Milankovitch Cycles

The orbital eccentricity, the obliquity and the precession of the equinoxes all affect the earth climate in regular predictable cycles which can often be detected in the geological record stratigraphically or chemically utilising mathematical analysis.

Orkney provides the opportunity to examine the control of the earth's orbit on the global energy budget in two contrasting periods

• the Devonian, between 380 and 360 Myr when the climatic cycles relate to changing lake levels and

• the Quaternary Ice Age, when ice sheets advanced and retreated according to changes in temperature and snowfall

The driver for these variations is the amount of solar radiation (insolation) reaching the surface of the Earth. Insolation changes with the positioning of the Earth and Sun, as well as changes in the orbital parameters of the Earth.

Milankovitch observed three fundamental cycles:

1. The orbital eccentricity
2. The obliquity
3. The precession of the equinoxes

The combined influences produces a complex pattern of insolation, resulting in increased seasonal contrast in one hemisphere of the Earth and reduced contrast in the other.

1.  The eccentricity of the orbit of the Earth around the Sun varies from circular to elliptical giving a change in seasonal incoming solar radiation of about 30%. This cycle takes 95,800 years.
 

2. Changes in the inclination (obliquity) of the Earth's axis alternate between extreme values of 21.39 degrees and 24.36 degrees with a periodicity of 41,000 years. An increase in axial tilt results in lengthening the period of winter darkness in Polar Regions. These changes therefore cause significant variation in the insolation at high latitudes.

3.  The precession of the equinoxes has a 21,700 year period. The northern hemisphere tilts towards the Sun at successively different points on the Earth's annual orbit. This affects the length of day and the summer and winter temperature.

The Eday Marl Cycles

Analysis of the calcite at 25cm (2,500yrs) intervals reveals the 41m section to have 4.3 eccentricity cycles.

The periods from the Fourier analysis of the data give:

102,500yrs  - Eccentricity

41,000yrs    - Obliquity

25,600yrs    - Precession

These numbers are in good agreement with the expected Milankovitch  numbers given the potential errors in the method. Ongoing sampling at 5cm intervals should give greater precision and