RESPONSE OF GLOBAL OCEAN OXYGENATION TO EARLY CENOZOIC CLIMATE EXTREMES (RESPIRE)

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The evolution of life on Earth has been tightly linked to the development of the planet's oceans and to its climate system. Scientists have built up a picture of Earth history, and of the animals and plants of past times, through detailed examination of ancient rock strata that have accumulated on the continents and in the oceans over the ages. Long periods of relative quiescence and of gradual change were punctuated by shorter intervals when Earth's environment changed abruptly. Intervals of rapid environmental change were often accompanied by unusually high levels of species extinctions and of changes in diversity, and were often followed by new patterns of species evolution. One well established aspect of Earth history is that past climates were often much warmer than at present. Furthermore, it is almost universally accepted that climate and mean global temperature are intimately related to the level of atmospheric CO2, albeit in a complex way. But no matter what the precise nature of the climate-CO2 relationship, one consequence of global warmth is that seawater oxygen levels are expected to be relatively low, for two reasons. The first is that all gases - including oxygen - are less soluble in warmer liquids than in cooler ones; the second is that the primary productivity of the oceans affects oxygen levels directly, as higher productivity leads to greater levels of oxygen consumption. Thus there is the reasonable expectation that seawater oxygenation will decline in the future, as the oceans warm and as rivers supply more nutrients. This expectation is backed up by the direct observation of substantially decreasing oxygen levels in many parts of the oceans over the last 50 or 60 years. Although a global phenomenon, oxygen levels are most sensitive in continental shelf waters. This is a concern, because most marine species live on the continental shelf, and they are highly susceptible to changes in seawater oxygenation. Humankind is acutely at risk from the consequences of shelf deoxygenation: more than one billion people depend on marine food as their primary protein source. However, it is notoriously difficult to predict accurately the speed, severity and trajectory of future deoxygenation. One very powerful way of improving the reliability of forecasts is to refine predictive models by 'tuning' them using observations of past seawater oxygenation. This project (RESPIRE) will define the oxygenation history of seawater covering a period of just over 30 million years, from around 56 million years ago to 25 million years ago. The Earth's surface environment cooled substantially during this period, both gradually and also in a few discrete jumps. Because there are no direct records of past seawater oxygenation, we will use geochemical proxies whose values reflect oxygenation levels. Although these geochemical measurements are very difficult and time consuming, we have many years' experience in their development and application and we have shown that the proxies can act as robust archives of past oxygenation for short time intervals. The challenge now is to generate longer-term records that will help us to better understand the controls on past - and future - seawater oxygenation.An additional and highly important aspect of low-oxygen marine environments is that they are a pre-requisite for the formation of hydrocarbon source rocks, which supply most of the world's current energy demand. Because RESPIRE will involve close co-operation between field geologists, geochemists, climate modellers and industry geologists, the project will provide a forum to better define the relationship between past seawater deoxygenation and the accumulation of organic matter from which hydrocarbons are derived. RESPIRE will be the first study to establish the longer-term oxygenation history of seawater by providing an integrated, interdisciplinary assessment of how seawater oxygenation is linked to global Earth System processes.