Global warming and abrupt ocean circulation changes at the
Paleocene/Eocene boundary (55 Ma)
1. motivation
2. tool / numerical model setup
12
10
8
6
4
benthic δ18O [‰]
0
1
2
temperature [oC]
(for an ice-free ocean)
Paleotemperature proxies show an exceptional, short-lived (∼100 ka)
warm climate aberration about 55 Ma ago known as the
Paleocene/Eocene Thermal Maximum (PETM). Previous studies suggest
that this warm climate event was caused by a release of methane gas
(CH4) from melting clathrates in marine sediments (e.g. Dickens et al.
1995).
3
4. P/E control simulation: ocean circulation
To study the climate at the Paleocene/Eocene boundary, we use the fully coupled
atmosphere-ocean-sea ice GCM ECHAM5/MPI-OM. The resolution in the atmospheric part
is T31 with 19 vertical levels. For MPI-OM, we choose a curvilinear grid with 144x87
points and 40 vertical levels.
The topography is interpolated from a 2ox2o reconstruction derived by Bice and
Marotzke (2002). For simplicity, we first assume globally uniform vegetation and soil
properties (woody savanna), as well as constant orbital parameters.
MPI-OM
left: 65 million years of climate
change: global deep-sea oxygen
isotope ratio based on more than 40
DSDP and ODP sites; modified from
Zachos et al. (2001);
below: methane clathrate from ocean
sediments and ‘burning ice’; pictures
from www.rcom.marum.de.
In our control simulation, deepwater formation occurs in the proto-Labrador
Sea as well as more widespread around Antarctica. The North Atlantic
deepwater flows southward as a western boundary current at about 2km
depth. This fits with the deepwater track Nunes and Norris (2006) inferred
from δ13C for the PETM, but not the pre-PETM. However, the few δ13C data
points are located relatively far away from our modelled deepwater track.
convective depth:
ECHAM5
OASIS
4
5
0
20
10
30
40
Model setup; bathymetry and orography as
used to simulate the Paleocene/Eocene
boundary.
60
50
Million years ago
-6000
0
3000
0
[m]
3000
55.0
55.2
55.4
2
3
4
Atlantic
5
0
-2
-1
1
0
δ13C [0/00]
2
Relative change in carbon isotope
ratios of benthic foraminifera between
different locations (colours) indicate a
‘switch’ of the deepwater flow;
modified from Nunes and Norris
(2006)
Dickens, G.R., J.R. O’Neil, D.K. Rea, and R.M. Owen,1995: Dissociation of oceanic methane hydrate as a cause of the carbon-isotope
excursion at the end of the Paleocene, Paleoceanography, 10, 965-971.
Bice, K.L. and J. Marotzke, 2002: Could changing ocean circulation have destabilized methane hydrate at the Paleocene/Eocene
boundary? Paleoceanography,17, doi:10.1029/2001PA000678.
Tripati, A. and H. Elderfield, 2005: Deep-sea temperature and circulation changes at the Paleocene-Eocene thermal maximum,
Science, 308, 1894–1898.
500
depth [km]
1
20
2
10
0
3
-10
18
15
12
9
6
3
2
3
4
2
3
4
Arctic Ocean
Pacific
5
0
500
60
30
60
30
0
latitude [deg. North]
90
1
Max Planck Institute for Meteorology,
2000
0
upper left: 200a mean of the annual maximum of the monthly mean convective depth;
lower left: global meridional overturning circulation (averaged over the last 200a);
upper right: 200a mean of the top 690m average velocities; bathymetry plotted in the background;
lower right: 200a mean of the velocities averaged over the 690m to 2650m depth layer.
3
[oC]
8
4
‘Wedell’ Sea
0
6
12 18 24 30 36
7
35
30
25
20
15
10
5
60
even using the (for PETM standards) moderate CO2 concentration of
560ppm, the simulated P/E climate is very warm (mostly due to a
low surface albedo);
0
30
60
30
latitude [deg. North]
90
40
deepwater formation occurs in the North Atlantic as well as
relatively widespread in the Southern Ocean;
30
20
next step: investigate climate and ocean circulation sensitivity to
greenhouse gas forcing.
10
0
-10
-90
60
0
30
30 60
latitude [deg. North]
90
Pearson, P. N. and M.R. Palmer, 2000: Atmospheric carbon dioxide concentrations over the past 60 million years, Nature, 406,
695–699.
Malte
4000
[m]
9
-90
Nunes, F. and R.D. Norris, 2006: Abrupt reversal in ocean overturning during the Palaeocene/Eocene warm period, Nature, 439,
60–63.
-90
-30
we performed a coupled atmosphere-ocean GCM simulation with
Paleocene/Eocene boundary conditions;
1000 1500 2000 [oC]
10
2
top left: time evolution of the horizontal mean
potential water temperature in different areas;
top right: surface temperature (averaged over
the last 200a of the 2000a simulation);
right: zonal mean surface temperature; black
line is the 200a mean; upper and lower bound
of the shading are given by the maximum and
minimum monthly mean surface temperatures
(also averaged over the last 200a).
5
5. summary and outlook
1
5
-20
surface temperature:
18
15
12
9
6
3
1
1000 1500 2000 [oC]
1
5
references:
18
15
12
9
6
3
1
0
time [years]
500 1000 1500 2000 [oC]
land surface temperature [oC]
depth [km]
0
time [years]
500 1000 1500 2000 [oC]
sea surface temperature [oC]
54.8
Greenhouse gas concentrations even before the carbon isotope excursion at the P/E
boundary are widely believed to have been higher than present (e.g. Pearson and Palmer
2000). For our control simulation, we are using a ‘moderate’ CO2 concentration of 560ppm.
CO2 concentration and land surface boundary conditions (mostly the surface albedo) add
up to an already very warm ice-free climate.
circulation, 690 to 2650m:
[Sv]
30
4
depth [km]
Millions of years ago
possible deepwater tracks
[m]
300 600 900 1200 1500
MOC:
3. P/E control simulation: temperature
Our objective is to study the climate system at the Paleocene/Eocene
boundary and to test the hypothesis that the melting of methane
clathrates was due to an abrupt change of the global ocean circulation
(Bice and Marotzke 2002; Tripati and Elderfield 2005; Nunes and Norris
2006).
circulation, surface to 690m:
1,2
Heinemann ,
Jochem
1
Marotzke
(malte.heinemann@zmaw.de)
2International
Max Planck Research School on Earth System Modelling, Hamburg,
Germany