Benthic biogeochemical processes change the chemistry and the mineralogy of the first centimetres below the water/sediment interface. These processes are linked to the mineralisation of organic matter, which fall on the sea floor. They control the benthic behaviour of redox species, OM or pollutants…The objectives of this study was to clarify the dynamic of the redox-sensitive species in modern sediments (Bay of Biscay, France) and to calibrate geochemical proxies used as paleotracers of past surficial redox conditions and consequently of past organic carbon rain rate.

Vertical distribution of major redox compounds has been analysed in pore waters (O2, NO3-, NH4+, Mn2+, Fe2+, SO42-) and in solid fraction (Mn-oxihydroxides, -carbonate, amorphous Fe-oxides, total sulfur and both organic and inorganic carbon) in order to study and quantify depth sequences of diagenetic reactions (nature and prevailing processes, fluxes, seasonal variations). The classical sequence of redox zones has been observed, but several secondary diagenetic reactions occur, and create alternative metabolic pathways. These processes involve mainly benthic cycle of nitrogen species and metals (Fe and Mn) and could produce oxidised compounds in anoxic horizon (example of anoxic NO3-production). Additionally, non steady-state diagenetic processes have been studied in recent turbidites (Capbreton Canyon). These turbites showed to be a natural laboratory to observe the transitory nature of diagenetic processes
The vertical distribution of trace metals (U, Mo, Cd, As, and REE) is related to the presence of major diagenetic phases, such as sulfides and reactive metal-oxides (Mn- and Fe-oxides). Although their diagenetic behaviour is controlled by redox conditions, their use as paleotracers needs further investigations.