Bjørn Sundby

Interactions of manganese with the nitrogen cycle: Alternative pathways to dinitrogen

Abstract The conversion of combined nitrogen (ammonia, nitrate, organic nitrogen) to dinitrogen (N 2 ) in marine sediments, an important link in the global nitrogen cycle, is traditionally assumed to take place only via the coupled bacterial nitrification-denitrification process. We provide field and laboratory evidence that N 2 can also be produced by the oxidation of NH 3 and organic-N With MnO 2 in air. The reduced manganese formed in this reaction readily reacts With O 2 , generating reactive Mn(III, IV) species to continue the oxidation of NH 3 and organic-N to N 2 . Free energy calculations indicate that these two reactions are more favorable as a couple than the oxidation of organic matter by O 2 alone. We also provide field evidence consistent With the reduction of NO 3 − to N 2 by dissolved Mn 2+ . These two reactions involving nitrogen and manganese species can take place in the presence and absence of O 2 , respectively. Our field evidence suggests that the oxidation of NH 3 and organic-N to N 2 by MnO 2 in the presence of O 2 can outcompete the oxidation of NH 3 to NO 3 in Mn-rich continental margin sediments and thereby short-circuit the nitrification/ denitrification process. The MnO 2 catalyzed reaction may account for up to 90% of the N 2 formation in continental margin sediments, the most important N 2 producing environments in the marine N cycle. The oxidation of NH 3 and organic-N by MnO 2 in the presence of O 2 can explain Why N 2 can form in oxic sediments; it can also explain Why denitrification rates measured by acetylene inhibition and labeled tracers can give lower estimates than direct measurements of N 2 production.

The effect of oxygen on release and uptake of cobalt, manganese, iron and phosphate at the sediment-water interface

The porewater of a sediment core taken at 6 m depth in Gullmarsfjorden, Sweden, was enriched in Fe, Mn, Co, and phosphate compared to the overlying bottom water. Yet, in situ measurements with a benthic flux-chamber, in which dissolved oxygen and pH were maintained near ambient values (regulated flux-chamber), showed that the sediment did not release any of these ions but instead removed Co, Mn, and Fe from the overlying water. In a parallel experiment, where dissolved oxygen and pH were not maintained but allowed to decrease as a result of benthic respiration, Co, Mn, Fe, and PO4 were released from the sediment. An accidental interruption of the stirring in the regulated chamber caused a pulse of dissolved Co, Mn, Fe, and PO4 to be released from the sediment. When the stirring was resumed, all four ions were again removed. The kinetics of the removal process was apparent first order with half-removal times of 3–5 days, similar to the removal kinetics of the radioactive tracers 59Fe and 54Mn from the water in a smaller chamber, run in parallel. The critical variable which controls the reactions at the sediment-water interface is the flux of oxygen from the water column into the sediment. When benthic chambers are used to measure fluxes of redox-sensitive ions, the oxygen flux must be maintained as close as possible to the actual in situ flux. If not, the measured fluxes may vary greatly in magnitude and even change direction.

Pathways of manganese in an open estuarine system

The distribution of Mn was examined in the bottom sediments and water column (suspended paniculate matter) of the Laurentian Trough. Gulf of St. Lawrence. A characteristic profile of Mn with depth in the sediment consisted of a Mn-enriched surface oxidized zone, less than 20 mm thick, and a Mn-depleted subsurface reducing zone. A subsurface Mn maximum occurred within the oxidized zone. Below this maximum the concentration dropped sharply to nearly constant residual levels in the reducing zone. The accumulating estuarine sediments are deficient in Mn compared to the river input of suspended matter and are definitely not the ultimate sink for manganese. Manganese escapes from the sediment by diffusion and resuspension, forming Mn-enriched, fine-grained particles which are flushed out in the estuarine circulation. 5.0 × 109gyr−1 of Mn, or 50% more than the river input of dissolved Mn. are exported to the open ocean. In spite of the efficient mobilization and export of Mn, the quantity exported is a small fraction (0.2%) of the total flux to the deep-sea sediments. This is related to the low levels of paniculate matter transported by the St. Lawrence River. The export phenomenon, however, is probably true of many coastal regions of muddy sediments and thus has interesting implications for the oceanic budget of Mn.

Photoferrotrophs thrive in an Archean Ocean analogue

Considerable discussion surrounds the potential role of anoxygenic phototrophic Fe(II)-oxidizing bacteria in both the genesis of Banded Iron Formations (BIFs) and early marine productivity. However, anoxygenic phototrophs have yet to be identified in modern environments with comparable chemistry and physical structure to the ancient Fe(II)-rich (ferruginous) oceans from which BIFs deposited. Lake Matano, Indonesia, the eighth deepest lake in the world, is such an environment. Here, sulfate is scarce (<20 μmol·liter−1), and it is completely removed by sulfate reduction within the deep, Fe(II)-rich chemocline. The sulfide produced is efficiently scavenged by the formation and precipitation of FeS, thereby maintaining very low sulfide concentrations within the chemocline and the deep ferruginous bottom waters. Low productivity in the surface water allows sunlight to penetrate to the >100-m-deep chemocline. Within this sulfide-poor, Fe(II)-rich, illuminated chemocline, we find a populous assemblage of anoxygenic phototrophic green sulfur bacteria (GSB). These GSB represent a large component of the Lake Matano phototrophic community, and bacteriochlorophyll e, a pigment produced by low-light-adapted GSB, is nearly as abundant as chlorophyll a in the lakes euphotic surface waters. The dearth of sulfide in the chemocline requires that the GSB are sustained by phototrophic oxidation of Fe(II), which is in abundant supply. By analogy, we propose that similar microbial communities, including populations of sulfate reducers and photoferrotrophic GSB, likely populated the chemoclines of ancient ferruginous oceans, driving the genesis of BIFs and fueling early marine productivity.

Interactions between metal oxides and species of nitrogen and iodine in bioturbated marine sediments

By using a gold amalgam (Au/Hg) voltammetric microelectrode, we have measured simulta- neously and with millimeter resolution the distributions of O 2, Mn(II), Fe(II), I(2I), and HS(2I) in bioturbated sediment cores from the Laurentian Trough. We also measured nitrate and ammonia in the pore water, total I and ascorbate- and HCl-extractable Fe and Mn in the solid-phase sediment, and fluxes of O 2, NO3 , and NH4 across the sediment-water interface. The concentrations of O2 and Mn(II) were below their respective detection limits of 3 and 5 mM between 4 and 12 mm depth, but a sharp iodide maximum occurred at the depth where upward diffusing Mn(II) was being removed. We propose that the iodide peak is maintained through the reduction of IO3 by Mn(II), reoxidation of I(2I) to IO3 in the oxic zone above the peak and oxidation to I2 below where it is ultimately trapped by reaction with organic matter. The iodide production rate is sufficient to account for the oxidation of all of the upward diffusing Mn(II) by IO3 . Nitrate plus nitrite (SNO3) decreased to a minimum within 10 mm of the sediment-water interface, in agreement with flux measurements which showed SNO3 uptake by the sediment. Below the minimum, SNO3 rebounded, and reached a maximum at 40- to 50-mm depth. This rebound is attributed to the anaerobic oxidation of ammonia by manganese oxides. Fe(II) was always first detected below the anoxic SNO3 maximum, and was accompanied by colloidal or complexed Fe(III). A sharp upward-directed ammonia gradient was recorded near the sediment-water interface, but no ammonia was released during the first 48 h of the incubations. If the ammonia removal were due to coupled bacterial nitrification- denitrification, more than one half of the total measured oxygen uptake (6.7 to 7.3 mmol/m 2 /d) would be required, and more organic carbon would be oxidized by nitrate than by oxygen. This scenario is not supported by nitrate flux calculations. Alternatively, the oxidation of ammonia to N2 by manganese oxides is a potential removal mechanism. It would require one quarter of the total oxygen flux. The high-resolution profiles of redox species support the conceptualization of bioturbated sediments as a spatially and temporally changing mosaic of redox reactions. They show evidence for a multitude of reactions whose relative importance will vary over time, and for reaction pathways complementing those usually considered in diagenetic studies. Copyright

Cadmium diagenesis in Laurentian Trough sediments

The depth distributions of Cd, Mn and Fe in pore water and sediment were determined on three replicate box cores collected at a 325 m deep station in the Laurentian Trough. The results reveal a surface layer in which the content of solid-phase Cd first decreases but then increases sharply with depth. In contrast, Mn decreases regularly over the same depth interval. In the oxygenated zone of this layer, Cd, probably associated with organic matter, is released to the pore water, resulting in concentrations that are more than an order of magnitude higher than in the overlying water column. Some of the dissolved Cd is returned to the water column and some migrates downward and is precipitated at depth. This part of the Cd cycle is virtually complete within the surface layer, the base of which corresponds to the base of the zone of Mn enrichment. In the subsurface layer, solid-phase Cd and Mn show little or no concentration change. However, dissolved Cd, which reaches often non-detectable concentrations (<0.05 nM) in this layer, increases once again deep within the anaerobic zone and attains concentrations even higher than in the surface layer. The presence of dissolved Cd complexes (revealed by 1.5 to 8 fold increases in electrochemically active Cd in pore water following UV-treatment) as well as inconsistent distributions of dissolved and solid-phase Cd, indicate complexities in diagenesis that merit further investigation.

Benthic fluxes of cadmium, copper, nickel, zinc and lead in the coastal environment

Fluxes of trace metals across the sediment-water interface were measured in situ at 6 m depth in Gullmarsfjorden, Sweden, using diver-operated stirred benthic flux-chambers. These were equipped so that dissolved oxygen and pH could be maintained near ambient seawater values (regulated chamber) or be allowed to change in response to benthic respiration (unregulated chamber). In the regulated chamber, Cd, Cu, Ni, and Zn were released from the sediment at constant rates both during a winter experiment (water temperature −1 °C) and during a fall experiment (+ 10°C). During the fall experiment, fluxes (in nmol m−2 d−1) of 13 (Cd), 118 (Cu), 209 (Ni), and 1400 (Zn) were measured. In winter, the release rates were lower by factors of 5 and 10 for Cu and Ni but not significantly different for Cd and Zn. Neither release nor uptake by the sediment could be demonstrated for Pb. The pore-water in a diver-collected core was depleted in Cd, Cu, and Zn and slightly enriched in Ni and Pb, relative to the ambient seawater. There was no correspondence between fluxes calculated from porewater profiles and actually measured fluxes; nor could the fluxes be directly related to the degradation rate of organic matter. In the unregulated chamber, initial trace metal release rates were lower than in the regulated chamber. As the oxygen concentration decreased, the metal fluxes decreased as well and were ultimately reversed as sulfide began to appear in the water. The fluxes of trace metals are sensitive to the oxygen regime in the flux chamber because the solubilization of these metals, which takes place in a thin oxic layer near the sediment surface, depends on the oxygen flux across the sediment-water interface.

Organic matter processing in continental shelf sediments—the subtidal pump revisited

Abstract Pressure variations along an uneven permeable bottom, generated by the passage of gravity waves and by bottom currents, force water into the sediment below regions of high pressure, such as ripple troughs, and draw it out at regions of low pressure, such as ripple crests. The resulting pore water circulation brings organic matter and oxygen to the interior of the sediment, creates horizontal concentration gradients that can be as strong as the vertical gradients, and increases the flux of pore water constituents across the sediment-water interface. Here we review the present knowledge of pore water circulation and organic carbon mineralization in sandy continental shelf sediments. We argue that the circulation of water through the pores of the sediment enhances the mineralization rate of dissolved and particulate organic matter and that continental shelf sediments play a more important role in the oceanic carbon cycle than is generally thought.

Diagenetic separation of cadmium and manganese in suboxic continental margin sediments

Abstract The vertical distribution of Cd differs from that of Mn in sediment cores taken from the continental margins of the Arctic and Atlantic oceans: the oxic surface sediment is depleted in Cd and enriched in Mn, whereas the subsurface sediment is enriched in Cd and depleted in Mn. In some cores the distributions of solid-phase and dissolved Mn are offset: the porewater profiles indicate that Mn is being precipitated relatively deep in the sediment, whereas the solid-phase distributions indicate that Mn has been precipitated very close to the sediment surface in the recent past. At one deep site, located near the base of the continental slope (3000 m), the offset may be caused by a relatively recent decrease in ocean productivity, a corresponding decrease in the organic carbon flux to the sediment, and a subsequent migration of the redox boundary deeper into the sediment. However, at a site where the upper sediment layers have accumulated during the last few decades, this explanation cannot account for the offset distributions. We propose that a fluctuating input of organic matter to the sediment on seasonal or decadal timescales causes the position of the redox boundary to fluctuate. The fluctuating redox boundary pumps dissolved Mn upward and concentrates it as Mn oxide in a distinct layer at the upper limit of the redox excursion; at the same time, it pumps Cd downward and concentrates it in a distinct layer at the lower limit of the redox excursion. The net result of a fluctuating redox boundary is to increase the spatial separation of Cd and Mn during the early diagenesis of suboxic sediments.

Suspended sediment fluctuations in the Tagus estuary on semi-diurnal and fortnightly time scales

Abstract Nine multi-ship synoptic surveys of the distribution of suspended sediment, each survey including the distribution at both low and high tide, were carried out over a 12-month period in the mesotidal Tagus estuary in Portugal. Additional measurements of the semi-diurnal fluctuations of suspended sediment concentration and current strength were made at fixed stations during a neap and a spring tide. During the study period, the river discharge of water and suspended sediment remained below the mean annual discharge and did not show a pronounced seasonal fluctuation. A turbidity maximum, defined as an area with suspended sediment concentrations greater than 50 mg l −1 , was absent during neap tides (1·3-m amplitude), but appeared and grew in both extent and turbidity as the tidal amplitude increased. The turbidity maximum was fully developed during spring tides (> 3-m amplitude) with concentrations greater than 50 mg l −1 throughout the entire estuary. Maximum concentrations, reaching as much as 1000 mg l −1 during spring tides, were always found in the inner shallow bay region of the estuary. In contrast to the salinity distribution, which fluctuated between partly stratified during neap tides and well mixed during spring tides, the vertical distribution of suspended matter in the turbidity maximum zone was always stratified with the highest concentrations near the bottom. The semi-diurnal fluctuation of the suspended sediment concentration was negligible during neap tides, but attained magnitudes during spring tides that were comparable to the fortnightly fluctuation. The fluctuation in suspended matter concentration is interpreted as a fortnightly erosion-sedimentation cycle, caused by a cyclic variation in the strength of the bottom currents. Superimposed on this fortnightly cycle is a semi-diurnal cycle. The amount of material involved in these cycles is equivalent to one years input of suspended sediment by the Tagus river during normal discharge conditions.