Linking climatic-driven iron toxicity and water stress to a massive mangrove dieback

Amassive mangrove dieback event occurred in 2015/2016along ~1000km of pristine coastline in the Gulf of Carpentaria, Australia. To gain insights into dieback drivers, we combinesedimentandwood chronologies to analyze geochemical and climatic changes.The unique combination of low rainfall and low sea level observed during the dieback event was unprecedented in the previous three decades. Multiple lines of evidence from iron (Fe) chronologies in wood and sediment, wood densities and mangrove water use efficiency suggest low water availability within the dead mangrove forest.Wood and sediment chronologies suggest a rapid and largemobilizationof sedimentary Fe, which was likely associated with pyrite oxidation within mangrove sediments. High resolution elemental analysis of wood cross sections revealed 30-90 fold increase inFeconcentrations in dead mangrove areasjust prior to mortality. Fe concentrations in wood samples correlated strongly with the El Niño Southern Oscillation (ENSO) index, suggestingENSO was amajor driver of Fe mobilization. LargeFe losses from sediments during the dieback are consistent with Fe uptake in the trees, further implying sediment pyrite oxidation. If our data are representative of the entire dieback region, we estimate that the dieback drove the mobilization and loss of 50 ± 173 Gg Fe, equivalent to 8-50% of annual global atmospheric Fe deposition into the oceans, which is one of the major drivers of surface ocean productivity.Overall, our observations support the hypothesis that the forest dieback was associated with low water availability and Fe toxicity driven by a strong ENSO event.