Clemente Recio

Origin of cordierite-bearing granites by assimilation in the Central Iberian Massif (CIM), Spain

Abstract The Central Iberian Massif (CIM) is characterized by the great abundance and variety of granitoids emplaced during Hercynian times. One of the most conspicuous is cordierite-bearing biotite granite (CG), which in the western part of the CIM is associated with other granites lacking cordierite [amphibole-bearing biotite granite (AG) and biotite granite (BG)] and with anatectic host-rocks [cordierite-rich nebulites (NB)]. Major-, trace- and rare-earth element data for AG and BG show that these granites have chemical affinities, and parallel or overlapping differentiation trends, suggesting two different granite magmas, with a common or similar deep-seated source. CG show two extreme facies: a marginal cordierite-rich facies (MCG) in direct contact with host-rocks (schists, migmatites), and an internal cordierite-poor facies (ICG) farther away from the host-rocks. Gradual transitions between MCG and ICG are commonly observed, as well as between ICG and BG. An anatectic hypothesis for CG is difficult to accept because: (a) CG have higher CaO contents relative to NB or other country rocks, a smaller Eu/Eu∗ ratio and similar Sr contents; and (b) the similar REE absolute contents and patterns in CG, BG and NB, and the absence of REE fractionation with respect to country rocks. Simple mixing equations are computed to test assimilation processes involving BG as a primary magma and NB as the contaminant in producing CG. CaO and MgO data have been used for this purpose since they are the most discriminating elements between BG and NB. The results of the computation indicate that: (a) assimilation is possible for nearly the whole CaO range in NB if CaO in the BG ranges between 1.91% and 2.86%, and MgO is between 0.93% and 1.18% (these extreme values are comprised within the observed range for CaO and MgO in BG); (b) CaO and MgO data for MCG are compatible with assimilation for nearly the whole range of data observed; and (c) the percentage of assimilation is

Denitrification in a hypersaline lake-aquifer system (Pétrola Basin, Central Spain): the role of recent organic matter and Cretaceous organic rich sediments.

Agricultural regions in semi-arid to arid climates with associated saline wetlands are one of the most vulnerable environments to nitrate pollution. The Pétrola Basin was declared vulnerable to NO3(-) pollution by the Regional Government in 1998, and the hypersaline lake was classified as a heavily modified body of water. The study assessed groundwater NO3(-) through the use of multi-isotopic tracers (δ(15)N, δ(34)S, δ(13)C, δ(18)O) coupled to hydrochemistry in the aquifer connected to the eutrophic lake. Hydrogeologically, the basin shows two main flow components: regional groundwater flow from recharge areas (Zone 1) to the lake (Zone 2), and a density-driven flow from surface water to the underlying aquifer (Zone 3). In Zones 1 and 2, δ(15)NNO3 and δ(18)ONO3 suggest that NO3(-) from slightly volatilized ammonium synthetic fertilizers is only partially denitrified. The natural attenuation of NO3(-) can occur by heterotrophic reactions. However, autotrophic reactions cannot be ruled out. In Zone 3, the freshwater-saltwater interface (down to 12-16 m below the ground surface) is a reactive zone for NO3(-) attenuation. Tritium data suggest that the absence of NO3(-) in the deepest zones of the aquifer under the lake can be attributed to a regional groundwater flow with long residence time. In hypersaline lakes the geometry of the density-driven flow can play an important role in the transport of chemical species that can be related to denitrification processes.

Tracking climate change in oligotrophic mountain lakes: Recent hydrology and productivity synergies in Lago de Sanabria (NW Iberian Peninsula)

Mountain lakes are particularly sensitive to global change as their oligotrophic conditions may be rapidly altered after reaching an ecological threshold, due to increasing human impact and climate change. Sanabria Lake, the largest mountain lake in the Iberian Peninsula and with a recent history of increased human impact in its watershed, provides an opportunity to investigate recent trends in an oligotrophic, hydrologically-open mountain lake, and their relationship with climate, hydrological variability and human pressure. We conducted the first systematic and detailed survey of stable isotope compositions of Sanabria Lake and Tera River together with limnological analyses during 2009-2011. δ18Olakewater and δDlakewater seasonal fluctuations are strongly linked to river discharges, and follow the monthly mean isotopic composition of precipitation, which is controlled by NAO dynamics. δ13CPOM and δ13CDIC revealed higher contribution of allochthonous organic matter in winter and spring due to higher river inflow and lower primary productivity. Increased phytoplankton biomass in late summer correlated significantly with higher pH and Chl-a, and higher nutrient input and lower river inflow. However, the small δ13CPOM seasonal amplitude underlines the stability of the oligotrophic conditions and the isotopic variation in POM and DIC reflect small seasonal fluctuations mostly as a consequence of strong throughflow. The stability of hydrology and productivity patterns is consistent with Holocene and last millennium reconstructions of past limnological changes in Sanabria Lake. The results of this study indicate that trophic state in this hydrologically-open mountain lake is strongly controlled by climate variability, but recent changes in human-land uses have increased sediment delivery and nutrients supply to the lake and have to be considered for management policies. Monitoring surveys including isotope techniques provide snapshots of modern isotope variability, and serve as a benchmark for assessing the environmental impacts of future developments and long-term climate changes in mountain lakes.

Biogeochemical characterization of a Mediterranean shallow lake using stable isotopes: Laguna del Cristo (NW Iberian Peninsula)

The present multi-isotopic study (δ18O–δDwater, δ34S–δ18Odissolved-sulphate, δ13Cdissolved-inorganic-carbon, δ13C–δ18Oshells-modern-gastropods, δ13Cplants, and δ13Csedimentary-organic-matter) is aimed at assessing the hydrogeochemical changes and biogeochemical dynamics in a Mediterranean shallow lake fed by a Quaternary–Tertiary aquifer, the “Laguna del Cristo” (NW Iberian Peninsula), a system sensitive to climate fluctuations, between 2010 and 2011. Lake water is of the bicarbonate type, and there are no major pollutants. δ18O-δDwater values plot on a local evaporation line (δD=5.29δ18O–12.29) indicating that evaporative enrichment had a significant impact on lake water isotopic features. Periods of high water levels are characterized by lower δ34S–δ18Odissolved-sulphate and δ13Cdissolved-inorganic-carbon values and suggest sulphate derived from weathering of sulphides in the catchment area, delivered to the lake by surface run-off or via groundwater, and in situ decay of organic matter. During lower water levels, sulphate reduction and enhanced primary productivity lead to higher δ34Sdissolved-sulphate and δ13Cdissolved-inorganic-carbon values. Evaporation induced enrichment in 18Osulphate, 13Cdissolved-inorganic-carbon and 13C–18Oshells-Galba-Gyraulus. δ13Cplant confirms the C3 photosynthetic pathway. Enrichment in 13C submerged aquatic plants indicates that HCO3− is the main carbon source, except for 13C-depleted Potamogeton. The TOC, δ13Corg values, and TC/TN ratios in sediments all confirm the autochthonous character of organic matter contribution. This study provides a baseline for isotopic research into shallow, flow-through lakes fed by siliciclastic aquifers, and stresses the importance of evaporation and refilling (direct precipitation and groundwater discharge) in controlling the solute chemistry and stable isotopic composition in temperate regions with contrasting seasonal climates. The results also provide a snapshot of modern lake isotope variability that can be applied to paleoenvironmental reconstructions.