Impact of depositional regimes on biogeochemical cycling of iron in sediments of the Argentina Continental Margin – First results of RV SONNE expedition SO260

At the Argentina Continental Margin fundamental interactions between bottom currents, sediment deposition and how these processes control biogeochemical reactions and element cycling including iron can be studied. The focus of my thesis was to determine (1) the consequences of depositional environments (lower continental slope vs. contourite terrace) on biogeochemical processes and the (2) preservation and diagenetic cycling of solid Fe phases in the sediments. Additionally it was assessed (3) how sedimentary stable Fe isotope signatures (δ56Fe) are affected during early diagenesis. A sequential extraction protocol was applied to determine four reactive Fe phases including Fe carbonates, easily reducible Fe (oxyhydr)oxides, reducible Fe (oxyhydr)oxides and hardly reducible Fe oxides. Purification for sedimentary δ56Fe signals of Fe carbonates and easily reducible Fe (oxyhydr)oxides followed. The dataset was combined with pore-water data from RV SONNE cruise SO260 and provided stable carbon isotope data. Different extents of the redox zonation and location of the sulfate-methane-transition (SMT) were found. It is suggested that sedimentation rates are low at the lower continental slope with steady state conditions, causing a strong diagenetic overprint of Fe phases unlike the contourite site, where high sedimentation rates are evident and a diagenetic overprint is missing. Fe extraction data indicate that reactive Fe oxides are subject to reductive dissolution at the SMT. Yet significant amounts of reactive Fe oxides are preserved below for deep Fe reduction (potentially through Fe mediated anaerobic oxidation of methane), which is evidenced by dissolved Fe in methanic sediments. The δ56Fe of reactive Fe phases suggest significant microbial fractionation during deep Fe reduction at the lower continental slope, whereas at the contourite site microbial fractionation is not reflected in δ56Fe. It is concluded that the applicability of δ56Fe signatures as tracer of microbial Fe reduction might be dependent on the depositional regime (limited in high sedimentation areas)