REE in suspended particulate matter and sediment of the Zuari estuary and adjacent shelf, western India: influence of mining and estuarine turbidity

Abstract

Concentrations of Al, Fe, Mn and rare earth elements (REE) were measured in 122 samples of suspended particulate matter (SPM) and 70 surficial sediments from the Zuari estuary and the adjacent shelf to understand their distribution, provenance and estuarine processes. Concentrations of SPM were low in the upper estuary, increased seaward with high values in the lower estuary and then decreased at stations in the bay in all seasons. The distributions of mean ΣREE, Al and Fe along transect imitate each other and resemble inverted bowl-shaped pattern, with high and identical values at the lower estuary. The mean ΣREE, Al and Fe of sediment along transect showed two peak high values, one in the upper estuary and another in the bay amid low values corresponding to the lower estuary. The variations in the mean ratio of ΣREE_SPM/ΣREE_SED along transect resembled that of mean SPM at each station. The ΣREE of sediments in shallow shelf were close to that of the bay and, decreased seaward with increasing depth. PAAS-normalized REE patterns of every SPM/sediment sample revealed MREE- and HREE-enrichment with positive Ce and Eu anomalies. Ce^*/Ce was inversely correlated with Eu^*/Eu and salinity and, directly correlated with Mn concentrations. The results indicate that the REE of SPM/sediment is dominated by Fe, Mn ore dust and, its distribution along transect is controlled by the estuarine turbidity maximum (ETM). The ETM and seasonal circulation in the estuary controlled mixing and advective transport of particulates to the shelf during monsoon and, into the estuary during dry season. This study indicates sediment contribution to the shelf from tropical, minor rivers that are controlled by hydrodynamic conditions in the estuaries and should not be underestimated.