Australia: The Land Where Time Began

A biography of the Australian continent 

LIPs - Karoo & Ferrar traps

At the breakup of Gondwana in the Early Jurassic a major CFBP was emplaced that is now divided by the South Atlantic, the Karoo Traps in South Africa and the Farrar Traps in Antarctica which combined contain more than 2.5 x 10⁶ km³ of lava. It is indicated by the dating of the Karoo lavas that there was a brief interval of eruption 183 ± 1 Ma (Duncan et al., 1997), the range increasing to ± 2 Ma at 2σ when external errors are included (Pálfy & Smith, 2000). A U-Pb aging of a marine ash layer dated the middle Toarcian as 181.4 ± 1.2 Ma (Pálfy et al., 1997), and the age has been extrapolated (Pálfy & Smith, 2000) to about 183 Ma for the base of the Falciferum Zone, an interval that is associated with significant extinctions.

The extinction event of the early Toarcian was first identified in marine sections in NW Europe (Hallam, 1961) and the discovery subsequently that extinctions also occurred in South America (Aberhan & Fürsich, 1996) suggests that this was a global biotic crisis of particularly shallow marine molluscs  (Little & Benton, 1995). It appears that the development of oxygen-poor conditions is the most likely cause of the crisis (Hallam, 1987). It has been postulated once again that the eruption of volcanic CO₂, the consequent global warming, has been postulated as the ultimate origin of the “anoxic” event and therefore the extinction itself (Jenkyns, 1999). It has been reported widely that there was a 2-3‰ positive C isotope excursion from the Falciferum Zone (Jenkyns, 1988), which is generally regarded as a reflection of the burial of organically light organic C in the anoxic seas. A brief negative δ¹³C excursion of 2-3‰ magnitude has recently been reported (Hesselbo et al., 2000) in the early Falciferum immediately prior to the positive excursion. The magnitude of the event in the early Falciferum is considered to be too great to be accounted for by volcanic CO₂ emissions even by a province as large as the Karoo-Ferrar Traps, as was the case with the negative excursion at the P-Tr boundary. It has been speculated (Hesselbo et al., 2000) that the global warming of volcanic CO₂ may have triggered dissociation of up to a quarter of the gas hydrate reservoir, which is the same scenario that was proposed by Bowring et al. (1998) for the P-Tr events. The model of Hesselbo et al. (2000) neatly incorporates several aspects of the Toarcian geology. Wignall suggests it is important, however, that the highest resolution δ¹³C curve for the Falciferum Zone, which was obtained from belemnites, shows that were many more fluctuations in this interval with 4 isotopic minima (McArthur et al., 2000). The greatest excursion, which is probably the one that was identified by Hesselbo et al. (2000) occurred in the mid-Falciferum Zone. The interval of (volcanically triggered?) gas hydrate release occurred slightly later than the onset of oceanic anoxia and mass extinction.

Sources & Further reading

At the breakup of Gondwana in the Early Jurassic a major CFBP was emplaced that is now divided by the South Atlantic, the Karoo Traps in South Africa and the Farrar Traps in Antarctica which combined contain more than 2.5 x 10⁶ km³ of lava. It is indicated by the dating of the Karoo lavas that there was a brief interval of eruption 183 ± 1 Ma (Duncan et al., 1997), the range increasing to ± 2 Ma at 2σ when external errors are included (Pálfy & Smith, 2000). A U-Pb aging of a marine ash layer dated the middle Toarcian as 181.4 ± 1.2 Ma (Pálfy et al., 1997), and the age has been extrapolated (Pálfy & Smith, 2000) to about 183 Ma for the base of the Falciferum Zone, an interval that is associated with significant extinctions.

The extinction event of the early Toarcian was first identified in marine sections in NW Europe (Hallam, 1961) and the discovery subsequently that extinctions also occurred in South America (Aberhan & Fürsich, 1996) suggests that this was a global biotic crisis of particularly shallow marine molluscs  (Little & Benton, 1995). It appears that the development of oxygen-poor conditions is the most likely cause of the crisis (Hallam, 1987). It has been postulated once again that the eruption of volcanic CO₂, the consequent global warming, has been postulated as the ultimate origin of the “anoxic” event and therefore the extinction itself (Jenkyns, 1999). It has been reported widely that there was a 2-3‰ positive C isotope excursion from the Falciferum Zone (Jenkyns, 1988), which is generally regarded as a reflection of the burial of organically light organic C in the anoxic seas. A brief negative δ¹³C excursion of 2-3‰ magnitude has recently been reported (Hesselbo et al., 2000) in the early Falciferum immediately prior to the positive excursion. The magnitude of the event in the early Falciferum is considered to be too great to be accounted for by volcanic CO₂ emissions even by a province as large as the Karoo-Ferrar Traps, as was the case with the negative excursion at the P-Tr boundary. It has been speculated (Hesselbo et al., 2000) that the global warming of volcanic CO₂ may have triggered dissociation of up to a quarter of the gas hydrate reservoir, which is the same scenario that was proposed by Bowring et al. (1998) for the P-Tr events. The model of Hesselbo et al. (2000) neatly incorporates several aspects of the Toarcian geology. Wignall suggests it is important, however, that the highest resolution δ¹³C curve for the Falciferum Zone, which was obtained from belemnites, shows that were many more fluctuations in this interval with 4 isotopic minima (McArthur et al., 2000). The greatest excursion, which is probably the one that was identified by Hesselbo et al. (2000) occurred in the mid-Falciferum Zone. The interval of (volcanically triggered?) gas hydrate release occurred slightly later than the onset of oceanic anoxia and mass extinction.

Sources & Further reading  

Wignall, P. B. (2001). "Large igneous provinces and mass extinctions." Earth-Sci. Rev. 53: 1-33.