Relating Mass Extinction Events to Phanerozoic Geological Period Boundaries

 

What I have noticed while thinking about Phanerozoic climate cycles, in the context of vertical tectonics, is the interesting correlation between Earth climate and the times that have been adopted by geologists to mark the transition from one geological period to the next. These correlations seem to relate to periods when Earth climate is experiencing a rapid transition from icehouse to hothouse conditions and vice-versa. There also seem to be some correlations between the periods when Earth climate is experiencing extreme heat or extreme cold, although in most cases a more sensitive measure of time varying climate shows these generally involve short bursts of very rapid climate change. On reflection, there is some logic in an expectation that many species of flora and fauna will find it difficult to adapt to major changes in their natural environments, occasioned by rapidly changing climatic conditions. And given that it is the sudden changes in the nature of the fossil records, observed in differing locations over the Earth, that have been used to define these period boundaries, there would be a reasonable expectation for a correlation to exist between climate cycles and the geological period boundaries.  

These correlations can be seen in Fig 1. The proxies for climatic conditions have been taken from Veizer et al (2000). The various curves show data from the oxygen isotope analysis of fossil calcite marine deposits at varying levels of time sensitivity, allowing the ocean temperatures to be estimated at the time of deposition. For example, the light green curve marked as 5/10 indicates sampling window of 5 Ma that are averaged over a 10 Ma period. Using the abbreviated period boundaries, so that Cm/O indicates the boundary between the Cambrian and Ordovician periods at -485 Ma, it can be seen from Fig 1 that Earth climate is emerging from an extended period of hothouse conditions into icehouse conditions at each of the Cm/O, D/C, T/J, and K/Pg period boundaries. Most of the other period boundaries during the Phanerozoic can be seen to occur at times of rapid change in climate during either a hot-house or ice-house period. For example, the P/Tr boundary relates to very rapid decreases in temperature during the late-Paleozoic hothouse climate conditions. The O/S boundary lies at a point in time when temperature decreases suddenly during the mid-Paleozoic icehouse climate conditions - it is also a time of massive decreased temperature. Similarly, the J/K transition is associated with some sharp swings in Earth temperature during the mid-Mesozoic icehouse conditions. Note, the periods when Earth is experiencing ice-house climate connditions are shown by the blue bars at the top.      

Fig 1 plotting climate cycles and geological periods based upon an adaptation of Veizer’s Fig 1 (Nature 408, 698-701, 2000) correlating geological period boundaries with rapid changes in Earth climate. See text for details. The light blue shading indicastes periods and paleolatidudes of ice rafted debris during period when ice sheets existyed at the poles. The purple shading represenrts the frequency histogrammes of other glacial deposits.