Paleocene–Eocene Thermal Maximum

Whether they only occurred during the long-term warming, and whether they are causally related to apparently similar events in older intervals of the geological record (e.g. the Toarcian turnover of the Jurassic) are open issues.

[22] Proxy data from Esplugafereda in northeastern Spain shows a rapid +8 °C temperature rise, in accordance with existing regional records of marine and terrestrial environments.

First, a prominent negative excursion in the carbon isotope composition (δ13C) of carbon-bearing phases characterizes the PETM in numerous (>130) widespread locations from a range of environments.

This is because the total duration of the CIE, from the rapid drop in δ13C through the near recovery to initial conditions, relates to key parameters of our global carbon cycle, and because the onset provides insight to the source of 13C-depleted CO2.

At this location, the PETM CIE, from start to end, spans about 2 m. Long-term age constraints, through biostratigraphy and magnetostratigraphy, suggest an average Paleogene sedimentation rate of about 1.23 cm/1,000yrs.

[47] A different study, based on a revised orbital chronology and data from sediment cores in the South Atlantic and the Southern Ocean, calculated a slightly shorter duration of about 170,000 years.

[49] Age constraints at several deep-sea sites have been independently examined using 3He contents, assuming the flux of this cosmogenic nuclide is roughly constant over short time periods.

[51] Some authors have suggested that the magnitude of the CIE may be underestimated due to local processes in many sites causing a large proportion of allochthonous sediments to accumulate in their sedimentary rocks, contaminating and offsetting isotopic values derived from them.

[57] Very high precipitation is also evidenced in the Cambay Shale Formation of India by the deposition of thick lignitic seams as a consequence of increased soil erosion and organic matter burial.

[59] In Cap d'Ailly, in present-day Normandy, a transient dry spell occurred just before the negative CIE, after which much moister conditions predominated, with the local environment transitioning from a closed marsh to an open, eutrophic swamp with frequent algal blooms.

[65] East African sites display evidence of aridity punctuated by seasonal episodes of potent precipitation, revealing the global climate during the PETM not to be universally humid.

[67] Evidence from Forada in northeastern Italy suggests that arid and humid climatic intervals alternated over the course of the PETM concomitantly with precessional cycles in mid-latitudes, and that overall, net precipitation over the central-western Tethys Ocean decreased.

[68] The amount of freshwater in the Arctic Ocean increased, in part due to Northern Hemisphere rainfall patterns, fueled by poleward storm track migrations under global warming conditions.

[74] Euxinia struck the epicontinental North Sea Basin as well,[75] as shown by increases in sedimentary uranium, molybdenum, sulphur, and pyrite concentrations,[76] along with the presence of sulphur-bound isorenieratane.

[91] Acidification may have increased the efficiency of transport of photic zone water into the ocean depths, thus partially acting as a negative feedback that retarded the rate of atmospheric carbon dioxide buildup.

The study suggests that development of thick suboxic zones with high iron bioavailability, the result of dramatic changes in weathering and sedimentation rates, drove diversification of magnetite-forming organisms, likely including eukaryotes.

Regional extinctions in the North Atlantic can be attributed to increased deep-sea anoxia, which could be due to the slowdown of overturning ocean currents, or the release and rapid oxidation of large amounts of methane.

[163] In the Tremp-Graus Basin of northern Spain, fluvial systems grew and rates of deposition of alluvial sediments increased with a lag time of around 3,800 years after the PETM.

Temperatures were rising globally at a steady pace, and a mechanism must be invoked to produce an instantaneous spike which may have been accentuated or catalyzed by positive feedback (or activation of "tipping points"[166]).

The emplacement of a large cluster of kimberlite pipes at ~56 Ma in the Lac de Gras region of northern Canada may have provided the carbon that triggered early warming in the form of exsolved magmatic CO2.

Calculations indicate that the estimated 900–1,100 Pg[167] of carbon required for the initial approximately 3 °C of ocean water warming associated with the Paleocene-Eocene thermal maximum could have been released during the emplacement of a large kimberlite cluster.

[175] Volcanic eruptions of a large magnitude can impact global climate, reducing the amount of solar radiation reaching the Earth's surface, lowering temperatures in the troposphere, and changing atmospheric circulation patterns.

A cometary impact coincident with the P/E boundary can also help explain some enigmatic features associated with this event, such as the iridium anomaly at Zumaia, the abrupt appearance of a localized kaolinitic clay layer with abundant magnetic nanoparticles, and especially the nearly simultaneous onset of the carbon isotope excursion and the thermal maximum.

[189] The combustion of prodigious quantities of peat was once postulated, because there was probably a greater mass of carbon stored as living terrestrial biomass during the Paleocene than there is today since plants in fact grew more vigorously during the period of the PETM.

[197] Moreover, the small apparent change in TEX86 that precede the δ13C anomaly can easily (and more plausibly) be ascribed to local variability (especially on the Atlantic coastal plain, e.g. Sluijs, et al., 2007) as the TEX86 paleo-thermometer is prone to significant biological effects.

[200] One study, however, suggests that because seawater oxygen content was lower, sufficient methane clathrate deposits could have been present to make them a viable mechanism for explaining the isotopic changes.

[207] It was estimated in 2001 that it would take around 2,300 years for an increased temperature to diffuse warmth into the sea bed to a depth sufficient to cause a release of clathrates, although the exact time-frame is highly dependent on a number of poorly constrained assumptions.

[210] The timing of changes in ocean circulation with respect to the shift in carbon isotope ratios has been argued to support the proposition that warmer deep water caused methane hydrate release.

[218] Although the PETM is now commonly held to be a "case study" for global warming and massive carbon emission,[1][2][41] the cause, details, and overall significance of the event remain uncertain.

[222] Professor of Earth and planetary sciences James Zachos notes that IPCC projections for 2300 in the 'business-as-usual' scenario could "potentially bring global temperature to a level the planet has not seen in 50 million years" – during the early Eocene.