Relating Mass Extinction Events to Phanerozoic Climate Cycles

 

As current climate concerns recognise, it is likely that both faunal and floral life will experience considerable environment stress during periods of rapid climate change. So perhaps the observed sudden changes in the fossil records that came to form the basis of the geological period boundaries could have at least a part of their explanation in terms of these correlations with the rates of change in the climate.      

This suggests that high rates of climate changes might also be a contributing cause of the observed “mass extinction events” (MEE). A reasonably well supported summary of the recorded MEE is provided in Figure 2, adapted from the entry in Wikipedia. This shows the percentage extinctions of readily fossilized marine genera (these are the ones that have hard calcite shells). Especially over the past 300 Ma, where the fossil records are a little more complete, the relationship is quite uncanny. For example, the three most extreme MEE occur at: -250 Ma which falls on the Permian-Triassic boundary (P/T); -200 Ma on the Triassic-Jurassic boundary (T/J); -66 Ma on the Cretaceous-Paleogene boundary (K/Pg) – all associated with very rapid (in geological terms) decreases in Earth temperature. Even the relatively minor MEE at -150 Ma corresponds with the Jurassic-Cretaceous (J/K); and near the -35 Ma the Paleogene-Neogene (Pg/N) boundary.

Earlier MME are a little more difficult to separate but the well recorded events at: -445 Ma (O/S) is very close to the Ordovician-Silurian boundary; and -485 Ma (Cm/O) close to the Cambrian-Ordovician boundary; the -500 Ma and -520 Ma events occur either side of the Cambrian-Ordovician boundary

Fig 2 The percentage extinctions of readily fossilized marine genera (vertical axis) shown by the vertical blue bar chart related to the geological period boundaries over the phanerozoic eon.

One of the commonly advanced explanations for MEE events has been sudden drops in sea level – regression events. And noting that one of the primary drivers for drops in sea level is the indirect sucking-up of sea water into the ice sheets and permafrost, that characterise periods of icehouse climate, it might be anticipated that the periods at which Earth climate is experiencing rapid temperature reduction as it moves into an ice-house period would fall into this category, including - the Cm/O, D/C, P/T, T/J, K/Pg MEE. It is interesting to observe that in his discussion of mass extinctions, Hallam (1998) attributed a significant cause for Newell’s six MEE to sea level regression at each of the Cm/O, O/S, D/C, P/T, T/J, K/Pg MEE (see Fig 5 Hallam (1998). With the exception of the O/S MEE at -450Ma these assessments agree. However, the O/S MEE, along with the C/P. J/K Pg/N can be seen in Fig 3 to all occur at periods when Earth temperature is experienceing sudden drops. All of this suggests a very strong possibility that sudden and extreme climate change could have been responsible for most of the observed MEE over the past 500Ma.  

 There have been several other explanations for MEE in the past. There is some evidence of a strong correlation between recorded flood basalt eruptions and at least some of the periods when MEE occur. For example, the widespread MEE recorded at the P/T (-250 Ma) and T/J (-200 Ma) and K/Pg (-66 Ma) boundaries all coincided with times when massive flood basalt events occurred (see Matthews et al 2019) usually creating large igneous provinces. As discussed in earlier posts (ref needed), periods of rapid cooling of the lithosphere, occurring for example when climate moves from hot-house conditions to ice-house, would be expected to increase the predominance of tension induced thermal fractures of the lithosphere and the often associated release of magma through volcanism and release of flood basalts. And of course it is at these times that sea level regressions will be at their extreme linking both the regression and flood basalt explanations to their underlying cause from major changes of climate.  

Another possible cause, discussed by Alvarez et al (1980), is that of bolide impact events. It is generally accepted that the only credible possibility of a bolide impact being the cause is that of the K/Pg event, but even that is ambiguous (Raup 1991) so that Bolide impacts are generally dismissed as likely explanations - See Hallam,(1998). 

      

Fig 3. Summarises the links between MEE, the geological period boundaries and their possible unified causal link to periods of rapid climate change.

In the posts relating to the evidence for the Colorado Plateau, I concentrated upon the moments in time at which regions of the Earth's surface experienced a renewed pulse of sedimentation. This provided a fairly precise time signal at which a possibly very long periods of missing time, a so called unconformity, came to an end. Over what would have been very long periods when fossil bearing sediments were eroded during these unconformity missing times, starts to expain why so many of the recorded MEE seem to correspond with the transitions from hot-house to ice-house conditions. Perhaps some of these may have been rather slower extinctions of species than the step functions of Figure 2 suggest!