Causes and consequences of global cooling in the mid-Miocene

Throughout its history, the climate on Earth has fluctuated from episodes of extensive glaciation and sub-freezing temperatures, to periods when there was little or no polar ice on the planet. Extreme as these different conditions may appear, Earth's climate has nevertheless remained within the bounds that have allowed life to continue for a few thousand million years. The Earth system contains some very ingenious mechanisms that help regulate its climate so that it never becomes too hot, or too cold, to kill off life altogether. These mechanisms include chemical weathering of continental rocks, burial of organic carbon, and changes in the flora of the oceans and continents. It usually takes many millions of years for climate to shift to either generally warmer conditions or cooler conditions, and these conditions are then maintained for many tens of millions of years. However, recent research has shown that there have been a number episodes in the past when temperatures world-wide have suddenly soared over intervals as short as a few tens or hundreds of years, and that these high temperatures lasted for thousands or hundreds of thousands of years before recovering to pre-warming levels. In geological terms, these episodes lasted a relatively short time, and they were entirely unlike the more gradual, million-year changes. But we also know that many of these sudden episodes of severe global warming coincided with so-called 'mass extinctions' when very large numbers of plant and animal species suddenly died out. Equally, there have been relatively short periods when the Earth suddenly cooled and there was a rapid expansion of glacial environments. The subject of our study is a critical episode of sudden global cooling that occurred some 14 million years ago and resulted in major growth of the Antarctic ice sheet. This episode is one of the 4 major steps in the gradual cooling of Earth's surface that started around 50 million years ago, and which has continued to the present day. Despite its great importance, the causes of the sudden cooling are poorly understood. The purpose of our study is twofold. First, we want to understand why the Earth abruptly flipped from one state to another, from relatively warm to relatively cold. And second, we want to use this example of global cooling to examine how other aspects of the Earth system responded at that time. How do we find out this information, and where is it recorded? The sediments that are deposited in the world's oceans are derived from the continents and from the remains of marine organisms. The composition of these sediments has changed over time in response to changing environmental conditions. We can thus use the chemistry of marine sediments as proxies for seawater temperature, continental weathering rate, and so on. By looking in detail at the changes in composition across a section of sedimentary rock, which represents a 'slice' of time, we can then infer how temperature and weathering have varied over this interval. A succession of sedimentary rocks known as the Monterey Formation that is now exposed along the coast in California contains an excellent, high resolution record of changes in seawater chemistry over the major cooling period about 14 million years ago. We have already completed a successful pilot study that has established an outline geochemistry and an accurate, high-resolution timescale for this succession through the use of a built-in astronomical 'clock', so we can readily locate our position in absolute time. Our objective now is to determine which mechanism caused the Earth to cool at that time, and what the environmental consequences were then. At the present day, we face a major challenge in predicting the future course of Earth's climate. A further objective is to obtain reliable information about the behaviour of the Earth system in the past that will be able to inform us better of how to deal with current and future environmental issues.