Final Activity Report Summary - POMFLUX (Particulate organic matter and nutrient fluxes in European margins) Ocean biology and chemistry respond to climate, nutrient supplies and atmospheric inputs. Vice versa, marine biogeochemical processes influence atmospheric levels of greenhouse gases and, hence, climate. Reliable predictions of future climatic changes will depend on models that shall accurately depict the complex interactions among multiple factors affecting Earth's climate system, as well as on an accurate understanding of feedbacks within the terrestrial and ocean carbon cycles that affect future fate of atmospheric carbon dioxide (CO2). Half of the CO2 released by fossil fuel combustion and deforestation to the atmosphere has already been taken up by the ocean. Future climate scenarios will depend on whether this fraction stays the same, or whether feedbacks resulting from changes in the marine and terrestrial carbon cycles change them. While our understanding of the oceanic carbon cycle has improved radically during the last decade, we cannot yet predict probable ocean responses to global change. Similarly, we have not yet developed the capability to address the physical, chemical and biological feedbacks to atmospheric CO2. A quantitative understanding of the ocean carbon cycle is a necessary, but certainly not a sufficient, condition for addressing these issues. Credible projections of the ocean carbon cycle response to climate perturbation will not be possible without a much more detailed, mechanistic understanding of the processes controlling the partitioning of carbon among the marine, terrestrial and atmospheric reservoirs. One of the critical components needed to answer these questions is an improved understanding of the past, present and future variability of the ocean carbon cycle, particularly as it relates to the air-sea and land-sea exchange of carbon. Two of the major pathways followed by organic matter, i.e. the carbon-rich debris produced by once-living organisms, to travel from shallow towards deeper areas of the ocean are vertical settling through the pelagic region of the ocean, as described by Fasham et al. in 2001, and advective lateral transport near the bottom, mainly focussed along submarine canyons (Heussner et al., 1996; van Weering et al., 2001). The relevance of this transfer lies in the fact that it plays a key role in the withdrawal of carbon from reservoirs with a high rate of exchange with the atmosphere towards more isolated reservoirs, i.e. the marine sediments, where carbon can be retained during millennia. Furthermore, downward and advective organic carbon and nutrient fluxes are presumably key mechanisms affecting the functioning of deep-water benthic ecosystems as they represent the main source of energy reaching such environments, as referred to by Vetter and Dayton in 1998. These important ecosystems require urgent study because of their possible biological fragility, global relevance to carbon cycling and susceptibility to catastrophic events and global change. The pressure to improve understanding of the deep-water ecosystems is great because human exploitation of deep ocean margins is progressing rapidly both in Europe and beyond. During transit of vertical sinking particulate matter towards the sea floor, most, usually greater than 90 %, of Particulate organic carbon (POC) is returned to inorganic form in a process called 'remineralisation' and redistributed in the water column. This redistribution determines the depth profile of dissolved CO2, including its concentration in the surface mixed layer, and hence the rate at which the ocean can absorb CO2 from the atmosphere. The ability to quantitatively and mechanistically predict the depth profile of remineralisation is therefore critical to predicting the response of the global carbon cycle to environmental change.
