Redox cycling controls iron inputs to the interior of the eastern tropical Pacific from the Mexican continental margin

Continental margins associated with eastern boundary upwelling regimes and oxygen minimum zones exhibit large, deep plumes of iron and may be a significant source of iron to the oceans interior. Such systems include eastern boundary regimes off Peru and Namibia. The Arabian Sea, while not technically an eastern boundary regime, has similar features. Such plumes are thought to arise from an efficient shelf to basin shuttle that transports iron ultimately derived from fluvial sources. Here, we show that the eastern boundary region off southern Mexico, with a vast oxygen minimum zone, can be added to this list. Based on results from a cruise in 2016 on the RV Ron Brown and subsequent cruises on other vessels, we show that there is a deep plume of iron extending from the continental margin at depths well below the oxygen minimum zone. Formation processes probably resemble those off Peru: advection of Fe(II) off the Fe-rich shelf followed by chemoautotrophic oxidative scavenging mediated by nitrate and ultimately remobilization of Fe off the continental slope at depth. Eddies originating along the shelf-slope break and transported within the 13CW water mass help to facilitate this process. The Fe feature is not associated with an enrichment in manganese, a characteristic of the margin derived plumes, in contrast to hydrothermal inputs, which are usually associated with manganese. Ultimately, the source of iron is major rivers along the Mexican Pacific coast. This iron may be transported within a poleward undercurrent and support primary production within the California coastal upwelling.