not long after the onset of magma eruption, that is the killing phase. According to Wignall it is not yet known why this is the case, suggesting it might be connected with the volume being erupted, or possibly it might occur when a certain threshold is passed.

The understanding of the impacts of the large igneous provinces is aided by fine-tuning the chronologies of the CAMP pulses throughout the world and at the extinction event that occurred at the end of the Triassic. .

According to Blackburn in North America the extinctions occurred before the eruptions, which he says places the timing of the extinctions at the onset of the oldest eruptions of basalt in Morocco, as is suggested by this study, that has at least allowed a causal relationship.

Zircon U-Pb geochronology linking the end-Triassic extinction to the Central Atlantic Magmatic Province2

Major losses of marine and terrestrial diversity resulted from the end-Triassic extinction event, which allowed the dinosaurs to rise to dominance where they remained for the following 136 My. The existing geochronologic resolution is insufficient to suggest more than an approximate coincidence between the extinction event and the flood basalt volcanism, and does not provide eruptive rates that would be required to trigger major perturbations of the Climate. In this paper the authors2 provide new geochronologic constraints, based on U-Pb dates of zircon, on the age and duration of the flood basalt volcanism within the CAMP. Synchroneity is demonstrated by this chronology between the earliest volcanism and the extinctions, as well as testing and corroborating the existing astrochronologic time scale, and indicates that there were 4 pulses of magma release and the associated atmospheric flux that occurred over a period of 600,000 years, which indicates the expansion of volcanism was continuing as the biologic recovery was already under way.

Sources & Further reading

  1. Pratt, Sara E., June 2013, "Flood Basalts Triggered End-Triassic Extinction, Earth.
  2. Blackburn, Terrence J., Paul E. Olsen, Samuel A. Bowring, Noah M. McLean, Dennis V. Kent, John Puffer, Greg McHone, E. Troy Rasbury, and Mohammed Et-Touhami. "Zircon U-Pb Geochronology Links the End-Triassic Extinction with the Central Atlantic Magmatic Province." Science 340, no. 6135 (May 24, 2013 2013): 941-45.
Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated  13/06/2013
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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading