Tag: PETM

Milankovich precession and the Palaeocene-Eocene Thermal Maximum

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About 56 Ma ago there occurred some of the most dramatic biological changes since the mass extinction at the Cretaceous-Palaeogene boundary. They included rapid expansion and diversification of mammals and land plants, and a plunge in the number of deep-water foraminifera. Global cooling from the Cretaceous hothouse was rudely reversed by sudden global warming of about 5 to 10°C. Some climatologists have ascribed bugbear status to the Palaeocene-Eocene Thermal Maximum (PETM) as a possible scenario for future anthropogenic global warming. The widely accepted cause is a massive blurt into the Palaeocene atmosphere of greenhouse gases, but what caused it is enthusiastically debated. The climate shift is associated with a sudden decrease in the proportion of 13C in marine sediments: a negative spike in δ13C. Because photosynthesis favours the lighter 12C, organic matter has a low δ13C, so a great deal of buried organic carbon may have escaped from the ocean floor, most likely in the form of methane gas. However, massive burning of living terrestrial biomass would produce the same carbon-isotope signal, but absence of evidence for mass conflagration supports methane release. Methane is temporarily held in marine sediments in the form of gas hydrate (clathrate), an ice-like solid that forms at low temperatures on the deep seafloor. Warming of deep sea water or a decrease in pressure, if sea level falls, destabilise clathrates thereby releasing methane gas: the ‘clathrate gun hypothesis’. The main issue is what mechanism may have pulled the trigger for a monstrous methane release.

Massive leak of natural gas – mainly methane – off Sweden in the Baltic Sea, from the probably sabotaged Nord Stream pipeline. (Source: Swedish coastguard agency)

Many have favoured a major igneous event. Between 55.0 and 55.8 Ma basaltic magmatism– continuing today in Iceland – formed the North Atlantic Igneous Province. It involved large-scale intrusion of sills as well as outpourings of flood basalts and coincided with the initial rifting of Greenland from northern Europe (see: Smoking gun for end-Palaeocene warming: an igneous connection; July/August 2004). The occurrence of impact ejecta in end-Palaeocene sediments off the east coast of the US has spawned an extraterrestrial hypothesis for the warming, which could account for the negative spike in δ13C as the product of a burning terrestrial biosphere (see: Impact linked to the Palaeocene-Eocene boundary event; October 2016). Less headline-grabbing is the possibility that the event was part and parcel of the Milankovich effect: an inevitability in the complex interplay between the three astronomical components that affect Earth’s orbital and rotational behaviour: eccentricity, axial tilt and precession. A group of geoscientists from China and the US, led by Mingsong Li of Peking University, have investigated in minute detail the ups and downs of δ13C around 56 Ma in drill cores recovered from a sequence of Palaeocene and Eocene continental-shelf sediments in Maryland, USA (Li, M., Bralower, T.J. et al. 2022. Astrochronology of the Paleocene-Eocene Thermal Maximum on the Atlantic Coastal Plain. Nature Communications, v. 13, Article 5618; DOI: 10.1038/s41467-022-33390-x).

The study involved sampling sediment for carbon- and oxygen-isotope analysis at depth intervals between 3 and 10 cm over a 35 m section through the lower Eocene and uppermost Palaeocene. Calcium abundances in the core were logged at a resolution of 5 mm using an X-ray fluorescence instrument. The results link to variations in CaCO3 in the sediments across the PETM event. Another dataset involves semi-continuous measurements of magnetic susceptibility (MS) along the core. These measurements are able to indicate variations in delivery to the ocean of dissolved calcium and detrital magnetic minerals as climate and continental weathering vary through time. They are widely known to be good recorders of Milankovich cycles. After processing, the Ca and MS data sets show cyclical fluctuations relative to depth within the cores. ‘Tuning’ their frequencies to the familiar time series of Milankovich astronomical climate forcing reveals a close match to what would be expected if the climate fluctuations were paced by the 26 ka axial precession signal. My post of 17 June 2022 about the influence of precession over ‘iceberg armadas’ during the Pleistocene might be useful to re-read in this context. This correlation enabled the researchers to convert depth in the cores to time, so that the timing of fluctuations in carbon- and oxygen-isotope data that the PETM had created could be considered against various hypotheses for its cause. The ‘excursions’ of both began at the same time and reached the maxima of their changes from Palaeocene values over about 6,000 years. The authors consider that is far too long to countenance the release of methane as a result of asteroidal impact, or by massive burning of terrestrial vegetation. The other option that the beginning of the North Atlantic Igneous Province had been the trigger may also be ruled out on two grounds: the magmatism began earlier, and it continued for far longer. The onset of the PETM coincides with an extreme in precession-related climatic forcing. So Li et al. consider that a quirk in the Milankovich Effect could have played a role in triggering massive methane release. This might also explain features of the global calcium record in seafloor sediments as results of a brief period of ocean acidification during the PETM. Such an event would play havoc with carbonate-secreting organisms, such as foraminifera, by lowering the dissolved carbonate ion content on which they depend for their shells: hence their suffering considerable extinction. Of course, the other elements of astronomical forcing – eccentricity and axial tilt – would also have been operating on global climate at the time.  The long-term 100 and 405 ka eccentricity cycles may have played a role in amplifying warming, which may have resulted in increased burial of organic carbon and thus the amount of methane buried beneath the seabed.

When Greenland was a warm place

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On 14-15 August 2021 it rained for the first time since records began at the highest point on the Greenland ice cap. Summit Camp at 3.216 m is run by the US National Science Foundation, which set it up in 1989, and is famous for climate data gleaned from two deep ice cores there. This odd event came at a time when surface melting of the ice cap covered 870 thousand km2: over half of its total 1.7 million km2 extent: a sure sign of global warming. The average maximum temperature in August at Summit is -14°C, but since the mid 20th century the Arctic has been warming at about twice the global rate. Under naturally fluctuating climatic conditions during the Pleistocene, associated with glacial-interglacial cycles, Greenland may have been ice-free for extended periods, perhaps as long as a quarter of a million years around 1.1 Ma ago. If 75% of the up to 3 km thick ice on Greenland melted that would add 5 to 6 m to global sea level, perhaps as early as 2100 if current rates of climate warming persist.

The edge of the ice cap in NE Greenland (credit: Wikipedia)

The worst scenario is runaway warming on the scale of that which took place 56 Ma ago during the Palaeocene-Eocene Thermal Maximum (PETM) when global mean temperature rose by between 5 to 8°C at a rate comparable with what the planet is experiencing now as a result of growth in the world economy. In fact, the CO2 released during the PETM emerged at a rate that was only about tenth of modern anthropogenic emissions  Sediments that span the Palaeocene-Eocene boundary occur in NE Greenland, a study of which was recently published by scientists from Denmark, Greenland, the UK, Australia and Poland (Hovikoski, J. and 13 others 2021. Paleocene-Eocene volcanic segmentation of the Norwegian-Greenland seaway reorganized high-latitude ocean circulation. Communications Earth & Environment, v. 2, article 172; DOI: 10.1038/s43247-021-00249-w). The greenhouse world of NE Greenland that lay between 70 and 80°N then, as it still does, was an area alternating between shallow marine and terrestrial conditions, the latter characterised by coastal plain and floodplain sediments deposited in estuaries, deltas and lakes. They include coals derived from lush, wooded swamps, inhabited by hippo-like ungulates, primates and reptiles.

At that time the opening of the northern part of the North Atlantic had barely begun, with little chance for an equivalent of the Gulf Stream to have had a warming influence on the Arctic. Shortly after the PETM volcanism began in earnest, to form the flood basalts of the North Atlantic Igneous Province. Each lava flow is capped by red soil or bole: further evidence for a warm, humid climate and rapid chemical weathering. As well as lava build-up, tectonic forces resulted in uplift, effectively opening migration routes for animals and land plants to colonise the benign – for such high latitudes – conditions and perhaps escape the far hotter conditions further south.

The situation now is much different, with the potential for even more rapid melting of the Greenland ice cap to flood freshwater into the North Atlantic, as is currently beginning. Diluting surface seawater reduces its density and thus its tendency to sink, which is the main driving force that pulls warmer water towards high-latitudes in the form of the Gulf Stream. Slowing and even shutting down its influence may have an effect that contradicts the general tendency for global warming – a cooling trend at mid- to high latitudes with chaotic effects on atmospheric pressure systems, the jet stream and weather in general.

See also: Barham, M. et al. 2021. When Greenland was green: rapid global warming 55 million years ago shows us what the future may hold. The Conversation, 23 August 2021.

Chaos and the Palaeocene-Eocene thermal maximum

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The transition from the Palaeocene to Eocene Epochs (56 Ma) was marked by an abrupt increase in global mean temperature of about 5 to 8°C within about 10 to 20 thousand years. That is comparable to a rate of warming similar to that currently induced by human activities. The evidence comes from the oxygen isotopes and magnesium/calcium ratios in the tests of both surface- and bottom dwelling foraminifera. The event is matched by a similarly profound excursion in the δ13C of carbon-rich strata of that age, whose extreme negative value marks the release of a huge mass of previously buried organic carbon to the atmosphere. The Epoch-boundary coincides with the beginning of rapid diversification among mammals and plants that had survived the end-Cretaceous mass extinction some 10 Ma beforehand. The most likely cause was the release of methane, a more potent greenhouse gas than CO2, from gas hydrate buried just beneath the surface of sea-floor sediments on continental shelves. An estimated mass of 1.5 trillion tonnes of released methane has been suggested. Methane rapidly oxidizes to CO2 in the atmosphere, which dissolves to make rainwater slightly acid so that the oceans also become more acid; a likely cause for the mass extinction of foraminifera species at the boundary.

Since the discovery of the Palaeocene-Eocene Thermal Maximum (PETM) in the late-1990s a range of possible causes have been suggested. Releasing methane suddenly from sea-floor gas hydrates needs some kind of trigger, such as a steady increase in the temperature of ocean-bottom water to above the critical level for gas-hydrate stability. The late-Palaeocene witnessed slow global warming by between 3 to 5°C over 4 to 5 Ma. There are several hypotheses for this precursor warming, such as a direct CO2 release from the mantle by volcanic activity for which there are several candidates in the geological record of the Palaeocene. Such surface warming would have had to be transferred to the sea floor on continental shelves to destabilise gas hydrates, which implicates a change in oceanic current patterns. An extraterrestrial cause has also been considered (see Impact linked to the Palaeocene-Eocene boundary event, Earth-logs October 2016). Sediment cores from the North Atlantic off the eastern seaboard of the US have revealed impact debris including glass spherules and shocked mineral grains at the same level as the PETM, together with iridium in terrestrial sediments onshore of the same age: there are no such global signatures). But apart from two small craters in Texas and Jordan (12 and 5 km across, respectively) of roughly the same age, no impact event of the necessary magnitude for truly global influence is known. However, there may have been an altogether different triggering mechanism.

Since the confirmation of the Milanković-Croll hypothesis to explain the cyclical shifts in climate during the Pleistocene Epoch in terms of changes in Earth’s orbital characteristics induced by varying gravitational forces in the solar system, the findings have been used as an alternative means of dating other stratigraphic events that show cyclicity. In essence, the varying forces at work are inherently chaotic in a formally mathematical sense. Although Milanković cycles sometimes pop-up when ancient, repetitive stratigraphic sequences are analysed, consistently using the method as a tool to calibrate the geological record to an astronomical timescale breaks down for sediments older than about 50 Ma. Calculations disagree markedly beyond that time. Richard Zeebe and Lucas Lourens of the Universities of Hawaii and Utrecht tried an opposite approach, using the known geological records from deep-sea cores to calibrate the astronomical predictions and, in turn, used the solution to take the astronomical time scale further back than 50 Ma (Zeebe, R.E. & Lourens, L.J. 2019. Solar System chaos and the Paleocene–Eocene boundary age constrained by geology and astronomy. Science, v. 365, p. 926-929; DOI: 10.1126/science.aax0612). They reached back about 8 Ma, so putting the PETM in focus. As well as refining its age (56.01 ± 0.05 Ma) they showed that the PETM coincided with a 405 ka maximum in Earth’s orbital eccentricity lasting around 170 ka: a possible orbital trigger for the spike in temperature and δ13C together, with evidence for a period of chaos in the Solar System about 50 Ma ago. But, what did that chaos actually do, other than mess up orbital dating? To me it seems to suggest something narsty happening to the behaviour of the Giant Planets that are the Lords of the astronomical dance…

See also: Grabowski, M. 2019. Deep-sea sediments reveal solar system chaos: an advance in dating geologic archives. SOEST News

Tiny shrinking horses

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English: This reproduction of a painting of an...
Reconstruction of Sifrhippus. Image via Wikipedia

The earliest known ancestors of modern horses occur in Palaeogene mammal-rich terrestrial sediments of the northwestern US, particularly those of the Wind and Bighorn Basins. The first with clear horse-like features was Sifrhippus (formely Eohippus, or Hyracotherium), but famously it had four hoofed toes and was about the size of a household cat. Subsequent development to a single load-bearing toe has long formed one of the classic cases for evolution. Sifrhippus lived at the end of the Palaeocene. From the large numbers of well-preserved skeletons, this was a herding animal. The large numbers of fossils have also made it a candidate for testing a hypothesis that individuals of a mammal and bird species become smaller as climate warms: Bergmann’s Rule. The background to this view is that in modern warm-blooded or endothermic animal species individuals tend to be smaller the closer they are to the Equator.

The end of the Palaeocene was marked by a now well-documented rise in global surface temperature that left a marked sign of increased 13C in sediments spanning the Palaeocene-Eocene boundary, which is widely believed to have resulted from massive exhalations of methane from the seafloor. Bergmann’s Rule arose because there appears to be a general decrease in size of most mammal fossils through the P-E Thermal Maximum.  Sifrhippus lived through the event and indeed did undergo 30% decrease in size at the start of the carbon-isotope shift marking the PETM. Moreover, after the isotopic excursion its fossils indicate a 70% increase in size (Secord, R. and 8 others 2012. Evolution of the earliest horses driven by climate change in the Paleocene-Eocene Thermal Maximum. Science, v. 335, p. 959-962).

The study was of Sifrhippus and other mammals over a period representing several thousand generations. It broke new ground in two ways: it used the size of the horses’ teeth to estimate body mass, and teeth of a variety of mammals afforded systematic measurements of both carbon and oxygen isotopes. The carbon isotopic analyses pin-pointed the span of the PETM locally, while oxygen isotopes charted local changes in average temperature. The results show remarkable coherence with Bergmann’s Rule, but reveal other interesting aspects of the PETM in North America. Oxygen-isotope in the teeth of different mammal species give some idea of their diet and habitat. Sifrhippus shows the highest enrichment of 18O in its teeth, which suggests that it ate leaves from which water evaporation selectively removed the lighter 16O, i.e. in open, dry areas. Another ubiquitous fossil, Coryphodon, consistently has lower 18O than other mammals, signifying that it was water-loviong and ate aquatic plants, i.e. not subject to evaporation. Matching O-isotopes for the two species across the PETM shows a greater shift in 18O for Sifrhippus than for Coryphodon, which suggests that hidden in the O-isotope record of temperature is information about rainfall variations during the PETM. To further support Bergmann’s Rule, changes in the size of Sifrhippus, do not correlate with the aridity index. So it seem that heat alone was responsible for dwarfing – the other possibility considered by the researchers was that decreased availability or quality of diet could have been responsible.

Coryphodon
Reconstruction of Coryphodon. Image via Wikipedia

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