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

Abstract

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) in marsh sediments from depths generally greater than 27 cm support the occurrence of early diagenetic reactions consistent with suboxic to anoxic conditions whereby reductive dissolution of ferromanganese oxide/oxyhydroxide carrier phases, and/or Ce(IV) phases, release Ce(III) to the marsh pore waters. The negative Ce anomalies suggests that a suboxic or anoxic environment predominates to a depth of 9 cm, and then again below 27 cm depth in sediments from the M2 salt marsh core, and below a depth of 13 cm in the upper estuary marsh sediments (i.e., M3). In contrast, positive Ce anomalies (Ce/Ce* > 1) for much of the sediment from the M1 core, the mid depths of core M2 (i.e., 9 cm to 27 cm), and shallower than 13 cm in sediments from core M3 support oxic conditions where Ce is enriched in sediments from these depths by preferential scavenging from pore waters onto Fe/Mn oxides/oxyhydroxides. These trends in Ce/Ce* with depth in each sediment core are also consistent with known bioturbation that occurs within these marsh sediments. The relatively high REE contents of the sediments from location M3 are also related to the fact that these sediments are dominantly composed of clay-silt deposits that are enriched in organic matter. Finally, we find no conclusive evidence that the studied salt marsh sediments have been impacted by addition of REE from the local fertilizer manufacturing industry.