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Non-steady-state closed dynamic accumulation chambers are widely used to measure the respiration of terrestrial ecosystems, thanks to their low cost, low energy consumption and simple transportability, that allow measurements even in hostile and remote environments. However, the assessment of the accuracy and precision associated with the measurement system (independently of possible disturbances due to chamber-soil interactions) is rarely reported. This information is instead necessary for basic quality control, to compare data obtained by different devices and regression models and to provide Confidence Intervals (CIs) on the carbon flux values. This study quantifies the uncertainty associated with emission flux measurements, with a focus on very low fluxes. Calibration tests using different accumulation chambers and CO2 sensors were performed, and fluxes were calculated by means of different models (parametric, non-parametric and flux models). The results of this work show that the linear regression model has the best reproducibility when compared to the other tested models, regardless of the sensor used and the chamber volumes, while the second order polynomial regression has the best accuracy. We remark the importance of building a calibration curve in the range of the expected flux values, with an interval between the lowest and highest imposed flux that should not exceed two orders of magnitude. To evaluate the reproducibility of the measurement, performing replicates for each imposed flux value is essential. We also show that it is necessary to carefully identify the best time interval for interpolating the CO2 concentration curve in order to guarantee reproducibility and accuracy in flux estimates.

International audience; The trace-element geochemistry of speleothems is becoming increasingly used for reconstructing palaeoclimate, with a particularemphasis on elements whose concentrations vary according to hydrological conditions at the cave site (e.g. Mg, Sr, Baand U). An important step in interpreting trace-element abundances is understanding the underlying processes of their incorporation.This includes quantifying the fractionation between the solution and speleothem carbonate via partition coefficients(where the partitioning (D) of element X (DX) is the molar ratio [X/Ca] in the calcite divided by the molar ratio [X/Ca] in theparent water) and evaluating the degree of spatial variability across time-constant speleothem layers. Previous studies of howthese elements are incorporated into speleothems have focused primarily on stalagmites and their source waters in naturalcave settings, or have used synthetic solutions under cave-analogue laboratory conditions to produce similar dripstones. However,dripstones are not the only speleothem types capable of yielding useful palaeoclimate information. In this study, weinvestigate the incorporation of Mg, Sr, Ba and U into a subaqueous calcite speleothem (CD3) growing in a natural cave poolin Italy. Pool-water measurements extending back 15 years reveal a remarkably stable geochemical environment owing to thedeep cave setting, enabling the calculation of precise solution [X/Ca]. We determine the trace element variability of ‘modern’subaqueous calcite from a drill core taken through CD3 to derive DMg, DSr, DBa and DU then compare these with publishedcave, cave-analogue and seawater-analogue studies. The DMg for CD3 is anomalously high (0.042 ± 0.002) compared to previousestimates at similar temperatures (~8 °C). The DSr (0.100 ± 0.007) is similar to previously reported values, but datafrom this study as well as those from Tremaine and Froelich (2013) and Day and Henderson (2013) suggest that [Na/Sr] mightplay an important role in Sr incorporation through the potential for Na to outcompete Sr for calcite non-lattice sites. DBa inCD3 (0.086 ± 0.008) is similar to values derived by Day and Henderson (2013) under cave-analogue conditions, whilst DU (0.013 ± 0.002) is almost an order of magnitude lower, possibly due to the unusually slow speleothem growth rates(

Poster; Several small lakes occupy a number of levels in the Galleria delle Stalattiti of Corchia Cave, Italy. An important feature of two such lakes is the presence of actively growing, subaqueous calcite mounds. Drill cores from these mounds have been the subject of investigations for several years. 234U-238U ages from the base of these cores, combined with stable isotope data from U-Pb-dated stalagmites collected metres away, suggest that some of the mounds have been growing since ~1 Ma. To provide baseline data for interpreting the geochemical variations of these cores, we commenced a study of modern lake-water chemistry, including stable isotopes of H, C and O, trace elements and organic matter content. Besides demonstrating the exceptionally stable state of these lake systems, the combining of core-top geochemistry with source-water chemistry allows us to explore water-calcite isotope and trace element partitioning, and to evaluate the suitability of this unusual speleothem-forming environment for clumped-isotope palaeothermometry. Preliminary results will be presented in this poster.