Geochemistry of Rare Earth Elements in Benthic Layer Particulate Matter and Sediments of Lake Superior

Abstract

Samples of suspended particulate matter from the nepheloid layer and the benthic sediment layer (fluff zone), as well as core samples of the underlying sediment column, were collected from two locations in Lake Superior during manned submersible dives. All samples were analyzed for eight rare earth elements (REE's), biogenic silica, and organic carbon. The REE patterns indicate lithogenous material is a dominant component in all samples but that significant amounts of REE's derived from the water column are also present. Biogenous sources (diatom shells), on the other hand, contribute negligibly to the REE geochemistry of these samples. Statistical analyses of the geochemical data suggest that the mechanism by which REE's are removed from the water column involve scavenging by particulate phases of Fe. The Precambrian banded iron formations around Lake Superior represent an abundant source of dissolved and particulate Fe to the lake. Absolute concentrations of both REE's and Fe are greater in the fluff layer than in the nepheloid layer; however, the nepheloid layer contains 24–57% more REE's on a per unit Fe basis. This difference may be due to longer exposure of nepheloid particulate matter to lake waters than fluff, diminished scavenging efficiency at higher Fe levels, and/or to the fact that particulate matter in the fluff layer is derived from surficial sediments which have lost some REE's during early diagenesis. A comparison of the Ce-anomalies of the suspended layers with the sediments indicates the sediments have undergone some degree of early diagenesis.