Ciomadul is the youngest volcano in the Carpathian‐Pannonian Region, Eastern‐Central Europe, which last erupted 30 ka. This volcano is considered to be inactive, however, combined evidence from ...petrologic and magnetotelluric data, as well as seismic tomography studies, suggests the existence of a subvolcanic crystal mush with variable melt content. The volcanic area is characterized by high CO2 gas output rate, with a minimum of 8.7 × 103 t/year. We investigated 31 gas emissions at Ciomadul to constrain the origin of the volatiles. The δ13C–CO2 and 3He/4He compositions suggest the outgassing of a significant component of mantle‐derived fluids. The He isotope signature in the outgassing fluids (up to 3.10 Ra) is lower than the values in the peridotite xenoliths of the nearby alkaline basalt volcanic field (R/Ra 5.95 Ra ± 0.01), which are representative of a continental lithospheric mantle and significantly lower than MORB values. Considering the chemical characteristics of the Ciomadul dacite, including trace element and Sr–Nd and O isotope compositions, an upper crustal contamination is less probable, whereas the primary magmas could have been derived from an enriched mantle source. The low He isotopic ratios could indicate a strongly metasomatized mantle lithosphere. This could be due to infiltration of subduction‐related fluids and postmetasomatic ingrowth of radiogenic He. The metasomatic fluids are inferred to have contained subducted carbonate material resulting in a heavier carbon isotope composition (δ13C is in the range of −1.4‰ to −4.6‰) and an increase of CO2/3He ratio. Our study shows the magmatic contribution to the emitted gases.
Plain Language Summary
Determining the fluxes and composition of gases in active and dormant volcanoes could help to constrain their origin. Ciomadul is the youngest volcano of the Carpathian‐Pannonian Region, Eastern‐Central Europe, where the last eruption occurred 30 ka. Its eruption chronology is punctuated by long quiescence periods (even >100 kyr) separating the active phases; therefore, the long dormancy since the last eruption (30 ka) does not unambiguously indicate inactivity. Knowing if melt‐bearing magma resides in the crust is fundamental to evaluate the nature of the volcano. Isotopic compositions of helium (3He/4He) and carbon (δ13CCO2) are important tools for the study of the origin of the gases. We show that the isotope variation of the emitted gases suggests a metasomatized lithospheric mantle origin for the primary magmas. This is consistent with a degassing deep magma body existing beneath Ciomadul, and this long‐dormant volcano cannot be considered as extinct.
Key Points
CO2 emissions at Ciomadul, Eastern‐Central Europe, suggest a still‐active plumbing system beneath the volcano in spite of long dormancy
The CO2 and He isotope compositions provide evidence for significant contribution of magma‐derived volatiles, up to 80%
Isotopic signatures of gases indicate that primary magmas could have derived from a mantle source modified by subduction‐related fluids
Duvalo “volcano” is a site of anomalous geogenic degassing close to Ohrid (North Macedonia) not related to volcanic activity, despite its name. CO2 flux measurements made with the accumulation ...chamber (321 sites over ∼50,000 m2) showed fluxes up to nearly 60,000 g m−2 d−1, sustaining a total output of ∼67 t d−1. Soil gas samples were taken at 50 cm depth from sites with high CO2 fluxes and analyzed for their chemical and isotope composition. The gas is mainly composed by CO2 (>90%) with significant concentrations of H2S (up to 0.55%) and CH4 (up to 0.32%). The isotope compositions of He (R/RA 0.10) and of CO2 (δ13C ∼ 0‰) exclude significant mantle contribution, while δ13C‐CH4 (∼−35‰) and δ2H‐CH4 (∼−170‰) suggest a thermogenic origin for CH4. The area is characterized by intense seismic activity and Duvalo corresponds to an active tectonic structure bordering the Ohrid graben. The production of H2S within the stratigraphic sequence may be explained by thermochemical reduction of sulfate. The uprising H2S is partially oxidized to sulfuric acid that, reacting with carbonate rocks, releases CO2. The tectonic structure of the area favors fluid circulation, sustaining H2S production and oxidation, CO2 production and allowing the escape of the gases to the atmosphere. In the end, Duvalo represents a tectonic‐related CO2 degassing area whose gases originate mostly, if not exclusively, in the shallowest part of the crust (<10 km). This finding highlights that even systems with trivial mantle contribution may sustain intense CO2 degassing (>1,000 t km−2 d−1).
Plain Language Summary
The carbon cycle is an important piece of the puzzle of the present climate change. While anthropogenic sources of atmospheric carbon are reasonably constrained, geological sources are much less. Among the latter, carbon release to the atmosphere in seismically active areas, though known from decades, is less studied. Here we estimate the total emission of carbon dioxide from Duvalo, an area near Ohrid (North Macedonia). The obtained value (67 metric tons per day) is comparable with some active volcanic areas in the region. Although local inhabitants call this area Duvalo volcano, no recent volcanic activity is recognized here. The composition of the gas released by this system seems also to rule out geothermal activity or deep contributions from the Earth's mantle. This study shows that natural degassing systems with relatively shallow crustal sources (few kilometers), may sustain intense gas emissions from the soil.
Key Points
The total CO2 output from Duvalo “volcano” has been estimated for the first time
Gas compositions rule out significant mantle contributions or recent volcanic activity while geothermal activity is improbable
An active fault system favors indirect gas production and upflow to the Earth's surface
Tritium (3H) has become synonymous with modern groundwater and is used in a myriad of applications, ranging from sustainability investigations to contaminant transport and groundwater vulnerability. ...This study uses measured 3H groundwater activities from 722 sample locations across South Africa to construct a 3H groundwater distribution surface. Environmental covariables are tested using geostatistical analysis to constrain external controls on 3H variability, namely: (a) depth to groundwater, (b) distance from the ocean, and (c) summer versus winter rainfall proportion. The inclusion of covariables in the “fit” of residual variograms improved prediction variance significantly yet does not mitigate issues with sample density. The distribution of 3H in groundwater agrees well to expected controls, with proximal (<100 km) coastal regions, winter rainfall zones, and greater depth to groundwater predicted to have lower 3H activities. Conversely, inland localities with shallower depth to groundwater and/or summer rainfall are predicted to have elevated 3H activities. Some 3H high and low anomalies cannot be explained by known phenomena and may simply be regions of variable recharge and/or longer isolated groundwater flow paths. Regions that receive modern groundwater recharge are more vulnerable to climate change as well as modern pollution. Less actively recharged groundwater may be more resilient to climate change yet represents a potentially nonrenewable resource for ion in South Africa. The application of 3H distributions in the assessment of hydrological resilience is pertinent to effective groundwater management studies.
Plain Language Summary
Scientists, who try to understand the water cycle, use isotopes to track how water moves from rain to rivers and groundwater. In this study, we use one isotope called tritium, which is a heavy and unstable form of hydrogen (3H), to identify rainwater that has reached groundwater in modern times (50–100 years). Modern groundwater is not only a renewable resource, but it is also vulnerable to climate change and modern pollution. We collected 722 tritium samples and used a model to predict how much tritium is in groundwater across the country of South Africa. The model found that coastal areas, that receive winter rainfall and/or have deeper groundwater generally have less tritium than inland, summer rainfall, and/or shallow groundwater areas. This is partially explained by the amount of tritium in the rain that these regions receive or the time it takes for the rain to reach the aquifer. The central Karoo region and north eastern regions of South Africa had the most actively recharged groundwater and that the west coast and northern Karoo had the least. Understanding how much water reaches groundwater helps scientists advise policy makers, who create strategies to use water sustainably and protect it from pollution.
Key Points
722 groundwater 3H samples across South Africa
Lower 3H activity in groundwater with depth, coastal proximity, and winter rainfall region
Higher 3H activity in groundwater with inland localities and summer rainfall regions
Ecosystems that offer carbon sequestration by leaching bicarbonate to groundwater are valuable natural capital. One region that may offer this service is the west coast of South Africa. Over 20 % is ...covered by soil mounds (“heuweltjies”) up to 40 m diameter, 2 m high, inhabited by the southern harvester termite Microhodotermes viator and enriched in soil organic and inorganic carbon and soluble minerals. We aimed to generate radiogenic and stable isotope data for soils and groundwater in a region where these data are absent, to 1) verify the atmosphere-soil-groundwater link, and 2) resolve the timing and pattern of calcite dissolution and water infiltration in the landscape. Results show that soil and groundwater sulfate have the same marine aerosol source. Episodic calcite dissolution in mound centers, which increased during periods of global cooling, has been set against background input of marine aerosols since before the Last Glacial according to radiocarbon (14C) ages. Our data push back soil organic carbon 14C ages of inhabited termite mounds to 13-19 ka (kiloannum, thousand years before present), nest carbonate 14C ages to 33 ka, and mound soil carbonate 14C ages to 34 ka, making these the oldest active termite features ever dated. These ages are consistent with soil organic carbon and carbonate 14C ages of regional, non-mound, coastal petrocalcic horizons formed by accumulation of carbonate leached from their overlying aeolian dune fields. Harvesting activities of termites inject younger organic material around nests >1 m deep, leading to continuous renewal of important soil carbon reservoirs at depth. Termite bioturbation increases the system's ability to dissolve carbonate. The central, bioturbated part of the mounds have greater infiltration depths and greater calcite dissolution, whereas surrounding soils experienced more surface runoff. Calcareous termite mounds offer a mechanism to sequester CO2 through dissolution and leaching of soil carbonate-bicarbonate to groundwater.
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•M. viator mounds and nests are the oldest active termite features ever dated.•Termites inject of younger organic material at depth and invert age-depth profile.•Accumulated aerosols in the mounds leach to groundwater during high rainfall.•Deeper carbonate leaching associated with periods of global cooling.•Carbonate dissolution is a C sequestration mechanism.
► Results of 9
years of systematic precipitation sampling and isotope analysis. ► Extrapolation of the data of the surrounding stations to Debrecen is misleading. ► LMWL for the study site was found ...to be
δ
2H
=
(6.55
±
0.22)
δ
18O–7.74
±
1.97. ► Deuterium excess shows the effects of secondary evaporation better than
δ
18O values. ► The
δ
18O–T function has similar slope than that of other continental stations.
The stable isotopic composition of hydrogen and oxygen of precipitation from Debrecen, Eastern Hungary was analysed in event-based samples collected from the beginning of 2001 to the end of 2009.
During the monitoring period, the
δ
18O values varied between −22.3‰ and 6.64‰ and the
δ
2H values between −176.8‰ and 10.7‰. The LMWL for the monthly based data is close to the GMWL, but shows the effect of secondary evaporation of falling raindrops with lower intercept and slope. LMWL of each year shows highly different parameters due to differences in precipitation amount and summer temperatures, especially in the extreme years of 2002 and 2003. On the basis of our data, deuterium excess is considered to be the best parameter to reveal the extremities of dry and warm periods. Deuterium excess also proved to be a useful tool to show the different formation histories of certain precipitation events. Good correlation of
δ
18O with temperature was obtained for the samples. The slope of the
δ
18O–T functions for the whole sampling period was 0.32
±
0.03‰/°C for the monthly samples, however, a slope of 0.37
±
0.03‰/°C was obtained if monthly mean temperatures were replaced with the monthly mean temperatures of the rainy days. Considering the temperature dependency of the
δ
18O values in the past, it can be concluded that Δ
δ
18O/ΔT relationship using monthly mean temperatures of the rainy days might be a better approach than monthly mean temperatures.
Namaqualand, South Africa, is a global biodiversity hotspot but local populations are affected by challenging economic conditions largely because of poor access to water. In this study groundwater ...types are characterised and sources of salts and salinisation processes are identified using hydrochemistry and δ18O, δ2H and 87Sr/86Sr data. Analysis of δ18O and δ2H data suggests that evaporation does not play a major role in salinisation of the groundwater. However, major ion chemistry and 87Sr/86Sr ratios indicate that salts present in the groundwater are linked to dry deposition of marine aerosols and ion-exchange reactions in soils in the alluvial aquifer systems. The hydrochemical variability of the groundwater in the basement aquifer system suggests that there are strong local controls linked to weathering processes in individual basement rock types. The region is also notable for the high density of heuweltjies, biophysical features associated with increased nutrient levels, associated with termite activity. Electromagnetic scanning as well as measurement of water-soluble soil electrical conductivity values on and off heuweltjies, show that heuweltjies are saline with salinity increasing with depth. The level of groundwater salinity correlates with the level of heuweltjie salinity. Precipitation records from the last 150 years provide support for the hypothesis that accumulated salts, and in particular, heuweltjie salts are flushed into the groundwater system during sporadic large volume precipitation events. Thus, heuweltjies and hence termite activity, could potentially represent a previously unrecognized contributor to groundwater salinisation across Namaqualand and in other parts of the world.
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•Evaporation does not appear to be a significant contributor to groundwater salinisation.•Salts are derived from marine aerosols, ion-exchange reactions and weathering processes.•Salts are concentrated in heuweltjies, palaeo-biophysical features, common in Namaqualand.•Heuweltjies may thus be an important unrecognized source of groundwater salts.•Flushing of heuweltjie salts likely controlled by incidence of high volume precipitation events.
Tritium, the radioactive isotope of hydrogen, has been used to understand groundwater recharge processes for decades. The current variation of tritium in the atmosphere is largely attributed to ...stratospheric production and fall out rates as well as global circulation phenomena controlling the hydrological cycle. Global controls on the variability in atmospheric tritium activity are poorly suited to explain local variation and tritium activities in precipitation are often assumed to be uniform over both local and regional catchments and watersheds. This assumption can result in both over and under estimation of modern recharge within an aquifer when using tritium as the recharge proxy. In order to minimize the inherent prediction residuals associated with tritium based recharge investigations, the variability of tritium in precipitation was modelled from 127 spatial precipitation samples taken over a two year period, combined with a 76 precipitation sample group-set taken over a one year period in a single location. Precipitation events were traced backward in time, from the point of collection, using HYSPLIT modelling to ascertain the origins of moisture content as well as the altitudes of moisture origin reached along the particle track. Tritium activities, collected over a one year period in Paarl, range from 0.45 to 4.16 TU and have a mean of 1.59 TU. Spatial storm events in the Western Cape in 2017 and 2018 had a range from 0 to 2.2 and 0.37 to 3.27 TU, respectively, with mean activities of 1.18 (n = 34) and 1.25 TU (n = 32). Both storm events had similar tritium variability (σ = 0.5 n = 35 and 0.48 n = 32). Regional precipitation events had the largest range of tritium activities (0.55–12.2 TU). Although not all tritium activities can be explained by interrogating the water mass origin, this study suggests that approximately 90% of events can be completely or partially attributed to the origin of the water mass. The variability of tritium, both spatially and temporally, was higher than expected, confirming that when uniform tritium inputs are used, the groundwater system would provide inaccurate modern recharge estimates. Higher spatial resolution of tritium variation in precipitation for a particular region will improve our ability to relate tritium activities in groundwater to local precipitation.
•Tritium in precipitation measured daily in precipitation for 1 year (n = 76).•Variability of tritium in annual rainfall: 0.45–4.16 TU.•HYSPLIT method explains about 90% of tritium variability through water mass origin evaluation.•High latitude and low altitude Southern Ocean sources of winter rainfall bring lower tritium activity.•Low latitude and high altitude sources in summer rainfall bring higher tritium.
Tritium, radiocarbon and radiocesium concentrations in water column samples in coastal waters offshore Fukushima and in the western North Pacific Ocean collected in 2011–2012 during the ...Ka'imikai-o-Kanaloa (KoK) cruise are compared with other published results. The highest levels in surface seawater were observed for 134Cs and 137Cs in seawater samples collected offshore Fukushima (up to 1.1 Bq L−1), which represent an increase by about three orders of magnitude when compared with the pre-Fukushima concentration. Tritium levels were much lower (up to 0.15 Bq L−1), representing an increase by about a factor of 6. The impact on the radiocarbon distribution was measurable, but the observed levels were only by about 9% above the global fallout background. The 137Cs (and similarly 134Cs) inventory in the water column of the investigated western North Pacific region was (2.7 ± 0.4) PBq, while for 3H it was only (0.3 ± 0.2) PBq. Direct releases of highly contaminated water from the damaged Fukushima NPP, as well as dry and wet depositions of these radionuclides over the western North Pacific considerably changed their distribution patterns in seawater. Presently we can distinguish Fukushima labeled waters from global fallout background thanks to short-lived 134Cs. However, in the long-term perspective when 134Cs will decay, new distribution patterns of 3H, 14C and 137Cs in the Pacific Ocean should be established for future oceanographic and climate change studies in the Pacific Ocean.
•Radiocesium, tritium and radiocarbon showed elevated levels in seawater of the western North Pacific.•The water column inventories of 137Cs and 3H were 2.7 ± 0.4 and 0.3 ± 0.2 PBq, respectively.•The radiocarbon levels were by about 9% above the global fallout background.•Released radionuclides will be useful in future oceanographic and climate studies.
Despite being the busiest transient sea in the world due to the Suez Canal, radionuclide distribution studies in seawater and sediment of the Red Sea remain rare. A sampling expedition in the Red Sea ...was conducted from June 9 to July 6, 2021, visiting a transect of several deep sampling stations located along the central axis of the basin from the Gulf of Aqaba to the southern Red Sea (near Farasan Island, Saudi Arabia). The collected seawater profile samples were analyzed for tritium, radiocarbon and oxygen-18. The observed tritium levels in surface waters of the Red Sea peaked at 0.3–0.4 TU, similar to the values observed in the western Arabian Sea (decay corrected). The values observed at waters below 150 m were around 0.2 TU, however, at depths of 450 and 750 m, tritium minima (<0.2 TU) were observed, which could be associated with a partial return flow of bottom waters from the southern to the northern Red Sea. At two stations at the depth of about 550 m, deep Δ14C minima were observed as well (−4‰ and −10‰), documenting ongoing transport of carbon in the water column, important for sink of anthropogenic carbon.
•Observed 3H and 14C water profiles in the Red Sea suggest a common origin of these tracers in the western Arabian Sea.•The vertical distribution of 3H and 14C indicates strong vertical mixing in the Gulf of Aqaba.•The distribution of isotope tracers in the Red Sea reflects different water masses and their circulation.•The results indicate the presence of a returning northward flow of Red Sea bottom waters at a depth of 450–750 m.
Abrupt and large‐scale climate changes have occurred repeatedly and within decades during the last glaciation. These events, where dramatic warming occurs over decades, are well represented in both ...Greenland ice core mineral dust and temperature records, suggesting a causal link. However, the feedbacks between atmospheric dust and climate change during these Dansgaard–Oeschger events are poorly known and the processes driving changes in atmospheric dust emission and transport remain elusive. Constraining dust provenance is key to resolving these gaps. Here, we present a multi‐technique analysis of Greenland dust provenance using novel and established, source diagnostic isotopic tracers as well as results from a regional climate model including dust cycle simulations. We show that the existing dominant model for the provenance of Greenland dust as sourced from combined East Asian dust and Pacific volcanics is not supported. Rather, our clay mineralogical and Hf–Sr–Nd and D/H isotopic analyses from last glacial Greenland dust and an extensive range of Northern Hemisphere potential dust sources reveal three most likely scenarios (in order of probability): direct dust sourcing from the Taklimakan Desert in western China, direct sourcing from European glacial sources, or a mix of dust originating from Europe and North Africa. Furthermore, our regional climate modeling demonstrates the plausibility of European or mixed European/North African sources for the first time. We suggest that the origin of dust to Greenland is potentially more complex than previously recognized, demonstrating more uncertainty in our understanding dust climate feedbacks during abrupt events than previously understood.
Plain Language Summary
Abrupt climate change represents an existential threat to civilization. However, the feedbacks that modulate these abrupt changes are poorly understood, undermining our ability to predict future events. Last glacial Greenland ice core records show abrupt climate events coupled to changes in abundance of atmospheric mineral dust, but how dust impacts these events is unclear as the processes involved in dust emission remain elusive. Here we apply multiple novel tracers of dust provenance as well as regional dust cycle modeling to address this uncertainty. We show that the dominant model of mixed East Asian and Pacific volcanic sources to Greenland dust is not supported. Instead, multiple other source scenarios are plausible, demonstrating far more uncertainty in dust climate feedbacks than previously understood.
Key Points
Hf–Nd isotope data demonstrate that Central Greenland last glacial dust is not sourced from mixed SE Asian and circum‐Pacific volcanic dust
Most likely sources are the Taklimakan in SE Asia, European and/or North African glacial dust deposits based on Hf–Sr–Nd and D/H isotopes
Model simulations show that sourcing of North Greenland Ice Core Project ice core dust from Europe is plausible, the addition of North African dust cannot be excluded
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