Milankovitch Cycles

Milankovitch Cycles are named after Milutin Milankovitch, a serbian engineer, who conducted research between the two World Wars on the hypothetical effects on global climate of changes in the Earth's orbital parameters (Smith, 1989). Three main varying orbital parameters affect the Earth's solar budget, namely:

• eccentricity of the Earth's orbit

• obliquity of the Earth's axis of rotation

• precession of the equinoxes

Each of these orbital factors shows a cyclical variation with a different periodicity. Precession of the equinoxes, with a periodicity of c. 23 000 years, is thought to be the most commonly expressed in the lithological record, and is the factor most relevant to Cenomanian cyclostratigraphy (Gale, 1995). Precession is a slow rotation of the Earth's axis in space, in the same way that a spinning top gently gyrates. The precession of the Earth's axis is caused by the gravitational attraction of the Sun and the Moon, with the result that the axis completes a single revolution once every 23 000 years. This means that each year the spring and autumn equinoxes, and thereby also the summer and winter solstices, occur at slightly different points in the Earth's orbit of the Sun. Since the Earth orbits the Sun in an elliptical path, then there will be times when the summer solstice for a given hemisphere is coincident with the greatest distance from the Sun in its annual orbital cycle, and times when the opposite is true. The predicted changes in Earth's solar budget are thought to be the cause of the regular climatic fluctuations that are apparently manifest in the rock-record. A human consequence ofprecession is that with time, the seasons will shift relative to the months of the year, so that after thousands of years, summer in the northern hemisphere will be in December and winter in June.

References

GALE, A S. 1995. Cyclostratigraphy and correlation of the Cenomanian Stage in Western Europe. In HOUSE, M R & GALE, A S (eds.), Orbital Forcing Timescales and Cyclostratigraphy, Geological Society Special Publication, No. 85, pp. 177-197.

SMITH, D G. 1989. Milankovitch cyclicity and the stratigraphic record - a review. Terra Nova, 1, 402-404.

Selected bed couplets of Gale (1995) and their key lithological and/or faunal characteristics

Bed Couplets of Gale (1995)	Key Characteristics
B13-18	carbonate-rich interval characterised by abundant Inoceramus virgatus
B19-22	clay-rich couplets characterised by Orbirhynchia mantelliana (the lowest of three widely developed acmes in the Grey Chalk Subgroup) and M. dixoni Zone ammonites (especially Mesoturrilites and Acompsoceras)
B23-24	massive, prominently weathering limestone with abundant Schloenbachia varians and diverse inoceramid bivalves (not to be confused with similar Schloenbachia-rich limestone in the M. mantelli Zone)
B41	dark, silty marl with diverse & partially restricted fauna (Chlamys arlesiensis, Oxytoma seminudum, Glandifera rustica); upper part of marl full of burrows with paler sediment infill
C1 ('Cast Bed')	dark, silty marl with abundant Entolium orbiculare at base and fauna of diverse small brachiopods (Grasirhynchia martini, Capillithyris squamosa, Kingena concinna), the bivalve Oxytoma seminudum and the belemnite Actinocamax primus
D1 (base of Jukes-Browne Bed VII)	marl with abundant Pycnodonte vesiculare
D1-8 (Jukes-Browne Bed VII)	poorly defined couplets of chalk and marly chalk containing concave-up laminated structures