1. High Latitude Insolation and Climate Response
23 April 2009 /// EAS 4803
Sean Miller

2. Introduction
Milankovitch: high latitude NH summer insolation drives glacial-interglacial cycles
Changes in precession (19-23 kyr cycle), obliquity (41 kyrs), eccentricity (100 kyrs) are important
Marine benthic foraminifera δ18O used to track global ice volume changes
Warm summers and cool winters are needed to melt Ice Sheets
These change over geologic time
Reflects ration of O18/O16 in seawater

3. Introduction Eccentricity: Orbit Shape Obliquity: Axial Tilt
Precession: Direction of Axis
All change slowly over time
Eccentricity – 100 kyr cycle
Obliquity – 41 kyr cycle
Precession – 23 kyr cycle

4. Data δ18O from Benthic Foraminifera 60ºN June Insolation
δ18O : Oxygen Isotope Ratio
Tracks global ice volume variability
Lighter δ18O values suggest more precipitation over ice sheet
Recorded every 1 kyr
780 ky of data
60ºN June Insolation
Insolation received during NH high latitude summer
Recorded every 1 kyr
1000 kyr of data
Calcium Carbonate Shells / Reflects T,H2O where forams lived

5. Methods Create matrix of data Detrend data Fourier Power Spectrum
plot periodograms
Cross-Spectral Analysis
cpsd
Coherence
mscohere

6. Time Series

7. Spectral Analysis Functions used: interp1, detrend, periodogram, xcov2 3 2 1 3 4
Pictured: Significant peaks and 95% significance level.
δ18O Signal of NH Ice Volume
1 – 87.0 kyr (Strongest)
2 – 39.1 kyr
3 – 23.0 kyr
4 – 18.6 kyr
60ºN June Insolation Changes
1 – 40.0 kyr
2 – 23.3 kyr (Strongest)
3 – 18.9 kyr

8. Cross-Spectral Analysis
Functions used: detrend, cpsd, mscohere
1 – 97.7 kyr  eccentricity?
2 – 41.1 kyr  obliquity
3 – 23.0 kyr  precession
The 23 year cycle has frequencies that show greatest variability among the two datasets.
Variability is also noted at 41.1 and 97.7 kyr.
All three peaks are coherent. 3 2 1

9. Results
Proxy data have cycles with 87, 39, 21, and 19 kyr periodicities
Insolation data have cycles with 40, 23, and 19 kyr periodicities
Cross-Spectral Analysis shows 98, 41, and 23 kyr periodicities

10. Discussion Strongest signal varied mainly at 23 kyrs 41 kyr signal
Suggests ice volume response to precession
41 kyr signal
Suggests ice volume response to obliquity
kyr signal
More complicated
No peak in 60ºN insolation data
Pronounced peak in paleoproxy record
Possibly from internal feedbacks within the ice sheets themselves
Ice sheets are non-linear systems

11. References
Berger A. and Loutre M.F., “Insolation values for the climate of the last 10 million years.” Quaternary Sciences Review, Vol. 10 No. 4 pp , 1991
Imbrie, J. Hays, J.D. Martinson, D.G., McIntyre, A., Mix, A. C, Morely, J.J, Pisias, N.G., Prell, W. L., and Shackleton, N. J., “The Orbital theory of Pleistocene climate: support from a revised chronology of the marine δ18O record. In Berger, A., Imbrie, J., Hays, J., Kulka, G., and Saltzman, B., (Eds.), Milonkovitch and Climate (Part 1)”. NATO ASI Ser. C, Math Phys. Sci., 126: , 1984
Ruddiman, William F., Earth’s Climate Past and Future, New York: W.H. Freeman and Company, 2008
Ruddiman, W.F. Orbital insolation, ice volume, and greenhouse gases. Quaternary Science Reviews, 22(15), pp