January 19, 2006 | David F. Coppedge

Can Caves Record Climate History?

Many geologists and climatologists have assumed that cave formations, forming slowly over long ages, preserve a record of climate changes.  These assumptions have been challenged by University of Texas researchers who experimented with water dripping from stalactites in a cave in Barbados.  Their work was published in GSA Bulletin.1
    Climate history could be inferred if the isotopic signatures of carbon and oxygen in the stalagmites and stalactites were purely a function of their abundances in the groundwater feeding the speleothems, and if speleothem growth were also a function of flow rate.  The problem is that these isotopic signatures are not in equilibrium.  They change dynamically, and are sensitive to variations even in the physical morphology of the growing formation.  Mickler et al. found that kinetic factors, such as the shape of a dripping stalactite, and how long the water has to flow down its surface before dripping onto the stalagmite below, can produce “large kinetic isotope effects” in the measurements.  The scientists put glass plates on three stalagmites within one square meter of each other, but measured very different signatures depending on the size of the stalactite above.
    Their measurements cast doubt on models in use to infer climate history.  Although they still believe that speleothem isotope measurements can still be used in such studies, they cautioned that other non-equilibrium effects must be taken into account:

Speleothem records may be influenced by kinetic isotope effects such that temperature-controlled equilibrium fractionation models alone cannot adequately explain the significance of the records.  Proper interpretation of these records may require that the non-equilibrium isotope effects … be calibrated to physical conditions in the cave, such as temperature, cave PCO2, drip rates, calcite precipitation rates, stalactite geometry, and drip water chemistry.   (Emphasis added in all quotes.)

They did not specify how such non-equilibrium kinetic effects might be calibrated.

1Mickler, Stern and Banner, “Large kinetic isotope effects in modern speleothems,” Geological Society of America Bulletin, Vol. 118, No. 1, pp. 65�81, doi: 10.1130/B25698.1.

This study points out some important principles of interpretation in scientific studies, especially those used to infer conditions in the unobservable past.  There are usually more factors influencing a measurement than many simplistic models assume.  Consider, for instance, how the experimental apparatus itself can alter a measurement.  The authors noted that even the shape of their glass plates differed from the natural situation: “Our experiment uses flat glass plates, which may affect the stable isotope composition of calcite along the growth layer by forcing the drip water to flow across a flat surface, unlike a natural convex stalagmite,” they said.  “This likely prolongs the residence time of drip water, relative to a natural stalagmite, allowing more time for CO2 degassing and calcite precipitation.”  Though they discounted the effect of this particular influence, there could well be other influences they did not consider.  After all, previous studies working on this subject apparently did not take speleothem geometry into account.
    Even putting a human being in a cave next to a stalagmite alters its environment.  A body gives off over 100 watts of heat, and alters the moisture of the atmosphere with his or her breath.  When making sensitive measurements of delicate isotopic ratios on a glass plate, how can they be sure all such influences have been calibrated, or even considered?  In addition, as they mentioned, the measurements are sensitive even to the geometry of the drip source.  Perhaps you can think of other influences that would be difficult to calibrate and easy to ignore: microbiology in the soil between the atmosphere and the cave, changes in the source of the water, the complex arrangement of cracks and channels in the cave roof, the presence of cave biota, or much more.  Consider how in recent decades a major paradigm shift in cave geology occurred when geologists began to consider the effect of bacteria on cave formation (05/10/2004).  Prior to that, they were not even thinking about it.  They were looking at geological forces alone, and had overlooked factors that turned out to have a huge effect on cave excavation processes and speleothem formation rates, leading to whole new ways of thinking about underground geology.  Meanwhile, summer-hire cave tour leaders were still regurgitating the old paradigm to the tourists.
    Regardless of how comfortable these scientists feel in their conclusion that isotopic ratios can reflect climate history with some degree of reliability, they admitted that the correlation of oxygen and carbon isotopic covariations was only about 55%.  Some studies showed no correlation, or even a negative correlation.  How can they know that the anomalous studies were not the more significant for true understanding?  Does the majority rule in science?  None of these questions should suggest that field studies and careful measurement are not profitable, but before swallowing a simplistic statement in a popular-level book or TV program about how we “know” something about prehistory, remember that even the best estimates are subject to drastic revision by subsequent findings.  Even if scientists could garner accurate measurements of every “known” parameter in the present, they cannot, in principle, know the unknowns.  In addition, the further that present processes are extrapolated into the past or future, the more uncertain they become – the more subject to assumptions that cannot be tested.  This is an inherent limitation of the scientific method.  Process is not the same as certainty.  When that is admitted, it is a good thing.  More often than not, interpretations are touted as facts, and the assumptions underlying the interpretations are not disclosed.
    For an interesting analysis of factors that call into question consensus views about cave formation and speleothem dating, order this DVD by Dr. Emil Silvestru, a specialist in karst geology with years of field experience: Geology and Cave Formation.  It also contains stunning photographs of some incredible crystalline wonders hidden in the darkness of these underground cathedrals (photo).

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Categories: Physics

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