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The implementation of climate change mitigation strategies based on the conservation and restoration of Blue Carbon ecosystems requires a deep understanding of the magnitude and variability in organic carbon (Corg) storage across and within these ecosystems. This study explored the variability in soil Corg stocks and burial rates across and within intertidal estuarine habitats of the Atlantic European coast and its relation to biotic and abiotic drivers. A total of 136 soil cores were collected across saltmarshes located at different tidal zones (high marsh, N = 45; low marsh, N = 30), seagrass meadows (N = 17) and tidal flats (N = 44), and from the inner to the outer sections of five estuaries characterized by different basin land uses. Soil Corg stocks were higher in high-marsh communities (65 ± 3 Mg ha−1) than in low-marsh communities (38 ± 3 Mg ha−1), seagrass meadows (40 ± 5 Mg ha−1) and unvegetated tidal flats (46 ± 3 Mg ha−1) whereas Corg burial rates also tended to be higher in high marshes (62 ± 13 g m−2 y−1) compared to low marshes (43 ± 15 g m−2 y−1) and tidal flats (35 ± 9 g m−2 y−1). Soil Corg stocks and burial rates decreased from inner to outer estuarine sections in most estuaries reflecting the decrease in the river influence towards the estuary mouth. Higher soil Corg stocks were related to higher content of silt and clay and higher proportion of forest and natural land within the river basin, pointing at new opportunities for protecting coastal natural carbon sinks based on the conservation and restoration of upland ecosystems. Our study contributes to the global inventory of Blue Carbon by adding data from unexplored regions and habitats in Europe, and by identifying drivers of variability across and within estuaries.

223Ra, 224Ra, 226Ra, and 228Ra isotopes have been measured in groundwaters from depths ranging 50–900 m in fractured crystalline bedrock (Forsmark, Sweden) to understand the reason for elevated (up to 150 μg/L) aqueous uranium (Uaq) at 400–650 m depth. Ra isotope data is interpreted alongside previously reported 222Rn, 234U, and 238U data, as well as PHREEQC geochemical modelling and uranium mineralogy. A novel, [223Ra/226Ra]GW-based approach (where brackets and “GW” subscript refer to expression of an activity ratio measured from groundwater) to groundwater residence time estimation shows that elevated [Uaq] is most common in Holocene-age groundwaters of marine origin. Although these groundwaters are geochemically reducing, the [223Ra/228Ra]corr (where “corr” subscript refers to a correction applied to compare [223Ra/228Ra]GW to the more commonly reported [226Ra/228Ra]GW) suggest that they interact with U-rich pegmatites containing Proterozoic- and Palaeozoic-age Ca-U(VI)-silicate minerals, which are undersaturated in the present groundwaters. Local aqueous U(VI) can be stabilized in Ca2UO2CO30 complexes at pe-values as low as −4.5 but is susceptible to reduction after a modest decrease in pe-value, alkalinity, or Ca concentration. The [223Ra/228Ra]corr and [224Ra/228Ra]GW also suggest that U(VI)aq precipitates as UO2+X at the interface betwee nmarine and non-marine groundwaters. From these data, local [Uaq] is proposed to be governed by on-going water-rock interaction involving old U(VI)-minerals.

223Ra, 224Ra, 226Ra, and 228Ra isotopes have been measured in groundwaters from depths ranging 50–900 m in fractured crystalline bedrock (Forsmark, Sweden) to understand the reason for elevated (up to 150 μg/L) aqueous uranium (Uaq) at 400–650 m depth. Ra isotope data is interpreted alongside previously reported 222Rn, 234U, and 238U data, as well as PHREEQC geochemical modelling and uranium mineralogy. A novel, [223Ra/226Ra]GW-based approach (where brackets and “GW” subscript refer to expression of an activity ratio measured from groundwater) to groundwater residence time estimation shows that elevated [Uaq] is most common in Holocene-age groundwaters of marine origin. Although these groundwaters are geochemically reducing, the [223Ra/228Ra]corr (where “corr” subscript refers to a correction applied to compare [223Ra/228Ra]GW to the more commonly reported [226Ra/228Ra]GW) suggest that they interact with U-rich pegmatites containing Proterozoic- and Palaeozoic-age Ca-U(VI)-silicate minerals, which are undersaturated in the present groundwaters. Local aqueous U(VI) can be stabilized in Ca2UO2CO30 complexes at pe-values as low as −4.5 but is susceptible to reduction after a modest decrease in pe-value, alkalinity, or Ca concentration. The [223Ra/228Ra]corr and [224Ra/228Ra]GW also suggest that U(VI)aq precipitates as UO2+X at the interface betwee nmarine and non-marine groundwaters. From these data, local [Uaq] is proposed to be governed by on-going water-rock interaction involving old U(VI)-minerals.

Continental shelves and shelf seas play a central role in the global carbon cycle. However, their importance with respect to trace element and isotope (TEI) inputs to ocean basins is less well understood. Here, we present major findings on shelf TEI biogeochemistry from the GEOTRACES programme as well as a proof of concept for a new method to estimate shelf TEI fluxes. The case studies focus on advances in our understanding of TEI cycling in the Arctic, transformations within a major river estuary (Amazon), shelf sediment micronutrient fluxes and basin-scale estimates of submarine groundwater discharge. The proposed shelf flux tracer is 228-radium (T1/2 =5.75 yr), which is continuously supplied to the shelf from coastal aquifers, sediment porewater exchange and rivers. Model-derived shelf 228Ra fluxes are combined with TEI/ 228Ra ratios to quantify ocean TEI fluxes from the western North Atlantic margin. The results from this new approach agree well with previous estimates for shelf Co, Fe, Mn and Zn inputs and exceed published estimates of atmospheric deposition by factors of approximately 3–23. Lastly, recommendations are made for additional GEOTRACES process studies and coastal margin-focused section cruises that will help refine the model and provide better insight on the mechanisms driving shelf-derived TEI fluxes to the ocean. This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.

Continental shelves and shelf seas play a central role in the global carbon cycle. However, their importance with respect to trace element and isotope (TEI) inputs to ocean basins is less well understood. Here, we present major findings on shelf TEI biogeochemistry from the GEOTRACES programme as well as a proof of concept for a new method to estimate shelf TEI fluxes. The case studies focus on advances in our understanding of TEI cycling in the Arctic, transformations within a major river estuary (Amazon), shelf sediment micronutrient fluxes and basin-scale estimates of submarine groundwater discharge. The proposed shelf flux tracer is 228-radium (T1/2 =5.75 yr), which is continuously supplied to the shelf from coastal aquifers, sediment porewater exchange and rivers. Model-derived shelf 228Ra fluxes are combined with TEI/ 228Ra ratios to quantify ocean TEI fluxes from the western North Atlantic margin. The results from this new approach agree well with previous estimates for shelf Co, Fe, Mn and Zn inputs and exceed published estimates of atmospheric deposition by factors of approximately 3–23. Lastly, recommendations are made for additional GEOTRACES process studies and coastal margin-focused section cruises that will help refine the model and provide better insight on the mechanisms driving shelf-derived TEI fluxes to the ocean. This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.

Seagrass meadows are sites of high rates of carbon sequestration and they potentially support 'blue carbon' strategies to mitigate anthropogenic CO2 emissions. Current uncertainties on the fate of carbon stocks following the loss or revegetation of seagrass meadows prevent the deployment of 'blue carbon' strategies. Here, we reconstruct the trajectories of carbon stocks associated with one of the longest monitored seagrass restoration projects globally. We demonstrate that sediment carbon stocks erode following seagrass loss and that revegetation projects effectively restore seagrass carbon sequestration capacity. We combine carbon chronosequences with 210Pb dating of seagrass sediments in a meadow that experienced losses until the end of 1980s and subsequent serial revegetation efforts. Inventories of excess 210Pb in seagrass sediments revealed that its accumulation, and thus sediments, coincided with the presence of seagrass vegetation. They also showed that the upper sediments eroded in areas that remained devoid of vegetation after seagrass loss. Seagrass revegetation enhanced autochthonous and allochthonous carbon deposition and burial. Carbon burial rates increased with the age of the restored sites, and 18 years after planting, they were similar to that in continuously vegetated meadows (26.4 ± 0.8 gCorg m-2 year-1). Synthesis. The results presented here demonstrate that loss of seagrass triggers the erosion of historic carbon deposits and that revegetation effectively restores seagrass carbon sequestration capacity. Thus, conservation and restoration of seagrass meadows are effective strategies for climate change mitigation.

Seagrass meadows are sites of high rates of carbon sequestration and they potentially support 'blue carbon' strategies to mitigate anthropogenic CO2 emissions. Current uncertainties on the fate of carbon stocks following the loss or revegetation of seagrass meadows prevent the deployment of 'blue carbon' strategies. Here, we reconstruct the trajectories of carbon stocks associated with one of the longest monitored seagrass restoration projects globally. We demonstrate that sediment carbon stocks erode following seagrass loss and that revegetation projects effectively restore seagrass carbon sequestration capacity. We combine carbon chronosequences with 210Pb dating of seagrass sediments in a meadow that experienced losses until the end of 1980s and subsequent serial revegetation efforts. Inventories of excess 210Pb in seagrass sediments revealed that its accumulation, and thus sediments, coincided with the presence of seagrass vegetation. They also showed that the upper sediments eroded in areas that remained devoid of vegetation after seagrass loss. Seagrass revegetation enhanced autochthonous and allochthonous carbon deposition and burial. Carbon burial rates increased with the age of the restored sites, and 18 years after planting, they were similar to that in continuously vegetated meadows (26.4 ± 0.8 gCorg m-2 year-1). Synthesis. The results presented here demonstrate that loss of seagrass triggers the erosion of historic carbon deposits and that revegetation effectively restores seagrass carbon sequestration capacity. Thus, conservation and restoration of seagrass meadows are effective strategies for climate change mitigation.

Climate conditions in the westernmost Mediterranean (Alboran Sea basin) over the last two millennia have been reconstructed through integration of molecular proxies applied for the first time in this region at such high resolution. Two temperature proxies, one based on isoprenoid membrane lipids of marine Thaumarchaeota (View the MathML source-tetraether index of compounds consisting of 86 carbons) and the other on alkenones produced by haptophytes (View the MathML source ratio) were applied to reconstruct sea surface temperature (SST). Both records reveal a progressive long term decline in SST over the last two millennia and an increased rate of warming during the second half of the 20th century. This is in accord with previous temperature reconstructions for the Northern Hemisphere. View the MathML source temperature values are higher than those inferred from View the MathML source, probably due to differences in the bloom season of haptophytes and Thaumarchaeota, and reflect summer SST. The branched vs. isoprenoid tetraether index (BIT index) suggests a low contribution of soil organic matter (OM) to the sedimentary OM. The stable carbon isotopic composition of long chain n-alkanes indicates a predominant C3 plant contribution, with no major change in vegetation over the last 2000 yr. The distribution of long chain 1,14-diols (most likely sourced by Proboscia spp. in this setting) provides insight into variation in upwelling conditions during the last 2000 yr and depicts a correlation with the North Atlantic Oscillation (NAO) index, providing evidence of enhanced wind induced upwelling during periods of a persistent positive mode of the NAO.

Climate conditions in the westernmost Mediterranean (Alboran Sea basin) over the last two millennia have been reconstructed through integration of molecular proxies applied for the first time in this region at such high resolution. Two temperature proxies, one based on isoprenoid membrane lipids of marine Thaumarchaeota (View the MathML source-tetraether index of compounds consisting of 86 carbons) and the other on alkenones produced by haptophytes (View the MathML source ratio) were applied to reconstruct sea surface temperature (SST). Both records reveal a progressive long term decline in SST over the last two millennia and an increased rate of warming during the second half of the 20th century. This is in accord with previous temperature reconstructions for the Northern Hemisphere. View the MathML source temperature values are higher than those inferred from View the MathML source, probably due to differences in the bloom season of haptophytes and Thaumarchaeota, and reflect summer SST. The branched vs. isoprenoid tetraether index (BIT index) suggests a low contribution of soil organic matter (OM) to the sedimentary OM. The stable carbon isotopic composition of long chain n-alkanes indicates a predominant C3 plant contribution, with no major change in vegetation over the last 2000 yr. The distribution of long chain 1,14-diols (most likely sourced by Proboscia spp. in this setting) provides insight into variation in upwelling conditions during the last 2000 yr and depicts a correlation with the North Atlantic Oscillation (NAO) index, providing evidence of enhanced wind induced upwelling during periods of a persistent positive mode of the NAO.

ID: 11526 PosterID: A0020 We are focused on determining the total concentrations of the anthropogenic radionuclides 239Pu, 240Pu, 237Np, and 137Cs in depth profiles from 2010 Atlantic GEOTRACES sections. Data from the equatorial region (A11) compared to GEOSECS data indicate water column inventories of 137Cs have increased over the last 40 years, while those of Pu are variable. By comparison, Pu and 237Np water column inventories are similar to regional soil core inventories, whereas 137Cs inventories are significantly higher, further suggesting continued supply of 137Cs to the open ocean. Water column 240Pu/239Pu inventory ratios are indicative of global fallout (~0.18). Deviations of the water column 237Np/239Pu inventory ratio from the average global fallout value (~0.48) can be used to estimate Pu-particulate fluxes, which are comparable to sediment trap data and may be used to assess scavenging at different locations along the cruise track. Data from mid and high latitude regions (A02) will be compare them to GEOSECS data; inventories and ratios will be used to identify additional contaminant sources, as water mass tracers, and to elucidate important processes such as scavenging and remineralization.

The natural cosmogenic radionuclide 7Be (T1/2 = 53.4 d) is supplied to the surface ocean from the atmosphere and, in the Arctic Ocean, can be used as a tracer of the efficiency with which sea ice intercepts the atmospheric fluxes of chemical species and of the importance of ice as a transport mechanism for particulate matter and chemical species. Analyses of 7Be in samples of surface water, surface sea ice, water beneath the ice, sea ice sediments, and precipitation from the Eurasian Basin of the Arctic Ocean show that the fraction of sea ice coverage determines the amount of 7Be in the surface water. When sea ice coverage is <40%, the 7Be inventory in the upper ocean (130 ± 19 Bq m−2) is in good agreement with that expected from the inventory from 7Be atmospheric flux (128 ± 21 Bq m−2). In contrast, when ice coverage is >80%, the water column inventory drops to 58 ± 20 Bq m−2. The 7Be inventory in sea ice is 39 ± 23 Bq m−2, and mass balance calculations show that sea ice can intercept 30 ± 18% of the atmospheric flux of 7Be during the studied period. We suggest that other atmospherically transported contaminants should be similarly intercepted. 7Be in the ice also can be used to estimate that the annual transport and release of sediment to the ablation area of the Fram Strait is ∼500 g m−2, a value comparable to previously measured fluxes in sediment traps deployed in the area.

The Arctic Ocean is characterized by being covered by sea ice with a large degree of seasonal variability between summer and winter. Along the whole life cycle of sea ice, diverse physical and chemical processes determine the concentration of the sea-ice sediments (SIS) and the chemical species entrapped in it and their final fate. Initially, sea ice incorporates particles (SIS) and associated chemical species (metals, nutrients, contaminants, etc.) during its formation mainly in the continental shelves, while dissolved solutes are excluded. As sea ice drifts offshore to the central Arctic Basin, it intercepts chemical species from the atmosphere and, the sediments in the ice may also incorporate some chemical solute compounds from the surface waters by scavenging (although this is likely small). Eventually, transported components, chemical species and SIS, are released to the underlying water column during melting. Thus, sea ice becomes an important transpo rt and distribution agent. However, the efficiency of interception of atmospheric fluxes by sea ice, the origin of the entrapped SIS and transit times of sea ice in the Arctic, as well as the importance of the transport of chemical species and particulate matter (SIS) and its release in the ablation area are all poorly understood. In an attempt to address these questions, a suite of natural (7Be and 210Po-210Pb) and artificial (137Cs, 239,240Pu) radionuclides were analysed in samples from precipitation, sea ice, surface water, water beneath ice and sea-ice sediments collected during the ARK XXII/2 expedition in 2007. The distributions of 7Be and 210Pb showed enrichment in sea ice (129 ± 90 and 5.1 ± 2.9 Bq·m-3, respectively) with respect to surface water (7.1 ± 1.3 and 1.1 ± 0.36 Bq·m-3, respectively). Given that only 4% of the total amount of 210Pb in sea ice comes from seawater and that any 7Be (T1/2 = 53 days) trapped in sea ice during its formation has decayed during drift, the direct atmospheric flux appear

The Arctic Ocean is characterized by being covered by sea ice with a large degree of seasonal variability between summer and winter. Along the whole life cycle of sea ice, diverse physical and chemical processes determine the concentration of the sea-ice sediments (SIS) and the chemical species entrapped in it and their final fate. Initially, sea ice incorporates particles (SIS) and associated chemical species (metals, nutrients, contaminants, etc.) during its formation mainly in the continental shelves, while dissolved solutes are excluded. As sea ice drifts offshore to the central Arctic Basin, it intercepts chemical species from the atmosphere and, the sediments in the ice may also incorporate some chemical solute compounds from the surface waters by scavenging (although this is likely small). Eventually, transported components, chemical species and SIS, are released to the underlying water column during melting. Thus, sea ice becomes an important transpo rt and distribution agent. However, the efficiency of interception of atmospheric fluxes by sea ice, the origin of the entrapped SIS and transit times of sea ice in the Arctic, as well as the importance of the transport of chemical species and particulate matter (SIS) and its release in the ablation area are all poorly understood. In an attempt to address these questions, a suite of natural (7Be and 210Po-210Pb) and artificial (137Cs, 239,240Pu) radionuclides were analysed in samples from precipitation, sea ice, surface water, water beneath ice and sea-ice sediments collected during the ARK XXII/2 expedition in 2007. The distributions of 7Be and 210Pb showed enrichment in sea ice (129 ± 90 and 5.1 ± 2.9 Bq·m-3, respectively) with respect to surface water (7.1 ± 1.3 and 1.1 ± 0.36 Bq·m-3, respectively). Given that only 4% of the total amount of 210Pb in sea ice comes from seawater and that any 7Be (T1/2 = 53 days) trapped in sea ice during its formation has decayed during drift, the direct atmospheric flux appear

The natural cosmogenic radionuclide 7Be (T1/2 = 53.4 d) is supplied to the surface ocean from the atmosphere and, in the Arctic Ocean, can be used as a tracer of the efficiency with which sea ice intercepts the atmospheric fluxes of chemical species and of the importance of ice as a transport mechanism for particulate matter and chemical species. Analyses of 7Be in samples of surface water, surface sea ice, water beneath the ice, sea ice sediments, and precipitation from the Eurasian Basin of the Arctic Ocean show that the fraction of sea ice coverage determines the amount of 7Be in the surface water. When sea ice coverage is <40%, the 7Be inventory in the upper ocean (130 ± 19 Bq m−2) is in good agreement with that expected from the inventory from 7Be atmospheric flux (128 ± 21 Bq m−2). In contrast, when ice coverage is >80%, the water column inventory drops to 58 ± 20 Bq m−2. The 7Be inventory in sea ice is 39 ± 23 Bq m−2, and mass balance calculations show that sea ice can intercept 30 ± 18% of the atmospheric flux of 7Be during the studied period. We suggest that other atmospherically transported contaminants should be similarly intercepted. 7Be in the ice also can be used to estimate that the annual transport and release of sediment to the ablation area of the Fram Strait is ∼500 g m−2, a value comparable to previously measured fluxes in sediment traps deployed in the area.

From its formation until melting, sea ice is subject to diverse chemical, physical and structural processes that affect its properties and the role that it plays in the Arctic Ocean. During sea ice formation, it incorporates particles and their associated chemical species (metals, nutrients, contaminants, etc.) but excludes chemical dissolved solutes. In the open Arctic Ocean, sea ice intercepts chemical species from the atmosphere and, presumably in a minor proportion, may also incorporate some chemical solute compounds from the surface waters that can be scavenged by sea-ice sediments. Thus, sea ice drift becomes an important and relatively rapid agent for the transport and distribution of particulate matter and chemical compounds in the Arctic Ocean. Eventually, these materials are released into the underlying water column during melting. This discharge occurs on a relatively short timescale, enhancing sedimentation rates and biological productivity in ice ablation areas. Pb-210 and Be-7 are natural particle-reactive radionuclides with half-lives of 22.3 y and 54 d, respectively, and can be used as tracers of sea-ice processes. A first aim of this work is to estimate the sediment load of sea ice and the associated sediment flux to the water column in the Fram Strait by melting. Through mass balance of the distribution of both Pb-210 and Be-7, considering their atmospheric inputs, interception of sea-ice and inventories in the ocean and ice, the load of sea-ice sediments would range from 5 to 100 g/m2. This would lead to an annual release of sediments in the Fram Strait of 10 to 240 g/m2·y. These estimates are consistent with sediment fluxes estimated by using sediments trap deployed in the area, as well as with direct observations of presence of sediments in sea ice. A second result derived from the study of the distribution of Be-7 in the Arctic Ocean is the efficiency of sea ice in intercepting and accumulating the atmospheric inputs of chemical species such as

From its formation until melting, sea ice is subject to diverse chemical, physical and structural processes that affect its properties and the role that it plays in the Arctic Ocean. During sea ice formation, it incorporates particles and their associated chemical species (metals, nutrients, contaminants, etc.) but excludes chemical dissolved solutes. In the open Arctic Ocean, sea ice intercepts chemical species from the atmosphere and, presumably in a minor proportion, may also incorporate some chemical solute compounds from the surface waters that can be scavenged by sea-ice sediments. Thus, sea ice drift becomes an important and relatively rapid agent for the transport and distribution of particulate matter and chemical compounds in the Arctic Ocean. Eventually, these materials are released into the underlying water column during melting. This discharge occurs on a relatively short timescale, enhancing sedimentation rates and biological productivity in ice ablation areas. Pb-210 and Be-7 are natural particle-reactive radionuclides with half-lives of 22.3 y and 54 d, respectively, and can be used as tracers of sea-ice processes. A first aim of this work is to estimate the sediment load of sea ice and the associated sediment flux to the water column in the Fram Strait by melting. Through mass balance of the distribution of both Pb-210 and Be-7, considering their atmospheric inputs, interception of sea-ice and inventories in the ocean and ice, the load of sea-ice sediments would range from 5 to 100 g/m2. This would lead to an annual release of sediments in the Fram Strait of 10 to 240 g/m2·y. These estimates are consistent with sediment fluxes estimated by using sediments trap deployed in the area, as well as with direct observations of presence of sediments in sea ice. A second result derived from the study of the distribution of Be-7 in the Arctic Ocean is the efficiency of sea ice in intercepting and accumulating the atmospheric inputs of chemical species such as

This article reports the results of a study of submarine groundwater discharge (SGD) to coastal waters of Majorca (NW Mediterranean). The overall aim is to evaluate the relevance of SGD of the island and chemically characterize the components that are supplied to the coastal waters through this pathway. Although other discharge areas are identified, we particularly focus on SGD in bays and areas of increased sea water residence time where effects of the discharges are expected to be most notable. Analysis at four selected embayments with different land-use characteristics indicated a link between human activities (mainly agriculture and urban) and compounds arriving to the coast. A pathway for these elements is the diffuse discharge along the shoreline, as suggested by the inverse relationship between salinity and nutrients in nearshore porewaters. A general survey was conducted at 46 sites around the island, and used dissolved radium as a qualitative indicator of SGD. Measurements of nutrients (P and N), pCO2 and TOC were performed to characterize the elements delivered to the coastal environment. Most nearshore samples showed 224Ra enrichment (mean ± SE, 7.0 ± 0.6 dpm 100 l−1) with respect to offshore waters (1.1 ± 0.2 dpm 100 l−1); however, 224Ra measurements along the coast were highly variable (1.0–38.1 dpm 100 l−1). Coastal samples with enhanced radium levels showed elevated pCO2 with respect to atmospheric concentrations, which together with high pCO2 in groundwater (>5,000 ppm) indicates that SGD is an important vector of CO2 to coastal waters. Moreover, a relationship between 224Ra and phytoplankton biomass was established, suggesting an important impact of SGD on coastal productivity. The results presented here provide a first approximation of the SGD effect in the coastal waters of Majorca, and indicate that SGD could be an important source of nutrients and CO2 to the coast, strongly influencing the productivity and biogeochemical cycling of the coastal w

Ra isotopes are a powerful tool for quantifying the flux of submarine groundwater discharge (SGD) into the sea. Previous studies of 223Ra and 224Ra mass balances in coastal embayments have shown that the Ra balance is dominated by supply via SGD, exchange with the open ocean and radioactive decay. The current study shows that a single time series over a tidal cycle at the principal inlet to Great South Bay (NY, US) is sufficient to determine the net flux of Ra across the inlet, and also can be used to estimate the decay of short-lived Ra in the bay. Estimates of the net Ra flux obtained from a single tidal time-series by using three different approaches agree with those determined from a more time-consuming survey of Ra within the bay, and may represent a first step of estimating SGD in bays and coastal lagoons.

Ra isotopes are a powerful tool for quantifying the flux of submarine groundwater discharge (SGD) into the sea. Previous studies of 223Ra and 224Ra mass balances in coastal embayments have shown that the Ra balance is dominated by supply via SGD, exchange with the open ocean and radioactive decay. The current study shows that a single time series over a tidal cycle at the principal inlet to Great South Bay (NY, US) is sufficient to determine the net flux of Ra across the inlet, and also can be used to estimate the decay of short-lived Ra in the bay. Estimates of the net Ra flux obtained from a single tidal time-series by using three different approaches agree with those determined from a more time-consuming survey of Ra within the bay, and may represent a first step of estimating SGD in bays and coastal lagoons.

This article reports the results of a study of submarine groundwater discharge (SGD) to coastal waters of Majorca (NW Mediterranean). The overall aim is to evaluate the relevance of SGD of the island and chemically characterize the components that are supplied to the coastal waters through this pathway. Although other discharge areas are identified, we particularly focus on SGD in bays and areas of increased sea water residence time where effects of the discharges are expected to be most notable. Analysis at four selected embayments with different land-use characteristics indicated a link between human activities (mainly agriculture and urban) and compounds arriving to the coast. A pathway for these elements is the diffuse discharge along the shoreline, as suggested by the inverse relationship between salinity and nutrients in nearshore porewaters. A general survey was conducted at 46 sites around the island, and used dissolved radium as a qualitative indicator of SGD. Measurements of nutrients (P and N), pCO2 and TOC were performed to characterize the elements delivered to the coastal environment. Most nearshore samples showed 224Ra enrichment (mean ± SE, 7.0 ± 0.6 dpm 100 l−1) with respect to offshore waters (1.1 ± 0.2 dpm 100 l−1); however, 224Ra measurements along the coast were highly variable (1.0–38.1 dpm 100 l−1). Coastal samples with enhanced radium levels showed elevated pCO2 with respect to atmospheric concentrations, which together with high pCO2 in groundwater (>5,000 ppm) indicates that SGD is an important vector of CO2 to coastal waters. Moreover, a relationship between 224Ra and phytoplankton biomass was established, suggesting an important impact of SGD on coastal productivity. The results presented here provide a first approximation of the SGD effect in the coastal waters of Majorca, and indicate that SGD could be an important source of nutrients and CO2 to the coast, strongly influencing the productivity and biogeochemical cycling of the coastal w

Artificial radionuclides enter the Mediterranean Sea mainly through atmospheric deposition following nuclear weapons tests and the Chernobyl accident, but also through the river discharge of nuclear facility effluents. Previous studies of artificial radionuclides impact of the Mediterranean Sea have focussed on shallow, coastal sediments. However, deep sea sediments have the potential to store and accumulate pollutants, including artificial radionuclides. Deep sea marine sediment cores were collected from Mediterranean Sea abyssal plains (depth > 2000 m) and analysed for 239,240Pu and 137Cs to elucidate the concentrations, inventories and sources of these radionuclides in the deepest areas of the Mediterranean. The activity — depth profiles of 210Pb, together with 14C dating, indicate that sediment mixing redistributes the artificial radionuclides within the first 2.5 cm of the sedimentary column. The excess 210Pb inventory was used to normalize 239,240Pu and 137Cs inventories for variable sediment fluxes. The 239,240Pu/210Pbxs ratio was uniform across the entire sea, with a mean value of 1.24 × 10− 3, indicating homogeneous fallout of 239,240Pu. The 137Cs/210Pbxs ratio showed differences between the eastern (0.049) and western basins (0.030), clearly significant impact of deep sea sediments from the Chernobyl accident. The inventory ratios of 239,240Pu/137Cs were 0.041 and 0.025 in the western and eastern basins respectively, greater than the fallout ratio, 0.021, showing more efficient scavenging of 239,240Pu in the water column and major sedimentation of 137Cs in the eastern basin. Although areas with water depths of > 2000 m constitute around 40% of the entire Mediterranean basin, the sediments in these regions only contained 2.7% of the 239,240Pu and 0.95% of the 137Cs deposited across the Sea in 2000. These data show that the accumulation of artificial radionuclides in deep Mediterranean environments is much lower than predicted by other studies from the analy

Artificial radionuclides enter the Mediterranean Sea mainly through atmospheric deposition following nuclear weapons tests and the Chernobyl accident, but also through the river discharge of nuclear facility effluents. Previous studies of artificial radionuclides impact of the Mediterranean Sea have focussed on shallow, coastal sediments. However, deep sea sediments have the potential to store and accumulate pollutants, including artificial radionuclides. Deep sea marine sediment cores were collected from Mediterranean Sea abyssal plains (depth > 2000 m) and analysed for 239,240Pu and 137Cs to elucidate the concentrations, inventories and sources of these radionuclides in the deepest areas of the Mediterranean. The activity — depth profiles of 210Pb, together with 14C dating, indicate that sediment mixing redistributes the artificial radionuclides within the first 2.5 cm of the sedimentary column. The excess 210Pb inventory was used to normalize 239,240Pu and 137Cs inventories for variable sediment fluxes. The 239,240Pu/210Pbxs ratio was uniform across the entire sea, with a mean value of 1.24 × 10− 3, indicating homogeneous fallout of 239,240Pu. The 137Cs/210Pbxs ratio showed differences between the eastern (0.049) and western basins (0.030), clearly significant impact of deep sea sediments from the Chernobyl accident. The inventory ratios of 239,240Pu/137Cs were 0.041 and 0.025 in the western and eastern basins respectively, greater than the fallout ratio, 0.021, showing more efficient scavenging of 239,240Pu in the water column and major sedimentation of 137Cs in the eastern basin. Although areas with water depths of > 2000 m constitute around 40% of the entire Mediterranean basin, the sediments in these regions only contained 2.7% of the 239,240Pu and 0.95% of the 137Cs deposited across the Sea in 2000. These data show that the accumulation of artificial radionuclides in deep Mediterranean environments is much lower than predicted by other studies from the analy

The four naturally-occurring radium isotopes (223Ra, 224Ra, 226Ra and 228Ra) were used to estimate the submarine groundwater discharge (SGD) in the Isola La Cura marsh area in the northern Venice Lagoon (Italy). By determining the radium contributors to the study area (river, coastal ocean and sediments) the radium excess in the lagoon water was quantified through a mass balance model. This radium excess is attributed to a submarine groundwater discharge source and represents the most important input of radium. Possible endmembers were considered from analysis of groundwater samples (subtidal and marsh piezometers, marsh wells and seepage meters) that were enriched in Ra by one to two orders of magnitude relative to surface waters. In particular, a permeable layer at 80 cm depth in the surrounding marsh is considered to be representative of the most likely SGD source, although similar radium activities were measured in other subtidal porewater samples collected in the Isola La Cura area. The estimated SGD flux to the study area ranged from 1 · 109 to 6 · 109 L·d− 1, the same order of magnitude as the overall riverine input to the lagoon (3 · 109 L·d− 1). A major fraction of this SGD flux is likely recirculated seawater, as evidenced by the endmember salinity. The water residence time of 2 days was estimated by both using the shortest-lived radium isotope and estimating the volume of water exchanged between the lagoon and the open sea during a tidal cycle (tidal prism approach). This SGD flux could be used to estimate the input of other chemical species (metals, nutrients, etc.) via SGD which might affect the Venice Lagoon ecosystem.

The four naturally-occurring radium isotopes (223Ra, 224Ra, 226Ra and 228Ra) were used to estimate the submarine groundwater discharge (SGD) in the Isola La Cura marsh area in the northern Venice Lagoon (Italy). By determining the radium contributors to the study area (river, coastal ocean and sediments) the radium excess in the lagoon water was quantified through a mass balance model. This radium excess is attributed to a submarine groundwater discharge source and represents the most important input of radium. Possible endmembers were considered from analysis of groundwater samples (subtidal and marsh piezometers, marsh wells and seepage meters) that were enriched in Ra by one to two orders of magnitude relative to surface waters. In particular, a permeable layer at 80 cm depth in the surrounding marsh is considered to be representative of the most likely SGD source, although similar radium activities were measured in other subtidal porewater samples collected in the Isola La Cura area. The estimated SGD flux to the study area ranged from 1 · 109 to 6 · 109 L·d− 1, the same order of magnitude as the overall riverine input to the lagoon (3 · 109 L·d− 1). A major fraction of this SGD flux is likely recirculated seawater, as evidenced by the endmember salinity. The water residence time of 2 days was estimated by both using the shortest-lived radium isotope and estimating the volume of water exchanged between the lagoon and the open sea during a tidal cycle (tidal prism approach). This SGD flux could be used to estimate the input of other chemical species (metals, nutrients, etc.) via SGD which might affect the Venice Lagoon ecosystem.

234Th (T1/2 = 24.1 d), 210Pb and 210Po (T1/2 = 138.4 d) are particle reactive radioisotopes that have been frequently used as tracers for particle cycling in the upper ocean. Particle export has usually been estimated using 234Th/238U disequilibrium, as 234Th is often deficient from its conservative parent 238U in the euphotic zone. Lately, however, some researchers are paying attention to the use of 210Po/210Pb disequilibrium as an additional tool to complement the knowledge about particle cycling, scavenging rates and the nature of the material settling out of the euphotic zone. In this work, we present a comparison of the use of both radiotracer pairs in an attempt to quantify the particle export from surface waters. To that end, we present data compiled from previous studies together with our own results from the Mediterranean. We test the advantages of integrating particle flux estimates over different time scales based on the different half-lives and scavenging affinities of these radioisotopes.

234Th (T1/2 = 24.1 d), 210Pb and 210Po (T1/2 = 138.4 d) are particle reactive radioisotopes that have been frequently used as tracers for particle cycling in the upper ocean. Particle export has usually been estimated using 234Th/238U disequilibrium, as 234Th is often deficient from its conservative parent 238U in the euphotic zone. Lately, however, some researchers are paying attention to the use of 210Po/210Pb disequilibrium as an additional tool to complement the knowledge about particle cycling, scavenging rates and the nature of the material settling out of the euphotic zone. In this work, we present a comparison of the use of both radiotracer pairs in an attempt to quantify the particle export from surface waters. To that end, we present data compiled from previous studies together with our own results from the Mediterranean. We test the advantages of integrating particle flux estimates over different time scales based on the different half-lives and scavenging affinities of these radioisotopes.

Nieto-Moreno, V., Martínez-Ruiz, F., Gallego-Torres, D., Gallego-Torres, D., Giralt, S., García-Orellana, J., Masqué, P., Masqué, P., Masqué, P., Sinninghe Damsté, J.S., Ortega-Huertas, M. (2015). Palaeoclimate and palaeoceanographic conditions in the westernmost mediterranean over the last millennium: An integrated organic and inorganic approach in Journal of the Geological Society, 172(2), 264-271. Available here.

Marbà, N., Arias-Ortiz, A., Masqué, P., Kendrick, G.A., Mazarrasa, I., Bastyan, G.R., Garcia-Orellana, J., Duarte, C.M. (2015). Impact of seagrass loss and subsequent revegetation on carbon sequestration and stocks in Journal of Ecology, 103(2), 296-302. Available here.