Fractionation change of hydrogen isotopes in trees due to atmospheric pollutants

Atmospheric pollution effects on hydrogen fractionation in trees are demonstrated for the first time in this study. The δ²H ring-cellulose series obtained for black spruce trees at a site near a SO₂-emitting smelter display short-term fluctuations superimposed on a first order −11‰ shift coincident with the onset of smelter operations. The isotopic depletion in trees exposed to various levels of SO₂ diminish with increasing distance relative to the location of the smelter, and it is not detected at the most distant selected stand, 116 km downwind from the point source. Both the spatial and temporal isotopic trends likely result from the combined effects of leaf transpiration, biochemical processes and water uptake by the root system. The spatial and temporal trends of δ²H values are the reverse of the δ¹³C trends previously obtained for the same tree ring series. These coupled isotopic fractionations underline an important response of trees to pollution stresses likely generated by ambient atmospheric SO₂ concentrations. The degradation of global air quality during the last 150 yr likely generated large scale modifications in the way terrestrial vegetation grows. In this respect, hydrogen dendrogeochemistry combined with other tracers such as C isotope ratios constitutes a new tool to evaluate the past behavior of forest ecosystems in terms of C uptake and acclimation to various types of atmospheric pollution.     

Spatial and temporal rockfall activity in a forest stand in the Swiss Prealps—A dendrogeomorphological case study

Rockfall is a major threat to settlements and transportation routes in large parts of the Alps. While protective forest stands in many locations undoubtedly reduce rockfall risk, little is known about the exact frequency and spatial distribution of rockfall activity in a given place or about how these parameters can be assessed. Therefore, the objective of the present study was to reconstruct rockfall events with dendrogeomorphological methods and to analyse the spatial and temporal rockfall activity in a subalpine forest stand. The study site is located in the transit zone of frequently passing, rather small rockfall fragments (mean diameter of 10 to 20 cm). In all, 33 stem discs from previously felled Picea abies trees found at the foot of Schwarzenberg in Diemtigtal (Swiss Prealps) were sampled, and a total number of 301 rockfall events were dated to between A.D. 1724 and 2002.Results showed that the spatial distribution of rockfall changed slightly with time, and that rockfall activity increased considerably over the last century. In contrast, rockfall magnitude presumably remained on a comparable level. The seasonal occurrence of rockfall showed a clear peak during the dormant season of trees, most probably in early spring. Furthermore, on a 10-year moving average basis, rockfall rates were positively correlated with mean annual as well as summer and winter temperatures. This means that higher temperatures resulted in increased rockfall activity. On the other hand, no correlation with annual or seasonal precipitation totals was revealed. Overall, this study provides an appropriate method for the detailed assessment of spatial and temporal variations in rockfall activity in a given place.     

Nutrients limiting the algal growth potential (AGP) in the Po River plume and an adjacent area, Northwest Adriatic Sea: Enrichment bioassays with the test algaeNitzschia closterium andThalassiosira pseudonana

From April 1993 to March 1994, 135 samples were collected at two sites in the coastal Adriatic Sea, both near (14.5 km) and far (63 km) from the Po River delta. The nutrient(s) limiting algal growth potential (AGP) were estimated by bioassay usingNitzshia closterium (local isolate) andThalassiosira pseudonama (strain 3-H). Estimates were also made by comparing the nutrient molar ratios, ΣN:P and ΣN:Si (whereΣN=NO₃ ^?+NO₂ ^?+NH₄ ⁺+urea), to the Redfield Ratio (16∶1, 1∶1). According to the bioassay results, phosphorus was the sole nutrient limiting AGP in 2% of the samples and was the most limiting nutrient in 69% of the samples; nitrogen was sole limiting nutrient in 18% of the samples. In 11% of samples, nitrogen and phosphorus were equally co-limiting. Omission of phosphorus from spike enrichments allowed, on average, only a 1.6-fold increase in biomass over that in the unenriched controls. Similar omission of nitrogen allowed a 4-fold increase, while silicon, iron, and micronutrients resulted in 14-fold, 18-fold, and >20-fold increases, respectively. In most of the samples, ΣN:P was much greater than 16, indicating a marked phosphorus deficiency, while ΣN:Si values suggested that silicon was the third most limiting nutrient in 35% of samples. In water collected for from the Po delta, the yield ofN. closterium was not limited by any nutrients other than the three major ones: P, N and Si. In these same waters,T. pseudonana was also potentially limited by iron and, to a lesser extent, by vitamins. The role of iron varied. In samples collected near the Po delta, iron acted as the third most limiting nutrient forN. closterium in June and September; it appeared 29 times out of 78 on the list of potentially limiting nutrients forT. pseudonana, including 5 times as the most limiting. Altogether, comparison with published results suggests that the roles of iron and silicon in AGP limitation have increased during the past three decades, and could become even more important if eutrophication in the Adriatic Sea continues to increase.     

Stress, failure and fluid flow deduced from earthquakes accompanying eruptions at Piton de la Fournaise volcano

In the present episode of eruptive activity, evidence from seismicity for sustained magma inflow from depth into the edifice of Piton de la Fournaise is lacking. Pre-eruptive main deformation and shallow seismicity help to identify very small volumes of magma that are in motion beneath the rim of the Dolomieu summit crater, and oriented along the azimuth of the future vents. Small magma pockets may reside in the cone above sea level, or may be expelled repeatedly, due to crystallisation in a small, low-velocity, aseismic region below sea level under the high-velocity central plug of the cone in which pre-eruptive earthquake swarms are located. In cross-section the hypocentres define two steep sheets diverging from the aseismic zone at sea level towards 1.5 km above sea level (or 1 km beneath the 2632 m high cone). However, failure induced by increased pressure in the suggested chamber does not account for the observed focal mechanisms.The occurrence and timing of magma transport are attested by eruption, and seismic activity may be related to magma transport. Focal mechanisms document strike-slip, not normal faulting or tensile failure. Vertical propagation of the edge of a feeder dike may enhance strike-slip motion above the edge, in a region where effective normal stress is decreased by thermally induced groundwater flow. The strike-slip mechanisms could also be caused by a tensile-shear widening of the horizontal section of vertical conduits.Fournaise strike-slip earthquakes occur in two orientations, with P axes orthogonal between them, within a single pre-eruptive event. Earthquakes are distributed in the same volume but mechanisms switch from one to another type systematically with time, indicating a reversal of stress conditions. The orientations of P axes with respect to the epicentral trend suggest that in the later parts of events leading to eruptions, a compression of the medium occurs after a dilation in the first part. The activated zone might respond successively to the arrival and the departure of the magma on its way from the reservoir at depth to the vent, radial to the cone.     

Determination of the earth rotation parameters from optical astrometry observations, 1962.0–1982.0

The optical astrometric data of the years 1962–1982 have been reduced once again at the Bureau International de l’Heure (BIH) in order to redetermine the Earth Rotation Parameters (ERP). This new reduction is based on serie largely revised by the stations since their use in the operational work of theBIH, and on some series which were not available previously. A total of 113 stations is considered, totaling nearly 500,000 measurements of time or latitude. TheERP are determined at five-day intervals. A new approach is developed: the catalog and local errors are analysed and corrected as group unknowns, which values are adjusted together with the main unknowns. The results obtained in the new reduction are compared to other series obtained by astrometry and space geodesy.     

A comparative analysis of Vondrak and Gaussian smoothing techniques

Summary Smoothing time series of measurements is usually employed to remove high frequency noise when it is necessary to interpolate them for dates where no measurements are available, and also in an attempt to free the series from their short-term instability in order to provide an improved representation of the measured quantity. Two methods are currently used in connection with the Earth's rotation studies: Vondrak's algorithm and a moving average with a Gaussian weighting function [Gaussian smoothing). This study describes their behaviour in terms of frequency filtering and of the response in the time domain. Examples of their application to the Earth Rotation Parameters are given.     

Multi-technique monitoring of ocean tide loading in northern France

We have organised afield study of ocean tide loading in the northwestern part of France, where tidal amplitudes are known to be among the highest in the world. GPS and gravimetric techniques have already proved their capability to measure such weak and high-frequency signals. In this study, these classical observations are complemented with less usual techniques, such as tiltmeter and Satellite Laser Ranging (SLR) measurements. We present here the preliminary results for a common period of observations spanning from 12–19 May 2004. Additional measurements from the French Transportable Laser Ranging Station (FTLRS) were available during September and October 2004. Observation residuals are computed as the difference between the observed and the predicted time signals. We obtain small RMS residuals for GPS measurements (2.5/3.1/4.5 mm for the eastward, northward and upward components), for absolute and relative gravimetry (9 nm/s² and 13 nm/s²) and for tiltmeters (0.05 μrad for EW component). We also fit the amplitude of the main M2 tidal constituent to FTLRS observations and we find a value of 3.731 cm, which is comparable to the theoretical value.     

Organotin accumulation in an estuarine food chain: comparing field measurements with model estimations

The bioaccumulation model OMEGA (optimal modelling for ecotoxicological applications) is used to explore accumulation of organotins in the Western Scheldt food chain, consisting of herbi-detritivores, primary and secondary carnivorous fish and a piscivorous bird. Organotins studied are tributyltin (TBT) and triphenyltin (TPT) and the respective di- and mono-organotin metabolites. Empirical elimination rate constants are compared to model predictions for organic substances and metals. It is found that field bioaccumulation ratios are higher than predicted based on elimination kinetics relevant for organic compounds. The results indicate that uptake of organotins mainly occurs via hydrophobic mechanisms, whereas elimination may occur via metal-like kinetics. This results in very low elimination rates, which are comparable to model predictions for metals.     

How does the bed surface impact low-magnitude bedload transport rates over gravel-bed rivers?

Bedload transport is a complex phenomenon that is not well understood, especially for poorly sorted sediment and low transport rates, which is what is typically found in alpine gravel-bed rivers. In this paper, the interaction between bedload rate, bed stability and flow is investigated using flume experiments. Significant differences in bedload rates were observed for experiments conducted on beds formed with the same gravel material but presenting diverse arrangements and bedforms. Tests were performed under regimes of low transport rate, which are mainly controlled by gravel-bed roughness. Different scales of roughness were identified using the statistical characteristics of detailed bed elevation measurements: grain, structure and large bedform scales. The role played by these different roughness scales in bedload dynamics was examined. For quasi-flat beds, bed stability was quantified using a combination of bed surface criteria describing grain and structure scales. It was found that bed stability affects the bedload rate directly and not only through its influence on the flow or on the incipient motion. For beds with large bedforms, the analysis of bedload dynamics also showed the importance of accounting for effective bed shear stress distributions. An empirical bedload model for low transport regimes was suggested. Compared with previous formulae developed for alpine rivers, this model accounts for bed stability and distribution of effective bed shear stress. It significantly improves the understanding of gravel dynamics over complex beds such as arranged beds or those with large bedforms. However, further tests are needed to use the model outside the range of conditions of this study. © 2019 John Wiley & Sons, Ltd.     

Comparison of ground-based and UAV a-UHF artificial tracer mobility monitoring methods on a braided river

Radio frequency identification (RFID) technologies, which allow wireless detection of individual buried or immersed tracers, represent a step forward in sediment tracking, especially passive integrated transponders (PIT tags) that have been widely used. Despite their widespread adoption in the scientific community, they typically have low efficiency when deployed in river systems with active bedload transport or deep wet channels, attributed to their technical specifications. A recent evaluation of active ultra-high frequency transponders (a-UHF tags) assessed their larger detection range and provided a methodology for their geopositioning. In this study, we test five different survey methods (one including an unmanned aerial vehicle [UAV]) in a sediment tracking study, and compare them in terms of recovery rate, field effort, geopositioning error, and efficiency. We then tested the method on a larger reach following a Q₅ flood and performed cross-comparisons between active and passive RFIDs. The results confirmed that the a-UHF RFID technology allowed rapid (1.5 h ha⁻¹) survey of a large area (<34 ha) of emerged bars and shallow water channels with recovery of a high percentage of tracers (72%) that had travelled large distances (mean ≈ 1000 m; max ≈ 3400 m). Moreover, the tracers were identified with low geopositioning error (mean ≈ 7.1 m, ≤1% of their travel distance). We also showed that a UAV-based survey was fast (0.38 h ha⁻¹), efficient (recovery rate = 84%), and low error (mean ≈ 4.2 m). Thus, a-UHF RFID technology permits the development of a variety of survey methods, depending on the study objectives and the human and financial resources available. This allows field efforts to be optimized by determining an appropriate balance between the high equipment cost of a-UHF tracers and the resulting reduced survey costs. © 2019 John Wiley & Sons, Ltd.