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

The characteristics of diagenesis and benthic biogeochemical cycling were s tudied at six stations in the Northern Adriatic during September, 1988. The objectives of this work were to quantify the mechanisms responsible for ma ss transport and to establish the stoichiometry of reactions involving carb on and nutrients. Stations were chosen to include sites near the Po delta t hat 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, alkalini ty, phosphate, silicate and radon were made at five of the study sites. Syn thesis of this data shows that fluxes of oxygen and nutrients are similar a t 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 silic a and radon. Cores were collected, and analyses of solid phases (organic ca rbon, nitrogen, sulfur, total and organic phosphorus, and Pb-210) and pore waters (alkalinity, pH, ammonia, sillica, calcium, iron, and phosphate) wer e carried out. Calculations indicated that pore waters were near equilibriu m 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. Co mparison 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 organ ic matter degradation were estimated and reflect the presence of fractions with mean lifetimes from a few months to several years. Diagenetic stoichio metry is dominated by degradation of organic carbon, which accounts for abo ut 95% of the TCO2 flux. Ultimately, oxygen is the principal terminal elect ron acceptor, although ferric iron, nitrate and sulfate must be important i ntermediates. Carbonate dissolution accounts for the remaining 5% of the TC O2 flux. The average C/N ratio of degrading organic material derived from p ore water profiles of TCO2 and ammonia is 5 +/- 2; this is 30-50% of the ra tio measured in solid phases, demonstrating preferential degradation of com pounds rich in N. However, the C/N ratio observed in flux measurements aver ages 11.0 +/- 1.7, suggesting that about half of the fixed nitrogen reminer alized is lost as N-2 during diagenesis. The TCO2/alkalinity diagenetic rat io 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. Se veral 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. (C) 1999 Else vier Science B.V. All rights reserved.