Reconstructing extreme climatic and geochemical conditions during the largest natural mangrove dieback on record
2020
Abstract. A massive mangrove dieback event occurred in 2015–2016 along ∼1000 km of pristine
coastline in the Gulf of Carpentaria, Australia. Here, we use sediment and wood chronologies to
gain insights into geochemical and climatic changes related to this dieback. The unique
combination of low rainfall and low sea level observed during the dieback event had been unprecedented
in the preceding 3 decades. A combination of iron ( Fe ) chronologies in wood and sediment, wood
density and estimates of mangrove water use efficiency all imply lower water availability within
the dead mangrove forest. Wood and sediment chronologies suggest a rapid, large mobilization of
sedimentary Fe , which is consistent with redox transitions promoted by changes in soil moisture
content. Elemental analysis of wood cross sections revealed a 30- to 90-fold increase in Fe
concentrations in dead mangroves just prior to their mortality. Mangrove wood uptake of Fe during
the dieback is consistent with large apparent losses of Fe from sediments, which potentially
caused an outwelling of Fe to the ocean. Although Fe toxicity may also have played a role in the
dieback, this possibility requires further study. We suggest that differences in wood and
sedimentary Fe between living and dead forest areas reflect sediment redox transitions that are, in
turn, associated with regional variability in groundwater flows. Overall, our observations provide
multiple lines of evidence that the forest dieback was driven by low water availability
coinciding with a strong El Nino–Southern Oscillation (ENSO) event and was associated with climate change.
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