Combining airborne thermal infrared images and radium isotopes to study submarine groundwater discharge along the French Mediterranean coastline

The French Mediterranean coastline, which includes karstic springs discharging into coastal seas and coastal lagoons. Study focus We investigated submarine groundwater discharge (SGD), an important vector for many chemical elements that may impact the quality of the coastal environment. First, we acquired airborne thermal infrared (TIR) images to detect terrestrial groundwater inputs. Then we report in situ data (salinity; temperature; radium isotopes). We use these data i) to confirm the presence of groundwater discharge and to characterize the different systems, and ii) to quantify SGD fluxes and estimate the residence time of the water bodies. New hydrological insights for the region Few studies have been conducted on SGD along the French Mediterranean coastline. The terrestrial groundwater spring inputs in La Palme and Salses-Leucate coastal lagoons are in the range (0.04–0.11) m 3 s −1 , ≤ 2% of the local river inputs. In comparison, total SGD estimates to La Palme lagoon (0.56–1.7 m 3 s −1 ) suggest that the recirculation of lagoon water through the sediment is two orders of magnitude greater than the terrestrial groundwater inputs. At the Calanque of Port-Miou, the terrestrial groundwater flux to the coastal seas was between 0.6 and 1.2 m 3 s −1 in July 2009. This study demonstrates the application of airborne TIR remote sensing for detecting surficial groundwater springs, and the inability of the method to detect deeper, submerged springs

Holocene Atmospheric Mercury Levels Reconstructed from Peat Bog Mercury Stable Isotopes

Environmental regulations on mercury (Hg)emissions and associated ecosystem restoration are closely linked to what Hg levels we consider natural. It is widely accepted that atmospheric Hg deposition has increased by a factor 3 ± 1 since preindustrial times. However, no long-term historical records of actual atmospheric gaseous elemental Hg (GEM) concentrations exist. In this study we report Hg stable isotope signatures in Pyrenean peat records (southwestern Europe) that are used as tracers of Hg deposition pathway (Δ200Hg, wet vs dry Hg deposition) and atmospheric Hg sources and cycling (δ202Hg, Δ199Hg). By anchoring peatderived GEM dry deposition to modern atmospheric GEM levels we are able to reconstruct the first millennial-scale atmospheric GEM concentration record. Reconstructed GEM levels from 1970 to 2010 agree with monitoring data, and maximum 20th century GEM levels of 3.9 ± 0.5 ng m−3 were 15 ± 4 times the natural Holocene background of 0.27 ± 0.11 ng m−3. We suggest that a −0.7‰ shift in δ202Hg during the medieval and Renaissance periods is caused by deforestation and associated biomass burning Hg emissions. Our findings suggest therefore that human impacts on the global mercury cycle are subtler and substantially larger than currently thought

Recent 210 Pb, 137 Cs and 241 Am accumulation in an ombrotrophic peatland from Amsterdam Island (Southern Indian Ocean)

Over the past 50 years, 210Pb, 137Cs and 241Am have been abundantly used in reconstructing recent sediment and peat chronologies. The study of global aerosol-climate interaction is also partially depending on our understanding of 222Rn-210Pb cycling, as radionuclides are useful aerosol tracers. However, in comparison with the Northern Hemisphere, few data are available for these radionuclides in the Southern Hemisphere, especially in the South Indian Ocean. A peat core was collected in an ombrotrophic peatland from the remote Amsterdam Island (AMS) and was analyzed for 210Pb, 137Cs and 241Am radionuclides using an underground ultra-low background gamma spectrometer. The 210Pb Constant Rate of Supply (CRS) model of peat accumulations is validated by peaks of artificial radionuclides (137Cs and 241Am) that are related to nuclear weapon tests. We compared the AMS 210Pb data with an updated 210Pb deposition database. The 210Pb flux of 98 ± 6 Bq·m−2·y−1 derived from the AMS core agrees with data from Madagascar and South Africa. The elevated flux observed at such a remote location may result from the enhanced 222Rn activity and frequent rainfall in AMS. This enhanced 222Rn activity itself may be explained by continental air masses passing over southern Africa and/or Madagascar. The 210Pb flux at AMS is higher than those derived from cores collected in coastal areas in Argentina and Chile, which are areas dominated by marine westerly winds with low 222Rn activities. We report a 137Cs inventory at AMS of 144 ± 13 Bq·m−2 (corrected to 1969). Our data thus contribute to the under-represented data coverage in the mid-latitudes of the Southern Hemisphere

Differentiating Lithogenic Supplies, Water Mass Transport, and Biological Processes On and Off the Kerguelen Plateau Using Rare Earth Element Concentrations and Neodymium Isotopic Compositions

Distributions of dissolved rare earth element (REE) concentrations and neodymium isotopic compositions (expressed as εNd) of seawater over and off the Kerguelen Plateau in the Southern Ocean are presented. The sampling took place during the austral spring bloom in October–November 2011 (KEOPS2 project, GEOTRACES process study) and aimed to further the investigations of the KEOPS1 austral summer study in terms of sources and transport of lithogenic material, and to investigate the impact of local biogeochemical cycles on the REE distributions. The REE signature of the coastal eastern Kerguelen Islands waters was characterized by negative europium anomalies (Eu/Eu*) and negative εNd in filtered samples. By contrast, the unfiltered sample showed a positive Eu/Eu* and more radiogenic εNd. These distinct signatures could reflect either differential dissolution of the local flood basalt minerals or differential leaching of local trachyte veins. The dissolved Kerguelen coastal REE patterns differ from those observed close to Heard Island, these latter featuring a positive Eu/Eu* and a less radiogenic εNd (Zhang et al., 2008). These differences enabled us to trace the transport of waters (tagged by the Kerguelen REE signature) 200 km downstream from the coastal area, north of the Polar Front. Northward transport of the central Plateau shallow waters, enriched by both local vertical supplies and lateral advection of inputs from Heard Island, was also evident. However, the transport of Kerguelen inputs southeastward across the Polar Front could not be discerned (possibly as a result of rapid dilution or scavenging of REE signatures), although evidence for such transport was found previously using Ra isotopes (Sanial et al., 2015). Comparison of the REE patterns at stations sampled prior, during and at the demise of the bloom revealed diverse fractionations, including production of significant lanthanum and europium anomalies, which are tentatively ascribed to chemical reactions with various inorganic and biogenic phases, including surface coatings, barite crystals, and biogenic silica

Multi-tracer reconnaissance of submarine groundwater discharge along the French Mediterranean coast

In order to evaluate the regional distribution of SGD along the French Mediterranean coast, the spatial distribution of radium, radon and salinity was investigated at a number of locations where groundwater was previously known to discharge into coastal waters. Field work was carried out during summer 2009 in the coastal lagoons of Salses-Leucate, La Palme and Tuau, and along the karstic coastline of 'Les Calanques' between Marseille and Cassis. The hydrogeology of the region is dominated by the karstic rocks of the coastal uplands. Mixing of coastal waters region is driven primarily by wind and air pressure changes. Radon and salinity in surface waters were mapped concurrently to obtain an overview of SGD locations and processes, and spot radium samples were collected to study fluxes of SGD, missing relationships with coastal waters and residence times of the lagoon waters. Radon was measured in situ with a surface-towed, continuously recoding multi-detector setup using Durridge Rad7 Radon-in-air monitors. Radium was sampled with managanese fibers by standard methods. Short and long lived isotopes were counted with RADECC delayed coincidence counters and gamma spectrometry in the underground laboratory at Ferrieres (Pyrenees) respectively.\ud \ud A brief overview of the study sites and results will be presented. For example, in the lagoon of La Palme, the spatial distribution of radionuclides varied dramatically over short period of time, likely associated with wind-driven input of saline groundwater from disused salt evaporation ponds, in addition to be groundwater input from karstic springs. In the Taui lagoon, the well-known submarine freshwater spring 'La Vise' in ca 25m of depth appears to have only a small impact on the radionuclide budget of the lagoon due to a very small flow rate. The Canal Du Midi may be additional source of radon during southerly winds, and small but systematic elevations of radon at typical lagoon salinity in shallow parts of the lagoon suggest the recirculation of seawater there. The residence time of water in the lagoons was found to be in the order of a few days, but varies with wind conditions

Karstic groundwater discharge and seawater recirculation through sediments in shallow coastal Mediterranean lagoons, determined from water, salt and radon budgets

Groundwater discharge to coastal water bodies is increasingly recognised to contribute significantly to coastal water and solute budgets. In order to evaluate the discharge of low-salinity groundwater of karstic origin and of recirculation of seawater through sediments into Mediterranean lagoons, a study was carried out in La Palme, Salses-Leucate and Thau lagoons on the French Mediterranean coastline in the dry summer months 2009, using radon and salinity as tracers.\ud \ud Whereas Salses-Leucate lagoon receives significant fluxes of karstic low-salinity groundwater, in La Palme and Thau lagoons, karstic groundwater fluxes are small, and have little effect on the lagoons' radon balance. A simultaneous water, salt and radon budget of the small La Palme lagoon (ca 50 ha surface area and 0.6 m mean depth) was used to simulate average salt and radon concentration over a one month period. The results indicate that despite its small flux (0.8–1.2% of lagoon volume per day) the discharge of low-salinity karstic groundwater contributes significantly to maintaining salinity lower than seawater in the seasonally closed lagoon, but makes only a minor contribution (7–18%) to the radon budget.\ud \ud Wind-driven seawater recirculation through sediments on the other hand is a major contributor, estimated at 22–60% of total radon fluxes to the lagoon, equivalent to a water flux of 1.4–7.0% of lagoon volume day− 1. The remaining balance of Radon is supplied by diffusion and in-water production from decay of its parent nuclide. Using a stand-alone radon model without considering a water and salt balance would have considerably overestimated the flux of groundwater of karstic origin.\ud \ud Radon can be regarded as a proxy for other dissolved solutes such as nutrients and contaminants transported with groundwater and seawater recirculation fluxes. Nutrient and contaminant enrichment of sediments in Mediterranean lagoons is well documented. Wind-driven seawater recirculation through these sediments as documented in this study may remobilise these nutrients and contaminants. It may thus play a considerable role in lagoonal biogeochemical budgets, and may require consideration in water quality management in Mediterranean coastal lagoons

Comparison of methods to determine extraction efficiencies of Ra isotopes and 227Ac from large volume seawater samples

International audienceRadium isotopes, other than 226Ra, and 227Ac are typically present at low activities in the open ocean. The analysis of these isotopes thus requires the collection of large volumes of seawater and high sensitivity, low background instruments. To obtain the required large volumes (hundreds to thousands of liters), these radionuclides are typically preconcentrated on cartridge-style filters impregnated with MnO2 (Mn-cartridges) deployed on in-situ pumps. This technique, however, requires the determination of the extraction efficiency of the Mn-cartridges for the radionuclides of interest. For Ra isotopes, we used two methods to estimate the extraction efficiency of these Mn-cartridges at two stations on the South-West Indian Ridge in the Southern Ocean (GEOTRACES GS02). Method (1) compares the 226Ra activities recovered on the Mn-cartridges versus the activities determined in Mn-fibers, through which seawater was passed at a flow rate < 1 L min−1 to quantitatively sorb Ra (Mn-fiber method) while method (2) combines the 226Ra activities determined from two Mn-cartridges placed in series on in-situ pumps (A-B method). The second method is also applied to determine the 227Ac extraction efficiency. We find a relatively wide-range of Ra and 227Ac extraction efficiencies across the dataset (from 44.8% to 99.6% for Ra, and from 23.7% to 77.5% for 227Ac). Overall, the yield of 227Ac extraction is lower than that of Ra (mean value of 49.3 ± 19.0% for 227Ac, n = 10, mean value of 79.2 ± 10.3% for Ra, n = 13, using the Mn-fiber method; and a mean value of 63.9 ± 12.5%, n = 11 using the A-B method). Our dataset suggests that the Ra extraction efficiencies using either the A-B method or the Mn-fiber method are in relatively good agreement. Consequently, the 223Raex, 224Raex and 228Ra activities determined from the Mn-cartridges by applying the two Ra extraction yields are similar. We also show that the 227Ac extraction efficiency can be estimated from the Ra extraction efficiency allowing the use of a single Mn-cartridge. Finally, we recommend to determine the Ra and 227Ac extraction efficiencies in each individual Mn-cartridge, rather than applying a single extraction efficiency to all the Mn-cartridges, since a significant variability in the extraction efficiencies was observed between the different Mn-cartridges

Combining airborne thermal infrared images and radium isotopes to study submarine groundwater discharge along the French Mediterranean coastline

The French Mediterranean coastline, which includes karstic springs discharging into coastal seas and coastal lagoons. Study focus We investigated submarine groundwater discharge (SGD), an important vector for many chemical elements that may impact the quality of the coastal environment. First, we acquired airborne thermal infrared (TIR) images to detect terrestrial groundwater inputs. Then we report in situ data (salinity; temperature; radium isotopes). We use these data i) to confirm the presence of groundwater discharge and to characterize the different systems, and ii) to quantify SGD fluxes and estimate the residence time of the water bodies. New hydrological insights for the region Few studies have been conducted on SGD along the French Mediterranean coastline. The terrestrial groundwater spring inputs in La Palme and Salses-Leucate coastal lagoons are in the range (0.04–0.11) m 3 s −1 , ≤ 2% of the local river inputs. In comparison, total SGD estimates to La Palme lagoon (0.56–1.7 m 3 s −1 ) suggest that the recirculation of lagoon water through the sediment is two orders of magnitude greater than the terrestrial groundwater inputs. At the Calanque of Port-Miou, the terrestrial groundwater flux to the coastal seas was between 0.6 and 1.2 m 3 s −1 in July 2009. This study demonstrates the application of airborne TIR remote sensing for detecting surficial groundwater springs, and the inability of the method to detect deeper, submerged springs

Use of radium isotopes to estimate mixing rates and trace sediment inputs to surface waters in northern Marguerite Bay, Antarctic Peninsula

In the western Antarctic Peninsula region, micronutrient injection facilitates strong plankton blooms that support productive food webs, unlike large areas of the low-productivity Southern Ocean.We use naturally occurring radioisotopes of radium to constrain rates of chemical fluxes into Ryder Bay (a small coastal embayment in northern Marguerite Bay), and hence to evaluate possible sources of sediment-derived micronutrients and estimate sediment-ocean mixing rates. We present the first coupled, short-lived radium isotope (223Ra and 224Ra) measurements from Antarctic waters, both present at very low activities (mean 0.155 and 3.21 dpmm-3, respectively), indicating much lower radium inputs than in other coastal environments. Longer-lived 228Ra activity was also lower than existing nearshore values, but higher than open ocean waters, indicating some degree of coastal radium input on timescales exceeding the week-to-month range reflected by 223Ra and 224Ra. Using a simple diffusion model along a shore to mid-bay transect, effective horizontal eddy diffusivity estimates ranged from 0.22–0.83m2 s-1 from 223Ra and 224Ra, respectively, much lower than already-low mixing estimates for the Southern Ocean. Significant radium enrichment and much faster mixing (18m2 s-1) was found near a marine-terminating glacier and consequently any sediment-derived micronutrient inputs in this location are more probably dominated by glacial processes than groundwater, land runoff, or marine sediment sources

Insight into the measurement of dissolved 227Ac in seawater using radium delayed coincidence counter

International audienceDue to the low abundance of 227 Ac in seawater, the analysis of this radionuclide requires the use of high-sensitivity, low-background instruments and the collection of large volume samples. A promising technique relies on the pre-concentration of 227 Ac in seawater using cartridges impregnated with manganese oxide (Mn-cartridges) that are mounted on in situ pumps, and its measurement on a Radium Delayed Coincidence Counter (RaDeCC), usually used to analyze short-lived radium isotopes. In this work, we present an evaluation of this technique, including 1) the study of the performance of the RaDeCC measurements for 227 Ac fixed on Mn-cartridges (backgrounds, detector efficiency, repeatability), and 2) the determination of the efficiency of seawater 227 Ac extraction of the Mn-cartridges and its reproducibility for the first time, by using Mn-cartridges placed in series. Overall, we found a Mn-cartridge extraction efficiency of 47 ± 12% (1 SD). Repeatability experiments allowed us to estimate the uncertainties of the entire measurement of 19% (1 SD). Finally, in the aim to validate the method, the 227 Ac activities thus obtained are compared 1) to the 227 Ac activities determined in several samples using Mn-fibers (assuming 100% yield of 227 Ac extraction) and 2) to the 231 Pa activities determined at the same stations during the GEOVIDE cruise (GEOTRACES GA01), 231 Pa being the parent nuclide of 227 Ac (Deng et al., 2018). Only few studies 227 Ac and 231 Pa have been published so far due to the difficulty to analyze these two nuclides. First, the 227 Ac activities determined using Mn-cartridges agree well with the 227 Ac activities determined using Mn-fibers. Second, at depths where 227 Ac is usually found to be in secular equilibrium with 231 Pa (0-2000 m), we found good agreement between 227 Ac and 231 Pa, which validates the method used to determine 227 Ac activities, including the estimate of the Mn-cartridge extraction efficiency