Journal article
Limitations of Hendy test criteria in judging the paleoclimatic suitability of speleothems and the need for replication
Journal of caves and karst studies, Vol.71(1), pp.73-80
04/2009
Abstract
Carbon and oxygen isotopes in calcite speleothems are powerful proxies for understanding past climate change. For calcite deposited under isotopic equilibrium conditions, variations in delta (super 18) O values directly reflect changes in cave temperature and the isotopic composition of meteoric water. Speleothem delta (super 13) C values have bedrock, atmospheric, and soil gas sources. Soil gases can be traced to the overlying vegetation, which is related to climate. Both delta (super 13) C and delta (super 18) O values are therefore potentially powerful tracers of climate change. Processes that could alter speleothem delta (super 13) C and/or delta (super 18) O values, and thereby mask primary environmental signals, fall in the categories of 1) kinetic processes, including deposition of calcite out of isotopic equilibrium, and 2) vadose processes, including evaporation of water at or near the land surface. In truth, there is no absolute test for the absence of these kinetic/vadose-zone processes. However, the Hendy Test is widely used for assessing whether isotopic equilibrium existed during the time of calcite deposition. Criterion (1) of the Hendy Test (i.e., that delta (super 18) O values remain constant along a single growth layer) may not be a valid control of equilibrium conditions because isotopic equilibrium could theoretically occur in the center of the speleothem at the same time that kinetic fractionation occurs at the flanks. Moreover, the concept of sampling along a single growth layer is flawed in both theory and practice. Criterion (2) of the Hendy Test (i.e., that there is no relationship between delta (super 13) C and delta (super 18) O) is based on the assumption that speleothem delta (super 13) C values are not linked to climate. However, speleothem delta (super 13) C values may well be linked to climate because climate provides a first-order control on soil productivity and the type of vegetation. Therefore, Hendy Test criterion (2) is not a prerequisite to isotopic equilibrium in all cases. We propose instead the Replication Test (i.e., the demonstration of similar isotopic profiles among two or more speleothems) for evaluating the likelihood of calcite deposition under isotopic equilibrium conditions. Replication of isotopic profiles among two or more speleothems is possible only if kinetic/vadose-zone processes are either: 1) absent or 2) have affected spatially separated speleothems in exactly the same way. Because the second scenario is highly unlikely, we propose that the Replication Test is effectively sufficient in ruling out kinetic/vadose-zone overprinting processes. We further suggest that the Replication Test is far more robust in testing for the absence of the wide range of processes described above than is the traditional Hendy Test.
Details
- Title: Subtitle
- Limitations of Hendy test criteria in judging the paleoclimatic suitability of speleothems and the need for replication
- Creators
- Jeffrey A Dorale - University of IowaLiu Zaihua - Institute of Geochemistry
- Resource Type
- Journal article
- Publication Details
- Journal of caves and karst studies, Vol.71(1), pp.73-80
- Publisher
- National Speleological Society
- ISSN
- 1090-6924
- eISSN
- 2331-3714
- Language
- English
- Date published
- 04/2009
- Academic Unit
- Earth and Environmental Sciences
- Record Identifier
- 9984229307002771
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