GLACIAL/INTERGLACIAL CYCLE OF ORGANIC CARBON RELEASE AND SEQUESTRATION IN MID-LATITUDE SHELVES

During the Quaternary glacial/interglacial cycles, the continental shelves experienced very dramatic environmental changes, passing from submergence to emergence i.e. from submarine deposition to subaerial exposure and erosion. So far the main contribution that such changes will produce on carbon cycle was identified in tropical environments due to the downwards displacement of carbonate factory and carsification of emerging reefs. In mid-latitude shelves however, a relevant role can be played by organic carbon being sequestrated in fine-grained sediment during sea level highstand and being released during sea level lowering and lowstand. An evidence of large amount of organic carbon within highstand deposits (prodelta and shelf mud) comes from high-frequency reflection seismics. In fact organic carbon discharged from rivers is buried within sediments where methane is usually generated. If methane concentration exceeds methane solubility, gas bubbles will form, hindering seismic wave penetration on sediment Shelves in both passive and active continental margins are suitable for storage of organic carbon, as passive margins host large river deltas whilst active margins are characterized by very narrow shelves with high sedimentation rates. However the highstand deposits have a very low preservation potential during a whole glacio-eustatic cycle. In fact margin architecture of mid-latitude shelves is essentially made up of forced regression and lowstand deposits, very different in geometry and seismic facies from present-day (highstand) shelf deposits. This fact is due to the fast sea level changes that cause the highstand deposits to be exposed to subaerial (and shoreline) erosion shortly after their deposition, so that they can be easily removed. Therefore the organic carbon trapped on highstand sediments will be released directly to the atmosphere during each glacial lowering of sea level. It is relevant to notice that such a phenomenon (release during glacial/lowstand, sequestration during interglacial/highstand) contrasts with the observed decrease of atmospheric methane concentration during glacial time. However it has to be taken into account as it may interfere with the other processes responsible of methane release or sequestration.