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.