Comparison of methods for a 3-D density inversion from airborne gravity gradiometry

In this study, we apply Tikhonov’s regularization algorithm for a 3-D density inversion from the gravity-gradiometry data. To reduce the non-uniqueness of the inverse solution (carried out without additional information from geological evidence), we implement the depth-weighting empirical function. However, the application of an empirical function in the inversion equation brings the bias problem of the regularization factor when a traditional Tikhonov’s algorithm is applied. To solve the bias problem of regularization factor selection, we present a standardized solution that comprises two parts for solving a 3-D constrained inversion equation, specifically the linear matrix transformation and Tikhonov’s regularization algorithm. Since traditional regularization techniques become numerically inefficient when dealing with large number of data, we further apply methods which include the Simultaneous Iterative Reconstruction Technique (SIRT) and the wavelet compression combined with Least Squares QR-decomposition (LSQR). In our simulation study, we demonstrate that SIRT as well as the wavelet compression plus LSQR algorithm improve the computation efficiency, while provide results which closely agree with that obtained from applying Tikhonov’s regularization. In particular, the algorithm of wavelet compression plus LSQR shows the best computing efficiency, because it combines the advantages of coefficients compression of big matrix and fast solution of sparse matrix. Similar findings are confirmed from the vertical gravity gradient data inversion for detecting potential deposits at the Kauring (near Perth, Western Australia) testing site.     

Evaluating the reliability of time series analysis to estimate variable riparian travel times by numerical groundwater modelling

The transition zones between rivers and adjacent riparian aquifers are locations of high biogeochemical activities that contribute to a removal of potentially hazardous substances in the aquatic system. The potential of the removal processes depends highly on subsurface water travel times, which can be determined by using the propagation of electrical conductivity (EC) signal from the river into the riparian aquifer. Although this method has been applied and verified in many studies, we observe possible limitations for the usage of EC fluctuation analysis. Our findings are based on EC time series analyses during storm events and artificial hydropeaks induced by watermill operations. Travel times derived by cross‐correlation analysis were compared with travel times calculated based on backward particle tracking of a calibrated transient numerical groundwater flow model. The cross‐correlation method produced only reasonable travel times for the artificial hydropeaks. In contrast, cross‐correlation analysis of the EC data during natural storm events resulted in implausibly negative or unrealistically low travel times for the bulk of the data sets. We conclude that the reason for this behaviour is, first, the low EC contrast between river and groundwater in connection with a strong damping of the infiltrating river EC signal into the subsurface during storm events. Second, the existence of old and less‐mineralized riparian water between the river and the monitoring well resulted in bank‐storage‐driven EC breakthrough curves with earlier arrival times and the subsequent estimation of implausible riparian travel times.     

Comparative application of four methods of groundwater vulnerability mapping in a Slovene karst catchment

Four methods of groundwater vulnerability mapping have been applied in a Slovene karst catchment and validated by tracer tests. The test site is characterised by high water table fluctuations, manifested in intermittent lakes and variable drainage divides. A first multi-tracer test (two injections) allowed subdivision of the catchment into zones of different degrees of contribution (‘inner zone’ and ‘outer zone’). For vulnerability mapping, only methods that consider the specific nature of karst aquifers such as heterogeneity and duality of infiltration processes, were selected: EPIK, PI, the ‘Simplified Method’ and the ‘Slovene Approach’. For validation, a second multi-tracer test (four injections) was carried out. The time of first detection and the normalised recovery were used as validation criteria. The results suggest that EPIK and the Simplified Method sometimes overestimate vulnerability, while PI and the Slovene Approach tend to deliver more realistic results, at least during low-flow conditions. The Slovene Approach gives the clearest guidance on how to deal with hydrologic variability, for example by assigning lower vulnerability to occasionally active sinking surface waters than to permanent ones. As a conclusion, commonly accepted validation techniques are needed and should be applied by default to evaluate different vulnerability mapping methods and the resulting maps.     

Early Cretaceous (late Berriasian to early Aptian) palaeoceanographic change along the northwestern Tethyan margin (Vocontian Trough,southeastern France): δ13C, δ18O and Sr-isotope belemnite and whole-rock records

Stable carbon, oxygen, and strontium isotope records were obtained from uppermost Hauterivian to lowermost Aptian belemnite rostra, which were collected in well-dated sections from the Vocontian Trough (southeastern France). This data set complements previously published belemnite-isotope records from the uppermost Berriasian-Hauterivian interval from the same basin. The belemnite carbon and oxygen isotope record is compared to the carbonate bulk-rock isotope record from the same sections, and from additional Italian sections. With regards to their long-term trends, both belemnite and whole-rock δ¹⁸O records are well correlated, except for the uppermost Hauterivian-lower Barremian interval, within which they deviate. This discrepancy is interpreted to be linked to the latest Hauterivian Faraoni oceanic anoxic event and its early Barremian aftermath. The Faraoni level is characterized by enhanced sea-water stratification, probably induced by the onset of a warmer and more humid climate along the northern Tethyan margin. The early Barremian was characterized by stronger vertical sea-water mixing reflected by a decrease in density contrast between sea-surface and deeper waters. The belemnite oxygen isotope record shows a more stable evolution with smaller fluctuations than its bulk-rock counterpart, which indicates that deeper water masses were not as much subjected to density fluctuations as sea-surface water. The comparison of belemnite and bulk-rock carbon isotope records allows observing the impact of regional influence exerted by platform carbonate ooze shedding on the carbon cycle. Discrepancies in the two records are observed during time of photozoan carbonate platform growth. The strontium isotopic record shows a gradual increase from the uppermost Berriasian to the uppermost lower Barremian followed by a rapid decrease until the uppermost Barremian and a renewed small increase within the lowermost Aptian. The major inflection point in the uppermost lower Barremian appears to predate the onset in the formation of the Ontong-Java volcanic plateau.     

Environmental variability in Lake Naivasha,Kenya, over the last two centuries

Lake Naivasha, Kenya, is one of a number of freshwater lakes in the East African Rift System. Since the beginning of the twentieth century, it has experienced greater anthropogenic influence as a result of increasingly intensive farming of coffee, tea, flowers, and other horticultural crops within its catchment. The water-level history of Lake Naivasha over the past 200 years was derived from a combination of instrumental records and sediment data. In this study, we analysed diatoms in a lake sediment core to infer past lacustrine conductivity and total phosphorus concentrations. We also measured total nitrogen and carbon concentrations in the sediments. Core chronology was established by ²¹⁰Pb dating and covered a ~186-year history of natural (climatic) and human-induced environmental changes. Three stratigraphic zones in the core were identified using diatom assemblages. There was a change from littoral/epiphytic diatoms such as Gomphonema gracile and Cymbella muelleri, which occurred during a prolonged dry period from ca. 1820 to 1896 AD, through a transition period, to the present planktonic Aulacoseira sp. that favors nutrient-rich waters. This marked change in the diatom assemblage was caused by climate change, and later a strong anthropogenic overprint on the lake system. Increases in sediment accumulation rates since 1928, from 0.01 to 0.08 g cm⁻² year⁻¹ correlate with an increase in diatom-inferred total phosphorus concentrations since the beginning of the twentieth century. The increase in phosphorus accumulation suggests increasing eutrophication of freshwater Lake Naivasha. This study identified two major periods in the lake’s history: (1) the period from 1820 to 1950 AD, during which the lake was affected mainly by natural climate variations, and (2) the period since 1950, during which the effects of anthropogenic activity overprinted those of natural climate variation.     

Assessing Greenland ice mass loss by means of point-mass modeling: a viable methodology

Greenland ice mass loss is one of the most serious phenomena of present-day global climate change. In this context, both the quantification of overall deglaciation rates and its spatial localization are highly significant. We have thoroughly investigated the technique of point-mass modeling in order to derive mass-balance patterns from GRACE (Gravity Recovery And Climate Experiment) gravimetry. The method infers mass variations on the Earth’s surface from gravitational signals at satellite altitude. In order to solve for point-mass changes, we applied least-squares adjustment. Due to downward continuation, numerical stabilization of the inversion process gains particular significance. We stabilized the ill-posed problem by Tikhonov regularization. Our simulation and real data experiments show that point-mass modeling provides both rational deglaciation rates and high-resolution spatial mass variation patterns.     

Contrasting time trends of organic contaminants in Antarctic pelagic and benthic food webs

We demonstrate that pelagic Antarctic seabirds show significant decreases in concentrations of some persistent organic pollutants. Trends in Adélie penguins and Southern fulmars fit in a general pattern revealed by a broad literature review. Downward trends are also visible in pelagic fish, contrasting sharply with steady or increasing concentrations in Antarctic benthic organisms. Transfer of contaminants between Antarctic pelagic and benthic food webs is associated with seasonal sea-ice dynamics which may influence the balance between the final receptors of contaminants under different climatic conditions. This complicates the predictability of future trends of emerging compounds in the Antarctic ecosystem, such as of the brominated compounds that we detected in Antarctic petrels. The discrepancy in trends between pelagic and benthic organisms shows that Antarctic biota are still final receptors of globally released organic contaminants and it remains questionable whether the total environmental burden of contaminants in the Antarctic ecosystem is declining.     

Thermodynamics, structure, dynamics, and freezing of Mg2SiO4 liquid at high pressure

We perform first principles molecular dynamics simulations of Mg₂SiO₄ liquid and crystalline forsterite. On compression by a factor of two, we find that the Grüneisen parameter of the liquid increases linearly from 0.6 to 1.2. Comparison of liquid and forsterite equations of state reveals a temperature-dependent density crossover at pressures of ∼12-17 GPa. Along the melting curve, which we calculate by integration of the Clapeyron equation, the density crossover occurs within the forsterite stability field at P = 13 GPa and T = 2550 K. The melting curve obtained from the root mean-square atomic displacement in forsterite using the Lindemann law fails to match experimental or calculated melting curves. We attribute this failure to the liquid structure that differs significantly from that of forsterite, and which changes markedly upon compression, with increases in the degree of polymerization and coordination. The mean Si coordination increases from 4 in the uncompressed system to 6 upon twofold compression. The self-diffusion coefficients increase with temperature and decrease monotonically with pressure, and are well described by the Arrhenian relation. We compare our equation of state to the available highpressure shock wave data for forsterite and wadsleyite. Our theoretical liquid Hugoniot is consistent with partial melting along the forsterite Hugoniot at pressures 150-170 GPa, and complete melting at 170 GPa. The wadsleyite Hugoniot is likely sub-liquidus at the highest experimental pressure to date (200 GPa).     

Seasonal and short-term controls of riparian oxygen dynamics and the implications for redox processes

Riparian zones are highly-dynamic transition zones between surface water (SW) and groundwater (GW) and function as key biogeochemical-reactors for solutes transitioning between both compartments. Infiltration of SW rich in dissolved oxygen (DO) into the riparian aquifer can supress removal processes of redox sensitive compounds like NO₃⁻, a nutrient harmful for the aquatic ecosystem at high concentrations. Seasonal and short-term variations of temperature and hydrologic conditions can influence biogeochemical reaction rates and thus the prevailing redox conditions in the riparian zone. We combined GW tracer-tests and a 1-year high-frequency dataset of DO with data-driven simulations of DO consumption to assess the effects of seasonal and event-scale variations in temperature and transit-times on the reactive transport of DO. Damköhler numbers for DO consumption (DA_DO) were used to characterize the system in terms of DO turnover potential. Our results suggest that seasonal and short-term variations in temperature are major controls for DO turnover and the resulting concentrations at our field site, while transit-times are of minor importance. Seasonal variations of temperature in GW lead to shifts from transport-limited (DA_DO > 1) to reaction-limited conditions (DA_DO < 1), while short-term events were found to have minor impacts on the state of the system, only resulting in slightly less transport-limited conditions due to decreasing temperature and transit-times. The data-driven analyses show that assuming constant water temperature along a flowpath can lead to an over- or underestimation of reaction rates by a factor of 2–3 due to different infiltrating water temperature at the SW–GW interface, whereas the assumption of constant transit-times results in incorrect estimates of NO₃⁻ removal potential based on DA_DO approach (40%–50% difference).