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The Global Ocean and the Paleocene-Eocene Thermal Maximum ATOC220 December 1, 2008 David Carozza david.carozza@mail.mcgill.ca. Summary. Warming up to the early Paleogene Fractionation and δ 13 C The carbon isotope excursion (CIE) Possible causes of the event 2 environmental changes

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  1. The Global Ocean and the Paleocene-Eocene Thermal MaximumATOC220December 1, 2008David Carozzadavid.carozza@mail.mcgill.ca

  2. Summary • Warming up to the early Paleogene • Fractionation and δ13C • The carbon isotope excursion (CIE) • Possible causes of the event • 2 environmental changes • Modelling

  3. The Big Picture: The Paleocene-Eocene Thermal Maximum • An extreme period of global climate change (warming followed by cooling) that occurred about 55 million years ago (5° to 9° increase in SST, 4-5° increase in deep waters) • Discovered through large negative isotope excursions in ocean and terrestrial records • Caused by the abrupt release of greenhouse gases (most likely methane) • Induced changes in the components of the Earth system • This is the most analogous event in Earth history to present-day climate change

  4. δ18O In the Paleogene Spike in δ18O indicates abrupt global warming Proxy is for deep water temperature Zachos et al. (2002)

  5. Dr. Ron Blakey http://jan.ucc.nau.edu/~rcb7/globehighres.html http://jan.ucc.nau.edu/~rcb7/globaltext2.html

  6. http://iodp.tamu.edu/scienceops/maps.html

  7. δ13C and Fractionation Isotopes of carbon are used to measure sources and fluxes of carbon throughout the Earth system Ruddiman (2001) p. 244

  8. The Discovery! A δ13C Excursion • Ocean sediment from ODP Site 690 (Maud Rise, Weddell Sea) first identified the carbon isotope excursion (CIE) (Kennett and Stott, 1991) • Magnitude and abruptness of event unprecedented • δ13C formula: [(13C/12C)sample/(13C/12C)standard – 1 ] x 1000 (measured in parts per thousand ‰) Figure from Nunes and Norris (2006)

  9. The CIE Throughout the World

  10. Causes of the PETM • CIE gives three clues about the PETM • Source (12C enriched carbon) • Magnitude • Models can be used to estimate this (840 to 6800 GtC) (Dickens, 1995; Panchuk et al., 2008) • Abruptness • Theories • Methane hydrate • Sill intrusions • Peat burning

  11. Methane Hydrate • Formation • Bacteria in sediments take up organic matter (strong preference for 12C) and release methane • Under sufficiently high pressure, low temperature, and high CH4 concentration, methane hydrate can form • Structure • Frozen water ice with CH4 embedded • δ13C = -60‰

  12. The Methane Hydrate Hypothesis • Theory is that the hydrates melted and methane was released into the ocean and atmosphere • Hydrate can be released by: • Increase in T (warming of water) • Decrease in P (tectonic activity or sea-level change) • This is the best hypothesis to date • Most calculations show that about 2500 GtC are needed to generate the observed CIE with it (reasonable considering the estimated size of the reservoir of methane hydrate) • Reservoir can also be affected quickly • Problem is that the trigger (force that caused the initial release) is still elusive

  13. Environmental Changes During the PETM • Global warming • Shift in global thermohaline circulation • Deepwater formation switch from southern hemisphere to northern hemisphere • Extinction of benthic foraminifera • Change in precipitation patterns • More rain at the poles than before • Decrease in ocean pH (acidification) • Enhanced surface ocean productivity • Due to increased nutrient load from rivers • Diversification of mammals

  14. A Shift in the THC • How do we know what the THC was 55 million years ago? • Carbon isotopes can be used as a tracer for nutrients • Older water masses contain higher nutrient concentrations • Nutrients are enriched in 12C because they originate from plankton (slide 7 shows that dead organic C has a δ13C of -22‰) • More positive δ13C • Younger deep water (region of deep water formation) • More negative δ13C • Older deep water Nunes and Norris (2006)

  15. THC through Time B) δ13C more positive in northern hemisphere A) δ13C more positive in southern hemisphere Nunes and Norris (2006)

  16. THC Results • Clear shift in location of deepwater formation • From Southern Hemisphere to Northern Hemisphere • Actual site of deepwater formation less certain due to similarity in the gradients • Global warming can cause abrupt shifts in the THC • Advocates of the methane hydrate theory argue that a THC shift like this one may have brought warmer water to intermediate depth and caused even more methane hydrate to melt Nunes and Norris (2006)

  17. Ocean pH Carbonic Acid Forms and Dissociates CO2 + H2O  H2CO3 H+ + HCO3- Bicarbonate Dissociates HCO3-  H+ + CO32- • Addition of CO2 causes pH and CO32- (see slide 20 of Roulet lecture) to fall and the lysocline and CCD will shoal http://www.klimaktiv.de/media/08/40_klima/gkss_pm_05_2008_q.idw.jpg

  18. The carbonate ion CO32- • Supersaturated in surface waters • Lysocline • Depth at which concentration of carbonate is less than saturation (dissolution begins at this level, mainly due to pressure increase) • Carbonate can still accumulate if the flux to the seafloor > rate of dissolution • Calcite Compensation Depth (CCD) • Depth at which there is more dissolution than influx of carbonate • Carbonate cannot accumulate below this level

  19. How do we know that the CCD and lysocline shoaled? • Shoaling causes • Change from sediment rich in carbonate (ooze) to sediment rich in clay (red) • Due to enhanced dissolution below the CCD • Shoaling causes • Increase in the thickness of the clay layer with increasing depth • Deeper sites remained below the CCD longer • This is exactly what Zachos et al. (2005) found

  20. Cores and Weight % (CaCO3) Weight % of CaCO3 falls to 0% at the CIE Zachos et al. (2005)

  21. Modelling the PETM • Allows theories of the source and amount of C emitted to be tested • 3 Examples • Dickens et al. (1997) required 840 GtC (CO2) into the atmosphere and generated an excursion of 2.5 ‰ • Panchuk et al. (2008) required 6800 GtC to reproduce the record of dissolution of calcium carbonate • Zachos et al. (2005) write of 4500 GtC being required to stop carbonate accumulation throughout the ocean • Why is there such a big difference? • Different models involving many different assumptions • Results based on maximizing the fit to different data sets

  22. Schematic of the Walker and Kasting (1992) Model of the Global Carbon Cycle I am using this model without the methane hydrate reservoir Dickens (1999)

  23. What does the model do? • Each of the boxes is a well-mixed region with characteristic variables (variable is constant throughout the reservoir) • Forced by fossil fuel emissions • Controlled by 32 equations • Phosphate (6) and total dissolved carbon (6) concentrations • Alkalinity (6), pCO2 (1), average atmospheric temperature (1), δ13C (8), biomass (1), pelagic carbonates (3)

  24. A Familiar Experiment • Emissions of 1.5 Gt C year-1 for 1000 years starting from t=20000 years with a δ13C of -60 ‰ • CO2 emitted directly into the atmosphere • Model then run for 150 000 • Timestep of 5 years

  25. δ13C Excursion of ~2‰ generated

  26. Atmospheric CO2 Because the CO2 is emitted so quickly it stays in the atmosphere and the CO2 concentration increases quickly Within a few thousand years the ocean is able to take up most of the CO2 and the CO2 concentration decreases

  27. Modelled Lysocline (CCD)

  28. Thank You! Please let me know if you have any additional questions or comments about this topic: david.carozza@mail.mcgill.ca

  29. References • Bowen et al. (2002), Mammalian dispersal at the Paleocene/Eocene boundary, Science, 295, 2062-2065. • Dickens (1999), A blast in the past, Nature, 401, 752-755. • Dickens (2000), Modeling the Global Carbon Cycle With a Gas Hydrate Capacitor: Significance for the Latest Paleocene Thermal Maximum, Natural Gas Hydrates: Occurrence, Distribution, and Detection, Geophysical Monograph. • Norris and Rohl (1999), Carbon cycling and chronology of climate warming during the Palaeocene/Eocene transition, Nature, 401, 775-778. • Nunes and Norris (2006), Abrupt reversal in ocean overturning during the Palaeocene/Eocene warm period, Nature, 439, 60-63. • Panchuk et al. (2008), Sedimentary response to Paleocene-Eocene Thermal Maximum carbon release: A model-data comparison, Geology, 36 (4), 315-318. • Ruddiman (2001), Earth’s Climate Past and Future, W.H. Freeman and Company. • Schmitz B. and Pujalte V. (2003), Sea-level, humidity, and land-erosion records across the initial Eocene thermal maximum from a continental marine transect in northern Spain, Geology, 31, 689-692. • Walker, J.C.G. and J.F. Kasting (1992), Effect of forest and fuel conservation on future levels of atmospheric carbon dioxide, PPP, 97: 151-189. (1992) Zachos et al. (2002), Shipboard Scientific Party, Leg 198 Preliminary Report. ODP Preliminary Report, 98. • Zachos et al. (2005), Rapid acidification of the ocean during the Paleocene-Eocene Thermal Maximum, Science, 308, 1611-1615.

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