Friday, 19 December 2014

Reading the signals in sediments... by Jonathan Dean

Jonathan Dean has just published a paper in the Journal of Hydrology, where he brought together measurements made at the British Geological Survey over two decades, to better understand how climate change is recorded in lake sediments. Here he discusses why this was such important work...

Nar Gölü in April 2014. The lake formed in an old volcano.
In the Stable Isotope Facility at the BGS, a lot of our work is focussed on using lake sediments to reconstruct how climate has changed in the past. In lakes, sediment deposited every year records what was going on in the lake, which is often related to climate, and by looking at variability in the chemical signature of lake sediments over time we can therefore reconstruct how climate changed in the past. But every lake records climate differently: some lakes have sediments that record how temperature is changing whereas others respond to how precipitation amounts are varying. Therefore, before we can use lake sediments to reconstruct climate, we need to calibrate the signal from the lake we are working on.

Over the past couple of decades, researchers from the Universities of Nottingham, Plymouth, Birmingham and Ankara have been collecting water samples and lake sediments from a lake in central Turkey called Nar Gölü. We then analysed the samples here at the BGS to look at how the oxygen isotope ratio has changed over recent times and what aspect of climate they are recording.

Isotopes are different types of an element, and oxygen has two main types. We compared how the oxygen isotope ratio in the lake sediments has changed over recent times compared to our measurements of lake level and the local meteorological record. Over the 2000s, lake level fell by 3 metres as precipitation decreased and high summer temperatures increased evaporation in central Turkey.

We were able to show that oxygen isotopes recorded this shift well: there was a strong correlation between the oxygen isotope ratio and the lake level (which was responding to the drying climate), with an increase in the ratio as the lake level fell.

Assuming this relationship was consistent into the past, we can therefore infer than if the oxygen isotope ratio in the sediments decreases the climate was getting wetter, whereas if the ratio increases the climate was getting drier.

Our study will allow us to better interpret a long sediment core sequence spanning roughly the last 15,000 years that we collected in 2010. It will hopefully also demonstrate to other scientists working with lake sediments that monitoring how the lake responds to climate change in the present allows us to better use lake sediments to reconstruct how climate changed in the past.

The photo on the right is a recent sediment core from Nar Gölü, with the top the present day, and a light and dark band representing 1 year of sedimentation.

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