SWEET Team Blog 4, December 2018

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Rachel – Understanding the Deep Oceans in a Warm World

Who I am and how my work ties in with SWEET

My name is Rachel Flint and I have just started a PhD with Tina van de Flierdt (Imperial College London) and Dan Lunt (University of Bristol), which will be closely aligned with the SWEET project. Like the rest of the SWEET team, I am interested in reconstructing aspects of the Earth’s climate system during the Early Eocene Climatic Optimum (EECO) – a particularly warm period which occurred around 50 million years ago.

Despite the current attempts to mediate current global warming, the Earth has already experienced an increase in average global temperature of about 1°C over the past 140 years (https://climate.nasa.gov/), and it is generally accepted that we are heading for further warming. Therefore, one aim of the SWEET project is to work out how the Earth’s climate system may function when CO2 levels are much higher. Climate models are often used to predict how the Earth’s system will change as it warms up over the next 100 years or so. However, these models are based on our understanding of the current, relatively cool climate. As a result, they may not provide entirely representative or realistic depictions of a future warm world. By using reconstructions of past warm climates based on solid geological data and comparing them to model simulations, we will be in a better position to evaluate the validity of these climate models.

My particular focus for the next four years will be on how the deep ocean circulation operated during the EECO, since deep ocean circulation is an important factor in Earth’s climate. Global and regional temperatures, as well as the positioning of the continents, play a significant role in determining where deep waters form and sink, how and where they travel through the oceans and where they meet the surface again. Furthermore, deep water masses accumulate nutrients and carbon throughout their lifetimes, and so how and where these are released back to the surface oceans and atmosphere may have important regional and global impacts.

What I will be doing

I will be based at Imperial for most of my time, performing chemical analysis on ancient fossil fish teeth from deep ocean sediment cores to reconstruct the deep ocean structure during the EECO. Otherwise, I will be working with Dan in Bristol to compare model simulations of the Early Eocene ocean with my reconstructions. With the fish teeth I will be analysing the isotopic composition of Neodymium (Nd), an element which is found to be an excellent tracer and means of identification for deep water masses. Once a fish has died and its teeth and bones have sunk to the seafloor, these hard parts undergo chemical changes which mean that they inherit the Nd composition of the bottom water mass at that location. By collecting this data from many different sites around the globe, I may be able to construct a picture of the deep ocean circulation during the Early Eocene.

The process of generating this data will involve quite a lot of lab work. The first step is to simply pick out the fish teeth from the deep ocean sediments. This is surprisingly tricky since you can barely see them with the naked eye – everything has to be done under the microscope with miniature paint brushes! I got my first taste of this during my trip to the University of Cardiff at the end of October, when I met with other members of the SWEET team to pick up some samples of theirs.

Once I have collected enough fish teeth for a particular site and time interval, the next stage will be to remove the components from the samples I am not interested in, dissolve the fish teeth and to isolate the Neodymium fraction, whose isotopic composition I will eventually measure using a mass spectrometer. This process will make up much of my PhD.

Currently, however, I am trying to get to grips with the methods by analysing sedimentary samples instead. While these will not tell us about the Nd composition of the water masses, they will tell us about the composition of the weathering input to the oceans, an important factor in determining a water mass’s Nd composition.

I have been working on this project for over two months now. As is the nature of starting anything new, I have been spending most of that time trying to get to grips with the subject matter (as well as trying to adjust to a new PhD lifestyle!). This has involved a lot of reading about the Eocene, finding out what we know about global ocean circulation at that time (not very much it seems!) and compiling all the Neodymium data already available. I received my first set of samples at the beginning of November in the post, sent from the SWEET team at Cardiff. These samples are from a deep ocean core in the middle of the Indian Ocean, and the sediments collected span the EECO interval we are interested in. Each sample is from a particular depth window in the core.

Sample analysis

Since receiving these samples, I have done a month of lab work on a subset of six of them to analyse the Neodymium composition of the sediments. These sediments are very carbonate rich (~95%), but unfortunately for us the Neodymium fraction is mainly in the silicate (i.e. sand/clay) fraction. It took about a week of lab work to dissolve all the carbonate away and leach out other components we didn’t need. This was then followed by a few days of digestion of the sediment in hydrofluoric acid (for the Breaking Bad fans out there, this is the stuff they use to dissolve bodies – you don’t want to spill any!). Next, I spent another week separating out the elements in the dissolved samples in a process called ‘column chemistry’, so that we could finally isolate the Neodymium and measure its isotopic composition on a mass spectrometer. The weeks of lab work certainly seemed worth it when my first few results came through!

In the new year I’m hoping to start picking out some fish teeth from my samples, and now that I’ve got some lab work experience I can start analysing them to get some Nd seawater values for the deep ocean in the Early Eocene.

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