Groundwater red beds in Holocene fluvial sediments as a product of iron and manganese redox cycling; Morava River, Czechia

Abstract

Modern fluvial deposits can be affected by early diagenetic element mobilization at redox boundaries between oxygenated and oxygen‐free zones near the groundwater level. The visible product of redox transformation of Fe is the sediment colour. Distinct red and black sediment layers have been studied in eight shallow (<4.2 m depth) sections in the floodplain of Morava River, Czechia. The aim was to investigate the composition, origin, rates of formation and stratigraphic significance of the red strata using bulk‐rock analytical methods, radiocarbon dating, optical and scanning electron microscopy coupled with in situ (energy‐dispersive X‐ray scanning electron microscopy and laser ablation inductively coupled plasma mass spectrometry) geochemistry. The coloured layers, in places well‐cemented, developed in permeable sands and gravels above the water table, close to the boundary with overlying less permeable floodplain sandy silts. Their colour is due to Fe and Mn oxyhydroxides (goethite, haematite, todorokite and birnessite) coatings of framework grains. Black, Mn‐rich layers occur stratigraphically higher than the red, Fe‐rich ones. The coatings are a few‐hundred microns thick at maximum, often botryoidal, composed of alternating Fe‐rich and Mn‐rich laminae enriched in As, Mo, Sb, P, Cu and U. The coatings formed under suboxic conditions due to Fe and Mn oxyhydroxide cycling, driven partly by microbial activity, at the groundwater‐related redox boundary which resulted in the distinct vertical arrangement of the black and red layers. Being markedly enriched in As bound to Fe‐oxyhydroxides and exposed to fluctuating redox conditions, the coloured strata can potentially release As to groundwater and represent serious environmental hazards. Radiocarbon ages suggest that the groundwater‐type red beds can occur extremely quickly, over a few hundred years. It is assumed that, under suitable circumstances, this reddening can be preserved in the rock record, and represent a specific model for the development of continental red beds.