engleski

Chronology of a Pleistocene cave ventilation event that impacted speleothem records and their geochemistry

Speleothems rarely grow continuously for more than some thousand years. However, the composite speleothem record based on multiple samples from the same cave can have tens or even hundred of thousands years in duration. The geological evolution of the cave systems (e.g., re-routing of drip flow, opening/colmatation of conduits affecting ventilation, etc) is often a common cause for changes in speleothem growth. These non-climate change related cave processes can also be recorded in the geochemical and petrological speleothem proxies, causing problems when interpreting paleoclimate from speleothem records. Identification of hydrological or inside-cave climate events unrelated to climate changes in speleothem records is difficult, although the study of the erosive and/or depositional record of caves often provide valuable information. Here we present a study of an extreme case of condensation corrosion in Eagle Cave (central Spain). Large speleothems were formed in this cave, although a sudden ceiling collapse caused a drastic change in cave ventilation that resulted in a net dissolution of speleothems over the main hall of the cave due to the condensation corrosion process. Speleothems affected by dissolution show blunt morphologies and their internal structure (i.e., layers) is visible on their surface. Cyclic thermal changes related to the new ventilation dynamics, forced the condensation of water droplets on the cave walls and the speleothems. The droplets had an acidic pH as a result of cave atmosphere CO2 diffusion in the solution, causing the corrosion of carbonates where the drops were standing or flowing. The dissolution of speleothems by this event is recorded through the cave, although the magnitude of the process varies locally. Based on the morphology of speleothems, the thickness of carbonate lost by dissolution was in the order of tens of centimetres for most speleothems. Progressively, the collapse sinkhole that enabled the large thermal variability in the cave was filled with gravitational sediments and the ventilation regime became once more stable enough for speleothems to have net accumulation. Thus, a distinctive series of speleothems overgrew the dissolved speleothems. Dating of several speleothems before and after the dissolution event framed the condensation corrosion event between 60 and 55 ka BP, suggesting that dissolution rates might have been as high as 100 μm/yr. Petrology and geochemistry of speleothems differed before and after the dissolution event as the result of ventilation dynamic differing from the previous stage of speleothem formation. This research highlights the potential of speleothem proxies to record non-climate change related processes. Acknowledgements: This research is a part of the research project “Inter-comparison of karst denudation measurement methods” (KADEME) (IP-2018-01-7080) financed by Croatian Science Foundation.

Eagle cave, collaps, unroofed cave, karst, KADEME

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