Caves in Climate Studies
BY HEATHER M. STOLL
 |
| 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.
 |
| 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.
 Many 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.
 No
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.
Copyright ©2004 Global
Underwater Explorers.
All rights reserved.
|
