Primeval 23 – Mid-Eocene Climatic Optimum

 

The Eocene Thermal Maximums and EECO existed for a combined 6 million years, until approximately 49 million years ago, when whatever had been keeping the world warm failed, and the world began descending into an ice-house stage. Isotopes of carbon and oxygen indicate a shift to a global cooling climate.(1)E. N. Speelman et al. (2009) “The Eocene Arctic Azolla bloom: environmental conditions, productivity, and carbon drawdown” Geobiology, Volume 7, Pages 155-170 The cause of the cooling has been attributed to a significant decrease of more than 2000 ppm in atmospheric carbon-dioxide concentrations.(2)Paul N. Pearson and Martin R. Palmer (2000) “Atmospheric carbon-dioxide concentrations over the past 60 million years” Nature, Volume 406, Pages 695-699 One proposed cause of the reduction in carbon-dioxide during the warming to cooling transition was the Azolla event. The increased warmth at the poles, the isolated Arctic basin during the early Eocene, and the significantly high amounts of carbon-dioxide possibly led to azolla blooms across the Arctic Ocean. Azolla is a genus of seven species of aquatic ferns, that are extremely reduced in form and specialized, looking nothing like conventional ferns but more resembling duckweed or some mosses. The isolation of the Arctic Ocean led to stagnant stratified waters and as the azolla sank to the sea floor, they became part of the sediments and effectively sequestered the carbon and nitrogen. The ability for the azolla to sequester carbon is exceptional, and the enhanced burial of azolla could have had a significant effect on the world atmospheric carbon content and may have been the event to begin the transition into an ice-house climate.

Azolla Caroliniana

Azolla Caroliniana

Global cooling continued until there was a major reversal from cooling to warming indicated in the Southern Ocean at around 42 to 41 million years ago.(3)Steven M. Bohaty and James C. Zachos (2003) “Significant Southern Ocean warming event in the late middle Eocene” Geology, Volume 31, Pages 1017-1020 Oxygen isotope analysis showed a large negative change in the proportion of heavier oxygen isotopes to lighter oxygen isotopes, which indicates an increase in global temperatures. This warming event is known as the Middle Eocene Climatic Optimum (MECO). The cause of the warming is considered to primarily be due to carbon-dioxide increases, since carbon isotope signatures rule out major methane release during this short term warming.

It is unclear where this carbon-dioxide came from, however several theories do exist. One theory is that the impact of India into Asia, and the subsequent mountain-building in the Himalayas could have produced the carbon-dioxide.(4)Paul N. Pearson (2010) “Increased Atmospheric CO2 During the Middle Eocene” Science, Volume 330, Pages 763-764 Another theory is that their could have been significant volcanic activity between Antarctica and Australia as the two continents rifted apart, however there are no significant volcanic ranges known to exist between the two continents. Whatever the cause this warming was fairly short lived, as benthic oxygen isotope records indicate a return to cooling at around 40 million years ago.(5)Mark Pagani et al. (2005) “Marked Decline in Atmospheric carbon-dioxide Concentrations During the Paleogene” Science, Volume 309, Pages 600-603

Popigai Crater in Northern Siberia

Popigai Crater in Northern Siberia

Once again it was a global warming event that does not have an adequate explanation. The world’s temperature warmed significantly for one to two million years, due to unexplained carbon emissions, and then once those emissions ended, the world continued on its cooling trend. The idea that the mountain-building in the Himalayas created the carbon-emissions seems to overlook the fact that the mountain-building has never stopped, why would the emission have suddenly stopped. This event took place approximately 9 million years after the end of the Eocene Maximum. It is possible that a civilization rebuilt and attempted to rebuild the warmer Earth again. Unlike the previous events this warming period is not documented as having either an extinction event, or an explosion of new species.

Cooling continued throughout the rest of the Late Eocene into the Eocene-Oligocene transition. During the cooling period, benthic oxygen isotopes show the possibility of ice creation and ice increase during this later cooling. The end of the Eocene and beginning of the Oligocene is marked with the massive expansion of area of the Antarctic ice sheet that was a major step into the ice-house climate.(6)Caroline H. Lear (2008) “Cooling and ice growth across the Eocene-Oligocene transition” Geology, Volume 36, Pages 251-254 Another major contribution to the expansion of the ice sheet was the creation of the Antarctic circumpolar current.(7)P. F. Barker and E. Thomas (2004) “Origin, signature and palaeoclimatic influence of the Antarctic Circumpolar Current” Earth-Science Reviews, Volume 66, Pages 143-162 The creation of the Antarctic circumpolar current would isolate the cold water around the Antarctic, which would reduce heat transport to the Antarctic(8)Matthew Hubera and Doron Nof (2006) “The ocean circulation in the southern hemisphere and its climatic impacts in the Eocene” Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 231, Pages 9-28 along with the creation of oceanic gyres that resulted in the upwelling of colder bottom waters.

Map of Chesapeake Crater

Map of Chesapeake Crater

The Eocene ended with the Eocene–Oligocene extinction event also called the Grande Coupure, which happened during what appears to have been an ice-age. The Grande Coupure seems to have been another multiple impact scenario involving asteroids impacting at Popigai Crater, Toms Canyon and Chesapeake Bay. Popigai Crater is a 100 km (62 miles) in diameter crater in Northern Siberia.(9)Alexander Deutsch and Christian Koeberl (2006) “Establishing the link between the Chesapeake Bay impact structure and the North American tektite strewn field: The Sr-Nd isotopic evidence” Meteoritics & Planetary Science, Volume 41, Number 5, Pages 689–703 Toms Canyon is an impact crater site where one or more asteroids struck the Atlantic continental shelf, about 160 km (99 miles) east of Atlantic City, New Jersey leaving a 22 km (14 miles) diameter crater.(10)David Rajmon (2009) Impact database 2010.1 The Chesapeake Bay impact crater(11)Earth Impact Database. University of New Brunswick was formed by an asteroid that impacted the eastern shore of North America about 35 million years ago. It is one of the best-preserved “wet-target” or marine impact craters, and the largest known impact crater in the U.S. Continued slumping of sediments over the rubble of the crater has helped shape the Chesapeake Bay.

During the Grande Coupure, most of the indigenous European mammal species became extinct, to be replaced by Asian mammals that migrated into Europe over the following 350,000 years.(12)J. J. Hooker (2004) “Eocene-Oligocene mammalian faunal turnover in the Hampshire Basin, UK: calibration to the global time scale and the major cooling event” Journal of the Geological Society, Volume 161, Number 2, Pages 161-172 Evidence in the world’s ocean current system indicates an abrupt cooling from 34.1 to 33.6 million years ago following the impacts. The leading scientific theory on climate cooling at this time is decrease in atmospheric carbon-dioxide, which slowly declined in the mid to late Eocene and possibly reached some threshold approximately 34 million years ago. This boundary is closely linked with the Oligocene Oi-1 event, an oxygen isotope excursion that marks the beginning of the permanent ice-sheet coverage on Antarctica.

References   [ + ]

1. E. N. Speelman et al. (2009) “The Eocene Arctic Azolla bloom: environmental conditions, productivity, and carbon drawdown” Geobiology, Volume 7, Pages 155-170
2. Paul N. Pearson and Martin R. Palmer (2000) “Atmospheric carbon-dioxide concentrations over the past 60 million years” Nature, Volume 406, Pages 695-699
3. Steven M. Bohaty and James C. Zachos (2003) “Significant Southern Ocean warming event in the late middle Eocene” Geology, Volume 31, Pages 1017-1020
4. Paul N. Pearson (2010) “Increased Atmospheric CO2 During the Middle Eocene” Science, Volume 330, Pages 763-764
5. Mark Pagani et al. (2005) “Marked Decline in Atmospheric carbon-dioxide Concentrations During the Paleogene” Science, Volume 309, Pages 600-603
6. Caroline H. Lear (2008) “Cooling and ice growth across the Eocene-Oligocene transition” Geology, Volume 36, Pages 251-254
7. P. F. Barker and E. Thomas (2004) “Origin, signature and palaeoclimatic influence of the Antarctic Circumpolar Current” Earth-Science Reviews, Volume 66, Pages 143-162
8. Matthew Hubera and Doron Nof (2006) “The ocean circulation in the southern hemisphere and its climatic impacts in the Eocene” Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 231, Pages 9-28
9. Alexander Deutsch and Christian Koeberl (2006) “Establishing the link between the Chesapeake Bay impact structure and the North American tektite strewn field: The Sr-Nd isotopic evidence” Meteoritics & Planetary Science, Volume 41, Number 5, Pages 689–703
10. David Rajmon (2009) Impact database 2010.1
11. Earth Impact Database. University of New Brunswick
12. J. J. Hooker (2004) “Eocene-Oligocene mammalian faunal turnover in the Hampshire Basin, UK: calibration to the global time scale and the major cooling event” Journal of the Geological Society, Volume 161, Number 2, Pages 161-172