Caves and Climate Change…. by Andi Smith

Andi

As we strive to understand modern day climate change and the possible impact humans are having on our environment, many scientists look to the past to provide evidence of natural climate evolution. Andrew Smith is one such scientist. Here he shares how analysing Spanish stalagmites has helped unlock the last 12,500 years of climatic changes as well as large scale rainfall dynamics throughout Europe...
 
So why caves?
 
Records of climate change can often be extracted from sediments, which have built up under different conditions to those we see today. Traditionally ice, ocean and lake cores have been the main areas of investigation, but it is now becoming increasingly common to use cave stalagmites to collect evidence of environmental change.

The formation of stalagmites
 

Here I am with some fine speleothem formations
in Shuttleworth Pot, Yorkshire, UK.

Cave formations or ‘speleothems’ are carbonate deposits that are gradually laid down by the deposition of limestone from cave drip waters. These waters originate as rainfall and enter the cave via the soil zone and then by exploiting existing bedrock fractures, further dissolving the surrounding limestone. As water enters the cave chamber it deposits it’s dissolved limestone load, creating stalactites that hang from the ceiling and stalagmites, which develop upwards from the floor.
 
Importantly, stalagmites develop year on year and whilst doing so incorporate the chemical signature of the water from which they have precipitated. Speleothems therefore encapsulate a wealth of chemical information, which can be used to provide evidence of changing rainfall patterns, vegetation productivity and regional aridity. In this current study, two stalagmites were extracted from a small cave in northern Spain. These speleothem indicate significant variations in rainfall during the last 12,500 years; offering an important record of environmental change, which may help us understand the development of human populations in this region.
 
Does the rain in Spain really fall on the plain?
 
Well…. honestly I’m not sure. However by analysing oxygen isotopes in the speleothem (a nice introduction to isotopes is provided in a previous blog post by BGS postdoctoral researcher J. Dean) we can assess how the amount of rainfall received in N. Spain has changed during the Holocene. A greater proportion of the lighter oxygen isotope (16O) in the record is driven by intense rainfall, whilst more of the heavier 18O can be attributed to lower rainfall amounts and enhanced evaporation. 
 

Automated drilling of the stalagmite creates a carbonate
powder which is then analysed for oxygen isotopes

Using the isotopic ratio we can identify periods of enhanced rainfall and extreme aridity during the Holocene. Our stalagmites indicate semi-glacial, arid conditions in the Younger Dryas (12,500 years BP), which rapidly subsided leading to very wet conditions by 8,500 BP. This change in rainfall is hugely important as it marks the start of the Holocene, a period of favourable climatic conditions which have supported rapid human development across the globe.
However, the Holocene has not always been climatically favourable. We identify a period of extreme aridity in N. Spain between 6000 and 4800 years BP, coinciding with the desertification of central Africa. It is around this time that human populations engaged in sedimentary farming activities in N. Spain, possibly related to the growing environmental pressures experienced during periods of drought. The latter part of the Holocene has been dominated by fluctuations in rainfall delivery and importantly, we identify dry climatic conditions around the timing of the Medieval Climate Anomaly and wetter conditions during the Little Ice Age, possibly related to variations in solar activity.
 
Whilst the speleothem records identify variations in rainfall amount at discrete time periods, it also becomes clear that Holocene rainfall dynamics have been influenced by a natural underlying climate cycle, with a periodicity of 1500 years.
 

Modern day environmental conditions near to the cave site in N. Spain

The 1500 year cycle in Europe’s climate
 
There has been great debate in the scientific community about the possible existence of an underlying (1500 year) cycle in European climate records, since the identification of such a cycle in Atlantic Ocean cores by Bond et al., (1997). This has sparked further investigation, centring on the forcing mechanisms that maybe capable of driving such a dominant climate cycle.

Our speleothem records provide further evidence to support this 1500 year cycle and are closely linked to the ocean core records of Bond. This close coupling indicates a singular control over both ocean circulation patterns and the delivery of rainfall to southern Europe. We identify that the North Atlantic Oscillation (NAO) has the capacity to influence both atmospheric and oceanic systems simultaneously. Our speleothem records therefore substantially add to the debate surrounding millennial scale cycles in Holocene climate and offer a coherent solution as to what forces such large scale cycles in European climate records.

Andi

Andi is a recently completed PhD student at Lancaster University and the BGS. Andi was supervised at Lancaster by Dr Peter Wynn and Prof Philip Barker, and at the BGS by Prof Melanie Leng and Dr Steve Noble. His thesis was entitled Speleothem Climate Capture – A Holocene Reconstruction of Northern Iberian Climate and Environmental Change.