Marine Isotope Stage - Developing A Timescale

Developing A Timescale

In 1957 Emiliani moved to the University of Miami to have access to core-drilling ships and equipment, and began to drill in the Caribbean and collect core data. A further important advance came in 1967, when Nicholas Shackleton suggested that the fluctuations over time in the marine isotope ratios that had then become evident were caused not so much by changes in water temperature, as Emiliani thought, but mainly by changes in the volume of ice-sheets, which when they expanded took up the lighter oxygen-16 isotypes in preference to the heavier oxygen-18. The cycles in the isotope ratio were found to correspond to terrestrial evidence of glacials and interglacials. A graph of the entire series of stages then revealed unsuspected advances and retreats of ice and also filled in the details of the stadials and interstadials. More recent ice core samples of today's glacial ice substantiated the cycles through studies of ancient pollen deposition. Currently a number of methods are making additional detail possible. Matching the stages to named periods proceeds as new dates are discovered and new regions are explored geologically. The marine isotopic records appear more complete and detailed than any terrestrial equivalents, and have enabled a timeline of glaciation for the Plio-Pleistocene to be identified. It is now believed that changes in the size of the major ice sheets such as the historical Laurentide ice sheet of North America are the main factor governing variations in the oxygen isotope ratios.

The MIS data also matches the astronomical data of Milankovitch cycles of orbital forcing or the effects of variations in insolation caused by cyclical slight changes in the tilt of the earth's axis of rotation - the "orbital theory". Indeed that the MIS data matched Milankovich's theory, which he formed during World War I, so well was a key factor in the theory gaining general acceptance, despite some remaining problems at certain points, notably the so-called 100,000-year problem. For relatively recent periods data from radiocarbon dating and dendrochronology also support the MIS data. The sediments also acquire depositional remanent magnetization which allows them to be correlated with earth's geomagnetic reversals. For older core samples, individual annual depositions cannot usually be distinguished, and dating is taken from the geomagnetic information in the cores. Other information, especially as to the ratios of gases such as carbon dioxide in the atmosphere, is provided by analysis of ice cores.

The SPECMAP Project, funded by the US National Science Foundation, has produced one standard chronology for oxygen isotope records, although there are others. This high resolution chronology was derived from several isotopic records, the composite curve was then smoothed, filtered and tuned to the known cycles of the astronomical variables. The use of a number of isotopic profiles was designed to eliminate 'noise' errors, that could have been contained within a single isotopic record. Another large research project funded by the US government in the 1970s and 1980s was Climate: Long range Investigation, Mapping, and Prediction or CLIMAP, which to a large degree succeeded in its aim of producing a map of the global climate at the Last Glacial Maximum, some 18,000 years ago, with some of the research also directed at the climate some 120,000 years ago, during the last interglacial. The theoretical advances and greatly improved data available by the 1970s enabled a "grand synthesis" to be made, best known from the 1976 paper Variations in the earth’s orbit: pacemaker of the ice ages (in Science), by J.D. Hays, Shackleton and John Imbrie, which is still very widely accepted today, and covers the MIS timescale and the causal effect of the orbital theory.