A 20-year time-series analysis of carbon oxidation rates in the dark waters of the Ionian Sea: the microbial response to the global changes.

In the marine ecosystem the biological pump controls the export of biogenic carbon from the surface layers down to the depth. In brief, through photosynthesis processes in the euphotic zone, this mechanism transforms the carbon dioxide into organic matter ; the latter sinks to the waters below and at the same time is consumed by respiration with the production of metabolic CO2. However, during the formation of dense waters, a significant quantity of organic products is conveyed from the surface within the water mass and remineralized inside it, thereby altering the normal flow of the biological pump. In 1990 a transitional climatic event (EMT, Eastern Mediterranean Transient) has changed not only the physical characteristics of the Mediterranean Sea but the microbiological, too. The organic load advected from Aegean Sea has fuelled the deep-sea biota that has utilized most of the oxygen carried by the new water masses for the biological oxidation. In this study the evolution of carbon dioxide production rates in the deep Ionian Sea, before and during the Eastern Mediterranean Transient (EMT), is reported. The check of the carbon oxidation evolution has shown that EMT event has constituted a natural test for a different approach to the oceanic carbon sequestration. Biogeochemical implications of accelerated rates of oxygen consumption and then of CO2 sequestration in the Ionian Sea will be analysed and discussed in the text.