No abstract

Oxygen minimum zones (OMZs), where the concentrations of oxygen are at analytical detection limits, but sulfate reduction is not occurring, are termed suboxic and typically nitrate serves as the terminal electron acceptor for heterotrophic respiration. Such respiration is highest near the top of OMZs where maxima in nitrite and other redox active species are observed. Within OMZs, the availability of free electrons for reduction reactions (pε) can range from −2 to 10 and at thermodynamic equilibrium could control the oxidation states of essential trace elements like iron and toxic ones like arsenic. In turn, this oxidation/reduction speciation affects trace element solubility and bioavailability, thus controlling their reactivity and transport. Field observations and mesoscale models demonstrate the increasing areal extent of oxygen minimum zones, but whether this in turn affects trace element cycling and fluxes remains to be verified. From current observations, it is difficult to determine the relative importance of in situ processes and lateral transport from boundaries to trace elements' redox cycling, which are typically highly redox-and biologically-active in these systems. Such processes could lead to the co-occurrence of redox species that are not controlled by a unique pε. In order to evaluate the effects of in situ redox processes versus advective/diffusive transport on trace elements, the cycling of nitrate/nitrite, iodate/iodide, Fe(II), As(III)/As(V), and hydrogen sulfide was examined in the low oxygen waters off the Peru shelf to the open South Pacific Ocean during the US GEOTRACES GP16 transect in 2013. Nitrite, Fe(II), and iodide were observed from the shelf to 95°W, while reduced forms of sulfur and arsenic were absent. Maxima in nitrite, Fe(II) and iodide were coincident, indicating the utilization of Fe(III), iodate and nitrate as terminal electron acceptors was possible. For iron, this finding was unexpected with regards to thermodynamics. However, closer inspection combined with advective/diffusive modeling of water column data and 228 Ra-based flux calculations, showed that benthic redox processes, coupled with westward isopycnal transport, influenced the chemical composition and redox speciation within the upper OMZ well offshore. This horizontal transport contributed to the coexistence of Fe(II) with nitrate, indicating that Fe redox chemistry is under kinetic, rather than thermodynamic, control.

Atmospheric deposition is an important input route of trace elements (TEs) to the global ocean. As atmospheric inputs impact phytoplankton community health and dynamics, atmospheric TE fluxes, and in particular atmospheric iron fluxes, are a key component of marine biogeochemical models. Trace element concentrations were determined in dry (aerosols) and wet (precipitation) deposition samples from the North Atlantic, north of 40°N, during the GEOVIDE cruise (GEOTRACES cruise GA01) in May/June 2014. Atmospheric aerosol loading in the study region was low (~2-500 ng m -3 ) throughout the cruise, as inferred from the very low aerosol Ti concentrations determined (0.0084-1.9 ng m -3 ). Wet deposition appeared to be of roughly equal or greater importance than dry deposition to the total depositional flux of TEs, which is consistent with other regions of the Atlantic Ocean outside of the influence of the Saharan plume.It can be challenging to convert aerosol chemical composition data into reliable flux estimates, due to the uncertainties associated with the parameterisation of dry deposition velocity, and precipitation rate.Therefore, the goal of this study was to compare TE flux estimates derived from two different techniques: (1) the traditional approach of summed wet and dry deposition TE fluxes, using concentration data, precipitation rates, and dry deposition velocities and, (2) using the inventory of the cosmogenic radioisotope beryllium-7 ( 7 Be) in the upper ocean as a proxy for atmospheric deposition. These two approaches yielded TE flux estimates that were in excellent agreement (within one standard deviation) for about half of the TEs under investigation. However, for the remaining TEs differences between the flux estimates ranged from two to forty times, with the traditional approach generally being the higher of the two estimates. Therefore, factors that may contribute to this variation, such as differences in the timescale of integration and selection of representative deposition velocities and precipitation rates, are discussed. Our results suggest that the 7 Be approach continues to show promise in this application, particularly in regions where precipitation samples cannot be routinely collected.

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 (226 Ra, 228 Ra) along a zonal transect at 12°S (US GEOTRACES GP16) in the eastern tropical South Pacific Ocean. We used 228 Ra 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 228 Ra activities were measured in surface water over the entire transect (~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 228 Ra shelf fluxes combined with water column dissolved TEI/ 228 Ra ratios were used to quantify the shelf-ocean input rates (normalized to shelf-area) for DMn (3.3 × 10 3 μmol m − 2 y − 1), DFe (1.5 × 10 3 μmol m − 2 y − 1), and DCo (1.0 × 10 2 μmol m − 2 y − 1). Second, co-occurring plumes of 228 Ra, 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 228 Ra 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 μmol m − 2 y − 1) and DFe (5.9 × 10 2 μmol m − 2 y − 1) fluxes based on their offshore concentration gradients. On the scale of the South Pacific continental margin between 0-20°S, the DMn shelf flux is approximately 2-3 orders of magnitude higher than the slope flux, while the DFe shelf/slope flux is~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.

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.