Milankovich precession and the Palaeocene-Eocene Thermal Maximum

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.

How did monkeys get to South America?

This is one of the great mysteries of palaeontology. There are plenty of monkey species in South and Central America and in Mexico. They are members of five families, collectively known as platyrrhine (‘flat-nosed’) primates, all having wide-spaced nostrils compared with the primates of the ‘Old World’. They are the catarrhines (‘hook-nosed). There are other differences, such as the unique prehensile tails of many ‘New World’ monkeys. The two monkey groups are genetically related, but their last common ancestor is estimated, using the ‘molecular clock’ approach, to have lived at least 31 Ma ago, in the Oligocene. The earliest platyrrhine primates of the Americas date to around the Eocene-Oligocene boundary (34 Ma). Interestingly, they are predated by the earliest rodent remains by only a few million years (41 Ma). Both primates and rodents had been inhabiting other continents long before this, so it is certain that, somehow, members of the two groups must have migrated to become isolated in the Americas. The problem lies with palaeogeography. By the late-Eocene the Americas were completely separated from Eurasia and Africa by the actively spreading Atlantic Ocean, then between 1500 to 2000 km wide. Complete isolation of the Americas dates from around 60 Ma ago, when the northernmost part of the North Atlantic began to open. The South Atlantic had become a wide ocean long before that, beginning in the far south during the early Cretaceous Period (138 Ma), with the mid-Atlantic Ridge steadily propagating northwards thereafter.

35 Ma
World palaeogeography at the Eocene-Oligocene boundary. The site of a recent fossil primate discovery in eastern Peru is marked by the yellow dot.

Since 60 Ma years ago it would have been impossible for the ancestors of ‘New World’ rodents and primates simply to have walked there. In any case the earliest known primate fossils from China are just 55 Ma old. Island hopping across the far northern, narrowest part of the North Atlantic during the Eocene may have been possible, although many islands there could have been subject to intense volcanic activity, as is Iceland today. The only alternative is a sea trip across the mighty Atlantic. Unless, that is, there is a hitherto undiscovered land bridge. The Walvis-Rio Grande Rise – a hotspot track – that spans the South Atlantic Ocean floor from Namibia to São Paulo in Brazil, has been the subject of some speculation since it is dotted with sea mounts and in places has micro-continental fragments. But it is too deep to have emerged as a result of falls in sea level. To suggest that the > 1500 km migration to the Americas of ancestral platyrrhine primates, or rodents for that matter, involved their being carried on drifting vegetation rafts obviously invites scepticism. For starters, why only two groups of animals? Or, could that imply a one-off event carrying only ancestral rodents and monkeys? It would need to be a special kind of raft: large enough to provide security against storm waves; immune to waterlogging, and carrying substantial food. On the plus side, there are powerful east-to-west currents in the equatorial Atlantic and trade winds going in the same direction, thanks to the Coriolis effect and ultimately Earth’s rotation. Islands as ‘way-points’ or temporary refuges are less convincing, for they would have to be heavily vegetated themselves to provide onward rafts. Apparently, in the absence of anything more plausible, Sherlock Holmes’s principle points to trans-Atlantic rafting.

This issue recently became ‘live’ again, with a fossil discovery in Peru, in an upper Amazon river bank close to at the Andean watershed but around 4000 km from the east coast of South America (Seiffert, E.R.  et al. 2020. A parapithecid stem anthropoid of African origin in the Paleogene of South America. Science, v. 368, p. 194-197; DOI: 10.1126/science.aba1135). The site had previously yielded both playrrhine monkey and rodent remains. To these have been added teeth with distinct similarities to those of fossils previously known only from Egypt, Libya and Tanzania: parapithecid anthropoids whose teeth are sufficiently different from those of platyrrhines to warrant a separate suborder, which includes baboons and primates. This is the only trace of parapithecids in South America and it may be assumed that, although they were possibly fellow-travellers with New World monkey ancestors, they were unable to compete and became extinct.

However, there is another possibility. Albeit with a sparse record of fossils resembling primates, North America does have at least one. George Gaylord Simpson (1902-1984), once the doyen of US palaeontologists, found a marmoset-like fossil in the early-Eocene of Wyoming, which he named Teilhardinia after the French Jesuit philosopher and palaeontologist Teihard de Chardin. It is about 56 Ma old and the size of a mouse. So was this diminutive the pioneer New World primate that crossed the northern North Atlantic? If so it would have had an equally perilous journey to reach South America, because the Isthmus of Panama was also open sea until around 4.5 Ma ago. With Teilhardinia, the plot thickens for there are several known species: in the US T. brandti from Wyoming and T. magnoliana from Mississippi; in Asia and Europe T. asiatica and T. belgica respectively. An embarrassment of riches that may well ignite: it has been suggested that North American Teilhardinia may have been the first of all primates and spread across the Eocene forests of North America, Europe and Asia. That hypothesis sort of implies that the entry of monkeys into South America may well have started with the tiny continent hopper who passed on its proclivities to its descendants in Africa

See also: Godinot, M. 2020. Rafting on a wide and wild ocean. Science, v. 368, p. 136-137; DOI: 10.1126/science.abb4107; Ancient teeth from Peru hint now-extinct monkeys crossed Atlantic from Africa. Science Daily, 9 April 2020. Oldest-known ancestor of modern primates may have come from North America, not Asia. Science Daily, 29 November 2018