Palaeoceanographic records from the NW Pacific, 16-0 Ma (using samples from Exp 350)

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Global climate has changed dramatically over the last 16 million years. Global temperatures at the start of this interval (part of what we call the middle Miocene Climatic Optimum) were perhaps 4-5 degrees C warmer than today, with large cold-blooded reptiles such as crocodiles observed as far north as the UK, and a much smaller-than-modern East Antarctic Ice Sheet (now the largest on the planet). This was followed around 14.5 million years ago (14.5 'Ma') by a major and permanent cooling step (the middle Miocene Climate transition, MCT) associated with the build up of ice sheet in East Antarctica and strengthening of deep water circulation and meriodional temperature gradients (what drives circulation in our current oceans). The next major phase of cooling and ice build up occurred after 5 million years ago culminating with the formation of large northern hemisphere ice sheets (that characterised our so-called 'ice ages'). Palaeoclimatologists investigate what drove these changes so that we can better understand what drives changes in our modern climate system. The ocean plays a major role in controlling climate. To understand drivers of climatic changes over the last 16 million years, we need long-term records of temperature from many different parts of the world's oceans (so that we can 'map' changes in oceanic temperature patterns) as well as information about global ice volume (the presence of ice sheets at high latitudes generates saline seawater which together with cool temperatures can drive 'thermohaline' ocean circulation by forming dense water masses). We can obtain these records by analysing the chemistry of the shells of microscopic marine organisms that collect in deep-sea sediments: Mg/Ca ratios in these shells can tell us about past water temperatures and their 18O/16O ratio can tell us about temperature and the amount of ice locked up on land. There is evidence that the Pacific experienced major oceanographical changes since 16 Ma but Mg/Ca and 18O/16O records from this ocean basin are limited; we therefore only have a loose understanding of changes in the Earth's largest ocean. A new Mg/Ca-18/16O record from the equatorial Pacific (Lear et al., in prep.) shows major changes in water temperature and global ice volume since 16 Ma but raises interesting questions about a) role of atmospheric CO2 in controlling climate (CO2 and temperature appear to be decoupled for the last 6 Myrs which raises pertinent questions give our current concerns over rising CO2 levels) and b) patterns of Pacific ocean circulation (no bottom water temperature changes were observed around 14 Ma despite inferred major ice sheet growth and evidence elsewhere for injection of cold deep Antarctic waters into the Pacific at this time). We plan to construct a new Mg/Ca-18O/16O record (from deepwaters and surface waters) from microfossil shells in the NW Pacific (particularly undersampled) in order to fill an important gap in our 'map' of temperature changes in the Pacific ocean and refine our records of global ice volume. We will also look at past effects of NW Pacific volcanism on primary productivity in the surface ocean. Explosive volcanism in the NW Pacific would have released vast amounts of volcanic ash. There is evidence to suggest that this ash is an effective fertiliser of surface oceans and can produce massive algal blooms within days. This has important implications for carbon storage in the ocean. We would like to use geochemical measurements in fossil foraminifera spanning ash layers in cores drilled in the NW Pacific, to determine whether there were changes in primary productivity associated with volcanic events so we can better understand the influence of volcanism on carbon cycling in the Pacific