Manganese and Iron Profiles During Early Diagenesis in Loch Etive, Scotland. Application of two Diagenetic Models

Abstract

Sediment cores were taken from two locations in Loch Etive, Scotland. The first location, Bonawe, 145m depth, was a site with high surficial sediment manganese (1·8% extractable Mn) beneath waters that were usually hypoxic, and the second location, Airds Bay, 75m depth, was a contrasting site with lower surficial manganese (0·29% extractable Mn) beneath a usually well mixed water column. Cores were analysed for porewater manganese, iron and ammonium, and solid phase, extractable non-lithogenic, precipitated manganese and iron. The manganese and iron profiles were analysed using two models released into the public domain by their authors (van Cappellen & Wang, 1996; Slomp et al., 1997). Both models assume steady state; the former is a multicomponent transport and reaction model for carbon, oxygen, nitrogen, sulphur, iron and manganese, whereas the other is an analytical single component model for the diagenesis of manganese or iron. At the low manganese, Airds Bay site, good fits to the measured manganese and iron concentration profiles were obtained by application of the Slomp model. At this site, the van Cappellen model gave an acceptable fit to the manganese profiles (but not as good as that of the Slomp model) and a poor fit to the iron profiles, with a modelled dissolved maximum approximately 5 times higher than measured, and strong decrease in solid phase iron with depth, which was not observed. At the high manganese Bonawe station the Slomp model again produced a good fit to the iron concentration profiles, but with the manganese profiles it was able to produce a good fit to either the dissolved or to the solid phase extractable, but not simultaneously to both. At this site the van Cappellen and Wang model again gave an acceptable fit to the manganese profiles but a poor fit to the iron profiles. The sub-oxic diagenetic processes, which gave profiles resembling the measured profiles, were then examined using the models to compare their contribution to that of the measured direct oxygen uptake. This showed that at the Bonawe site manganese reduction amounted to at least 40% of the oxygen uptake (van Cappellen model) and possibly up to 80% (Slomp model). The reductions of iron oxides (0·4%) and of sulphate (∼8%) were of less importance and the balance was assumed to be direct oxidation of carbon by oxygen. At the Airds Bay site direct oxidation of carbon by oxygen appeared to be the dominant process, with manganese oxide, iron oxide and sulphate accounting for only approximately 8%, 3% and 5% of oxygen uptake. The similarities between the results of the two models used give extra weight to the results, and the differences indicate where caution is needed and possibly more work.