Diagenesis of carbon and nutrients and benthic exchange in sediments of the Northern Adriatic Sea

The characteristics of diagenesis and benthic biogeochemical cycling were studied at six stations in the Northern Adriatic during September, 1988. The objectives of this work were to quantify the mechanisms responsible for mass transport and to establish the stoichiometry of reactions involving carbon and nutrients. Stations were chosen to include sites near the Po delta that have rapid sediment accumulation, sites south of the Po delta that lie beneath its nutrient-rich plume but beyond the region of rapid accumulation, and one site further offshore in a zone with little or no modern sediment accumulation. Benthic flux measurements of oxygen, TCO2, ammonia, alkalinity, phosphate, silicate and radon were made at five of the study sites. Synthesis of this data shows that fluxes of oxygen and nutrients are similar at both high and low accumulation rate sites, but that the offshore site has fluxes that are at least two times smaller for all parameters except silica and radon. Cores were collected, and analyses of solid phases (organic carbon, nitrogen, sulfur, total and organic phosphorus, and 210Pb) and pore waters (alkalinity, pH, ammonia, silica, calcium, iron, and phosphate) were carried out. Calculations indicated that pore waters were near equilibrium or slightly supersaturated with calcite, supersaturated with apatite, and undersaturated with vivianite. Pore water profiles of TCO2 and silica were used to calculate diffusive fluxes across the sediment–water interface. Comparison of these calculated fluxes to in situ flux measurements indicated solute transport at the rapid accumulation sites is dominated by diffusive fluxes, while at the other sites about half is accomplished by irrigation; the contrast reflects the abundance of macrofauna. Rate constants for organic matter degradation were estimated and reflect the presence of fractions with mean lifetimes from a few months to several years. Diagenetic stoichiometry is dominated by degradation of organic carbon, which accounts for about 95% of the TCO2 flux. Ultimately, oxygen is the principal terminal electron acceptor, although ferric iron, nitrate and sulfate must be important intermediates. Carbonate dissolution accounts for the remaining 5% of the TCO2 flux. The average C/N ratio of degrading organic material derived from pore water profiles of TCO2 and ammonia is 5±2; this is 30–50% of the ratio measured in solid phases, demonstrating preferential degradation of compounds rich in N. However, the C/N ratio observed in flux measurements averages 11.0±1.7, suggesting that about half of the fixed nitrogen remineralized is lost as N2 during diagenesis. The TCO2/alkalinity diagenetic ratio in the anoxic pore waters was uniform at all sites and averaged 1.11±0.02 mol/equivalent, a result 30% greater than predicted for the formation of FeS2 via sulfate reduction with ferric hydroxide as the iron source. Several explanations for this difference are possible. The observed ratio is more consistent with precipitation of FeS using ferric hydroxide as an iron source; additional factors may be preferential de-carboxylation of organic matter or proton uptake by DOC during alkalinity titrations.