Although several studies already dealt with N recycling in subduction zones, controversy still persists about the amount of N actually recycled to the deep mantle. From the study of fumaroles and hot springs in the Central America, Fischer et al. (2002) showed that subducted N can be efficiently transferred to the surface via arc volcanism and concluded that N is not recycled to the deep mantle. In contrast, occurrence of high amount of N in metamorphic microdiamonds from Kokchetav massif (Kazakhstan) indicates that it can be subducted to ultrahigh-pressures (Cartigny et al., 2001). The comparison between three sequences of subducted metasediments also demonstrates different behaviours of nitrogen during subduction. In the Catalina Schists (California) and the Erzgebirge Schists (Germany), N content decrease and δN increase with increasing metamorphic conditions indicate that N was strongly affected by devolatilization processes during subduction (Bebout and Fogel, 1992; Mingram and Brauer, 2001). On the contrary, a study of the Schistes Lustres metasediments (western Italian Alps) showed that N (together with other fluid-mobile elements, K, Rb, Cs, H) was entirely preserved during subduction down to 90 km depths (Busigny et al., 2003). All of these results can be reconciled if the thermal structure of the subduction zone is considered. While N is dramatically devolatilized in “warm” subduction zones, it can be deeply recycled in “cold” slab environment, which is the case of most current subduction zones. An important implication of this work concerns the evolution of N recycling through geological times. Because the thermal regime of the early Earth was hotter than today, the N recycling was certainly less efficient during this time. N was likely devolatilized and fractionated, producing an increase of the δN value of the remaining recycled nitrogen.
<p>Lake Lungo and Lake Ripasottile are two shallow (4-5 m) lakes located in the Rieti Basin, central Italy, that have been described previously as surface outcroppings of the groundwater table. In this work, the two lakes as well as springs and rivers that represent their potential source waters are characterized physio-chemically and isotopically, using a combination of environmental tracers. Temperature and pH were measured and water samples were analyzed for alkalinity, major ion concentration, and stable isotope (δ<sup>2</sup>H, δ<sup>18</sup>O, δ<sup>13</sup>C of dissolved inorganic carbon, and δ<sup>34</sup>S and δ<sup>18</sup>O of sulfate) composition. Chemical data were also investigated in terms of local meteorological data (air temperature, precipitation) to determine the sensitivity of lake parameters to changes in the surrounding environment. Groundwater represented by samples taken from Santa Susanna Spring was shown to be distinct with SO<sub>4</sub><sup>2- </sup>and Mg<sup>2+ </sup>content of 270 and 29 mg/L, respectively, and heavy sulfate isotopic composition (δ<sup>34</sup>S=15.2 ‰ and δ<sup>18</sup>O=10‰). Outflow from the Santa Susanna Spring enters Lake Ripasottile <em>via</em> a canal and both spring and lake water exhibits the same chemical distinctions and comparatively low seasonal variability. Major ion concentrations in Lake Lungo are similar to the Vicenna Riara Spring and are interpreted to represent the groundwater locally recharged within the plain. The δ<sup>13</sup>C<sub>DIC</sub> exhibit the same groupings as the other chemical parameters, providing supporting evidence of the source relationships. Lake Lungo exhibited exceptional ranges of δ<sup>13</sup>C<sub>DIC </sub>(±5 ‰) and δ<sup>2</sup>H, δ<sup>18</sup>O (±5 ‰ and ±7 ‰, respectively), attributed to sensitivity to seasonal changes. The hydrochemistry results, particularly major ion data, highlight how the two lakes, though geographically and morphologically similar, represent distinct hydrochemical facies. These data also show a different response in each lake to temperature and precipitation patterns in the basin that may be attributed to lake water retention time. The sensitivity of each lake to meteorological patterns can be used to understand the potential effects from long-term climate variability.</p>
Abstract Mechanisms by which hydrochemical changes occur after earthquakes are not well documented. We use the 2016–2017 central Italy seismic sequence, which caused notable hydrochemical transient variations in groundwater springs to address this topic, with special reference to effects on fractured carbonate aquifers. Hydrochemistry measured before and after the earthquakes at four springs at varying distances from the epicenters all showed immediate postmainshock peaks in trace element concentrations but little change in major elements. Most parameters returned to preearthquake values before the last events of the seismic sequence. The source of solutes, particularly trace elements, is longer residence time pore water stored in slow‐moving fractures or abandoned karstic flow paths. These fluids were expelled into the main flow paths after an increase in pore pressure, hydraulic conductivity, and shaking from coseismic aquifer stress. The weak response to the later earthquakes is explained by progressive depletion of high solute fluids as earlier shocks flushed out the stored fluids in the fractures. Spring δ 13 C DIC values closest to a deep magma source to the west became enriched relative to preearthquake values following the 24 August event. This enrichment indicates input from deeply sourced dissolved CO 2 gas after dilation of specific fault conduits. Differences in carbon isotopic responses between springs are attributed to proximity to the deep gaseous CO 2 source. Most of the transient chemical changes seen in the three fractured carbonate aquifers are attributed to local shaking and emptying of isolated pores and fractures and are not from rapid upward movement of deep fluids.
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