Particulate Mn Concentrations Research Articles

Biological Uptake, Water Mass Mixing and Scavenging Limit the Transport of Manganese‐Rich Waters From the East Antarctic Shelf

AbstractManganese (Mn) is an essential element involved in photosynthesis, yet its concentrations in Southern Ocean open waters are very low, arising from biological uptake and limited external inputs. At southern latitudes, waters overlying the Antarctic shelf are expected to have much higher Mn concentrations due to their proximity to external sources. In this study, we investigated the potential export of Mn‐rich Antarctic shelf waters toward depleted open Southern Ocean waters. Our results showed that while high Mn concentrations were observed over the shelf, biological uptake decreased dissolved Mn concentrations in surface waters north of the South Antarctic Circumpolar Current Front (<0.1 nmol kg−1), limiting its export. Conversely, mixing between Mn‐rich Antarctic Bottom Waters and Mn‐depleted Lower Circumpolar Deep Waters combined with oxidative and scavenging processes led to a decrease in dissolved Mn concentrations within bottom waters, with distance from the coast. Particulate Mn concentrations also showed a decreasing trend with distance from the coast. A comparison with other Antarctic coastal regions suggests this bottom water Mn removal may be widespread and that East Antarctica may be characterized by lower Mn concentrations compared to other regions. Still, subsurface dissolved Mn maxima (0.3–0.6 nmol kg−1) represented a potential reservoir for surface waters. We hypothesize that these high subsurface values result from external sources near the shelf. Overall, these results suggest that the moderate lateral export of trace metal‐enriched waters contributes to the extremely low and potentially limiting Mn concentrations previously reported further north in this Southern Ocean region.

Control of particulate manganese (Mn) cycling in halocline Arctic Ocean waters by putative Mn‐oxidizing bacterial dynamics

AbstractParticulate Mn, given its high adsorptive capacity and oxidation potential, has profound impacts on the cycling of various trace elements and organic matter in the ocean. Moreover, particulate Mn acts as a sink (via oxidation and adsorption) or as a source (via remineralization and photoreduction) term of bioactive dissolved Mn(II). In the Canadian Arctic Ocean, particulate Mn distributions in the water column revealed the presence of distinctively high particulate Mn concentrations and an overwhelming dominance of the non‐lithogenic component to the bulk particulate Mn pool. This phenomenon is of particular importance in halocline waters in the Canada Basin, the Canadian Arctic Archipelago and Baffin Bay, and near‐bottom samples in Baffin Bay. Enhanced microbially‐mediated Mn oxidation in the water column is suggested as the main mechanism driving the non‐lithogenic dominance. Indeed, the microbial community composition data associated with high non‐lithogenic particulate Mn (i.e., Mn oxides) display a high relative abundance of taxa (e.g., f.Pirellulaceae, o.Phycisphaerales, f.Cryomorphaceae, g. Moritella) that have been identified in Mn oxide enriched environments. Furthermore, numerous taxa identified in the Canada Basin halocline water, where non‐lithogenic particulate Mn peaked, are phylogenetically related to known (cultured) Mn‐oxidizing bacteria (MnOB; e.g., Rhodobacteraceae, Oceanospirillaceae, Rhizobiaceae, and other Alphaproteobacteria). Putative MnOB appears to proliferate in certain water masses having a unique set of environmental conditions: low light intensity—alleviating photoinhibition—and high dissolved Mn concentrations, the main drivers known to influence MnOB dynamic, and hence, Mn oxidation.

Manganese biogeochemistry in the Southern Ocean, from Tasmania to Antarctica

AbstractManganese (Mn) is an abundant element in the Earth's crust. However, its concentrations in open ocean seawater are low, where external inputs are scarce. In this study, we report the dissolved Mn and particulate Mn distributions in the Southern Ocean, measured along the GEOTRACES—SR3 transect, from Tasmania (Australia) to Antarctica in the Southern Ocean, during the austral summer 2018. Both dissolved Mn and particulate Mn concentrations were generally low away from localized sources (< 0.3 nmol L−1 and < 0.1 nmol L−1, respectively) along the transect. Our observations of a lower labile particulate fraction than previously measured suggest the Southern Ocean has a unique particulate Mn composition. Low surface dissolved Mn concentrations were attributed to biological uptake and few external sources. Our results suggest biological control of the Mn cycle was higher above the Antarctic continental slope, compared to the rest of the section, and our particulate Mn : P ratios indicated the presence of iron‐stressed diatoms south of the Sub‐Antarctic Front. We suggest low dissolved Mn surface concentrations may (co‐)limit phytoplankton growth in this region. Localized higher dissolved Mn concentrations were observed due to external sources, such as sedimentary and hydrothermal inputs. The presence of an eddy at the same latitude as a hydrothermal plume induced an upwelling of hydrothermally enriched waters up to 1000 m; however, no dissolved Mn inputs to the surface waters were observed. We suggest previous dissolved Mn inputs to the surface layer may be obscured by biological uptake.

Shelf Inputs and Lateral Transport of Mn, Co, and Ce in the Western North Pacific Ocean

The margin of the western North Pacific Ocean releases redox-active elements like Mn, Co, and Ce into the water column to undergo further transformation through oxide formation, scavenging, and reductive dissolution. Near the margin, the upper ocean waters enriched in these elements are characterized by high dissolved oxygen, low salinity, and low temperature, and are a source of the North Pacific Intermediate Water. High dissolved concentrations are observed across the Western Subarctic Gyre, with a rapid decrease in concentrations away from the margin and across the subarctic-subtropical front. The particulate concentrations of Mn, Co, and Ce are also high in the subarctic surface ocean and enriched relative to Ti and trivalent rare earth elements. Furthermore, the particles enriched in Mn, Co, and Ce coincide at the same depth range, suggesting that these elemental cycles are coupled through microbial oxidation in the subarctic gyre as the waters travel along the margin before being subducted at the subarctic-subtropical front. Away from the margin, the Mn, Co, and Ce cycles decouple, as Mn and Ce settle out as particles while dissolved Co is preserved and transported within the North Pacific Intermediate Water into the central North Pacific Ocean.

Vanadium cycling in the Western Arctic Ocean is influenced by shelf-basin connectivity

Water in the western Arctic Ocean tends to show lower dissolved vanadium concentrations than profiles observed elsewhere in the open ocean. Dissolved V in Pacific-derived basin waters was depleted by approximately 15–30% from the effective Pacific Ocean endmember. The depletion originates on western Arctic shelves and is not a result of mixing with a water mass with low V. While biological uptake may account for some of the V removal from the water column, adsorption onto particulate Fe is likely the dominant factor in removing V from shelf waters to the sediments. Once in the sediments, reduction should result in sequestering the V while Fe (and Mn) can be remobilized. A similar Fe-shuttling mechanism for V was previously described for the Peru margin (Scholz et al. 2011). Off the shelves, particulate Mn concentrations often exceed particulate Fe concentrations and thus may exert greater control on the V distribution in basin waters. Nonetheless, particulate V concentrations are much lower in basin waters and dissolved V thus behaves largely conservatively away from the shelf environment. Dissolved V concentrations in Atlantic-derived and Arctic deep waters were as much as 5 nmol/kg lower than those observed in deep waters of other ocean basins. The uniformity in deep water dissolved V between the sampled basins suggests that slow removal of V from the deep basins is probably not a factor in the deep water depletion. Vanadium-depleted incoming Atlantic waters (i.e., the source of Arctic deep waters) and/or removal of vanadium from incoming waters that pass over the shelves probably accounts for the deep water dissolved V depletion. Overall, our results demonstrate the utility of the V distribution as an additional tool to help understand the Arctic marine system. Furthermore, our work is pertinent to questions related to the net effect of marginal basin shelves on oceanic vanadium cycling, its isotopic balance, and how climate-induced changes in shelf biogeochemical cycling will impact vanadium cycling.

Manganese in the shell of the bivalve Mytilus edulis: Seawater Mn or physiological control?

Manganese in the shell calcite of marine bivalves has been suggested to reflect ambient seawater Mn concentrations, thus providing a high-resolution archive of past seawater Mn concentrations. However, a quantitative relationship between seawater Mn and shell Mn/Ca ratios, as well as clear understanding of which process(es) control(s) shell Mn/Ca, are still lacking. Blue mussels, Mytilus edulis, were grown in a one-year duration field experiment in the Menai Strait, U.K., to study the relationship between seawater particulate and dissolved Mn2+ concentrations and shell calcite Mn/Ca ratios.Shell Mn/Ca showed a well-defined intra-annual double-peak, with maximum values during early spring and early summer and low values during autumn and winter. Seawater particulate Mn peaked during winter and autumn, with a series of smaller peaks during spring and summer, whereas dissolved Mn2+ exhibited a marked single maximum during late-spring to early-summer, being low during the remainder of the year. Consequently, neither seawater particulate Mn nor dissolved Mn2+ concentrations explain the intra-annual variation of shell Mn/Ca ratios.A physiological control on shell Mn/Ca ratios is evident from the strong similarity and timing of the double-peaked intra-annual variations of Mn/Ca and shell growth rate (SGR), the latter corresponding to periods of increased metabolic activity (as indicated by respiration rate). It is thus likely that in M. edulis SGR influences shell Mn/Ca by altering the concentration or activity of Mn2+ within the extra-pallial fluid (EPF), by changing the flux of Mn into or the proportion of protein bound Mn within the EPF. By linking shell Mn/Ca ratios to the endogenous and environmental factors that determine growth and metabolic activity, this study helps to explain the lack of a consistent relationship between shell Mn/Ca in marine bivalve shell calcite and seawater particulate and dissolved Mn2+ concentrations.The use of Mn content from M. edulis shell calcite as a proxy for the dissolved and/or particulate Mn concentrations, and thus the biogeochemical processes that control them, remains elusive.

Relationships Between Geochemical Parameters (pH, DOC, SPM, EDTA Concentrations) and Trace Metal (Cd, Co, Cu, Fe, Mn, Ni, Pb, Zn) Concentrations in River Waters of Texas (USA)

Water samples from eight major Texas rivers were collected at different times during 1997–1998 to determine the dissolved and particulate trace metal concentrations, expected to show differences in climate patterns, river discharge and other hydrochemical conditions, and human activities along the different rivers. Specifically, two eastern Texas rivers (Sabine, Neches) lie in a region with high vegetation, flat topography, and high rainfall rates, while four Central Texas rivers (Trinity, Brazos, Colorado, and San Antonio) flow through large population centers. Relatively high dissolved organic carbon (DOC) concentrations in the eastern Texas rivers and lower pH led to higher Fe and Mn concentrations in river waters. The rivers that flow through large population centers showed elevated trace metal (e.g., Cd, Pb, Zn) concentrations partly due to anthropogenically produced organic ligands such as ethylenediaminetetraacetic acid (EDTA) present in these rivers. Trace metal levels were reduced below dams/reservoirs along several Texas rivers. Statistical analysis revealed four major factors (suspended particulate matter [SPM], EDTA, pH, and DOC) that can explain most of the observed variability of trace metal concentrations in these rivers. SPM concentrations directly controlled particulate metal contents. Variation in pH correlated with changes of dissolved Co, Fe, Mn, and Ni, and particulate Mn concentrations, while DOC concentrations were significantly related to dissolved Fe concentrations. Most importantly, it was found that, more than pH, EDTA concentrations exerted a major control on dissolved concentrations of Cd and Zn, and, to a lesser extent, Cu, Ni, and Pb.

Sources and fate of manganese in a tidal basin of the German Wadden Sea

Dissolved and particulate Mn concentrations were investigated on a seasonal scale in surface waters of the NW German Wadden Sea (Spiekeroog Island) in 2002 and 2003. As the Wadden Sea forms the transition zone between the terrestrial and marine realms, Mn was analysed in coastal freshwater tributaries and in the adjoining German Bight as well. Additionally, sediments and porewaters of the tidal flat sediments were analysed for Mn partitioning and microbial activity. Dissolved Mn concentrations show strong tidal and seasonal variation with elevated concentrations during summer at low tide. Summer values in the Wadden Sea (av. 0.7 μM) are distinctly higher than in the central areas of the German Bight (av. 0.02 μM), suggesting a possible impact of the Wadden Sea environment on the Mn budget of the North Sea. Seasonality is also observed for particulate Mn in the Wadden Sea (winter av. 800 mg kg − 1 ; summer av. 1360 mg kg − 1 ). Although particles are relatively Mn-poor during winter, the high SPM load during this season causes elevated excess concentrations of particulate Mn, which in part exceed those of the dissolved phase. Therefore, winter values cannot be ignored in balance calculations for the Wadden Sea system. Porewater Mn concentrations differ depending on sediment type and season. Maximum concentrations are found in surface sediments at a mixed flat site (190 μM) during summer, while winter values are distinctly lower. This indicates that enhanced microbial activity owing to higher temperature during summer leads to increased reduction of Mn-oxides in surface sediments and enhances the corresponding diffusive and advective Mn flux across the sediment-water interface. Draining of Mn-rich porewaters from sediments is also documented by analyses of tidal creek waters, which are highly enriched in Mn during summer. Furthermore, an important Mn source is freshwater discharged into the Wadden Sea via a flood-gate. The concentration of dissolved Mn in freshwater was highly variable during the sampling campaigns in 2002 and 2003, averaging 4 μM. In contrast, particulate Mn displayed a seasonal behaviour with increasing contents during summer. On the basis of salinity variations in the Wadden Sea, the total amount of Mn contributed to the Wadden Sea via freshwater was estimated. This balance shows the importance of the freshwater environment for the Mn inventory of the Wadden Sea. During winter the total Mn inventory of the Wadden Sea water column may be explained almost completely by freshwater discharge, whereas in summer the porewater system forms the dominating source.

Trace metal distributions in shelf waters of the northwestern Black Sea

Measurements of dissolved and leachable particulate trace metals (Mn, Fe, Co, Pb, Cd, Zn, Cu and Ni) and total particulate Mn and Fe were made on seawater samples collected from the northwestern Black Sea during the EROS 2000 expedition conducted in July–August 1995. The investigation concentrated on waters of the shelf and shelf edge, but included one deeper water (1440 m) station. In the oxic layer of the deep station, the suspended particulate fractions of Mn and Fe were a major part of the total metal mass, consistent with the presence of the “Fine Particle Layer” which forms on the shelf and spreads all over the Black Sea with intensities decreasing from the coast. Dissolved and total particulate concentrations were, respectively, Mn, 0.69–9.6, 1.2–29; Fe, 0.79–3.03, 2.3–7.4 nM. Dissolved Cu and Ni concentrations were relatively high (1–8 and 8–12 nM, respectively), and did not show any depletion in surface oxic waters, possibly as a result of strong organic complexation. Dissolved Pb concentrations (100–200 pM) were higher than were generally found on the shelf. This was attributed to atmospheric inputs combined with less efficient scavenging of metals in these low SPM waters. The distribution of dissolved Co closely resembled that of dissolved Mn reflecting coupling through oxidation of Mn. Concentrations of dissolved Cd and Zn were low in surface water (0.07–0.09 and 0.9–2.0 nM, respectively), and increases in concentrations with depth were sharply reversed around the top of the redoxcline. For most metals (Mn, Fe, Co, Pb, Cu, Cd, Zn) dissolved concentrations were low in the anoxic layers as a result of solubility by formation of, or association with, solid sulphide phases. Dissolved Ni was not affected by sulphide precipitation. At most of the shelf stations there were clear enhancements of dissolved Mn and Fe in the deepest waters, consistent with other evidence that significant benthic fluxes of these metals arise through the redox conditions in the region of the sediment-water interface. In the shelf water column, dissolved Mn and Fe concentrations ranged between 1.2 and 1350 and between 0.4 and 181 nM, respectively; the highest concentrations were found near the bottom. Particulate concentrations of Mn and Fe were high, implying high oxidation rates of Mn(II) and Fe(II) and/or high supply rates from rivers. Total particulate concentrations of Mn and Fe were 0.7–1050 and 2.3–2650 nM, respectively; the highest concentrations were found in surface and bottom waters. The distributions of particulate Mn and Fe were consistent with the isopycnal transport of Mn and Fe oxyhydroxides from the shelf by the coastal circulation. Distributions of other trace metals (Co, Pb, Cu, Ni, Cd, Zn) were considerably influenced by riverine inputs. Relatively high dissolved and available particulate metal concentrations were generally found in surface waters at stations directly influenced by the Danube River. Some trace metals (Co, Ni, Cd and Zn) were influenced by Mn and Fe cycling and increases in their dissolved concentrations occurred at a number of stations near the sediment–water interface. Dissolved and available particulate metal concentrations (nM) at stations on the shelf were, respectively: 0.171–1.80, 0.003–0.437 (Co); 0.014–0.614, 0.010–1.48 (Pb); 7.6–28.8, 0.048–3.75 (Cu); 11.0–17.5, 0.018–2.10 (Ni); 0.033–0.161, 0.003–0.063 (Cd); 1.01–8.33, 0.135–7.58 (Zn).

The impact of scavenging on trace metal budgets in Puget Sound

The distributions of dissolved and particulate Mn, Pb, Cu and Zn in Puget Sound, its tributaries, and its sediments were determined to illustrate the impact of scavenging reactions on trace metal budgets of a large fjordlike estuary. About 75% of the dissolved Pb discharged into the main basin of Puget Sound was scavenged from the dissolved phase. The high particulate Pb concentrations in regions of high particulate Mn concentrations and the significant correlations between Pb and Mn concentrations on suspended particles demonstrate the importance of hydrous manganese oxide phases in scavenging dissolved Pb. Since particles are effectively retained within the main basin, scavenging results in the sedimentation of about 70% of the total Pb added to the main basin. Only isolated instances of scavenging of dissolved Cu and Zn were observed and the discharge rates of dissolved Cu and Zn from known sources were equal to their rates of advective removal within the errors of the budget. The remineralization of organically-bound particulate Cu is partly responsible for its quasi-conservative behavior within the main basin of Puget Sound. The fates of Cu and Zn are controlled mainly by physical processes such as advection and settling of particles. Advection removes about 60% of the total Cu and Zn added to the main basin while 40% is deposited in the sediments of Puget Sound.