Ancient floods on the Thames... by Raquel Santos

Raquel

Dr Raquel Santos is an organic geochemist working within the Centre for Environmental Geochemistry. Today we discover the exciting project she's working on with BGS colleague Dr Christopher Vane investigating past flooding events and the carbon cycle of the London Thames, an area housing a fifth of the UK population...

My current work with Chris is exploring if GDGTs (Glycerol Dialkyl Glycerol Tetraethers - more about these later!) will identify past climate variations and flooding events by determining changes in the organic matter in the soils.

Molecular structure of branched GDGTs (right) and Crenarchaeol (left) used to calculate the BIT index in tidal Thames.

Location of the surface samples from the intertidal foreshore of the River Thames to be analysed in this project. (c) Google

The project will increase our knowledge of how human activities have impacted on the transport of carbon from the land into the North Sea, ultimately leading to a better understanding of the global carbon cycle. It is also important to understand the fate of the terrestrial organic matter in the Thames estuary because the particulate fraction (the percentage that has not dissolved in the water) has the potential to adsorb organic micropollutants (pesticides, pharmaceutical residues, hormones, etc) and heavy metals (mercury, lead, etc) that can cause health concerns.  

GDGTs explained!

The Organic Chemistry Team; from right to left:
Alex Kim, Vicky Moss-Hayes, Chris Vane, Raquel Santos

GDGTs are membrane lipids produced by Archaea and bacteria that provide information on the biogeochemistry and environmental conditions of modern and past environments. They have been used specially in paleoclimate reconstructions to recreate changes in sea surface temperatures, mean annual temperatures and soil acidity (pH). Another important application of these compounds is to track the amount of soil organic matter input into the aquatic environments using the Branched Isoprenoid Tetraether index (BIT index). The BIT index is the ratio between branched GDGTs that are produced by soil bacteria, and Crenarchaeol, an isoprenoid GDGT produced by marine Archaea of the phylum Thaumarchaeota. The index was first described by Hopmans et al., 2004 and was originally used to estimate the amount of soil organic matter input to marine environments. Recently, the BIT index has been applied in rivers and lakes. Although, it has been mainly applied in large river settings (e.g. yellow river, Amazon, etc.), it has fewer applications in smaller rivers like river Têt in France where it showed to be a useful proxy for flood reconstruction.

Why use GDGTs to track soil organic matter?

Most studies that aim to track the origin and fate of organic carbon in estuaries have been predominantly based on bulk organic parameters. However, the interpretation of these data can be compromised by the wide range of potential end-member compositions and preferential remineralisation. Some of these limitations can be overcome with the application of molecular biomarkers that are specific to organisms. GDGTs are ubiquitous compounds that are specific to certain groups of organisms and that are not expected to be strongly affected by the industrial activities that take place along the river margins. Therefore, they are good biomarkers to be used at these settings.

If you would like to contact me, please write to Dr. Raquel Santos.

For information about current and future projects at the Organic Geochemical Laboratory, please write to Dr. Christopher Vane.