Nutrient, Oxygen and Carbon Ratios, CO₂ Sequestrationand Anthropogenic Forcing in the Mediterranean Sea

Abstract

The climate and the environment, together with present tectonic morphology, constitute main constraints for the Mediterranean Sea geochemistry. The resulting homogeneity of deep water characteristics (temperature and salinity) and concentrations (nutrients, oxygen, alkalinity, total inorganic carbon) allow estimation of carbon, oxygen and nutrient ratios, new production and carbon sequestration at the basin scale. In the eastern Mediterranean, the C/P and O₂/P molar ratios are about 200% and the N/P ratio 50% higher than the classical Redfield ratios (RR) [1]. Total new production, determined either from the carbon budget or from oxygen and nutrient ratios, is consequently twice as great as previous estimates based on the phosphate budget and RR. Anomalies in nutrient, oxygen and carbon ratios probably denote a link between marine dynamics, the continental environment (phosphate and silicate inputs) and the marine ecosystem with predominating diatoms, nitrogen fixation and phosphate limitation. Over the twentieth century, the determined CO₂ sequestration was mainly derived from marine dynamics (winter deep water formation and Mediterranean deep outflow towards the Atlantic intermediate waters), while the biological pump (new production) only represented about 3% of the total sequestration. Nevertheless, the recent increase of phosphorus pollution by anthropogenic inputs to the sea favours biological CO₂ uptake, together with an appreciable sequestration of the atmospheric CO₂ in the early twenty-first century, and an ecosystem shift from dominant diatoms to non-siliceous species.