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Radium isotopes have been used to trace nutrient, carbon, and trace metal fluxes inputs from ocean margins. However, these approaches require a full accounting of radium sources to the coastal ocean including rivers. Here, we aim to quantify river radium inputs into the Arctic Ocean for the first time for 226Ra and to refine the estimates for 228Ra. Using new and existing data, we find that the estimated combined (dissolved plus desorbed) annual 226Ra and 228Ra fluxes to the Arctic Ocean are [7.0–9.4] × 1014 dpm y−1 and [15–18] × 1014 dpm y−1, respectively. Of these totals, 44% and 60% of the river 226Ra and 228Ra, respectively are from suspended sediment desorption, which were estimated from laboratory incubation experiments. Using Ra isotope data from 20 major rivers around the world, we derived global annual 226Ra and 228Ra fluxes of [7.4–17] × 1015 and [15–27] × 1015 dpm y−1, respectively. As climate change spurs rapid Arctic warming, hydrological cycles are intensifying and coastal ice cover and permafrost are diminishing. These river radium inputs to the Arctic Ocean will serve as a valuable baseline as we attempt to understand the changes that warming temperatures are having on fluxes of biogeochemically important elements to the Arctic coastal zone.

When rivers mix into the ocean, radium isotopes are released into the dissolved phase allowing us to apply these isotopes as powerful tracers of river plumes spreading into the ocean. We report 223Ra and 224Ra radium activities that were determined in the Amazon River mouth and along the Amazon plume that extends off the coasts of Brazil and French Guyana into the Atlantic Ocean. We summarize 223Ra and 224Ra data from AmasSeds (1991), AMANDES (GEOTRACES process study GApr01; 2007–2008) and M147 (2018) projects, which were conducted in different seasons corresponding to different Amazon discharge rates. We determined the 224Raex/223Ra activity ratios (AR) along the Amazon plume to derive apparent ages and to estimate the residence time of the Amazon waters on the Brazilian continental shelf. Our data suggest that it takes 9–14 d for the Amazon waters to reach the northern continental shelf off French Guyana and 12–21 d to reach the eastern part of the Brazilian continental shelf. These time scales are in good agreement with those derived from a high-resolution numerical simulation of the regional ocean dynamics. We do not find any relationship between the discharge rate of the Amazon River and the residence time of the waters on the Brazilian continental shelf, suggesting that the residence time of the Amazon waters is primarily driven by the ambient northwestward current. Using the apparent ages along the plume, we estimate an average velocity of 26 cm s−1 for the northward transport of the Amazon waters on the continental shelf.

International audience; The Mississippi Bight, east of the Mississippi River, is a complex coastal ecosystem that, like the better-known Louisiana Shelf to the west, experiences seasonal bottom water hypoxia. However, input of allochthonous nutrients from the Mississippi River to the Mississippi Bight appears to be limited, begging the question of what drives seasonal hypoxia in this system. Prior research has suggested submarine groundwater discharge (SGD) could be an overlooked component of the Mississippi Bight biogeochemical system. We thus examined the hypothesis that SGD provides a "bottom up" driver for seasonal hypoxia in this area. We used a multi-tracer approach based on known SGD indicators (dissolved Ra, Ba, Si, methane) to: i) demonstrate the presence of SGD as a constituent contributor to Bight bottom waters, ii) constrain the SGD flux of macronutrients, and, iii) investigate the hypoxia-SGD linkage. We found excess SGD tracers in saline bottom waters relative to surface waters, suggesting a bottom source. Examination of other sources for the constituent enrichments besides SGD (e. g., rivers, produced waters from oil wells) appear inadequate to close the bottom water chemical mass balances. Additionally, inverse correlations between DO and SGD indicators in bottom waters support a common mechanism supplying dissolved Ra, Ba, and Si, and decreasing DO concentrations in these waters. Two different approaches to modeling the bottom water Ra distribution both suggest a seepage rate of ~0.055 m 3 m − 2 d − 1 , in line with previous estimates in similar systems. Our more complex model, involving four mass balances, suggests that as much as 10-20% of the bottom water in the Bight circulates through the underlying permeable sediments on a time scale of ~10 days. This circulated water emerges as SGD with completely altered chemistry. More specifically, SGD appears in some cases to be the dominant contributor of nutrients to Bight bottom waters. Additionally, the potential oxygen demand of reduced species within SGD likely contributes significantly to the development of seasonal hypoxia in Bight bottom waters. Further work is needed to better resolve sources of nutrients and additional reduced species within the Mississippi Bight SGD as well as the variability and pathways of this supply. Nonetheless, the bottom-up influence of SGD on the Mississippi Bight appears to be a significant and overlooked aspect of this system. We suggest that such a bottom-up influence may be a generally important feature of coastal ecosystems.

Qinghai Lake, the largest lake in China with a surface area of 4260 km2, lies in the northeastern Qinghai-Tibet Plateau (QTP) at a mean altitude of 3194 m above sea level. The lake has a water volume of 71.6 × 109 m3, a salinity ranges of 4.18 ± 0.14 ppt to 12.3 ± 0.14 ppt during sampling period which is about a half of typical seawater. The aquifer systems at the lake shoreline have obvious saline water and freshwater mixing zones with similar hydrogeological and biogeochemical settings to coastal aquifers. This study aims to estimate lacustrine groundwater discharge (LGD) and the associated chemical loadings to western Qinghai Lake. Naturally occurring radium isotopes (223Ra, 224Ra, 226Ra and 228Ra), nutrients, heavy metals and dissolved inorganic carbon (DIC) were systematically investigated in different water endmembers of western Qinghai Lake. LGD was estimated with radium inventory model and radium eddy diffusion model, leading to a LGD estimate of 7.67 × 106 m3 d−1 and 8.52 × 106 m3 d−1, respectively. The fresh groundwater discharge obtained based on regional water budget is about 5.97 × 106 m3 d−1 during sampling period, accounting for 73.74% (ranging from 53.22 to 80.43%) of total LGD. This study reveals that the fresh groundwater component is dominant in LGD of Qinghai Lake, which is quite different from most coastal scenarios. Furthermore, the chemical loadings derived from fresh-LGD and recirculated-LGD are evaluated. The preliminary results indicate that fresh-LGD is an important source of nutrient, barium, strontium, uranium, and DIC to the lake, and may significantly impact biogeochemical and ecological processes in the lake. This study presents the first LGD estimate in the largest QTP saline lake in China, and would be instructive to further similar studies in saline lakes in QTP and elsewhere in the world.

Intrusion of saltwater into freshwater coastal aquifers poisons an essential resource. Such intrusions are occurring along coastlines worldwide due largely to the over-pumping of freshwater and sea level rise. Saltwater intrusion impacts drinking water, agriculture and industry, and causes profound changes in the biogeochemistry of the affected aquifers, the dynamic systems called subterranean estuaries. Subterranean estuaries receive freshwater from land and saltwater from the ocean and expose this fluid mixture to intense biogeochemical dynamics as it interacts with the aquifer and aquiclude solids. Increased saltwater intrusion alters the ionic strength and oxidative capacity of these systems, resulting in elevated concentrations of certain chemical species in the groundwater, which flows from subterranean estuaries into the ocean as submarine groundwater discharge (SGD). These highly altered fluids are enriched in nutrients, carbon, trace gases, sulfide, metals, and radionuclides. Seawater intrusion expands the subterranean estuary. Climate change amplifies sea level variations on short and seasonal time scales. These changes may result in higher SGD fluxes, further accelerating release of nutrients and thus promoting biological productivity in nutrient-depleted waters. But this process may also adversely affect the environment and alter the local ecology. Research on saltwater intrusion and SGD has largely been undertaken by different groups. We demonstrate that these two processes are linked in ways that neither group has articulated effectively to date.

Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 125(5), (2020): e2019JC016007, doi:10.1029/2019JC016007.

Data from GEOTRACES cruises GN01 (HLY1502) and GN04 (PS94) have been archived at the Biological & Chemical Oceanography Data Management Office (BCO-DMO; https://www.bco-dmo.org/deployment/638807) and PANGAEA (https://www.pangaea.de/?q=PS94&f.campaign%5B%5D=PS94) websites, respectively. The inorganic carbon data are available at the NOAA Ocean Carbon Data System (OCADS ; doi :10.3334/CDIAC/OTG.CLIVAR_ARC01_33HQ20150809). In addition to these national data archives, the data used in this paper is available as a supplementary downloadable excel file; International audience; A major surface circulation feature of the Arctic Ocean is the Transpolar Drift (TPD), a current that transports river‐influenced shelf water from the Laptev and East Siberian Seas toward the center of the basin and Fram Strait. In 2015, the international GEOTRACES program included a high‐resolution pan‐Arctic survey of carbon, nutrients, and a suite of trace elements and isotopes (TEIs). The cruises bisected the TPD at two locations in the central basin, which were defined by maxima in meteoric water and dissolved organic carbon concentrations that spanned 600 km horizontally and ~25‐50 m vertically. Dissolved TEIs such as Fe, Co, Ni, Cu, Hg, Nd, and Th, which are generally particle‐reactive but can be complexed by organic matter, were observed at concentrations much higher than expected for the open ocean setting. Other trace element concentrations such as Al, V, Ga, and Pb were lower than expected due to scavenging over the productive East Siberian and Laptev shelf seas. Using a combination of radionuclide tracers and ice drift modeling, the transport rate for the core of the TPD was estimated at 0.9 ± 0.4 Sv (106 m3 s‐1). This rate was used to derive the mass flux for TEIs that were enriched in the TPD, revealing the importance of lateral transport in supplying materials beneath the ice to the central Arctic Ocean and potentially to the North Atlantic Ocean via Fram Strait. Continued intensification of the Arctic hydrologic cycle and permafrost degradation will likely lead to an increase in the flux of TEIs into the Arctic Ocean.

International audience; Hydrothermal activity in the deep ocean generates plumes of metal-rich particles capable of removing certain trace elements from seawater by adsorption and sedimentation. This removal process, known as scavenging, can be probed using the insoluble radiogenic isotopes of thorium (Th), which are produced at a known rate in the water column via the decay of soluble uranium (234 Th, 230 Th) and radium (228 Th) isotopes. We present dissolved and particulate measurements of these three thorium isotopes in a hydrothermal plume observed in the southeast Pacific Ocean on the GEOTRACES GP16 section. Since their half-lives vary from days (234 Th) to years (228 Th) to tens of thousands of years (230 Th), the combination of their signals can be used to understand scavenging processes occurring on a wide range of timescales. Scavenging is a multi-step process involving adsorption and desorption onto particles, followed by particle aggregation, sinking, and eventual sedimentation. We use thorium isotopes to study how hydrothermal activity affects these steps. The rate constants for net adsorption of 234 Th determined here are comparable to previous estimates from hydrothermal plumes in the Atlantic and North Pacific Oceans. The partitioning of 234 Th and 230 Th between large and small particles is more similar in the hydrothermal plume than above it, indicating faster aggregation of particles within the hydrothermal plume at stations nearby the East Pacific Rise than in waters outside the plume. In addition to rapid scavenging and aggregation near the ridge axis, we also infer continuous off-axis scavenging from observations and modeling of 228 Th/ 228 Ra activity ratios. The degree of depletion of the three thorium isotopes increases in order of half-life, with total 234 Th activity close to that of its parent 238 U, but 230 Th showing nearly 70% depletion compared to expected values from reversible scavenging. By modeling the variations in depletion for the different isotopes, we show that much of the 230 Th removal is inherited from scavenging events happening long before the most recent hydrothermal inputs.

Methane concentrations can be high in coastal groundwater, resulting in methane export driven by submarine groundwater discharge. However, the magnitude of this methane flux depends significantly on the rate of methanotrophy, the often overlooked process of microbial methane consumption that occurs within coastal aquifer sediments. Here we describe a zone of methanogenesis within the freshwater lens of a barrier island aquifer and investigate the methane source/sink behavior of the barrier island system as a whole. The median concentration of methane dissolved in fresh groundwater beneath the center of the island was 0.6 mM, supported by high rates of potential methanogenesis (22 mmol m−2 day−1). However, rates of microbial methane consumption were also elevated in surrounding sediments (18 mmol m−2 day−1). Groundwater flowing from the zone of methanogenesis to the point of discharge into the ocean had a long residence time within methanotrophic sediments (∼195 days) such that the majority of the methane produced within the barrier island aquifer was likely consumed there.

Processes leading to the development of coastal hypoxia are usually attributed to anthropogenic nutrient loading. We present new evidence documenting a natural process capable of producing large-scale water masses undersaturated with respect to dissolved oxygen, independent of human-caused nutrient loading and subsequent nutrient-enhanced productivity. Along the heavily developed coastline of Myrtle Beach, South Carolina, the offshore discharge of saline, anoxic groundwater generates undersaturated bottom water masses which can be advected into nearshore waters. Highly elevated radium isotope activities measured in low-oxygen bottom waters cannot be supported by any known terrestrial source. Rather, discharge of cold, anoxic, high-radium water from offshore carbonate aquifers is the likely source. Density stratification inhibits mixing of these discharged waters during inshore transport. During a low-oxygen event in August 2012, this restricted mixing is likely the mechanism for the observed oxygen deficit in bottom waters. This discovery suggests benthic layers undersaturated with respect to dissolved oxygen may be considerably more prevalent than currently recognized, particularly around the carbonate-platform of the US East Coast.

This study examined dissolved Mo and sedimentary Mo along with hydrochemical parameters in the western Taiwan Strait (WTS) in May and August 2012. The results demonstrate that dissolved Mo could be depleted of as high as 10–20 nM during our May sampling period when the nutrient-enriched Min-Zhe coastal current ceased and spring blooms developed. The negative correlation between Chl-a and dissolved Mo suggests the possible involvement of high algal productivity in removing dissolved Mo out of the water column. Specific oceanographic settings (little currents) permitted a high sedimentary enrichment of Mo (>6 µg/g Mo) within the highly productive waters outside the Jiulong River mouth. Possibly, the high algal productivities and consequent organic matter sinks provide a pathway of Mo burial from water columns into sediments. Dissolved Mo was relatively high in groundwater samples, but we observed that submarine groundwater discharges (SGDs) only contributed to a relatively small percentage of the total dissolved Mo pool in WTS. It is probably attributable to the immediate removal of SGD-released Mo ions via adsorption onto newly formed Mn oxides once exposed to oxygenated seawater, followed by an elevated sedimentary Mo accumulation near the SGDs (∼5 µg/g). In addition to metal oxide particle scavenging and sulfide precipitation, we estimated that biological uptake along with Mo adsorption onto organic matter carriers could finally provide more than 10% of the annual sedimentary Mo accumulation in WTS.

The GEOTRACES Intermediate Data Product 2017 (IDP2017) is the second publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2016. The IDP2017 includes data from the Atlantic, Pacific, Arctic, Southern and Indian oceans, with about twice the data volume of the previous IDP2014. For the first time, the IDP2017 contains data for a large suite of biogeochemical parameters as well as aerosol and rain data characterising atmospheric trace element and isotope (TEI) sources. The TEI data in the IDP2017 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at crossover stations. The IDP2017 consists of two parts: (1) a compilation of digital data for more than 450 TEIs as well as standard hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing an on-line atlas that includes more than 590 section plots and 130 animated 3D scenes. The digital data are provided in several formats, including ASCII, Excel spreadsheet, netCDF, and Ocean Data View collection. Users can download the full data packages or make their own custom selections with a new on-line data extraction service. In addition to the actual data values, the IDP2017 also contains data quality flags and 1-σ data error values where available. Quality flags and error values are useful for data filtering and for statistical analysis. Metadata about data originators, analytical methods and original publications related to the data are linked in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2017 as section plots and rotating 3D scenes. The basin-wide 3D scenes combine data from many cruises and provide quick overviews of large-scale tracer distributions. These 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of tracer plumes near ocean margins or along ridges. The IDP2017 is the result of a truly international effort involving 326 researchers from 25 countries. This publication provides the critical reference for unpublished data, as well as for studies that make use of a large cross-section of data from the IDP2017. This article is part of a special issue entitled: Conway GEOTRACES - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González., National Science Foundation (U.S.) (Grant OCE-0608600), National Science Foundation (U.S.) (Grant OCE0938349), National Science Foundation (U.S.) (Grant OCE-1243377), National Science Foundation (U.S.) (Grant OCE-1546580)

Radium (Ra) isotopes are enriched in hydrothermal fluids, have a wide range of half-lives and are minimally impacted by scavenging removal processes; therefore, they have the ability to provide key information on the fate of trace elements in both near- and far- field hydrothermal plumes. To expand our understanding of Ra isotopes and their usefulness as tracers of hydrothermal activity, we measured Ra isotopes in both high and low temperature vent fluids, and in neutrally buoyant plumes. The time scales of plume transport were derived from Ra isotopes in neutrally buoyant plumes emanating from 15°S on the East Pacific Rise (EPR) and the TAG vent field on the Mid Atlantic Ridge (MAR) relative to their source ratio in high-temperature vent fluids. In near-field ( 223 Ra: t 1/2 = 11.4 d, 224 Ra: t 1/2 = 3.66 d) suggest plume ages of ~ 1 month and ~ 5 d over the EPR and MAR, respectively. Farther afield (> 100 km from vent), the distribution of the longer-lived 228 Ra (t 1/2 = 5.75 y) is indicative of hydrothermal plumes with different points of origin along the EPR. Radium-228 derived plume ages were used to calculate the residence time of hydrothermal dissolved Fe, which was in the range of 9–20 y (with an upper limit of 50 y depending on model assumptions). These are shorter than typical deep ocean Fe residence times, a result of scavenging removal of Fe from the neutrally buoyant plume during transport. This study highlights the utility of Ra isotopes in providing time-scales of hydrothermally derived trace element cycling in the deep ocean.

International audience; Continental margins play a central role in the composition of seawater by being an important source of trace element essentials to the functioning of the ocean ecosystems. Here, we measured long-lived radium isotopes (Ra-226, Ra-228) along a zonal transect at 12 degrees S (US GEOTRACES GP16) in the eastern tropical South Pacific Ocean. We used Ra-228 to quantify the trace element and isotope (TEI) fluxes (DMn, DFe, and DCo) delivered from the Peruvian continental i) shelf and ii) slope. First, elevated Ra-228 activities were measured in surface water over the entire transect (similar to 8500 km), evidence that the continental shelf is an important source of sediment-derived TEIs not only to coastal areas, but to central Pacific Ocean waters. Modeled Ra-228 shelf fluxes combined with water column dissolved TEI/Ra-228 ratios were used to quantify the shelf-ocean input rates (normalized to shelf-area) for DMn (3.3 x 10(3) mu mol m(-2) y(-1)), DFe (1.5 x 10(3) mu mol m(-2) y(-1)), and DCo (1.0 x 10(2) mu mol m(-2) y(-1)). Second, co-occurring plumes of Ra-228, DFe, and DMn extended over 1800 km from the margin at 1000-2500 m depth, indicative of a continental slope sediment TEI input to the intermediate water column. The Ra-228 gradient allowed us to derive an effective horizontal eddy diffusion coefficient (K-h) of 46 m(2) s(-1), which in turn permitted the calculation of slope sediment DMn (6.4 mu mol m(-2) y(-1)) and DFe (5.9 x 10(2) mu mol m(-2) y(-1)) fluxes based on their offshore concentration gradients. On the scale of the South Pacific continental margin between 0-20 degrees S, the DMn shelf flux is approximately 2-3 orders of magnitude higher than the slope flux, while the DFe shelf/slope flux is similar to 3:1. Both shelf and slope sediment derived DMn was transported over a significant distance towards the ocean interior, while DFe concentration gradients were steep, consistent with longer water column residence time for DMn as compared to DFe in marine systems. These findings highlight the importance of considering the continental slope-ocean boundary in the oceanic budgets of biologically-important trace elements.

Submarine groundwater discharge (SGD) is herein recognized as a significant pathway of material transport from land to the coastal SW Atlantic Ocean and thus, it can be a relevant factor affecting the marine biogeochemical cycles in the region. This paper focuses on the initial measurements of 226Ra, 228Ra and 222Rn made in Patagonia’s coastal zone of Chubut and Santa Cruz provinces (42°S–48°S, Argentina). 226Ra activity ranged from 2.9 to 73.5 dpm 100 L−1, and 228Ra activity ranged from 11.9 to 311.0 dpm 100 L−1 in groundwater wells. The radium activities found in Patagonia’s marine coastal regions and adjacent shelf indicate significant enrichment throughout the coastal waters. Groundwater samples presented the largest 222Rn activity and ranged from 2.66 to 1083 dpm L−1. Conversely, in the coastal marine environment, the 222Rn activity ranged from 1.03 to 6.23 dpm L−1. The Patagonian coastal aquifer showed a larger enrichment in 228Ra than in 226Ra, which is a typical feature for sites where SGD is dominant, probably playing a significant role in the biogeochemistry of these coastal waters. Fil: Torres, Américo Iadran. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Centro para el Estudio de Sistemas Marinos; Argentina Fil: Andrade, Carlos F.. Fundação Universidade Federal do Rio Grande; Brasil Fil: Moore, Willard S.. University of South Carolina; Estados Unidos Fil: Faleschini, Mauricio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Centro Nacional Patagónico. Centro para el Estudio de Sistemas Marinos; Argentina Fil: Esteves, Jose Luis. Fundación Patagonia Natural; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Niencheski, Luis F. H.. Fundação Universidade Federal do Rio Grande; Brasil Fil: Depetris Gallino, Pedro Jose. Academia Nacional de Ciencias; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

The shores of the Pearl River estuary are home to 35 million people. Their wastes are discharged into the large river delta-front estuary (LDE), one of the most highly polluted systems in the world. Here we construct a radium reactive transport model to estimate the terrestrial groundwater discharge (TGD) into the highly urbanized Pearl River LDE. We find the TGD comprises only approximately 0.9% in term of water discharge compared to the river discharge. The TGD in the Pearl River LDE delivers significant chemical fluxes to the coast, which are comparable to the fluvial loadings from Pearl River and other world major rivers. Of particular importance is the flux of ammonium because of its considerable role in Pearl River estuary eutrophication and hypoxia. Unlike the ammonium in many other aquifers, the ammonium in the Pearl River aquifer system is natural and originated from organic matter remineralization by sulfate reduction in the extremely reducing environment. The TGD derived NH4+ is as much as 5% of the upstream Pearl River fluvial loading and 42% of the anthropogenic inputs. This high groundwater NH4+ flux may greatly intensify the eutrophication, shift the trophic states, and lead to alarming hypoxia within the affected ecosystems in the Pearl River LDE. The large TGD derived chemical fluxes will lead to deterioration of water and will potentially affect human health.

Shelf inputs in the Arctic Ocean appear to be increasing, which could change the nutrient balance of the central basin., Rising temperatures in the Arctic Ocean region are responsible for changes such as reduced ice cover, permafrost thawing, and increased river discharge, which, together, alter nutrient and carbon cycles over the vast Arctic continental shelf. We show that the concentration of radium-228, sourced to seawater through sediment-water exchange processes, has increased substantially in surface waters of the central Arctic Ocean over the past decade. A mass balance model for 228Ra suggests that this increase is due to an intensification of shelf-derived material inputs to the central basin, a source that would also carry elevated concentrations of dissolved organic carbon and nutrients. Therefore, we suggest that significant changes in the nutrient, carbon, and trace metal balances of the Arctic Ocean are underway, with the potential to affect biological productivity and species assemblages in Arctic surface waters.

The first full transarctic section of 228Ra in surface waters measured during GEOTRACES cruises PS94 and HLY1502 (2015) shows a consistent distribution with maximum activities in the transpolar drift. Activities in the central Arctic have increased from 2007 through 2011 to 2015. The increased 228Ra input is attributed to stronger wave action on shelves resulting from a longer ice‐free season. A concomitant decrease in the 228Th/228Ra ratio likely results from more rapid transit of surface waters depleted in 228Th by scavenging over the shelf. The 228Ra activities observed in intermediate waters (

article i nfo The application of thorium-228 towardsunderstandingparticle dynamics intheopen ocean islimitedbecauseof its low natural abundanceinseawaterand associated sampling and analytical challenges.Here we describea fast and nondestructivemethod for measuringboth dissolved and particulate 228 Thactivities inthe openocean using Radium Delayed-Coincidence Counters (RaDeCC). Particulate and dissolved samples were collected from the

Actinium-227 (t 1/2 = 21.77 y) is produced by decay of protactinium-231 (t 1/2 = 32,760 y). Abyssal sediments are enriched in 231 Pa and thus provide a source of the more soluble 227 Ac to the deep-sea, where it can be used as a tracer of deep ocean diapycnal mixing. In this study, we examine the distribution of dissolved and particulate 227 Ac within a neutrally buoyant hydrothermal plume overlying the TAG vent field (26.14°N, 44.83°W, Mid-Atlantic Ridge) collected during the U.S. GEOTRACES North Atlantic Transect (2011). Elevated activities of both particulate and dissolved 227 Ac within the plume indicate that hydrothermal vents may be a previously unrecognized source of this isotope to the marine environment. We contend that vent fluids are the main source of the 227 Ac enrichment. Based on the activities of dissolved and particulate 227 Ac within the plume, a 3 He dilution model was used to derive a hydrothermal 227 Ac end-member activity of 1.5 × 10 4 –2.5 × 10 4 dpm m − 3 . Assuming that TAG is characteristic of all vents along the Mid-Atlantic Ridge, the hydrothermal source of 227 Ac to the deep Atlantic is estimated to be 1.9–5.8% of the flux from Atlantic sediments. Although this hydrothermal flux may not have a large impact on the total oceanic inventory of 227 Ac, it must be considered when using this isotope as a tracer of deep ocean mixing in areas where hydrothermal activity is present.

Radium isotopes are produced in sediments via the decay of thorium isotopes and are generally soluble in seawater. As such, isotopes of the radium quartet (224Ra, 223Ra, 228Ra, 226Ra) have been used as tracers of ocean boundary inputs and mixing processes. We measured radium isotopes on the US GEOTRACES North Atlantic cruises in 2010 and 2011, which crossed a number of key ocean boundary features including the Mediterranean outflow, the Mauritanian upwelling and oxygen minimum zones (OMZ) along western Africa, a hydrothermal plume over the mid-Atlantic Ridge (MAR), and the broad continental margin of the north-eastern United States. Radium isotope features at these locations are discussed in the context of their potential to quantify fluxes of trace elements and isotopes (TEIs) from common sources. Notable observations include: (1) a Mediterranean outflow spreading rate of 0.52–0.60 cm/s derived from 228Ra, (2) evidence of substantial sediment–water interaction in the benthic boundary layer along the OMZ, (3) decoupling between 223Ra and the other Ra isotope sources over the MAR, and (4) significant continental inputs (e.g. submarine groundwater discharge—SGD) in the western Atlantic. Lastly, 228Ra inventories have remained constant over the past 30 years, which suggests that SGD is steady-state for the North Atlantic on decadal time scales.

Radium isotopes ( 223 Ra, t 1/2 = 11.4 d; 224 Ra, t 1/2 = 3.66 d; 226 Ra, t 1/2 = 1600 y; and 228 Ra, t 1/2 = 5.75 y) are considered excellent tracers of groundwater movement and discharge in coastal systems. However, spatial and temporal variability in porewater radium activity have raised questions about the accuracy of these tracers. To better understand the factors affecting radium variability in coastal systems, measurements of porewater and surface water radium activity were made at an island in North Inlet Salt Marsh in Georgetown, South Carolina, from November 2009 to February 2011. Water salinity, temperature, pH, and redox potential were also recorded, and sediment samples were collected for analysis of bulk 228 Ra and 226 Ra activity. Hydraulic head observations during 2007–2008 from piezometers on the island were used to generate independent estimates of groundwater fluxes. Porewater radium activities decreased with depth below the marsh surface and increased with distance from the creek banks. Salinity measurements were lower and redox potential higher near the marsh creeks. The stratigraphy of the island is typical of intertidal wetlands in the southeastern U.S., with a mud layer overlying a confined sandy aquifer; the observed patterns in porewater radium, salinity, and redox potential were consistent with (1) shorter porewater residence times in the permeable sand aquifer than in the low-permeability mud, (2) differences in grain size between the mud and sand, and (3) greater tidal exchange near the creeks. Temporal variations in porewater radium activity were not associated with salinity, pH, and redox potential although temperature provided significant control (P 2 228 Ra and 226 Ra activity. Lower mean sea water levels resulted in greater calculated groundwater discharge and were also associated with lower average porewater 224 Ra and 223 Ra activity, in that groundwater discharge variations strongly affected short-lived radium activity at this site. The 228 Ra/ 226 Ra activity ratios in the surface water and porewater signified that the confined aquifer, rather than the surficial mud, was the primary source of radium to the surface water. Our results highlight the importance of understanding the hydrology of any coastal system when interpreting radium results. It is also essential to identify and measure the correct porewater end-member(s) (i.e. source aquifers) when calculating radium budgets.

Approximately 40% of the total global rate of nitrogen fixation is the result of human activities, and most of this anthropogenic nitrogen is used to fertilize agricultural fields. Approximately 23% of the applied agricultural nitrogen is delivered to the coastal zone, often reducing water quality and driving eutrophication. Nitrogen cycling in coastal sediments can mitigate eutrophication by removing bioavailable nitrogen. However, some of these processes generate nitrous oxide, a potent greenhouse gas, as a by-product. Here we report the discovery of a nitrous oxide production hot spot in shallow barrier island sands. Nitrous oxide concentrations, production and consumption rates, vertical diffusion fluxes, and flux to the atmosphere were measured across triplicate depth profiles. Using a mass balance approach, rates of net nitrous oxide production were estimated to be 40 µmol m−2 d−1. This production was driven by a hot spot of nitrate consumption that removed bioavailable nitrogen from the coastal environment at a rate of 10 mmol m−2 d−1, a rate that is comparable with the highest rates of denitrification reported for coastal sediments.

Numerous investigators have used the advection–diffusion equation and the distributions of the four isotopes of radium to model coastal mixing rates. However, there is a dichotomy in approach. Some models use the short-lived 224 Ra ( T 1/2 =3.7 days) and 223 Ra ( T 1/2 =11 days) to determine horizontal mixing coefficients and the long-lived 226 Ra ( T 1/2 =1600 years) and 228 Ra ( T 1/2 =5.75 years) to test for the effects of advection. Because the timescales of mixing in most coastal systems (10–80 km offshore) are on the order of months or less, radioactive decay and particle removal of the long-lived isotopes are neglected in these models. If the long-lived isotopes display a linear gradient with distance offshore, this is taken as evidence that conservative mixing rather than advection controls the radium distribution. In this case it is appropriate to neglect the advection term and calculate mixing rates based on the exponential decay of the short-lived isotopes. Other models assume that radioactive decay of all of the isotopes impacts their distribution offshore. These models set up equations that separate the effects of mixing from advection and attempt to force mixing rates derived from all of the Ra isotopes to match. There are several reasons these models are doomed to fail. First, 224 Ra and 223 Ra look at different scales of mixing-there is no reason they must give the same rate. Secondly, on the timescale of coastal mixing decay of 226 Ra and 228 Ra is essentially zero. Equations that include decay terms for 226 Ra and 228 Ra become unrealistic when we consider that the measurement error is considerably greater than decay. In fact particle removal of 226 Ra is more important than radioactive decay. Finally, these models require steady-state conditions on the timescales of the decay terms. In order to use such models, the source functions and distributions of 228 Ra and 226 Ra would have to be constant for several times their half lives, approximately 25 and 4000 years, respectively. Seasonal and annual variability in supply and mixing rearranges the distributions, violating the steady-state assumption. Thus, models utilizing 226 Ra and 228 Ra radioactive decay are useless in the coastal zone.

Submarine groundwater discharge (SGD) varies significantly across time scales ranging from hours to years, but studies that allow quantitative comparisons between different time scales are few. Most of these studies have focused on beach settings, where the combined variations in fresh and saline SGD can be difficult to interpret. We calculated variations in saline SGD based on a 1 year record of hydraulic head in a salt marsh, where we could isolate variations in saline, tidally driven SGD. Observed SGD varied by an order of magnitude over the course of the year. Groundwater discharge was proportional to tidal amplitude and varied by at least a factor of 2 between spring and neap tides. Monthly average SGD was inversely proportional to average sea level; it increased by nearly a factor of 2 as sea level declined by ∼50 cm from late summer to late winter. This variation was far larger than that predicted by analytic models, owing to the flat topography and inundation of the marsh platform. The effect of short-term (days) variations in sea level associated with wind events and storms was small in comparison. SGD is probably proportional to tidal amplitude in nearly all coastal settings, including beaches. Seasonal variations in sea level may not affect the volume of SGD as significantly in coastal settings where the slope of the intertidal zone is relatively constant, but such variations have the potential to strongly affect the composition of SGD.

Ecological zonation of salt marsh macrophytes is strongly influenced by hydrologic factors, but these factors are poorly understood. We examined groundwater flow patterns through surficial sediments in two saltmarshes in the southeastern United States to quantify hydrologic differences between distinct ecological zones. Both sites included tall- or medium-form Spartina alterniflora near the creek bank; short-form Spartina alterniflora in the mid-marsh; salt flats and Salicornia virginica in the high marsh; and Juncus roemarianus in brackish-to-fresh areas adjacent to uplands. Both sites had relatively small, sandy uplands and similar stratigraphy consisting of marsh muds overlying a deeper sand layer. We found significant hydrologic differences between the four ecological zones. In the zones colonized by S. alterniflora, the vertical flow direction oscillated with semi-diurnal tides. Net flow (14-day average) through the tall S. alterniflora zones was downward, whereas the short S. alterniflora zones included significant periods of net upward groundwater flow. An examination of tidal efficiency at these sites suggested that the net flow patterns rather than tidal damping controlled the width of the tall S. alterniflora zone. In contrast to the S. alterniflora zones, hypersaline zones populated by S. virginica were characterized by sustained periods (days) of continuous upward flow of saline water during neap tides. The fresher zone populated by J. roemarianus showed physical flow patterns that were similar to the hypersaline zones, but the upwelling porewaters were fresh rather than saline. These flow patterns were influenced by the hydrogeologic framework of the marshes, particularly differences in hydraulic head between the upland water table and the tidal creeks. We observed increases in hydraulic head of approximately 40 cm from the creek to the upland in the sand layers below both marshes, which is consistent with previous observations that sandy aquifers below fine-grained marsh soils act as conduits for flow from uplands to tidal creeks. This hydrologic framework supports relatively good drainage near the creek, increased waterlogging in the mid-marsh, and the development of hypersalinity adjacent to the freshwater upland. These hydrologic differences in turn support distinct ecological zones.

To evaluate geochemical impacts of the subterranean estuary (STE) on the Jiulong River estuary, China, we estimated seasonal fluxes of subterranean water discharge into the estuary based on the mass balance of radium isotopes and net subterranean export fluxes of dissolved inorganic C (DIC), N (DIN), Si (DSi), soluble reactive phosphorus (SRP), and total alkalinity (TA). Based on 226Ra data, the subterranean discharge (in 107 m3 d−1) was estimated to be 0.29–0.60 in the spring, 0.69–1.44 in the summer, 0.45–0.93 in the fall, and 0.26–0.54 in the winter. This was equivalent to 8–19% of the concomitant river discharge. The net spatially integrated material fluxes from the STE into the estuary were equivalent up to 45–110% of the concomitant riverine fluxes for DIC and TA, around 14–32% for DSi and 7–19% for DIN, and negligible for SRP. Paradoxically, the mixing lines along the salinity gradient revealed no apparent additions of these species. These additions are not revealed because the STE is a relatively small spatially-averaged source (at most 11% of the total input at steady state) that spreads throughout the estuary as a non-point source in contrast to the major point sources of the river and the ocean for the estuary and a true open ocean endmember is likely lacking. Greater water flushing in the summer might dilute the STE effect on the mixing lines even more. The great spatial variation in salinity in the estuary introduced the major uncertainty in our estimates of the flushing time, which further affected the estimate of the subterranean discharge and associated material fluxes. Additionally, the great spatial variation in the STE endmember caused the relatively large ranges in these flux estimates. Despite apparent conservative mixing of DIC, DIN, and DSi in estuaries, net subterranean exports must be taken into account in evaluating geochemical impacts of estuarine exports on shelf waters.

We collected samples from January 2010 to December 2011 to determine the concentrations and fluxes of uranium in the Changjiang (Yangtze) and Huanghe (Yellow) Rivers in China. The dissolved U concentrations (DUC) were measured by inductively coupled plasma mass spectrometry (ICP-MS) at the freshwater end members of the Changjiang and Huanghe Rivers. The DUCs ranged from 1.32 to 4.06 nmol/L and 13.85 to 29.99 nmol/L in the Changjiang and Huanghe Rivers, respectively. The temporal variations of DUC followed the seasonal change, with high values in the two rivers occurring during the dry seasons and low values during the flood seasons. A strong negative correlation was observed between DUC and discharge in the Changjiang River (R2 = 0.69), but a weak correlation (R2 = 0.35) was found in the Huanghe River. The correlations between the major ions and the U in the rivers indicated that the primary source of uranium was from the weathering of carbonate and evaporite in the Changjiang Basin. The weathering of evaporite-bearing sequences and the erosion of loess dominated the U sources of the Huanghe River. Carbonate ligands in the dry season and phosphate ligands in the wet season were the primary factors controlling the accumulation and transportation of dissolved uranium in the Changjiang River. The soils of the Huanghe Basin contained five times more leachable uranium compared to the soils of the Changjiang Basin, which may explain the high DUC in the Huanghe River. The weighted-mean-concentrations of uranium were 2.78 nmol/L in the Changjiang River and 22.07 nmol/L in the Huanghe River. This lead to annual dissolved U fluxes (DUF) of 2.3 × 106 mol/yr in the Changjiang River and 4.1 × 105 mol/yr in the Huanghe River. The sum of the U fluxes in the two rivers represented 11.9% of the global U riverine flux into the sea comparing with 2.5% of the global runoff into the sea. The 234U/238U activity ratio of the Huanghe River had higher values (1.455–1.418) compared to the Changjiang River (1.424–1.374) from the dry season to the wet season, respectively.

We discovered a hotspot of elevated nitrate concentration (median = 431 μM) in shallow beach pore water that extended across the entire length of two barrier islands in the southeastern United States of America. We investigated this feature by surveying groundwater geochemistry, measuring fluctuations in in situ dissolved oxygen (DO) concentrations, modeling groundwater flow, and quantifying nitrification rates. Nitrification of groundwater ammonium was the only possible nitrate source, with a measured potential rate of 0.84 μmol m−2 h−1. However, the observed nitrate concentrations were far in excess of the predicted maximum achievable by aerobic nitrification assuming a 2 : 1 ratio of O : N and around 200 μM DO in air-saturated seawater. Groundwater DO concentrations within the hotspot (65 cm depth) were consistently 20–50 μM. The nitrate hotspot was located at the top of the water table beneath dry, undersaturated sand that allowed the penetration of air and the dissolution of excess oxygen into the pore fluids. The total dissolved nitrogen concentration of the hotspot was higher than anywhere else on the island, indicating nitrogen accumulation within the hotspot, most likely via ammonium adsorption. Vertical dispersion was the dominant pathway for nitrate loss from the hotspot. This nitrate was consumed in underlying anoxic sand, coupling microbial pathways of nitrogen oxidation and reduction and removing bioavailable nitrogen from the beach ecosystem.

Along the continental margins, rivers and submarine groundwater supply nutrients, trace elements, and radionuclides to the coastal ocean, supporting coastal ecosystems and, increasingly, causing harmful algal blooms and eutrophication. While the global magnitude of gauged riverine water discharge is well known, the magnitude of submarine groundwater discharge (SGD) is poorly constrained. Using an inverse model combined with a global compilation of 228Ra observations, we show that the SGD integrated over the Atlantic and Indo-Pacific Oceans between 60°S and 70°N is (12 ± 3) × 1013 m3 yr−1, which is 3 to 4 times greater than the freshwater fluxes into the oceans by rivers. Unlike the rivers, where more than half of the total flux is discharged into the Atlantic, about 70% of SGD flows into the Indo-Pacific Oceans. We suggest that SGD is the dominant pathway for dissolved terrestrial materials to the global ocean, and this necessitates revisions for the budgets of chemical elements including carbon.

Chemistry of Southern Brazilian Continental Shelf (SBS) waters, between 28.5°S and 34°S, is evaluated in relation to the mixing of thermohaline defined water masses and concomitant water column processes. Data on inorganic nutrient and trace metal (Ba, Cd, Co, Cu, Fe, Mn, U, V and Zn) concentrations and radium isotopic activities from previous reports and recent analyses are presented and discussed with the aim of providing a state of knowledge on processes governing their sources, distribution, transport and fate on the SBS. Nutrients concentrations/distributions on the shelf are controlled mainly by Tropical Water/Subtropical Shelf Water, seasonally modified as a result of summer upwelling in the northern shelf region, and by Plata Plume Water, which is seasonally modified by discharges of submarine groundwater (SGD), the Patos Lagoon and by the greater northern penetration of the Rio de la Plata plume during winter. Radium-226 activity varies little across the shelf. Radium-228 activity, associated dominantly with SGD, decreases conservatively, with respect to salinity, across shelf transects, converging on a typical ocean endmember activity. The low salinity endmember activity, however, depends on the location of the shelf transect controlled by the variability of coastal SGD. Because SGD is so important to the coastal shelf region, 228 Ra activity appears to provide a better tracer of water mass interactions than thermohaline characteristics. Using metal– 228 Ra relationships, sources, transport and fate of trace metals are better constrained enabling the following conclusion: The major source of dissolved Co, Mn and Fe to the shelf is SGD, along the coastal Holocene barrier system, followed by removal as coastal water mixes across the shelf. Cu and Si concentrations are explained as conservative mixtures of three endmembers: SGD, surface freshwater discharge and oceanic. Cd and Zn are largely explained similarly. Vanadium is enriched in coastal waters during the summer presumably due to seasonal remobilization from sediments but exhibits conservative mixing behavior across the shelf. Barium behavior is the most unexpected and is speculated to be the result of biochemical removal in highly productive coastal waters followed by release from decaying phytoplankton detritus as it is advected across the shelf.

Multiple techniques, including thermal infrared aerial remote sensing, geophysical and geological data, geochemical characterization and radium isotopes, were used to evaluate the role of groundwater as a source of dissolved nutrients, carbon, and trace gases to the Okatee River estuary, South Carolina. Thermal infrared aerial remote sensing surveys illustrated the presence of multiple submarine groundwater discharge sites in Okatee headwaters. Significant relationships were observed between groundwater geochemical constituents and 226 Ra activity in groundwater with higher 226 Ra activity correlated to higher concentrations of organics, dissolved inorganic carbon, nutrients, and trace gases to the Okatee system. A system-level radium mass balance confirmed a substantial submarine groundwater discharge contribution of these constituents to the Okatee River. Diffusive benthic flux measurements and potential denitrification rate assays tracked the fate of constituents in creek bank sediments. Diffusive benthic fluxes were substantially lower than calculated radium-based submarine groundwater discharge inputs, showing that advection of groundwater-derived nutrients dominated fluxes in the system. While a considerable potential for denitrification in tidal creek bank sediments was noted, in situ denitrification rates were nitrate-limited, making intertidal sediments an inefficient nitrogen sink in this system. Groundwater geochemical data indicated significant differences in groundwater chemical composition and radium activity ratios between the eastern and western sides of the river; these likely arose from the distinct hydrological regimes observed in each area. Groundwater from the western side of the Okatee headwaters was characterized by higher concentrations of dissolved organic and inorganic carbon, dissolved organic nitrogen, inorganic nutrients and reduced metabolites and trace gases, i.e. methane and nitrous oxide, than groundwater from the eastern side. Differences in microbial sulfate reduction, organic matter supply, and/or groundwater residence time likely contributed to this pattern. The contrasting features of the east and west sub-marsh zones highlight the need for multiple techniques for characterization of submarine groundwater discharge sources and the impact of biogeochemical processes on the delivery of nutrients and carbon to coastal areas via submarine groundwater discharge.

US National Science Foundation [OCE-0926559, OCE-1231211]; National Basic Research Program ("973" Program) of China [2009CB421203]; Natural Science Foundation of China (NSFC) [41076041]; National Natural Science Foundation of China [41121091]

State Key Laboratory of Estuarine and Coastal Re-search of China [SKLEC-2012KYYW05, 2011KYYW04]; 111 project [B08022]

Large amounts of fission products were released from the Fukushima nuclear accident after a devastating earthquake and subsequent tsunami hit the northeast coast of Japan on 11 March 2011. The radioactive mass was sent high into the atmosphere by hydrogen explosions and fires in the reactor buildings at Fukushima Daiichi Nuclear Power Plant and spread all over the world. A relatively complete detection of both gaseous and particulate fission products was conducted during 15 March and 30 May 2011 at Orangeburg, South Carolina, located along the southeast coast of the United States, 11,000 km from the accident site. The histograms of gaseous and particulate radionuclides were obtained, and the major radioactivity plateaus were found between 18 March and 7 April 2011. The maximum levels of particulate and gaseous 131I were 1.0 ± 0.1 and 5.0 ± 0.4 mBq m-3, respectively. The maximum radioactivities of 134Cs and 137Cs were 10 times less than that of the particulate 131I. The average activity ratio of 134Cs to 137Cs was determined as 0.98 ± 0.26 throughout the observation. It was found that the plateaus and spikes in the histogram curves corresponded to the nuclear release events at Fukushima Daiichi. The arrival times of the particulate and gaseous nuclear fallout were determined to be 8 and 10 d, respectively. The deposition rates of gaseous and particulate iodine and the mass transfer between the two phases were discussed based on the radioactivity ratios of the fission products. By comparing with the radionuclide effluent concentrations issued in NRC 10 CFR 20, it was concluded that the Fukushima fallout presented negligible radiation risk to the public at Orangeburg as well as in the southeastern coastal region of the U.S.