Global warming and abrupt ocean circulation changes at the Paleocene/Eocene boundary (55 Ma) Malte Heinemann 1,2 , Jochem Marotzke 1 ([email protected]) 1 Max Planck Institute for Meteorology, 2 International 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 (CH 4 ) from melting clathrates in marine sediments (e.g. Dickens et al. 1995). δ 13 C [ 0 / 00 ] 54.8 55.4 55.2 55. 0 Millions of years ago -2 0 -1 1 2 before PETM during PETM 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 2 o x2 o 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. -6000 0 3000 3000 [m] 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 CO 2 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’ CO 2 concentration of 560ppm. CO 2 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 δ 13 C for the PETM, but not the pre-PETM. However, the few δ 13 C 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). 0 6 [ o C] 24 18 12 36 30 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. 7 [ o C] 8 9 10 0 2000 1000 500 1500 6 [ o C] 9 15 18 0 2000 1000 500 1500 3 12 depth [km] 1 2 3 4 5 6 [ o C] 9 15 18 time [years] 0 2000 1000 500 1500 3 12 depth [km] 1 2 3 4 5 6 [ o C] 9 15 18 time [years] 0 2000 1000 500 1500 3 12 1 2 3 4 5 1 2 3 4 5 Arctic Ocean ‘Wedell’ Sea Pacific Atlantic latitude [deg. North] -90 0 90 60 30 30 60 5 10 35 30 25 20 15 sea surface temperature [ o C] latitude [deg. North] -90 0 90 60 30 30 60 0 10 40 30 20 land surface temperature [ o C] -10 0 2000 4000 [m] circulation, surface to 690m: circulation, 690 to 2650m: 0 600 300 900 1500 1200 [m] convective depth: latitude [deg. North] -90 0 90 60 30 30 60 depth [km] 1 2 3 4 5 [Sv] 30 -30 -20 -10 0 10 20 MOC: surface temperature: temperature [ o C] (for an ice-free ocean) benthic δ 18 O [‰] Million years ago 12 10 8 6 4 0 10 20 30 40 50 60 0 3 2 1 4 5 4. P/E control simulation: ocean circulation