The potential of carbonate speleothems from Mediterranean gypsum caves for palaeoclimate and palaeoenvironmental reconstructions

• Main Author: Columbu, Andrea
• Format: Doctoral or Postdoctoral Thesis
• Language: unknown
• Published: 2016
• Subjects: palaeoclimate; speleothems; landscape evolution; geochronology; U Th dating; caves; stable isotopes; karst; geomorphology; hydrogeology; Greenland; Stella; Basino
• Online Access: http://hdl.handle.net/11343/123733

© 2016 Dr. Andrea Columbu The principal aim of this thesis is to assess to what extent carbonate speleothems from gypsum caves can be applied to the study of past environments and climates. This is achieved by discussing four distinctive case studies. Each develops a single application for the use of these speleothems, involving geological, environmental, geochemical and climatic issues. The major outcomes are listed below. The first case of study (geological application) demonstrated that climate strictly controls the speleogenesis in gypsum, and the excavation of cave levels occurs at intra-Milankovitch timescale. Five speleothems were sampled at Monte Tondo (North Italy), an area affected by rapid uplifting. The age of the speleothems was correlated with the main warm/wet climate pulses over the last ~130,000 years. The age of speleothems provided a minimum age for the formation of the caves in which they grew; caves were formed during periods of relative cold/dry climate, when the local base level (Senio River) established a new altitudinal position and the flux of regolith from the vegetation-free valley flanks form river terrace deposits at the valley bottom. These observations together allowed the creation of a rigid climate-driven speleogenetic model that might be generally applicable to karst systems in different geological and climatic conditions. The second case of study (palaeoenvironmental application) aimed to test if speleothems from other gypsum caves also grew during periods of warm and wet climate. Speleothems were sampled nearby Monte Tondo (Spipola-Acquafredda, Stella-Basino and Castelnuovo karst systems) and Spain (Sorbas karst system). The age of the samples (sixteen in total) coincided with the main interglacials (MIS 7e, MIS 5e and Holocene) and Greenland Interstadials (GIS 24, 21, 20 and 19) over the last ~250,000 years, confirming the hypothesis. Climate controlled the formation of speleothems because of high concentration of CO2 in soils and enhanced hydrogeological circulation in the karst network, essential prerequisite to trigger supersaturation with respect calcium carbonate in gypsum groundwater. All the samples were also analysed for δ18O; at multimillenial timescale, speleothem δ18O appeared to be driven by palaeovariation of rainfall amount. The third case of study (geochemical application) had several findings: 1) speleothems from gypsum caves are comparable to speleothems from calcareous caves when the aim is the construction of U-Th chronologies; 2) radiocarbon analyses demonstrated that, in the area of study (Monte Tondo), percolating water received most of carbon from overlying soils rather than carbonate beds within the gypsum sequence, dissolution of bedrock occurred under open-system conditions, and SOM turnover rate is slightly above the average of rates for soils studied elsewhere (~600 years); 3) speleothems from gypsum caves report a peculiar Sr-Mg composition if compared with those grew in calcareous environment; the gypsiferous bedrock is responsible for the high content of Sr, while a separate source procures Mg; PCP is not the principal driver for the co- variability of Mg-Sr. This study set the background for the use of speleothems from gypsum caves as palaeoclimate archives. The last case of study (palaeoclimate application) presents growth-rate, δ18O, δ13C and trace elements timeseries for a flowstone grew during the Last Interglacial (~133-124 ka), aiming to understand if these properties were affected by important climate changes occurred during this period (i.e. penultimate deglaciation and intra-LIG rainfall/temperature pulsations) at different timescales. The effect of climate was found negligible, because hydrogeological and karst processes had the main impact on growth rate and geochemical composition. This study should warn future researchers about the existence of important limitations when using carbonate speleothems from gypsum caves for rigid palaeoclimate reconstructions; the high solubility of gypsum bedrock, and consequent rapid modification of karst routes, must to be taken into account when using these samples for palaeoclimatic studies.