Transcript to - Earth System Modelling Group

The Global Ocean and the
Paleocene-Eocene Thermal
Maximum
ATOC220
December 1, 2008
David Carozza
[email protected]
Summary
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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 PaleoceneEocene 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
Spike in δ18O
indicates abrupt
global warming
Zachos et al.
(2002)
Proxy is for deep
water temperature
http://jan.ucc.nau.edu/
Dr.
Ron Blakey
http://jan.ucc.nau.edu/~rcb
~rcb7/globaltext2.html
7/globehighres.html
http://iodp.tamu.edu/scienceops/maps.html
δ13C and Fractionation
Isotopes of carbon are used to
measure sources and fluxes of
carbon throughout the Earth system
Ruddiman (2001) p. 244
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)
-6.0
Organic soil
Tremp Basin,
Spain
Schmitz and
Pujalte (2003)
-4.7
Soil
carbonate
Hengyand,
China
Bowen et al.
(2002)
Terrestrial
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
A) δ13C more
positive in southern
hemisphere
Nunes and Norris (2006)
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 pH
Carbonic Acid Forms and Dissociates
CO2 + H2O  H2CO3  H+ + HCO3Bicarbonate 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
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)
Weight % of CaCO3
falls to 0% at the CIE
Zachos et al. (2005)
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
I am using this model
without the methane
hydrate reservoir
Dickens (1999)
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
δ13C
Excursion 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: [email protected]
References
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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.