The dynamics of central Main Ethiopian Rift waters: Evidence from δD, δO and Sr/Sr ratios

Water samples from cold and geothermal boreholes, hot springs, lakes and rivers were analyzed for δD, δ¹⁸O and ⁸⁷Sr/⁸⁶Sr compositions in order to investigate lake water–groundwater mixing processes, water–rock interactions, and to evaluate groundwater flow paths in the central Main Ethiopian Rift (MER) of the Ziway–Shala basin. Different ranges of isotopic values were recorded for different water types: hot springs show δ¹⁸O −3.36 to +3.69 and δD −15.85 to +24.23, deep Aluto-Langano geothermal wells show δ¹⁸O −4.65 to −1.24 and δD −12.39 to −9.31, groundwater wells show δ¹⁸O −3.99 to +5.14 and δD −19.69 to +32.27, whereas the lakes show δ¹⁸O and δD in the range +3.98 to +7.92 and +26.19 to +45.71, respectively. The intersection of the Local Meteoric Water Line (LMWL: δD = 7 δ¹⁸O + 11.2, R² = 0.94, n = 42) and the Local Evaporation Line (LEL: δD = 5.63δ¹⁸O + 8, n = 14, R² = 0.82) was used to estimate the average isotopic composition of recharge water into the basin (δD = −5.15 and δ¹⁸O = −2.34). These values are depleted if compared with the modern-day average precipitation, presumably indicating paleo-groundwater components recharged during previous humid climatic phases. The measured stable isotope values indicate that the geothermal wells, some of the hot springs and groundwater wells mainly consist of meteoric water. The Sr isotopic signatures in all waters are within the range of the Sr isotopic composition of the rift basalts and rhyolites. The variability of Sr isotopic data also pinpoints complex water–rock interaction and mixing processes in groundwater and surface water. The ⁸⁷Sr/⁸⁶Sr ratio ranges from 0.70445 to 0.70756 in the hot springs, from 0.70426 to 0.70537 in two deep geothermal wells, and from 0.70673 to 0.70721 in the rift lakes Ziway, Langano, Shala and Awasa. The radiogenic composition recorded by the lakes indicates that the input water was predominantly affected by progressive interaction with rhyolitic volcanics and lacustrine sediments.     

Flow dynamics at the continental scale: Streamflow correlation and hydrological similarity

Streamflow variability in space and time critically affects anthropic water uses and ecosystem services. Unfortunately, spatiotemporal patterns of flow regimes are often unknown, as discharge measurements are usually recorded at a limited number of hydrometric stations unevenly distributed along river networks. Advances in understanding the physical processes that control the spatial patterns of river flows are therefore necessary to predict water availability at ungauged locations or to extrapolate pointwise streamflow observations. This work explores the use of the spatial correlation of river flows as a metric to quantify the similarity between hydrological responses of two catchments. Following a stochastic framework, 340,000 cross‐correlations between pairs of daily streamflows time series are predicted at a seasonal timescale across the contiguous United States using 413 catchments of the MOPEX dataset. Model predictions of streamflow correlation obtained in absence of run‐off information are successfully used to identify catchment outlets sharing similar discharge dynamics and flow regimes across a broad range of geomorphoclimatic conditions, without relying on calibration. The selection of reference streamgauges based on predicted streamflow correlation generally outperforms the selection based on spatial proximity, especially as the density of available gauged sections decreases. Interestingly, correlated outlets share a broad spectrum of hydrological signatures (mean discharge, flow variability, and recession properties), suggesting that catchments forced by analogous frequency and intensity of effective rainfall events might exhibit common geomorphoecological traits leading to similar hydrological responses. The proposed framework provides a physical basis to assist the regionalization of flow dynamics and to interpret the spatial variability of flow regimes along stream networks.     

Central Mediterranean tephrochronology between 313 and 366 ka: New insights from the Fucino palaeolake sediment succession

Thirty-two tephra layers were identified in the time-interval 313–366 ka (Marine Isotope Stages 9–10) of the Quaternary lacustrine succession of the Fucino Basin, central Italy. Twenty-seven of these tephra layers yielded suitable geochemical material to explore their volcanic origins. Investigations also included the acquisition of geochemical data of some relevant, chronologically compatible proximal units from Italian volcanoes. The record contains tephra from some well-known eruptions and eruptive sequences of Roman and Roccamonfina volcanoes, such as the Magliano Romano Plinian Fall, the Orvieto–Bagnoregio Ignimbrite, the Lower White Trachytic Tuff and the Brown Leucitic Tuff. In addition, the record documents eruptions currently undescribed in proximal (i.e. near-vent) sections, suggesting a more complex history of the major eruptions of the Colli Albani, Sabatini, Vulsini and Roccamonfina volcanoes between 313 and 366 ka. Six of the investigated tephra layers were directly dated by single-crystal-fusion ⁴⁰Ar/³⁹Ar dating, providing the basis for a Bayesian age–depth model and a reassessment of the chronologies for both already known and dated eruptive units and for so far undated eruptions. The results provide a significant contribution for improving knowledge on the peri-Tyrrhenian explosive activity as well as for extending the Mediterranean tephrostratigraphical framework, which was previously based on limited proximal and distal archives for that time interval.     

Asbestos fibre identification vs. evaluation of asbestos hazard in ophiolitic rock mélanges,a case study from the Ligurian Alps (Italy)

In recent years, the high incidence of harmful health effects through inhalation of airborne asbestos from amphibole-bearing rock mélanges has been thoroughly documented. Here, we present a field-based, multi-scale geological approach aimed at illustrating the occurrence of amphibole fibrous mineralisation in an ophiolitic suite from the Ligurian Alps (Italy) and discussing the implication on in situ determination of the asbestos hazard. The rock mélange is composed of plurimetre-sized blocks of different lithotypes (metagabbro, serpentinite, chloritoschist) juxtaposed by the meaning of tectonic structures. The geological-structural survey revealed that the fibrous mineralisation is localised in specific structural sites of the rock volume, including veins and schistosity. Both micro-chemical and crystal structure analyses on selected fibrous samples revealed that actinolite fibres grow in veins within the metagabbro and in chloritoschists, while fibrous tremolite occurs in serpentinite schistosity. The morphological features of these amphibole fibres have been analysed in TEM images and used for classifying them as “asbestiform” or “non-asbestiform”. The results show that the asbestos hazard determination is not unequivocally identified when different procedures for asbestos fibre identification and classification are applied. This may have impact on normatives and regulations in defining environmental hazards due to asbestos occurrence.     

Evidence of diverse depletion and metasomatic events in harzburgite–lherzolite mantle xenoliths from the Iberian plate (Olot, NE Spain): Implications for lithosphere accretionary processes

Mantle xenoliths from the Olot volcanic district (NE Spain) comprise a bi-modal suite consisting of protogranular spinel lherzolites (cpx 12–14%) sometimes with pargasitic amphibole, and highly refractory spinel harzburgites (cpx ≤ 1%) with coarse-grained granular textures. The lherzolites range from slightly depleted to moderately LREE-enriched with flat HREE patterns between 1.5 and 2.7 × chondrite (Ch). In contrast, the harzburgites are extremely depleted in HREE (down to 0.2 × Ch) and strongly LREE-enriched (La_N/Yb_N = 12.3–17.2). LA-ICP-MS analyses of clinopyroxene and amphibole of the lherzolites highlight variable degrees of LREE depletion (HREE up to 13 × Ch, La_N/Yb_N down to 0.01), with the exception of a single sample in which both clinopyroxene and amphibole are LREE-enriched (La_N/Yb_N up to 19). In the harzburgites, clinopyroxenes display totally different REE distributions, characterized by extreme HREE depletion (down to 0.4 × Ch) and upward convex positively fractionated middle-light REE patterns (Nd_N/Yb_N up to 20.7 × Ch; La_N/Yb_N up to 12 × Ch). Sr–Nd–Hf isotopic data for both whole-rocks and cpx separates, coherently indicate depleted mantle (DM) compositions for the lherzolites (εSr = − 15 to − 26, εNd = + 9 to + 17, εHf = + 18 to + 68) and enriched mantle (EM) compositions for the harzburgites (εSr = − 10 to + 36, εNd = − 1 to − 6, εHf = + 3 to + 8). Modelling of the clinopyroxene REE data and isotopic systematics suggest that some lherzolites were affected by pre-Paleozoic (0.6–1 By) low-degree partial melting processes, while others probably reflect some extent of refertilization of the mantle protolith by metasomatizing melts similar to the Triassic rift-related tholeiites reported from several Pyrenean localities. The harzburgites represent extreme refractory residua, resulting from a complex depletion history due to multistage melt extraction as often observed in the cratonic mantle. The distinctive REE patterns and isotopic systematics of their clinopyroxenes suggest that the harzburgites were formed by the interaction of an ultra-depleted peridotite matrix with highly alkaline basic melts similar in composition to the Permo-Triassic alkaline lamprophyres which are widespread within the Iberian plate. Lherzolites possibly represent younger lithosphere (accreted asthenosphere?) up-lifted and juxtaposed to the older subcontinental lithospheric mantle (harzburgites) during the post-Variscan rifting of the Iberian margin. These two genetically different, but adjoining, mantle domains intimately mingled along the northern Iberian margin during the subsequent plate convergence processes, leading to the close association of harzburgites and lherzolites observed in the Olot mantle xenoliths and in some Pyrenean peridotite massifs.     

Debris-avalanche deposits of the Milo Lahar sequence and the opening of the Valle del Bove on Etna volcano (Italy)

Previously undescribed debris-avalanche deposits occur in two locations downslope from the open end of the Valle del Bove. These outcrops comprise unstratified, ungraded deposits of metre-scale lava blocks in a matrix of weathered and fractured lava clasts. The avalanche deposits are unconformably overlain by matrix- to clast-supported conglomerates, representing debris-flow and interbedded fluvial deposits, that constitute most of the Milo Lahar sequence. We present evidence that the Milo Lahar sequence, which crops out just at the exit of the Valle del Bove, formed during the opening and enlargement of this depression. The presence of the avalanche deposits at the base of the Milo Lahar sequence indicates that catastrophic landslides were involved in the formation of the Valle del Bove. The composition of lavas in the debris avalanche deposits is similar to that of most of the Ellittico volcanic sequence exposed along the northern wall of the Valle del Bove. Radiocarbon dates of 8400 and 5300 years BP from the base and top, respectively, of the debris-flow sequence indicate that the Milo Lahars are correlative with the exposed part of the Chiancone deposit. The basal lahars of the Chiancone, which contain lava blocks whose compositions partially overlap that of blocks in the avalanche deposits, may have formed by water concentration in the distal end of the avalanche causing transformation to debris, or alternatively by reworking of the avalanche deposit.     

Soil C and N response to changes in winter precipitation in a subalpine forest ecosystem,NW Italy

Among the potential effects of climate change on subalpine forest ecosystems during the winter season, the shift in snowline towards higher altitudes and the increase in frequency of rain events on the snowpack are of particular interest. Here, we present the results of a 2‐year field experiment conducted in a forest stand (Larix decidua) in NW Italy at 2020 m a.s.l. From 2009 to 2011, we monitored soil physical characteristics (temperature and moisture), and soil and soil solution chemistry, in particular carbon (C) and nitrogen (N) forms and their change in time, as affected by simulated late snowpack accumulation and rain on snow events. Late snowpack accumulation determined a stronger effect on soil thermal and moisture regimes than rain on snow events. Also soil chemistry was significantly affected by late snowfall simulation. Although microbial biomass C and N were not reduced by soil freezing, soil contents of the more labile dissolved organic carbon and inorganic N increased when the soil was affected by mild/hard freezing. Variations in the soil solution were shifted with respect to those observed in soil, with an increase in N‐NO₃^? concentrations occurring during spring and summer. This study highlights the potential N loss in subalpine soils under changing environmental conditions driven by a changing climate. Copyright © 2013 John Wiley & Sons, Ltd.