Streamflow variability and its relationship with climate indices in western rivers of Argentina

The objective of this study was to examine the streamflow variability of Argentinean Andean basins (22°–52°S). Trends and step changes of seven hydrological variables were analysed. In addition, relationships between the hydrological variables and Pacific Decadal Oscillation (PDO), Niño 3.4, and Southern Annular Mode (SAM) indices were analysed. Most streamflow variables showed upward trends in the northwest and central-western basins, while downward trends were identified in the Patagonia (southwestern) region. Streamflow of the central-western and Patagonian basins was positively correlated with the Niño 3.4 index. Moreover, an inverse relationship with the SAM was found in watersheds south of 37°S. Positive step changes associated with the PDO phases in the north and central-western basins in the mid-1970s were detected, while negative step changes resulted in Patagonia between 1970 and 2000. This research provides new evidence of the influence of major climate modes on streamflow variability in the western rivers of Argentina.     

Clinopyroxene/liquid trace element partitioning in natural trachyte–trachyphonolite systems: insights from Campi Flegrei (southern Italy)

Trace element partition coefficients between clinopyroxenes and associated glassy matrix (^Cpx/L D) have been determined for 13 REE, HFSE^4+,5+, U, Th, Sr, Pb, Sc and V from combined LA-ICP-MS/EMP analyses in selected trachytes and trachyphonolites from Campi Flegrei. Composition of clinopyroxene and glass is pretty homogeneous in the trachyphonolites, pointing to an overall attainment of the equilibrium conditions. In trachytes, conversely, phases show some compositional heterogeneity (due to the presence of clinopyroxene xenocrysts) that requested a more careful petrographic and geochemical inspection of the samples to assess the equilibrium clinopyroxene composition. In the trachyte clinopyroxenes, REE are compatible from Nd to Lu (^Cpx/L D up to 2.9), like Y, Ti, Sc and V. The ^Cpx/L D for Eu is lower than those of the adjacent REE, highlighting Eu²⁺ contribution. High D values are also shown by U, Th, Pb, Zr, Hf, Nb and Ta relatively to basaltic and andesitic systems, whereas the D _Sr is roughly similar to that found for less evolved magmas. Trachyphonolites are characterized by an overall decrease of the ^Cpx/L D for highly-charged cations (with the exception of V), and by a slight increase of D _Sr. REE are still compatible from Nd to Lu (^Cpx/L D up to 2.1), like Ti, Y, Sc and V. This variation is also predicted for REE and Y by models based on the elastic strain theory, being consistent with the slightly lower polymerization degree estimated for the trachyphonolites. However, the observed ^Cpx/L D _(REE,Y) are matched by the modelled ones only considering very low T (≤825°C), which are believed unlikely. This mismatch cannot be attributed to effects induced by the water-rich composition of the trachyte–trachyphonolite suite, since they would lower the observed ^Cpx/L D _(REE,Y). Moreover, the anomalous inflections of measured ^Cpx/L D for HREE suggests some crystal-chemical control, such as the entrance of these elements in a site distinct from M2. It is concluded that the large ^Cpx/L D determined for trachytes and trachyphonolites are likely induced by hitherto unconstrained changes of the Z³⁺ activities related to the composition of melt and/or solid. All these considerations strongly highlight the importance of a direct characterization of trace element partitioning in natural samples from magmatic systems poorly characterized by experimental studies. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Nitrogen and carbon cycling in the North Sea and exchange with the North Atlantic—A model study,Part II: Carbon budget and fluxes

The 3-d coupled physical–biogeochemical model ECOHAM (version 3) was applied to the Northwest-European Shelf (47°41′–63°53′N, 15°5′W–13°55′E) for the years 1993–1996. Carbon fluxes were calculated for the years 1995 and 1996 for the inner shelf region, the North Sea (511,725 km²). This period was chosen because it corresponds to a shift from a very high winter-time North Atlantic Oscillation Index (NAOI) in 1994/1995, to an extremely low one in 1995/1996, with consequences for the North Sea physics and biogeochemistry. During the first half of 1996, the observed mean SST was about 1 °C lower than in 1995; in the southern part of the North Sea the difference was even larger (up to 3 °C). Due to a different wind regime, the normally prevailing anti-clockwise circulation, as found in winter 1995, was replaced by more complicated circulation patterns in winter 1996. Decreased precipitation over the drainage area of the continental rivers led to a reduction in the total (inorganic and organic) riverine carbon load to the North Sea from 476 Gmol C yr⁻¹ in 1995 to 340 Gmol C yr⁻¹ in 1996. In addition, the North Sea took up 503 Gmol C yr⁻¹ of CO₂ from the atmosphere. According to our calculations, the North Sea was a sink for atmospheric CO₂, at a rate of 0.98 mol C m⁻² yr⁻¹, for both years. The North Sea is divided into two sub-systems: the shallow southern North Sea (SNS; 190,765 km²) and the deeper northern North Sea (NNS; 320,960 km²). According to our findings the SNS is a net-autotrophic system (net ecosystem production NEP>0) but released CO₂ to the atmosphere: 159 Gmol C yr⁻¹ in 1995 and 59 Gmol C yr⁻¹ in 1996. There, the temperature-driven release of CO₂ outcompetes the biological CO₂ drawdown. In the NNS, where respiratory processes prevail (NEP<0), 662 and 562 Gmol C yr⁻¹ were taken up from the atmosphere in 1995 and 1996, respectively. Stratification separates the productive, upper layer from the deeper layers of the water column where respiration/remineralization takes place. Duration and stability of the stratification are determined by the meteorological conditions, in relation to the NAO. Our results suggest that this mechanism controlling the nutrient supply to the upper layer in the northern and central North Sea has a larger impact on the carbon fluxes than changes in lateral transport due to NAOI variations. The North Sea as a whole imports organic carbon and exports inorganic carbon across the outer boundaries, and was found to be net-heterotrophic, more markedly in 1996 than in 1995.     

Composition and processes of the mantle lithosphere in northeastern Brazil and Fernando de Noronha: evidence from mantle xenoliths

Spinel–peridotite facies mantle xenoliths in Cenozoic alkali basalts of the Pico Cabuji volcano (Rio Grande do Norte State, Northeast Brazil) and the adjacent South Atlantic oceanic island of Fernando de Noronha are studied for: (1) the information they provide on the composition of the lithospheric component in the erupted basalt geochemistry, and (2) to check the effects of the Fernando de Noronha plume track on the mantle lithosphere. Xenoliths from Pico Cabuji are protogranular lherzolites and porphyroclastic harzburgites recording average equilibrium temperatures of 825 ± 116 and 1248 ± 19 °C, respectively. Pressure in the porphyroclastic xenoliths ranges from 1.9 to 2.7 GPa (Ca-in-olivine geobarometer). Both groups show major element chemical variation trends in whole-rock and Ti and HREE (Er, Yb) variations in clinopyroxene consistent with fractional melting and basalt extraction. REE (rare earth element) profiles of clinopyroxenes vary from LREE (La, Ce) enriched (spoon shaped) to LREE depleted in the protogranular group, whereas they are slightly convex upward in most porphyroclastic clinopyroxenes. HFSE (Ti and Zr) negative anomalies are in general modest in the clinopyroxenes of both groups. Xenoliths from Fernando de Noronha have textural variations similar to those of Pico Cabuji. Protogranular and porphyroclastic samples have similar temperature (1035 ± 80 °C) and the pressure is 1–1.9 and 2.3 GPa, respectively. Whole-rock chemical variation trends overlap and extend further than those of Pico Cabuji. The trace element profiles of the clinopyroxenes of the porphyroclastic xenoliths are enriched in La up to 30 × PM and are smoothly fractionated from LREE to HREE, with deep, negative, Zr and Ti anomalies. The geochemical heterogeneities of the xenoliths from both localities are interpreted in terms of reactive porous percolation. The porphyroclastic xenoliths from Pico Cabuji represent the lower part of a mantle column (the head of a mantle diapir, at the transition conductive–adiabatic mantle), where OIB infiltration triggers melting, and the protogranular xenoliths the top of the mantle column, chromatographically enriched by percolation at a low melt/rock ratio. This interpretation may also apply for Fernando de Noronha, but the different geochemical signature recorded by the clinopyroxenes requires a different composition of the infiltrated melt. Nd and Sr isotopes of the Pico Cabuji porphyroclastic clinopyroxenes (¹⁴³Nd/¹⁴⁴Nd= 0.51339–0.51255, ⁸⁷Sr/⁸⁶Sr=0.70275–0.70319) and of Fernando de Noronha (¹⁴³Nd/¹⁴⁴Nd=0.51323–0.51285, ⁸⁷Sr/⁸⁶Sr=0.70323–0.70465) plot on distinct arrays originating from a similar, isotopically depleted composition and trending to low Nd–low Sr (EMI) and low Nd–high Sr (EMII), respectively. Correlation of the isotope variation with geochemical parameters indicates that the isotopic variation was induced by the metasomatic component, of EMI type at Pico Cabuji and of EMII type at Fernando de Noronha. These different components enriched a lithosphere isotopically similar to DMM (depleted MORB mantle) at both localities. At Fernando de Noronha, the isotopic signature of the metasomatic component is similar to that of the ∼ 8 Ma old lavas of the Remedios Formation, suggesting that this is the age of metasomatism. At Pico Cabuji, the mantle xenoliths do not record the high ⁸⁷Sr/⁸⁶Sr component present in the basalts. We speculate that the EMII component derives from a lithospheric reservoir, which was not thermally affected during mantle metasomatism at Pico Cabuji, but was mobilized by the hotspot thermal influence at Fernando de Noronha. This interpretation provides a plausible explanation for the presence of distinct metasomatic components at the two localities, which would be difficult to reconcile with their genetic relationship with the same plume. Received: 12 June 1999 / Accepted: 13 December 1999     

Mechanistic and Kinetic Study of the Gas-Phase Reaction of Atomic Chlorine with Cyclohexanone using an Absolute and a Relative Technique; Influence of Temperature

The reaction of Cl with cyclohexanone (1) was investigated, for the first time, as a function of temperature (273–333 K) and at a low total pressure (1 Torr) with helium as a carrier gas using a discharge flow-mass spectrometry technique (DF-MS). The resulting Arrhenius expression is proposed, k ₁= (7.7 ± 4.1) × 10^–10 exp[–(540 ± 169)/T]. We also report a mechanistic study with the quantitative determination of the products of the reaction of Cl with cyclohexanone. The absolute rate constant derived from this study at 1 Torr of total pressure and room temperature is (1.3 ± 0.2) × 10^–10 cm³ molecule^–1 s^–1. A yield of 0.94 ± 0.10 was found for the H-abstraction channel giving HCl. In relative studies, using a newly constructed relative rate system, the decay of cyclohexanone was followed by gas chromatography coupled with flame-ionisation detection. These relative measurements were performed at atmospheric pressure with synthetic air and room temperature. Rate constant measured using the relative method for reaction (1) is: (1.7 ± 0.3) × 10^–10 cm³ molecule^–1 s^–1. Finally, results and atmospheric implications are discussed and compared with the reactivity with OH radicals.     

Hydrogeochemical and Flow Modeling of Aquitard Percolation in the Cariri Valley-Northeast Brazil

Flow modeling and hydrogeochemical inversion calculations were performed on the aquifer systems of the Crato-Juazeiro Graben in the Araripe Sedimentary Basin of the Cariri Valley, Northeast Brazil, in order to assess aquifer interconnections through an aquitard of the Araripe Plateau. Steady state hydraulic head numeric calculations with the finite difference method used Modflow software with hydrologically based boundary conditions and a piezometric surface as initial condition. Hydrogeochemical inversion with PHREEQC was performed for two water samples representing typical initial and final waters along the flow path. Flow simulation shows that approximately 20% of all recharge water from the superior aquifer system passes through the Santana aquitard reaching the middle aquifer system. Quantification of mineral dissolution/precipitation processes, ion exchange and microbiological redox reactions taking place in the aquifer systems also indicates hydraulic interconnection of the superior and middle aquifer systems through the Santana aquitard.     

Wood productivity of Prosopis flexuosa D.C. woodlands in the central Monte: Influence of population structure and tree-growth habit

A balance between forest production and protection is hard to achieve in arid zones due to their low potential for wood production. Prosopis flexuosa woodlands are the major woody formations in the Monte desert and are currently in a degraded state due to intense use. The main degradation factors in the study area are overgrazing and firewood extraction. We developed allometric models to estimate the aerial biomass of P. flexuosa, compared annual growth rates of one- and multi-stemmed individuals through dendrochronological methods, and estimated the productivity of four structurally different woodlands in the central Monte. Total dry weight was best estimated by power equations. Annual increments in basal area and dry weight were initially larger for multi- than one-stemmed individuals. However, whereas multi-stemmed individuals rapidly decreased their growth rates after 60 years of age, one-stemmed trees maintained steady growth rates during the first 100 years. Depending on woodland density and tree size, total woodland biomass varied between 4000 and 15 000 kg ha⁻¹. Wood productivity was similar in all four woodlands studied (121.6-173.7 kg ha⁻¹ year⁻¹). Our results reveal the importance of tree growth habit to productivity, and suggest that regulated extraction of firewood and poles from old multi-stemmed individuals could optimize wood productivity and contribute to the sustainable use and conservation of these woodlands.