Dissolved iron exports from an estuary surrounded by coastal wetlands: can small estuaries be a significant source of Fe to the ocean?

A creek draining a coastal wetland was studied to determine dissolved iron fluxes to the ocean, and whether these fluxes may be driven by submarine groundwater discharge (SGD). Dissolved metals (i.e., Fe, Mn, Ba, and U), natural groundwater tracers (²²⁴Ra and ²²⁶Ra), and dissolved organic carbon (DOC) were measured every 1 h for 30 h during both summer (wet season) and winter (dry season). The average dissolved Fe concentrations at the mouth of the creek were 8020 and 5630 nM during the summer and winter, respectively. Dissolved Fe concentrations in groundwater were 4060 nM and SGD-derived Fe fluxes were 1.2 mol km^{− 2} year^{− 1} (catchment area) or 205 mol km^{− 2} year^{− 1} (creek area). Groundwater seepage played a minor role on the total coastal wetland creek export of Fe to the ocean. The surface water dissolved Fe fluxes to the ocean were approximately 9 × 10⁵ mol km^{− 2} (estuary area) year^{− 1} and ~ 50,000 mol km² year^{− 1} on a catchment area basis. If our results are comparable to other small estuarine systems and considering the global areas of coastal wetlands (~ 660,000 km²), short creeks that drain coastal wetlands may discharge a large amount of dissolved Fe to the ocean (32.2 Gmol year^{− 1} or ~ 1.8 Tg year^{− 1}) which is comparable to the global river estimates (~ 1.5 Tg year^{− 1}). The high dissolved Fe concentrations associated with high DOC concentrations and no significant Fe removal within the estuarine gradient seem to be responsible for these large potential Fe fluxes.