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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.

The discharge of nutrient-rich meltwater from the Greenland Ice Sheet has emerged as a potentially important contributor to regional marine primary production and nutrient cycling. While significant, this direct nutrient input by the ice sheet may be secondary to the upwelling of deep-ocean-sourced nutrients driven by the release of meltwater at depth in glacial fjords. Here, we present a comprehensive suite of micro- and macronutrient observations collected in Sermilik Fjord at the margin of Helheim, one of Greenland’s largest glaciers, and quantitatively decompose glacial and ocean contributions to fjord dissolved nutrient inventories. We show that the substantial enrichment in nitrate, phosphate and silicate observed in the upper 250 m of the glacial fjord is the result of upwelling of warm subtropical waters present at depth throughout the fjord. These nutrient-enriched fjord waters are subsequently exported subsurface to the continental shelf. The upwelled nutrient transport within Sermilik rivals exports by the largest Arctic rivers and the ice sheet as a whole, suggesting that glacier-induced pumping of deep nutrients may constitute a major source of macronutrients to the surrounding coastal ocean. The importance of this mechanism is likely to grow given projected increases in surface melt of the ice sheet.

There are 440 operational nuclear reactors in the world, with approximately one-half situated along the coastline. This includes the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), which experienced multiple reactor meltdowns in March 2011 followed by the release of radioactivity to the marine environment. While surface inputs to the ocean via atmospheric deposition and rivers are usually well monitored after a nuclear accident, no study has focused on subterranean pathways. During our study period, we found the highest cesium-137 (137Cs) levels (up to 23,000 Bq·m−3) outside of the FDNPP site not in the ocean, rivers, or potable groundwater, but in groundwater beneath sand beaches over tens of kilometers away from the FDNPP. Here, we present evidence of a previously unknown, ongoing source of Fukushima-derived 137Cs to the coastal ocean. We postulate that these beach sands were contaminated in 2011 through wave- and tide-driven exchange and sorption of highly radioactive Cs from seawater. Subsequent desorption of 137Cs and fluid exchange from the beach sands was quantified using naturally occurring radium isotopes. This estimated ocean 137Cs source (0.6 TBq·y−1) is of similar magnitude as the ongoing releases of 137Cs from the FDNPP site for 2013–2016, as well as the input of Fukushima-derived dissolved 137Cs via rivers. Although this ongoing source is not at present a public health issue for Japan, the release of Cs of this type and scale needs to be considered in nuclear power plant monitoring and scenarios involving future accidents.

Groundwater-derived solute fluxes to the ocean have long been assumed static and subordinate to riverine fluxes, if not neglected entirely, in marine isotope budgets. Here we present concentration and isotope data for Li, Mg, Ca, Sr, and Ba in coastal groundwaters to constrain the importance of groundwater discharge in mediating the magnitude and isotopic composition of terrestrially derived solute fluxes to the ocean. Data were extrapolated globally using three independent volumetric estimates of groundwater discharge to coastal waters, from which we estimate that groundwater-derived solute fluxes represent, at a minimum, 5% of riverine fluxes for Li, Mg, Ca, Sr, and Ba. The isotopic compositions of the groundwater-derived Mg, Ca, and Sr fluxes are distinct from global riverine averages, while Li and Ba fluxes are isotopically indistinguishable from rivers. These differences reflect a strong dependence on coastal lithology that should be considered a priority for parameterization in Earth-system models. Groundwater discharge is a mechanism that transports chemicals from inland systems to the ocean, but it has been considered of secondary influence compared to rivers. Here the authors assess the global significance of groundwater discharge, finding that it has a unique and important contribution to ocean chemistry and Earth-system models.

The micronutrient iron is thought to limit primary production in large regions of the global ocean. Meltwater measurements suggest that the Greenland ice sheet serves as a significant source of potentially bioavailable iron to the surrounding coastal ocean The micronutrient iron is thought to limit primary productivity in large regions of the global ocean 1 . Ice sheets and glaciers have been shown to deliver bioavailable iron to the coastal and open ocean in the form of sediment released from the base of icebergs 2 , 3 and glacially derived dust 4 . More direct measurements from glacial runoff are limited, but iron concentrations are thought to be in the nanomolar range 5 . Here we present measurements of dissolved and particulate iron concentrations in glacial meltwater from the southwest margin of the Greenland ice sheet. We report micromolar concentrations of dissolved and particulate iron. Particulate iron concentrations were on average an order of magnitude higher than those of dissolved iron, and around 50% of this particulate iron was deemed to be potentially bioavailable, on the basis of experimental leaching. If our observations are scalable to the entire ice sheet, then the annual flux of dissolved and potentially bioavailable particulate iron to the North Atlantic Ocean would be approximately 0.3 Tg. This is comparable to dust-derived soluble iron inputs to the North Atlantic. We suggest that glacial runoff serves as a significant source of bioavailable iron to surrounding coastal oceans, which is likely to increase as melting of the Greenland ice sheet escalates under climate warming.

A seasonal study of radium-derived submarine groundwater discharge (SGD) and associated nitrogen fluxes was carried out in a salt marsh estuary between 2001 and 2003 (Pamet River Estuary, Massachusetts). Twelve hour time series of salinity and radium at the estuary inlet were used to determine the relative importance of fresh versus saline SGD, respectively. The distinct radium $(^228{\\rm Ra}$ : $^226{\\rm Ra})$ isotopic signature of marsh peat pore water and aquifer-derived brackish groundwater was used to further partition the Ra-derived SGD estimate. Of these three groundwater sources, only the marsh-derived groundwater was constant across time. The ratio of brackish to fresh SGD was inversely correlated with water table elevation in the aquifer, suggesting that Ra-derived SGD was enhanced during dry periods. The various SGD fluxes were responsible for an average annual dissolved inorganic nitrogen (DIN) input of between 1.7 mol m⁻² yr⁻¹ and 7.1 mol m⁻² yr⁻¹ and a soluble reactive phosphate (SRP) flux of 0.13-0.54 mol m⁻² yr⁻¹. Approximately 30% of the SGD-derived DIN and SRP flux is exported to coastal waters (Cape Cod Bay), whereas 70% is retained by the salt marsh ecosystem.

Using radium (Ra) isotopes, we estimate that the average submarine groundwater discharge (SGD) flux (marine plus terrestrial groundwater) into the southwest Florida Shelf (SWFS) was 20 ± 10 × 10⁷ and 18 ± 8 × 10⁷ m³ d−1 in July and October 2009, respectively. The terrestrial groundwater flux was the same order of magnitude as the local river discharge in July 2009. Shelf-water total alkalinity (TAlk) and dissolved inorganic carbon (DIC) concentrations could not be explained by river inputs alone, suggesting a groundwater source. We estimated SGD fluxes of TAlk and DIC using the SGD flux derived from a shelf-water 226Ra budget and TAlk and DIC concentration differences between the groundwater and seawater. These fluxes were also determined by the observed TAlk : 226Ra and DIC : 226Ra relationships in the shelf water, and the 226Ra flux sustained by SGD. These TAlk and DIC fluxes were 11–71 times more than the combined input of local rivers, suggesting that SGD was the dominant source of TAlk and DIC to the SWFS during 2009. SGD is an important component of the inorganic carbon budget for the coastal ocean.

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’.

Iron fertilization, naturally The ocean's importance in storing carbon is widely recognized, as is the importance of iron as a limiting nutrient in much of the global ocean. But quantifying the increase in long-term carbon storage in response to the lifting of iron limitation has proved difficult. The CROZEX experiment, a cruise on-board the RRS Discovery , set out to test the hypothesis that the observed north–south gradient in phytoplankton concentrations near the Crozet Islands in the Southern Ocean is induced by natural iron fertilization that results in enhanced organic carbon flux to the deep ocean. The data support the hypothesis: carbon export fluxes to the deep from the fertile waters were two to three times greater than those fluxes from an adjacent high-nutrient low-chlorophyll area not fertilized by iron. The efficiency of carbon export was somewhat greater than that reported in experiments where iron is added artificially, a possible consequence of large losses of the artificially added iron, but smaller compared to that reported from a naturally induced bloom, possibly related to the importance of horizontal iron supply. It is found that carbon export fluxes to the deep ocean from a highly productive, naturally iron-fertilized region of the sub-Antarctic Southern Ocean are two to three times larger than the carbon export fluxes from an adjacent high-nutrient low-chlorophyll area not fertilized by iron. These findings support the hypothesis that increased iron supply to the glacial sub-Antarctic may have directly enhanced carbon export to the deep ocean. The addition of iron to high-nutrient, low-chlorophyll regions induces phytoplankton blooms that take up carbon 1 , 2 , 3 . Carbon export from the surface layer and, in particular, the ability of the ocean and sediments to sequester carbon for many years remains, however, poorly quantified 3 . Here we report data from the CROZEX experiment 4 in the Southern Ocean, which was conducted to test the hypothesis that the observed north–south gradient in phytoplankton concentrations in the vicinity of the Crozet Islands is induced by natural iron fertilization that results in enhanced organic carbon flux to the deep ocean. We report annual particulate carbon fluxes out of the surface layer, at three kilometres below the ocean surface and to the ocean floor. We find that carbon fluxes from a highly productive, naturally iron-fertilized region of the sub-Antarctic Southern Ocean are two to three times larger than the carbon fluxes from an adjacent high-nutrient, low-chlorophyll area not fertilized by iron. Our findings support the hypothesis that increased iron supply to the glacial sub-Antarctic may have directly enhanced carbon export to the deep ocean 5 . The CROZEX sequestration efficiency 6 (the amount of carbon sequestered below the depth of winter mixing for a given iron supply) of 8,600 mol mol -1 was 18 times greater than that of a phytoplankton bloom induced artificially by adding iron 7 , but 77 times smaller than that of another bloom 8 initiated, like CROZEX, by a natural supply of iron. Large losses of purposefully added iron can explain the lower efficiency of the induced bloom 6 . The discrepancy between the blooms naturally supplied with iron may result in part from an underestimate of horizontal iron supply.

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.