Nearshore regions of Lake Superior: Multi-element signatures of mining discharges and a test of Pb-210 deposition under conditions of variable sediment mass flux

Document Type

Article

Publication Date

1-1-1999

Abstract

Around the turn-of-the century, mining activities greatly increased sediment accumulation and metal fluxes in nearshore regions of Lake Superior. In the low-energy environment of Portage Lake, within the Keweenaw Waterway estuary, sediment accumulation increased 33X, whereas elemental Cu flux increased 312X. One difficulty in establishing the dispersion of mining discharges is that stamp sands were derived from local ore deposits, hence few elements are 'unique' to the source materials. One approach is to search for multi-elemental 'signatures', in concentration and flux profiles. For example, several rare earth elements of the lanthanide series are characteristic of source materials and have the potential to identify stamp sand material across Lake Superior. Although conditions of variable mass loading from multiple sources can produce complicating dilution effects in concentration profiles, multivariate techniques are capable of deciphering original source signals. Here non-destructive neutron activation analysis was utilized to construct elemental flux and concentration profiles, then multivariate techniques (Factor Analysis, End-member Analysis) were used to illustrate how partial mass flux signatures can be assigned to two different types of ore lodes (conglomerate, amygdaloid) and to background (erosional) sedimentation. Temporal patterns were verified through archived company discharge records. Also exploited were the varve-like deposition of slime clays to independently check 210Pb determinations under conditions of Variable sediment mass flux and to demonstrate constant excess 210Pb delivery to sediments in the presence of massive slime clay loading. The results suggest assumptions of 210Pb dating may apply under conditions where sediment accumulation is highly variable.

Publication Title

Journal of Great Lakes Research

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