IODP Exp. 362 - Carbonate accumulation events in the Eocene Indian Ocean

The Eocene period, 56-34 million years ago, was around 14 degrees Celsius warmer than today with no ice sheets on Antarctica or Greenland and atmospheric carbon dioxide concentrations of between 500 and 1500 ppm (compared to pre-1750 AD levels of ~280 ppm). In the past, the termination of the Eocene period (c. 33.9 million years ago) was thought to have represented the first appearance of major ice sheets on Antarctica, however, recent research has suggested that middle/late Eocene climate may have been very dynamic including substantial ice growth on Antarctica, followed by larger, more permanent ice caps developing on Antarctica at the end of the Eocene at the same time as a major decrease in atmospheric carbon dioxide concentrations.

Computer models suggest that the depth at which calcium carbonate (which forms the shells of the marine plankton that sink to the ocean floor to form sediment) dissolves in the ocean (the calcium carbonate compensation depth, CCD) is closely linked to ocean acidity, which in turn is linked to atmospheric crbon dioxide. While reliable records of past-carbon dioxide concentrations remain scarce, researchers have identified significant fluctuations of the Pacific CCD during the middle/late Eocene (leading to sedimentary carbonate accumulation events or CAEs), which were likely associated with changes in atmospheric carbon dioxide concentrations. In the modern ocean, the depth of the CCD is similar in the Indian and Pacific Oceans. However, the depth of the CCD is also dependent on the temperature and pressure of the deep ocean, and on oceanic circulation which controls the age of deep waters in ocean basins - important because the concentration of dissolved carbon dioxide in sea water increases with the time since it was last exposed to the atmosphere. Whilst researchers think CAEs were likely global events, no studies from the Indian Ocean have yet confirmed that the CCD varied in a similar way there, where middle/late Eocene oceanic conditions may have been significantly different.

This proposal is based on sediment cores collected from the sea floor at IODP Site U1480 in the eastern equatorial Indian Ocean (4148 m water depth). The middle/late Eocene sediment section contained bands of calcareous ooze alternating with darker clay-rich sediments. We propose that these calcareous bands represent deepening of the CCD that is correlated with at least some of the Pacific CAEs, which could confirm the global nature of these events. In order to test this global hypothesis, we propose generating a record of sedimentary calcium carbonate content, where a <10% concentration threshold is usually used to define the position of the CCD.

In order the best characterise the dramatic changes in sedimentary regime that lead to the preservation of the striking light and dark banding, we also propose to analyse the sediments using a high-resolution analytical scanning electron microscope. We will document changes in sediment fabrics and grainsize across the transitions from light to dark layers and produce accompanying high-resolution chemcial element maps. Analysing at the sediment at this microscale will allow us to interpret what kind of palaeoenvironmental changes were associated with such dramatic fluctuations of the sedimentary regime, using microfossils and sediment geochemistry. Further, we will investigate whether millimeter-scale laminations in the light calcareous ooze bands are associated with any changes in grain size that could be associated with variations in deep water from the Southern Ocean, which was likely invigorated during cooler climate periods.

From a UK-perspective, the primary societal and economic impacts of this proposal will be through public engagement and skills gained by the research team. Firstly, persistent communication of climate science to the public is necessary to stimulate debate and promote dialogue around the subject of climate change. Large climate feedbacks and systems, for example the relationship between ocean carbonate chemistry and marine sedimentation patterns, can seem obscure and not relevant to the UK general public and so effort is required to reduce this perceived lack of relevance and to increase the national understanding of the Earth's climate as a large-scale interlinked system. This kind of understanding is essential if the public are to be persuaded that mitigation against global anthropogenic climate change is necessary. As part of this proposal we will use the result fo the research to engae with the general public through STEM events (Mitchison is a STEM Ambassador) and Soapbox Science events (Pike has been part of the organising team in Cardiff and will be presenting at Oxford Soapbox: Art & Science in 2017)

Secondly, by the end of the project the researcher, Mitchison, will have significantly improved her skills in creating sediment thin sections, and using Cardiff's state-of-the-art analytical scanning electron microscope and microanalysis system. Mitchison and Pike will have increased their knowledge of ocean acidification and Eocene climate evolution. These new skills will likely contribute to career enhancement for all involved in the project, particularly Mitchison and will, in turn, increase the scientific understanding of and skills available to the UK scientific community as a whole.