Dissolved manganese (Mn) is a biologically essential element. Moreover, its oxidised form is involved in removing itself and several other trace elements from ocean waters. Here we report the longest thus far 17 500 km length full-depth ocean section of dissolved Mn in the West Atlantic Ocean, comprising 1320 data values of high accuracy. This is the GA02 transect that is part of the GEOTRACES programme, which aims to understand trace element distributions. The goal of this study is to combine these new observations with a new, state-of-the-art, modelling to give a first assessment of the main sources and redistribution of Mn throughout the ocean. To this end, we simulate the distribution of dissolved Mn using a global-scale circulation model. This first model includes simple parameterisations to account for the sources, processes and sinks of Mn in the ocean. Oxidation and (photo)reduction, aggregation, settling, as well as biological uptake and remineralisation by plankton, are included in the model. Our model provides, together with the observations, the following insights: -The high surface concentrations of manganese are caused by the combination of photoreduction and sources to the upper ocean. The most important sources are sediments, dust, and, more locally, rivers. -Observations and model simulations suggest that surface Mn in the Atlantic Ocean moves downwards into the southward flowing North Atlantic Deep Water (NADW), but because of strong removal rates there is no elevated concentration of Mn visible any more in the NADW south of 40 • N.-The model predicts lower dissolved Mn in surface waters of the Pacific Ocean than the observed concentrations. The intense Oxygen Minimum Zone (OMZ) in subsurface waters is deemed to be a major source of dissolved Mn also mixing upwards into surface waters, but the OMZ is not well represented by the model. Improved high resolution simulation of the OMZ may solve this problem.-There is a mainly homogeneous background concentration of dissolved Mn of about 0.10 nM to 0.15 nM throughout most of the deep ocean. The model reproduces this by means of a threshold on particulate manganese oxides of 25 pM, suggesting that a minimal concentration of particulate Mn is needed before aggregation and removal become efficient.-The observed distinct hydrothermal signals are produced by assuming both a strong source and a strong removal of Mn near hydrothermal vents.

Quantifying fluxes of trace elements and their isotopes (TEIs) at the ocean's sediment–water boundary is a pre-eminent challenge to understand their role in the present, past and future ocean. There are multiple processes that drive the uptake and release of TEIs, and properties that determine their rates are unevenly distributed (e.g. sediment composition, redox conditions and (bio)physical dynamics). These factors complicate our efforts to find, measure and extrapolate TEI fluxes across ocean basins. GEOTRACES observations are unveiling the oceanic distributions of many TEIs for the first time. These data evidence the influence of the sediment–water boundary on many TEI cycles, and underline the fact that our knowledge of the source–sink fluxes that sustain oceanic distributions is largely missing. Present flux measurements provide low spatial coverage and only part of the empirical basis needed to predict TEI flux variations. Many of the advances and present challenges facing TEI flux measurements are linked to process studies that collect sediment cores, pore waters, sinking material or seawater in close contact with sediments. However, such sampling has not routinely been viable on GEOTRACES expeditions. In this article, we recommend approaches to address these issues: firstly, with an interrogation of emergent data using isotopic mass-balance and inverse modelling techniques; and secondly, by innovating pursuits of direct TEI flux measurements. We exemplify the value of GEOTRACES data with a new inverse model estimate of benthic Al flux in the North Atlantic Ocean. Furthermore, we review viable flux measurement techniques tailored to the sediment–water boundary. We propose that such activities are aimed at regions that intersect the GEOTRACES Science Plan on the basis of seven criteria that may influence TEI fluxes: sediment provenance, composition, organic carbon supply, redox conditions, sedimentation rate, bathymetry and the benthic nepheloid inventory.This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.

a b s t r a c tThe concentrations of dissolved aluminium (dissolved Al) were studied along the West Atlantic GEOTRACES GA02 transect from 64°N to 50°S. Concentrations ranged from~0.5 nmol kg −1 in the high latitude surface waters to~48 nmol kg −1 in surface waters around 25°N. Elevated surface water concentrations due to atmospheric dust loading have little influence on the deep water distribution. However, just below the thermocline, both Northern and Southern Hemisphere Subtropical Mode Waters are elevated in Al, most likely related to atmospheric dust deposition in the respective source regions. In the deep ocean, high concentrations of up to 35 nmol kg −1 were observed in North Atlantic Deep Water as a result of Al input via sediment resuspension. Comparatively low deep water concentrations were associated with water masses of Antarctic origin. During water mass advection, Al loss by scavenging overrules input via remineralisation and sediment resuspension at the basin wide scale. Nevertheless, sediment resuspension is more important than previously realised for the deep ocean Al distribution and even more intensive sampling is needed in bottom waters to constrain the spatial heterogeneity in the global deep ocean. This thus far longest (17,500 km) full depth ocean section shows that the distribution of Al can be explained by its input sources and the combination of association with particles and release from those particles at depth, the latter most likely when the particles remineralise. The association of Al with particles can be due to incorporation of Al into biogenic silica or scavenging of Al onto biogenic particles. The interaction between Al and biogenic particles can lead to the coupled cycling of Al and silicate that is observed in some ocean regions. However, in other regions this coupling is not observed due to (i) advective processes bringing in older water masses that are depleted in Al, (ii) unfavourable scavenging conditions in the water column, (iii) low surface concentrations of Al or (iv) additional Al sources, notably sediment resuspension.

Abstract. The marine biological carbon pump is dominated by the vertical transfer of particulate organic carbon (POC) from the surface ocean to its interior. The efficiency of this transfer plays an important role in controlling the amount of atmospheric carbon that is sequestered in the ocean. Furthermore, the abundance and composition of POC is critical for the removal of numerous trace elements by scavenging, a number of which, such as iron, are essential for the growth of marine organisms, including phytoplankton. Observations and laboratory experiments have shown that POC is composed of numerous organic compounds that can have very different reactivities. However, this variable reactivity of POC has never been extensively considered, especially in modelling studies. Here, we introduced in the global ocean biogeochemical model NEMO-PISCES a description of the variable composition of POC based on the theoretical reactivity continuum model proposed by Boudreau and Ruddick (1991). Our model experiments show that accounting for a variable lability of POC increases POC concentrations in the ocean's interior by 1 to 2 orders of magnitude. This increase is mainly the consequence of a better preservation of small particles that sink slowly from the surface. Comparison with observations is significantly improved both in abundance and in size distribution. Furthermore, the amount of carbon that reaches the sediments is increased by more than a factor of 2, which is in better agreement with global estimates of the sediment oxygen demand. The impact on the major macronutrients (nitrate and phosphate) remains modest. However, iron (Fe) distribution is strongly altered, especially in the upper mesopelagic zone as a result of more intense scavenging: vertical gradients in Fe are milder in the upper ocean, which appears to be closer to observations. Thus, our study shows that the variable lability of POC can play a critical role in the marine biogeochemical cycles which advocates for more dedicated in situ and laboratory experiments.

International audienceA model of aluminium has been developed and implemented in an Ocean General Circulation Model (NEMO-PISCES). In the model, aluminium enters the ocean by means of dust deposition. The internal oceanic processes are described by advection, mixing and reversible scavenging. The model has been evaluated against a number of selected high-quality datasets covering much of the world ocean, especially those from the West Atlantic Geotraces cruises of 2010 and 2011. Generally, the model results are in fair agreement with the observations. However, the model does not describe well the vertical distribution of dissolved Al in the North Atlantic Ocean. The model may require changes in the physical forcing and the vertical dependence of the sinking velocity of biogenic silica to account for other discrepancies. To explore the model behaviour, sensitivity experiments have been performed, in which we changed the key parameters of the scavenging process as well as the input of aluminium into the ocean. This resulted in a better understanding of aluminium in the ocean, and it is now clear which parameter has what effect on the dissolved aluminium distribution and which processes might be missing in the model, among which boundary scavenging and biological incorporation of aluminium into diatoms

The importance of the atmospheric deposition of biologically essential trace elements, especially iron, is widely recognized, as are the difficulties of accurately quantifying the rates of trace element wet and dry deposition and their fractional solubility. This paper summarizes some of the recent progress in this field, particularly that driven by the GEOTRACES, and other, international research programmes. The utility and limitations of models used to estimate atmospheric deposition flux, for example, from the surface ocean distribution of tracers such as dissolved aluminium, are discussed and a relatively new technique for quantifying atmospheric deposition using the short-lived radionuclide beryllium-7 is highlighted. It is proposed that this field will advance more rapidly by using a multi-tracer approach, and that aerosol deposition models should be ground-truthed against observed aerosol concentration data. It is also important to improve our understanding of the mechanisms and rates that control the fractional solubility of these tracers. Aerosol provenance and chemistry (humidity, acidity and organic ligand characteristics) play important roles in governing tracer solubility. Many of these factors are likely to be influenced by changes in atmospheric composition in the future. Intercalibration exercises for aerosol chemistry and fractional solubility are an essential component of the GEOTRACES programme.This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.

In this paper, we set forth a 3-D ocean model of the radioactive trace isotopes 230 Th and 231 Pa. The interest arises from the fact that these isotopes are extensively used for investigating particle transport in the ocean and reconstructing past ocean circulation. The tracers are reversibly scavenged by biogenic and lithogenic particles.Our simulations of 230 Th and 231 Pa are based on the NEMO-PISCES ocean biogeochemistry general circulation model, which includes biogenic particles, namely small and big particulate organic carbon, calcium carbonate and biogenic silica.Small and big lithogenic particles from dust deposition are included in our model as well. Their distributions generally compare well with the small and big lithogenic particle concentrations from recent observations from the GEOTRACES programme, except for boundary nepheloid layers for which, as up to today, there are no non-trivial, prognostic models available on a global scale. Our simulations reproduce 230 Th and 231 Pa dissolved concentrations: they compare well with recent GEOTRACES observations in many parts of the ocean. Particulate 230 Th and 231 Pa concentrations are significantly improved compared to previous studies, but they are still too low because of missing particles from nepheloid layers. Our simulation reproduces the main characteristics of the 231 Pa/ 230 Th ratio observed in the sediments, and supports a moderate affinity of 231 Pa to biogenic silica as suggested by recent observations, relative to 230 Th.Future model development may further improve understanding, especially when this will include a more complete representation of all particles, including different size classes, manganese hydroxides and nepheloid layers. This can be done based on our model, as its source code is readily available.

The distribution of dissolved aluminium in the West Atlantic Ocean shows a mirror image with that of dissolved silicic acid, hinting at intricate interactions between the ocean cycling of Al and Si. The marine biogeochemistry of Al is of interest because of its potential impact on diatom opal remineralisation, hence Si availability. Furthermore, the dissolved Al concentration at the surface ocean has been used as a tracer for dust input, dust being the most important source of the bio-essential trace element iron to the ocean. Previously, the dissolved concentration of Al was simulated reasonably well with only a dust source, and scavenging by adsorption on settling biogenic debris as the only removal process. Here we explore the impacts of (i) a sediment source of Al in the Northern Hemisphere (especially north of ∼40 • N), (ii) the imposed velocity field, and (iii) biological incorporation of Al on the modelled Al distribution in the ocean. The sediment source clearly improves the model results, and using a different velocity field shows the importance of advection on the simulated Al distribution. Biological incorporation appears to be a potentially important removal process. However, conclusive independent data to constrain the Al : Si incorporation ratio by growing diatoms are missing. Therefore, this study does not provide a definitive answer to the question of the relative importance of Al removal by incorporation compared to removal by adsorptive scavenging.