Paul Wignall did an outstanding job in this book bringing us up-to-date on mass extinctions with his encyclopedic knowledge. His emphasis on the linkage between mass extinctions and supercontinent existence is also shared by another theory, the Gravity Theory of Mass Extinction (GTME). The GTME can resolve many of the apparently contradictory issues raised in this book.
For example, on page 149, the point is made that the Karoo-Ferrar eruptions seem to have begun 200k years after the 2nd Extinction and the timing mismatch is also seen for the Emeishan Traps, Siberian Traps and CAMP eruptions. This time lag, if accurate, indicates that the volcanism was not the primary factor responsible for the corresponding extinctions.
Briefly, the GTME posits that the Earth’s core elements (inner and outer cores and the densest part of the lower mantle) can and have moved off-center when the center of mass (COM) of the total continental mass moves to a higher latitude from the equator. This movement occurred when Pangea formed. Pangea’s COM moved significantly below and above the equator during the last 300 my (per Plate tectonics may control geomagnetic reversal frequency by Petrelis, Besse and Valet, 2011). This latitudinal movement shifted the core elements away from Pangea, based upon the Law of Conservation of Angular Momentum, reducing surface gravity on Pangea and increasing it antipodally within the Panthalassa Ocean. This is why some fauna, particularly dinosaurs, reached immense proportions.
When Pangea’s COM moved to lower latitudes the core elements moved back toward Earth-centricity increasing surface gravity on Pangea and causing extinction. This retrograde core movement initiated the flow of lava plumes at the core/mantle boundary which would take hundreds of thousands of years to reach the surface. This is why all of the mass extinctions initially begin a considerable time before the corresponding flood basalt eruptions occur, as noted on page 149. This also answers the question of why the most massive flood basalt eruptions occur when supercontinents exist. However, this also applies to the period prior to 300mya because the same principle applies, for example, when there are 2 supercontinents such as Gondwana and Laurasia. Even after Pangea started breaking apart, the net latitudinal, continental movement was sufficient to move the core elements initiating the Deccan Traps and the NAIP.
The core element movement away from Pangea also caused higher sea level around the supercontinent due to lower surface gravity there. During the retrograde core movement as surface gravity increased on Pangea, sea levels fell as it did in all the other mass extinctions. The lower sea level, warm water and still relatively low surface gravity caused the disassociation of methane from the hydrates at the bottom of the sea. Note that the lower surface gravity decreases the density of the sea water and thereby reduces its pressure per unit of depth. The combination of warm water and lower water pressure released the methane. This is why episodes of carbon isotope excursions accompany mass extinctions and why benthic life forms are severely affected even before the thermogenic gas release from volcanic eruptions occurs. Therefore, their demise is from methane caused anoxia, not from carbon dioxide from volcanism, at least initially. The delayed volcanism did exacerbate these conditions extending the extinctions.
When surface gravity increased on Pangea marine life forms that had shells or skeletons, including conodonts which had teeth, were negatively affected because the increase in water pressure per unit depth in the water column, as mentioned above, limited their vertical movement. Therefore, during mass extinctions benthic life forms are negatively affected by the release of methane and those that moved vertically in the water column are negatively affected by the surface gravity increase which causes a water pressure increase per unit depth.
The GTME explains the extinction of the crurotarsan archosaurs at the end of the Triassic (page 114) as being the result of their sprawling and pillar-erect posture when surface gravity increased allowing the dinosaurs, which had an erect posture similar to mammals, to flourish.
I strongly recommend this book for all who are interested in the incredible journey of this planet that made human life possible today.
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