Chronostratigraphy – the backbone of paleoclimatology
An accurate time-calibration of sediment sequences provides the basis for all reconstructions of ocean and climate history. It is indispensable for analyzing the pace and dynamics of natural climate variability and allows for accurate stratigraphic correlations between globally distributed climate proxy-records. Chronostratigraphy is also essential for balancing paleo-fluxes of different sediment components (in units per area and time), which are a prerequisite for quantifying physical and biogeochemical processes in the earth system. Their reliability strongly depends on the density and accuracy of age control points.
The absolute dating methods most widely used and accepted are based on the natural radioactivity of isotopes measured in shells of marine fossils or certain minerals. Since the rate of radioactive decay of particular isotopes (their half-life) is known, the age can be estimated from the relative proportions of the remaining radioactive material and its decay products. The best-known absolute dating technique is carbon-14 dating. However, this method cannot be used for materials older than ca. 50,000 years, because the half-life of 14C is only 5730 years. For older sequences, radiometric dating involves the use of isotope series, such as rubidium/strontium, thorium/lead, potassium/argon, argon/argon, or uranium/lead, all of which have longer half-lives.
The astronomical tuning technique is at present the most accurate dating method for sediment records spanning the time interval of the last 35 million years for which astronomers provide a valid and precise orbital solution for variations in Earth’s orbital parameters – a method that is applied and refined at AWI (e.g. Tiedemann et al., 2007a). Changes in the eccentricity of Earth’s orbit are marked by main periods of 413,000 and 100,000 years, and the tilt and precession of Earth’s axis are dominated by periods of 41,000 and 23,000/19,000 years, respectively. These astronomical records provide the basis to date Neogene sediment records by matching patterns of cyclic variation in climate proxy records with patterns of changes in solar radiation that are controlled by cyclic variations in Earth’s orbital parameters. The astronomical tuning method is based on the fact that cyclic changes in climate proxy records statistically respond convincingly to variations in insolation (see figure). The application of this tuning procedure has been making a high-precision time calibration possible because the “astronomical clock” is very accurate and cyclic changes in orbital parameters give very small-scale time markers on geological timescales. At best, the astronomical tuning could provide an age control point every 10,000 or 20,000 years, corresponding to half of a precession or obliquity cycle. The orbital tuning method is far more precise than that achieved by radiometric dating alone and provides a reliable and absolute timescale for magnetic reversal stratigraphy, biostratigraphy, oxygen isotope stratigraphy, and, of course, records of climate and oceanographic variability which transfer the astronomical record of varying insolation into quasi-cyclic sedimentological variability.
This figure illustrates the relationship between past changes in insolation caused by cyclic variations in Earth’s orbit and the pace of climate change registered by a planktonic oxygen isotope record (Bassinot et al., 1994). The shape of Earth’s orbit around the sun varies from nearly circular to elliptical with cycles of 413,000 and 100,000 years. The angle of Earth’s tilt varies between 22.2° and 24.5° with a cycle of 41,000 years. The wobbling motion of the Earth (like a spinning top) in combination with eccentricity (Earth-sun distance) influences insolation on precessional periods. Today, Earth is closest to the sun on January 3 and most distant on July 4.
