Pliocene history of the Indian Monsoon: new data from IODP Expedition 353

It is important to understand how Earth's climate varied during past periods of higher atmospheric CO2, so we can better predict how the climate might evolve in the near future due to human activity. Of particular concern is the future behaviour of the Asian Monsoon, which in the modern world provides water for, and influences the lives of, billions of people. The response of the monsoons to higher CO2 and global temperatures is unknown, but latest modelling results suggest that the summer monsoon will get stronger and more prolonged, raising the possibility of severe and widespread flooding, as seen in India in 2013 and Pakistan in 2015.

Past temperature and rainfall patterns associated with the Indian Monsoon can be reconstructed by studying marine sediments recovered from deep-sea cores in the basins surrounding the Indian subcontinent. New sediment cores have been recovered from the NW Indian Ocean region, including the Bay of Bengal and the Andaman Sea, during International Ocean Discovery Program (IODP) Expedition 353 (Dec 2014-Jan 2015). The aim of this expedition is to try and understand the past behaviour of the Indian Monsoon from the Holocene to the late Miocene (present day to 10 million years ago), by taking cores in a part of the Indian Ocean that has never been scientifically drilled before. By examining the geochemistry and microfossil content of the sedimentary cores, and indeed the composition of the sediments themselves, many interesting features of the ancient ocean and monsoons that influence them can be reconstructed.

For the overarching project, we propose to examine the geochemistry and sedimentology of sediments from two sites cored during IODP Exp. 353, Sites U1445 of the NE coast of India and Site U1448 in the Andaman Sea, with the aim of reconstructing the rainfall and temperature patterns associated with the Indian Monsoon during the enigmatic Pliocene period (~5.4-2.6 million years ago). This time period was the most recent in Earth's history where atmospheric CO2 levels were similar to those predicted for our near future, and so it may be a good model for predicting how the Asian monsoons will behave as the Earth heats up in the coming century. By taking sediment samples of Pliocene age and applying sophisticated organic geochemical techniques, we will reconstruct the sea surface temperature (SST) and the rainfall intensity at very high resolution, which will increase our understanding of how the monsoon affected the Indian during a time of great global environmental change. Using this new organic geochemistry data, we will be able to see how the Indian monsoons responded to repeated heating and cooling cycles (glacials and interglacials) triggered by the change in Earth's orbit around the sun.

Specifically in this grant, we first need to determine exactly how old the sediments are before we can apply the organic geochemistry techniques. To do this we will generate stable oxygen isotope data from the shells of microfossils (foraminifera), which we can then compare to a global reference set that has been calibrated to orbital-forcing, in order to generate an "orbitally-tuned" aged model. This age model will be the foundation block on which all subsequent data, and our deeper understanding of the Indian Monsoon, is built.

The overarching project will create impact beyond the academic community, in both the world of policy and outreach (see Pathways to Impact).

The Indian monsoon provides water for drinking, washing and irrigation for billions of people, but can also bring devastating floods or droughts depending on its strength and duration from year to year. Therefore, understanding its stability in the coming centuries in the context of a warming climate is of utmost concern. During the warm Pliocene, when CO2 levels were the same as they currently are in 2015 (~400 ppmv), sea level was 20 m higher and the world was on average 3-4C warmer. Therefore, the Pliocene study outlined in this proposal will add to our knowledge of the past behaviour of the monsoons under similar conditions to those of our very near future, which will help scientists to ground-truth their predictive models for monsoon strength under anthropogenic conditions. These predictions are important for policy makers, who must decide how best to mitigate the effects of changing monsoonal rain patterns over the countries of southern Asia. The consequences of continuing emissions over the coming century should be taken into account by policy makers when considering where to set caps on emissions and which emission scenario industrialised countries should seek to follow.

This project will also have impact in the sphere of public outreach, through the PIs involvement with local schools, higher education institutes and the wider public. During the expedition Dr Littler maintained a blog (http://deepseadiscovery.wordpress.com/), which detailed the goals and achievements of the cruise, as well as the nitty-gritty of what it is like to live and work at sea for months on end as a research scientist. Dr Littler has continued to make updates to the blog during the onshore sampling party and beyond into the research phase of the project, in order to give followers an insight into the life-cycle of a large multifaceted research project. While at sea, Dr Littler led two Skype seminars with students at the Camborne School of Mines and Geography Department, University of Exeter, and with the Department of Earth Sciences at the University of Oxford. This allowed students to gain a first-hand look at how science is done at sea and encouraged them to appreciate the valuable insights into past climates that scientific ocean drilling affords us. Dr Littler was also involved with several Skype seminars with primary and secondary schools whilst at sea, in collaboration with the onboard educators. Upon returning to dry land in 2015, Dr Littler has resumed the outreach work she carries out with local schools in Cornwall, armed with new photographs, videos and materials collected during her time on Expedition 353, and will continue to do this through the lifetime of this grant.