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PP23B-1346: Climate and Ocean Circulation at the Paleocene/Eocene Boundary and Their Sensitivity to Atmospheric CO2

Malte Heinemann1,2, Jochem Marotzke1

(malte.heinemann@zmaw.de)1 Max Planck Institute for Meteorology, 2International Max Planck Research School on Earth System Modelling, Hamburg, Germany

reconstructed deepwater track(Nunes & Norris 2006):

simulated deepwater track (690m-2650m) for pre-PETM conditions

PETM

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proxy data:Thomas et al. (2002)1)

Tripati and Elderfield (2004) 2) 3)

Zachos et al. (2003) 4)

Zachos et al. (2006) 5)

Sluis et al. (2006) 6)

The Paleocene/Eocene Boundary (PEB) was marked by an extraordinary, short-lived global warming event due to increased atmospheric greenhouse gas (GHG) concentrations. This event, the Paleocene/Eocene Thermal Maximum (PETM), was superimposed on a long-term global warming trend.

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We aim at two objectives: First, to numerically simulate the already warm climate before the PETM. Second, to investigate the climate response to a variation of CO2. We focus on possible long-term changes of the global ocean circulation to test the hypothesis that such changes could have triggered the dissociation of methane hydrates in marine sediments and thus caused the subsequent GHG warming.

To study the climate at the PEB, we use the fully coupled atmosphere-ocean-sea ice GCM ECHAM5/MPI-OM. The resolution in the atmospheric part is T31 (~3.75°x3.75°) with 19 vertical levels. For MPI-OM, we choose a curvilinear grid with poles on paleo-South America and Asia, 144x87 points (~1° to 4°) and 40 vertical levels.The topography is interpolated from a 2°x2° reconstruction derived by Bice and Marotzke(2002). For simplicity, we first assume globally uniform vegetation and soil properties (woody savannah), as well as constant orbital parameters.

1. motivation

2. tool / numerical model setup

-6000 0 30003000[m]

MPI-OM ECHAM5

Model setup; bathymetry and orography as used to simulate the Paleocene/Eocene boundary.

OASIS

global deep-sea oxygen isotope ratio based on more than 40 DSDP and ODP sites; modified from Zachos et al. (2001);

references: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.Nunes, F. and R.D. Norris, 2006: Abrupt reversal in ocean overturning during the Palaeocene/Eocene warm period, Nature, Vol. 439, pp 60–63. Pearson, P. N. and M.R. Palmer, 2000: Atmospheric carbon dioxide concentrations over the past 60 million years, Nature, Vol. 406, pp 695–699.Sluis, A. et al., 2006: Subtropical Arctic Ocean temperatures during the Paleocene/Eocene thermal maximum, Nature, Vol. 441, pp 610-613.Thomas, D.J., J.C. Zachos, T.J. Bralower, E. Thomas, and S. Bohaty, 2002: Warming the fuel for the fire: Evidence for the thermal dissociation of methane hydrate during the Paleocene-Eocene thermal maximum, Geology, Vol. 30, No. 12, pp 1067-1070.Tripati, A. K. and H. Elderfield, 2004: Abrupt hydrographic changes in the equatorial Pacific and subtropical Atlantic from foraminiferal Mg/Ca indicate greenhouse origin for the thermal maximum at the Paleocene-Eocene Boundary, Geochemistry Geophysics Geosystems, Vol. 5, No. 2. Zachos, J.C. et al., 2003: A Transient Rise in Tropical Sea Surface Temperature During the Paleocene-Eocene Termal Maximum, Science, Vol. 302, pp 1551-1554. Zachos, J.C. et al. 2006: Extreme warming of mid-latitude coastal ocean during the Paleocene-Eocene Thermal Maximum: Inferences from TEX86 and isotope data, Geology, Vol. 34, No. 9, pp 737-740.

3. pre-PETM climate and ocean circulation 4. sensitivity to atmospheric CO2

Greenhouse gas concentrations even before the carbon isotope excursion at the PEB are widely believed to have been higher than present (e.g., Pearson and Palmer 2000). For our control simulation, we use a ‘moderate’ CO2 concentration of 560ppm. CO2 concentration and land surface boundary conditions (mostly the surface albedo) add up to a warm sea-ice free climate.

200a mean of the simulated upper 50m temperature; circles indicate proxy data paleo locations

In our control simulation, deepwater formation occurs mainly in the proto-Labrador Sea. This North Atlantic Deep Water (NADW) flows southward as a western boundary current at about 2km depth below the open Panama Strait. This fits with the deepwater track Nunes and Norris (2006) inferred from δ13C for the PETM, but not for the pre-PETM.

5. summary and outlook

200a mean of the simulated sea surface salinity

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control run: CO2=560ppm840ppm1120ppm

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The Paleocene/Eocene Boundary (PEB) provides a testbed to run and evaluate our climate model under extreme conditions;

the simulated pre-PETM Arctic Ocean surface is much colder than suggested by proxy data (18°C reconstructed by Sluis et al. 2006 5°C simulated);pre-PETM deepwater formation mainly occurs in the proto-Labrador Sea, in contrast to the reconstruction by Nunes and Norris (2006);for reduced CO2, NADW formation becomes even stronger; in addition, South Pacific Deep Water formation sets in;

CO2 increase causes model instability (ongoing work; could be a true runaway or just a model artefact).

To test the hypothesis that a sudden switch of the ocean circulation caused a massive dissolution of methane hydrates, we investigate the sensitivity of the ocean circulation to a variation of CO2. A decrease to a CO2 concentration of 280ppm causes a deeper Meridional Overturning Circulation (MOC). In the Pacific, southern deepwater formation sets in.

convective depth(200a mean of annual maximum)Global MOC (200a mean) Atlantic MOC Pacific MOC

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Increasing the CO2 concentration to 840ppm and 1120ppm currently yields an instable climate. The planetary albedo reduces due to decreasing cloud cover. Moreover, increasing absolute humidity causes an additional greenhouse warming. This runaway effect is currently under investigation and might be a model artefact.

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Five of the six pre-PETM sea surface temperature (SST) reconstructions shown in the upper left figure lie within the simulated seasonal variation of the zonal mean SST. Some small deviations between the zonal mean and the reconstructions may be due to zonal temperature variations (e.g. location within the equatorial Pacific cold tongue). The simulated Arctic Ocean surface is about 13°C colder than reconstructed (site )! In contrast to all the other basins, the weakly connected Arctic deep ocean is not very well equilibrated but still warms up significantly. Moreover, warm and salty deeper water lies beneath the cold and fresh surface water:

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