When the mites go up…

Guest Commentary from Andy Baker, U. of Birmingham

It doesn’t seem obvious really. Going underground into caves, removing stalagmites and analysing their isotopic composition isn’t the first thing you would do to look for past climate information. But for nearly 40 years, there has been an active, and growing research community that investigates the climate records preserved in these archives. Stalagmites have recently received high profile use in climate reconstructions, for example records from China and Norway have featured in Moberg’s last millennium temperature reconstruction; in a northern hemisphere temperature reconstruction of the last 500 years and even been debated here on RealClimate. So it seems timely to review why on (or even under) earth should research go underground to look at surface climate.

To do that, we need briefly to explain how stalagmites are formed. Most simplistically, to grow a stalagmite you need water, and that water has to be saturated with carbon dioxide. Then this water drips from a cave roof, the carbon dioxide in the water will ‘degas’ into the atmosphere, and as part of that process calcium carbonate will form, which will form a stalagmite. Both the presence of water, and the fact that the water is saturated with carbon dioxide, can provide information about the surface climate. The water was, at one time in the past, surface rain or snow, and should contain information about that rain or snow through the composition of its isotopes. And the carbon dioxide saturation comes, not from the atmosphere, but the soil above the cave. Soil carbon dioxide concentrations are orders of magnitude greater than atmospheric, and there is a complex relationship between the concentration of soil carbon dioxide in cave drip waters, temperature and soil moisture.

Thus there could be some climate signal preserved within stalagmites, the question is how to decode it. The surface rain will interact with the surface soil and vegetation, which may alter any climate signal containing in the rainwater, or create new soil derived signals, or probably a mixture of the two. After a period of time that will depend on climate, seasonality, vegetation, etc. the water will reach the ground water. In the ground water, it will probably mix with waters of differing ages, smoothing any climate signal it contains. The nature of ground water flow may also introduce non-linearities into the signal. The simplest example is the overflowing bath scenario – imagine that filling your bath up at an increasing rate represents increasing rainfall, and that the bath is your groundwater store, and the plughole and the overflow are the outlets feeding stalagmites. The plughole stalagmite will respond first to the rainfall, the overflow stalagmite will be delayed untill the bath is full. As the bath fills and the storage time increases, the plughole stalagmite will preserve an increasingly smoothed water signal.

Therefore, it is a complex system. Over the last 40 years of stalagmite palaeoclimatology, the bulk of the research community was interested in the timescales of ice ages, as one of the big advantages of working with stalagmites is that they can be dated by the natural decay of uranium and thorium isotopes back to around 500,000 years. Over those timescales and temperature changes of 10°C or more, stalagmites are pretty convincing at recording the 1st order climate changes (e.g. glacial-interglacial changes, Dansgaard-Oeschger events, etc..) as the climate signal is much greater than the noise induced in the soil/vegetation-groundwater-cave system, and several papers a year can be read in journals such as Science and Nature (in particular, the Hulu cave record was truly exceptional).

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