The anoxia develops as a result of the microbial decomposition of organic carbon in the sediments. This consumes the dissolved oxygen in the pore water and the core becomes anoxic. The organic carbon can come from the death of plankton in the water column, or inputs via sewage and river run off of organic rich material.

Hello! Oxic to anoxic variability is controlled by the availability of dissolved oxygen within the aquatic sediment environment. As oxygen is depleted, redox reactions proceed with the use of alternative terminal electron acceptors in the following order: O2 > NO3- > Fe3+ > Mn4+ > SO42- > CO2

Please refer to the seminal work by R. A. BERNER for more information as follows:

BERNER, R. A.: A New Geochemical Classification of Sedimentary Environments. J. Sediment. Petrol., 1981, 51(2); pp. 359 – 365.

are any sediments fully oxidized? It was my impression that the redox gradient C. Hamilton describes occurs everywhere in water saturated sediments, the transition depth depending on hydraulic conductivity assisting diffusion of oxygen and oxidized species and the spatial distribution of the various elements to be used as electron acceptors. For example, in some tropical systems there is little to no iron. In some fresh water system little to no SO4.

following the discussion above I thnk there are sediments with a very thin aerobic layer (typically shallow nutrient rich coastal waters such as the waddensea in which the aerobic layers is only 0-a few millimeters) to oligotrophic nutrient and OC poor systems were the aerobic layers can reach into the ground water which itself can be aerobic.

oxic and anoxic conditions in marine environment are controlled by availability of dissolved oxygen in water column, more abundant life in water column will exhaust dissolved oxygen making waters as reducing in nature, flux of sediments from land also make water column as reducing, deep water ventilation also plays important role in governing the Eh of marine environment

Yu (1991) showed that the

pH of acidic sediments increased from 4.5 to near 5.5

after they were submerged for 7 d and reached a final

stable pH of 6.4 after about 14 d. When submerged,

sediment pH is influenced by several reactions (Yu,

1991). The impact of these reactions on sediment pH

changes as the redox status changes. In general, reduction

reactions in aqueous systems take up protons, or

make the sediment more basic.

The most simple answer is perhaps the presence of oxygen during the formation of these marine sediments. You have to look for low-temperature oxide or sulphide mineral to evaluate the deposition history. For detail mineralogy please see Deer, Howie and Zussmans' Mineralogy book

Don't forget that the quality and quantity of the organic carbon that reaches the sediments affects the oxygen and its penetration into the sediments.

The control of redox status (oxic-suboxic-anoxic) in marine sediments is controlled by the most favourable thermodynamic/kinetic reaction coupled with the bacteria population that mediated most of early diagenetic reactions. In some environments the sequence above presented may be well defined in depth while in other environments an overlapping may occur due to biotubation, excess of one of electron acceptors (eg. Hydrothermal vents), diminishing quantity of organic matter, ... Other aspects like permeability, tortuosity, compaction may also influence the early diagenesis.

If you are looking at longer timescales and want to examine some macroscopic causes for a facies change from inferred oxic to anoxic sediments, I would suggest further investigating the influence of primary productivity. Episodes of enhanced primary productivity (evidenced by high TOC) can cause dysoxic/anoxic bottom waters via the above methods and could manifest as a facies change. These episodes can be initiated by terrestrial sources of enhanced nutrient supply (riverine input) and/or marine sources by hydrothermal input. I would also pay attention to geography and ocean circulation.

The amount of dissolved oxygen in a parcel of water mostly depends on two factors, the solubility of oxygen at the air/water interface, and the amount of organic carbon respired since time of initial exchange. When it comes to sediment, it mostly depends on oxygen concentration at the seafloor, diffusion into the sediment, and respiration within the sediment. There is a very wide range given these parameters. For example, one can find modern sites in the central Pacific Ocean that have oxygen in pore waters for meters (because of very limited organic input) and on the slope of the eastern Pacific Ocean (offshore of Peru) that have no oxygen at the seafloor (because of very high organic input through the water column, as well as old water age).

One prerequisite for well developed anoxic conditions is that the bottom-near waters remain stratified preventing oxygenation through overturn. Such a stratification can be due to salinity or temperature stability.

The Baltic Sea is a typical example of a large water body frequently experiencing anoxia in its deep basin due to inflow of more saline North Sea waters through the Danish Straits and further enhanced by lengthy periods of warm climates, e.g. during the Medieval Warm Period with strong algal blooms (high bioproductivity).

The amount of light reaching the sediments via the water column and organic input are important determinants of the oxygenated status of sediments.

Incidences of up-welling also affect oxygenation of sediments.

I can accept that light is an important factor in production of organic matter, but the development of anoxia is in no way dependent on light. Strong bacterial degradation of abundant organic matter in the sediment especially in poorly oxygenated waters is the main requirement. Poor exchange of bottom water is also a prerequisite for the formation of near bottom anoxia, although anoxia deeper in the sediment is only dependent on the amount of organic matter. .

Oxic and anoxic sediments are controlled by both marine conditions and terrestrial input. In case of marine conditions surface water productivity, upwelling, oxygen minimum zone, deep and shallow water ventilations play the role for deciding oxic or suboxic conditions. In case of terrestial input, flux of sediments coming from nearby landmass depends upon precipitation conditions which if intense can make marine anoxic sediment conditions while as oxic conditions will remain under low intensity rainfall and sediment flux to oceans from nearby landmass.

J.N. Pattan, Ishfaq Ahmad Mir, G. Parthiban, Supriya G. Karapurkar, V.M. Matta, P.D. Naidu, S.W.A. Naqvi, 2013. Coupling between suboxic condition in sediments of the western Bay of Bengal and southwest monsoon intensification: A geochemical study. Chemical Geology, 343, 55–66.

I would just like to add a quick supplementary comment - although the answers above are all excellent. I work in bone diagenesis and have examined bone specimens from a wide range or "waterlogged" marine and terrestrial sediments. One factor at the very local scale seems to be anoxic micro-niches where the oxygen potential can be very low compared to the surrounding sediment. This seems to be a result of slow oxygen diffusion rates and poor exchange of pore water. As an example, I have found pyrite framboids in the pore structures of an obviously very modern bone surface-collected from a beach. There was no evidence that this was ever enclosed in anoxic sediments since the colour was pristine, white.

Fluctuation of sea on the shore which control availability and non availability atmospheric oxygen on sediments

@ Saka Opeloye: Sir, I failed to understand the efficiency of impact of Fluctuation of sea on sediment oxygen concentration. Is it applicable for Kyrs scale ?

Gordon makes a valid point in that anoxic conditions can prevail in a multitude of environments ranging from microscopic enclosures devoid of oxygen or in fjordlike environments with complete anoxia in depressions. Sapropelic muds cover a large area of teh bottom of the Black Sea, etc.

Framboidal pyrite/marcasite is well known in mid Holocene Ancylus Clays in the Baltic Sea: Ignatius, Heikki; Kukkonen, E.; Winterhalter, B. 1968. Notes on a pyritic zone in upper Ancylus sediments from the Bothnian Sea. Bull. Geol. Soc. Finland, n:o 40, p. 131-134.