In later posts it will be reasoned that a major driver of processes taking place on and deep within the Earth could derive from the Earth’s various interactions with our celestial neighbours. At short timescales we are all familiar with the ways in which fluctuations of solar radiation reaching the Earth surface can have such dramatic effects on weather and climate. Whether at periods measured in hours, those of circadian origin or those determined from seasonal changes, many surface phenomena appear to be driven by the cyclic changes of insolation. Many forms of concrete and asphalt failure (see Croll, 2006, 2007b, 2008b) are believed to derive from thermal mechanical interactions driven by short term variations of insolation energy. Analogous forms of behaviour are known (Leffingwell, 1915, Mackay et al, 2002), and believed (Croll, 2004, 2006, 2007c,d, 2008a) to derive from seasonal variations of insolation affecting the response of ice and permafrost. So it would not require a major leap of imagination to consider whether very long period fluctuations of insolation energy could also be playing a major role in determining certain geological phenomena near the surface and deep within the Earth’s crust.
Ice Ages over Phanerozoic:
In earlier paper (Croll, 2007a) I attempted to explain the cyclic nature of many important Earth tectonic processes in terms of the periodic nature of the glacials and interglacials occurring when the Earth is experiencing an ice age. At the time I had not fully appreciated just how long a surface thermal wave would take to penetrate to the lower boundary of the crust. Accordingly, I implied that major changes to the thermal conditions within the crust could be generated by the surface thermal perturbations arising from the glacial and inter-glacial cycles having periodicities in the range of 20s to 100s of ka. This was only true to a limited extent. As Figure 1 shows, a sinusoidal change in surface temperature having period of say 100 ka will fully penetrate typical crustal rocks to depths of only around 1.75 km to 3.50 km, depending upon the thermal diffusion and other properties of the rock. While enough to drive many near surface processes this degree of penetration would be insufficient to account for some of the deeper tectonic phenomena ascribed to these circa 100 ka cycles. In addition, I had not fully appreciated the evidence relating to the periodicity at which the Earth experiences ice ages. This has now become more readily available and does seem to make it plausible to contemplate a celestial driver for many of the important processes that have and continue to shape the Earth’s lithosphere.
Ice Ages over Phanerozoic:
In earlier paper (Croll, 2007a) I attempted to explain the cyclic nature of many important Earth tectonic processes in terms of the periodic nature of the glacials and interglacials occurring when the Earth is experiencing an ice age. At the time I had not fully appreciated just how long a surface thermal wave would take to penetrate to the lower boundary of the crust. Accordingly, I implied that major changes to the thermal conditions within the crust could be generated by the surface thermal perturbations arising from the glacial and inter-glacial cycles having periodicities in the range of 20s to 100s of ka. This was only true to a limited extent. As Figure 1 shows, a sinusoidal change in surface temperature having period of say 100 ka will fully penetrate typical crustal rocks to depths of only around 1.75 km to 3.50 km, depending upon the thermal diffusion and other properties of the rock. While enough to drive many near surface processes this degree of penetration would be insufficient to account for some of the deeper tectonic phenomena ascribed to these circa 100 ka cycles. In addition, I had not fully appreciated the evidence relating to the periodicity at which the Earth experiences ice ages. This has now become more readily available and does seem to make it plausible to contemplate a celestial driver for many of the important processes that have and continue to shape the Earth’s lithosphere.
From the work of Shaviv (2002) and Shaviv and Veizer (2003) it appears that ice ages also display a periodicity thought by some to largely derived from the interaction arising from the Solar System’s orbital passage through the Milky Way. There is strong evidence to suggest that one of the primary drivers of long term Earth climate is changes in the Cosmic Ray Flux (CRF). In Figure 2 of Shaviv and Veizer (2003), reproduced here also as Figure 2, the geologically reconstructed smoothed temperature anomalies based upon running means of δ18O values found in calcite shells, as reported by Veizer et al (2000), are plotted over the Phanerozoic eon as the lower continuous red line. The periods over which low temperature anomalies occur, often lasting many 10’s Ma, can be seen to correspond closely to the paleoclimate reconstructions of the known ice ages, shown by the blue bars at the top of Figure 2. The upper continuous black
curve of Figure 2 summarised the reconstructed CRF values determined by iron meteorite
exposure ages reported by Shaviz, 2002. Based upon these CRF values a
prediction can be made of the ΔT values; these are shown by the continuous
black line in the lower part of Figure 2. What is remarkable is the degree to
which the fluctuations in CRF values appear to determine the temperature
anomalies. Apart from the period of circa 30 Ma occurring around 250 Mbp, the
close correlation between CRF predicted values and ΔT geological
reconstructions of the temperature variations cannot reasonably be put down to
chance. It is interesting to observe that this relatively poor correlation
around 250 Mbp corresponds with a time when a scarcity of fossils, during one
of the largest extinction events in Earth history, meant that few δ18O
measurements have been possible. Not repeated here, since it is not of central
concern to the theme of these posts, are the relatively poor correlations
between reconstructed pCO2
levels and temperature anomalies. Some of the highest levels of pCO2 are shown to occur
(Figure 2, Shaviv and Veizer, 2003) at the periods of lowest recorded
temperature anomalies and known ice ages around 440 Mbp and 170 Mbp. Values of
CO2 over these earlier ice ages
appear to have been considerably higher than those currently existing. Not unexpectedly, these findings triggered an almost immediate response from those committed to pCO2 being the primary driver of climate change, Royer et alia (2004). They pointed out that with appropriate adjustments to allow for the influence of seawater pH on δ18O values, the pCO2 levels become somewhat more closely aligned to the temperature anomalies. But all this is perhaps a discussion for another day!
While the periodic ice ages over the past 600 Ma are the most obvious symptoms of this cyclic process there is also considerable paleantological evidence to suggest that they have been occurring, possibly with even more extreme variations in temperature, for very much longer than this. If as suggested above an important driver for the occurrence of ice ages is variations in cosmic ray flux, and if as seems likely such variations have been occurring for periods longer than the Phanerozoic, then it is more than likely the Earth has been experiencing ice ages over equally long periods. As shown in Figure 1 cyclic surface temperature changes with periods of the order of 120Ma could be expected to penetrate the full depth of the Earth's crust.
While the periodic ice ages over the past 600 Ma are the most obvious symptoms of this cyclic process there is also considerable paleantological evidence to suggest that they have been occurring, possibly with even more extreme variations in temperature, for very much longer than this. If as suggested above an important driver for the occurrence of ice ages is variations in cosmic ray flux, and if as seems likely such variations have been occurring for periods longer than the Phanerozoic, then it is more than likely the Earth has been experiencing ice ages over equally long periods. As shown in Figure 1 cyclic surface temperature changes with periods of the order of 120Ma could be expected to penetrate the full depth of the Earth's crust.
And of course there are now widely recognised shorter period variations in temperature anomalies that are determined by Earth’s interaction with our nearer celestial neighbours. Glacials and inter-glacials during ice ages:Changes in the Earth’s elliptic orbit around the sun together with the cyclic changes in the tilt and precession of the axis of spin relative to the orbital plane, are regarded as the chief sources of the massive changes in climate that have seen the development of periodic glacials and inter-glacials over the duration of an ice ages.
This periodic model explaining the known cyclic changes of Earth temperatures was
first put forward by Croll (1864, 1875) and although discredited in the next 60 or so years was finally given a more soundly based astronomical confirmation by Milankovic (1920, 1941). The so called Croll-Milankovic model is now regarded as a major source for the very large changes in level of solar radiation reaching the Earth’s surface and for the associated large changes in temperature gradient through the upper few km of the Earth’s crust. One of the direct consequences of these cycles can be seen in the evidence of permafrost penetration during a glacial period (Washburn, 1979).
With periods of around ti = 20 ka for the variations of tilt angle and tp = 40 ka for the precession of the axis of spin, and around to = 110 ka for the change from the eccentric elliptic orbit to the basically circular orbit of the Earth, the intensity of the temperature changes at a given location on the Earth’s surface are anticipated to show a time dependence like those shown in Figure 3. The phasing of the orbital cycles along with the tilt and precession of the spin axis will result in changes being differentially experienced within the northern and southern hemispheres. How intense are these changes in temperature and how deep they penetrate into the Earth’s crust are still being debated, but as shown in Figure 1 they could be having significant influence down to depths of around 4 km.
first put forward by Croll (1864, 1875) and although discredited in the next 60 or so years was finally given a more soundly based astronomical confirmation by Milankovic (1920, 1941). The so called Croll-Milankovic model is now regarded as a major source for the very large changes in level of solar radiation reaching the Earth’s surface and for the associated large changes in temperature gradient through the upper few km of the Earth’s crust. One of the direct consequences of these cycles can be seen in the evidence of permafrost penetration during a glacial period (Washburn, 1979).
With periods of around ti = 20 ka for the variations of tilt angle and tp = 40 ka for the precession of the axis of spin, and around to = 110 ka for the change from the eccentric elliptic orbit to the basically circular orbit of the Earth, the intensity of the temperature changes at a given location on the Earth’s surface are anticipated to show a time dependence like those shown in Figure 3. The phasing of the orbital cycles along with the tilt and precession of the spin axis will result in changes being differentially experienced within the northern and southern hemispheres. How intense are these changes in temperature and how deep they penetrate into the Earth’s crust are still being debated, but as shown in Figure 1 they could be having significant influence down to depths of around 4 km.
Much of the above post has been taken from the paper "On the Causes of Vertical Motions of Lithosphere", James G A Croll, Frontiers meeting, Geological Society of London, November, 2011.
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