Mangrove sediments as long-term mercury sinks: Evidence from millennial to decadal time scales

The mercury (Hg) cycle in estuaries has been globally discussed, although Holocene deposition in mangrove sediments remains unknown. Herein, a sediment core from a mangrove system in southeastern Brazil was 14C-dated to evaluate millennial Hg deposition. The highest Hg concentrations (1010–2540 ng g−1) in surface sediments were explained by emissions from a chlor-alkali industry (1964 CE). However, Hg levels were also high in pre-industrial periods, associated to fine grain-size and algal organic deposition. Less anomalous Hg concentrations in bottom sediments indicate Holocene ages (~1940–3324 cal yr BP), potentially associated to Serra do Mar mountains weathering. This study reveals the capacity of mangrove to retain Hg over millennial time scales, acting as significant and long-term Hg sinks. Therefore, the use of Hg as an Anthropocene marker must be considered cautiously in coastal systems that act as Hg sinks in times when environmental changes were not caused by human activities. •A mangrove sediment core was 14C-dated to assess millennial Hg deposition.•Hg was high in surface sediments due to chlor-alkali plant emissions.•Holocene ages were obtained by radiocarbon ages.•Mangroves are reservoirs for high Hg concentrations.•Applying mercury as an Anthropocene marker in mangroves is not simple.