Acidic drainage drives anomalous rare earth element signatures in intertidal mangrove sediments

Sedimentary rare earth element (REE) signatures can provide powerful insights into nearshore biogeochemical processes and anthropogenic influences. Despite this, there is limited research investigating REE behaviour in sediments influenced by coastal acid sulfate soils (CASS). Here, we explore REE abundance and fractionation in intertidal mangrove sediments that received CASS drainage for ~15–20y within the Hastings Catchment in NSW, Australia. Sediments close to the CASS discharge point (1.5), with a high proportion (63–100%) of REEs residing in the reactive (1M HCl extractable) sediment fraction. Interestingly, the degree of MREE enrichment was significantly correlated with Ce anomalies (r2=0.72, P1) are consistent with reactive Fe(III) oxides/oxyhydroxides driving REE retention in these sediments. This study indicates that CASS drainage alters REE signatures in receiving sediments by (1) providing a source of REEs, thereby enhancing sedimentary REE concentrations, and (2) causing accumulation of reactive Fe(III) phases with a high affinity for REEs. Together, these two factors drive the development of distinctive REE signatures in CASS-impacted sediments. The recognition of such signatures may provide a promising tool for identifying coastal sediments receiving anthropogenic CASS drainage inputs. [Display omitted] •REE examined in intertidal sediments exposed to long-term (~20y) acidic drainage•Acidic drainage drives REE enrichment in reactive sediment fraction.•Distinctive REE fractionation observed in sediments closest to acidic source•REE signature reflects sediments rich in reactive Fe(III) due to acidic influence.•Site uninfluenced by CASS has REE fractionation typical of coastal sediments.