Along-path evolution of biogeochemical and carbonate system properties in the intermediate water of the Western Mediterranean

It is well known that in the Mediterranean major oceanic processes may occur, of which the most importantexample is deep water formation (DWF) that sustains the basin-wide thermohaline circulation cell. This is alsothe main oceanic mechanism that sustains the physical carbon pump: once carbon dioxide is dissolved in surfacewater, it can enter into the ocean carbon cycle and be sequestered and transported to different depths and parts ofthe ocean thanks to the processes of DWF and thermohaline circulation. The deep waters that are formed via DWFhave long residence times, thus trapping great parts of the anthropogenic carbon released to the atmosphere.The most ubiquitous water mass of the Mediterranean Sea is the IntermediateWater (IW). It forms via salinificationand densification of the surface waters in the Levantine Basin and in the Cretan Sea. After sinking to its equilibriumdepth, the IW spreads throughout the whole Mediterranean Sea. The core of this water mass can be easily identifiedby its absolute maximum in salinity (and relative maximum in temperature), when looking at vertical profiles ofthese properties. While flowing back towards the Western Mediterranean (WMED), it tends to gradually lose itscharacteristics, due to dilution with adjacent water masses, becoming thus less salty and less warm. Not onlysalinity and temperature change along the IW path across the WMED, but also its biogeochemical and carbonatesystem properties: these non-conservative properties change also as a consequence of bio-chemical processes.The aim of this investigation is to understand the evolution of physical (temperature, salinity), biogeochemical(AOU, nutrients and DOM) and carbonate system properties (TA, pH) of the IW along its pathway through theWMED, assessing the role of changes induced by physical mixing of the IW with adjacent water masses and thoseinduced by biological and biochemical processes. To discriminate between the acting processes and assess theirrelative roles, a mixing analysis has been performed.Along IW path the increase of AOU with age of the water mass indicates the preponderance of respiration over theproduction. The most important increase of AOU occurs while the IW enters and circulates within the TyrrhenianSea. In all other areas persistent strong hypoxic conditions are maintained with scarce changes. The decreasein TA is consistent with the change in salinity, as expected given the strong linear relationship between these twoparameters, while the decrease of pH and the increases of the concentrations of the main inorganic nutrients (+77%,+34 and +26 %, for phosphates, nitrates and silicates, respectively) indicated an active remineralization of organicmatter in this water mass. The process of acidification of IW also causes a shift of the carbonate equilibrium,toward more acidic species, and a decrease of the saturation state of calcite and aragonite, indicating a reductionof the oversaturation of these calcium carbonate minerals along IW path. The concomitant increase of the Revellebuffer factor suggest that this water is potentially less efficient to take up atmospheric CO2 with aging.