Caves in Climate Studies

Caves in Climate Studies

BY HEATHER M. STOLL

Peacock Springs
Peacock Springs . Photo ©Steve Auer
Amid rising concern over greenhouse warming, scientists from all disciplines have turned a great deal of their attention to studying climate change. Geologists are no exception; they are busy hunting down records of how the earth's climate has varied in the past. By unraveling the sequence of events for past climate changes, they refine our understanding of the climate system, its stability, and feedback processes. No time period has captured as much interest as the relatively recent Quaternary (the last 1.8 million years), with its large oscillations from frigid glacial to warm interglacial climates such as the present. Cave deposits have recently emerged as one of the more promising sources of Quaternary climate records from the terrestrial realm. For the last several decades, scientists studying Quaternary climate changes have been stuck in the mud, so to speak. Well, marine sediments, to put it more politely. In most locations, tiny carbonate and silica shells constantly accumulate in an ever-thicker pile of sediments on the ocean floor; these enable geologists to use the chemistry, sedimentology, and species composition of these sediments to learn about past climate changes. However, in all fairness, scientists have been very interested in complementing marine records with climate records from the continents, because, among other reasons, we live there. The problem is that most climate-related deposits on land (e.g., glacial landforms, like moraines) are, by nature, discontinuous; so, it is difficult to identify the context of the climate changes they represent.
George Irvine
Speleotherm Cross-section
Cave deposits represent one of the very few continuous records of climate change available for the continents. Speleothems (stalactites, stalagmites, flowstones, and other formations precipitated in caves) are characteristic of karst zones where groundwaters, enriched in CO2 from soil respiration, dissolve carbonate in their host rock, and reprecipitate it when groundwater enters cavities and releases this CO2. In many cases, continuous precipitation of speleothems creates successive layers of carbonate (analogous to tree rings or coral growth bands) during thousands or tens of thousands of years. The properties of each layer of speleothem carbonate provide information about the climatic and hydrologic conditions at the time that layer formed. By taking a core of the speleothem and measuring the properties of each successive layer in a speleothem, it is possible to obtain a continuous record of climatic variations in that region. By radiometrically dating (carbon-14 or U-Th) parts of the speleothem, we can identify the timescale of these climatic variations. Graph 1Many properties of cave speleothems can provide information on climatically driven processes. Most frequently we study speleothem chemistry, both the ratio of stable (non-radioactive) isotopes of oxygen and carbon, and the abundance of minor elements, such as Magnesium (Mg), Strontium (Sr), Barium (Ba), Uranium (U), and Manganese (Mn). The chemistry of speleothems is controlled primarily by the chemistry of the drip waters from which they precipitate. In turn, the chemistry of these drip waters depends on weathering rates, temperature, and surface vegetationÑfactors that are all influenced by climate. However, unraveling the climatic significance of chemical variations in speleothems is more complicated than interpreting similar variations in marine carbonates (like corals or foraminiferal shells). This is because the chemistry of the ocean, from which marine carbonate precipitates, is much more homogeneous than the chemistry of the ground water, from which speleothem carbonate precipitates. Typically, we need to study the modern hydrogeology of the cave system, especially the chemistry of the cave drip waters, to understand how the drip water chemistry responds to climate variations. Alternatively, it is possible to compare recent (last 100 year) variations in speleothem chemistry with historical records of regional climate to "calibrate" the chemical indicators of climate variations.


Among paleoclimatologists, another popular property of speleothems is their luminescence, which depends on the amount of organic acids released to cave drip waters from surface vegetation. The release of these acids depends on the intensity of sunlight, rainfall, and soil biotic activity, all related to climate. As with speleothem chemistry, the relative importance of different climatic variables can be calibrated for each cave environment by comparing recent variations in speleothem luminosity with historical records of regional climate. Detailed measurements of luminescence might require complicated laser techniques, but basic results can be obtained by photographing a speleothem section under an ultraviolet lamp (the same one that makes otherwise dull-looking minerals turn pink and green, which most of us have seen at one time or another in a natural history museum). Occasionally, seasonal cycles in plant productivity make thin luminescent bands that can be counted like tree rings.

Aside from the continuous climate record provided by speleothem chemistry and luminescence, speleothems can also be useful in identifying certain discrete events in caves, like marine incursions and earthquakes (which can offset the speleothem growth axis). Other important episodes in the history of caves, such as major floods and roof collapses, can be constrained by dating overlying and underlying speleothem deposits. Unfortunately, all of these climate approaches require speleothem material to be removed from caves for analysis. However, recent advances in microcoring technology make it possible to extract small cores from the speleothem centers so that the boreholes can be subsequently filled and capped.

Figure 2No previous climate studies (that I am aware of) have used speleothems from underwater caves. However, most mid-latitude and low-latitude coastal platforms were exposed to karst processes repeatedly during Quaternary glacial episodes. For the last several million years, glaciers have come and gone about every 100,000 years. But the glaciers tend to hang around for much longer than they stay away. As a result, the continental shelves have been above sea level, where caves and conduits can worm their way through the carbonates, for most of the past several million years (see Figure). Imagine going for a dive to someplace currently 66 feet (20 meters) underwater off the coast of Florida. If we hang out there for an extended vacation (say the 120,000-year duration of a glacial cycle), we would find ourselves waving at the fish for only 10% of the time (about 12,000 years) and dry under the sun for about 90% of that time (about 108,000 years). Our current situation (waving at fish) is unusual, and the sunny, cave-making situation the norm. Drowned karst systems, like those off the coast of Florida, and like those hypothesized but as-yet-undiscovered in Northern Spain, may provide important deposits for future climate studies.

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