Creating an Orbitally Tuned Chronology

Overview

Geomorphological evidence of past glaciations - orbital changes suspected Louis Agassiz- first proposed past ice age Joseph Adhemar- first to suggest precession control James Croll- linked reduced winter sunlight to increased snow accumulation A Brief History of Orbital Theory “This is the work of Ice!” 1837 Developed theory and predicted multiple glaciations

A Brief History of Orbital Theory Milutin Milankovitch - First hypothesized that summer insolation at 65 o N 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 i.e. Eccentricity modulates precession Precession has greatest influence at low latitudes Anti-phased across hemispheres

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 hemispheres 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

Climate Proxy & Continuous Record Emiliani Pleistocene temperatures

C 14 dating in foraminifera U Th 230 dating coral reefs Ar 40 - Ar 39 dating palaeomagnetic reversals Absolute Age Dating Techniques

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  18 O Magnetic Susceptibility

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 Simple Ice Sheet Model

Lisiecki & Raymo LR04 Stack Combined 57 d18O records to make “global” record

Lisiecki & Raymo LR04 Stack Distribution and number of records vary through time

Lisiecki & Raymo LR04 Stack

Alignment to the LR04 Stack

LR04 Site 1267 Alignment to the LR04 Stack

The early Pliocene problem

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 Conclusions