ALBERT

Description: Hydrogen (δ2H) and oxygen (δ18O) isotopes of water extracted from speleothem fluid inclusions are important proxies used for paleoclimate reconstruction. In our study we use a cavity ring-down laser spectroscopy system for analysis and modified the approach of Affolter et al. (2014) for sample extraction. The method is based on crushing of small sub-gram speleothem samples in a heated and continuously water-vapour purged extraction line. The following points were identified: Injection of reference water shows a precision (1σ) of 0.4–0.5 ‰ for δ18O values and 1.1–1.9 ‰ for δ2H values for water amounts of 0.1–0.5 μl, which improves with increasing water amount to 0.1–0.3 ‰ and 0.2–0.7 ‰, respectively, above 1 μl. The accuracy of measurements of water injections and water-filled glass capillaries crushed in the system is better than 0.08 ‰ for δ18O and 0.3 ‰ for δ2H values. The reproducibility (1σ) based on replicate analysis of speleothem fluid inclusion samples with water amounts 〉0.2 μl is 0.5 ‰ for δ18O and 1.2 ‰ for δ2H values, respectively. Isotopic differences between the water vapour background of the extraction system and the fluid inclusions have no significant impact on the measured fluid inclusion isotope values if they are within 10 ‰ for δ18O and 50 ‰ for δ2H values of the background. Tests of potential adsorption effects with inclusion free spar calcite confirm that the isotope values are unaffected by adsorption for water contents of about 1 μl (fluid inclusion) water per g of carbonate or above. Fluid inclusion analysis on three different modern to late Holocene speleothems from caves in northwest Germany resulted in δ18O and δ2H values that follow the relationship as defined by the meteoric water line and that correspond to the local drip water. Yet, due to potential isotope exchange reactions for oxygen atoms, hydrogen isotope measurements are preferentially to be used for temperature reconstructions. We demonstrate this in a case study with a Romanian stalagmite, for which we reconstruct the 20th century warming with an amplitude of approximately 1 °C, with a precision for each data point of better than ±0.5 °C.

Description: Up to 50% of the signal delays of L-band signals used in the Global Navigation Satellite System comes from the topside ionosphere and plasmasphere. In this study, we apply an Ensemble Kalman Filter (EnKF) approach to estimate the 4D electron density of the topside ionosphere and plasmasphere based on space-based STEC data. NeQuick model is used as background. The STEC measurements of eleven LEO satellites are used for the reconstructions. The majority of the approaches, working with EnKF, uses physics-based models for the propagation step. In our work, we investigate the question how the propagation step can be realized, in the case that a physical propagation model is not available or discarded due to computational burden. We explore different propagation models and compare them with the iterative reconstruction technique SMART+ for two periods of the year 2015 covering quiet to perturbed ionospheric conditions. We check the capability of the estimations to reproduce assimilated STEC as well as to reconstruct independent STEC measurements. The comparison with the assimilated STEC shows that during both periods all methods reduce the statistics (Median, SD, RMS) of the STEC residuals in comparison to the background model by up to 86%. In summary, the results indicate that the methods EnKF with exponential decay as propagation model (EnKFexp) and SMART+ perform best, reducing the independent STEC residuals by up to 64%, compared to the NeQuick model.

Description: The global and regional ionospheric maps are often used for a wide range of applications in geosciences, in particular to support precise positioning, but also for geophysical and atmospheric studies. There are currently many analysis centers and research groups providing operational and test VTEC maps. However, IGS ACs and other groups use different mathematical models and estimation techniques resulting different resolutions, accuracies and time delays of their products. Therefore, there is a need to compare and validate existing VTEC models. In this presentation, we present the overview talk about the work within the last 4 years of the IAG Joint Working Group (JWG 4.3.4) on validation of VTEC models for high-precision and high resolution applications. Among others, we evaluated (1) the accuracy and consistency of the IAAC GIMs during high and low solar activity periods of the 24th solar cycle, (2) the accuracy the two most popular ionospheric mapping functions - SLM and MSLM, (3) deterministic and stochastic approaches to VTEC modelling, (4) GIMs performance in single point and precise point positioning GNSS applications, (5) the accuracy and consistency of GNSS-derived VTEC maps and empirical models, (6) GIMs performance using external data (JASON) and GNSS, (7) the accuracy of new global ionosphere models.

Description: The rising demand for potable water in densely populated coastal regions has recently promoted growing research interest in detecting offshore freshened groundwater (OFG) worldwide. Recent geophysical studies along the continental margins offshore Israel, New Zealand, Malta, and the United States of America provide some examples of integrating geophysical and borehole data to constrain the spatial extent of OFGs and estimate their pore-water salinity. However, occurrences of OFGs and the interaction between terrestrial, carbonate-hosted groundwater systems with seawater are understudied in many coastal regions by complicated seafloor morphology. In this study, we investigate whether OFG can exist offshore a semi-arid carbonate coastline along the Maltese Islands and explore the possibility of sustainably exploiting these reservoirs as an unconventional source of potable water to relieve freshwater scarcity. We present an integration of 2-D resistivity models derived from marine controlled source electromagnetic (CSEM) measurements with 2-D and 3-D seismic data, core samples, borehole data, and geochemical measurements. Electrical resistivity models identify localized resistive anomalies (〉 10 ohm-m) offshore the northeastern coast of Gozo (the second-largest island in the Maltese archipelago). Furthermore, a resistive body is observed at ~ 300 m below sea level close to the coast of Gozo which extends northeastwards and disappears at ~ 8 km offshore. If the anomalous resistive body is associated with pore-water salinity variations or, alternatively, caused by lithological changes, will be discussed through an integrative geological model developed along each profile.

Description: The high-quality dual-frequency phase measurements of Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) system provide valuable opportunities to examine the Earth’s ionosphere. DORIS data from the Jason-3 satellite has also been available in Near-Real-Time (NRT) with a delay of a few hours. Such data are perfectly suited for independent validation and combination of Real-Time Global Ionospheric Maps (RT-GIM) derived from GNSS measurements. In this work, we analyzed the feasibility of using DORIS data to estimate the accuracy of GNSS-generated ionospheric models. To this end, the concept of DORIS differential Slant Total Electron Content (dSTEC) assessment is proposed. The consistency between DORIS and GNSS dSTEC assessments in the quality analysis of RT-GIMs was checked, and the overall Pearson correlation coefficient reaches 0.81 during the one-year test period. The DORIS dSTEC assessment can be used not only to estimate the accuracy of individual GIMs, but also to determine their weighting within a combination strategy. The performance of DORIS-dSTEC and GNSS-dSTEC combined GIMs is assessed by comparison to Jason-3 VTEC from the mission altimeter. The standard deviations are 4.71 TECu and 4.80 TECu for DORIS-dSTEC and GNSS-dSTEC combined GIMs, indicating the slightly better performance of DORIS-dSTEC combined RT-GIM in Jason-3 VTEC assessment. Overall, NRT DORIS data can be used to independently validate and combine GNSS-derived ionospheric maps. In the future, it is also envisaged that DORIS data can be directly incorporated into ionosphere modeling. To this end, the provision of NRT data from other DORIS missions is planned (e.g., Sentinel-3).