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Abstract:
Speleothems from cold high-elevation (and high-latitude) caves are sensitive paleoenvironmental archives, because they form close to the freezing point of water, and Alpine or Arctic soils – the source of carbon dioxide for carbonic-acid dissolution – are thin and experience a short vegetation period. Climatic cooling is therefore likely to disrupt the growth of these speleothems. However, growth hiatuses may paradoxically be overcome when atmospheric cooling leads to lowering of the equilibrium-line altitude and expansion of temperate glaciers over the karst system, which prevents the latter from freezing. Caves in mountain regions of the mid to high latitudes had a much higher chance of being covered by glaciers during cold climatic periods than caves at lower elevations, which were often in the permafrost zone. These periods without frost in caves covered by temperate glacier ice can be recorded by so-called subglacial speleothems if the host rock contains disseminated pyrite. Widely present in impure limestones, dolostones, and marbles, oxidation of this sulfide mineral gives rise to sulfuric-acid dissolution of the host rock, replacing the carbonic-acid dissolution that operates during warm climate periods.
This review summarizes the research history of subglacial speleothems, beginning with seminal studies of Castleguard Cave, which extends beneath the Columbia Icefield in Western Canada. Subsequent work on a number of caves in the Eastern and Western Alps in Europe has shown that speleothems from such subglacial environments are widespread and provide unprecedented opportunities to obtain records of environmental change covering the long glacial periods. However, the proxy system behavior of these settings has yet to be fully developed. We therefore propose a suite of diagnostic criteria, some untested, by which to identify and interpret subglacial speleothems, with recommendations for future research. The combination of high δ13C (at or above host rock values) and low δ18O provide a robust first-order proxy for subglacial environments, while trace-element and sulfate stable-isotope (δ34S and δ18O) data can elucidate the dissolution pathway and redox conditions resulting from ice cover. When studies of subglacial speleothems are combined with conventional warm-climate speleothems controlled by soil dynamics, as well as locally present cryogenic cave carbonates (an indicator of the presence of paleo-cave ice accumulations), the possibility of exploiting the full paleoclimate potential of Alpine and Arctic caves on glacial-interglacial timescales opens up.
