Barite bio-organo-mineralization processes at mesopelagic depths

Important to our understanding of marine biogeochemistry and climates cycles is the transfer to depth of particulate organic carbon (C) via the biological pump. The surface and mesopelagic zones (upper 1000 m) are of crucial significance in determining the efficiency of the transfer and the fate of biogenic material, since mineralization length scales are important indicators of the capacity of intermediate layers for longer term C sequestration. A quantitative representation of this process is thus essential to every simulation of the oceans’ role in the global C cycle.

The particulate barium barite (Ba-BaSO4) in suspended matter is a proxy of C mineralization fluxes. The picture emerging today is that barite precipitation takes place inside micro-environments (e.g. biogenic aggregates) sinking out of the surface layers into mesopelagic waters. Barite formation appears closely linked (directly or indirectly) with prokaryotic degradation of Organic Matter (OM) and thus with C remineralization rate. As revealed by barite proxy in contrasting environments, the extent of mesopelagic C mineralization appears closely linked to specific ecosystem’s characteristics (e.g. differences in phytoplankton community composition, grazing pressure, trophic interactions and types of aggregates formed). However, we are still far from a detailed understanding of the processes controlling formation and fate of aggregates beyond the surface layer. Indeed, uncertainties both on particles and plankton communities characteristics and on the exact processes controlling barite precipitation are limiting our understanding of particle formation, transport and fate, thereby impeding both a predictive understanding of the flux of organic C to the deep and a wider use of the barite proxy. Here we point out the need for a mechanistic understanding of the barite formation process and of the Ba trace-element cycle in order to validate both the modern and paleo–applications. Overall, the factors controlling the whole processes reported above need to be better constrained, especially at meso- and submeso- scale. It is clearly a challenge to connect the flux of sinking particles to surface water processes and to assess their variability with regards to climate and anthropogenic changes.