The early diagenesis of iron in pelagic sediments: a multidisciplinary approach

Biogeochemical reactions of iron within pelagic sediments from the eastern and western equatorial Atlantic are investigated by means of pore water chemistry, chemical leaching experiments and total elemental determinations, color reflectance spectroscopy, rock magnetic measurements and TEM observations of the magnetic fraction. Results indicate that in the presence of nitrate, ascorbate (a weak reducing agent) leachable iron decreased with depth from the sediment surface. Within this upper sediment region, iron assimilation by bacteria is indicated as magnetosomes were found by TEM observations throughout the whole core. Extractions with a strong reducing agent (dithionite), representing iron bound to iron oxides, correlated linearly with the concentration of highly coercive magnetic minerals and with the reflection intensity of red color within the same iron redox zone. The abrupt decrease of red color reflection intensity, relative to the amount of iron oxides below the iron redox boundary, is the result of a decrease in the specific surface area of iron oxyhydroxide/oxides. Magnetic parameters imply a smaller average grain size of the magnetic fraction below the iron redox boundary, arising from an increase in biogenic magnetite formed by magnetotactic (assimilatory) bacteria, dissolution of very fine-grained magnetite, and the gradual decrease of coarser grained terrigenous (titano-) magnetite with depth. The early diagenetically formed magnetic fraction withstands subsequent dissolution and gives pronounced peaks of magnetic parameters within sediments with high carbonate and low terrigenous matter content.