Biogeochemical processes at the sediment-water interface in a Northeastern Atlantic abyssal locality (Porcupine Abyssal Plain)

Temporal and spatial variability in the composition of suspended particulate material (SPM) was found at a Northeastern Atlantic abyssal locality on the Porcupine Abyssal Plain at a depth of 4850 m. Total SPM was distinctly higher in July 1997 than in March-April 1997 and October 1996. The 20 m of water immediately overlying the seabed was strongly influenced by resuspension and the suspended particulate matter was enriched with iron and manganese. Two water distinct layers, which were characterized by being enriched with SPM, Fe-p Mn-p and Al-p were encountered during all sampling periods at depths of 4000 m and 4800 m. The enrichment of SPM with biogenic elements such as Ba, Ca and Sr implies it has a high biogenic content. Increases were observed in the concentrations of these biogenic elements throughout the sampling period, which indicates that:the biogenic content of the SPM was steadily increased. SPM content and element/aluminum ratios increased in the spring and summer of 1997 compared to autumn 1996, probably as a result of increases in primary productivity, coincidental increases in near-bottom current velocities and possibly other processes such as increases in bacterial activity and bioturbation. Authigenic formation of solid phases of Fe and Mn were also influencing the spatial variability. Chemical reactions involving organic carbon and oxygen, which lead to the incorporation of iron and manganese in various forms into sediments were deduced. It is proposed that there was formation of lepidocrocite (gamma -FeOOH) in the presence of organic carbon and oxygen, and also that iron may also have been transformed into Fe2O3. Re-precipitation of Fe and Mn is strongly enhanced by their enrichment in the SPM in the bottom waters. The sediments predominantly consist of biogenic carbonates and can be characterized as foraminiferal ooze. The proportion of these biogenic carbonates proved to be higher in the more recent sediments, either as result of recent increases in total primary productivity or because of carbonate dissolution within the subsurface sediments. Parallel increases of Mn, Fe and carbonates in the sediment layers supports the idea that the metallic ions are being adsorbed on to calcite surfaces, rather than being co-precipitated in the carbonate phases. Reprecipitation of iron and manganese in other solid phases with the involvement of organic carbon (C-org) in the oxygen penetration zone is suggested. (C) 2001 Elsevier Science Ltd. All rights reserved.