Carbonate saturation and the effect of pressure on the alkalinity of interstitial waters from the Guatemala Basin

Interstitial water samples were collected from the Guatemala Basin using an in situ sampler and by centrifuging box core sediment samples. Results from these two sampling methods for Mn, Si, PO₄ agree well. There is a systematic difference in the alkalinity values, however, which suggests that CaCO₃ (s) precipitates from the box core samples when they are brought from in situ pressure at 1 atm. Thus the alkalinity on box core samples is less than that on samples collected in situ. The magnitude of the alkalinity decrease can be calculated using basic thermodynamic principles and the observed and predicted differences agree well.Both sampling methods show a sharp drop in pH just below the sediment water interface which can be explained by the oxidation of organic matter by O₂ in the absence of CaCO₃. Alkalinity increases during the reduction of MnO₂(s) and release of Mn²⁺ to the interstitial water. The result is that interstitial waters become undersaturated with CaCO₃ immediately below the sediment/water interface and then return to or nearly to saturation at depth.     

Interstitial water chemistry in the sediments of Saanich Inlet

Measurements of nutrients and trace metals are used to examine the processes controlling their distributions in the interstitial waters of Saanich Inlet. Samples were collected using both in situ and squeezing techniques with excellent agreement. Additional measurements of porosity, organic carbon and sedimentation rate by ²¹⁰Pb are used in conjunction with the nutrient measurements to test the equation for the diagenesis of organic matter in fine-grained, organic-rich and rapidly-accumulating sediments.Organic carbon and sulfate decrease with depth in the sediment whereas ammonia and alkalinity increase. In the zone of sulfate reduction (0–20 cm) the rate constants for sulfate reduction ($\text{k}{}_{\mathrm{s}}$), ammonia production ($\text{k}{}_{\mathrm{N}}$) and organic carbon decomposition ($\text{k}{}_{\mathrm{c}}$) agree within a factor of two. Our calculations indicate, however, that this is fortuitous since the observed decrease in paniculate organic carbon is insufficient to account for the sulfate consumption. Sulfate must also be consumed by reaction with methane diffusing up from the underlying sediments. The rate constant for sulfate reduction using particulate organic carbon is lower than a modelled rate encompassing all organic species, including methane.The rate constant for ammonia production ($\text{k}{}_{\mathrm{N}}$) decreases by an order of magnitude when sulfate is completely depleted and methane production dominates.Thermodynamic calculations suggest that the interstitial waters are saturated or supersaturated with respect to all forms of iron ‘monosulfides’, apatite and rhodochrosite.     

Metal-solid interactions in the marine environment: Estimating apparent equilibrium binding constants

Surface chemistry concepts developed for heterogeneous oxide surfaces are illustrated with Cu, Cd, and Zn adsorption data on a well defined, model surface—goethite (αFeOOH)—and then applied to define the interaction or Zn with interfacial sediments from the Guatemala Basin at MANOP site H.Apparent equilibrium binding constants which describe metal associations with a surface, whose sites are composed of a spectrum of binding energies, are independent of adsorption density when sites are available in excess, but dependent on adsorption density when sites are limited. This conclusion makes it especially important to properly apply laboratory experimental results to describe adsorption processes in the natural environment. Therefore, a means for estimating the maximum adsorption density of an element on natural particles is discussed and used to estimate the quantity of Zn in interfacial sediments at MANOP site H which can be ascribed to the process of adsorption.     

The use of electron backscatter diffraction and orientation contrast imaging as tools for sulphide textural studies: example from the Greens Creek deposit (Alaska)

The Greens Creek polymetallic massive sulphide deposit is hosted in a typical polyphase deformed lower greenschist facies orogenic setting. The structure of the host rocks is well constrained, exhibiting a series of three superimposed ductile deformations followed by two brittle episodes. The ore is found both in fold hinges where early-formed depositional features are preserved and in fold limbs where primary features are typically strongly modified or obliterated. Samples from both settings have been investigated using electron backscatter diffraction (EBSD) coupled with forescatter orientation contrast (OC) imaging in order to observe the effects of deformation and lower greenschist facies metamorphism on pyrite. Results suggest that colloform pyrite may preserve information relevant to palaeoenvironment, that apparently simple textures are generally more complex, and that pyrite can deform plastically by dislocation glide and creep processes at lower temperatures and/or strain rates than generally accepted. This analysis indicates that EBSD and OC imaging provide powerful tools for observing textural relationships in pyrite that are not shown by more traditional methods. They should become routine tools for pyrite texture analysis.Editorial handling: D. Lentz     

Habitat requirements for submerged aquatic vegetation in Chesapeake Bay: Water quality, light regime, and physical-chemical factors

We developed an algorithm for calculating habitat suitability for seagrasses and related submerged aquatic vegetation (SAV) at coastal sites where monitoring data are available for five water quality variables that govern light availability at the leaf surface. We developed independent estimates of the minimum light required for SAV survival both as a percentage of surface light passing though the water column to the depth of SAV growth (PLW _min) and as a percentage of light reaching reaching leaves through the epiphyte layer (PLL _min). Value were computed by applying, as inputs to this algorithm, statistically dervived values for water quality variables that correspond to thresholds for SAV presence in Chesapeake Bay. These estimates ofPLW _min andPLL _min compared well with the values established from a literature review. Calcultations account for tidal range, and total light attenuation is partitioned into water column and epiphyte contributions. Water column attenuation is further partitioned into effects of chlorophylla (chla), total suspended solids (TSS) and other substances. We used this algorithm to predict potential SAV presence throughout the Bay where calculated light available at plant leaves exceededPLL _min. Predictions closely matched results of aerial photographic monitoring surveys of SAV distribution. Correspondence between predictions and observations was particularly strong in the mesohaline and polythaline regions, which contain 75–80% of all potential SAV sites in this estuary. The method also allows for independent assessment of effects of physical and chemical factors other than light in limiting SAV growth and survival. Although this algorithm was developed with data from Chesapeake Bay, its general structure allows it to be calibrated and used as a quantitative tool for applying water quality data to define suitability of specific sites as habitats for SAV survival in diverse coastal environments worldwide.     

A comparative analysis of eutrophication patterns in a temperate coastal lagoon

The coastal bays and lagoons of Maryland extend the full length of the state's Atlantic coast and compose a substantial ecosystem at the land-sea margin that is characterized by shallow depth, a well-mixed water column, slow exchange with the coastal ocean, and minimal freshwater input from the land. For at least 25 years, various types of measurements have been made intermittently in these systems, but almost no effort has been made to determine if water quality or habitat conditions have changed over the years or if distinctive spatial gradients in these features have developed in response to changing land uses. The purpose of this work was to examine this fragmented database and determine if such patterns have emerged and how they may be related to land uses. Turbidity, dissolved inorganic phosphate, algal biomass, and primary production rates in most areas of the coastal bays followed a regular seasonal pattern, which was well correlated with water temperature. Nitrate concentrations were low (<5 μM), and only modestly higher in tributary creeks (<20 μM). Additionally, there was little indication of the spring bloom typical of river-dominated systems. There does appear to be a strong spatial gradient in water quality conditions (more eutrophic in the upper bays, especially in tributary creeks). Comparisons of water quality data collected between 1970 and 1991 indicate little temporal change in most areas and some small improvements in a few areas, probably related to decreases in point-source discharges. Seagrass communities were once extensive in these systems but at present are restricted to the eastern portion of the lower bays where water clarity is sufficient to support plant survival. Even in these areas, seagrass densities have recently decreased. Examination of diel dissolved oxygen data collected in the summer indicates progressively larger diel excursions from lower to upper bays and from open bays to tributary subsystems; however, hypoxic conditions (<2 mg 1^?1) were rarely observed in any location. Nitrogen input data (point, surface runoff, groundwater and atmospheric deposition to surface waters) were assembled for seven regions of the coastal bay system; annual loading rates ranged from 2.4 g N m^?2 yr^?1 to 39.7 g N m^?2 yr^?1. Compared with a sampling of loading rates to other coastal systems, those to the upper and lower bays were low while those to tributaries were moderate to high. Regression analysis indicated significant relationships between annual nitrogen loading rates and average annual total nitrogen and chlorophyll a concentrations in the water column. Similar analyses also indicated significant relationships between chlorophyll a and the magnitude of diel dissolved oxygen changes in the water column. It is concluded that these simple models, which could be improved with a well-designed monitoring program, could be used as quantitative management tools to relate habitat conditions to nutrient loading rates.