The Pleistocene Epoch started about 2.6 million years ago and probably ended about 12,000 years ago, although some say that it is too soon to conclude this. The Pleistocene climate was very unstable, fluctuating between arctic and temperate conditions with corresponding ice advances and retreats (glacials and interglacial periods respectively). The number of glacials and their age was originally identified by examination of successive glacial deposits left behind after each ice advance. This had the inherent disadvantage that each new glaciation largely destroyed the deposits from the previous glaciation, making it difficult to identify the individual episodes. Six named glacial periods were originally identified using this method. Each glacial and interglacial period was named after regional events in the northern hemisphere but the names which were applied differed between regions making correlation difficult.
Marine Isotope Stages
In the mid 1950’s a method of estimating temperature changes during the Pleistocene was developed by measuring the ratio of two oxygen atoms with the same atomic number but with different mass (isotopes). These ratios were measured in shell fragments recovered from cores taken from sea-bed sediments. The ratio varied depending on the temperature of the seawater when the shells were being formed. This method enabled Ice Age temperature changes to be identified to a higher resolution and further back in time than the sedimentary technique and allowed more glaciations to be identified. These temperature variations were used to define Marine Isotope Stages (MIS).
The greater resolution of temperature changes, made possible by using MIS, promoted arguments concerning the scale of temperature change required to constitute a glacial or interglacial period with the result that many sub-stages were proposed. There are currently between ten and twelve major Pleistocene glacial periods which are recognised from MIS, with minor glacials and interglacials. 103 MIS’s have been recognised extending back to about 2.58 million years which is defined as the beginning of the Pleistocene. These are increasingly referred to by their MIS number, rather than names, where even numbers refer to glacial periods and MIS 1 is the current interglacial
This method has its critics. It is indirect indication of surface temperatures and only samples a few locations on the seabed which may not be representative of global temperatures. A problem recently highlighted is that it seems that the glacials and interglacials were not synchronous around the world, so there is some doubt as to whether the MIS are valid globally (Gibbard & Hughes, 2020).
Milankovitch Cycles
There is some evidence which suggests a global uniformity in the cause of these temperature changes. A correlation was recognised between the MIS and regular variations in the heat reaching the Earth from the Sun caused by changes in the distance of the Earth from the Sun. These changes are controlled by three types of variation in the Earth’s orbit around the Sun, its eccentricity, its precession and its tilt. They interact to produce combined cyclical variations which are called the Milankovitch Cycles after the person who worked out their periodicity. Three groups of cycles have been recognised based on their periodicity, 21-24,000 years, 41,000 years and 100,000 years. (The effects of Milankovitch Cycles have been recognised elsewhere in the geological record – see the note on “How were flints in chalk formed” which describes the effect of these cycles on the occurrence of flint bands).
Milankovitch also noted that the effect of changes in the Sun’s heat would vary across latitudes with the greatest effect being experienced at high latitudes where the total heat input is low. So near the poles, a small variation in the total heat input has a proportionally higher effect on the heat reaching the ground surface. In the tropics, where the Sun is overhead and the intensity of solar heat is great, small changes are not noticeable (Imagine being in a hall lit by 1000 candles. If a quarter was extinguished, you probably wouldn’t notice much of a difference. If you were in a hall lit by 4 candles and a quarter was extinguished, you probably would notice a difference).
The MIS temperature profiles show that the warming phases at the end of each glaciation occurred quicker than the cooling phases implying that the ice melts were relatively rapid. This is supported by the prevalence of meltwater sands and gravels seen in the geological record.
There is some evidence of very rapid temperature increases of 15°C over tens of years in the northern hemisphere during the Ice Age, with the magnitude reducing towards the equator and simultaneous but less marked cooling in the southern hemisphere. These are the Dansgaard-Oeschger Events (Masson-Delmotte, (2013) but not mentioned in Masson-Delmotte et al., (2021)).
The temperature profiles also indicate that the length of the glacial-interglacial cycles has increased, and the temperature variation has become more pronounced, since about 0.9 million years ago and appear to have switched from following the 41,000-year Milankovitch Cycle to following the 100,000-year Cycle. Bridgland, (2006) correlated end of glaciation River Terrace deposits with the 100,000 year Milankovitch Cycle. This suggests that, for the last 900,000 years, the glacial cycle has comprised approximately a 20,000-year interglacial and an 80,000-year glacial.
During the last 100,000 years glacial cycle there have been longer periods of interglacial than glacial conditions, although superimposed on an overall freezing trend (Fig.5.7, Murton & Ballantyne, 2017). This interpretation is supported by the fossil and archaeological records which record the movements of groups of large mammals following the waxing and waning of the ice sheet, and which were accompanied by hunter gatherer humans.
Andrew Coleman
Rev. 11/04/2025
References:
Bridgland, D. R. (2006). The Middle and Upper Pleistocene sequence in the Lower Thames: a record of Milankovitch climatic fluctuation and early human occupation of southern Britain. Proceedings of the Geologists’ Association, 117(3), 281–305. https://doi.org/10.1016/S0016-7878(06)80036-2
Gibbard, P. L., & Hughes, P. D. (2020). Terrestrial stratigraphical division in the quaternary and its correlation. Journal of the Geological Society, 178(2). https://doi.org/10.1144/jgs2020-134
Masson-Delmotte et al. (2021). IPCC, 2021: Climate Change 2021: The Physical Science Basis. Contribution of Working Group 1 to the 6th Assessment Report of the Intergovernmental Panel on Climate Change.
Masson-Delmotte, V. , et al. (2013). Fifth assessment report of the IPCC, Physical Science Basis, Ch. 5.
Murton, J. B., & Ballantyne, C. K. (2017). Periglacial and permafrost ground models for Great Britain. In Geological Society Engineering Geology Special Publication (Vol. 28, Issue 1, pp. 501–579). Geological Society of London. https://doi.org/10.1144/EGSP28.5
Geomusings 