The Palaeocene–Eocene (P-E) boundary at 55.8 Ma marks the most dramatic biological changes since the mass extinction at the Cretaceous-Palaeogene boundary 10 million years earlier. They included the rapid expansions of mammals and land plants and major extinction of deep-water foraminifera. It was a time of sudden global warming (5-10°C in 10-20 ka) superimposed on the general Cenozoic cooling from the ‘hothouse’ of the Cretaceous Period. It coincided with a decrease in the proportion of 13C in marine carbonates. Because photosynthesis, the source of organic carbon, favours light 12C, such a negative δ13C “spike” is generally ascribed to an unusually high release of organic carbon to the atmosphere. The end-Palaeocene warming may have resulted from a massive release of methane from gas-hydrate buried in shallow seafloor sediments. But another process may yield such a signature; massive burning of organic material at the land surface. Since its discovery, the P-E thermal maximum has been likened to the situation that we may face should CO2 emissions from fossil-fuel burning continue to rise without control. Unsurprisingly, funds are more easily available for research on this topic than, say, ‘Snowball Earth’ events.
Three seafloor sediment cores off the east coast of the US that include the P-E boundary have been found to contain evidence for an impact that occurred at the time of the δ13C “spike” (Schaller, M.F. et al. 2016. Impact ejecta at the Paleocene-Eocene boundary. Science, v. 354, p. 225-229). The evidence is dominated by tiny spherules and tear-shaped blobs of glass, some of which contain tiny crystals of shocked and high-temperature forms of silica (SiO2). These form part of the suite of features that have been used to prove the influence of asteroid impacts. Two other onshore sites have yielded iridium anomalies at the boundary, so it does look like there was an impact at the time. The question is, was it large enough either to cause vast amounts of methane to blurt out from shall-water gas hydrates or set the biosphere in fire? Two craters whose age approximates that of the P-E boundary are known, one in Texas the other in Jordan, with diameters of 12 and 5 km respectively; far too small to have had any global effect. So either a suitably substantial crater of the right age is hidden somewhere by younger sediments or the association is coincidental – the impact that created the Texan crater could conceivably have flung glassy ejecta to the area of the three seafloor drilling sites.
Almost coinciding with the spherule-based paper’s publication another stole its potential thunder. Researchers at Southampton University used a mathematical model to investigate how a methane release event might have unfolded (Minshull, T.A. et al. 2016. Mechanistic insights into a hydrate contribution to the Paleocene-Eocene carbon cycle perturbation from coupled thermohydraulic simulations. Geophysical Research Letters, v. 43, p. 8637-8644, DOI: 10.1002/2016GL069676). Their findings challenge the hypothesized role of methane hydrates in causing the sudden warming at the P-E boundary. But that leaves out the biosphere burning, which probably would have neded a truly spectacular impact.
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.