Artificially induced migration of redox layers in a coastal sediment from the Northern Adriatic

Abstract

Abstract. Long-term experimental studies suggest that, under transient anoxic conditions, redox fronts within the sediment shift upwards, causing sequential rise and fall of benthic fluxes of reduced species (Mn(II), Fe(II) and S(-II)). Infaunal benthic organisms are associated with different redox fronts as micro-habitats and must be affected by such changes during natural hypoxia events. In order to document the geochemical evolution of the sediment during prolonged anoxia in the framework of an in situ experiment designed to mimic natural conditions, benthic chambers were deployed on the seafloor of the Northern Adriatic and sampled after 9, 30 and 315 days of incubation. Oxygen and sulfide were measured continuously in the early stages (9 days) of the experiment. High-resolution pore water profiles were sampled by DET probes and redox-sensitive species (S(VI), Mn(II) and Fe(II)) and alkalinity were measured.Starting oxygen saturation was about 80 % within the chamber. After 7 days, anoxia was established in the bottom waters within the chambers. Mn(II) and Fe(II) started diffusing towards the anoxic water column until they reached the surficial sediment. Being reoxidized there, Mn and Fe reprecipitated, giving a rusty coloration to the seafloor. Infaunal species appeared at the sediment surface. After 20 days, all macro-organisms were dead. Decomposition of macroorganisms at the sediment-water interface generated S(-II) within the entire height of the chamber, leading to a downward flux of sulfides into the sediment, where they were quickly oxidized by metallic oxides or precipitated as FeS. S(-II) was below detection in the water column and pore waters at the end of the experiment. Our results suggest that S(-II) enrichment in the water column of coastal systems, which are episodically anoxic, is strongly controlled by the biomass of benthic macrofauna and its decay during anoxia, whereas its residence time in the water column is controlled by iron availability (as solid oxides or as dissolved reduced cations) within the sediment, even without water circulation.

Aerobic Biodegradation at a Seawater-Sediment Interface of a New Bioplastic 100% Based on Natural Polymers

“…It has been seen that under conditions of induced and prolonged anoxia, the progressive reduction of oxygen leads to an upward migration of alternative reduced final acceptors (Fe 2+ , Mn 2+ , and SO 4 2– ). The presence of sulfides at the interface, mainly due to the death and anaerobic degradation of the organism in the sediment, proved to be temporary since the presence of iron in the form of mineral oxides or in a reduced and dissolved form (Fe 2+ ) precipitated as FeS . Our results are coherent with the mechanism described here because the samples have undergone a fast biodegradation in a very short timeframe.…”

“…The presence of sulfides at the interface, mainly due to the death and anaerobic degradation of the organism in the sediment, proved to be temporary since the presence of iron in the form of mineral oxides or in a reduced and dissolved form (Fe 2+ ) precipitated as FeS. 48 Our results are coherent with the mechanism described here because the samples have undergone a fast biodegradation in a very short timeframe. This could have released a huge amount of carbon and nitrogen that would have promoted uncontrolled microbial growth and subsequent decay of this biomass, promoting a transient ipo-or anoxic environment.…”

Early Diagenesis in the Hypoxic and Acidified Zone of the Northern Gulf of Mexico: Is Organic Matter Recycling in Sediments Disconnected From the Water Column?

“…Hypoxic areas are known as "Dead Zones" due to the scarcity of fishable species that leave the area when oxygen is below the hypoxic threshold of 63 µmol L −1 (Rabalais et al, 2001b). Below this threshold, burrowing invertebrates are stressed and ultimately die as oxygen levels decrease and persist over time (Rabalais et al, 2002;Levin et al, 2009;Middelburg and Levin, 2009;Metzger et al, 2014;Riedel et al, 2014). It can thus be hypothesized that the bioturbation function of the ecosystem, which relies on macrofaunal invertebrates' activity, is largely affected by hypoxia.…”

Ichnology of the Winnipeg Formation, southeast Saskatchewan: a glimpse into the marine infaunal ecology of the Great Ordovician Biodiversification Event

“…Adámek & Maršálek ; Metzger et al . ). The timing of appearance of the mixed layer in the geological record is still debatable (e.g.…”