Future evolution of a tidal inlet due to changes in wave climate, Sea level and lagoon morphology (Óbidos lagoon, Portugal)

Tidal inlets are extremely dynamic, as a result of an often delicate balance between the effects of tides, waves and other forcings. Since the morphology of these inlets can affect navigation, water quality and ecosystem dynamics, there is a clear need to anticipate their evolution in order to promote adequate management decisions. Over decadal time scales, the position and size of tidal inlets are expected to evolve with the conditions that affect them, for instance as a result of climate change. A process-based morphodynamic modeling system is validated and used to analyze the effects of sea level rise, an expected shift in the wave direction and the reduction of the upper lagoon surface area by sedimentation on a small tidal inlet (Óbidos lagoon, Portugal). A new approach to define yearly wave regimes is first developed, which includes a seasonal behavior, random inter-annual variability and the possibility to extrapolate trends. Once validated, this approach is used to produce yearly time series of wave spectra for the present and for the end of the 21st century, considering the local rotation trends computed using hindcast results for the past 57 years. Predictions of the mean sea level for 2100 are based on previous studies, while the bathymetry of the upper lagoon for the same year is obtained by extrapolation of past trends. Results show, and data confirm, that the Óbidos lagoon inlet has three stable configurations, largely determined by the inter-annual variations in the wave characteristics. Both sea level rise and the reduction of the lagoon surface area will promote the accretion of the inlet. In contrast, the predicted rotation of the wave regime, within foreseeable limits, will have a negligible impact on the inlet morphology.     

The monsoon imprint during the ‘atypical’ MIS 13 as seen through north and equatorial Indian Ocean records

Previous studies have suggested that Marine Isotope Stage (MIS) 13, recognized as atypical in many paleoclimate records, is marked by the development of anomalously strong summer monsoons in the northern tropical areas. To test this hypothesis, we performed a multi-proxy study on three marine records from the tropical Indian Ocean in order to reconstruct and analyse changes in the summer Indian monsoon winds and precipitations during MIS 13. Our data confirm the existence of a low-salinity event during MIS 13 in the equatorial Indian Ocean but we argue that this event should not be considered as “atypical”. Taking only into account a smaller precession does not make it possible to explain such precipitation episode. However, when considering also the larger obliquity in a more complete orbitally driven monsoon “model,” one can successfully explain this event. In addition, our data suggest that intense summer monsoon winds, although not atypical in strength, prevailed during MIS 13 in the western Arabian Sea. These strong monsoon winds, transporting important moisture, together with the effect of insolation and Eurasian ice sheet, are likely one of the factors responsible for the intense monsoon precipitation signal recorded in China loess, as suggested by model simulations.     

Carbon dioxide in silicate melts: A molecular dynamics simulation study

We have performed a series of molecular dynamics simulations aimed at the evaluation of the solubility of CO₂ in silicate melts of natural composition (from felsic to ultramafic). In making in contact within the simulation cell a supercritical CO₂ phase with a silicate melt of a given composition, we have been able to evaluate (i) the solubility of CO₂ in the P-T range 1473-2273 K and 20-150 kbar, (ii) the density change experienced by the CO₂-bearing melt, (iii) the respective concentrations of CO₂ and species in the melt, (iv) the lifetime and the diffusivity of these species and (v) the structure of the melt around the carbonate groups. The main results are the following:(1) The solubility of CO₂ increases markedly with the pressure in the three investigated melts (a rhyolite, a mid-ocean ridge basalt and a kimberlite) from about ∼2 wt% CO₂ at 20 kbar to ∼25 wt% at 100 kbar and 2273 K. The solubility is found to be weakly dependent on the melt composition (as far as the present compositions are concerned) and it is only at very high pressure (above ∼100 kbar) that a clear hierarchy between solubilities occurs (rhyolite < MORB < kimberlite). Furthermore at a given pressure the calculated solubility is negatively correlated with the temperature.(2) In CO₂-saturated melts, the proportion of carbonate ions is positively correlated with the pressure at isothermal condition and is negatively correlated with the temperature at isobaric condition (and vice versa for molecular CO₂). Furthermore, at fixed (P, T) conditions the proportion of carbonate ions is higher in CO₂-undersaturated melts than in the CO₂-saturated melt. Although the proportion of molecular CO₂ decreases when the degree of depolymerization of the melt increases, it is still significant in CO₂-saturated basic and ultrabasic compositions at high temperatures. This finding is at variance with experimental data on CO₂-bearing glasses which show no evidence of molecular CO₂ as soon as the degree of depolymerization of the melt is high (e.g. basalt). These conflicting results can be reconciled with each other by noticing that a simple low temperature extrapolation of the simulation data predicts that the proportion of molecular CO₂ in basaltic melts might be negligible in the glass at room temperature.(3) The carbonate ions are found to be transient species in the liquid phase, with a lifetime increasing exponentially with the inverse of the temperature. Contrarily to a usual assumption, the diffusivity of carbonate ions into the liquid silicate is not vanishingly small with respect to that of CO₂ molecules: in MORB they differ from each other by a factor of ∼6 at 1473 K and only a factor of ∼2 at 2273 K. Although the bulk diffusivity of CO₂ is governed primarily by the diffusivity of CO₂ molecules, the carbonate ions contribute significantly to the diffusivity of CO₂ in depolymerized melts.(4) Concerning the structure of the CO₂-bearing silicate melt, the carbonate ions are found to be preferentially associated with NBO’s of the melt, with an affinity for NBOs which exceeds that for BOs by almost one order of magnitude. This result explains why the concentration in carbonate ions is positively correlated with the degree of depolymerization of the melt and diminishes drastically in fully polymerized melts where the number of NBO’s is close to zero. Furthermore, the network modifier cations are not randomly distributed in the close vicinity of carbonate groups but exhibit a preferential ordering which depends at once on the nature of the cation and on the melt composition. However at the high temperatures investigated here, there is no evidence of long lived complexes between carbonate groups and metal cations.     

Dissolved organic nitrogen as an indicator of livestock impacts on soil biochemical quality

Soil degradation in the Mediterranean and other arid and semi-arid regions of the world is caused mainly by cultivation and grazing. A consequence of de-vegetation due to overgrazing has been a decrease in organic matter (litter) input to soil and a decrease of aggregate size and stability making soils more susceptible to erosion and to organic matter losses. This study provided evidence linking the Dissolved Organic Nitrogen (DON) export from river basins to livestock grazing intensity and the resulting decrease in vegetation. Koiliaris River Basin in Crete was selected to study the effects of livestock grazing on water quality because it offers a unique morphologic situation due to its karstic hydrogeology draining the upland grazing areas through karstic springs. Mass balance calculations of N loads indicated that organic N is behaving as a conservative substance. It is postulated that the two potential mechanisms of Mineralization–Immobilization-Turnover and Direct Uptake did not operate in the degraded soils of the karst and arguments are presented justifying the hypothesis. De-vegetated soils of the area had lower C and N content, the same bacterial count, but lower microbial activity, lower fungi counts and species richness and lower mineralizable N compared to naturally vegetated soils. DON was the predominant N species in both extracted soluble N pools. De-vegetated soils had lower decomposition potential compared to vegetated soils. Mineralization and plant uptake appeared to be restricted and leaching of soluble low aromaticity organic matter was favored. A linear relationship between DON export and livestock N load was obtained for five Greek basins suggesting a mechanism that operates on regional scales. The de-vegetation of grazing lands in Koiliaris River highland calcaric leptosols was shown to be a primary factor causing the decline of soil biochemical quality and DON can be used as a reliable indicator for livestock grazing impacts to soil biochemical quality.     

Analytical solutions of tracer transport in fractured rock associated with precipitation-dissolution reactions

Precipitation-dissolution reactions are important for a number of applications such as isotopic tracer transport in the subsurface. Analytical solutions have been developed for tracer transport in both single-fracture and multiple-fracture systems associated with these reactions under transient and steady-state transport conditions. These solutions also take into account advective transport in fractures and molecular diffusion in the rock matrix. For studying distributions of disturbed tracer concentration (the difference between actual concentration and its equilibrium value), effects of precipitation-dissolution reactions are mathematically equivalent to a “decay” process with a decay constant proportional to the corresponding bulk reaction rate. This important feature significantly simplifies the derivation procedure by taking advantage of the existence of analytical solutions for tracer transport associated with radioactive decay in fractured rock. It is also useful for interpreting tracer breakthrough curves, because the impact of a decay process is relatively easy to analyze. Several illustrative examples are presented, which show that the results are sensitive to fracture spacing, matrix diffusion coefficient (fracture surface area), and bulk reaction rate (or “decay” constant), indicating that the relevant flow and transport parameters may be estimated by analyzing tracer signals.     

Seasonal dynamic of a shallow freshwater lens due to irrigation in the coastal plain of Ravenna,Italy

Irrigation in low-lying coastal plains may enhance the formation of fresh groundwater lenses, which counteract salinization of groundwater and soil. This study presents seasonal dynamics of such a freshwater lens and discusses its influence on the salinity distribution of the unconfined aquifer in the coastal plain of Ravenna, Italy, combining field observations with numerical modeling (SEAWAT). The lens originates from an irrigation ditch used as a water reservoir for spray irrigation. The geometry of the freshwater lens shows seasonal differences because of freshwater infiltration during the irrigation season and upconing of deeper saltwater for the remainder of the year. The extent of the freshwater lens is controlled by the presence of nearby drainage ditches. Irrigation also results in a temperature anomaly in the aquifer because of the infiltration of warm water during the irrigation season. The surficial zone in the vicinity of the irrigation ditch is increased considerably in thickness. Finally, different irrigation alternatives and the influence of sea-level rise are simulated. This shows that it is necessary to integrate irrigation planning into the water management strategy of the coastal zone to have maximum benefits for freshening of the aquifer and to make optimal use of the existing infrastructure.     

Impact of Oyster Farming on Diagenetic Processes and the Phosphorus Cycle in Two Estuaries (Brittany,France)

This study aims to compare the impact of oyster cultures on diagenetic processes and the phosphorus cycle in the sediments of the Aber Benoît and the Rivière d’Auray, estuary of Brittany, France. Our results showed clear evidence of the seasonal impact of oyster cultures on sediment characteristics (grain size and organic matter parameters) and the phosphorus cycle, especially in the Aber Benoît. At this site, seasonal variations in sulfide and Fe concentrations in pore waters, as well as Fe–P concentrations in the solid phase, highlighted a shift from a system governed by iron reduction (Reference) to a system governed by sulfate reduction (beneath oyster). This could be partly explained by the increase in labile organic matter (i.e., biodeposits) beneath oysters, whose mineralization by sulfate led to high sulfide concentrations in pore waters (up to 4,475 µmol l^?1). In turn, sulfide caused an enhanced release of phosphate in the summer, as adsorption sites for phosphate decreased through the formation of iron–sulfide compounds (FeS and FeS₂). In the Aber Benoît, dissolved Fe/PO₄ ratios could be used as an indicator of phosphate release into oxic water. Low Fe/PO₄ ratios in the summer indicated higher effluxes of phosphate toward the water column (up to 47 µmol m^?2 h^?1). At other periods, Fe/PO₄ ratios higher than 2 mol/mol indicated very low phosphate fluxes. In contrast, in the Rivière d’Auray, the occurrence of macroalgae, stranding regularly all over the site, clearly masked the impact of oyster cultures on sediment properties and the phosphorus cycle and made the use of Fe/PO₄ ratios more difficult in terms of indicators of phosphate release.     

In situ observation of crystal growth in a basalt melt and the development of crystal size distribution in igneous rocks

The speciation of CO₂ in dacite, phonolite, basaltic andesite, and alkali silicate melt was studied by synchrotron infrared spectroscopy in diamond anvil cells to 1,000 °C and more than 200 kbar. Upon compression to 110 kbar at room temperature, a conversion of molecular CO₂ into a metastable carbonate species was observed for dacite and phonolite glass. Upon heating under high pressure, molecular CO₂ re-appeared. Infrared extinction coefficients of both carbonate and molecular CO₂ decrease with temperature. This effect can be quantitatively modeled as the result of a reduced occupancy of the vibrational ground state. In alkali silicate (NBO/t = 0.98) and basaltic andesite (NBO/t = 0.42) melt, only carbonate was detected up to the highest temperatures studied. For dacite (NBO/t = 0.09) and phonolite melts (NBO/t = 0.14), the equilibrium CO₂ + O^2? = CO₃ ^2? in the melt shifts toward CO₂ with increasing temperature, with ln K = ?4.57 (±1.68) + 5.05 (±1.44) 10³ T ^?1 for dacite melt (ΔH = ?42 kJ mol^?1) and ln K = ?6.13 (±2.41) + 7.82 (±2.41) 10³ T ^?1 for phonolite melt (ΔH = ?65 kJ mol^?1), where K is the molar ratio of carbonate over molecular CO₂ and T is temperature in Kelvin. Together with published data from annealing experiments, these results suggest that ΔS and ΔH are linear functions of NBO/t. Based on this relationship, a general model for CO₂ speciation in silicate melts is developed, with ln K = a + b/T, where T is temperature in Kelvin and a = ?2.69 ? 21.38 (NBO/t), b = 1,480 + 38,810 (NBO/t). The model shows that at temperatures around 1,500 °C, even depolymerized melts such as basalt contain appreciable amounts of molecular CO₂, and therefore, the diffusion coefficient of CO₂ is only slightly dependent on composition at such high temperatures. However, at temperatures close to 1,000 °C, the model predicts a much stronger dependence of CO₂ solubility and speciation on melt composition, in accordance with available solubility data.     

Effects of ion excitation on charge transfer reactions of the Mars, Venus, and Earth ionospheres

The absolute reaction cross sections and reaction rate coefficients as a function of photoionisation energy for 25 ion-molecule reactions (charge transfer reactions except for one) have been measured between the most abundant species present as ions or neutral in the Mars, Venus and Earth ionospheres: O₂, N₂, NO, CO, Ar and CO₂.This study shows the strong influence of electronic as well as vibrational internal energy on most ion-molecule reactions. In particular endothermic charge transfer reactions are driven by electronic excitation of O₂⁺ and NO⁺ ions in their a⁴Π_u and a³Σ⁺ metastable states, respectively. Moreover, it is shown that lifetimes of these metastable states are sufficient to survive the mean free path in the lowest part of ionospheres and therefore express their enhanced reactivity. The reactions of O₂⁺ with NO as well as the reactions of CO₂⁺ with NO, O₂, CO and to a less extent N₂ are driven by vibrational excitation. N₂⁺ and CO⁺ reactions vary much less with photon energy than the other ones, except for the case of reactions with Ar. The effects of the molecular ion internal energy content on their reactivity must be included in the ionospheric models for most of the reactions investigated in the present work. It is also the case for the effect of collision energy on the CO⁺+M reactions as we expect that a significant proportion of these CO⁺ could be produced with translational energy by dissociation of doubly charged CO₂²⁺, in particular in the Mars ionosphere. Recommended effective rate constant values are given as a function of VUV photon energy.