Iron (Fe) is an essential trace metal element for marine organisms, which limits primary productivity in “high-nutrient, low-chlorophyll” (HNLC) regions of the global ocean. In the Peruvian oxygen minimum zone (OMZ), Fe released from shelf sediments has the potential to fuel phytoplankton growth if this supply reaches the euphotic layer. However, our understanding of the controls on temporal Fe supply within the Peruvian OMZ, for example, under varying El Niño-Southern Oscillation phases, is limited due partly to strong spatial variation in Fe dynamics. To address these uncertainties, here we report dissolved (dFe) and particulate (pFe) Fe concentrations, as well as Fe isotopic composition (δ⁵⁶Fe), along two coastal transects at 12°S and 14°S off Peru. An offshore plume with Fe-enriched waters indicates Fe release from the Peruvian shelf. Repeat measurements revealed that within one month, the highest Fe concentrations on the Peruvian shelf declined sharply from 13 to 4 nmol kg⁻¹ for dFe, and from 122 to 13 nmol kg⁻¹ for pFe. We attribute this rapid decline to changes in the speed of the Peru-Chile Undercurrent. Between April and May 2017, the Peru-Chile Undercurrent intensified and delivered more oxygen to the seafloor, therefore temporarily inhibiting Fe release from sediments into the water column. In addition to constraining dissolved and particulate concentrations we determined the Fe isotopic composition for dissolved (δ⁵⁶dFe) and particulate (δ⁵⁶pFe) which are informative concerning the specific processes affecting Fe distribution. Released δ⁵⁶dFe signatures from the Peruvian shelf are markedly low due to sedimentary reductive dissolution. Such low δ⁵⁶dFe signatures were evident in the offshore plume, coupled with elevated dFe and pFe concentrations, confirming reducing sediments as a dominant source of Fe for the plume. δ⁵⁶dFe in the core plume increased in response to declined Fe releasing by sediments resulted from intensified PCUC. Organic ligands binding dissolved Fe, the formation of authigenic Fe phases and scavenging were the main processes affecting offshore Fe dynamics. Our findings reveal that offshore Fe fluxes are sensitive to El Niño variability over short timescales. Understanding these rapid fluctuations is critical for predicting how coastal ocean productivity and thus Oxygen Minimum Zone dynamics will respond to future climate change.

 

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