Fe redox cycling in Iberian continental margin sediments (NE Atlantic)

Abstract

In this paper, data are presented on the vertical distribution of pore water Fe 2+ , ferrozine-extractable Fe 2+ and solid phase Fe in sediments along four across-slope transects including the Nazare canyon at the Iberian margin. Sorbed Fe 2+ cannot be measured directly and is operationally defined as the fraction that can be extracted with ferrozine. Our objectives were (1) to investigate the potential role of Fe 2+ sorption in the Fe redox cycle, (2) to quantify Fe redox cycling and (3) to determine its rate limiting factors, with emphasis on differences between stations across the slope and in the canyon. In all sediments pore water Fe 2+ and ferrozine-extractable Fe 2+ concentrations increased simultaneously with depth until a maximum was reached and upon which the pore water Fe 2+ concentration rapidly declined, presumably due to precipitation as iron-sulfide. The ferrozine-extractable Fe 2+ concentration, however, either slowly diminished or remained unchanged when going deeper into the sediment, suggesting ongoing reaction at the sorption surfaces or much slower desorption than adsorption kinetics. Upon Fe reduction, Fe 2+ is released into the pore water where it either directly precipitates and/or adsorbs onto available surfaces of the sediment matrix, including organic matter. Through sorption, authigenic ferrous mineral formation is delayed and Fe 2+ may be transported deeper into the sediment. There, sorbed Fe 2+ can act as a deep source for authigenic ferrous mineral formation. A simple steady-state model was formulated that includes Fe 2+ sorption as a first-order kinetic reaction to estimate Fe reaction rates. Fe oxidation and reduction rates were most intense at the shelf, where organic carbon mineralization rates are high, and decreased with water depth. In the canyon, where deposition fluxes were high, Fe reaction rates increased with water depth until a maximum at the foot of the canyon at 3097 m and decreased again to the abyssal at 4280 m. Turnover times estimated for pore water Fe 2+ , ferrozine-extractable Fe 2+ and solid phase Fe both in the oxidized and reduced layer indicated that sediment mixing was the most important rate limiting factor for Fe cycling at all stations of the Iberian margin. The contribution of Fe reduction to organic matter mineralization was 5% at the 104-m and 113-m stations on the main transect and 6% at the 3097-m station at the foot of the canyon. At the other stations Fe reduction contributed less than 4% to the mineralization of organic matter.