Early diagenesis and redox transitions

On a short timescale, coastal eutrophication and contaminant fate motivate research in early diagenesis,while burial of reduced substances such as organic matter is relevant for the long-term evolution of oxygen in the atmosphere (Berner and Canfield 1989).

Bioirrigation
An ubiqueouis feature in aquatic sediments is the sequential depletion of terminal electron acceptors (oxygen, nitrate, Mn & Fe oxides, sulfate). However, particularly in the coastal ocean, sediments can exhibit a high degree of heterogeneity, to a large part due to the presence of benthic fauna. One of the effects of burrowing organisms on sediment biogeochemistry is through their enhancement of transport of both solids and solutes. This is particularly important in the coastal ocean, where benthic organisms are most abundant such that bioirrigation can even strongly affect global biogeochemical cycles (Meile and Van Cappellen 2003).

Burrowed sediment-water interface: core top and simulated oxygen distribution in a burrow

Redox transition zones
The Orca Basin is a depression located on the northern slope of the Gulf of Mexico with a strong density stratification due to the increase of salinity from 35 to about 260 ‰. The stratification lowers transport rates dramatically, hence increasing the residence time of settling particulate organic matter at the brine-water interface. The depth range over which the transition between oxic and anerobic reactions occur extends over several meters, instead of the centimeter scale redox gradients usually observed in sediments but redox sequences in such stratified water bodies are similar to those encountered in sediments.

In contrast to these meter-scale redox transition zone in the Orca basin, microbial mats exhibit a layers of distinct chemical characteristics and microbial populations at the microscale. As ancient forms of living communities, microbial mats are of particular interest from a evolution of life perspective.

References:
Berner, R.A., and Canfield, D.E. 1989. A new model for atmospheric oxygen over phanerozoic time, Am. J. Sci. 289 , 333-361.
Meile, C. and Van Cappellen, P. 2003. Global estimates of enhanced solute transport in marine sediments. Limnol. Oceanogr. 48(2), 777-786.