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To investigate the role of the seepage zone in transport, chemical speciation, and attenuation of nitrogen loads carried by submarine groundwater discharge, we collected nearshore groundwater samples (n = 328) and examined the distribution and isotopic signature δ¹⁵N of nitrate and ammonium. In addition, we estimated nutrient fluxes from terrestrial and marine groundwater sources. We discuss our results in the context of three aquifer zones: a fresh groundwater zone, a shallow salinity transition zone (STZ), and a deep STZ. Groundwater plumes containing nitrate and ammonium occurred in the freshwater zone, whereas the deep STZ carried almost exclusively ammonium. The distributions of redox-cycled elements were consistent with theoretical thermodynamic stability of chemical species, with sharp interfaces between water masses of distinct oxidation : reduction potential, suggesting that microbial transformations of nitrogen were rapid relative to dispersive mixing. In limited locations in which overlap occurs between distribution of nitrate with that of ammonium and dissolved Fe²⁺, changes in concentration and in δ¹⁵N suggest loss of all species. Concurrent removal of ${\\rm{NO}}_{\\rm{3}}^{\\rm{ - }} $ and ${\\rm{NH}}_{4}^{\\rm{ + }} $, both in freshwater and the deep STZ, might occur through a range of mechanisms, including heterotrophic or autotrophic denitrification, coupled nitrfication : denitrification, anammox, or Mn oxidation of ${\\rm{NH}}_{4}^{\\rm{ + }} $. Loss of nitrogen was not apparent in the shallow STZ, perhaps because of short water residence time. Despite organic C- poor conditions, the nearshore aquifer and subterranean estuary are biogeochemically active zones, where attenuation of N loads can occur. Extent of attenuation is controlled by the degree of mixing of biogeochemically dissimilar water masses, highlighting the critical role of hydrogeology in N biogeochemistry. Mixing is related in part to thinning of the freshwater lens before discharge and to dispersion at the fresh : saline groundwater interface, features common to all submarine groundwater discharge zones.

In order to assess the role of submarine groundwater discharge (SGD) and its impact on the carbonate system on the northern South China Sea (NSCS) shelf, we measured seawater concentrations of four radium isotopes 223,224,226,228Ra along with carbonate system parameters in June–July, 2008. Complementary groundwater sampling was conducted in coastal areas in December 2008 and October 2010 to constrain the groundwater end-members. The distribution of Ra isotopes in the NSCS was largely controlled by the Pearl River plume and coastal upwelling. Long-lived Ra isotopes (228Ra and 226Ra) were enriched in the river plume but low in the offshore surface water and subsurface water/upwelling zone. In contrast, short-lived Ra isotopes (224Ra and 223Ra) were elevated in the subsurface water/upwelling zone as well as in the river plume but depleted in the offshore surface water. In order to quantify SGD, we adopted two independent mathematical approaches. Using a three end-member mixing model with total alkalinity (TAlk) and Ra isotopes, we derived a SGD flux into the NSCS shelf of 2.3–3.7 × 108 m3 day−1. Our second approach involved a simple mass balance of 228Ra and 226Ra and resulted in a first order but consistent SGD flux estimate of 2.2–3.7 × 108 m3 day−1. These fluxes were equivalent to 12–21 % of the Pearl River discharge, but the source of the SGD was mostly recirculated seawater. Despite the relatively small SGD volume flow compared to the river, the associated material fluxes were substantial given their elevated concentrations of dissolved inorganic solutes. In this case, dissolved inorganic carbon (DIC) flux through SGD was 153–347 × 109 mol yr−1, or ~23–53 % of the riverine DIC export flux. Our estimates of the groundwater-derived phosphate flux ranged 3–68 × 107 mol yr−1, which may be responsible for new production on the shelf up to 0.3–6.3 mmol C m−2 d−1. This rate of new production would at most consume 11 % of the DIC contribution delivered by SGD. Hence, SGD may play an important role in the carbon balance over the NSCS shelf.

The international GEOTRACES program has been instrumental in demonstrating how marine sediments are a critical source of dissolved Fe to the world's oceans. Here, we present dissolved iron (dFe) from the GEOTRACES North Pacific GP15 section, which, alongside other sediment‐source tracers (including dissolved δ56Fe, Mn, 228Ra, and particulate Fe), allows for identification of the dFe provenance of three distinct dFe depth maxima at the Alaskan margin. Two of these (shelf and abyssal depths) are of local Alaskan sedimentary origin. The third, a mid‐depth dFe maximum with an absence of 228Ra, is an advected signal that, based on tracer data from Western Pacific GEOTRACES transects and circulation models, must be advected from sedimentary sources on the Asian margin, ∼5,000 km away. This study illustrates the importance of measuring diagnostic sedimentary tracers like radium when assigning local margins as sedimentary sources of marine trace metal budgets. Plain Language Summary Iron is an essential, yet limiting, micronutrient for marine primary producers, and thus influences patterns of global oceanic primary productivity and carbon exchange. In recent years, the International GEOTRACES program has highlighted that marine sediments, hydrothermal vents, and atmospheric dust all supply dissolved iron to the oceans. Here, we investigated the sources of dissolved iron to the Eastern North Pacific Ocean, using samples collected on the U.S. GEOTRACES GP15 Pacific Meridional Cruise that sailed from Alaska to Tahiti in 2018. We identified three elevated dissolved iron features close to the Alaskan continental margin, with two originating from local sedimentary sources (shelf and abyss). The third, an intermediate depth dissolved iron plume that extends south into the gyre, is not of local sedimentary origin, but instead results from long‐distance transport of dissolved iron from Asian marginal sediment sources. A critical aspect of this study is the use of multiple chemical tracers such as radium, iron, and manganese, coupled with ocean circulation models, to correctly attribute the sources of trace metals to the ocean. Key Points Three distinct dFe maxima were identified close to the Alaskan margin at different depths. Two are of local origin (shelf and abyssal) An intermediate depth dFe plume with an absence of Mn and Ra, is a distal advected signal from Asian margin sedimentary sources (5,000 km away) A multiple tracer approach of Fe, Mn, and Ra was necessary to rule out what appeared to be a local margin source

Radium has four naturally occurring isotopes that have proven useful in constraining water mass source, age, and mixing rates in the coastal and open ocean. In this study, we used radium isotopes to determine the fate and flux of runoff-derived cesium from the Fukushima Dai-ichi Nuclear Power Plant (FNPP). During a June 2011 cruise, the highest cesium (Cs) concentrations were found along the eastern shelf of northern Japan, from Fukushima south, to the edge of the Kuroshio Current, and in an eddy ~ 130 km from the FNPP site. Locations with the highest cesium also had some of the highest radium activities, suggesting much of the direct ocean discharges of Cs remained in the coastal zone 2–3 months after the accident. We used a short-lived Ra isotope (223Ra, t1/2 = 11.4 d) to derive an average water mass age (Tr) in the coastal zone of 32 days. To ground-truth the Ra age model, we conducted a direct, station-by-station comparison of water mass ages with a numerical oceanographic model and found them to be in excellent agreement (model avg. Tr = 27 days). From these independent Tr values and the inventory of Cs within the water column at the time of our cruise, we were able to calculate an offshore 134Cs flux of 3.9–4.6 × 1013 Bq d−1. Radium-228 (t1/2 = 5.75 yr) was used to derive a vertical eddy diffusivity (Kz) of 0.7 m2 d−1 (0.1 cm2 s−1); from this Kz and 134Cs inventory, we estimated a 134Cs flux across the pycnocline of 1.8 × 104 Bq d−1 for the same time period. On average, our results show that horizontal mixing loss of Cs from the coastal zone was ~ 109 greater than vertical exchange below the surface mixed layer. Finally, a mixing/dilution model that utilized our Ra-based and oceanographic model water mass ages produced a direct ocean discharge of 134Cs from the FNPP of 11–16 PBq at the time of the peak release in early April 2011. Our results can be used to calculate discharge of other water-soluble radionuclides that were released to the ocean directly from the Fukushima NPP.

In the Southern Ocean near the Antarctic Peninsula, Antarctic Circumpolar Current (ACC) fronts interact with shelf waters facilitating lateral transport of shelf‐derived components such as iron into high‐nutrient offshore regions. To trace these shelf‐derived components and estimate lateral mixing rates of shelf water, we used naturally occurring radium isotopes. Short‐lived radium isotopes were used to quantify the rates of shelf water entrainment while Fe/228Ra ratios were used to calculate the Fe flux. In the summer of 2006 we found rapid mixing and significant lateral iron export, namely, a dissolved iron flux of 1.1 × 105 mol d−1 and total acid leachable iron flux of 1.1 × 106 mol d−1 all of which is transported in the mixed layer from the shelf region offshore. This dissolved iron flux is significant, especially considering that the bloom observed in the offshore region (0.5–2 mg chl a m−3) had an iron demand of 1.1 to 4 × 105 mol Fe. Net vertical export fluxes of particulate Fe derived from 234Th/238U disequilibrium and Fe/234Th ratios accounted for only about 25% of the dissolved iron flux. On the other hand, vertical upward mixing of iron rich deeper waters provided only 7% of the lateral dissolved iron flux. We found that similarly to other studies in iron‐fertilized regions of the Southern Ocean, lateral fluxes overwhelm vertical inputs and vertical export from the water column and support significant phytoplankton blooms in the offshore regions of the Drake Passage.

We studied the effect of iron addition on particle export in the Southern Ocean by measuring changes in the distribution of thorium-234 during a 4-week iron (Fe) enrichment experiment conducted in the high-silicate high-nitrate waters just south of the Southern Antarctic Circumpolar Current Front at 172.5°W. Decreases in ^{234}\\text{Th}$ activity with time in the fertilized mixed layer (0-50 m) exceeded those in unfertilized waters, indicating enhanced export of ^{234}\\text{Th}$ on sinking particles after Fe enrichment. The addition of Fe also affected export below the fertilized patch by increasing the efficiency of particle export through the 100-m depth horizon. Extensive temporal and vertical Lagrangian sampling allowed us to make a detailed examination of the ^{234}\\text{Th}$ flux model, which was used to quantify the fluxes of particulate organic carbon (POC) and biogenic silica ($\\text{bSiO}_{2}$). Iron addition increased the flux of both POC and $\\text{bSiO}_{2}$ out of the mixed layer by about 300%. The flux at 100 m increased by more than 700% and 600% for POC and $\\text{bSiO}_{2}$, respectively. The absolute magnitude of the POC and $\\text{bSiO}_{2}$ fluxes were not large relative to natural blooms at these latitudes or to those found in association with the termination of blooms in other ocean regions. Our results support the hypothesis that Fe addition leads directly to significant particle export and sequestration of C in the deep ocean. This is a key link between ocean Fe inputs and past changes in atmospheric CO2 and climate.

An unresolved issue in ocean and climate sciences is whether changes to the surface ocean input of the micronutrient iron can alter the flux of carbon to the deep ocean. During the Southern Ocean Iron Experiment, we measured an increase in the flux of particulate carbon from the surface mixed layer, as well as changes in particle cycling below the iron-fertilized patch. The flux of carbon was similar in magnitude to that of natural blooms in the Southern Ocean and thus small relative to global carbon budgets and proposed geoengineering plans to sequester atmospheric carbon dioxide in the deep sea.

The element silicon (Si) is required for the growth of silicified organisms in marine environments, such as diatoms. These organisms consume vast amounts of Si together with N, P, and C, connecting the biogeochemical cycles of these elements. Thus, understanding the Si cycle in the ocean is critical for understanding wider issues such as carbon sequestration by the ocean's biological pump. In this review, we show that recent advances in process studies indicate that total Si inputs and outputs, to and from the world ocean, are 57 % and 37 % higher, respectively, than previous estimates. We also update the total ocean silicic acid inventory value, which is about 24 % higher than previously estimated. These changes are significant, modifying factors such as the geochemical residence time of Si, which is now about 8000 years, 2 times faster than previously assumed. In addition, we present an updated value of the global annual pelagic biogenic silica production (255 Tmol Si yr−1) based on new data from 49 field studies and 18 model outputs, and we provide a first estimate of the global annual benthic biogenic silica production due to sponges (6 Tmol Si yr−1). Given these important modifications, we hypothesize that the modern ocean Si cycle is at approximately steady state with inputs =14.8(±2.6) Tmol Si yr−1 and outputs =15.6(±2.4) Tmol Si yr−1. Potential impacts of global change on the marine Si cycle are discussed.

Because of rapid increases in population, anthropogenic sources of nitrogen have adversely impacted the water quality of coastal ponds on Cape Cod. A major source of \"new\" nitrogen to these estuaries is groundwater, which intercepts septic tank fields in its flow path to the coastline. Many attempts have been made to quantify this process; however, groundwater discharge is often patchy in nature and is therefore difficult to study by use of traditional techniques such as seepage meters. In Waquoit Bay, MA, we tested an approach based on radium, which is naturally enriched in aquifer fluids and has four isotopes with half-lives ranging from 4 d to 1600 yr. Groundwater entering the bay was low in salinity and contained several orders of magnitude greater radium and dissolved inorganic nitrogen (DIN) relative to ambient bay water. Using a mass-balance approach for radium, we calculated a submarine groundwater flux of ∼37,000 m3d-1, which compared well with aquifer recharge rates calculated from rainfall. From the DIN content of the groundwater, we estimated that ∼2100 mol N d-1was directly input to the estuary. However, this nitrogen flux was small in comparison with literature values for DIN fluxes from the heavily populated subestuaries. Furthermore, our results suggest that groundwater flux of DIN was assimilated by plant biomass during the summer but may be exported from the embayment to coastal waters during the winter months.

Fresh groundwater discharge is an important vehicle for nitrogen transport to coastal waters. Talbor et al, in their study, investigates the flux of dissolved inorganic nitrogen to the head of Waquoit Bay through both the fresh water and intermediate-salinity of portions of the near-shore aquifer and investigate the behavior of N during transport through the subterranean estuary. The results suggest that DIN flux through the subterranean estuary of Waquoit Bay may be significant; relative to flux due to freshwater discharge.