Global warming and abrupt ocean circulation changes at the Paleocene/Eocene boundary (55 Ma)

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

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).

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).

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possible deepwater tracks

1. motivation

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)

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.

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.

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MPI-OM ECHAM5

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

2. tool / numerical model setup

3. P/E control simulation: temperature

5. summary and outlook

references: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.Nunes, F. and R.D. Norris, 2006: Abrupt reversal in ocean overturning during the Palaeocene/Eocene warm period, Nature, 439, 60–63. Pearson, P. N. and M.R. Palmer, 2000: Atmospheric carbon dioxide concentrations over the past 60 million years, Nature, 406, 695–699.

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);

OASIS

deepwater formation occurs in the North Atlantic as well as relatively widespread in the Southern Ocean;

next step: investigate climate and ocean circulation sensitivity to greenhouse gas forcing.

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.

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.

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).

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we performed a coupled atmosphere-ocean GCM simulation with Paleocene/Eocene boundary conditions;

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.

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4. P/E control simulation: ocean circulation