Chaos and the Palaeocene-Eocene thermal maximum

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

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.

Reconstruction of Coryphodon. Image via Wikipedia