Cenozoic climate change (past 70 Myr) Cenozoic environmental change Climatic variations over the past 70 million years • oxygen isotope records • geochemical records Potential controls of Cenozoic cooling trend • continental drift • CO2 • ocean heat transport Shorter (orbital) time scale variations Onset of Northern Hemisphere glaciation • oxygen isotope records • monsoon record Potential controls of onset of glaciation • uplift • ocean gateway closure African climate change over the past 3 Myr Zachos et al. (2001, Science) The Oxygen-Isotope "Paleothermometer” The basics: How ice volume is recorded • when water evaporates from the ocean, the lighter H216O molecules evaporate first, leaving the heavier H218O molecules behind • when there are large ice sheets, then the ocean gets "richer" in the heavier H218O while the ice sheets are made up of the lighter H216O • There are two isotopes of oxygen, 16O (the most abundant) and 18O (less abundant, and heavier) • 16O and 18O generally behave the same chemically and physically, but with some important exceptions • water molecules in the ocean, atmosphere, and ice sheets consequently are also of two kinds (H216O and H218O) • • marine organisms reflect the relative abundance of H216O and H218O in the calcium carbonate (CaCO3) of their shells this change in the relative abundance of H216O and H218O is recorded in marine sediments: • abundant 18O = more ice; less abundant 18O = less ice • when the organisms die and sink to the bottom of the ocean, they preserve in the sediments of the ocean a record of the relative abundance of H216O and H218O in the ocean when • there is also a direct temperature effect on the relative abundances of H216O and H218O • abundant 18O = cold oceans; less abundant 18O = warm oceans they were alive Cenozic temperature changes from Mg/Ca Lear et al. (2000, Science) Cenozoic paleogeography Zachos et al. (2001, Science) 1 CO2 Variations over the past 600 million yrs CO2 and climatre variations over the past 600 million yrs Crowley & Berner (2001) CO2 variations during the Cenozoic Cretaceous boron isotopes → surface water pH → CO2 Pearson & Palmer (2000, Nature) Barron et al. (1995, Paleoceanography) Modern CO2 effects 2 x CO2 4 x CO2 6 x CO2 Mid-Cretaceous Geography Barron et al. (1995, Paleoceanography) Barron et al. (1995, Paleoceanography) 2 CO2 effects + ocean heat transport increase Cretaceous ocean-atmosphere climate simulation 4 x CO2 + 4 x OHT Barron et al. (1995, Paleoceanography) Otto-Bliesner et al. (2002, JGR) Cretaceous ocean-atmosphere climate simulation Otto-Bliesner et al. (2002, JGR) Orbital variations Zachos et al. (2001, Science) Late Paleocene thermal maximum Zachos et al. (2001, Science) Changing variability Zachos et al. (2001, Science) 3 Tropical Atlantic δ18O and dust flux Winter monsoon onset (China loess) Xiong et al. (2003, GRL) Tiedemann et al. (1994, Paleoceanography) Tibetan uplift and the Asian monsoon An et al. (2001, Nature) Impact of uplift on N. Hemisphere circulation Climate simulations of uplift effects An et al. (2001, Nature) Positive feedback from uplift-induced cooling Ruddiman (2001) 4 Impact of THC “short circuit” Driscoll & Haug (1998, Science) E. Africa vegetation changes Impact of Panama Isthmus emergence Haug & Teidemann (1998, Nature) African climate and hominid evolution Cane and Molnar (2001, Nature) de Menocal (1995, Science) 5