A new twist to Pleistocene climate cycles

The combined gravitational pulls of the sun and moon modulate variations in local tidal range. High spring tides occur when the two bodies are opposed at full moon or in roughly the same direction at new Moon. When the positions of sun and moon are at right angles (1st quarter and 3rd quarter) their gravitational pulls partly cancel each other to give neap tides. Consequently, there are two tidal cycles every lunar month.  In a similar way, the varying gravitational pulls of the planets during their orbital cycles impart a repetitive harmony to Earths astronomical behaviour. But their combined effects are on the order of tens of thousand years. Milutin Milankovich (1879-1958), a Serbian engineer, pondered on the possible causes of Earth’s climatic variations, particularly the repetition of ice ages. He was inspired by 19th century astronomers’ suggestion that maybe the gravitational effects of other planets might be a fruitful line of research. Milankovich focussed on how the shape of Earth’s orbit, the tilt of its rotational axis and the way the axis wobbles like that of a spinning top affect the amount of solar heating at all points on the surface: the effects of varying eccentricity, obliquity and precession, respectively.

 Earlier astronomers had calculated cycles of gravitational effects on Earth of the orbits of Jupiter and Saturn of the three attributes of Earth’s astronomical behaviour and found periods of about 100, 41 and 23 thousand years (ka) respectively. The other 3 inner planets and the much more distant giants Uranus and Neptune also have gravitational effects on Earth, but they are negligible compared with those of the two nearest giant planets, because gravitation force varies with mass and inversely with the square of distance. Sadly, Milankovich was long dead when his hypothesis of astronomical climate forcing was verified in 1976 by frequency analysis of the record of oxygen isotopes in foraminifera found in two ocean sediment core from the Southern Indian Ocean. It revealed that all three periods interfered in complex ways during the Late Pleistocene, to dominate variations in sea-surface temperatures and the fluctuating volume of continental ice sheets for which δ18O is a proxy (see: Odds and ends about Milankovich and climate change; February 2017).

Precession of the axis of a spinning top and that of the Earth. At present the northern end of Earth’s axis points to what we now call the Pole Star. Around 11.5 ka from now it will point to the star Vega

This was as revolutionary for climatology as plate tectonics was for geology. We now know that in the early Pleistocene glacial-interglacial cycles were in lockstep with the 41 ka period of axial obliquity, and since 700 ka followed closely – but not perfectly – the 100 ka orbital eccentricity forcing. The transitional period between 1.25 and 0.7 Ma (the Mid-Pleistocene Transition or MPT) suggested neither one nor the other. Milankovich established that axial tilt variations have the greatest influence on solar heating, so the early 41 ka cycles were no surprise. But the dominance of orbital eccentricity on the last 700 ka certainly presented a puzzle, for it has by far the weakest influence on solar heating: 10 times less than those of axial obliquity and precession. The other oddity concerns the actual effect of axial precession on climate change. There are no obvious 23 ka cycles in the climate record, despite the precession signal being clear in frequency analysis and its effect on solar heating being almost as powerful as obliquity and ten times greater than that of orbital eccentricity. Precessional wobbling of the axis controls the time of year when one hemisphere or the other is closest to the Sun. At one extreme it will be the Northern and 11.5 ka later it will be the Southern. The times of solstices and equinoxes also change relative to the calendar that we use today.

There is an important, if obvious, point about astronomical forcing of climate. It is always there, with much the same complicated interactions between the factors: human activities have absolutely no bearing on them. Climatic ‘surprises’ are likely to continue!

Changes in ice-rafted debris (IRD) since 1.7 Ma in a sediment core from the North Atlantic (orange fill) compared with its oxygen-isotope (δ18O) record of changes in continental ice cover (blue fill). At the top are the modelled variations in 23 ka axial precession (lilac) and 41 ka obliquity (green). The red circles mark major interglacial episodes, blue diamonds show the onset of significant ice rafting and orange diamonds are terminations of ice-rafting (TIR). (Credit: Barker et al., Fig. 2)

Sea temperature and ice-sheet volume are not the only things that changed during the Pleistocene. Another kind of record from oceanic sediments concerns the varying proportion in the muddy layers of abnormally coarse sand grains and even small pebbles that have been carried by icebergs; they are known as ice-rafted debris (IRD). The North Atlantic Ocean floor has plenty of evidence for them appearing and disappearing on a layer-by-layer basis. They were first recognised in 1988 by an oceanographer called Helmut Heinrich, who proposed that six major layers rich in IRD in North Atlantic cores bear witness to iceberg ‘armadas’ launched by collapse, or ablation, at the front of surging ice sheets on Scandinavia, Greenland and eastern Canada. Heinrich events, along with Dansgaard-Oeschger events (rapid climatic warming followed by slower cooling) in the progression to the last glacial maximum have been ascribed to a variety of processes  operating on a ‘millennial’ scale. However, ocean-floor sediment cores are full of lesser fluctuations in IRD, back to at least 1.7 Ma ago. That record offers a better chance of explaining fluctuations in ice-sheet ablation. A joint European-US group has investigated their potential over the last decade or so (Barker, S. et al. 2022. Persistent influence of precession on northern ice sheet variability since the early Pleistocene. Science, v. 376, p. 961-967; DOI: 10.1126/science.abm4033). The authors noted that in each glacial cycle since 1.7 Ma the start of ice rafting consistently occurred during a time of decreasing axial obliquity. Yet the largest ablation events were linked to minima in the precession cycles. In the last 700 ka, such extreme events are associated with the terminations of each ice age.

In the earlier part of the record, the 41 ka obliquity ‘signal’ was sufficient to drive glacial-interglacial cycles, hence their much greater regularity and symmetry than those that followed the Mid-Pleistocene Transition. The earlier ice sheets in the Northern Hemisphere also had consistently smaller extents than those after the MPT. Although the records show a role for precession in pre-MPT times in the form of ice-rafting events, the lesser effect of precession on summer warming at higher latitudes, compared with that of axial obliquity, gave it no decisive influence. After 700 ka the northern ice sheets extended much further south – as far as 40°N in North America – where summer warming would always have been commensurately greater than at high northern latitudes. So they were more susceptible to melting during the increased summer warming driven by the precession cycles. When maximum summer heating induced by axial precession in the Northern Hemisphere coincided with that of obliquity the ice sheets as a whole would have become prone to catastrophic collapse.

It is hard to say whether these revelations have a bearing on future climate. Of course, astronomical forcing will continue relentlessly, irrespective of anthropogenic greenhouse gas emissions. Earth has been in an interglacial for the last 11.5 ka, since the Younger Dryas; i.e. about half a precession cycle ago. The combination of obliquity- and precession-driven influences suggest that climate should be cooling and has been since 6,000 years ago, until the Industrial Revolution intervened. Can the gravitational pull of the giant planets prevent a runaway greenhouse effect, or will human effects defy astronomical forces that continually distort Earth’s astronomical behaviour?

The Mid-Pleistocene Transition: when glacial cycles changed to 100 ka

Before about a million years ago the Earth’s overall climate repeatedly swung from warm to cool roughly every 41 thousand years. This cyclicity is best shown by the variation of oxygen isotopes in sea-floor sediments. That evidence stems from the tendency during evaporation at the ocean surface for isotopically light  oxygen (16O) in seawater to preferentially enter atmospheric water vapour relative to 18O.  During cool episodes more water vapour that falls as snow at high latitudes fails to melt, so that glaciers grow. Continental ice sheets therefore extract and store 16O so that the proportion of the heavier 18O increases in the oceans. This shift shows up in the calcium carbonate (CaCO­3) shells of surface-dwelling organisms whose shells are preserved in sea-floor sediment. When the climate warms, the ice sheets melt and return the excess of 16O back to ocean-surface water, again marked by changed oxygen isotope proportions in plankton shells. The first systematic study of sea-floor oxygen isotopes over time revolutionised ideas about ancient climates in much the same way as sea-floor magnetic stripes revealed the existence of plate tectonics. Both provided incontrovertible explanations for changes observed in the geological record. In the case of oxygen isotopes climatic cyclicity could be linked to changes in the Earth’s orbital and rotational behaviour: the Milankovich Effect.

Glacial-interglacial cycles during the Pleistocene

The 41 ka cycles reflect periodic changes in the angle of the Earth’s rotational axis (obliquity), which have the greatest effect on how much solar heating occurs at high latitudes. However, between about 1200 and 600 ka the fairly regular, moderately intense 41 ka climate cycles shifted to more extreme, complex and longer 100 ka cycles at the ‘Mid-Pleistocene Transition’ (MPT). They crudely match cyclical variations in the shape of Earth’s orbit (eccentricity), but that has by far the least influence over seasonal solar heating. Moreover, modelling of the combined astronomical climate influences through the transition show little, if any, sign of any significant change in external climatic forcing. Thirty years of pondering on this climatic enigma has forced climatologists to wonder if the MPT was due to some sort of change in the surface part of the Earth system itself.

There are means of addressing the general processes at the Earth’s surface and how they may have changed by using other aspects of sea-floor geochemistry (Yehudai, M. and 8 others 2021. Evidence for a Northern Hemispheric trigger of the 100,000-y glacial cyclicity. Proceedings of the National Academy of Sciences, v. 118, article e2020260118; DOI: 10.1073/pnas.2020260118). For instance the ratio between the abundance of the strontium isotope 87Sr to that of 86Sr in marine sediments tells us about the progress of continental weathering around a particular ocean basin. The 87Sr/ 86Sr ratio is higher in rocks making up the bulk of the crystalline continental crust than that in basalts of the oceanic crust. That ratio is currently uniform throughout all ocean water. During the Cenozoic Era the ratio steadily increased in sea-floor sediments, reflecting the continual weathering and erosion of the continents. In the warm Pliocene (5.3 to 2.8 Ma) 87Sr/ 86Sr remained more or less constant, but began increasing again at the start of the Pleistocene with the onset of glaciation in the Northern Hemisphere. At about 1450 ka it began to increase more rapidly suggesting increased weathering, and then settled back to its earlier Pleistocene rate after 1100 ka. Another geochemical contrast between the continental and oceanic crust lies in the degree to which the ratio of two isotopes of neodymium (143Nd/144Nd) in rocks deviates from that in the Earth’s mantle – modelled from meteorite geochemistry – a measure signified by ЄNd. Magmatic rocks and young continental rocks have positive ЄNd values, but going back in time continental crust has increasingly negative ЄNd.

Yehudai et al analysed cores from deep-sea sediments that had been drilled between 41°N and 43°S in the Atlantic Ocean floor. They targeted layers designated as glacial and interglacial from their oxygen isotope geochemistry at different levels in the cores to check how ЄNd varied with time. The broad variations within each core look much the same, although at increasingly negative values from south to north, except in one case. The data from the most northerly Atlantic core show far more negative values of ЄNd, in both glacial and interglacial layers at around 950 ka ago, than do cores further to the south. The authors interpret this anomaly as showing a sudden increase in the amount of very old continental rocks – with highly negative ЄNd – that had become exposed at and ground from the base of the great northern ice sheets of North America, Greenland and Scandinavia. At present, the shield areas where the great ice sheets occurred until about 11 ka are almost entirely crystalline Precambrian basement, including the most ancient rocks that are known. Although broadly speaking the shields now have low relief, they are extremely rugged terrains of knobbly basement outcrops and depressions filled with millions of lakes. In the earlier Cenozoic they were covered by younger sedimentary rocks and soils formed by deep weathering, with less-negative ЄNd values. The authors conclude that around 950 ka that younger cover had largely been removed by glacial every every 41 ka or so since about 2.6 Ma ago, when glaciation of the Northern Hemisphere began.

The surface on which the North American ice sheet moved – typical Canadian Shield.

So what follows from that ЄNd anomaly? Yehudai et al suggest that in earlier Pleistocene times each successive ice sheet rested on soft rock; i.e. their bases were well lubricated. As a result, glaciers quickly reached the coast to break up and melt as icebergs drifted south. Exposure of the deeper, very resistant crystalline basement resulted in much more rugged base, as can be seen in northern Canada and Scandinavia today. Friction at their bases suddenly increased, so that much more ice was able to build up on the great shields surrounding the Arctic Ocean than had previously been possible. Shortly after 950 ka the sea-floor cores also reveal that deep ocean circulation weakened significantly in the following 100 ka. The influence on climate of regular, 41 ka changes in the tilt of the Earth’s rotational axis could therefore not be sustained in the later Pleistocene. The ice sheets could neither melt nor slide into the sea sufficiently quickly; indeed, bigger and more durable ice sheets would reflect away more solar heating than was previously possible as glacial gave way to interglacial. The 41 ka astronomical ‘pacemaker’ still operated, but ineffectually. A new and much more complex climate cyclicity set in. Insofar as climate change became stabilised, an overall ~100 ka pulsation emerged. Whether or not this fortuitously had the same pace as the weak influence of Earth’s changing orbital eccentricity remains to be addressed. The climate system just might be too complicated and sensitive for us ever to tell: it may even have little relevance in a climatically uncertain future.

See also: Why did glacial cycles intensify a million years ago? Science Daily, 8 November 2021.

The mid-Pleistocene transition

As shown by oxygen-isotope records from marine sediments, before about 1.25 Ma global climate cycled between cold and warm episodes roughly every 41 ka. Between 1.25 to 0.7 Ma these glacial-interglacial pulses lengthened to the ~100 ka periods that have characterised the last seven cycles that were also marked by larger volume of Northern Hemisphere ice-sheet cover during glacial maxima. Both periodicities have been empirically linked to regular changes in the Earth’s astronomical behaviour and their effects on the annual amount of energy received from the Sun, as predicted by Milutin Milankovich. As long ago as 1976 early investigation of changes of oxygen isotopes with depth in deep-sea sediments had revealed that their patterns closely matched Milankovich’s  hypothesis. The 41 ka periodicity matches the rate at which the Earth’s axial tilt changes, while the ~100 ka signal matches that for variation in the eccentricity of Earth’s orbit. 19 and 24 ka cycles were also found in the analysis that reflect those involved in the gyroscope-like precession of the axis of rotation. Surprisingly, the 100 ka cycling follows by far the weakest astronomical effect on solar warming yet the climate fluctuations of the last 700 ka are by far the largest of the last 2.5 million years. In fact the 2 to 8 % changes in solar heat input implicated in the climate cycles are 10 times greater than those predicted even for times when all the astronomical influences act in concert. That and other deviations from Milankovich’s hypothesis suggest that some of Earth’s surface processes act to amplify the astronomical drivers. Moreover, they probably lie behind the mid-Pleistocene transition from 41 to 100 ka cyclicity. What are they? Changes in albedo related to ice- and cloud cover, and shifts in the release and absorption of carbon dioxide and other greenhouse gases are among many suggested factors. As with many geoscientific conundrums, only more and better quality data about changes recorded in sediments that may be proxies for climatic variations are likely to resolve this one.

Adam Hazenfratz of ETH in Zurich and colleagues from several other European countries and the US have compiled details about changing surface- and deep-ocean temperatures and salinity – from δ18O and Mg/Ca ratios in foraminifera shells from a core into Southern Ocean-floor sediments – that go back 1.5 Ma (Hazenfratz, A.P. and 9 others 2019. The residence time of Southern Ocean surface waters and the 100,000-year ice age cycle. Science, v. 363, p. 1080-1084; DOI: 10.1126/science.aat7067). Differences in temperature and salinity (and thus density) gradients show up at different times in this critical sediment record. In turn, they record gross shifts in ocean circulation at high southern latitudes that may have affected the CO2 released from and absorbed by sea water. Specifically, Hazenfratz et al. teased out fluctuations in the rate of  mixing of dense, cold and salty water supplied to the Southern Ocean by deep currents with less dense surface water. Cold, dense water is able to dissolve more CO2 than does warmer surface water so that when it forms near the surface at high latitudes it draws down this greenhouse gas from the atmosphere and carries it into long-term storage in the deep ocean when it sinks. Deep-water formation therefore tends to force down mean global surface temperature, the more so the longer it resides at depth. When deep water wells to the surface and warms up it releases some of its CO2 content to produce an opposite, warming influence on global climate. So, when mixing of deep and surface waters is enhanced the net result is global warming, whereas if mixing is hindered global climate undergoes cooling.

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The Southern Ocean, where most dissolved and gaseous carbon dioxide are emitted and absorbed by seawater (Credit: British Antarctic Survey)

The critical factor in the rate of mixing deep with surface water is the density of that at the surface. When its salinity and density are low the surface water layer acts as a lid on what lies beneath, thereby increasing the residence time of deep water and the CO2 that it contains. This surface ‘freshening’ in the Southern Ocean seems to have begun at around 1.25 Ma and became well established 700 ka ago; that is, during the mid-Pleistocene climate transition. The phenomenon helped to lessen the greenhouse effect after 700 ka so that frigid conditions lasted longer and more glacial ice was able to accumulate, especially on the northern continents. This would have made it more difficult for the 41 ka astronomically paced changes in solar heating to have restored the rate of deep-water mixing to release sufficient CO2 to return the climate to interglacial conditions That would lengthen the glacial-interglacial cycles. The link between the new 100 ka cyclicity and very weak forcing by the varying eccentricity of Earth’s orbit may be fortuitous. So how might anthropogenic global warming affect this process? Increased melting of the Antarctic ice sheet may further freshen surface waters of the Southern Ocean, thereby slowing its mixing with deep, CO2-rich deep water and the release of stored greenhouse gases. As yet, no process leading to the decreased density of surface waters between 1.25 and 0.7 Ma has been suggested, but it seems that something similar may attend global warming.

Related articles: Menviel, L. 2019. The southern amplifier. Science, v. 363, p. 1040-1041; DOI: 10.1126/science.aaw7196; The deep Southern Ocean is key to more intense ice ages (Phys.org)

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