Quantifying dating uncertainties in layer-counted paleoclimate proxy archives

Paleoclimate proxy records have non-negligible uncertainties that arise from both the proxy measurement and the dating processes. Knowledge of the dating uncertainties is important for a rigorous propagation to further analyses; for example for identification and dating of stadial-interstadial transitions in Greenland ice core records during glacial intervals, for comparing the variability in different proxy archives, and for model-data comparisons in general. In this study we develop a statistical framework to quantify and propagate dating uncertainties in layer-counted proxy archives using the example of the Greenland Ice Core Chronology 2005 (GICC05). We express the number of layers per depth interval as the sum of a structured component that represents both underlying physical processes and biases in layer counting, described by a regression model, and a noise component that represents the fluctuations of the underlying physical processes, as well as unbiased counting errors. The age-depth relationship of the joint dating uncertainties can then be described by a multivariate Gaussian process from which realizations of the chronology can be sampled. We show how the effect of an unknown counting bias can be incorporated in our framework and present refined estimates of the occurrence times of Dansgaard-Oeschger events evidenced in Greenland ice cores together with a complete uncertainty quantification of these timings.

Depth to water table correction for initial carbon-14 activities in groundwater mean residence time estimation

Carbon-14 (14C) is routinely used to determine mean residence times (MRTs) of groundwater. 14C-based MRT calculations typically assume that the unsaturated zone is in equilibrium with the atmosphere, controlling the input 14C activity. However, multiple studies have shown that unsaturated zone 14C activities are lower than atmospheric values. Despite the availability of unsaturated zone 14C data, no attempt has been made to generalise initial 14C activities with depth to the water table. We utilise measurements of unsaturated zone 14C activities from 13 studies to produce a 14C–depth relationship to estimate initial 14C activities. The technique only requires the depth to the water table at the time of sampling or an estimate of depth to water in the recharge zone to determine the input 14C activity, making it straightforward to apply. Applying this new relationship to two Australian datasets (113 14C measurements in groundwater) shows that MRT estimates were up to 9250 years younger when the 14C–depth correction was applied relative to conventional MRTs. These findings may have important implications for groundwater samples that suggest the mixing of young and old waters and the determination of the relative proportions of young and waters, whereby the estimated fraction of older water may be much younger than previously assumed. Owing to the simplicity of the application of the technique, this approach can be easily incorporated into existing correction schemes to assess the sensitivity of unsaturated zone 14C to MRTs derived from 14C data.

Continuous color model as a tool to improve speleothem age model development

Because they can archive a variety of geochemical proxies and be precisely and accurately dated with the U-Th decay series chronometer, stalagmites are widely used for paleoclimate reconstructions. However, limitations in the use of this chronometer arise because U-Th dating is analytically time consuming, expensive, and requires a relatively large sample size. These limitations restrict the number of absolute dates usually obtained, which can result in significant uncertainties in the age model and inhibit the ability to archive high resolution records of environmental variability, particularly in those stalagmites where there are variations in growth rate not constrained by U-Th dates. Here, we explore the relationship between stalagmite color and growth rate. Consequently, we evaluate the use of a simple, practically non-destructive approach to model the age-depth relationship of stalagmites using the sample color to provide a continuous record of growth rate. The method was developed by comparing high-resolution color images with pre-determined U-Th dates along the growth axes of seven stalagmites. The obtained results suggest that prior to dating, a color-derived, continuous growth rate model may be used to identify important changes in growth rate which may aid in the determination of the most efficacious locations for U-Th dating. Further, continuous color-derived interpolations between U-Th derived dates may be superior to traditional linear interpolation methods. Such an approach has the potential to greatly improve a researcher’s ability to efficiently choose sampling locations for more precise, albeit laborious and costly, U-Th dating.

Research on Technology and Application of Buried Faults Identification in Urban Underground Space

With theurbanization rate's rising and three-dimensional expansion and development of urban, the identification of underground buried faults has become the key factor of earthquake risk in urban underground space and surface area. As a typical method of detecting blind faults in underground space, shallow seismic prospecting technology plays an important role in judging and avoiding potential risks such as underground faults in the process of urban expansion and site selection. In this paper, shallow seismic prospecting technology is adopted, and optimized processing technologies such as parameter test, tomographic correction, pre-stack denoising, fidelity and consistency processing, correction iteration, migration imaging, and time-depth relationship deduction are adopted. Underground faults are identified and interpreted in the studied urban area, and fault risk assessment is carried out based on fault characteristics, scale, distribution and overlying strata, thus providing suggestions for regional pattern and construction of urban planning.

Depth to water table correction for initial carbon-14 activities in groundwater mean residence time estimation

Carbon-14 (14C) is routinely used to determine mean residence times (MRTs) of groundwater. 14C-based MRT calculations typically assume that the unsaturated zone is in equilibrium with the atmosphere, controlling the input 14C activity. However, multiple studies have shown that unsaturated zone 14C activities are lower than atmospheric values. Despite the availability of unsaturated zone 14C data, no attempt has been made to generalise initial 14C activities with depth to the water table. We utilise measurements of unsaturated zone 14C activities from 13 studies to produce a 14C-depth relationship to estimate initial 14C activities. The technique only requires the depth to the water table at the time of sampling, or an estimate of depth to water in the recharge zone to determine the input 14C activity, making it straightforward to apply. Applying this new relationship to two Australian datasets (113 14C measurements in groundwater) shows that MRT estimates were up to 9250 years younger when the 14C-depth correction was applied relative to conventional MRTs. These findings may have important implications for groundwater samples that suggest the mixing of young and old waters and the determination of the relative proportions of young and waters, whereby the estimated fraction of older water may be much younger than previously assumed. Owing to the simplicity of the application of the technique, this approach can be easily incorporated into existing correction schemes to assess the sensitivity of 14Cuz to MRTs derived from 14C data.

An improved representation of ephemeral pool hydrology in a semi-distributed hydrologic model

<p>Ephemeral pools are seasonally flooded geographically isolated wetlands with distinct hydrology i.e., they are filled in winter and spring with inflow from snowmelt, and precipitation and dry out during summer. Ephemeral pools offer a variety of biodiversity benefits notably providing breeding habitat for several amphibian and invertebrate species. The quality of their ecosystem services is mainly controlled by their hydroperiod which is regulated by hydrology i.e., inflow /outflow of the pools. The classic water budget modeling approach with a simplified representation of the flow exchange between the pool and surface-subsurface zones may not adequately reveal their sensitivity to anthropogenic interventions and climatic changes. On the other hand, the generic volume-area-depth relationship of isolated wetlands in deterministic hydrologic models may not adequately reveal their dynamic water level fluctuations. The objective of this study, in the first place, is to improve the representation of ephemeral pools in the semi-distributed SWAT hydrological model, notably in the pothole module which is used for modeling isolated wetlands. The developed model will then be used to analyze the impact of land use and climate changes on dynamics of hydroperiods of ephemeral pools of the Saumon River watershed (68 km<sup>2</sup>) in the Canadian Shield of the Outaouais region (Quebec, Canada). A detailed bathymetry survey along with a long series (one to five years) of daily water level measurements available at ten pools allowed to replace the simplified linear volume-area relationship with the measured rating curve for the ephemeral pools in this region. The calibration process of the revised model is performed using the standard SWAT calibration code (SWAT-CUP) coupled to a Particle Swarm Optimization (PSO) algorithm adjusting evaporation and seepage coefficients of the revised module for all isolated wetlands of the region. This double calibration ensures representation of both the watershed hydrology (10 years of river flow rates) and the water level fluctuations in the pools. The simulation results show that the revised SWAT version can adequately reproduce the dynamic water level behavior of the monitored pools as well as streamflow discharges. The model is currently used with scenarios of human and climatic disturbances to understand their impact on the filling-drying cycle of ephemeral pools and on the integrated hydrologic system at the watershed scale.</p>

Seismicity Detection in Skopje Region Using Tomographic Methods and 3-D Modelling

<p>A novel geotomography technique has been applied at the epicentral area around capitol of Macedonia - Skopje, using selected earthquakes that occurred over a period of 57 years and were recorded on temporary and permanent seismograph stations. This study will test the tomography method for the first time in investigation of the crustal shape and structures in our tectonic environment using specially designed datasets covering 1964-1967 and 2016-2020 periods.</p><p>In the initial phase, the analysis will show the potential of the geotomography application in revealing detailed velocity perturbation in the lithosphere. Then, the events are relocated in the 3-D models and new cross-sections of the crust produced by a simultaneous approach. The images can help in constraining the velocity vs depth relationship and thus can contribute towards redefinition of the earthquake zones. The results are discussed in terms of general stress and seismic regime and their temporal changes.</p><p>Better understanding of the seismicity and tectonics processes in the Skopje region will lead to an overall improvement of the earthquake hazard assessment at local and national level, as well as further integration in research programs with other geophysical methods.</p><p> </p>

Chronological Assessment of the Balta Alba Kurgan Loess-Paleosol Section (Romania) – A Comparative Study on Different Dating Methods for a Robust and Precise Age Model

Loess-paleosol sequences (LPSs) are important terrestrial archives of paleoenvironmental and paleoclimatic information. One of the main obstacles for the investigation and interpretation of these archives is the uncertainty of their age-depth relationship. In this study, four different dating techniques were applied to the Late Pleistocene to Holocene LPS Balta Alba Kurgan (Romania) in order to achieve a robust chronology. Luminescence dating includes analysis of different grain-size fractions of both quartz and potassium feldspar and the best results are obtained using fine-grained quartz blue‐stimulated and polymineral post-infrared infrared-stimulated luminescence measurements. Radiocarbon (14C) dating is based on the analysis of bulk organic carbon (OC) and compound-specific radiocarbon analysis (CSRA). Bulk OC and leaf wax-derived n-alkane 14C ages provide reliable age constraints for the past c. 25–27 kyr. CSRA reveals post-depositional incorporation of roots and microbial OC into the LPS limiting the applicability of 14C dating in older parts of the sequence. Magnetic stratigraphy data reveal good correlation of magnetic susceptibility and the relative paleointensity of the Earth’s magnetic field with one another as well as reference records and regional data. In contrast, the application of paleomagnetic secular variation stratigraphy is limited by a lack of regional reference data. The identification of the Campanian Ignimbrite/Y-5 tephra layer in the outcrop provides an independent time marker against which results from the other dating methods have been tested. The most accurate age constraints from each method are used for two Bayesian age-depth modeling approaches. The systematic comparison of the individual results exemplifies the advantages and disadvantages of the respective methods. Taken as a whole, the two age-depth models agree very well, our study also demonstrates that the multi-method approach can improve the accuracy and precision of dating loess sequences.

Predictive Factors to Assess the Difficulty to Extract Retained Lower Third Molars

Background: Third molar eruption occurs in a very limited space. Several difficulty scales have been used to determine the complexity when extracting retained molars, which are key for surgical planning and prediction. A scale including indicators such as quality of mucosa and bone, as well as shape and number of roots is introduced. Purpose: Evaluate the difficulty in extracting retained lower third molars, using the scale proposed by Romero-Ruiz, and thus estimate the presence of intraoperative complications and surgical time. Methods: An observational descriptive cross-sectional study was carried out, with a sample of 100 extractions of retained lower third molars in patients between 16 and 40 years of age. The following variables were evaluated: spatial relationship, depth, relationship with mandible ramus/space, integrity of bone and mucosa, roots, dental follicle, and surgical time. The data were summarized in absolute frequency tables and analyzed with Pearson's Chi2 test (p < 0.05). Results: 71 % of third molars were classified as “difficult” on the scale. There were significant differences in terms of surgical time-age (p = 0.002), presence of complications-location of the third molar (p = 0.015), presence of complications-follicle size (p = 0.022), difficulty-sex (p = 0.011 ), difficulty-age (p = 0.068). Conclusions: This scale can be used to plan extraction treatments for retained lower third molars to reduce surgical times and anticipate complications.

Fabrication of Ordered Micro/Nanostructures Using Probe-Based Force-Controlled Micromachining System

This paper presents a probe-based force-controlled nanoindentation method to fabricate ordered micro/nanostructures. Both the experimental and finite element simulation approaches are employed to investigate the influence of the interval between the adjacent indentations and the rotation angle of the probe on the formed micro/nanostructures. The non-contacting part between indenter and the sample material and the height of the material pile-up are two competing factors to determine the depth relationship between the adjacent indentations. For the one array indentations, nanostructures with good depth consistency and periodicity can be formed after the depth of the indentation becoming stable, and the variation of the rotation angle results in the large difference between the morphology of the formed nanostructures at the bottom of the one array indentation. In addition, for the indentation arrays, the nanostructures with good consistency and periodicity of the shape and depth can be generated with the spacing greater than 1 μm. Finally, Raman tests are also carried out based on the obtained ordered micro/nanostructures with Rhodamine probe molecule. The indentations arrays with a smaller spacing lead to better the enhancement effect of the substrate, which has the potential applications in the fields of biological or chemical molecular detection.