Relating Phases of Deposition to Climate Cycles

Earlier posts, particularly those from 2012 to 2016, have suggested that a possible explanation for the periodic rises and falls of the Earth’s crust (here referred to as the lithosphere to include the gradually more liquid like upper mantle below the more solid upper rock) could relate to the very long-term cycles of Earth climate. This hypothesised relationship is predicated upon the effects of the changes in the disposition of ice, water and the troposphere overlying the lithosphere, and how they affect the rate at which the heat within the Earth’s core and mantle are escaping to the surface – referred to as the geothermal flux. If for example, the lithosphere is overlain with sea water this has the effect of increasing the geothermal flux due to the oceans capacity to suck out and absorb the Earth’s heat. In contrast, an overlying thick ice sheet with its interstitial pockets of water will reduce the geothermal flux by acting a little like the insulating effects provided by a tea cosy keeping the tea in a teapot hot. Or if the lithosphere is overlain with the atmospheric gases, it will experience a low heat flux due to the low thermal capacity of the atmosphere to suck out the Earth’s heat energy.

Because the heat flux is directly proportional to the thermal gradient, and because the temperature of both the mantle and the Earth’s surface are pretty much fixed, a high heat flux will relate to a thin lithosphere while a low heat flux will be associated with a thick lithosphere. In this model these thickness changes are postulated as being made possible through very long terms phase changes at the underside of the lithosphere. And since the more solid phases of lithosphere rock have a lower density than the mantle, a thick lithosphere will have its upper surface floating higher than a thin one. Hence, if very long-term climate cycles have the effect of altering the surface disposition of air, water and ice they may well help to explain how over wide regions the crust can rise to form mountains while being eroded and sink beneath the oceans to have mega sequences of sediments continuously laid down.  

All very interesting, but where is the evidence that this model has any relevance?

As previously suggested the Colorado Plateau provides an incredibly rich record of Earth’s vertical tectonics over at least the last 1.6 Ba and especially over the Phanerozoic eon - the last 540 Ma. As the previous few posts have demonstrated the exposed geological record tells us that this area has been subjected to periodic rises and erosion as well as periodic subsidence and sedimentation. But do these periodicities have any relationship with Earth climate cycles?  

Fig 9 Cycles of average earth surface temperatures over the Phanerozoic showing the correlation between: 1. the onset of ice-house periods and pulses of epeirogenic uplift and mountain building (upper bar charts); and 2. the onset of hot-house periods and pulses of deposition.      

 Fig 9 summarises the pulses in deposition for which the most robust temporal signals of vertical position, relating to the vertical movements of the Earth’s lithosphere, are the commencement of deposition caused by subsidence beneath average mean sea level (amsl) following a hiatus, marked by the existence of unconformities, in which there was either no sedimentation occurring or erosion after uplift has removed the evidence of any sedimentation that had occurred. At the intra-cratonic location of the Grand Canyon, subsidence below sea level, likely combined with indeterminate moderate to large rises in sea level, saw the start of new pulses in sedimentation occurring over the Phanerozoic at -525 Ma, -385 Ma, -265 Ma, and -140 Ma. These ages at which renewed pulses of sedimentation started, immediately above unconformities, are well defined and consequently marked with strong black bars on the lower bar chart of Fig 9. The black bars indicate the periods known to have produced continuous deposition. However, when sedimentation ceased or when erosion started are rather less well defined and so the ending of the deposition is marked with alternating black and yellow pulses indicating either non-deposition or uplift and erosion. 

Also plotted in Fig 9 are the geological reconstructions of the average surface temperatures over this same time period⁸ along with predictions of surface temperatures based upon analysis of the variations in cosmic ray flux⁹ experienced by the solar system. What is noteworthy in these plots are: the close, and possibly causal^8,9, relationships between the intensity of cosmic rays and climate cycles, and; the correlation between the onset of deposition as recorded by strata immediately above recorded major unconformities and their consistent phasing within the climate cycles. In each case, deposition is seen to commence soon (in geological terms) after earth climate emerges from an ice-house period, shown by the black sections of the lower of the upper bars in Fig 9, and enters into a period of hot house (shown perhaps confusingly as blue sections). After a long period of glacial and inter-glacial cycles during the ice-house period, it might be anticipated that ice erosion will have reduced continental land surface elevations in the vicinity of ice sheets to near sea level. This means that moderate rises in average sea levels, due to the full melting of ice sheet and permafrost accompanying the transition from average ice-house climate to hot-house, might be expected to inundate the low continental land surfaces – a clear precondition for the onset of marine sedimentation.