November 15, 2006

Stalagmite Story

Do stalagmites grow from the ceiling or the floor of cave? Time is up – they grow from the floor of caves (stalactites grow from the ceiling), but the key is that they grow over long periods of time. Some stalagmites are thousands of years old and if they are in just the right type of cave, they can preserve a signal of temperature and precipitation levels over the time they grew.

A team of scientist from Austria and Germany located three stalagmites in the Spannagel Cave located around 2,500 m above sea level at the end of the Tux Valley in Tyrol (Austria) close to the Hintertux glacier. The temperature of the cave stays near freezing and the relative humidity in the cave is always at or near 100%. The stalagmites grew at a rate between 17 and 75 millionths of a meter per year and are nearly 10,000 years old.

The trick to extracting a temperature signal from the stalagmites involves measuring the oxygen 18 isotope (d18O) levels of each small layer from a sample taken from the features in the Spannagel Cave. Given the near perfect conditions in the cave, the d18O levels are determined by the temperature of the drip water at the time the layer was formed. Vollweiler et al. report that the resulting record “may thus be interpreted as a temperature signal with low d18O values corresponding to warmer temperatures and high d18O values corresponding to colder temperatures.”

The results are seen in Figure 1 (below) and the Little Ice Age (LIA) and Medieval Warm Period (MWP) stand out prominently along with other major climate features of the Holocene. The authors note that the record “exhibits substantial oscillations with low d18O values between 7.5 and 5.9 kyr (Holocene Climatic Optimum), 3.8 and 3.6, and 1.2 and 0.7 kyr (MWP) and high d18O between 7.9 and 7.5, 5.9 and 5, 3.5 and 3 kyr, and 600 and 150 yr (LIA).” In discussing warm periods of the past, they note “Between 2.25 and 1.7 kyr, a time known as the Roman Warm Period (RWP), some of the Alpine passes were ice-free also in winter.” Further, they note that the record “has a pronounced peak at 2.2 kyr, synchronous with Hannibal’s crossing of the Alps in 218 BC.” The stalagmite is screaming to us that many periods in the past 9,000 years were warmer than present-day conditions!


Figure 1. 9,000 year d18O record from Austria. The grey areas indicate the uncertainty ranges for the composite parts of the curve (see text). Note that the d18O values are inverted so that the line on the graph goes up during warming and down during periods of cooling (from Vollweiller et al., 2006).

The team of scientists notes that variations in vegetation, glacier extension, timberline, tree-ring width, and lake and river histories indicate major climate oscillations during the past few thousand years. Figure 2 shows a comparison of the d18O record with European glacial extension and lake level data. All three records clearly show (a) the Little Ice Age, (b) the Medieval Warm Period, and (c) periods in the past that are warmer than today.


Figure 2. Stalagmite d18O data (red) compared to an extension record of the Great Aletsch Glacier (Switzerland, top) and the west-central European lake level record (bottom) for the last 3,500 years.

So are the stalagmites telling us a story that only applies for conditions around the cave or for conditions that affected much of the planet? Two articles have appeared recently in major journals with results that reinforce the stalagmite story, but from very distant parts of the world. Achim Bräuning of the University of Stuttgart headed off to southern Tibet to core trees to study climate variations preserved in the ring widths and composition. Bräuning notes that other tree ring series from Tibet “have shown that the period from 1150 to 1380 was characterized by higher temperatures than today” and that “The succeeding period from 1430 until the late nineteenth century witnessed a series of cold intervals indicated by growth reductions and changes in the isotopic composition in juniper trees.” Bräuning concludes that “Larch trees from a third glacier forefield in southeastern Tibet show evidence of glacier activity from 1580 to 1590, from the end of the eighteenth to the beginning of the nineteenth century and from 1860 to 1880.” The Little Ice Age seems to come through loud and clear in the trees of southern Tibet.

Finally, a pair of scientists from the United States and Canada cored a lake in southeastern British Columbia and examined variations in pollen spores through layers of the core. Hallett and Hills found that between 700 and 150 years ago, the Kootenay Valley had returned to a wet closed forest “that appears to be a response to Little Ice Age cooling.”

Al Gore and the Intergovernmental Panel on Climate Change rolled out the Hockey Stick that showed little variation in climate until the great warm-up of the last 100 years. But as we see in a cave in Austria, trees in Tibet, and pollen in a lake in British Columbia, there is overwhelming evidence of considerable climate variability over the past 1,000 to 10,000 years.

References:

Bräuning, A., 2006. Tree-ring evidence of ‘Little Ice Age’ glacier advances in southern Tibet, The Holocene, 16, 369-380

Hallett, D.J., and L.V. Hills, 2006. Holocene vegetation dynamics, fire history, lake level and climate change in the Kootenay Valley, southeastern British Columbia, Canada, Journal of Paleolimatology, 35, 351-371.

Vollweiler, N., D. Scholz, C. Mühlinghaus, A. Mangini, and C. Spötl, 2006. A precisely dated climate record for the last 9 kyr from three high alpine stalagmites, Spannagel Cave, Austria, Geophysical Research Letters, 33, L20703, doi:10.1029/2006GL027662.




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