At https://phys.org/news/2023-08-late-pleistocene-glaciations-terminated-earth.html … the headline here is that new research finds Late Pleistocene glaciations weere terminated by earth’s axis tilt rather than orbital eccentricity. They are not, of course, thinking in terms of pole shift, or pole tilt. This may be the most simplistic method of creating an ice age, as far as the layman is concerned, but this is not on the table. Instead, it is a variation on Milankovitch. They are questioning the idea of 100,000 year timescales for glaciations. Or so it would seem, looking at it from face value. Eccentricity is in fact the weakest part of the Milankovitch effects. Obliquity refers to the varying tilt of the planet, somewhere between 22.1 and 24.5 degrees over 41,000 years. This is apparently the stronger effect. On top of that we have precession. In simple terms this is the direction earth’s axis is pointed – which can make the seasons more extreme in one hemisphere than in the other one.
Eccentricity has been the favoured driver of glacial cycles. Newer research has suggested that obliquity or precession cycles, are the main driver, 41,000 and 23,000 year cycles. The new research was designed to test this idea and they have come up with precession as the effect to most likely create the glacial cycles. Note … they only looked at the three aspects of Milankovitch. Catastrophism was ignored even though electromagnetism may actually provide a mechanism, via earth’s dipole, for movement at the axis of rotation. Likewise, the journey of the sun around its barycentre, very nearly 100,000 years, is also ignored. Cycles, themselves, are wonders of computer simulation.
Having said that, the process by which they arrived at their conclusion is interesting in itself. For example, the use of oxygen isotopes in foraminifera shells. These are found in sediment cores from deep sea locations. They were previously used to support the eccentricity effect but are now employed for the obliquity argument. Hence, that suggests the oxygen isotopes are better understood – or have also been subject to realignment of theory. Late Pleistocene data is said to show a rapid deviation in oxygen ratios. This is said to indicate a change in deep sea temperatures – as a result of variations in ice volumes.
Also, the dating of orbital change relied upon speleotherm records from China. Speleotherms are mineral deposits in caves – such as the formation of flowstone by a steady dripping process from the cave roof, or calcite stalactites etc. These are said to have produced a 640,000 year record. Just the job for evaluating the last few ice ages. Speleotherms are notoriously difficult to pin down and by incorporating them into IntCal dating methodology [in the Holocene] they have upended tree ring data by making it much older. The problem is that speleotherms are thought to be a gradual process with no significant ups and downs. Phases of more rapid formation, shall we say. In spite of that it is likely they are reasonably close to reality – apart from the odd catastrophic incident that might have impinged on the foraminifera and speleotherm records. Therefore, not so far removed from the 100,000 year cycle. A bit of faith is involved here. Even though they are working within the mainstream parameters.
The researchers identified 9 glacial termination events. Some 3 of these were partial events – leaving 6 full terminations [glacial to inter-glacial]. See https://doi.org/10.1038/s41561-023-01235-x …