Frederick L. Sayles

Oxygen penetration depths and fluxes in marine sediments

A compilation of numerous studies of oxygen profiles in marine sediments and estimated fluxes across the sediment-water interface supports the existence of a simple relationship between oxygen penetration depth (L), benthic oxygen flux (FO20), and bottom water oxygen concentration([O2]BW). This relationship is: L=2ϕDs[O2]BWFO20, where Φ and Ds are the porosity and diffusivity of O2 in sediment, respectively. The relationship holds well for continental margin environments, but not in open ocean basins. The success of this simple relationship in describing continental margin field data suggests that the pore water profiles are at steady state, the depth distribution and reactivity of organic carbon is nearly uniform, and pore water irrigation is generally negligible. This work also demonstrates the consistency of the data recently collected by several independent works.

CACO3 DISSOLUTION IN SEDIMENTS OF THE CEARA RISE, WESTERN EQUATORIAL ATLANTIC

We have used porewater sampling by in situ techniques, including whole-core squeezing, as well as by shipboard sectioning and whole-core squeezing to estimate the rates of sedimentary organic matter oxidation and CaCO3 dissolution at seven sites on the Ceara Rise in the western equatorial Atlantic Ocean. Porewater NO3− profiles at all sites show a pattern indicative of active organic matter oxidation in the upper 15–20 cm of the sediments and in a buried, organic-rich layer. The organic C oxidation rate generally decreases with increasing water depth, from a value of 22 μmol/cm2/y at the shallowest site (3279 m) to 14 μmol/cm2/y at the deepest site (4675 m). Over this depth range, the bottomwaters vary from moderately supersaturated with respect to calcite to strongly undersaturated. High-resolution alkalinity profiles, measured in porewaters collected by in situ whole-core squeezing, yield estimated Ca2+ fluxes of 11 μmol/cm2/y at a site located at the depth of the calcite saturation horizon, and 7.6 μzmol/cm2/y at a moderately undersaturated site. Ca2+ fluxes calculated from profiles in porewaters collected by relatively coarse-resolution in situ sampling methods clearly indicate that there is CaCO3 dissolution above the calcite saturation horizon. The dissolution of aragonite may contribute to the dissolution flux at the shallowest site. These Ca2+ fluxes, as well as fluxes estimated from a model of sedimentary organic matter oxidation and calcite dissolution, indicate that 36–66% of the CaCO3 rain to the seafloor dissolves at sites at and above the calcite saturation horizon, while 52–75% of the rain dissolves at sites below this depth. When these results are incorporated into the oceanic CaCO3 budget of Milliman 1993, they indicate that 35% of CaCO3 production is preserved in the deep sea; they suggest a CaCO3 accumulation rate that is 27% lower than that estimated by Milliman 1993. Our Corg oxidation/CaCO3 dissolution model indicates that a large fraction of the CaCO3 dissolution that is occurring on the Ceara Rise is attributable to the neutralization of metabolic acids produced during organic matter oxidation. The efficiency with which organic matter oxidation dissolves CaCO3 (that is, the ratio, CaCO3 dissolution attributable to organic matter oxidation:organic matter oxidation rate) generally increases as degree of undersaturation of bottomwaters increases. However, there are deviations from the general trend that can be attributed to site-to-site variations in the kinetics of organic matter oxidation and calcite dissolution. This result indicates that the dissolution of CaCO3 as a result of organic matter oxidation in the deep sea may mask the effects of variations in surface water CaCO3 productivity and bottomwater chemistry on the accumulation rate of CaCO3 in deep-sea sediments.

The composition and diagenesis of interstitial solutions—I. Fluxes across the seawater-sediment interface in the Atlantic Ocean☆

Abstract Studies of the composition of interstitial solutions of marine sediments have been carried out utilizing in situ sampling techniques. Samples were obtained from the Caribbean, North Atlantic and South Atlantic. In virtually all cases, diagenesis has led to the uptake of Mg 2+ and K + and the release of Ca 2+ , HCO − 3 and Na + by the solid phases. SO 2− 4 is slightly enriched at nearly all stations, reduction being observed only at continental margin stations. Cl − is conservative within experimental precision. The reactions controlling the fluxes of most components across the water-sediment interface occur almost entirely in the upper 100 cm of sediment. Contributions of Mg 2+ , Ca 2+ , K + and HCO − 3 from below 100 cm amount to less than 15% of the calculated fluxes across the interface. Reactions in the upper 30 cm account for 70–90% of the fluxes of these components across the interface. Only Na + has a deep source, gradients often being linear in the upper 2m of sediment. Calculated fluxes across the sediment-water interface are of the same order of magnitude as river inputs for the components studied. In the case of Mg 2+ and K + , 60–100% of the river input can be balanced by diagenetic uptake in the sediment. For Ca 2+ and HCO − 3 additions to seawater augment the river supply by 25–50%. When the uptake of Mg 2+ and K + by the sediment is calculated by integrating the fluxes across the interface, calculated concentrations of both of these elements are inconsistent with published average concentrations for the types of sediment studied.

The composition and diagenesis of interstitial solutions—II. Fluxes and diagenesis at the water-sediment interface in the high latitude North and South Atlantic☆

Studies of the major element composition of in situ sampled pore waters are reported for the North Atlantic and Southern Ocean between Africa and Antarctica. The pattern of diagenetic modification of pore water composition is similar throughout the entire Atlantic. Enrichment of Na+, Ca2+ and alkalinity and depletion of Mg2+ and K+ are nearly universal. Only siliceous oozes consistently provide very limited evidence of cation diagenesis. The changes observed and the calculated fluxes across the seawater-sediment interface are much the same as those reported previously for other areas of the Atlantic and Caribbean. Fluxes of the major cations across the interface continue to be indicated as a major factor in the geochemical cycling of these elements, particularly Na+, Mg2+ and K+. Diagenetic modelling indicates that aerobic oxidation of organic matter and consequent dissolution of CaCO3 is a dominant reaction throughout the North Atlantic. The data indicate that O2 oxidation to at least 30 cm is prevalent at nearly all stations. Dissolution of CaCO3 in response to the introduction of metabolic CO2 can lead to significant post-depositional modification of the sediments. The modelling also indicates a 1:1 stoichiometric relationship between Na+ release and Mg2+ uptake by sedimentary components. Although dissolved silica concentrations in biogenic siliceous sediments are among the highest yet reported (>700μM), calculations demonstrate that solubility control cannot be through equilibria with the mineral sepiolite. Further, the influence of cation-silicate surface phases, generally, upon solubility is contraindicated by Si(OH)4-Mg2+-H+ relationships. Evaluation of the influence of fluid advection on pore water profiles indicates that at the stations studied in the North Atlantic, it is small. A more general consideration of the potential contribution of fluid advection to shaping interstitial water profiles demonstrates that advection can be a dominant factor. In such circumstances, serious misinterpretation of the nature of diagenetic reactions may result from a lack of knowledge and consideration of fluid advection.

Temporal changes in the hydrography and chemistry of the Cariaco Trench

The Cariaco Trench has been considered a classic example of a marine anoxic basin for nearly 30 years. Although most workers assume that the chemistry of the basin is at steady-state, detailed data sets collected for hydrographic and nutrient parameters in 1973 and 1982 indicate strong temporal variations on a decadal time scale. Over this 8 year period deep-water temperature increased by 0.06°C, salinity by 0.01%, and hydrogen sulfide by 14 μM. Silica concentrations may also have increased by up to 14 μM. We can explain most of the temporal variability of these parameters in the Cariaco Trench deep waters using a time-dependent box model that includes the effects of vertical eddy diffusion and flux of the chemical species from the sediments. Deep-water temperature and salinity increases are primarily the result of downward diffusion of heat and salt, while geothermal heat flux seems to be responsible for only a small fraction of the observed temperature increase. Hydrogen sulfide profiles can be predicted fairly well by the model if a sediment-water flux of 2.7 × 10−6 μmol cm−2 s−1 (measured using pore waters collected with the Woods Hole Insitu Marine Probe) is used. For hydrogen sulfide, in situ production seems to be much less important than diffusion on sulfide from the pore waters. On the other hand, silica profiles cannot be adequately modeled unless an in situ production term is included.

The equilibration of clay minerals with sea water: exchange reactions☆

Abstract Studies of seawater-sediment and seawater-clay mineral exchange equilibria demonstrate that rinsing procedures employed in many previous studies have grossly shifted the exchange equilibria away from the true seawater conditions. Exchange complements have been determined here by measurement of compositional changes in seawater that result from reaction with clays, thereby avoiding rinsing. These data show that exchangeable Na + is normally greater than or equal to exchangeable Mg 2+ on clays and sediments in exchange equilibrium with seawater. On introduction to seawater, fluvial clays are shown to give up their exchangeable Ca 2+ for Na + , a process of importance in the geochemical budget of Na + .

The crystallization of magnesite from aqueous solution

It has been suggested that the highly hydrated character of the Mg2+ ion in aqueous solution is responsible for the often encountered difficulty of precipitating stable, anhydrous phases of magnesium carbonate and calcium-magnesium carbonate. In an effort to investigate this, a study of magnesite crystallization kinetics was undertaken, utilizing the reaction of hydromagnesite plus CO2 to yield magnesite at 126°C. The reactions were characterized by prolonged initial quiescent periods prior to the onset of detectable crystallization. The length of the initial period was found to vary with Mg concentration, pCO2 and ionic strength. Contrary to classical kinetics, the reaction studied was inhibited by increased Mg concentration. Ionic strength and pCO2 acted as positive catalysts.

A sampler for the in situ collection of marine sedimentary pore waters

Abstract An in situ sampler for pore waters of marine sediments is described. Filtered pore water from six depths in the upper 2 m of sediment and bottom water are collected and stored in capillary tubing, thereby preserving sampling sequence. Tests demonstrate the collection of uncontaminated samples.

In situ Sampler for Marine Sedimentary Pore Waters: Evidence for Potassium Depletion and Calcium Enrichment

A device for sampling the interstitial waters of deep-sea sediments in situ has been developed and tested. The sampler collects a series of samples over a depth of 1.5 meters in the sediment and thus makes possible the accurate delineation of chemical gradients existing in the pore waters. Samples collected in the North Atlantic indicate that significant gradients of K+ and Ca2+ exist in the sediments sampled. Interstitial solutions sampled between Ireland and Cape Cod, Massachusetts, are characterized by the depletion of K+ and the enrichment of Ca2+.

Cation-exchange characteristics of Amazon River suspended sediment and its reaction with seawater

Abstract The cation-exchange characteristics of Amazon River suspended sediment have been studied in order to determine the contribution of exchangeable cations to the geochemical fluxes from the river. Sediment samples were obtained throughout most of the Amazon Basin. The range of exchangeable cation compositions is very narrow in the river and in seawater as well. In river water, the exchangeable cation complement (equivalent basis, exclusive of H+) is 80% Ca2+, 17% Mg2+, 3% Na+ plus K+. In seawater Na+ and Mg2+ are about equal (38%) while Ca2+ ~ 15% and K+ ~ 9%. On reaction with seawater, river suspended sediment took up an amount of Na+ equal to nearly one-third of the dissolved river load, as well as amounts corresponding to 15–20% of the dissolved fluvial K+ and Mg2+. These estimates reflect an unusually high suspended-sediment:dissolved-solids ratio of 6.4 at the time of sampling. At a more representative world average ratio of four, the uptake of Na+ would be 20% of the dissolved fluvial load, and that for K+ and Mg2+ about 10%. Over the annual cycle of the Amazon, it is estimated that ion exchange has a still smaller effect, as a consequence of the low average suspended-solids:dissolved-solids ratio of 1.7. Variations in the ratio X Ca X Mg , the equivalent fraction of exchangeable Ca2+ and Mg2+, throughout the river, can be described by a single isotherm. This same isotherm accurately describes the distribution of exchangeable Ca2+ and Mg2+ on sediment equilibrated with seawater, despite that a high proportion of exchange sites is occupied by Na+ and K+.