Sedimentary dynamics of coastal organic matter: An assessment of the porewater size/reactivity model by spectroscopic techniques

Sedimentary organic matter (SOM) mineralization is the driving force in diagenesis activity. This biogeochemical process, which controls the mobility of a huge number of elements and especially SOM transformations in surface and deep (down to −50 cm) sediments, was assessed in this study via seasonal monitoring with sediment core sampling in Toulon Bay (SE France). The SOM diagenetic degradation resulted in an important production of nutrients (NO3−/NH4+, PO43−, Si(OH)4, etc.) in porewater associated with dissolved organic matter (DOM) characteristic modifications, which are seasonally dependent. Within this seasonal variation, different elemental ratios (C/P, C/Si) as well as absorbance and fluorescence characteristics were observed in surface and deep sediments, illustrating two pools of SOM, i.e., “fresh” and “buried”. Ultra-filtration results revealed the high proportion of low-molecular-weight DOM and the high-molecular-weight DOM (<3 kDa) was primarily characterized by protein-like fluorophores. The obtained results confirmed and extended the PoreWater Size/Reactivity model that describes diagenetic SOM transformations through the identification of two SOM pools and their reactivity.