Eocene Thermal Maximum 2 (ETM-2), also called H-1 or the Elmo (Eocene Layer of Mysterious Origin) event, was a transient period of global warming that occurred around either 54.09 Ma[1][2] or 53.69 Ma.[3][4][5] It appears to be the second major hyperthermal that punctuated the long-term warming trend from the Late Paleocene through the Early Eocene (58 to 50 Ma).[6]

The hyperthermals were geologically brief time intervals (<200,000 years) of global warming and massive input of isotopically light carbon into the atmosphere.[7][8] The most extreme and best-studied event, the Paleocene-Eocene Thermal Maximum (PETM or ETM-1), occurred about 1.8 million years before ETM-2, at approximately 55.5 Ma. Other hyperthermals likely followed ETM-2 at nominally 53.6 Ma (H-2), 53.3 (I-1), 53.2 (I-2) and 52.8 Ma (informally called K, X or ETM-3). The number, nomenclature, absolute ages and relative global impact of the Eocene hyperthermals are the source of much current research.[9][10] In any case, the hyperthermals appear to have ushered in the Early Eocene Climatic Optimum, the warmest sustained interval of the Cenozoic Era.[11] They also definitely precede the Azolla event at about 49 Ma.

Timing

ETM-2 is clearly recognized in sediment sequences by analyzing the stable carbon isotope composition of carbon-bearing material.[3][9][10] The 13C/12C ratio of calcium carbonate or organic matter drops significantly across the event.[12] This is similar to what happens when one examines sediment across the PETM, although the magnitude of the negative carbon isotope excursion is not as large. The timing of Earth system perturbations during ETM-2 and PETM also appear different.[5] Specifically, the onset of ETM-2 may have been longer (perhaps 30,000 years) while the recovery seems to have been shorter (perhaps <50,000 years).[5] (Note, however, that the timing of short-term carbon cycle perturbations during both events remains difficult to constrain.)

A thin clay-rich horizon marks ETM-2 in marine sediment from widely separated locations. In sections recovered from the deep sea (for example those recovered by Ocean Drilling Program Leg 208 on Walvis Ridge), this layer is caused by dissolution of calcium carbonate.[5] However, in sections deposited along continental margins (for example those now exposed along the Waiau Toa / Clarence River, New Zealand), the clay-rich horizon represents dilution by excess accumulation of terrestrial material entering the ocean.[4] Similar changes in sediment accumulation are found across the PETM.[4] In sediment from Lomonosov Ridge in the Arctic Ocean, intervals across both ETM-2 and PETM show signs of higher temperature, lower salinity and lower dissolved oxygen.[8]

Causes

ETM-2 resulted from a surge in atmospheric carbon dioxide, which climbed to over 1,000 ppm during the hyperthermal event.[13] The PETM and ETM-2 are thought to have a similar generic origin,[4][8][5] although this idea is at the edge of current research. During both events, a tremendous amount of 13C-depleted carbon rapidly entered the ocean and atmosphere. This decreased the 13C/12C ratio of carbon-bearing sedimentary components, and dissolved carbonate in the deep ocean. Somehow the carbon input was coupled to an increase in Earth surface temperature and a greater seasonality in precipitation, which explains the excess terrestrial sediment discharge along continental margins. Possible explanations for changes during ETM-2 are the same as those for the PETM, and are discussed in that article.

The H-2 event appears to be a "minor" hyperthermal that follows ETM-2 (H-1) by about 100,000 years. This has led to speculation that the two events are somehow coupled and paced by changes in orbital eccentricity.[4][5]

Sea surface temperatures (SSTs) climbed by 2–4 °C and salinity by ~1–2 ppt in subtropical waters during ETM-2.[14]

Effects

Ocean acidification did occur during ETM2 as it did in the PETM, but the magnitude of the drop in pH was significantly lower.[15]

As in the case of the PETM, reversible dwarfing of mammals has been noted to have occurred during the ETM-2.[16][17]

See also

References

  1. ↑ Westerhold, Thomas; Röhl, Ursula; Laskar, Jacques; Raffi, Isabella; Bowles, Julie; Laurens, Lucas J.; Zachos, James C. (6 April 2007). "On the duration of magnetochrons C24r and C25n and the timing of early Eocene global warming events: Implications from the Ocean Drilling Program Leg 208 Walvis Ridge depth transect". Paleoceanography and Paleoclimatology. 22 (2). Bibcode:2007PalOc..22.2201W. doi:10.1029/2006PA001322. Retrieved 4 December 2022.
  2. ↑ Galeotti, Simone; Sprovieri, Mario; Rio, Domenico; Moretti, Matteo; Francescone, Federica; Sabatino, Nadia; Fornaciari, Eliana; Giusberti, Luca; Lanci, Luca (1 August 2019). "Stratigraphy of early to middle Eocene hyperthermals from Possagno (Southern Alps, Italy) and comparison with global carbon isotope records". Palaeogeography, Palaeoclimatology, Palaeoecology. 527: 39–52. Bibcode:2019PPP...527...39G. doi:10.1016/j.palaeo.2019.04.027. S2CID 149669059. Retrieved 4 December 2022.
  3. 1 2 Lourens, L.J.; Sluijs, A.; Kroon, D.; Zachos, J.C.; Thomas, E.; Röhl, U.; Bowles, J.; Raffi, I. (2005). "Astronomical pacing of late Palaeocene to early Eocene global warming events". Nature. 435 (7045): 1083–1087. Bibcode:2005Natur.435.1083L. doi:10.1038/nature03814. hdl:1874/11299. PMID 15944716. S2CID 2139892.
  4. 1 2 3 4 5 Nicolo, M.J.; Dickens, G.R.; Hollis, C.J.; Zachos, J.C. (2007). "Multiple early Eocene hyperthermals: Their sedimentary expression on the New Zealand continental margin and in the deep sea". Geology. 35 (8): 699–702. Bibcode:2007Geo....35..699N. doi:10.1130/G23648A.1.
  5. 1 2 3 4 5 6 Stap, L.; Lourens, L.J.; Thomas, E.; Sluijs, A.; Bohaty, S.; Zachos, J.C. (2010). "High-resolution deep-sea carbon and oxygen isotope records of Eocene Thermal Maximum 2 and H2". Geology. 38 (7): 607–610. Bibcode:2010Geo....38..607S. doi:10.1130/G30777.1. hdl:1874/385773. S2CID 41123449.
  6. ↑ Zachos, J.C.; Dickens, G.R.; Zeebe, R.E. (2008). "An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics". Nature. 451 (7176): 279–283. Bibcode:2008Natur.451..279Z. doi:10.1038/nature06588. PMID 18202643.
  7. ↑ Li, Yuanji; Sun, Pingchang; Falcon-Lang, Howard J.; Liu, Zhaojun; Zhang, Baoyong; Zhang, Qiang; Wang, Junxian; Xu, Yinbo (15 January 2023). "Eocene hyperthermal events drove episodes of vegetation turnover in the Fushun Basin, northeast China: Evidence from a palaeoclimate analysis of palynological assemblages". Palaeogeography, Palaeoclimatology, Palaeoecology. 610: 111317. Bibcode:2023PPP...610k1317L. doi:10.1016/j.palaeo.2022.111317. Retrieved 3 December 2022 – via Elsevier Science Direct.
  8. 1 2 3 Sluijs, A.; Schouten, S.; Donders, T.H.; Schoon. P.L.; Röhl, U.; Reichart, G.-J.; Sangiorgi, F.; Kim, J.-H.; Sinninghe Damsté, J.S.; Brinkhuis, H. (2009). "Warm and wet conditions in the Arctic region during Eocene Thermal Maximum 2". Nature Geoscience. 2 (11): 777–780. Bibcode:2009NatGe...2..777S. doi:10.1038/ngeo668. hdl:1874/39397. S2CID 130137472.
  9. 1 2 Slotnick, B.S.; Dickens. G.R.; Nicolo, M.J.; Hollis, C.J.; Crampton, J.S.; Zachos, J.C.; Sluijs, A. (2012). "Large amplitude variations in carbon cycling and terrestrial weathering during the latest Paleocene and earliest Eocene: The record at Mead Stream, New Zealand". Journal of Geology. 120 (5): 487–505. Bibcode:2012JG....120..487S. doi:10.1086/666743. hdl:1911/88269. S2CID 55327247.
  10. 1 2 Abels, H.A..; Clyde, H.C.; Gingerich, P.D.; Hilgen, F.J.; Fricke, H.C.; Bowen, G.J.; Lourens, L.J. (2012). "Terrestrial carbon isotope excursions and biotic change during Palaeogene hyperthermals". Nature Geoscience. 5 (8): 326–329. Bibcode:2012NatGe...5..326A. doi:10.1038/NGEO1427.
  11. ↑ Slotnick, B. S.; Dickens, G. R.; Hollis, C. J.; Crampton, J. S.; Strong, C. Percy; Phillips, A. (17 September 2015). "The onset of the Early Eocene Climatic Optimum at Branch Stream, Clarence River valley, New Zealand". New Zealand Journal of Geology and Geophysics. 58 (3): 262–280. doi:10.1080/00288306.2015.1063514. S2CID 130982094. Retrieved 20 September 2022 – via Taylor and Francis.
  12. ↑ Clementz, Mark; Bajpai, S.; Ravikant, V.; Thewissen, J. G. M.; Saravanan, N.; Singh, I. B.; Prasad, V. (1 January 2011). "Early Eocene warming events and the timing of terrestrial faunal exchange between India and Asia". Geology. 39 (1): 15–18. Bibcode:2011Geo....39...15C. doi:10.1130/G31585.1. Retrieved 6 April 2023.
  13. ↑ Cui, Ying; Schubert, Brian A. (15 November 2017). "Atmospheric pCO2 reconstructed across five early Eocene global warming events". Earth and Planetary Science Letters. 478: 225–233. doi:10.1016/j.epsl.2017.08.038. ISSN 0012-821X. Retrieved 30 December 2023 – via Elsevier Science Direct.
  14. ↑ Harper, Dustin T.; Zeebe, Richard; Hönisch, Bärbel; Schrader, Cindy D.; Lourens, Lucas J.; Zachos, James C. (20 December 2017). "Subtropical sea-surface warming and increased salinity during Eocene Thermal Maximum 2". Geology. 46 (2): 187–190. doi:10.1130/G39658.1. Retrieved 25 June 2023.
  15. ↑ Harper, D. T.; Hönisch, B.; Zeebe, R. E.; Shaffer, G.; Haynes, L. L.; Thomas, E.; Zachos, James C. (18 December 2019). "The Magnitude of Surface Ocean Acidification and Carbon Release During Eocene Thermal Maximum 2 (ETM‐2) and the Paleocene‐Eocene Thermal Maximum (PETM)". Paleoceanography and Paleoclimatology. 35 (2). doi:10.1029/2019PA003699. ISSN 2572-4517. Retrieved 31 December 2023.
  16. ↑ D'Ambrosia, Abigail R.; Clyde, William C.; Fricke, Henry C.; Gingerich, Philip D.; Abels, Hemmo A. (15 March 2017). "Repetitive mammalian dwarfing during ancient greenhouse warming events". Science Advances. 3 (3): e1601430. Bibcode:2017SciA....3E1430D. doi:10.1126/sciadv.1601430. PMC 5351980. PMID 28345031.
  17. ↑ Erickson, J. (1 November 2013). "Global warming led to dwarfism in mammals — twice". University of Michigan. Retrieved 12 November 2013.
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