Primeval 20 – Eocene Epoch

 

The Paleocene–Eocene Thermal Maximum (PETM) also called the Eocene Thermal Maximum 1 (ETM-1) refers to a climate event that began at the beginning of the Eocene epoch. The absolute age and duration of the event remain uncertain, but are thought to be close to 55.8 million years ago and about 170,000 years long.(1)Ursula Röhl et al (2000) “New chronology for the late Paleocene thermal maximum and its environmental implications” Geology, Volume 28, Number 10, Pages 927–930 The PETM has become a focal point of considerable geoscientific research because it provides the best known past analog by which to understand impacts of global warming and massive carbon input to the ocean and atmosphere today, including ocean acidification.(2)Gerald R. Dickens et al (1997) “A blast of gas in the latest Paleocene; simulating first-order effects of massive dissociation of oceanic methane hydrate” Geology, Volume 25, Number 3, Pages 259–262

Necrolemur, an Early Primate

Necrolemur, an Early Primate

The onset of the PETM has been linked to an initial 5°C temperature rise and extreme changes in Earth’s carbon cycle.(3)Francesca A. McInherney and Scott L. Wing (2011) “A perturbation of carbon cycle, climate, and biosphere with implications for the future” Annual Reviews of Earth Science, Volume 39, Pages 489–516 The PETM is marked by a large decrease in Carbon-13/Carbon-12 ratio of marine and terrestrial carbonates and of organic carbon deposited in ocean basins. Carbon-12 and Carbon-13 are the two stable isotopes of carbon that occur in the Earth’s biosphere. Carbon-12 is the more common form, usually accounting for 98.9% of carbon, while Carbon-13 usually accounts for about 1.1%. The ratio of the two isotopes is called Delta Carbon Thirteen (δ¹³C) and is used in geochemistry, paleoclimatology and paleoceanography to determine the makeup of the ancient atmosphere, as carbon isotopes change depending on what compounds the carbon is part of. The PETM was responsible for the largest known extinction of deep-sea animals in the past 90 million years, and is associated with the warming of Antarctic surface waters by around 5 °C.(4)James P. Kennett and Lowell D. Stott (1991) “Abrupt deep-sea warming, palaeoceanographic changes and benthic extinctions at the end of the Paleocene” Nature, Volume 353, Number 6341, Pages 225–229 Numerous other changes can be observed in stratigraphic sections containing the PETM, including the sudden appearance in Europe and North America of modern mammals, including deer, horses, and early primates.

Earth surface temperatures increased by about 6°C from the late Paleocene through the early Eocene, culminating in the “Early Eocene Climatic Optimum” (EECO).(5)James C. Zachos et al. (2008) “An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics” Nature, Volume 451, Number 7176, Pages 279–83 Within this long-term gradual warming were several “hyper-thermals.” These were under 200,000 year long rapid global warming events accompanying major changes in the environment and massive carbon increases. The PETM was the most extreme and most-likely the first hyper-thermal within the Cenozoic. Another hyper-thermal clearly occurred at approximately 53.7 million years ago, and is now called ETM-2. Additional hyper-thermals probably took place at about 53.6 (H-2), 53.3 (I-1), 53.2 (I-2) and 52.8 million years ago (ETM-3).

Arctic Ocean During the PETM

Arctic Ocean During the PETM

At the start of the PETM, average global temperatures increased by approximately 6°C (11°F) within about 20,000 years.(6)Ellen Thomas and Nicholas J. Shackleton (1996) “The Paleocene-Eocene benthic foraminiferal extinction and stable isotope anomalies” Geological Society London Special Publications, Volume 101, Number 1, Pages 401 Precise limits on the global temperature rise during the PETM, and whether this varied significantly with latitude remain open issues. Certainly the central Arctic Ocean was ice-free before, during and after the PETM. This has been confirmed by the composition of sediment cores recovered during the Arctic Coring Expedition (ACEX) at 87°N on Lomonosov Ridge.(7)Kathryn Moran et al. (2006) “The Cenozoic palaeoenvironment of the Arctic Ocean” Nature, Volume 441, Pages 601–605 Temperature increases during the PETM are also indicated by the brief presence of subtropical plankton, and a marked increase in TEX86.(8)Appy Sluijs et al (2006) “Subtropical Arctic Ocean temperatures during the Palaeocene/Eocene thermal maximum” Nature, Volume 441, Number 7093, Pages 610–613 The TEX86 record is intriguing, though, because it suggests a 6°C (11°F) rise of average summer temperatures from around 17°C (63°F) before the PETM to around 23°C (73°F) during the PETM. TEX86 is an organic paleo-thermometer based upon the membrane lipids of mesophilic marine Thaumarchaeota.

Clear evidence for massive addition of Carbon-12 without Carbon-13 at the beginning of the PETM comes more than 130 locations from a widespread range of environments. Additionally carbonate dissolution marks the PETM in sections from the deep-sea. The total mass of carbon infused into the ocean and atmosphere during the PETM remains the source of debate. In theory, it can be estimated from the magnitude of the Delta Carbon Thirteen fluctuation, the amount of carbonate dissolution on the seafloor, or ideally both. However, the shift in the Delta Carbon Thirteen across the PETM depends on the location and the carbon-bearing phase analyzed. In some records of bulk carbonate, it is about 2%; in some records of terrestrial carbonate or organic matter it exceeds 6%.(9)Richard D. Norris and Ursula Röhl (1999) “Carbon cycling and chronology of climate warming during the Palaeocene/Eocene transition” Nature, Volume 401, Number 6755, Pages 775–778 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.(10)Richard E. Zeebe et al. (2009) “Carbon-dioxide forcing alone insufficient to explain Palaeocene–Eocene Thermal Maximum warming” Nature Geoscience, Volume 2, Pages 576–580 With available information, estimates of the carbon addition range from about 2000 to 7000 gigatons.(11)Ying Cui et al. (2011) “Slow release of fossil carbon during the Palaeocene-Eocene thermal maximum” Nature Geoscience, Volume 4, Pages 481–485

Climate Change During the PETM

Climate Change During the PETM

The timing of the PETM excursion is of considerable interest to scientists as it is a model of what is currently happening within the biosphere, and it could help us predict what will happen next. The total duration of the PETM can be estimated in several ways, although the general approach used in to examine core samples from oceanic ridges and continental shelves.(12)Luca Giusberti (2007) “Mode and tempo of the Paleocene-Eocene thermal maximum in an expanded section from the Venetian pre-Alps” Geological Society of America, Volume 119, Pages 391–412 In deep-marine locations sedimentation decreased across the PETM, presumably because of carbonate dissolution on the seafloor. In shallow-marine locations sedimentation increased across the PETM, presumably because of increasing delivery of river sediment during the event.

Age constraints at several deep-sea sites have been independently examined using Helium-3 contents, assuming the flux of Helium-3 is roughly constant over short time periods. This approach also suggests a rapid onset for the PETM less than 20,000 years. However the Helium-3 records support a fast recovery to near initial conditions in under 100,000 years, much faster than predictions based on flushing via weathering inputs and carbonate and organic outputs.(13)K. A. Farley and S. F. Eltgroth (2003) “An alternative age model for the Paleocene—Eocene thermal maximum using extraterrestrial 3He” Earth and Planetary Science Letters, Volume 208, Number 3-4, Pages 135–148

The climate also became much wetter with increased evaporation rates peaking in the tropics. Deuterium isotopes reveal that much more of this moisture was transported polewards than normal. This resulted in the largely isolated Arctic Ocean becoming more like a giant freshwater lake in as northern hemisphere rainfall was channeled towards it through the rivers of North America, Asia, and Europe.(14)Mark Pagani et al. (2006) “Arctic hydrology during global warming at the Palaeocene/Eocene thermal maximum” Nature, Volume 442, Numbers 7103, Pages 671–675

eocene_marsh_fauna_hq_by_zdenek_burian_1976

Life During the PETM

Although the cause of the initial warming has been attributed to a massive injection of carbon into the atmosphere, either as carbon-dioxide or methane, the source of the carbon has yet to be found. In order to balance the mass of carbon and produce the observed Delta Carbon Thirteen value, at least 1500 gigatons of carbon would have to have been degassed from the mantle via volcanoes over the course of the two 1000 year steps. This is about 200 times the rate of degassing for the rest of the Paleocene. There is no indication that such a burst of volcanic activity has occurred at any point in Earth’s history. A briefly popular theory held that a Carbon-12-rich comet struck the earth and initiated the warming event. Even allowing for feedback processes, this would require at least 100 gigatons of extraterrestrial carbon.(15)Dennis V. Kent et al. (2003) “A case for a comet impact trigger for the Paleocene/Eocene thermal maximum and carbon isotope excursion” Earth and Planetary Science Letters, Volume 211, Number 1-2, Pages 13–26 Such a catastrophic impact should have left its crater somewhere on the globe.

Another popular theory is that methane could have been released from the ocean floor, which would have caused the increase in global temperature. However, there are several major problems with the methane hypothesis. The most conservative interpretation for surface-water planktonic foram shows the Delta Carbon Thirteen excursion before their deep-sea counterparts, meaning the effects occurred from the top down, and not the bottom up. If the anomalous Carbon-12 entered the atmospheric first, and then diffused into the surface ocean waters, which mix with the deeper ocean waters over much longer time-scales, we would expect to observe the planktonics shifting toward lighter values before the benthics. Moreover, careful examination of the data set shows that there is not a single intermediate planktonic foram value, implying that the fluctuation and Delta Carbon Thirteen anomaly happened over the lifespan of a single foram, much too fast for the nominal 10,000 year release needed for the methane hypothesis to work.(16)Deborah J. Thomas et al. (2002) “Warming the fuel for the fire: Evidence for the thermal dissociation of methane hydrate during the Paleocene-Eocene thermal maximum” Geology, Volume 30, Number 12, Pages 1067–1070

Carbon Effect on Oceanic Lysocline Level

Carbon Effect on Oceanic Lysocline Level

Ultimately the cause of the PETM and the five hyper-thermals that followed it over the first 3 million years of the Eocene Period remain unknown. The effects are well documented, it is now widely accepted that the PETM represents a “case study” for global warming and massive carbon input to Earth’s surface. Massive quantities of carbon-dioxide were released into the atmosphere, and then sank into the oceans. As a result the lysocline level of the ocean rose to around 2 km (1.2 miles) deep,(17)J. C. Zachos and Lee R. Kump (2005) “Carbon cycle feedbacks and the initiation of Antarctic glaciation in the earliest Oligocene” Global and Planetary Change, Volume 47, Number 1, Pages 51–66 killing most animals living lower than that depth. The lysocline level marks the depth at which carbonate starts to dissolve. Currently this is at about 4 km (2.5 miles), comparable to the average depth of the oceans. This depth depends on several things, including temperature and the amount of carbon-dioxide dissolved in the ocean. Adding carbon-dioxide initially shallows the lysocline, resulting in the dissolution of deep water carbonates.

The PETM is accompanied by a mass extinction of 35% to 50% of deep-sea life over the course of 1,000 years. Contrarily, planktonic foraminifera diversified, and surface algaes bloomed. There is no evidence of any increased extinction rate among the terrestrial biota. Many major mammalian orders, including the Artiodactyla, horses, and primates, appeared and spread around the globe within 13,000 to 22,000 years of the start of the PETM.(18)Philip D. Gingerich (2003) “Mammalian responses to climate change at the Paleocene-Eocene boundary: Polecat Bench record in the northern Bighorn Basin, Wyoming” In Scott L. Wing (Editor) Causes and Consequences of Globally Warm Climates in the Early Paleogene, Volume 369, Pages 463–78

References   [ + ]

1. Ursula Röhl et al (2000) “New chronology for the late Paleocene thermal maximum and its environmental implications” Geology, Volume 28, Number 10, Pages 927–930
2. Gerald R. Dickens et al (1997) “A blast of gas in the latest Paleocene; simulating first-order effects of massive dissociation of oceanic methane hydrate” Geology, Volume 25, Number 3, Pages 259–262
3. Francesca A. McInherney and Scott L. Wing (2011) “A perturbation of carbon cycle, climate, and biosphere with implications for the future” Annual Reviews of Earth Science, Volume 39, Pages 489–516
4. James P. Kennett and Lowell D. Stott (1991) “Abrupt deep-sea warming, palaeoceanographic changes and benthic extinctions at the end of the Paleocene” Nature, Volume 353, Number 6341, Pages 225–229
5. James C. Zachos et al. (2008) “An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics” Nature, Volume 451, Number 7176, Pages 279–83
6. Ellen Thomas and Nicholas J. Shackleton (1996) “The Paleocene-Eocene benthic foraminiferal extinction and stable isotope anomalies” Geological Society London Special Publications, Volume 101, Number 1, Pages 401
7. Kathryn Moran et al. (2006) “The Cenozoic palaeoenvironment of the Arctic Ocean” Nature, Volume 441, Pages 601–605
8. Appy Sluijs et al (2006) “Subtropical Arctic Ocean temperatures during the Palaeocene/Eocene thermal maximum” Nature, Volume 441, Number 7093, Pages 610–613
9. Richard D. Norris and Ursula Röhl (1999) “Carbon cycling and chronology of climate warming during the Palaeocene/Eocene transition” Nature, Volume 401, Number 6755, Pages 775–778
10. Richard E. Zeebe et al. (2009) “Carbon-dioxide forcing alone insufficient to explain Palaeocene–Eocene Thermal Maximum warming” Nature Geoscience, Volume 2, Pages 576–580
11. Ying Cui et al. (2011) “Slow release of fossil carbon during the Palaeocene-Eocene thermal maximum” Nature Geoscience, Volume 4, Pages 481–485
12. Luca Giusberti (2007) “Mode and tempo of the Paleocene-Eocene thermal maximum in an expanded section from the Venetian pre-Alps” Geological Society of America, Volume 119, Pages 391–412
13. K. A. Farley and S. F. Eltgroth (2003) “An alternative age model for the Paleocene—Eocene thermal maximum using extraterrestrial 3He” Earth and Planetary Science Letters, Volume 208, Number 3-4, Pages 135–148
14. Mark Pagani et al. (2006) “Arctic hydrology during global warming at the Palaeocene/Eocene thermal maximum” Nature, Volume 442, Numbers 7103, Pages 671–675
15. Dennis V. Kent et al. (2003) “A case for a comet impact trigger for the Paleocene/Eocene thermal maximum and carbon isotope excursion” Earth and Planetary Science Letters, Volume 211, Number 1-2, Pages 13–26
16. Deborah J. Thomas et al. (2002) “Warming the fuel for the fire: Evidence for the thermal dissociation of methane hydrate during the Paleocene-Eocene thermal maximum” Geology, Volume 30, Number 12, Pages 1067–1070
17. J. C. Zachos and Lee R. Kump (2005) “Carbon cycle feedbacks and the initiation of Antarctic glaciation in the earliest Oligocene” Global and Planetary Change, Volume 47, Number 1, Pages 51–66
18. Philip D. Gingerich (2003) “Mammalian responses to climate change at the Paleocene-Eocene boundary: Polecat Bench record in the northern Bighorn Basin, Wyoming” In Scott L. Wing (Editor) Causes and Consequences of Globally Warm Climates in the Early Paleogene, Volume 369, Pages 463–78