Molecular Fractionation of Dissolved Organic Matter in a Shallow Subterranean Estuary: The Role of the Iron Curtain
Marine Dissolved Organic Matter (DOM) is one of the largest active carbon pools on Earth. The Earth’s continents are major sources for terrigenous DOM, which is transported with other constituents such as iron via rivers or groundwater into the ocean. We know from previous studies that DOM of terrestrial origin is more likely to coagulate with iron than DOM of marine origin. What implications could these findings have for the subterranean estuary, a coastal aquifer where meteoric groundwater mixes with seawater, thus we find DOM of varying origin and composition?
We found that the extent of coagulation decreased along the studied transect from land to sea. We suggest that fractions more prone to iron coagulation (such as polyphenols, PCAs with aliphatic chains, and phenolic and highly unsaturated compounds), which are more abundant in DOM of terrestrial origin and less and less abundant towards the sea, precipitate with iron. In consequence, the extent of coagulation overall decreases with increasing distance from land to sea.
The iron curtain, the iron-rich redox boundary typical for subterranean estuaries, might serve as an inorganic modulator for the supply of DOM from groundwaters to the sea, and the subterranean estuary might have the potential to act as a temporal storage or even sink for terrigenous aromatic DOM compounds. This process could be of global importance, considering that subterranean estuaries are characteristic for sandy beaches.
Reference: Linkhorst A, Dittmar T, Waska H (2016), Molecular Fractionation of Dissolved Organic Matter in a Shallow Subterranean Estuary: The Role of the Iron Curtain. Environ. Sci. Technol. doi: 10.1021/acs.est.6b03608