Geological Evidence for Precession Image: NASA Atmospheric circulation is orbitally driven but atmospheric pressure is also a variable of precession Our planet is a dynamic multi-phase system and, as pointed out before, the different layers of gases, liquids, solids, and plasma respond differentially to the varying mass arrangements in space and the ensuing changes to the rotational motion. We are just beginning to learn of the possible implications. Mörner relates a 15 millisecond change in the rotational speed to a sea level rise or fall of 1 meter (Mörner). Evidence for periodic sea level fluctuations comes from sedimentary rocks deposited in former marine basins. Changes to the water column are reflected by stacked patterns of depositional sequences bounded by erosional surfaces. Signs of periodicity are also evident in rocks on the continents in places where seasons contrasted starkly. In the 1930s and early 40s, Milush Milankovitch set out to prove the relationship between glacial episodes and orbital cycles. He first observed distinct rhythmic patterns in the sedimentary rocks of his Eastern European homeland, a periglacial environment during the Pliocene and the Pleistocene. Recognizing the same pattern in other locations of the world, he then matched the sedimentation rate with the three major orbital periods. His ideas were accepted once variances in oxygen isotope ratios were discovered and correlated to the changing volumes of glacial ice and the corresponding changes in sea level (deBoer & Smith). This link could be established because oxygen atoms incorporated into the calcite of seashells or skeletal parts of marine organisms have values of oxygen isotope ratios equal to those of the ocean water at the time of precipitation. Seawater is enriched with delta 18 Oxygen isotopes during a glacial interval as ice formation prefers the lighter form of oxygen. Data analysis and graphic representation of oxygen isotope ratios essentially produced a sinuous pattern called a sea level curve. The valleys and peaks of such a curve coincide with the paces of the ice ages. While eccentricity occurs at a frequency of 100 000 to 2 Ma years, obliquity and precession are much shorter, with cycles of 41 000 years and     19 000 to 26 000 years, respectively. Cyclic sequences caused by precession are more readily recognized in the rock record because the shorter depositional events are less likely to be overprinted by other sedimentary episodes. Furthermore, as the equator shifts, the changes in one hemisphere are matched with an equal and opposite effect on the other half of the globe. The modifications compress or expand the existing climatic zones causing regional changes in ecological systems. Vegetational successions resulting from climatic variations during the Quaternary era are well documented for the Northern Hemisphere with the help of pollen analysis. For example, a core drilling sample from the bottom of the Pula maar lake in Hungaria signals a long history of floral boundary shifts: sub-tropical forest compositions alternated repeatedly with boreal forest flora in concordance with the many advances and retreats of the ice sheets (Willis, Kleczkowski & Crowhurst). The recognition of former climate zones is an important step in the reconstruction of paleoclimates and the configuration of the length of transitional periods. Another rhythmical depositional pattern reflecting strong seasonality in terrestrial or marine rocks are laminations or varves. These are stratified layers in undisturbed sediment that vary in organic content and thickness as biologic activity and sediment accumulation rates are climate-influenced. The two tone layering is indicative of either changing oxygen levels (aeration) or biogenic production. Often the layer boundaries coincide with a full period of an orbital cycle. The timing of orbital frequencies has been calculated mathematically with a high degree of certainty for the last 440 Ma years (deBoer& Smith). The organic particles in the sediment may be dated to verify the theory and the calculations. An example of varves. Image: tvl1.geo.uc.edu Dark, organic-rich layers alternate with light clastic sediment.Varves reflect strong seasonality. Precessional frequencies are more readily observed in lower latitude regions where the shift of the equator changes the intensity of direct insolation. Variations in incoming sunlight causes changes to both the atmosphere and the hydrosphere. A difference in heat flow across the equator also affects the heat dissipation from the tropical regions towards the poles. The convection-driven atmospheric circulation cells (Hadley cells) expand or compress moving climate zones north or south over large regions up to 30 degrees latitude. This in turn will have an effect on the ocean bottom currents which are driven by the winds and ultimately, on Earth's rotational motion. El Nino and La Nina influenced weather patterns experienced presently in the Western Hemisphere are thought to be due to precession (Mörner). In our current ice-house situation we are interested in preventing changes to the volume of the Antarctic Ice Sheet to protect our present degree of seasonality which makes for comfortable living conditions. Therefore, scientists monitor the behavior of the ice and study its history in times when different atmospheric conditions prevailed. The information provided by the rock record helps construct climatic models from which the stability of our ecosystems can be infered as Earth reacts to changes in space. See also on the next page a physical model for the rotational properties of Earth