Rare earth element distributions in salt marsh sediment cores reveal evidence of environmental lability during bioturbation and diagenetic processes

• Published in: Chemical geology 2021-12, Vol.584, p.120503, Article 120503
• Main Authors: Costa, Larissa, Johannesson, Karen, Mirlean, Nicolai, Quintana, Guilherme
• Format: Article
• Language: English
• Subjects: Ce anomalies; Estuary; Oxyhydroxides; Rare earth elements; Salt marsh
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2021.120503

The distribution and accumulation of rare earth elements (REE) in the labile fraction of sediment cores collected from salt marshes in the Patos Lagoon estuary from southern Brazil were investigated. Sediment cores (ca. 40 cm) were obtained from three locations within the estuary to capture possible changes in REE content across the salinity gradient (i.e., where saline, brackish, and freshwater dominate). Salt marsh sediments from all three coring locations were enriched in the light REE (LREE) over the heavy REE (HREE) when normalized to the North American shale composite (NASC). Shale normalized values for the LREE of marsh sediments from sites M2 and M3 in the mid- and upper estuary commonly approximates unity indicating these sediments chiefly exhibit a terrigenous signature. In contrast, all 14 naturally occurring REE are depleted in the sediments from the M1 coring location in the lower estuary compared to shale. Sediments from the mid-estuary (M2 core location) where typical salinity values are ca. 10 practical salinity units, exhibit the greatest shale normalized LREE enrichments. The higher LREE contents of the M2 sediments likely reflect preferential removal of LREE from the water column owing to salt-induced coagulation of river-borne colloids that occurs during estuarine mixing processes. Sediments samples collected from the salt marsh in the lower estuary nearer the Atlantic Ocean (i.e., M1), have substantially lower REE contents than salt marsh sediments from sites M2 and M3 in the mid- and upper estuary, respectively. The more sand-rich lower estuary sediments are exposed to higher salinity water from the South Atlantic compared to the generally finer grained sediments from the mid- and upper estuary, which are dominated by brackish and freshwater conditions, respectively. Negative Ce-anomalies (i.e., Ce/Ce*  1) for much of the sediment from