The Global Ocean and the Paleocene-Eocene Thermal

Maximum

ATOC220December 1, 2008

David Carozzadavid.carozza@mail.mcgill.ca

Summary

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

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

δ18O In the Paleogene

Zachos et al. (2002)

Spike in δ18O indicates abrupt global warming

Proxy is for deep water temperature

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

http://jan.ucc.nau.edu/~rcb7/globaltext2.html

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

δ13C and Fractionation

Ruddiman (2001) p. 244

Isotopes of carbon are used to measure sources and fluxes of carbon throughout the Earth system

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)

The CIE Throughout the World

Excursion (‰) Source Location Reference

Marine

-2.6 Benthic Antarctica Kennett and Stott (1991)

-2.8 Surface North Atlantic Norris and Rohl (1999)

Terrestrial

-6.0 Organic soil Tremp Basin, Spain

Schmitz and Pujalte (2003)

-4.7 Soil carbonate

Hengyand, China

Bowen et al. (2002)

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

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‰

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

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

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)

THC through Time

Nunes and Norris (2006)

A) δ13C more positive in southern hemisphere

B) δ13C more positive in northern hemisphere

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)

Ocean pHCarbonic Acid Forms and Dissociates

CO2 + H2O H2CO3 H+ + HCO3- Bicarbonate Dissociates HCO3- H+ + CO3

2-

• 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

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

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

Cores and Weight % (CaCO3)

Zachos et al. (2005)

Weight % of CaCO3 falls to 0% at the CIE

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

Schematic of the Walker and Kasting (1992) Model of the Global Carbon Cycle

Dickens (1999)I am using this model without the methane hydrate reservoir

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)

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

δ13CExcursion of ~2‰ generated

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

Modelled Lysocline (CCD)

Thank You!

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

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.