Norman Silverberg

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.

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.

Early diagenesis of cadmium and cobalt in sediments of the Laurentian Trough

Abstract The depth distributions of hydroxylamine/acetic acid-extractable Co and Cd were determined in box-cores from a series of stations along the 1200 Km length of the Laurentian Trough. These were compared with the concentrations of total Co and Cd and the distributions of extractable Mn and Fe in the cores, as well as with the total Cd concentrations in sedimenting particles collected with a sediment trap. The results indicate a strongly contrasting behaviour of the two elements during early diagenesis, consistent with their respective chemistries and particle-associations. Cobalt occurs mainly in a non-reactive form and is buried with the accumulating sediments. A mobile component of Co is associated with Mn- and Fehydroxides and, like these compounds, follows a redox-sensitive pattern of dissolution in the reducing zone of the sediment—vertical migration—enrichment by precipitation in the oxidized surface layer—and redistribution along the bottom in paniculate form. Paniculate Cd arrives at the sediment surface bound to biogenic material, is rapidly solubilized during aerobic degradation of the organic matter, and migrates vertically both into the reducing zone of the sediments where it precipitates, and back into the water column where it may be recycled through biological processes. In an estuary, Co will be concentrated in the oxidized layer of the sediments and tend to migrate landward, while Cd will be most abundant in sediments underlying regions where plankton production is high relative to dilution by terrigenous particles.

SULFATE REDUCTION IN DEEP COASTAL MARINE SEDIMENTS

(Received October 31, 1986; revision accepted May 13, 1987) ABSTRACT Edenborn, H.M., Silverberg, N., Mucci, A. and Sundby, B., 1987. Sulfate reduction in deep coastal marine sediments. Mar. Chem., 21: 329-345. Sulfate reduction rates in sediments of four deep stations in the Saguenay Fjord and the Laurentian Trough, Gulf of St. Lawrence, are among the lowest reported for the coastal environ- ment. Maximum rates were 0.4-7.0nmolcm-3day -1. The low rates are due to relatively low sedimentation rates and continuously low temperatures. Regional differences in both integrated and maximum sulfate reduction rates in the sediment correlate with sediment trap measurements of sedimentation rate and organic carbon flux. Sulfate reduction accounts for the degradation of 5-26% of the estimated downward flux of organic matter to these sediments. Unlike the absolute rate of sulfate reduction, the relative proportion of the carbon flux that is degraded via sulfate reduction is not directly correlated with the sedimentation rate but is a function of organic matter composition, intensity of bioturbation, and the abundance of sub-oxic electron acceptors. Thus, the lowest proportion of carbon degrada- tion via sulfate reduction occurred at a Gulf site, where a combination of low sedimentation and bioturbation rates allowed a long residence time for organic matter near the sediment surface and, in consequence, a low flux of labile carbon into the sulfate reduction zone. The highest proportion was observed at a station with a similar organic carbon flux but with higher rates of sedimentation and bioturbation. At a third site, with the highest rates of sulfate reduction as well as the highest rates of sedimentation and bioturbation, the contribution of sulfate reduction to organic matter degradation was only intermediate. This is attributable to the exhaustion of the supply of pore- water sulfate. In deep coastal environments the proportion of organic matter degraded via sulfate reduction can be highly variable both spatially and temporally. INTRODUCTION

Factors influencing particulate matter geochemistry in the St. Lawrence estuary turbidity maximum

635 samples of suspended particulate matter (SPM), collected in the St. Lawrence river and estuary during periods of high and low river flow from a series of individual and anchor stations on a transect traversing the turbidity maximum zone, as well as two sediment box cores, were analyzed for Al, Si, Ca, Mg, Fe and Mn. An abrupt change in elemental composition occurs when traversing the front at the landward edge of the turbidity maximum. As the SPM concentration increases across the front from 20–200 mg l−1, the Ca/Al and Mg/Al ratios of the SPM increase and the Si/Al, Fe/Al and Mn/Al ratios decrease. The almost 50% decrease of the Mn/Al ratio is not related to changes in salinity. Within the turbidity maximum the tidal-averaged Si/Al, Ca/Al, Mg/Al and Fe/Al ratios of the SPM do not differ significantly from the landward to the seaward end of the turbidity zone, but on one tidal station the ratios of Si, Ca and Fe to Al are significantly lower at high river flow than at low flow. The Mn/Al ratio is insensitive to the extreme variations of either salinity (0.6–30‰) or SPM concentrations (10–480 mg l−1) within the turbidity zone. A tendency for higher Mn/Al ratios to be associated with near-bottom SPM, observed in the center of the turbidity zone during the low river flow period, is well developed in the lower reaches of the zone. Diagenetic mobilization within the rare fine-grained bottom sediments of the turbidity maximum is responsible for changes in Mn and Fe content of particulate matter, and early settling of coarse-grained components and size sorting within the zone are responsible for other compositional changes. Local sources, desorption and precipitation are apparently of secondary importance. The depletion of both Mn and Fe in the SPM and sediment of the upper estuary implies a net seaward escape of diagenetically mobilized metal.

Observations on the diagenetic behavior of arsenic in a deep coastal sediment

Vertical profiles of total dissolved arsenic, manganese and iron, pH, Eh and rates of sulfate reduction were determined in a freshly-collected box core from a 335m depth station in the Laurentian Trough. The relationships observed between the profiles were further examined in the laboratory by measuring these same parameters with time in surficial sediment slurries as the Eh decreased in response to biological activity or chemical alteration.Both field and laboratory observations have shown that arsenic is released predominantly as As(III) into reducing sediment porewaters. This occurs after the dissolution of manganese oxides and at the same time as the dissolution of iron oxyhydroxides and the onset of sulfate reduction. Laboratory experiments indicated that sulfate reduction and the production of sulfide ions are not solely responsible for the release of arsenic to the porewaters, although this process is necessary to create and maintain a highly reducing environment conducive to rapid iron dissolution.The diagenesis of arsenic in Laurentain Trough sediments involves the simultaneous release of arsenic and iron at a subsurface depth, followed by its removal from porewaters by precipitation and adsorption reactions after migration by diffusion along concentration gradients. A qualitative model is presented to describe the behavior of arsenic in coastal marine sediments.

Comparative diagenesis at three sites on the Canadian continental margin

Diagenesis of carbon, oxygen, nitrogen, and manganese at three sites on the Canadian continental margin is quantitatively compared and contrasted using results from a computer code (CANDI) published by Boudreau (1996a), The data at Station 3 (Cabot Strait) are well explained by the steady state output from CANDI, assuming a porewater balance created by diffusion and reaction only, whereas the data from Stations 4 (Emerald Basin-Scotia Shelf) and 5 (Scotia Slope) are not consistent, in one way or another, with this simple model. The deviations between model and data at Station 4 are best explained by nonsteady-state diagenesis. Model fits to the Station 5 ΣCO 2 observations are improved dramatically by adding some irrigation at this site, but the ΣNH 3 distribution appears to be subject to an additional anomalous transport to the O 2 zone and subsequent oxidation to NO 3 . The mechanism for this latter phenomena is unknown and in need of future research. In addition, the O 2 and ΣCO 2 profiles at all sites require the existence of at least two reactive organic matter types; furthermore, the initial amounts of these OM types at each station is strongly dependent on the intensity of particle bioturbation. Ammonia is preferentially regenerated at Station 3 at a high ratio of about 25 N to 106 C. The net kinetics of the deeper removal of Mn 2+ appear to be fractional-order with respect to the concentration of this species, suggesting multiple removal processes, Finally, an oxidant balance, assuming steady state, indicates a considerable difference in the use of oxidants at each station even though the O 2 fluxes are similar.

Gravity core shortening and pore water chemical gradients

Abstract Profiles of pore water alkalinity and dissolved Fe, Mn, and PO 4 −3 were obtained for virtually undisturbed box cores and for gravity cores taken at the same stations. Linear alkalinity gradients in the box cores were preserved in the gravity cores, but the steepened gradients in the gravity cores indicate significant shortening. The ratio of the alkalinity gradients provides a measure of the degree of shortening. The relative shortening could also be clearly seen in the Mn, Fe, and PO 4 −3 profiles. We observed virtually no compaction and a linear shortening. The results agree with the mechanism proposed in early studies on gravity corer behaviour, namely the stretching of the sediment in advance of the corer and the recovery of a thinner section of sediment than the in situ interval. We observed no noticeable differences in pore water chemistry between box and gravity core samples, only the increased gradients due to thinning. The shortening (or thinning) factor at individual stations varied between 1.4 and 3.4, however, and for precise work it is suggested that a concurrent box core or other independent measures of the in situ gradients be obtained to correct for the true shortening.

Oceanography of a Large-Scale Estuarine System

This paper is an attempt to synthesize current knowledge about tides and tidal modelling efforts in the St. Lawrence Estuary. A sequence of numerical models, increasing in complexity from simple one-dimensional to baroclinic, is discussed. The performance of these models and their limitations are pointed out. In the last section, some recommendations are given for future work to improve our present knowledge of the tidal properties over the estuary. These include complementary in situ observations of offshore tidal elevations and principal tidal currents, in addition to new numerical modelling experiments combining all the positive attributes of the previous models.

Observations on trace element hypersaline geochemistry in surficial deposits of evaporation ponds of Exportadora de Sal, Guerrero Negro, Baja California Sur, México

Trace element concentrations were determined for 28 samples of surficial deposits collected over a salinity gradient of 39x to approximately 250xin evaporation ponds of a large industrial NaCl production facility located near Guerrero Negro (Peninsula of Baja California, Mexico). Grain size and lithology are used to describe the mixed chemical–detrital material. Major earth and trace elements in the sediments were measured by a combination of an inductively coupled plasma mass spectrometry (ICPMS) for As, Ba, Cd, Cu, Mg, Ni, Pb, U, and Zn, flame atomic absorption spectrophotometry for Al, Fe, and Mn, isotope dilution ICPMS for Hg, and instrumental neutron activation analysis (INAA) for Cs, Ca, Sr, Cr, Sb, and Sc. Changes in grain-size, mineralogy, and chemical composition suggest that suspended particulate matter supplied to the system, mainly by seawater pumped in from the adjacent Ojo de Liebre Lagoon, and to a smaller extent, carried into the ponds by wind from adjacent dunes and by wave-induced erosion of retaining dikes, dominates the sediment in the first two concentration ponds. Freshly deposited fine sediments in these ponds are enriched in organic matter, biogenic carbonates, Al, Fe, Mn, and most of the trace elements, and are similar to surface sediments of the adjacent lagoon. In ponds 3 and 4, intensive evaporationinduced precipitation of calcium and magnesium carbonates occurs causing scavenging of Al, Fe, Mn carried by colloidal particles. The sediment in ponds 4 and 5 includes a thick microbial mat layer, which is enriched in organic carbon, and the accumulation of As and Cd is observed there. In ponds 7 through 11, the deposits generally are low in organic matter and trace elements (except for Sb) and are formed by intensive gypsum and anhydrite precipitation, with an admixture of terrigeneous material in ponds 8–10 supplied by a dyke erosion or carried in with wind from adjacent sand dunes. D 2002 Elsevier Science B.V. All rights reserved.