Seasonal and diel patterns of total gaseous mercury concentration in the atmosphere of the Central Valley of Costa Rica

Monitoring of Hg in the atmosphere near volcanoes is limited with no previous data for Costa Rica. Seasonal and daily patterns of total gaseous mercury (TGM) were observed at the main sampling location at the Universidad Nacional, Heredia, Costa Rica. The area (lat. 10.000230 long. −84.109499) is located in the Central Valley of Costa Rica and is 27 km SE of the Poas volcano (lat. 10.199486 long. −84.231388). Measurements were made from May 2008 to May 2009 at this location with some additional values obtained at other sites near the Poas volcano including San Luis and Grecia as well as near, Turrialba and Irazu volcanoes. Total gaseous Hg was determined in samples collected at a height of 2 m using the Tekran 2537A (Tekran Inc.) gas-phase Hg vapor analyzer. Meteorological data (temperature, relative humidity, wind speed, wind direction, radiation and precipitation) were obtained from the airport weather station located at Alajuela. Monthly precipitation is typically 85 mm during the dry season (December to April) with winds from the west. The wet season begins in late April and continues to December with monthly rainfall of 328 mm and winds from the NE. The annual mean temperature is 20 °C. With the onset of the wet season TGM increased from typical values near 10 to 905 ng m⁻³. Measurements made within 5 km of the Poas volcano were higher than at Heredia at that time. Diel values measured at the university site increased until midday along with temperature and radiation. Relative humidity showed a reciprocal pattern. It was found that high values of TGM were not related to wind velocity or direction. The strong diel pattern increased with sunrise, peaked at midday and was lowest during the night time. It would seem that Hg⁰ from the volcano is oxidized and is deposited to the soils during the dry season when winds are blowing from the volcano. With the onset of heavy rains in April, Hg in the soil is reduced and re-volatilized resulting in the high levels in the atmosphere. Values at other volcano sites were provided. The role of atmospheric pollutants such as H₂O₂ and O₃ should be included in future studies as they may result in oxidation of reduced Hg. The instability in the air masses may also be a factor and local pollution sources may result in high levels that is emitted from the volcanoes of Hg being circulated to ground level as radiation intensity increases.     

Ductile crust exhumation and extensional detachments in the central Aegean (Cyclades and Evvia Islands).

Abstract

The Aegean continental domain is known to be the site of widespread “back-arc” extension since at least 13 Ma, on the basis of seismotectonic, stratigraphic and fault analysis studies. This extension is documented to overprint structures related to the Mesozoic-Cenozoic Hellenic orogeny. Features attributed to early thrusting include the overall ductile deformation within two broad belts that have suffered HP/LT metamorphism across the Aegean. This study presents a structural analysis of the central Aegean area (Cyclades and Evvia Islands), examining in particular the relationship between ductile and brittle deformation, both in the field and on a regional scale. Extension appears to be responsible for most of the ductile deformation within HP rock units that have experienced penetrative greenschist facies and higher grade metamorphic over-printing. On each studied island, progressive extensional deformation has occurred through the development of a major normal-sense detachment zone down to depths of about 18-25 km. Large displacement along the detachment zone accounts for rapid cooling and exhumation of ductile lower crust to form a local metamorphic dome or core complex. Structural and stratigraphic features support a progressive migration of normal faulting away from the dome axis, and a rotation of previously active faults toward low dips, as in kinematic models recently suggested for the development of extensional detachment systems. All the studied domes, except that seen on los Island, show a dominant top-to-the north or north-east sense of shear, while on the southern flank of many of them, an opposite sense of shear is observed, displaying the same progressive evolution from ductile to brittle rock behaviour. This opposite sense of shear is thought not to result from shearing along a major conjugate detachment zone, as in some recent models, but from the accommodation in the ductile crust of upward bending of the brittle upper crust in the footwall of the north-dipping detachment. Available radiometric and stratigraphie data indicate an early minimum age (22-19 Ma) for the onset of extension. The relationship between early metamorphic domes and shallow-dipping detachments, on one hand, and Messinian-Quaternary steep normal faults and grabens, on the other hand, is best explained with the progressive and continuous development of new normal faults away from the domes axes, rather than with a two-stage evolutionary model (core-complex stage, then Basin-and-Range stage) of the type invoked for the North American Cordillera.     

Tectonometamorphic history of the Gruf complex (Central Alps): exhumation of a granulite–migmatite complex with the Bergell pluton

We describe field occurrences of sapphirine-bearing granulites, charnockites and migmatites in the Gruf complex, Central Alps and present a new geological map and a structural analysis of the entire Gruf complex for the first time. We have carried out an accurate analysis of the relationships between granulite facies metamorphism, migmatisation and deformation within the complex, in relation to the intrusion of the Bergell pluton. Granulites and charnockites display fabrics different from those defined by the regional foliation and lineation, which are, typically for migmatites, disordered on the mesoscale. On a regional scale, strike variations are also related to the structural complexity of migmatites within which no major antiform could be identified. Irregular interfingering of sub-parallel leucosome veins and back-veining along the contact between the Gruf migmatites and the Bergell tonalite are evidence for contemporaneous emplacement and crystallisation at about 740 °C and 6.5–7.5 kbar in Oligocene times (ca 30 Ma). Metamorphic conditions in the charnockites and granulites (>920 °C for 8.5–9.5 kbar) largely exceed these regional metamorphic conditions and are dated at 282–260 Ma. We propose that the ascending Bergell pluton entrained the polymetamorphic, granulitic lower crust enclosed within the peripheral migmatitic Gruf complex.     

The position of the Rupelian/Chattian boundary in the southern Upper Rhine Graben based on new records of microfossils

This study analyses the regressive phase of the marine Froidefontaine Subgroup and the subsequent fluvio-lacustrine Niederroedern formation in the southern Upper Rhine Graben during the Late Rupelian and the position of the Rupelian/Chattian boundary. The study is based on the sedimentary record and several microfossil groups from two boreholes, with a focus on new records of fish otoliths, Bolboforma and Charophyta. The biostratigraphic evaluation of these groups provides evidence for a position of the Rupelian/Chattian boundary within the upper Niederroedern Formation. This is contrary to the results from a previous sedimentological approach, which places the boundary at the base of the Niederroedern Formation. The fish fauna indicates a biogeographic relation to southern France, the Molasse Basin of Switzerland and southern Germany.     

Simulation Study of a Robust Algorithm for Soil Moisture and Surface Roughness Estimation Using L‐Band Radar Backscatter

Abstract

Various inversion algorithms have been developed to obtain estimates of soil moisture and surface roughness parameters from multifrequency, multiangle, and multipolarization radar reflectances. Since the penetration depth for radar signals increases with wavelength, an inversion algorithm using widely separated frequencies does not yield comparable probing depths. Furthermore, existing algorithms assume a linear relationship between the radar backscatter coefficient (in dB) and soil parameters, such as the volumetric soil moisture, soil surface roughness and surface slope. This assumption is valid only over a narrow range of soil parameters, thereby restricting its operational use under realistic conditions. Our research specifically explored the use of inversion algorithms based on L‐Band radar reflectances at 1 GHz and 2 GHz frequencies in order to retain relatively consistent probing depths. In order to extend the range of applicability, a non‐linear exponential‐type relationship was developed between radar reflectance at a specified frequency, polarization and incidence angle combination, and soil parameters of interest, viz., soil moisture, surface roughness, and surface slope. An over‐constrained inversion algorithm using a six‐parameter combination was found to yield relatively accurate estimates of soil parameters over a wide range of soil conditions even in the presence of system error.     

Solar Stereoscopy with STEREO/EUVI A and B Spacecraft from Small (6°) to Large (170°) Spacecraft Separation Angles

We performed for the first time stereoscopic triangulation of coronal loops in active regions over the entire range of spacecraft separation angles (?? _sep??6^°,43^°,89^°,127^°,and 170^°). The accuracy of stereoscopic correlation depends mostly on the viewing angle with respect to the solar surface for each spacecraft, which affects the stereoscopic correspondence identification of loops in image pairs. From a simple theoretical model we predict an optimum range of ?? _sep??22^°??C?125^°, which is also experimentally confirmed. The best accuracy is generally obtained when an active region passes the central meridian (viewed from Earth), which yields a symmetric view for both STEREO spacecraft and causes minimum horizontal foreshortening. For the extended angular range of ?? _sep??6^°??C?127^° we find a mean 3D misalignment angle of ?? _PF??21^°??C?39^° of stereoscopically triangulated loops with magnetic potential-field models, and ?? _FFF??15^°??C?21^° for a force-free field model, which is partly caused by stereoscopic uncertainties ?? _SE??9^°. We predict optimum conditions for solar stereoscopy during the time intervals of 2012??C?2014, 2016??C?2017, and 2021??C?2023.     

The Multiscale TROPIcal CatchmentS critical zone observatory M-TROPICS dataset I: The Nyong River Basin,Cameroon

Simultaneously acquiring time series of climate, hydrology and hydrochemical data over decades on river systems is pivotal to understand the complex interactions involving rock, soil water, air and biota in the Critical Zone, to build integrated modelling and to propose predictive scenarios. Among the Critical Zone Observatories (CZOs) implemented in the past 25 years, only a few are located in the humid Tropics despite the importance of these regions in terms of population density, fast-changing land use, biodiversity hotspots, biomass stock on continents, size of river systems, etc. Since 1994, weathering and erosion processes and fluxes have been investigated at both local (experimental watershed) and regional scales in the Nyong River Basin (Cameroon) which belongs to the Critical Zone Observatories network named Multiscale TROPIcal CatchmentS (M-TROPICS). The data shared by M-TROPICS in Cameroon are: (1) rainfall; (2) air temperature, air relative humidity, wind speed and direction, and global radiation; (3) stream and river water level; (4) pH, electrical conductivity, water temperature and suspended particulate matter (SPM) concentration; (5) major ion, alkalinity and dissolved organic carbon (DOC) concentrations. The dataset already contributed to describe the water partitioning in these tropical humid watersheds, to better understand the factors controlling chemical weathering and physical erosion in tropical ecosystems, particularly the role of organic matter. The dataset also contributed to calculate elemental weathering fluxes and saprolite production rate and to propose denudation rates on tropical cratonic landscapes. Hydrological modelling allowed quantification of the geographical water sources contributing to streamflow. DOC data were used to determine greenhouse-gas emissions and carbon budgets from African inland waters. However, long-term solute concentrations at the outlet of a small tributary of the Nyong River exhibit non-stationary behaviour over the last 26 years. The processes governing those fluctuations are not yet fully understood and might be related to changes in the hydrological regime, land-cover and land-use. The latter highlights the need for longer time-series and continued support for CZOs particularly in the humid tropics.     

Moisture transport and seasonal variations in the stable isotopic composition of rainfall in Central American and Andean Páramo during El Niño conditions (2015–2016)

High‐elevation tropical grassland systems, called Páramo, provide essential ecosystem services such as water storage and supply for surrounding and lowland areas. Páramo systems are threatened by climate and land use changes. Rainfall generation processes and moisture transport pathways influencing precipitation in the Páramo are poorly understood but needed to estimate the impact of these changes, particularly during El Niño conditions, which largely affect hydrometeorological conditions in tropical regions. To fill this knowledge gap, we present a stable isotope analysis of rainfall samples collected on a daily to weekly basis between January 2015 and May 2016 during the strongest El Niño event recorded in history (2014–2016) in two Páramo regions of Central America (Chirripó, Costa Rica) and the northern Andes (Cajas, south Ecuador). Isotopic compositions were used to identify how rainfall generation processes (convective and orographic) change seasonally at each study site. Hybrid Single Particle Lagrangian Integrated Trajectory model (HYSPLIT) air mass back trajectory analysis was used to identify preferential moisture transport pathways to each Páramo site. Our results show the strong influence of north‐east trade winds to transport moisture from the Caribbean Sea to Chirripó and the South American low‐level jet to transport moisture from the Amazon forest to Cajas. These moisture contributions were also related to the formation of convective rainfall associated with the passage of the Intertropical Convergence Zone over Costa Rica and Ecuador during the wetter seasons and to orographic precipitation during the transition and drier seasons. Our findings provide essential baseline information for further research applications of water stable isotopes as tracers of rainfall generation processes and transport in the Páramo and other montane ecosystems in the tropics.     

Tracer hydrology of the data-scarce and heterogeneous Central American Isthmus

Numerous socio-economic activities depend on the seasonal rainfall and groundwater recharge cycle across the Central American Isthmus. Population growth and unregulated land use changes resulted in extensive surface water pollution and a large dependency on groundwater resources. This work combines stable isotope variations in rainfall, surface water, and groundwater of Costa Rica, Nicaragua, El Salvador, and Honduras to develop a regionalized rainfall isoscape, isotopic lapse rates, spatial–temporal isotopic variations, and air mass back trajectories determining potential mean recharge elevations, moisture circulation patterns, and surface water–groundwater interactions. Intra-seasonal rainfall modes resulted in two isotopically depleted incursions (W-shaped isotopic pattern) during the wet season and two enriched pulses during the mid-summer drought and the months of the strongest trade winds. Notable isotopic sub-cloud fractionation and near-surface secondary evaporation were identified as common denominators within the Central American Dry Corridor. Groundwater and surface water isotope ratios depicted the strong orographic separation into the Caribbean and Pacific domains, mainly induced by the governing moisture transport from the Caribbean Sea, complex rainfall producing systems across the N-S mountain range, and the subsequent mixing with local evapotranspiration, and, to a lesser degree, the eastern Pacific Ocean fluxes. Groundwater recharge was characterized by (a) depleted recharge in highland areas (72.3%), (b) rapid recharge via preferential flow paths (13.1%), and enriched recharge due to near-surface secondary fractionation (14.6%). Median recharge elevation ranged from 1,104 to 1,979 m a.s.l. These results are intended to enhance forest conservation practices, inform water protection regulations, and facilitate water security and sustainability planning in the Central American Isthmus.     

Re–Os geochronology of a Mesoproterozoic sedimentary succession,Taoudeni basin,Mauritania: Implications for basin-wide correlations and Re–Os organic-rich sediments systematics

The exceptionally well-preserved sedimentary rocks of the Taoudeni basin, NW Africa represent one of the world's most widespread (> 1 M km²) Proterozoic successions. Hitherto, the sedimentary rocks were considered to be Mid Tonian based on Rb–Sr illite and glauconite geochronology of the Atar Group. However, new Re–Os organic-rich sediment (ORS) geochronology from two drill cores indicates that the Proterozoic Atar Group is ～ 200 Ma older (1107 ± 12 Ma, 1109 ± 22 Ma and 1105 ± 37 Ma). The Re–Os geochronology suggests that the Rb–Sr geochronology records the age of diagenetic events possibly associated with the Pan African collision.The new Re–Os geochronology data provide absolute age constraints for recent carbon isotope chemostratigraphy which suggests that the Atar Group is Mesoproterozoic and not Neoproterozoic.The new Re–Os ORS geochronology supports previous studies that suggest that rapid hydrocarbon generation (flash pyrolysis) from contact metamorphism of a dolerite sill does not significantly disturb the Re–Os ORS systematics. Modelled contact conditions suggest that the Re–Os ORS systematics remain undisturbed at ～ 650 °C at the sill/shale contact and ≥ 280 °C 20 m from the sill/shale contact.Moreover, the Re–Os geochronology indicates that the West African craton has a depositional history that predates 1100 Ma and that ORS can be correlated on a basin-wide scale. In addition, the Re–Os depositional ages for the ORS of the Taoudeni basin are comparable to those of ORS from the São Francisco craton, suggesting that these cratons are correlatable. This postulate is further supported by identical Os_i values for the Atar Group and the Vazante Group of the São Francisco craton.