ALBERT

Beschreibung: In order to better understand the microtextural changes associated with serpentinization reactions, natural millimeter-sized olivine grains were experimentally reacted with alkaline NaOH and NaHCO3 solutions at a temperature of 200 °C and for durations of 3 to 12 months. During hydration experiments, dissolution and precipitation were intimately correlated in time and space, with reaction products growing in situ, either as layered veins or as nearly continuous surface cover. In contrast, carbonation experiments showed a strong decoupling between both processes leading to essentially delocalized precipitation of the reaction products away from dissolution sites. Textural analyses of the samples using scanning electron microscopy, Raman spectroscopy, and X-ray synchrotron microtomography provided experimental evidence for a cause-and-effect relationship between in situ precipitation and intracrystalline reaction-induced cracking in olivine. Juvenile cracks typically nucleated at the tip of dissolution notches or on diamond-shaped pores filled with reaction products, and propagated through the olivine crystal lattice during the course of the reaction. The occurrence of new cracks at the tip of diamond-shaped pores, but also of tiny subspherical pores lining up along microcracks, indicated that fracturation and porosity networks were mutually driven, making serpentinization an extremely efficient alteration process over time. Alternatively, our data suggested that some form of porosity also developed in absence of fracturation, thus further highlighting the remarkable efficiency and versatility of serpentinization processes.

Beschreibung: We characterize and monitor subsidence of the Bursa Plain (southern Marmara region of Turkey), which has been interpreted as resulting from tectonic motions in the region. We quantify the subsidence using Interferometric Synthetic Aperture Radar (InSAR) time-series analysis. The Stanford Method for Persistent Scatterers InSAR package (StaMPS) is employed to process series of Sentinel 1 A-B radar images acquired between 2014 and 2017 along both ascending and descending orbits. The vertical velocity field obtained after decomposition of line-of-sight velocity fields on the two tracks reveals that the Bursa plain is subsiding at rates up to 25 mm/yr. The most prominent subsidence signal in the basin forms an east-west elongated ellipse of deformation in the east, and is bounded by a Quaternary alluvial plain undergoing average vertical subsidence at ~10 mm/yr. Another localized subsidence signal is located 5 km north of the city, following the Bursa alluvial fan, and is subsiding at velocities up to 25 mm/yr. The comparison between temporal variations of the subsiding surface displacements and variations of the water pressure head in the aquifer allows estimation of the compressibility of the aquifer, α . It falls in the range of 0.5 × 10 − 6 − 2 × 10 − 6 Pa−1, which corresponds to typical values for clay and sand sediments. We find a clear correlation between subsidence patterns and the lithology, suggesting a strong lithological control over subsidence. In addition, the maximum rate of ground subsidence occurs where agricultural activity relies on groundwater exploitation. The InSAR time series within the observation period is well correlated with changes in the depth of the ground water. These observations indicate that the recent acceleration of subsidence is mainly due to anthropogenic activities rather than tectonic motion.