Milutin Milankovitch, of Serbian origin, was at once an astronomer, geophysicist and climatologist. In 1911, Milankovitch was interested in glacial periods of the Pleistocene, a geological epoch from 1.8 million years to 11,500 years before our era. It is characterized by prolonged glaciations, glaciers covering the continents, interrupted by short interglacial periods with temperate climate. Milankovitch established a mathematical theory of climate. In his calculations, he included information on small variations in the inclination of the axis of the Earth, and small orbital changes caused by the gravity of other planets, Jupiter and Saturn essentially, each of these orbital variations with a definite period. Milankovitch proposed that changes in the intensity of the solar radiation received by the Earth are due to three basic factors: the eccentricity with a period of 413,000 and 100,000 years, the inclination with a period of 41,000 years, the precession with periods of 23,000 and 19,000 years. The tables he drew are still true, confirmed by more recent calculations.
Since 1976, inspection of deep ocean cores has confirmed the Milankovitch theory. We can reconstruct changes in ice volume using measurements of oxygen isotope in the calcite shells of foraminifera. Indeed, variations in 18O[i] in seawater can be correlated to changes in volume of ice. During the glacial period, the sea level was 130 m below the current level. Accordingly 18O of the ocean was 1.5 per thousand higher than what it is today. Measuring 18O in the shells of foraminifera allows to reconstruct changes in the volume of ice on the scale of million years. Thus, the climate could be restored over a period of 5.3 million years, by means of a graphical correlation algorithm that takes into account the constraints on sedimentation rates, more than 50 cores coming from abysses in the three oceans.
The origins of glacial-interglacial oscillation from the Milankovitch cycles can be understood from these two extreme cases:
1) for the occurrence of an interglacial period, an extreme orbital configuration results from high eccentricity (the Earth’s orbit is an ellipse), large inclination and low Earth – Sun distance in summer. It follows very contrasting seasons, thus warming as the main variable is the amount of radiation received in summer at high latitudes of the northern hemisphere.
2) on the contrary, for the glacial period the orbit of the Earth is nearly circular (low eccentricity) with a low inclination and a large Earth – Sun distance in summer. This results in a low seasonal contrast and a favorable cooling configuration.
On longer time scales, sediment cores show that the cycles of glacial and interglacial periods are episodes of a long ice age that began with the glaciation of Antarctica about 40 million years ago. However, these glacial and interglacial cycles primarily began about 3 million years ago with the growth of continental ice caps in the Northern Hemisphere. During the period from 3.0 to 0.8 million years before our era, the period of 41,000 years prevailed, corresponding to changes in the inclination of the Earth. Over the past 800,000 years, the period of oscillation of glacial-interglacial that dominates is 100,000 years, corresponding to the evolution of the eccentricity of the Earth, which is however of lower amplitude that is the obliquity.
[i] Relationship between benthic foraminifera δ18O and volume of polar ice
Ice of the polar caps is depleted in H2 18O about 30-40 ‰ (δ18O ~ – 40 ‰ = – 4%) compared to ocean water. Indeed, the water vapor transported from low latitudes toward the poles undergoes isotopic fractionation (depletion of 18O, heavy oxygen isotope consisting essentially of 16O) during successive condensations, all the more important as temperature decreases. As the total amount of H2 18O (oceans + caps) is constant, the more bulky the caps (at constant δ18O), the more concentrated in H2 18O seawater is, by difference. The volume of the caps and the δ18O in seawater therefore vary in the same direction and proportionally.
Relationship between δ18O in the oceans and δ18O of benthic foraminifera
Benthic foraminifera are protozoa that live on the ocean floor and synthesize a carbonate shell with isotopic composition that depends on the water temperature and δ18O of water. As the water temperature varies only slightly at depths, variations in δ18O in the shells depend only of those of water.