Monday, 11 December 2017

Using fossilised algae to detect historical mega-droughts...by Heather Moorhouse

DeepCHALLA is an International Continental Scientific Drilling Programme project investigating ~250,000 years of climate change using lake sediment cores from Challa, a 92m crater lake on the Kenyan-Tanzanian border. Dr Heather Moorhouse from Lancaster University explains how fossilised diatoms have been purified from the sediments ready for isotope analysis at the Stable Isotope Facility, British Geological Survey.

The DeepCHALLA project is a large, international consortium of scientists investigating ~250,000 years of climate and ecosystem change in equatorial east Africa using sediment cores from lake Challa. My role, along with Principal Investigators Prof. Philip Barker at Lancaster University and Prof. Mel Lang at BGS is to test whether mega-droughts (lasting up to thousands of years) from ~130-190,000 years before present, may have resulted in the dispersal of our hominin ancestors out of Africa. Further, this region is drought-sensitive and improved understanding of past climate will help predict and prepare the area for future climate change as our planet warms.

In order to investigate the historical climate of the region, we are using diatoms found in the lake sediment cores. Diatoms are a common and abundant member of the phytoplankton community; the microscopic single-celled organisms found in all surface waters, which produce energy from sunlight. They bloom in Challa in the summer and when they die, they sink to the lake floor and form noticeable diatom-rich layers in the sediments, which accumulate over time. Because Challa is a deep crater lake with little shoreline or shallow lake habitats, it is a relatively simple system leading to low diatom diversity, dominated by two species; Afrocymbella and Nitzschia species.

From L-R: Two of the DeepCHALLA lake sediment core sections - the lighter layers are rich in diatoms; Difference
 between a sample rich in diatoms (left) and a sample with little diatoms and more mineral matter (right).
In particular, we are interested in the oxygen isotopes that the diatoms have up-taken from the lake water. Heavier oxygen isotopes indicate higher evaporation rates and so, drier conditions, whereas lighter isotopes indicate more rainfall. Diatoms produce silica or glass cell walls, which protect the isotopes from degradation and thus make ideal proxies for climate reconstructions. Additionally, In terms of investigating isotopes from diatoms the low diversity at Challa is a good thing, as sometimes the size of the species can influence what isotopes they uptake and cause confusion when interpreting results.

Sediment samples were collected this summer from the lake Challa sediment cores from Gent, Belgium (see my previous blog). Since summer, I have been busy in the lab at Lancaster trying to purify ~290 sediment samples so that just diatoms remain. This involves dosing the sediment with hydrochloric acid to remove carbonates, hydrogen peroxide and nitric acid to remove organic material and sieving to remove large particles. Because the sediments of lake Challa are so rich in diatoms, most samples have been processed quite quickly.

SEM image of fossilized diatoms from lake sediment 39 metres deep.
Image shows diatom fragments, Afrocymbella species.
It is important that the diatom samples are as pure as possible as any additional organic or minerogenic material can alter the isotope results. In order to double check the cleanliness of the diatoms, I looked at all the cleaned samples under a light microscope and determined the percentage of diatoms to contaminants. A further subset of samples was investigated using a Scanning Electron Microscope (SEM) at Lancaster University, which has a greater magnification to that of a light microscope. Any potential contaminants were scanned using the EDX detector attached to the SEM, which describes the elemental composition of the item in question and again is another great tool to help detect impurity. Luckily most of my samples consisted of diatoms or diatom fragments, and so, are ready to undergo isotope mass spectrometry at BGS, which will begin at the start of next year. Watch this space for what I hope will be some exciting results.

Special thanks to Dr Sara Baldock at Lancaster University for help with the SEM.
 
 

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