Early Ordovician continental break-up in Variscan Europe: Nd---Sr isotope and trace element evidence from bimodal igneous associations of the Southern Massif Central, France

The high-grade Marvejols Group and the low-grade Albigeois-Cévennes sedimentary sequence contain bimodal igneous rocks of Early Ordovician age which are representative of a widespread thermal event in the European Variscides. Comparison of their Nd---Sr isotope and trace-element characteristics provides additional evidence for their origin in an ensialic extensional setting. As an alternative to the back-arc model proposed by previous authors, we propose that these associations record a continental break-up episode unrelated to contemporaneous subduction. In this model, the widespread Early Paleozoic bimodal magmatism marks the birth of a Mid-European oceanic arm. We propose that the break-up was controlled by both transtensional processes and mantle-plume activity.

High grade associations, such as the Marvejols Group, and low-grade bimodal associations, such as the Albigeois-Cévennes, might be regarded as representing opposite rifted passive margins. The “northern”, Marvejols-type margin was involved in a Late Ordovician-Silurian subduction-like process, with HP-HT metamorphism. In contrast, the “southern” Albigeois/Cévennes margin remained largely unaffected, possibly as part of the overriding plate of the subduction zone.

“Pseudo-calc-alkaline” signatures unrelated to subduction processes may occur in magmatic rocks associated with continental break-up episodes. In this case, negative Nb anomalies are produced by the addition of crustal components enriched in Th and LREE relative to Nb. This inferred alternative origin of Nb anomalies has important bearing on the paleogeodynamic settings based on geochemical data.     

Conditions for deposition of annually laminated sediments in small meromictic lakes: a case study of Lake Suminko (northern Poland)

A three-year field study was conducted in Lake Suminko, Poland, to gain an understanding of the limnological variables that influence the formation and spatial extent of annually laminated sediments in the lake. The water body is divided into three depth strata, the mixolimnion, chemocline and monimolimnion, each defined by distinct values of temperature, electrical conductivity and oxygen concentration. Typical for meromictic lakes, the monimolimnion remains perennially anoxic and is rich in dissolved solids and nutrients. The annual pattern of particle flux in Lake Suminko is closely related to biochemical processes in the euphotic zone. During winter months we observed very low accumulation rates of non-carbonaceous matter, while during the rest of the year, three periods of calcite deposition were recorded (April, July–August, and October–November). The periods of high calcite deposition corresponded with algal blooms and oxygen concentration maxima. Two principal factors contribute to meromixis in Lake Suminko: (1) the basin is sheltered, preventing wind mixing, and (2) biochemical processes in the lake associated with high lacustrine productivity. Meromictic conditions must control the formation and preservation of laminated sediments in the lake because preserved laminations occur only in areas where the lake floor lies below monimolimnetic waters.     

Paleoclimatic record of a long deep sea core from the eastern Mediterranean

A deep-sea core over 16 m long from the crestal area of the Mediterranean Ridge has been investigated with different techniques, including quantitative micropaleontology, stable isotopes (measured on the epipelagic species Globigerinoides ruber and on the mesopelagic species Globorotalia inflata), and clay mineralogy. The resulting record of climatic fluctuations can be cross correlated to other Mediterranean cores by means of isochronous lithologies (tephra layers and sapropels). The climatic record of the Mediterranean is similar in character, phase, and chronology to the records investigated in the equatorial Pacific and in the Caribbean. Isotope stages 1 to 17 have been recognized. Cyclically repeated stagnant cycles resulting in sapropel deposition complicate both the isotopic and the faunal signal. The isotopic investigations reveal that the temperature change in the surface layers of the eastern Mediterranean was no greater than 8°C in the late “glacial” Pleistocene. The chronostratigraphic and biostratigraphic interpretation of Core KS09 indicate that the mean sedimentation rate was 2.4 cm/1000 years, a value very close to the 2.5 cm/1000 years calculated for the entire Quaternary section at DSDP Site 125, also located in the crestal area of the Mediterranean Ridge in the Ionian Basin. The base of KS09 is likely to be very close to the Brunhes/Matuyama boundary dated at 0.7 my.     

Submicromolar Oxygen Profiles at the Oxic–Anoxic Boundary of Temperate Lakes

Elements involved in biogeochemical cycles undergo rapid turnover at the oxic–anoxic interface of stratified lakes. Here, the presence or absence of oxygen governs abiotic and biotic processes and rates. However, achieving a detailed sampling resolution to precisely locate the oxic–anoxic interface is difficult due to a lack of fast, drift-free sensors in the working range of 10 to a few 1,000 nmol O₂ L^?1. Here, we demonstrate that conventional amperometric and optical microsensors can be used to resolve submicromolar oxygen concentrations in a continuous profiling mode. The amperometric drift was drastically reduced by anoxic preconditioning. In situ offset correction in the anoxic layer and a high amplification scheme allowed for an excellent detection limit of < 10 nmol L^?1. The optical microsensors also showed a similar performance with a detection limit of < 20 nmol L^?1. Their drift stability allowed for a laboratory calibration in combination with a minor in situ anoxic offset correction. The two different sensor systems showed virtually identical profiles during parallel use in stratified lakes. Both sensors were able to resolve the fine-scale structure at the oxic–anoxic interface and revealed hitherto unnoticed extended zones of submicromolar oxygen concentrations even below a steep oxycline. The zones extended up to several meters and showed substantial vertical variability. These results underline the need of a precise localization of the oxic–anoxic interface on a submicromolar scale in order to constrain the relevant aerobic and anaerobic redox processes.     

Conceptual hydrogeological model of volcanic Easter Island (Chile) after chemical and isotopic surveys

Most human activities and hydrogeological information on small young volcanic islands are near the coastal area. There are almost no hydrological data from inland areas, where permanent springs and/or boreholes may be rare or nonexistent. A major concern is the excessive salinity of near-the-coast wells. Obtaining a conceptual hydrogeological model is crucial for groundwater resources development and management. Surveys of water seepages and rain for chemical and environmental isotope contents may provide information on the whole island groundwater flow conditions, in spite of remaining geological and hydrogeological uncertainties. New data from Easter Island (Isla de Pascua), in the Pacific Ocean, are considered. Whether Easter Island has a central low permeability volcanic “core” sustaining an elevated water table remains unknown. Average recharge is estimated at 300–400 mm/year, with a low salinity of 15–50 mg/L Cl. There is an apron of highly permeable volcanics that extends to the coast. The salinity of near-the-coast wells, >1,000 mg/L Cl, is marine in origin. This is the result of a thick mixing zone of island groundwater and encroached seawater, locally enhanced by upconings below pumping wells. This conceptual model explains what is observed, in the absence of inland boreholes and springs.     

Assessment of spatial transferability of process‐based hydrological model parameters in two neighbouring catchments in the Himalayan Region

Estimating the hydrological regime of ungauged catchments in the Himalayan region is challenging due to a lack of sufficient monitoring stations. In this paper, the spatial transferability of the model parameters of the process‐oriented J2000 hydrological model was investigated in 2 glaciated subcatchments of the Koshi river basin in eastern Nepal. The catchments have a high degree of similarity with respect to their static landscape features. The model was first calibrated (1986–1991) and validated (1992–1997) in the Dudh Koshi subcatchment. The calibrated and validated model parameters were then transferred to the nearby Tamor catchment (2001–2009). Sensitivity and uncertainty analyses were carried out for both subcatchments to discover the sensitivity range of the parameters in the two catchments. The model represented the overall hydrograph well in both subcatchments, including baseflow, rising and falling limbs; however, the peak flows were underestimated. The efficiency results according to both Nash–Sutcliffe (ENS) and the coefficient of determination (r²) were above 0.84 in both catchments (1986–1997 in Dudh Koshi and 2001–2009 in Tamor). The ranking of the parameters in respect to their sensitivity matched well for both catchments while taking ENS and log Nash–Sutcliffe (LNS) efficiencies into account. However, there were some differences in sensitivity to ENS and LNS for moderately and less‐sensitive parameters, although the majority (13 out of 16 for ENS and 16 out of 16 for LNS) had a sensitivity response in a similar range. The generalized uncertainty likelihood estimation results suggest that the parameter uncertainty are most of the time within the range and the ensemble mean matches very good (ENS: 0.84) with observed discharge. The results indicate that transfer of the J2000 parameters to a neighbouring catchment in the Himalayan region with similar physiographic landscape characteristics is viable. This indicates the possibility of applying a calibrated process‐based J2000 model to other ungauged catchments in the Himalayan region, which could provide important insights into the hydrological system dynamics and provide much needed information to support water resources planning and management.     

The record of Miocene impacts in the Argentine Pampas

Abstract— Argentine Pampean sediments represent a nearly continuous record of deposition since the late Miocene (～10 Ma). Previous studies described five localized concentrations of vesicular impact glasses from the Holocene to late Pliocene. Two more occurrences from the late Miocene are reported here: one near Chasicó (CH) with an ⁴⁰Ar/³⁹Ar age of 9.24 ± 0.09 Ma, and the other near Bahía Blanca (BB) with an age of 5.28 ± 0.04 Ma. In contrast with andesitic and dacitic impact glasses from other localities in the Pampas, the CH and BB glasses are more mafic. They also exhibit higher degrees of melting with relatively few xenoycrysts but extensive quench crystals. In addition to evidence for extreme heating (>1700 °C), shock features are observed (e.g., planar deformation features [PDFs] and diaplectic quartz and feldspar) in impact glasses from both deposits. Geochemical analyses reveal unusually high levels of Ba (～7700 ppm) in some samples, which is consistent with an interpretation that these impacts excavated marine sequences known to be at depth. These two new impact glass occurrences raise to seven the number of late Cenozoic impacts for which there is evidence preserved in the Pampean sediments. This seemingly high number of significant impacts over a 10⁶ km² area in a time span of 10 Myr is consistent with the number of bolides larger than 100 m expected to enter the atmosphere but is contrary to calculated survival rates following atmospheric disruption. The Pampean record suggests, therefore, that either atmospheric entry models need to be reconsidered or that the Earth has received an enhanced flux of impactors during portions of the late Cenozoic. Evidence for the resulting collisions may be best preserved and revealed in rare dissected regions of continuous, low‐energy deposition such as the Pampas. Additionally, the rare earth element (REE) concentrations of the target sediments and impact melts associated with the Chasicó event resemble the HNa/K australites of similar age. This suggests the possibility that those enigmatic tektites could have originated as high‐angle, distal ejecta from an impact in Argentina, thereby accounting for their rarity and notable chemical and physical differences from other Australasian impact glasses.