The rare earth element chemistry of estuarine surface sediments in the Chesapeake Bay

Abstract

Concentrations of rare earth elements (REE) were measured in the fine fraction of shallow surface sediments from the lower Chesapeake Bay as studies suggest that coagulation of colloids dominates the removal of REE from the dissolved load in low salinity regions of estuaries. REE in sediments showed spatial heterogeneity, both laterally and longitudinally. We observed the influence of salinity on the spatial and temporal variability in total REE content and on Eu-anomalies. Lower total REE was found in sediments deposited along the Eastern Shore. The chemistry of the mid-Bay Islands was distinct with higher overall REE content and slightly more positive Ce-anomaly values. Positive Ce/Ce* in the mid-Bay are attributed to Coriolis acceleration restricting freshwater flow towards the west away from the Islands leading to oxidative removal of Ce from the pore waters to the sediments. The presence of positive Eu/Eu* in sediments deposited along the Eastern Shore is linked to salinity. We suggest that unique local conditions along the Eastern Shore, possibly including submarine groundwater discharge of nutrient-rich reducing waters, combined with organic matter decay and the predominance of oceanic water flow toward this region leads to the development of a strong localized salinity gradient. This study demonstrates the utility of REE sediment chemistry in identifying and resolving local and estuarine-wide geochemical processes.