Impact of the Geometry of Submarine Landscapes on Deep-Sea Biogeochemistry

Production of organic carbon by phytoplankton in the surface ocean, followed by transport of some of this organic carbon on sinking particulate matter from the surface ocean into underlying sediments, extracts carbon dioxide from and injects oxygen into the atmosphere. For long geological time scales of thousands up to hundreds of millions of years it is believed that changes in the magnitude of organic-carbon deposition in deep-sea sediments can influence the chemical composition of the atmosphere. Organic-carbon burial in deep-sea sediments must, therefore, be one of the key processes of the 'life-supporting system' on Earth. Consequently, an understanding of the mechanisms controlling the flux of carbon from the oceanic water column into underlying sediments and the burial of carbon in the sediments is of crucial importance. A number of possible controls on these carbon fluxes into deep-sea sediments have been studied. However, to date the impact of submarine landscape geometry has received virtually no attention. This is despite comprehensive and pervasive submarine landscape changes that must have occurred as a result of the rearrangements of continents and oceanic crust during the last hundreds of millions of years. Mid-ocean ridges, but also to some degree abyssal plains, are structured by submarine hills and mountains. Such kilometre-scale seafloor elevations are a major source of environmental variability in the deep sea. In addition to their mere presence, the interaction of the elevations with quasi-steady background (residual) and tidal flow introduces complexity in the environment. This enhanced complexity is expected to influence a range of environmental parameters and processes, including larval dispersal of deep-sea organisms, biodiversity (an important indirect control on sediment biogeochemistry), the absolute magnitude of sedimentary carbon burial, and the relative proportions of organic carbon being remineralised in deep-ocean waters and surface sediments versus organic carbon being buried in deeper sediments. This project will elucidate for the first time the link between three fundamental aspects of kilometre-scale flow/topography interactions and organic-carbon dynamics in the deep ocean: (1) The influence of the PRESENCE OF A SEAMOUNT on the transport of organic carbon through the water column and its fate in deep-sea sediments. (2) The impact of GEOGRAPHICAL LATITUDE: Quasi-steady background flow interacts with the seamount, with the shape of the resulting flow field depending on the impact of the Earth's rotation which, in turn, depends on the geographical latitude of the seamount. Are there latitudinal differences in the impact of topographically controlled flow fields on carbon dynamics? (3) The impact of TIDES: Tidal current velocities vary spatially in the deep sea and may have varied temporally over ice age cycles, thereby introducing spatiotemporal variability in the magnitude of tidal impact. How do different tidal forcings influence carbon dynamics at kilometre-scale seafloor elevations? We propose to elucidate these three problems by comparing seamounts of similar dimensions which differ in terms of their geographical latitude and tidal forcing: In the Northeast Atlantic the Senghor Seamount at 17degN and the Ampere Seamount at 35degN have similar open-ocean tidal forcing and can be compared in terms of the impact of the geographical latitude; the Ampere Seamount at 35degN and the Eratosthenes Seamount in the Eastern Mediterranean at 33.5degN are at similar geographical latitude and can be compared in terms of tidal forcing, with the tides in the Eastern Mediterranean being much weaker than the tides in the Northeast Atlantic. The main anticipated achievement of this project is a much advanced understanding of the fundamental controls of seafloor geometry on deep-sea biogeochemistry and biodiversity.