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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'.

Trace elements (TEs) play important roles as micronutrients in modulating marine productivity in the global ocean. The South Atlantic around 40 ∘ S is a prominent region of high productivity and a transition zone between the nitrate-depleted subtropical gyre and the iron-limited Southern Ocean. However, the sources and fluxes of trace elements to this region remain unclear. In this study, the distribution of the naturally occurring radioisotope 228 Ra in the water column of the South Atlantic (Cape Basin and Argentine Basin) has been investigated along a 40 ∘ S zonal transect to estimate ocean mixing and trace element supply to the surface ocean. Ra-228 profiles have been used to determine the horizontal and vertical mixing rates in the near-surface open ocean. In the Argentine Basin, horizontal mixing from the continental shelf to the open ocean shows an eddy diffusion of Kx=1.8±1.4 (10 6 cm 2 s -1) and an integrated advection velocity w=0.6±0.3 cm s -1. In the Cape Basin, horizontal mixing is Kx=2.7±0.8 (10 7 cm 2 s -1) and vertical mixing Kz = 1.0–1.7 cm 2 s -1 in the upper 600 m layer. Three different approaches (228 Ra diffusion, 228 Ra advection, and 228Ra/TE ratio) have been applied to estimate the dissolved trace element fluxes from the shelf to the open ocean. These approaches bracket the possible range of off-shelf fluxes from the Argentine Basin margin to be 4–21 (×103) nmol Co m -2 d -1 , 8–19 (×104) nmol Fe m -2 d -1 and 2.7–6.3 (×104) nmol Zn m -2 d -1. Off-shelf fluxes from the Cape Basin margin are 4.3–6.2 (×103) nmol Co m -2 d -1 , 1.2–3.1 (×104) nmol Fe m -2 d -1 , and 0.9–1.2 (×104) nmol Zn m -2 d -1. On average, at 40 ∘ S in the Atlantic, vertical mixing supplies 0.1–1.2 nmol Co m -2 d -1 , 6–9 nmol Fe m -2 d -1 , and 5–7 nmol Zn m -2 d -1 to the euphotic zone. Compared with atmospheric dust and continental shelf inputs, vertical mixing is a more important source for supplying dissolved trace elements to the surface 40 ∘ S Atlantic transect. It is insufficient, however, to provide the trace elements removed by biological uptake, particularly for Fe. Other inputs (e.g. particulate or from winter deep mixing) are required to balance the trace element budgets in this region. [ABSTRACT FROM AUTHOR]

Trace elements play important roles as micronutrients in modulating marine productivity in the global ocean. The South Atlantic around 40°S is a prominent region of high productivity and a transition zone between the nitrate-depleted Subtropical Gyre and the iron-limited Southern Ocean. However, the sources and fluxes of trace elements to this region remain unclear. In this study, the distribution of the naturally occurring radioisotope 228Ra in the water column of the South Atlantic (Cape Basin and Argentine Basin) has been investigated along a 40°S zonal transect to estimate ocean mixing and trace element supply to the surface ocean. Ra-228 profiles have been used to determine the horizontal and vertical mixing rates in the near-surface open ocean. In the Argentine Basin, horizontal mixing from the continental shelf to the open ocean shows an eddy diffusion of Kx = 1.7 ± 1.4 (106 cm² s-1) and an integrated advection velocity w = 0.6 ± 0.3 cm s-1. In the Cape Basin, horizontal mixing is Kx = 2.7 ± 0.8 (107 cm² s-1) and vertical mixing Kz = 1.0 - 1.5 cm² s-1 in the upper 600 m layer. Three different approaches (228Ra-diffusion, 228Ra-advection and 228Ra/TE-ratio) have been applied to estimate the dissolved trace-element fluxes from shelf to open ocean. These approaches bracket the possible range of off-shelf fluxes from the Argentine margin to be: 3.8 - 22 (x10³) nmol Co m-2 d-1, 7.9 - 20 (x104) nmol Fe m-2 d-1 and 2.7 - 6.5 (x104) nmol Zn m-2 d-1. Off-shelf fluxes from the Cape margin are: 4.3 - 6.2 (x10³) nmol Co m-2 d-1, 1.2 - 3.1 (x104) nmol Fe m-2 d-1 and 0.9 - 1.2 (x104) nmol Zn m-2 d-1. On average, at 40°S in the Atlantic, vertical mixing supplies 0.4 - 1.2 nmol Co m-2 d-1, 3.6 - 11 nmol Fe m-2 d-1, and 13 - 16 nmol Zn m-2 d-1 to the euphotic zone. Compared with atmospheric dust and continental shelf inputs, vertical mixing is a more important source for supplying dissolved trace elements to the surface 40°S Atlantic. It is insufficient, however, to provide the trace elements removed by biological uptake. Other inputs (e.g. particulate, or from [ABSTRACT FROM AUTHOR]

Continental margins are disproportionally important for global primary production, fisheries and CO2 uptake. However, across the Northeast Atlantic shelves, there has been an ongoing summertime decline of key biota—large diatoms, dinoflagellates and copepods—that traditionally fuel higher tropic levels such as fish, sea birds and marine mammals. Here, we combine multiple time series with in situ process studies to link these declines to summer nutrient stress and increasing proportions of picophytoplankton that can comprise up to 90% of the combined pico‐ and nanophytoplankton biomass in coastal areas. Among the pico‐fraction, it is the cyanobacterium Synechococcus that flourishes when iron and nitrogen resupply to surface waters are diminished. Our field data show how traits beyond small size give Synechococcus a competitive edge over pico‐ and nanoeukaryotes. Key is their ability to grow at low irradiances near the nutricline, which is aided by their superior light‐harvesting system and high affinity to iron. However, minute size and lack of essential biomolecules (e.g. omega‐3 polyunsaturated fatty acids and sterols) render Synechococcus poor primary producers to sustain shelf sea food webs efficiently. The combination of earlier spring blooms and lower summer food quantity and quality creates an increasing period of suboptimal feeding conditions for zooplankton at a time of year when their metabolic demand is highest. We suggest that this nutrition‐related mismatch has contributed to the widespread, ~50% decline in summer copepod abundance we observe over the last 60 years. With Synechococcus clades being prominent from the tropics to the Arctic and their abundances increasing worldwide, our study informs projections of future food web dynamics in coastal and shelf areas where droughts and stratification lead to increasing nutrient starvation of surface waters. [ABSTRACT FROM AUTHOR]

There has been a growing interest in the cause and impact of hypoxic regions known as "dead zones" that have increasingly appeared along the west coast of the United States and have caused widespread destruction to the crab and fishing industry in this upwelling region. Here, we present results that demonstrate that the hypoxic conditions in the water column over the continental shelf result in a marked increase in iron(ll) concentrations, which contribute to elevated dissolved and labile particulate iron concentrations. These elevated dissolved iron(ll) concentrations resuft from two factors: (1) the hypoxic water column allows extremely elevated iron(lI) concentrations in reducing porewaters to exist close to the sediment water interlace, leading to an increased flux of iron(lI) from the sediments; (2) the low oxygen, low pH, and low temperatures within the bottom boundary layer act in concert to markedly slow down the oxidation rate of Fe(ll). During upwelling conditions, this process can result in a greatly enhanced source of Fe available to upwell to surlace waters, potentially increasing phytoplankton productivity, which can, in turn, lead to enhanced export flux, driving the system further into hypoxic or suboxic conditions. [ABSTRACT FROM AUTHOR]

Nutrients and nutrient-like dissolved trace metals (dTMs) are essential for the functioning of marine organisms and therefore form an important part of ocean biogeochemical cycles. Here, we report on the seasonal distributions of dissolved zinc (dZn), nickel (dNi), copper (dCu), cadmium (dCd), aluminum (dAl), and nutrients on the Northeast Atlantic continental margin (Celtic Sea), which is representative for temperate shelf seas globally. Variations in surface water dTM and nutrient concentrations were mainly regulated by seasonal changes in biological processes. The stoichiometry of dTMs (especially for dCu and dZn) and nutrients on the continental shelf was additionally affected by fluvial inputs. Nutrients and dTMs at depth on the continental slope were determined by water mass mixing driven by ocean circulation, without an important role for local remineralization processes. The Mediterranean Outflow Waters are especially important for delivering Mediterranean-sourced dTMs to the Northeast Atlantic Ocean and drive dTM:nutrient kinks at a depth of ~1000 m. These results highlight the importance of riverine inputs, seasonality of primary production and ocean circulation on the distributions of nutrients and nutrient-like dTMs in temperate continental margin seas. Future climate related changes in the forcing factors may impact the availability of nutrients and dTMs to marine organisms in highly productive continental shelf regions and consequently the regional carbon cycle. • Biological processes and fluvial inputs drive seasonal variations in surface dissolved trace metals (TMs). • Dissolved trace metal concentrations and metal:P ratios at depth are controlled by water mass mixing. • Mediterranean Outflow Waters provide a strong imprint on TM concentrations and metal:P ratios in the Northeast Atlantic. [ABSTRACT FROM AUTHOR]