Transcript Precession
Creating an Orbitally Tuned Chronology Overview A Brief History of Orbital Theory Geomorphological evidence of past glaciations - orbital changes suspected Louis Agassiz- first proposed past ice age “This is the work of Ice!” 1837 Joseph Adhemar- first to suggest precession control James Croll- linked reduced winter sunlight to increased snow accumulation Developed theory and predicted multiple glaciations A Brief History of Orbital Theory Milutin Milankovitch - First hypothesized that summer insolation at 65oN as most important control on ice sheets - Detailed calculations of insolation Orbital Cycles Precession and Eccentricity • • • • • Eccentricity Only orbital cycle to change the total insolation Precession Effect of precession depends on ellipticity of orbit Precession has greatest influence at low latitudes Anti-phased across hemispheres i.e. Eccentricity modulates precession Obliquity Obliquity has greatest influence at high latitudes In phase across hemispheres Incoming Solar Radiation - Insolation Obliquity - Largest effect at high latitudes - In phase across Precession - Largest effect at low latitudes - Anti-phased across hemispheres Orbital Signal in Climate Records Signal vs Noise Signal - original forcing recorded in proxy record Noise - distortion of signal - additional signal not related to orbital forcing orbital forcing - climate response Understanding of how climate works Tool for creating chronologies Ingredients for understanding orbital climate change • Proxy of climate change • Continuous record • Absolute age dating technique Emiliani 1955- Pleistocene temperatures Climate Proxy & Continuous Record Absolute Age Dating Techniques C14 dating in foraminifera U234 - Th230 dating coral reefs Ar40 - Ar39 dating palaeomagnetic reversals Hays, Imbrie & Shackleton, 1976 Continuous climate proxy records Independent chronology Hays, Imbrie & Shackleton, 1976 Spectral analysis shows significant peaks at orbital frequencies Shackleton et al., 1990 Placed Brunhes-Matuyama magnetic reversal 5-7% older than accepted radiometric dates Ingredients for creating an orbitally tuned chronology • Assumptions • Tuning target • Tuning parameter Assumptions • Orbital signal is present • Time lag • Nature of orbital forcing - climate response • Continuous and complete record Tuning Target Tuning Parameter Sapropels d18O Magnetic Susceptibility Simple Ice Sheet Model y = ice volume t = time b = nonlinearity coefficient Tm = time lag x = forcing Simple Ice Sheet Model y = ice volume t = time b = nonlinearity coefficient Tm = time lag x = forcing Lisiecki & Raymo 2005 - LR04 Stack Combined 57 d18O records to make “global” record Lisiecki & Raymo 2005 - LR04 Stack Distribution and number of records vary through time Lisiecki & Raymo 2005 - LR04 Stack Lisiecki & Raymo 2005 - LR04 Stack Alignment to the LR04 Stack Alignment to the LR04 Stack LR04 Site 1267 The early Pliocene problem Conclusions • Characteristics of orbital cycles • Ingredients needed to understand orbital scale climate change • Importance of chronology & stratigraphy • How to use our understanding of orbital climate change to create age models