James E. Mackin

Diagenesis of Fe and S in Amazon inner shelf muds: apparent dominance of Fe reduction and implications for the genesis of ironstones

It is commonly observed that SO42− reduction dominates the anaerobic decomposition of organic matter and determines redox properties in organic-rich shelf environments. Some areas of inner shelf muds near the mouth of the Amazon River are apparent exceptions to such a generalization. These deposits are sufficiently rich in organic carbon (0.6 ± 0.1%) to support NH4+ production rates in the upper 0–20 cm of 0.4–6 mmol m−2 day−1 (average of 3.0 ± 0.3), comparable to reactive sediments on other shelves. Anoxic conditions usually occur near the sediment-water interface, as evidenced by the absence of NO3− and the presence of dissolved Mn and Fe a few centimeters below the surface. In contrast to other similarly reactive deposits, Fe-reduction zones in Amazon shelf muds (and presumably other comparable environments) are often vertically extensive (>∼1 m) and characterized by high concentrations of dissolved Fe (0.3−0.7 mM) and elevated levels of HCO3− (12–14 meq l−1). These extensive Fe-reduction zones occur with or without evidence of significant SO42− depletion (surface values ∼20–28 mM) in essentially unbioturbated deposits. Total solid phase S seldom exceeds 0.1% weight (often 7) and allows Fe and, to a lesser extent, Mn reduction to dominate the sediment redox properties. Pore waters are supersaturated with respect to a variety of authigenic minerals including siderite and vivianite, often typical of nonsulfidic sediments and low SO42− environments. Authigenic siderite was also observed in SEM photographs. These Fe2+-generating deposits are interbedded with or lie up-current from Fe-coated quartz and ‘glauconitic’ sands (superficial oolites?), implying Fe precipitation on sand nuclei during mud/sand mixing or lateral export and segregation of Fe colloids from disturbed muds. The overall facies relations and authigenic mineral suites are, in many ways, similar to Paleozoic oolitic ironstones and may represent an incipient or actual analogue.

Early chemical diagenesis, sediment-water solute exchange, and storage of reactive organic matter near the mouth of the Changjiang, East China Sea

Abstract A substantial proportion of the material delivered to the modern oceans is supplied by a few large rivers such as the Changjiang. Early diagenetic reactions in surficial bottom sediments determine in large part both the eventual influence of these rivers on the sea and the nature of sedimentary deposits formed. The region off the mouth of the Changjiang exemplifies the interplay between physical, chemical, and biological factors which can produce particular spatial patterns of diagenesis and sediment-water exchange. To examine these patterns measurement of pore water solute profiles, sediment-water solute fluxes, and solute reaction rates in the upper few decimeters of sediment were made at 27 stations near the Changjiang in the East China Sea. Direct measurements of dissolved Si(OH)4, NH4+, and NO3−, fluxes from or into bottom sediments made during summer and autumn periods (15 to 24°C) range from 0.13 to 13.2, −2.6 to 3.4, and −1.4 to 3.2mmol m−2 day−1, respectively. Net solute flux from the sea floor is often lowest from deposits having the highest interstitial solute concentrations. In addition, bottom regions having the highest build up of reaction products or depletion of reactants in pore waters (with respect to overlying water) actually have the lowest rates of reaction. These same areas of elevated (products) or depleted (reactants) pore water solute concentrations, low reaction rates, and low net rate of solute exchange which are located near the mouth of the Changjiang are sites of high sedimentation rates and depauperate benthic communities. High water turbidity and resuspension apparently hinder water column production and input of reactive organic matter or other biogenic material which drive many diagenetic reactions. Rapid sedimentation or disturbance hinders benthic community development, lowers biogenic reworking, and allows build up or depletion of reaction products or reactants in bottom sediments. Offshore areas of lower sedimentation, higher productivity, and active bottom communities are sites of high initial reaction rates and increased sediment-water solute exchange compared with rapid sedimentation regions. A diagenetic paradox resulting from the interaction between benthic communities and the physical environment of sedimentation is that proportionally the greatest storage of diagenetic products related to organic matter decomposition can occur in sediments that are initially the least diagenetically reactive.

Dissolved Al in sediments and waters of the East China Sea : implications for authigenic mineral formation

Abstract Numerous previous studies indicate that several different authigenic aluminosilicates form in the oceans. In this study we show, using dissolved Al distributions in sediments and waters from the nearshore regions of the East China Sea, that the process of aluminosilicate formation probably begins rapidly upon contact of detrital clays with seawater. Statistical analyses of dissolved Al-Si-H + relations in surface sediments indicate that the minerals forming in East China Sea sediments low in dissolved Fe are dioctahedral chlorites with an average composition EX 0.91 Mg 0.77 Al 5.0 Si 2.7 O 10 (OH) 8 (where EX = exchangeable + 1 cation). This composition is also consistent with dissolved Al and Si measurements as a function of salinity in turbid overlying waters. Results suggest a dissolution—reprecipitation mechanism for clay mineral reconstitution. This mechanism can help to explain why different authigenic clays are found in different areas of the oceans. In the East China Sea the total amount of authigenic clays present must constitute a very minor fraction of the bottom sediments. Thus, the formation of these minerals has a relatively small impact upon dissolved Si distributions. Clay mineral reconstitution in nearshore regions may provide a mechanism for buffering sediments and overlying waters with respect to pH, as the composition of minerals formed should be a direct function of the H + activity in the surrounding environment.

Diagenesis of dissolved aluminum in organic-rich estuarine sediments

Abstract The sedimentary geochemistry of dissolved Al is complicated by a number of different reactions. In this study we show that complexation by organic matter, adsorption to Fe-oxyhydroxides, and reaction with Si in solution have important effects on the distribution of dissolved Al in sediments. In the absence of physical resuspension of sediment into overlying waters, dissolved Al is rapidly consumed at the sediment-water interface and is subsequently released upon reduction of Fe-oxyhydroxides. This release does not cause noticeable perturbations in dissolved Al concentrations in sediments because of rapid consumption reactions which mask the true mobility of Al. Results suggest that one of the consumption reactions may be due to formation of an Fe-Al-silicate. The amount of authigenic aluminosilicate formed in estuarine sediments must be very small relative to the detrital component. In the deep-sea, however, the long residence time of Fe-oxyhydroxides at the sediment-water interface, with resulting greater accumulation of adsorbed Al may explain the abundance of Al in Fe-smectites reported from many different areas.

Processes affecting the behavior of dissolved aluminum in estuarine waters

Abstract Although it is well-known that reactions occur within estuaries to alter the flux of dissolved aluminum from rivers into the oceans, the nature and relative importance of these reactions are not well defined. In this study we show that sediment-water interactions can have a significant influence on dissolved Al distributions in estuaries. Undisturbed sediments will act as a sink for dissolved Al because of diffusion across the sediment-water interface and reaction of Al within the sediment. Resuspension of sediments will cause a release of dissolved Al into relatively Si-depleted estuarine waters. Flocculation of colloidal material may cause a net consumption of dissolved Al in estuaries. Our results indicate, however, that this process may not, in many cases, be the primary cause for curvature in Al-Cl − profiles. An alternative model, whereby Al is displaced from organic matter complexes and adsorption sites in the estuarine zone and reacts with Si and cations in solution, is consistent with the data presented in this study as well as many other studies. In this case, the extent of net Al removal in estuaries will be determined by both the amount and nature of the dissolved organic matter present.

The effects of clay mineral reactions on dissolved Al distributions in sediments and waters of the Amazon continental shelf

Reactions involving clay minerals may exert a major control on some aspects of marine water and sediment chemistry. The potential of clay mineral reactions in this regard was investigated in Amazon continental shelf muds and overlying waters using highly sensitive dissolved Al analyses. Data are restricted to low-Fe pore waters from undisturbed and incubated (4–11 days, T = 28 ± 1 °C) surface sediment at 9 stations, a surface water transect through the Amazon River plume, and water column profiles determined at coring sites. Approximately constant relations between dissolved Al, Si, and H+ in pore waters imply that aluminous authigenic clays (Si/Al = 0.83, H+/Al = 0.43) are forming in muddy regions of the Amazon shelf. Equilibrium models based upon the pore water data also predict the correct magnitude of dissolved A1 concentrations in the Amazon River plume in the absence of high biological productivity, indicating that authigenic clays control some characteristics of overlying water chemistry. In water column profiles, dissolved Al increases with depth at high salinities apparently because of sediment resuspension into low dissolved Si waters and subsequent clay dissolution. The results of this study confirm predictions based upon previous laboratory and field studies of dissolved Al behavior. They also point out some of the possible complexities of clay reconstitution reactions where aluminous authigenic clays form from more siliceous precursors in nearshore sediments and waters.

The effects of iron reduction and nonsteady-state diagenesis on iodine, ammonium, and boron distributions in sediments from the Amazon continental shelf

The sediments off the mouth of the Amazon River are unique in terms of modern depositional environments because Fe reduction dominates redox properties for several meters into the seabed. This unique character offers the opportunity to investigate the effects of Fe reduction on reactive solutes like iodine and boron. Also, direct measurements of solute distributions and reaction rates, reported in this paper for iodine and NH4+, constrain the timing of major physical disturbances, which are partly responsible for poising the sediments in the Fe reduction stage. Major enrichments of dissolved iodine and boron occur in 1–3 m cores from the region, in the absence of high organic matter decomposition rates. Coherent relationships between boron and iodine, high dissolved I/N and I/Br production ratios, and the lack of a strong correlation of iodine production with dissolved NH4+ all indicate that Fe reduction exerts the major control on iodine and boron distributions. Nonsteady-state models of dissolved iodine and NH4+ and solid-phase iodine distributions in long cores, using measured surface (0–20 cm) reaction rates, indicate that the surface mixed layer (as defined by210Pb; which includes the upper 1.5 m of sediment in some areas) was formed within 50–250 days prior to our sampling (May–June 1983; high Amazon River flow). These data imply that physical sediment reworking on the Amazon shelf has a strong seasonal character, with greatest disturbances occurring during rising river flow. The full implications of these reworking events, for the persistence of the unique authigenic mineral assemblages found in the sediments and for the shelf sediment budget, will require future seasonal studies.

Preservation of reactive organic matter in marine sediments

Abstract It is generally assumed that the reactivity of organic matter and the amount preserved in sedimentary deposits necessarily increases with total sedimentation rate. In some environments, such as deltas, where supply of unreactive terrigenous debris may vary independently of reactive organic matter input, the amount of reactive organic material preserved can in fact theoretically correlate either directly or inversely with sedimentation rate. The amount preserved can be shown quantitatively by transport-reaction models to depend on (1) the relative importance of electron acceptor concentration in the overlying water, (2) advection during sedimentation, (3) dilution by sedimentation, (4) solute diffusion, (5) initial flux of organic matter, and (6) the magnitudes of reaction rate constants. Both single reaction rate constant and multiple reaction rate constant models suggest that, at steady state, maximum preservation with respect to a given oxidant occurs when D s k = w 2 , where D s = whole sediment diffusion coefficient of the electron acceptor, k = first-order rate constant of the dominant organic fraction, and w = sedimentation rate. This is the likely basis for the reported correlation between average reactivity, k , of carbon in a deposit, percent carbon preservation, and w 1.5 –w 2 . Because of the variety of factors which determine these relationships, such correlations are probably valid only within specific classes of depositional environments.

The infinite dilution diffusion coefficient for A1(OH)4− at 25°C☆

The infinite dilution diffusion coefficient for Al(OH)4− necessary to calculate fluxes of dissolved Al between sediments and overlying waters, was determined at 25°C. Measurements were made using the diaphragm-cell method by diffusing Al(OH)4− spiked KBr solutions against KCL over a range of ionic strengths. The mean of 9 separate measurements gives 1.04 ± .02 × 10−5cm2/s as the infinite dilution diffusion coefficient for Al(OH)4− at 25°C.