R. Middag

Controls of Trace Metals in Seawater

This chapter presents a general overview of the major controls of trace metals in seawater, developed from the extensive research on trace metals over the last few decades. The reader should be given a first-order understanding and insight into trace metal biogeochemistry in the oceans, rather than presented with a comprehensive review of the distribution of each trace metal. Each of the trace metals discussed will undoubtedly prove to have unique characteristics and subtle differences from this version, yet the comparison with these characteristics will serve as a good springboard to a more complete understanding.

Dissolved iron in the Arctic Ocean: Important role of hydrothermal sources, shelf input and scavenging removal

Arctic Ocean waters exchange with the North Atlantic, and thus dissolved iron (DFe) in the Arctic has implications for the global Fe cycle. We present deep water (>250 m) DFe concentrations of the Central Arctic Ocean (Nansen, Amundsen and Makarov Basins). The DFe concentration in the deep waters varies considerably between these basins, with the lowest DFe concentrations (0.2-0.4 nM) in the Makarov Basin, higher concentrations (similar to 0.45 nM) in the Amundsen Basin and highest concentrations (similar to 0.6-0.7 nM) in the Nansen Basin. Atlantic input from the shelf seas and slopes enhances the DFe concentration in the Nansen Basin. Moreover, hydrothermal activity at the Gakkel Ridge causes a significant and widespread enrichment of DFe in the Eurasian Basins, at a depth of 2000-3000 m. Below this maximum, the important role of scavenging and absence of input sources are reflected in a strong relation with dissolved Mn (DMn) and in very low ( 3000 m) Amundsen and Makarov Basins. The depth profiles of DFe in the Arctic Ocean, notably in the Makarov Basin, deviate from the DFe distribution pattern observed in other parts of the world ocean.

The effects of continental margins and water mass circulation on the distribution of dissolved aluminum and manganese in Drake Passage

A total of 232 samples were analyzed for concentrations of dissolved aluminum ([Al]) and manganese ([Mn]) in Drake Passage. Both [Al] and [Mn] were extremely low (similar to 0.3 and 0.1 nM, respectively) in the surface layer of the middle Drake Passage, most likely due to limited input and biological uptake/scavenging. Elevated [Al] (>14 nM) and [Mn] (>2 nM) over the South American continental shelf are related to land run-off, whereas elevated concentrations (>1 nM and >2 nM, respectively) near the Antarctic Peninsula are most likely related to sediment re-suspension. Re-suspension of sedimentary particles and pore waters influences the distribution of [Al] and [Mn] over the continental slopes on both sides of Drake Passage. The influence of the continental margins and accumulated dust input potentially explains the higher [Al] observed eastward in the Atlantic section of the Southern Ocean. In the northern Drake Passage, elevated [Al] (similar to 0.8 nM) and [Mn] (similar to 0.3 nM) near the seafloor are most likely the result of bottom sediment re-suspension by the relatively strong currents. In the deep southern Drake Passage sediment re-suspension associated with the inflow of Weddell Sea Deep Water appears to cause elevated [Al] (>1 nM) and [Mn] (similar to 0.4 nM). In the deep northern Drake Passage, North Atlantic Deep Water brings in elevated [Al] and Southeast Pacific Deep Slope Water brings in the signature of Pacific hydrothermal vents. Elevated [Mn] and delta He-3 were correlated in this water layer and are most likely originating from the volcanically active ridges in the Pacific Ocean.

Aluminium in an ocean general circulation model compared with the West Atlantic Geotraces cruises.

A 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.

On the effects of circulation, sediment resuspension and biological incorporation by diatoms in an ocean model of aluminium

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.

Return of naturally sourced Pb to Atlantic surface waters

Anthropogenic emissions completely overwhelmed natural marine lead (Pb) sources during the past century, predominantly due to leaded petrol usage. Here, based on Pb isotope measurements, we reassess the importance of natural and anthropogenic Pb sources to the tropical North Atlantic following the nearly complete global cessation of leaded petrol use. Significant proportions of up to 30–50% of natural Pb, derived from mineral dust, are observed in Atlantic surface waters, reflecting the success of the global effort to reduce anthropogenic Pb emissions. The observation of mineral dust derived Pb in surface waters is governed by the elevated atmospheric mineral dust concentration of the North African dust plume and the dominance of dry deposition for the atmospheric aerosol flux to surface waters. Given these specific regional conditions, emissions from anthropogenic activities will remain the dominant global marine Pb source, even in the absence of leaded petrol combustion.

Manganese in the west Atlantic Ocean in the context of the first global ocean circulation model of manganese

Abstract. 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 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 and 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 contributing 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 0.10–0.15 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.