Constraining astronomical models with geological data

点击翻译按钮获取中文摘要

Just as tree rings can be used to count astronomical years back into the past, and ancient coral growth rings can be used to estimate the number of days per year in the past, layers of undisturbed sedimentary rocks can potentially record cyclical variations in the orbital configuration of Earth and other planets. These astronomical variations affect the amount of sunshine received at the top of the atmosphere, filtered through the climate system (e.g. ice ages), which finally influences the amount and type of sediment deposited in, e.g., the deep sea. Changes of these astronomical parameters: the Earth's eccentricity, obliquity (tilt), and climatic precession (wobble of the spin axis similar to a spinning top), occur on time scales of thousands to hundreds of thousands of years. The necessary geological records from the deep sea have only recently become available through the international Ocean Drilling Program, which recovers sediment cores from hundreds of metres below the sea bed in waters several kilometers deep. However, although we expect seasons to change very regularly every year, the solar system configuration of predominantly the inner planets is less regular on time scales of millions of years - it has been shown that it is 'chaotic'. This means that it is not possible to calculate aspects such as the changing tilt of the Earth's axis back in time indefinitely. This hinders geologist's efforts to use these changes as 'metronome' or pace-maker to estimate time. The aim of this research is to use characteristic patterns in the sedimentary record to extract astronomical parameters that can be used to extend the time scale over which model calculations are valid. This is important in order to constrain basic physical parameters that cannot be obtained otherwise by astronomers. More specifically, we hope to find several distinct occurrences of chaotic behaviour from deep sea sediment data during the past 30-50 million years that, in the extreme case, would even allow astronomers to very accurately test the predictions of Einstein's general relativity. The research proposed here would be accomplished through a novel collaboration between astronomers and Earth scientists. One additional aim of our proposed research is to constrain the evolution of the astronomical Earth model: due to the dissipative tides in the oceans, the solid parts of the Earth, Moon and Sun, as well as redistribution of mass on Earth due to, e.g. ice-ages and mantle convection, the rotation of the Earth is changing, and generally slowing down with time. Due to conservation of energy, this process also results in an increasing distance between the Earth and Moon, currently a few centimetres per year. In order to accurately calculate the orbital configuration and variations of the solar system, and the Earth in particular, it is important to constrain how strong this change in the Earth's rotation has been in the past. Again, just as tree rings can give information about dry and humid years, the detailed pattern of climate indicators recorded in sediments can be used to extract parameters of the Earth model in the past. We can now use newly available sediment cores, and additional measurements, to obtain important information for astronomers.