Molecular composition of dissolved organic matter in the Mediterranean Sea

The molecular composition of marine dissolved organic matter (DOM) is still poorly understood, particularly in the Mediterranean Sea. In this work, DOM from the open Mediterranean Sea and the adjacent Northeast Atlantic Ocean was isolated by solid-phase extraction (SPE-DOM) and molecularly characterized using Fourier-transform ion cyclotron resonance mass spectrometry. We assessed the gradual reworking of the SPE-DOM transported by the shallow overturning circulation of the Mediterranean Sea by following the increase in molecular weight (+20 Da), oxygenation (+5%), degradation index (Ideg +22%), and the proportional decrease of unsaturated aliphatic compounds (+34%) along the Levantine Intermediate Water. This reworked SPE-DOM that leaves the Mediterranean Sea through the Strait of Gibraltar strongly contrasts with the fresh material transported by the inflow of Atlantic water (Ideg -25%). In the deep eastern and western overturning cells, the molecular composition of the deep waters varied according to their area and/or time of formation. SPE-DOM of the waters formed in the Aegean Sea during the Eastern Mediterranean Transient (EMT) was more processed than the DOM in pre-EMT waters formed in the Adriatic Sea (molecular weight and the proportion of unsaturated aliphatic compounds were increased by 5 Da and 9%, respectively). Furthermore, pre-EMT waters contain more reworked SPE-DOM (Ideg +7%) than post-EMT waters formed also in the Adriatic Sea. In summary, our study shows that the Mediterranean Sea constitutes a laboratory basin where degradation processes and diagenetic transformations of DOM can be observed on close spatial and temporal scales.