Abstract
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 Niño–Southern Oscillation (ENSO) event and was associated with climate change.
Highlights
Mangroves provide a wide range of ecosystem services, including nursery habitat, carbon sequestration and coastal protection (Barbier et al, 2011; Donato et al, 2011)
Climate change is a major threat to mangroves, which adds to existing stressors imposed by deforestation and over-exploitation (Hamilton and Casey, 2016; Richards and Friess, 2016)
The climate records over the last 3 decades reveal an unprecedented combination of low sea levels and low annual rainfall
Summary
Mangroves provide a wide range of ecosystem services, including nursery habitat, carbon sequestration and coastal protection (Barbier et al, 2011; Donato et al, 2011). In Australia, an extensive mangrove dieback event in the Gulf of Carpentaria from December 2015 to January 2016. Sippo et al.: Reconstructing extreme climatic and geochemical conditions during mangrove dieback coincided with extreme drought and low regional sea levels. This extreme climatic event drove the largest recorded mangrove mortality event (∼ 1000 km coastline, ∼ 7400 ha) attributed to natural causes (Duke et al, 2017; Harris et al, 2017; Sippo et al, 2018) and led to extensive changes in the coastal carbon cycle (Sippo et al, 2019, 2020) and coastal food webs (Harada et al, 2020). Two other large-scale mangrove dieback events occurred at the same time: one in Exmouth (Lovelock et al, 2017) and the other in Kakadu National Park, Australia (Asbridge et al, 2019)
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