Fe-binding organic ligands in coastal and frontal regions of the western Antarctic Peninsula

Organic ligands are a key factor determining the availability of dissolved iron (DFe) in the high-nutrient low-chlorophyll (HNLC) areas of the Southern Ocean. In this study, organic speciation of Fe is investigated along a natural gradient of the western Antarctic Peninsula, from an ice-covered shelf to the open ocean. An electrochemical approach, competitive ligand exchange – adsorptive cathodic stripping voltammetry (CLE-AdCSV), was applied. Our results indicated that organic ligands in the surface water on the shelf are associated with ice-algal exudates, possibly combined with melting of sea ice. Organic ligands in the deeper shelf water are supplied via the resuspension of slope or shelf sediments. Further offshore, organic ligands are most likely related to the development of phytoplankton blooms in open ocean waters. On the shelf, total ligand concentrations ([Lt]) were between 1.2 and 6.4 nM eq. Fe. The organic ligands offshore ranged between 1.0 and 3.0 nM eq. Fe. The southern boundary of the Antarctic Circumpolar Current (SB ACC) separated the organic ligands on the shelf from bloom-associated ligands offshore. Overall, organic ligand concentrations always exceeded DFe concentrations (excess ligand concentration, [L′] = 0.8–5.0 nM eq. Fe). The [L′] made up to 80 % of [Lt], suggesting that any additional Fe input can be stabilized in the dissolved form via organic complexation. The denser modified Circumpolar Deep Water (mCDW) on the shelf showed the highest complexation capacity of Fe (αFe'L; the product of [L′] and conditional binding strength of ligands, KFe'Lcond). Since Fe is also supplied by shelf sediments and glacial discharge, the high complexation capacity over the shelf can keep Fe dissolved and available for local primary productivity later in the season upon sea-ice melting.</p

Acoustic imaging of the Dvurechenskii mud volcano in the Black Sea

In the CRIMEA project submarine gas emitting sites in the Black Sea are investigated in order to quantify methane transfer through the water column into the atmosphere. One target area is the Dvurechenskii mud volcano (DMV) in the Sorokin Trough south-east of the Crimea peninsula. The occurrence of gas hydrates and high methane concentrations in the sediment of this mud volcano are known. A seismic wide-angle experiment was performed at the DMV with twelve Ocean Bottom Hydrophones and Seismometers and a GI gun source with frequencies around 100 Hz. By using Kirchhoff depth migration the seismogram sections are transformed to images, which extent to 4 km laterally and 600 metres in depth. The images show the conduit of the DMVand the nearby sediment layers. The DMV has a diameter of 800-1000 m at the sea floor and its conduit has the same form and diameter up to 600 m depth. Several plane sediment layers are disrupted by the conduit, and strong reflectors are identified in 100 m and 400 m depth in the conduit. The lower bowl shaped reflectors are interpreted as collapsed parts of the disrupted sediment layers, which sunk in the lighter material of the conduit. This is also a possible explanation for the upper reflections. Compressional wave velocities are obtained from Kirchhoff migration, and the model is refined by using seismic ray tracing. Bulk density and shear wave velocity can also be obtained by analyzing the data. With the help of these elastic parameters and by using the Frenkel-Gassmann theory, the free gas saturation of the sediment pore space and the gas hydrate saturation can be quantified

Dissolved aluminium in the ocean conveyor of theWest Atlantic Ocean: Effects of the biological cycle, scavenging, sediment resuspension and hydrography

The concentrations of dissolved aluminium (dissolved Al) were studied along the West Atlantic GEOTRACESGA02 transect from 64°N to 50°S. Concentrations ranged from~0.5 nmol kg-1 in the high latitude surface watersto ~48 nmol kg-1 in surfacewaters around 25°N. Elevated surfacewater concentrations due to atmospheric dustloading have little influence on the deep water distribution. However, just belowthe thermocline, both Northernand Southern Hemisphere Subtropical Mode Waters are elevated in Al, most likely related to atmospheric dustdeposition in the respective source regions.In the deep ocean, high concentrations of up to 35 nmol kg-1 were observed in North Atlantic DeepWater as aresult of Al input via sediment resuspension. Comparatively lowdeepwater concentrationswere associatedwithwater masses of Antarctic origin. During water mass advection, Al loss by scavenging overrules input viaremineralisation and sediment resuspension at the basin wide scale. Nevertheless, sediment resuspension ismore important than previously realised for the deep ocean Al distribution and even more intensive samplingis 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 itsinput sources and the combination of association with particles and release from those particles at depth, thelattermost likelywhen the particles remineralise. The association of Alwith particles can be due to incorporationof Al into biogenic silica or scavenging of Al onto biogenic particles. The interaction between Al and biogenicparticles can lead to the coupled cycling of Al and silicate that is observed in some ocean regions. However, inother regions this coupling is not observed due to (i) advective processes bringing in older water masses thatare depleted in Al, (ii) unfavourable scavenging conditions in the water column, (iii) low surface concentrationsof Al or (iv) additional Al sources, notably sediment resuspension

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

Biogeochemistry of iron in coastal Antarctica: isotopic insights for external sources and biological uptake in the Amundsen Sea polynyas

Seasonal phytoplankton blooms in the Antarctic Amundsen Sea Polynyas are thought to be supported by an external supply of iron (Fe) from circumpolar deep waters, benthic sediments, and/or ice shelf meltwaters. However, largely due to the limited amount of Fe data reported for the Amundsen Sea Polynyas, understanding of the sources and processes that affect the biogeochemistry of Fe in this region (notably within the ice shelf system) remains limited. Here, we present the first investigation of dissolved Fe isotope distributions (δ56Fe) along the conveyer belt of waters into and through the Amundsen Sea, via the Dotson Ice Shelf, from samples collected during austral summer (2017–2018). Our dataset allows us to characterize and compare the dissolved δ56Fe signatures of incoming modified Circumpolar Deep Water (mCDW) and of sedimentary sources on the continental shelf. The range in dissolved δ56Fe (–1 to +0.1 ‰) observed in the Amundsen Sea close to the seafloor, coupled with elevated dissolved Fe concentrations (up to 1.6 nmol/L), suggests that Fe is released from shelf sediments via a combination of reductive and non-reductive processes, with non-reductive dissolution input being relatively more important (20–56 %) than reductive dissolution (4–12 %). Near the Dotson Ice Shelf, the δ56Fe in the mCDW inflow (–0.70 ‰) was lower than the mCDW outflow (–0.23 ‰), whereas any change in dissolved Fe concentrations was negligible. We speculate that this shift in dissolved δ56Fe underneath the ice shelf is driven by a combination of enhanced preservation (and addition) of lithogenic colloidal Fe(III) and/or complexation with Fe-binding ligands, together with a differential loss of Fe2+. We also found distinct δ56Fe signatures in surface waters of the polynya, with apparent preferential uptake of isotopically light Fe in a bloom dominated by diatoms leading to a relatively heavy remnant dissolved δ56Fe signature of +1.06 ‰, compared to a bloom dominated by haptophytes where more modest and variable isotope fractionation was observed. The different isotopic composition between the two regions could be related to the dominance of different species, but this remains speculative. Despite prominent biological uptake, we suggest that other factors such as rapid recycling (e.g., adsorption and regeneration), bacterial regeneration, and complexation with organic ligands, together with the supply of lithogenic particles also play important roles in setting surface dissolved δ56Fe in the Amundsen Sea Polynyas. Overall, this study provides a further understanding of the external Fe sources and the biogeochemical processes in the Amundsen Sea and thus a baseline on how changing conditions in Antarctica can affect Fe cycling in the Southern Ocean and beyond

A First Global Oceanic Compilation of Observational Dissolved Aluminum Data With Regional Statistical Data Treatment

Large national and international observational efforts over recent decades have provided extensive and invaluable datasets of a range of ocean variables. Compiled large datasets, structured, or unstructured, are a powerful tool that allow scientists to access and synthesize data collected over large spatial and temporal scales. The data treatment approaches for any element in the ocean could lead to new global perspectives of their distribution patterns and to a better understanding of large-scale oceanic processes and their impact on other biogeochemical cycles, which may not be evident otherwise. Ocean chemistry Big Data analysis may not just be limited to distribution patterns, but may be used to assess how sampling efforts and analytical methodologies can be improved. Furthermore, a systematic global scale assessment of data is important to evaluate the gaps in knowledge and to provide avenues for future research. In this context, here we provide an extensive compilation of oceanic aluminum (Al) concentration data from global ocean basins, including data available in the GEOTRACES Intermediate Data product (Schlitzer et al., 2018), but also thus far unpublished data

Dissolved Cd, Co, Cu, Fe, Mn, Ni and Zn in the Arctic Ocean

During the Polarstern (PS94) expedition, summer 2015, part of the international GEOTRACES program, sources and sinks of dissolved (D) Cd, Co, Cu, Fe, Mn, Ni and Zn were studied in the central Arctic Ocean. In the Polar Surface Water in which the TransPolar Drift (TPD) is situated, salinity and δ18O derived fractions indicated a distinct riverine source for silicate DCo, DCu, DFe, DMn and DNi. Linear relationships between DMn and the meteoric fraction depended on source distance, likely due to Mn-precipitation during transport. In the upper 50 m of the Makarov Basin, outside the TPD core, DCo, DMn, DNi, DCd and DCu were enriched by Pacific waters, whereas DFe seemed diluted. DCo, DFe, DMn and DZn were relatively high in the Barents Sea and led to enrichment of Atlantic water flowing into the Nansen Basin. Deep concentrations of all metals were significantly lower in the Makarov Basin compared to the Nansen and Amundsen, the Eurasian, Basins. The Gakkel ridge hydrothermal input and higher continental slope convection are explanations for higher metal concentrations in the Eurasian Basins. Although scavenging rates are lower in the Makarov Basin compared to the Eurasian Basins, the residence time is longer and therefore scavenging can decrease the dissolved concentrations with time. This study provides a baseline to assess future change, and additionally identifies processes driving trace metal distributions. Our results underline the importance of fluvial input as well as shelf sources and internal cycling, notably scavenging, for the distribution of bio-active metals in the Arctic Ocean