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Objective This paper presents a statistical test of the orbital forcing of glacial terminations.PowerPoint Presentation

Objective This paper presents a statistical test of the orbital forcing of glacial terminations.

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This paper presents a statistical test of the orbital forcing of glacial terminations.

Main Result

Glacial periods terminated every two or third obliquity cycle at times of high obliquity.

Obliquity pacing of the late Pleistocene

glacial terminations

P. Huybers, C. Wunsch

Data: Uses the EOF1 of 10 marine del18O records, which is a proxy for ice volume and ocean temperature. It's on a depth-derived age-model that has no orbital information built into the model.

Null Hypothesis (H0): glacial terminations independent of an orbital period.

Alternative hypothesis (H1): glacial terminations are paced by a certain orbital period.

Orbital periods: obliquity, precession, eccentricity

To apply the test it is necessary to determine the PDF of H0 and H1.

Model: Ice volume variability is represented by a random walk with terminations:

V(t+1)=V(t)+n(t)

if V(t) >= T0(=90 kyr) => terminate, and ice volume is linearly reset to 0 in 10kyr.

n(t)~N(mean=1,std=2)

I.C. V(t=-700kyr) between [0,T0]

Note that if n(t)=1, cycles would last 100kyr (no need for oscillator).

Do 10000 realizations of the random walk. and H1.

A PDF for H0 is constructed as follows: for each realization we consider 8 consecutive terminations and calculate

whereӨ is the phase between termination initiations and the orbital period.

The PDF of H1 is estimated using R and considering that glacial terminations always occur at the same phase of the orbital period, but that phase observations are subject to identification (+-1kyr) and age-model (+-9kr) error.

Test and H1.

Phase

Max likelihood

of H1

Critical value

of R (H0)

Obliquity

Precession

Eccentricity

Rejects H0 age-model uncertainty ~1/2 cycle H0 is not rejected

“Model” of del and H1.18O variations based on obliquity forcing.

V(t+1)=V(t)+n(t)

if V(t) >= T0-a*Obl(t) => terminate (the larger the obliquity, the easier)

Adjustable parameters: a, T0, I.C.

Set to a=15, T0=105, V(t=-700)=30

n(t) can be deterministic (=1) or stochastic (~N(1,2))

Deterministic and H1.

periods at 70, 29 and 23

are combinations of 41 and

100 kyr.

wide

narrow

One realization

Stochastic

Times between terminations

averages 100kr, but has

a tri-modal distribution

with peaks at 2, 3 and 4

obliquity cycles.

Triang: obs duration between

terminations.

Speculation about cycle-skipping: and H1.

Increased obliquity increases high latitude insolation and causes heating of ice-sheets, eventually warming the ice-bedrock interface (~10kyr).

=> If ice is thin, obliquity has small effect.

=> If ice is thick basal melting provides lubrication of the ice-bedrock interface and increases ice flux into ocean.

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