PETM Overview & Bibliography

Posted by: chaamjamal on: October 28, 2018

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PALEOCENE-EOCENE THERMAL MAXIMUM 55 MILLION YEARS AGO

RELATED POST: https://tambonthongchai.com/2020/03/20/an-atmosphere-bias-part-2/ 

WHAT WE KNOW FROM PALEO CLIMATOLOGY

1. Time, duration, & data: Paleoclimate data from carbonate and organic matter deposits in terrestrial and ocean sediments show that there was a 10,000 year (or so) period of global warming about 55 to 56 million years ago where the Paleocene age ends and the Eocene age begins. The warming is found in the atmosphere, in sea surface temperature, and in the deep ocean.
2. Global Warming: Temperatures in the deep ocean rose by 4ºC from 11ºC to 15ºC while sea surface temperature (SST), estimated from oxygen isotope excursion and Mg/Ca records, warmed by 8ºC to 10ºC with temperatures as high as 33ºC in the mid-latitudes and 23ºC in the Arctic. Global mean surface temperatures rose by 5ºC to 9ºC. Although these changes are described as “abrupt” in the long paleo context, it should be noted that a warming of 10ºC over a period of  10,000 years corresponds to a warming rate of 0.1ºC per century compared with our current warming rate of 0.5ºC/century.
3. Carbon Isotopic excursion: The data also show that over the same period of time, there were isotopic excursions of carbon13. (An isotopic excursion is a temporary divergence from the long term average.) In the 10,000-year excursion, carbon13 levels in both oceans and atmosphere fell by 0.2% to 0.4% from the norm. It is significant that the carbon isotope excursion is found in both the atmosphere and the oceans. The excursion implies that carbon in the current carbon cycle had been combined with carbon from a distant past or geological carbon that had yet not been exposed to the atmosphere. 
4. Oceanic oxygen depletion: Warming of deep waters was followed by oxygen deficiency in the deep ocean as seen in the extinction of 30–50% of deep‐sea benthic foraminiferal species. Oxygen depletion implies that warming was associated with oxidation of some kind.
5. Ocean Acidification: Coincident with oceanic oxygen depletion, a rapid decline in pH and evidence of shoaling of the calcite compensation depth down to depths greater than 3km of the ocean are found in the data. The data indicate a very large global oxidation event in the ocean that generated large quantities of carbon dioxide.
6. Increase in Atmospheric Carbon Dioxide: Atmospheric CO2 levels estimated from oxygen17 isotopic signatures in tooth enamel show a large uncertainty range from 230 to 630 ppm that is thought to have increased by more than 70% in course of the 10,000-year PETM event. The attempt to describe the observed warming in terms of the greenhouse effect of atmospheric CO2 and thereby to draw theoretical parallels with the current warming episode has not yielded useful results because it yields gross anomalies in terms of climate sensitivity and also because some of the warming events recorded came before the increase in atmospheric CO2.

WHAT WE DON’T KNOW BUT HAVE THEORIES FOR

1. What was the large deep-ocean oxidation event that warmed the ocean, depleted its oxygen, increased its inorganic carbon concentration, injected carbon dioxide into the atmosphere? The main body of research points to methane hydrates as the source of the carbon. It is proposed that the the hydrates were dissociated into methane by unspecified heat sources possibly geothermal, that then caused the methane to oxidize thus consuming the ocean’s oxygen and generating even more heat in a chain reaction. It is clear however, that much of the methane survived into the atmosphere where their further oxidation by atmospheric oxygen continued. An alternative theory identifies the mantle as a direct source of both carbon and heat (Svenson 2004). At least one study (Kent 2003) has presented evidence that bears the signature of a comet strike that may have initiated the ocean warming, carbonification, and oxidation sequence.
2. What was the role of the greenhouse effect of atmospheric carbon dioxide? The proposed heat trapping effect of atmospheric CO2 could not have initiated the PETM warming because the oceanic carbon enrichment and oxidation events preceded the rise in atmospheric CO2; and the rise in atmospheric CO2 is poorly quantified. Also, using the IPCC climate sensitivity range of ECS = [1.5, 4.5] in conjunction with the best guess for the rise in atmospheric CO2 concentration does not explain the amount of surface warming. It is therefore possible that other sources of heat, possibly geothermal, may have been at work.
3. Does the rate of carbon dioxide injection into the atmosphere compare with that of the current episode of anthropogenic global warming? Paleo climatology likes to refer to it as “a massive carbon injection”  and indeed a great deal of carbon dioxide was injected into the atmosphere; but the time frame is very long as these things occurred over 10,000 to 20,000 years compared with the century of two in today’s time scale. Currently, the CO2 injection rate is around 10 GTCY (gigatonnes of carbon equivalent per year). The corresponding figures for the PETM event is somewhere between 0.2 and 0.6 GTC per year. Yet, if the total amount injected had occurred in 100 years instead of 10,000 years, the corresponding annual rate would have been 20 to 60 GTCY.
4. Is there an analogy between AGW and PETM that will give us better insight and understanding of AGW and help us to design better climate action and climate adaptation policies? This analogy is often claimed and there may be some generalities about warming that will be useful to us but there are gross departures in the details of the two events that make it difficult to draw a parallel that can relate events in one to events in the other. The fundamental issue is that while the AGW event is thought to have been initiated in the atmosphere driven by humans digging up and burning fossil fuels in the industrial economy, the PETM event was initiated by nature in the deep ocean where inexplicably, an enormous amount of carbon was released possibly from the ocean bed either from methane hydrates or from the mantle. The oxidation of the carbon simultaneously consumed the ocean’s oxygen, caused ocean acidification, and caused atmospheric CO2 to rise. The parallel between these events and AGW often drawn by climate activists, require that these sequence of events in reverse – starting with CO2 release by humans into the atmosphere – can be understood in PETM terms.
5. The role of the earth’s internal geological carbon and geothermal heat in climate: The current episode of climate change is understood purely in terms of solar radiation arriving at the top of the atmosphere and the proposed role of carbon dioxide emissions from the use of fossil fuels in leveraging surface temperature. There is no role in this mechanism for the earth itself either in terms of its internal geothermal heat or of natural emissions of carbon from within the earth. The PETM is a cautionary tale in this regard because there, the predominant role of the earth’s internal heat and carbon emissions is acknowledged. The possible role of the earth in the current event is discussed in three related posts [Ocean Heat Content] ,  [Unprecedented Warming of the Arctic] [Carbon Cycle Measurement Problems Solved with Circular Reasoning] .
6. What was the impact of the PETM events on the flora and fauna? Mass extinctions of some species (benthic foraminifera) and expansion of other species (subtropical dinoflagellates) are recorded in the paleo data. The most dramatic change and the one most relevant to humans is that the PETM is credited with the rapid expansion of mammals on land and the first appearance of the modern orders of mammals. For details please see the various works of Paleontologist Philip Dean Gingerich.

PETM BIBLIOGRAPHY

Featured Authors

Gerald Dickens, James Zachos, Philip Gingerich, & Dennis Kent

Phil Gingerich University of Michigan with writer Rob Kunzig near Powell, Wyoming.

1. 1995: Dickens, Gerald R., et al. “Dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the Paleocene.” Paleoceanography and Paleoclimatology10.6 (1995): 965-971. Isotopic records across the “Latest Paleocene Thermal Maximum“ (LPTM) indicate that bottom water temperature increased by more than 4°C during a brief time interval (<10⁴years) of the latest Paleocene (∼55.6 Ma). There also was a coeval −2 to −3‰ excursion in the δ¹³C of the ocean/atmosphere inorganic carbon reservoir. Given the large mass of this reservoir, a rapid δ¹³C shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere. However, a direct consequence of warming bottom water temperature from 11 to 15°C over 10⁴ years would be a significant change in sediment thermal gradients and dissociation of oceanic CH₄ hydrate at locations with intermediate water depths. In terms of the present‐day oceanic CH₄ hydrate reservoir, thermal dissociation of oceanic CH₄ hydrate during the LPTM could have released greater than 1.1 to 2.1 × 10¹⁸ g of carbon with a δ¹³C of approximately −60‰. The release and subsequent oxidation of this amount of carbon is sufficient to explain a −2 to −3‰ excursion in δ¹³C across the LPTM. Fate of CH₄ in oceanic hydrates must be considered in developing models of the climatic and paleoceanographic regimes that operated during the LPTM.
2. 1997: Dickens, Gerald R., Maria M. Castillo, and James CG Walker. “A blast of gas in the latest Paleocene: Simulating first-order effects of massive dissociation of oceanic methane hydrate.” Geology 25.3 (1997): 259-262. Carbonate and organic matter deposited during the latest Paleocene thermal maximum is characterized by a remarkable −2.5‰ excursion in δ¹³C that occurred over ∼10⁴ yr and returned to near initial values in an exponential pattern over ∼2 × 10⁵ yr. It has been hypothesized that this excursion signifies transfer of 1.4 to 2.8 × 10¹⁸ g of CH₄ from oceanic hydrates to the combined ocean-atmosphere inorganic carbon reservoir. A scenario with 1.12 × 10¹⁸ g of CH₄ is numerically simulated here within the framework of the present-day global carbon cycle to test the plausibility of the hypothesis. We find that (1) the δ¹³C of the deep ocean, shallow ocean, and atmosphere decreases by −2.3‰ over 10⁴ yr and returns to initial values in an exponential pattern over ∼2 × 10⁵ yr; (2) the depth of the lysocline shoals by up to 400 m over 10⁴ yr, and this rise is most pronounced in one ocean region; and (3) global surface temperature increases by ∼2 °C over 10⁴ yr and returns to initial values over ∼2 × 10⁶ yr. The first effect is quantitatively consistent with the geologic record; the latter two effects are qualitatively consistent with observations. Thus, significant CH₄ release from oceanic hydrates is a plausible explanation for observed carbon cycle perturbations during the thermal maximum. This conclusion is of broad interest because the flux of CH₄ invoked during the maximum is of similar magnitude to that released to the atmosphere from present-day anthropogenic CH₄ sources.
3. 2002: Thomas, Deborah J., et al. “Warming the fuel for the fire: Evidence for the thermal dissociation of methane hydrate during the Paleocene-Eocene thermal maximum.” Geology30.12 (2002): 1067-1070.  Dramatic warming and upheaval of the carbon system at the end of the Paleocene Epoch have been linked to massive dissociation of sedimentary methane hydrate. However, testing the Paleocene-Eocene thermal maximum hydrate dissociation hypothesis has been hindered by the inability of available proxy records to resolve the initial sequence of events. The cause of the Paleocene-Eocene thermal maximum carbon isotope excursion remains speculative, primarily due to uncertainties in the timing and duration of the Paleocene-Eocene thermal maximum. We present new high-resolution stable isotope records based on analyses of single planktonic and benthic foraminiferal shells from Ocean Drilling Program Site 690 (Weddell Sea, Southern Ocean), demonstrating that the initial carbon isotope excursion was geologically instantaneous and was preceded by a brief period of gradual surface-water warming. Both of these findings support the thermal dissociation of methane hydrate as the cause of the Paleocene-Eocene thermal maximum carbon isotope excursion. Furthermore, the data reveal that the methane-derived carbon was mixed from the surface ocean downward, suggesting that a significant fraction of the initial dissociated hydrate methane reached the atmosphere prior to oxidation.
4. 2001: Katz, Miriam, et al. Uncorking the bottle: What triggered the Paleocene/Eocene thermal maximum methane release?   Paleoceanography 16.6 (2001): 549-562. The Paleocene/Eocene thermal maximum (PETM) was a time of rapid global warming in both marine and continental realms that has been attributed to a massive methane (CH₄) release from marine gas hydrate reservoirs. Previously proposed mechanisms for this methane release rely on a change in deepwater source region(s) to increase water temperatures rapidly enough to trigger the massive thermal dissociation of gas hydrate reservoirs beneath the seafloor. To establish constraints on thermal dissociation, we model heat flow through the sediment column and show the effect of the temperature change on the gas hydrate stability zone through time. In addition, we provide seismic evidence tied to borehole data for methane release along portions of the U.S. continental slope; the release sites are proximal to a buried Mesozoic reef front. Our model results, release site locations, published isotopic records, and ocean circulation models neither confirm nor refute thermal dissociation as the trigger for the PETM methane release. In the absence of definitive evidence to confirm thermal dissociation, we investigate an alternative hypothesis in which continental slope failure resulted in a catastrophic methane release. Seismic and isotopic evidence indicates that Antarctic source deepwater circulation and seafloor erosion caused slope retreat along the western margins of the North Atlantic in the late Paleocene. Continued erosion or seismic activity along the oversteepened continental margin may have allowed methane to escape from gas reservoirs trapped between the frozen hydrate‐bearing sediments and the underlying buried Mesozoic reef front, precipitating the Paleocene/Eocene boundary methane release. An important implication of this scenario is that the methane release caused (rather than resulted from) the transient temperature increase of the PETM. Neither thermal dissociation nor mechanical disruption of sediments can be identified unequivocally as the triggering mechanism for methane release with existing data. Further documentation with high‐resolution benthic foraminiferal isotopic records and with seismic profiles tied to borehole data is needed to clarify whether erosion, thermal dissociation, or a combination of these two was the triggering mechanism for the PETM methane release.
5. 2002: Bralower, Timothy J. “Evidence of surface water oligotrophy during the Paleocene‐Eocene thermal maximum: Nannofossil assemblage data from Ocean Drilling Program Site 690, Maud Rise, Weddell Sea.” Paleoceanography 17.2 (2002): 13-1. Nannoplankton assemblages at Ocean Drilling Program Site 690 (Maud Rise, Weddell Sea) experienced an abrupt and dramatic transformation at the onset of the Paleocene‐Eocene Thermal Maximum (PETM) at ∼55 m.y. The major assemblage shift suggests a change from colder, more productive surface waters to warmer, more oligotrophic conditions. Significant restructuring of assemblages during the later part of the PETM indicates that nannoplankton communities were not stable and that surface water conditions changed, although they remained warm and oligotrophic. Combined with benthic foraminiferal assemblage data, nannoplankton assemblage results suggest increased sequestration of nutrients in shelf environments and starvation of the open ocean. Although the PETM was a short‐lived event, it appears to have had long‐term effects on nannoplankton, leading to the extinction of Fasciculithus, a dominant Paleocene genus. The Cretaceous and early Paleogene was a time of remarkable transformation of marine communities [e.g., Vermeij, 1977]. Some of the most dramatic evolutionary changes took place in the protistans. Groups such as the diatoms and planktic foraminifera became fundamental parts of marine food chains during this time. Other groups such as the calcareous nannoplankton and radiolarians underwent wholesale changes in species composition and assemblage structure. The underlying causes of the long‐term evolutionary changes that took place are not well understood [e.g., Roth, 1987; Leckie, 1987; Leckie et al., 2002]. Research over the last decade, however, has established that these groups were also affected by environmental changes that took place over short timescales. In particular, short‐lived (<1 m.y.) global warming events sparked significant biotic turnover in association with dramatic changes in global carbon cycling [e.g., Schlanger et al., 1987; Leckie, 1989; Elder, 1991; Kennett and Stott, 1991; Coccioni et al., 1992; Erba, 1994; Koch et al., 1995; Kelly et al., 1996; Thomas and Shackleton, 1996; Aubry, 1998; Premoli Silva and Sliter, 1999; Premoli Silva et al., 1999]. One of the most extreme and abrupt warming episodes occurred close to the Paleocene/Eocene boundary at ∼55 Ma [Kennett and Stott, 1991; Bralower et al., 1995; Thomas and Shackleton, 1996]. This event, which is known as the Paleocene‐Eocene Thermal Maximum (PETM) [e.g., Zachos et al., 1993], lasted for a period of ∼210 kyr [Norris and Röhl, 1999; Röhl et al., 2000]. The deep and surface oceans warmed by ∼5° and ∼4°–8°C, respectively, during the PETM. The carbon isotopic composition of the ocean and atmosphere decreased by 3–4‰ coeval with the warming event, suggesting a massive perturbation to the global carbon cycle [Kennett and Stott, 1991; Koch et al., 1992; Bains et al., 1999; Norris and Röhl, 1999]. The large magnitude and rate of onset of the carbon isotope excursion (CIE) are most consistent with the sudden dissociation of methane hydrates from continental shelves and slopes [Dickens et al., 1995, 1997; Katz et al., 1999]; CH4 would have immediately contributed to greenhouse warming. The PETM climatic changes affected biota on a global scale, triggering abrupt turnover of benthic and planktic organisms in the ocean [e.g., Kennett and Stott, 1991; Kelly et al., 1996; Speijer and Morsi, 2002], and the rapid radiation of mammals on land [e.g., Gingerich et al., 1980; Maas et al., 1995; Hooker, 1996; Clyde and Gingerich, 1998]. Deep‐sea environmental changes led to an abrupt extinction in benthic foraminiferal communities [e.g., Thomas, 1990; Pak and Miller, 1992; Thomas and Shackleton, 1996; Thomas, 1998]. This benthic foraminiferal extinction (BFE) event [e.g., Tjalsma and Lohmann, 1983] has been well documented in a range of different environments and latitudes [e.g., Kaiho et al., 1996; Speijer et al., 1996]. The response of surface‐dwelling marine organisms to PETM environmental changes appears to have been fundamentally different: tropical planktic foraminifers radiated dramatically during this event [Kelly et al., 1996, 1998]. There have been few high‐resolution investigations of the response of phytoplankton groups such as the calcareous nannoplankton to the PETM. Most previous investigations have considered only long‐term changes in assemblages through the late Paleocene‐early Eocene interval [e.g., Aubry, 1998]. Interpretations of geochemical and biotic investigations disagree as to whether the PETM was characterized by increased or decreased surface water productivity. Tropical plankton at Pacific Site 865 suggests increased oligotrophy [Kelly et al., 1996]; benthic foraminiferal assemblages in open ocean sites also suggest reduced food supply under oligotrophic surface water conditions, whereas assemblages in marginally marine and shelf sites are interpreted as indicating high food supply likely as a result of eutrophic conditions [Thomas and Shackleton, 1996; Speijer and Schmitz, 1998; Thomas, 1998; Thomas et al., 2000]. A widespread bloom of the dinoflagellate Apectodinium in sections deposited in coastal environments is also consistent with high productivity [Crouch et al., 2001]. Bains et al. [2000] interpreted an increase in Ba accumulation rates in the PETM at several open‐ocean sites as evidence for high productivity; these authors concluded that elevated productivity led to increased CO2 draw down, curbing a potential runaway greenhouse. To attempt to resolve the contrast between biotic and geochemical proxies of productivity and to more fully constrain the effects of the PETM on marine phytoplankton, we have carried out a detailed study of calcareous nannofossil assemblages across the PETM at Site 690 (Maud Rise, Weddell Sea; Figure 1). This site contains one of the highest‐quality deep‐sea records of the PETM event. Upper Paleocene sediments are composed of ooze representing nannofossil zone NP9, planktic foraminiferal zones AP4 and AP5, and part of magnetic polarity zone C24r [Aubry et al., 1996]. White to pale brown lithologic cycles caused by oscillations of CaCO3 and clay content appear to correspond to precessional orbital rhythms [Röhl et al., 2000]. These cycles can be used to construct a timescale for Site 690 [Cramer, 2001; D. Thomas, manuscript in preparation, 2002], allowing us to monitor paleoceanographic changes at millennial resolution.
6. 2003: Kent, Dennis V., et al. “A case for a comet impact trigger for the Paleocene/Eocene thermal maximum and carbon isotope excursion.” Earth and Planetary Science Letters 211.1-2 (2003): 13-26. We hypothesize that the rapid onset of the carbon isotope excursion (CIE) at the Paleocene/Eocene boundary (∼55 Ma) may have resulted from the accretion of a significant amount of 12C-enriched carbon from the impact of a ∼10 km comet, an event that would also trigger greenhouse warming leading to the Paleocene/Eocene thermal maximum and, possibly, thermal dissociation of seafloor methane hydrate. Indirect evidence of an impact is the unusual abundance of magnetic nanoparticles in kaolinite-rich shelf sediments that closely coincide with the onset and nadir of the CIE at three drill sites on the Atlantic Coastal Plain. After considering various alternative mechanisms that could have produced the magnetic nanoparticle assemblage and by analogy with the reported detection of iron-rich nanophase material at the Cretaceous/Tertiary boundary, we suggest that the CIE occurrence was derived from an impact plume condensate. The sudden increase in kaolinite is thus thought to represent the redeposition on the marine shelf of a rapidly weathered impact ejecta dust blanket. Published reports of a small but significant iridium anomaly at or close to the Paleocene/Eocene boundary provide supportive evidence for an impact.
7. 2003: Zachos, James C., et al. “A transient rise in tropical sea surface temperature during the Paleocene-Eocene thermal maximum.” Science 302.5650 (2003): 1551-1554. The Paleocene-Eocene Thermal Maximum (PETM) has been attributed to a rapid rise in greenhouse gas levels. If so, warming should have occurred at all latitudes, although amplified toward the poles. Existing records reveal an increase in high-latitude sea surface temperatures (SSTs) (8° to 10°C) and in bottom water temperatures (4° to 5°C). To date, however, the character of the tropical SST response during this event remains unconstrained. Here we address this deficiency by using paired oxygen isotope and minor element (magnesium/calcium) ratios of planktonic foraminifera from a tropical Pacific core to estimate changes in SST. Using mixed-layer foraminifera, we found that the combined proxies imply a 4° to 5°C rise in Pacific SST during the PETM. These results would necessitate a rise in atmospheric pCO2 to levels three to four times as high as those estimated for the late Paleocene.
8. *2004: Svensen, Henrik, et al. “Release of methane from a volcanic basin as a mechanism for initial Eocene global warming.” Nature 429.6991 (2004): 542. A 200,000-yr interval of extreme global warming marked the start of the Eocene epoch about 55 million years ago. Negative carbon- and oxygen-isotope excursions in marine and terrestrial sediments show that this event was linked to a massive and rapid (∼10,000 yr) input of isotopically depleted carbon^1,2. It has been suggested previously that extensive melting of gas hydrates buried in marine sediments may represent the carbon source^3,4 and has caused the global climate change. Large-scale hydrate melting, however, requires a hitherto unknown triggering mechanism. Here we present evidence for the presence of thousands of hydrothermal vent complexes identified on seismic reflection profiles from the Vøring and Møre basins in the Norwegian Sea. We propose that intrusion of voluminous mantle-derived melts in carbon-rich sedimentary strata in the northeast Atlantic may have caused an explosive release of methane—transported to the ocean or atmosphere through the vent complexes—close to the Palaeocene/Eocene boundary. Similar volcanic and metamorphic processes may explain climate events associated with other large igneous provinces such as the Siberian Traps (∼250 million years ago) and the Karoo Igneous Province (∼183 million years ago).
9. 2004: Bowen, Gabriel J., et al. “A humid climate state during the Palaeocene/Eocene thermal maximum.” Nature 432.7016 (2004): 495. An abrupt climate warming of 5 to 10 °C during the Palaeocene/Eocene boundary thermal maximum (PETM) 55 Myr ago is linked to the catastrophic release of ∼1,050–2,100 Gt of carbon from sea-floor methane hydrate reservoirs¹. Although atmospheric methane, and the carbon dioxide derived from its oxidation, probably contributed to PETM warming, neither the magnitude nor the timing of the climate change is consistent with direct greenhouse forcing by the carbon derived from methane hydrate. Here we demonstrate significant differences between marine^2,3 and terrestrial^4,5,6 carbon isotope records spanning the PETM. We use models of key carbon cycle processes^7,8,9 to identify the cause of these differences. Our results provide evidence for a previously unrecognized discrete shift in the state of the climate system during the PETM, characterized by large increases in mid-latitude tropospheric humidity and enhanced cycling of carbon through terrestrial ecosystems. A more humid atmosphere helps to explain PETM temperatures, but the ultimate mechanisms underlying the shift remain unknown.
10. 2005: Tripati, Aradhna, and Henry Elderfield. “Deep-sea temperature and circulation changes at the Paleocene-Eocene thermal maximum.” Science 308.5730 (2005): 1894-1898. A rapid increase in greenhouse gas levels is thought to have fueled global warming at the Paleocene-Eocene Thermal Maximum (PETM). Foraminiferal magnesium/calcium ratios indicate that bottom waters warmed by 4° to 5°C, similar to tropical and subtropical surface ocean waters, implying no amplification of warming in high-latitude regions of deep-water formation under ice-free conditions. Intermediate waters warmed before the carbon isotope excursion, in association with down-welling in the North Pacific and reduced Southern Ocean convection, supporting changing circulation as the trigger for methane hydrate release. A switch to deep convection in the North Pacific at the PETM onset could have amplified and sustained warming.
11. 2005: Zachos, James C., et al. “Rapid acidification of the ocean during the Paleocene-Eocene thermal maximum.” Science308.5728 (2005): 1611-1615. The Paleocene-Eocene thermal maximum (PETM) has been attributed to the rapid release of ∼2000 × 109 metric tons of carbon in the form of methane. In theory, oxidation and ocean absorption of this carbon should have lowered deep-sea pH, thereby triggering a rapid (<10,000-year) shoaling of the calcite compensation depth (CCD), followed by gradual recovery. Here we present geochemical data from five new South Atlantic deep-sea sections that constrain the timing and extent of massive sea-floor carbonate dissolution coincident with the PETM. The sections, from between 2.7 and 4.8 kilometers water depth, are marked by a prominent clay layer, the character of which indicates that the CCD shoaled rapidly (<10,000 years) by more than 2 kilometers and recovered gradually (>100,000 years). These findings indicate that a large mass of carbon (»2000 × 109 metric tons of carbon) dissolved in the ocean at the Paleocene-Eocene boundary and that permanent sequestration of this carbon occurred through silicate weathering feedback.
12. 2006: Higgins, John A., and Daniel P. Schrag. “Beyond methane: towards a theory for the Paleocene–Eocene thermal maximum.” Earth and Planetary Science Letters 245.3-4 (2006): 523-537. Extreme global warmth and an abrupt negative carbon isotope excursion during the Paleocene–Eocene Thermal Maximum (PETM) have been attributed to a massive release of methane hydrate from sediments on the continental slope [1]. However, the magnitude of the warming (5 to 6 °C [2],[3]) and rise in the depth of the CCD (> 2 km; [4]) indicate that the size of the carbon addition was larger than can be accounted for by the methane hydrate hypothesis. Additional carbon sources associated with methane hydrate release (e.g. pore-water venting and turbidite oxidation) are also insufficient. We find that the oxidation of at least 5000 Gt C of organic carbon is the most likely explanation for the observed geochemical and climatic changes during the PETM, for which there are several potential mechanisms. Production of thermogenic CH₄ and CO₂during contact metamorphism associated with the intrusion of a large igneous province into organic rich sediments [5] is capable of supplying large amounts of carbon, but is inconsistent with the lack of extensive carbon loss in metamorphosed sediments, as well as the abrupt onset and termination of carbon release during the PETM. A global conflagration of Paleocene peatlands [6] highlights a large terrestrial carbon source, but massive carbon release by fire seems unlikely as it would require that all peatlands burn at once and then for only 10 to 30 ky. In addition, this hypothesis requires an order of magnitude increase in the amount of carbon stored in peat. The isolation of a large epicontinental seaway by tectonic uplift associated with volcanism or continental collision, followed by desiccation and bacterial respiration of the aerated organic matter is another potential mechanism for the rapid release of large amounts of CO₂. In addition to the oxidation of the underlying marine sediments, the desiccation of a major epicontinental seaway would remove a large source of moisture for the continental interior, resulting in the desiccation and bacterial oxidation of adjacent terrestrial wetlands.
13. 2006: Zachos, James C., et al. “Extreme warming of mid-latitude coastal ocean during the Paleocene-Eocene Thermal Maximum: Inferences from TEX86 and isotope data.” Geology34.9 (2006): 737-740. Changes in sea surface temperature (SST) during the Paleocene-Eocene Thermal Maximum (PETM) have been estimated primarily from oxygen isotope and Mg/Ca records generated from deep-sea cores. Here we present a record of sea surface temperature change across the Paleocene-Eocene boundary for a nearshore, shallow marine section located on the eastern margin of North America. The SST record, as inferred from TEX86 data, indicates a minimum of 8 °C of warming, with peak temperatures in excess of 33 °C. Similar SSTs are estimated from planktonic foraminifer oxygen isotope records, although the excursion is slightly larger. The slight offset in the oxygen isotope record may reflect on seasonally higher runoff and lower salinity.
14. 2006: Sluijs, Appy, et al. “Subtropical Arctic Ocean temperatures during the Palaeocene/Eocene thermal maximum.” Nature441.7093 (2006): 610. The Palaeocene/Eocene thermal maximum, ∼55 million years ago, was a brief period of widespread, extreme climatic warming1,2,3, that was associated with massive atmospheric greenhouse gas input4. Although aspects of the resulting environmental changes are well documented at low latitudes, no data were available to quantify simultaneous changes in the Arctic region. Here we identify the Palaeocene/Eocene thermal maximum in a marine sedimentary sequence obtained during the Arctic Coring Expedition5. We show that sea surface temperatures near the North Pole increased from ∼18 °C to over 23 °C during this event. Such warm values imply the absence of ice and thus exclude the influence of ice-albedo feedbacks on this Arctic warming. At the same time, sea level rose while anoxic and euxinic conditions developed in the ocean’s bottom waters and photic zone, respectively. Increasing temperature and sea level match expectations based on palaeoclimate model simulations6, but the absolute polar temperatures that we derive before, during and after the event are more than 10 °C warmer than those model-predicted. This suggests that higher-than-modern greenhouse gas concentrations must have operated in conjunction with other feedback mechanisms—perhaps polar stratospheric clouds7 or hurricane-induced ocean mixing8—to amplify early Palaeogene polar temperatures.
15. 2006: Gingerich, Philip D. “Environment and evolution through the Paleocene–Eocene thermal maximum.” Trends in ecology & evolution 21.5 (2006): 246-253. The modern orders of mammals, Artiodactyla, Perissodactyla and Primates (APP taxa), first appear in the fossil record at the Paleocene–Eocene boundary, c. 55 million years ago. Their appearance on all three northern continents has been linked to diversification and dispersal in response to rapid environmental change at the beginning of a worldwide 100 000–200 000-year Paleocene–Eocene thermal maximum (PETM) and carbon isotope excursion. As I discuss here, global environmental events such as the PETM have had profound effects on evolution in the geological past and must be considered when modeling the history of life. The PETM is also relevant when considering the causes and consequences of global greenhouse warming.
16. 2007: Röhl, Ursula, et al. “On the duration of the Paleocene‐Eocene thermal maximum (PETM).” Geochemistry, Geophysics, Geosystems 8.12 (2007). The Paleocene‐Eocene thermal maximum (PETM) is one of the best known examples of a transient climate perturbation, associated with a brief, but intense, interval of global warming and a massive perturbation of the global carbon cycle from injection of isotopically light carbon into the ocean‐atmosphere system. One key to quantifying the mass of carbon released, identifying the source(s), and understanding the ultimate fate of this carbon is to develop high‐resolution age models. Two independent strategies have been employed, cycle stratigraphy and analysis of extraterrestrial helium (HeET), both of which were first tested on Ocean Drilling Program (ODP) Site 690. These two methods are in agreement for the onset of the PETM and initial recovery, or the clay layer (“main body”), but seem to differ in the final recovery phase of the event above the clay layer, where the carbonate contents rise and carbon isotope values return toward background values. Here we present a state‐of‐the‐art age model for the PETM derived from a new orbital chronology developed with cycle stratigraphic records from sites drilled during ODP Leg 208 (Walvis Ridge, Southeastern Atlantic) integrated with published records from Site 690 (Weddell Sea, Southern Ocean, ODP Leg 113). During Leg 208, five Paleocene‐Eocene (P‐E) boundary sections (Sites 1262 to 1267) were recovered in multiple holes over a depth transect of more than 2200 m at the Walvis Ridge, yielding the first stratigraphically complete P‐E deep‐sea sequence with moderate to relatively high sedimentation rates (1 to 3 cm/ka, where “a” is years). A detailed chronology was developed with nondestructive X‐ray fluorescence (XRF) core scanning records on the scale of precession cycles, with a total duration of the PETM now estimated to be ∼170 ka. The revised cycle stratigraphic record confirms original estimates for the duration of the onset and initial recovery but suggests a new duration for the final recovery that is intermediate to the previous estimates by cycle stratigraphy and HeET. The Paleocene Eocene thermal maximum (PETM) is one of the most abrupt and transient climatic events documented in the geologic record [e.g., Zachos et al., 2001, 2005]. This event was associated with pronounced warming of the oceans and atmosphere, changes in ocean chemistry, and reorganization of the global carbon cycle [Kennett and Stott, 1991; Koch et al., 1992; Thomas et al., 2002; Zachos et al., 2003, 2005; Tripati and Elderfield, 2005; Sluijs et al., 2006]. Warming of deep waters and subsequent oxygen deficiency may have been responsible for extinction of 30–50% of deep‐sea benthic foraminiferal species [Thomas and Shackleton, 1996] and planktonic biota were affected by changes in surface water habitats [e.g., Kelly et al., 1996; Bralower et al., 2002; Kelly, 2002; Raffi et al., 2005; Gibbs et al., 2006a, 2006b]; global warming also may have led to a pulse of speciation or migration among mammalian groups [e.g., Koch et al., 1992, Bowen et al., 2001; Gingerich, 2003]. The PETM corresponds to a significant (∼3.5–4.5‰) negative carbon isotope excursion (CIE) recorded in marine and terrestrial sections [e.g., Kennett and Stott, 1991; Koch et al., 1992; Bralower et al., 1997; Zachos et al., 2004, 2005; Schouten et al., 2007]. The source and triggering mechanism of this event are still the focus of much debate [e.g., Lourens et al., 2005; Sluijs et al., 2007; Storey et al., 2007]. An orbital trigger for the PETM and similar (but less severe) events has been suggested [Lourens et al., 2005], but the specific orbital parameter association is still not completely resolved [Westerhold et al., 2007]. Other mechanisms that might explain the abruptness of the CIE include the input of methane into the ocean and atmosphere from the dissociation of methane hydrates in continental margin sediments or from the cracking of coal during rifting of the northern North Atlantic Ocean [Dickens et al., 1995, 1997; Svensen et al., 2004]. Identifying potential triggering mechanisms for the PETM, as well as understanding the relationship between forcing and consequences requires a very precise and high‐resolution chronology. For example, quantifying the climate sensitivity requires robust estimates of the mass of carbon released, and hence the rate of the CIE. Until recently, however, estimates of the absolute age of the onset and the duration of the event were poorly constrained, varying between 54.88 and 55.50 Ma, and 100 and 250 ka, respectively [e.g., Kennett and Stott, 1991; Koch et al., 1992; Aubry et al., 1996; Röhl and Abrams, 2000; Röhl et al., 2000; Farley and Eltgroth, 2003; Giusberti et al., 2007]. By using an astronomically calibrated but floating timescale, the age of the onset (54.93 to 54.98 Ma) and the duration (150 to 220 ka) of the CIE were initially determined at Ocean Drilling Program (ODP) Site 1051 [Norris and Röhl, 1999] then refined using combined records from Sites 690 and 1051 [Röhl et al., 2000]. However, because the onset of the PETM in pelagic sequences is marked by a pronounced dissolution layer or condensed interval and the recovery by a lithologically uniform carbonate‐rich interval, an alternative constant flux age model was developed [Farley and Eltgroth, 2003]. This model is based on the concentrations of extraterrestrial He (3HeET) and the assumption that the flux of this isotope to the Earth remained constant during the PETM. Both age models are in agreement for the duration of the main body of the PETM (70–80 ka for the “core”, the onset, peak, and initial recovery phase (rapid rise in δ13C, but low carbonate; here termed phase 1)), but diverge for the final recovery phase of the CIE (slow rise in δ13C, high carbonate; here termed phase II), with orbital age models producing 140 ka for this interval and He age models 30 ka. Identification of cycles in the Ca (or Fe) records in the recovery interval of the Site 690 section is complicated due to the high and uniform carbonate content of the sediments. A new era in Cenozoic paleoceanography was launched with the recovery of Paleogene sediments in multisite depth transects during Ocean Drilling Program Legs 198 (Shatsky Rise, Pacific Ocean [Bralower et al., 2002; Westerhold and Röhl, 2006]) and 208 (Walvis Ridge, Southeast Atlantic Ocean [Zachos et al., 2004]). These expeditions yielded the first high‐quality, stratigraphically complete sedimentary sequences of the early Paleogene, recovered in offset, multiple‐hole sites. The lithologic and geochemical records generated from these cores exhibit the highly cyclic nature of early Paleogene climate, while also demonstrating that the early Eocene Greenhouse World was punctuated by multiple transient global warming events, or hyperthermals [Thomas et al., 2000; Zachos et al., 2004]. The occurrence of multiple hyperthermals within the late Paleocene–early Eocene suggests a repeated trigger as their cause. Recently, X‐ray fluorescence (XRF) core scanning records from ODP Leg 208 sites and from ODP Site 1051 spanning a ∼4.3 million year interval of the late Paleocene to early Eocene were used to establish a longer time series and to develop a robust and improved chronology of magnetochrons [Westerhold et al., 2007] which is consistent with records from the Bighorn Basin [Wing et al., 2000; Clyde et al., 2007]. One of the obstacles to developing age models for PETM sections is providing a exact definition of the termination of the CIE on a global scale, e.g., at Site 690, the location of the termination is somewhat subjective because of the asymptotic shape of the CIE. In addition, the low signal‐to‐noise ratio of the XRF Ca concentrations in this high‐carbonate interval has made cycle extraction difficult and somewhat subjective. Here we develop a revised chronology for the PETM using high‐resolution geochemical data from the ODP Leg 208 depth transect in combination with new Barium (Ba) XRF intensity data of the expanded section at ODP Site 690 from the Weddell Sea, Southern Ocean (Figure 1). The Barium (Ba) records, in combination with Fe, Ca, and carbon isotope data from the Leg 208 sites and Site 690, show similar patterns that allow for refinement of correlation and age calibrations. These new data provide much better constraints on the durations of each phase of the CIE, particularly the recovery phases (I and II). These records will also allow for a more accurate recalibration of the He isotope chronology from Site 690 [Farley and Eltgroth, 2003]. Moreover, we propose that the definition of the termination of the CIE be based on a combination of cyclostratigraphic proxies derived from XRF scanner and other methods rather than carbon isotopes which gradually become uniform, thus making it difficult to define a globally recognizable termination point for the recovery2009: Zeebe, Richard E., James C. Zachos, and Gerald R. Dickens. “Carbon dioxide forcing alone insufficient to explain Palaeocene–Eocene Thermal Maximum warming.” Nature Geoscience 2.8 (2009): 576.
17. 2008: Panchuk, K., A. Ridgwell, and L. R. Kump. “Sedimentary response to Paleocene-Eocene Thermal Maximum carbon release: A model-data comparison.” Geology 36.4 (2008): 315-318. Possible sources of carbon that may have caused global warming at the Paleocene-Eocene boundary are constrained using an intermediate complexity Earth-system model configured with early Eocene paleogeography. We find that 6800 Pg C (δ¹³C of –22‰) is the smallest pulse modeled here to reasonably reproduce observations of the extent of seafloor CaCO₃ dissolution. This pulse could not have been solely the result of methane hydrate destabilization, suggesting that additional sources of CO₂ such as volcanic CO₂, the oxidation of sedimentary organic carbon, or thermogenic methane must also have contributed. Observed contrasts in dissolution intensity between Atlantic and Pacific sites are reproduced in the model by reducing bioturbation in the Atlantic during the event, simulating a potential consequence of the spread of low-oxygen bottom waters.
18. 2009: Zeebe, Richard E., James C. Zachos, and Gerald R. Dickens. “Carbon dioxide forcing alone insufficient to explain Palaeocene–Eocene Thermal Maximum warming.” Nature Geoscience 2.8 (2009): 576. The Palaeocene–Eocene Thermal Maximum (about 55 Myr ago) represents a possible analogue for the future and thus may provide insight into climate system sensitivity and feedbacks^1,2. The key feature of this event is the release of a large mass of ¹³C-depleted carbon into the carbon reservoirs at the Earth’s surface, although the source remains an open issue^3,4. Concurrently, global surface temperatures rose by 5–9 ^∘C within a few thousand years^5,6,7,8,9. Here we use published palaeorecords of deep-sea carbonate dissolution^{10,11,12,13,14}and stable carbon isotope composition^10,15,16,17 along with a carbon cycle model to constrain the initial carbon pulse to a magnitude of 3,000 Pg C or less, with an isotopic composition lighter than −50‰. As a result, atmospheric carbon dioxide concentrations increased during the main event by less than about 70% compared with pre-event levels. At accepted values for the climate sensitivity to a doubling of the atmospheric CO₂ concentration¹, this rise in CO₂ can explain only between 1 and 3.5 ^∘C of the warming inferred from proxy records. We conclude that in addition to direct CO₂ forcing, other processes and/or feedbacks that are hitherto unknown must have caused a substantial portion of the warming during the Palaeocene–Eocene Thermal Maximum. Once these processes have been identified, their potential effect on future climate change needs to be taken into account.
19. 2011: Dickens, Gerald R. “Down the rabbit hole: Toward appropriate discussion of methane release from gas hydrate systems during the Paleocene-Eocene thermal maximum and other past hyperthermal events.” Climate of the Past 7.3 (2011): 831-846. Enormous amounts of ¹³C-depleted carbon rapidly entered the exogenic carbon cycle during the onset of the Paleocene-Eocene thermal maximum (PETM), as attested to by a prominent negative carbon isotope (δ¹³C) excursion and deep-sea carbonate dissolution. A widely cited explanation for this carbon input has been thermal dissociation of gas hydrate on continental slopes, followed by release of CH₄ from the seafloor and its subsequent oxidation to CO₂ in the ocean or atmosphere. Increasingly, papers have argued against this mechanism, but without fully considering existing ideas and available data. Moreover, other explanations have been presented as plausible alternatives, even though they conflict with geological observations, they raise major conceptual problems, or both. Methane release from gas hydrates remains a congruous explanation for the δ¹³C excursion across the PETM, although it requires an unconventional framework for global carbon and sulfur cycling, and it lacks proof. These issues are addressed here in the hope that they will prompt appropriate discussions regarding the extraordinary carbon injection at the start of the PETM and during other events in Earth’s history.
20. 2011: Cui, Ying, et al. “Slow release of fossil carbon during the Palaeocene–Eocene Thermal Maximum.” Nature Geoscience4.7 (2011): 481. The transient global warming event known as the Palaeocene–Eocene Thermal Maximum occurred about 55.9 Myr ago. The warming was accompanied by a rapid shift in the isotopic signature of sedimentary carbonates, suggesting that the event was triggered by a massive release of carbon to the ocean–atmosphere system. However, the source, rate of emission and total amount of carbon involved remain poorly constrained. Here we use an expanded marine sedimentary section from Spitsbergen to reconstruct the carbon isotope excursion as recorded in marine organic matter. We find that the total magnitude of the carbon isotope excursion in the ocean–atmosphere system was about 4‰. We then force an Earth system model of intermediate complexity to conform to our isotope record, allowing us to generate a continuous estimate of the rate of carbon emissions to the atmosphere. Our simulations show that the peak rate of carbon addition was probably in the range of 0.3–1.7 Pg C yr⁻¹, much slower than the present rate of carbon emissions.
21. 2011: McInerney, Francesca A., and Scott L. Wing. “The Paleocene-Eocene Thermal Maximum: A perturbation of carbon cycle, climate, and biosphere with implications for the future.” Annual Review of Earth and Planetary Sciences 39 (2011): 489-516. During the Paleocene-Eocene Thermal Maximum (PETM), ∼56 Mya, thousands of petagrams of carbon were released into the ocean-atmosphere system with attendant changes in the carbon cycle, climate, ocean chemistry, and marine and continental ecosystems. The period of carbon release is thought to have lasted <20 ka, the duration of the whole event was ∼200 ka, and the global temperature increase was 5–8°C. Terrestrial and marine organisms experienced large shifts in geographic ranges, rapid evolution, and changes in trophic ecology, but few groups suffered major extinctions with the exception of benthic foraminifera. The PETM provides valuable insights into the carbon cycle, climate system, and biotic responses to environmental change that are relevant to long-term future global changes.
22. 2016: Gehler, Alexander, Philip D. Gingerich, and Andreas Pack. “Temperature and atmospheric CO2 concentration estimates through the PETM using triple oxygen isotope analysis of mammalian bioapatite.” Proceedings of the National Academy of Sciences 113.28 (2016): 7739-7744. The Paleocene–Eocene Thermal Maximum (PETM) is a remarkable climatic and environmental event that occurred 56 Ma ago and has importance for understanding possible future climate change. The Paleocene–Eocene transition is marked by a rapid temperature rise contemporaneous with a large negative carbon isotope excursion (CIE). Both the temperature and the isotopic excursion are well-documented by terrestrial and marine proxies. The CIE was the result of a massive release of carbon into the atmosphere. However, the carbon source and quantities of CO₂ and CH₄ greenhouse gases that contributed to global warming are poorly constrained and highly debated. Here we combine an established oxygen isotope paleothermometer with a newly developed triple oxygen isotope paleo-CO₂ barometer. We attempt to quantify the source of greenhouse gases released during the Paleocene–Eocene transition by analyzing bioapatite of terrestrial mammals. Our results are consistent with previous estimates of PETM temperature change and suggest that not only CO₂ but also massive release of seabed methane was the driver for CIE and PETM.