The Paleocene–Eocene thermal maximum (PETM), alternatively Eocene thermal maximum 1 (ETM1) and formerly known as the Initial Eocene or Late Paleocene thermal maximum, was a geologically brief time interval characterized by a global average temperature rise and massive input of carbon into the ocean and atmosphere. The event began, now formally codified, at the precise time boundary between the Paleocene and Eocene geological epochs. The PETM lasted about 200,000 years, and occurred about 56 million years ago.
The PETM arguably represents our best past analogue by which to understand how global warming and the carbon cycle operate in a greenhouse world. The time interval is marked by a prominent negative excursion in carbon stable isotope () records from around the globe; more specifically, a large decrease in the 13C/12C ratio of marine and terrestrial carbonates and organic carbon has been found and correlated across hundreds of locations. The magnitude and timing of the PETM () excursion, which attest to the massive past carbon release to the ocean and atmosphere, and the source of this carbon remain topics of considerable current geoscience research.
What has become clear over the last few decades is that stratigraphic sections across the PETM reveal numerous changes beyond warming and carbon emission. Consistent with an Epoch boundary, fossil records of many organisms show major turnovers. In the marine realm, a mass extinction of benthic foraminifera, a global expansion of subtropical dinoflagellates, and an appearance of excursion taxa, including within planktic foraminifera and calcareous nannofossils, all occurred during the beginning stages of the PETM. On land, many modern mammal orders (including primates) suddenly appear in Europe and in North America.
Setting The configuration of oceans and continents was somewhat different during the early Paleogene relative to the present day. The Panama Isthmus did not yet connect North America and South America, and this allowed direct low-latitude circulation between the Pacific and Atlantic Oceans. The Drake Passage, which now separates South America and Antarctica, was closed, and this perhaps prevented thermal isolation of Antarctica. The Arctic was also more restricted. Although various proxies for past atmospheric carbon dioxide concentrations across the Cenozoic do not agree in absolute terms, all suggest that levels in the early Paleogene before and after the PETM were much higher than at present-day. In any case, significant terrestrial ice sheets and sea-ice did not exist during the late Paleocene through early Eocene.
Earth surface temperatures gradually increased by about from the late Paleocene through the early Eocene. Superimposed on this long-term, gradual warming were at least three (and probably more) "hyperthermals". These can be defined as geologically brief (<200,000 year) events characterized by rapid global warming, major changes in the environment, and massive carbon addition. Though not the first within the Cenozoic, the PETM was the most extreme hyperthermal, and stands out as a major change in the lithologic, biotic and geochemical composition of sediment in hundreds of records across Earth. Other hyperthermals clearly occurred around 53.7 million years ago (now called ETM-2 and also referred to as H-1, or the Elmo event) and around 53.6 million years ago (H-2), 53.3 (I-1), 53.2 (I-2) and 52.8 million years ago (informally called K, X or ETM-3). The number, nomenclature, absolute ages, and relative global impact of the Eocene hyperthermals remain a source of current research. 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.
Global warming
A study in 2020 estimated the global mean surface temperature (GMST) with 66% confidence during the latest Paleocene (c. 57 Mya) as , PETM (56 Mya) as and Early Eocene Climatic Optimum (EECO) (53.3 to 49.1 Mya) as . Estimates of the amount of average global temperature rise at the start of the PETM range from approximately 3 to 6 °C to between 5 and 8 °C. This warming was superimposed on "long-term" early Paleogene warming, and is based on several lines of evidence. There is a prominent (>1‰) negative excursion in the of foraminifera shells, both those made in surface and deep ocean water. Because there was little or no polar ice in the early Paleogene, the shift in very probably signifies a rise in ocean temperature. The temperature rise is also supported by the spread of warmth-loving taxa to higher latitudes, changes in plant leaf shape and size, the Mg/Ca ratios of foraminifera, and the ratios of certain organic compounds, such as TEXH86. Additionally, modelling suggests the climate became more equable, with a 5 °C decrease in mean annual temperature range over most continental interiors.
Proxy data from Esplugafereda in northeastern Spain shows a rapid temperature rise, in accordance with existing regional records of marine and terrestrial environments. In the Fushun Basin, temperatures rose from . Southern California had a mean annual temperature of about 17 °C ± 4.4 °C. In Antarctica, at least part of the year saw minimum temperatures of 15 °C.
TEXH86 values indicate that the average sea surface temperature (SST) reached over in the tropics during the PETM, enough to cause heat stress even in organisms resistant to extreme thermal stress, such as dinoflagellates, of which a significant number of species went extinct. Oxygen isotope ratios from Tanzania suggest that tropical SSTs may have been even higher, exceeding . Ocean Drilling Program Site 1209 from the tropical western Pacific shows an increase in SST from before the PETM to ~40 °C. In the eastern Tethys, SSTs rose by . Low latitude Indian Ocean Mg/Ca records show seawater at all depths warmed by about . In the Pacific Ocean, tropical SSTs increased by about 4-5 °C. TEXL86 values from deposits in New Zealand, then located between 50°S and 60°S in the southwestern Pacific, indicate SSTs of to , an increase of over from an average of to at the boundary between the Selandian and Thanetian. The extreme warmth of the southwestern Pacific extended into the Australo-Antarctic Gulf. Sediment core samples from the East Tasman Plateau, then located at a palaeolatitude of ~65 °S, show an increase in SSTs from about during the PETM. In the North Sea, SSTs jumped by , reaching highs of ~33 °C, while in the West Siberian Sea, SSTs climbed to ~27 °C.
Certainly, the central Arctic Ocean was ice-free before, during, and after the PETM. This can be ascertained from the composition of sediment cores recovered during the Arctic Coring Expedition (ACEX) at 87°N on Lomonosov Ridge. Moreover, temperatures increased during the PETM, as indicated by the brief presence of subtropical dinoflagellates (Apectodinium spp.}, and a marked increase in TEX86. The latter record is unusual, though, because it suggests a rise from ~ before the PETM to ~ during the PETM. Assuming the TEX86 record reflects summer temperatures, it still implies much warmer temperatures on the North Pole compared to the present day, but no significant latitudinal amplification relative to surrounding time.
The above considerations are important because, in many global warming simulations, high latitude temperatures increase much more at the poles through an ice–albedo feedback. It may be the case, however, that during the PETM, this feedback was largely absent because of limited polar ice, so temperatures on the Equator and at the poles increased similarly. Notable is the absence of documented greater warming in polar regions compared to other regions. This implies a non-existing ice-albedo feedback, suggesting no sea or land ice was present in the late Paleocene.
Precise limits on the global temperature rise during the PETM and whether this varied significantly with latitude remain open issues. Oxygen isotope and Mg/Ca of carbonate shells precipitated in surface waters of the ocean are commonly used measurements for reconstructing past temperature; however, both paleotemperature proxies can be compromised at low latitude locations, because re-crystallization of carbonate on the seafloor renders lower values than when formed. On the other hand, these and other temperature proxies (e.g., TEX86) are impacted at high latitudes because of seasonality; that is, the "temperature recorder" is biased toward summer, and therefore higher values, when the production of carbonate and organic carbon occurred.
Carbon cycle disturbance Clear evidence for massive addition of 13C-depleted carbon at the onset of the PETM comes from two observations. First, a prominent negative excursion in the carbon isotope composition () of carbon-bearing phases characterizes the PETM in numerous (>130) widespread locations from a range of environments. Second, carbonate dissolution marks the PETM in sections from the deep sea.
The total mass of carbon injected to the ocean and atmosphere during the PETM remains the source of debate. In theory, it can be estimated from the magnitude of the negative carbon isotope excursion (CIE), the amount of carbonate dissolution on the seafloor, or ideally both. However, the shift in the across the PETM depends on the location and the carbon-bearing phase analyzed. In some records of bulk carbonate, it is about 2‰ (per mil); in some records of terrestrial carbonate or organic matter it exceeds 6‰. Carbonate dissolution also varies throughout different ocean basins. It was extreme in parts of the north and central Atlantic Ocean, but far less pronounced in the Pacific Ocean. With available information, estimates of the carbon addition range from about 2,000 to 7,000 gigatons.
Timing of carbon addition and warming The timing of the PETM excursion is of considerable interest. This is because the total duration of the CIE, from the rapid drop in 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 carbon dioxide.
The total duration of the CIE can be estimated in several ways. The iconic sediment interval for examining and dating the PETM is a core recovered in 1987 by the Ocean Drilling Program at Hole 690B at Maud Rise in the southern Atlantic Ocean. At this location, the PETM CIE, from start to end, spans about . Long-term age constraints, through biostratigraphy and magnetostratigraphy, suggest an average Paleogene sedimentation rate of about per 1,000 years. Assuming a constant sedimentation rate, the entire event, from onset though termination, was therefore estimated at 200,000 years. Subsequently, it was noted that the CIE spanned 10 or 11 subtle cycles in various sediment properties, such as Fe content. Assuming these cycles represent precession, a similar but slightly longer age was calculated by Rohl et al. 2000. If a massive amount of 13C-depleted carbon dioxide is rapidly injected into the modern ocean or atmosphere and projected into the future, a ~200,000 year CIE results because of slow flushing through quasi steady-state inputs (weathering and volcanism) and outputs (carbonate and organic) of carbon. A different study, based on a revised orbital chronology and data from sediment cores in the southern Atlantic calculated a slightly shorter duration of around 170,000 years.
An around 200,000 year duration for the CIE is estimated from models of global carbon cycling.