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

Around the turn-of-the century, mining activities greatly increased sedimen t accumulation and metal fluxes in nearshore regions of Lake Superior. In t he low-energy environment of Portage Lake, Within the Keweenaw Waterway est uary, Sediment accumulation increased 33X, whereas elemental CLI flux incre ased 312X. One difficulty in establishing the dispersion of mining discharg es is that stamp sands were derived from local ore deposits, hence few elem ents are "unique" to the source materials. One approach is to search for mu lti-elemental "signatures " in concentration and flux profiles, For example , several rare earth elements of the lanthanide series are characteristic o f source materials and have the potential to identify stamp sand material a cross Lake Superior. Although conditions of variable mass loading from mult iple sources can produce complicating dilution effects inconcentration prof iles, multivariate techniques are capable of deciphering original source si gnals. Here non-destructive neutron activation analysis was utilized to con struct elemental flux and concentration profiles, then multivariate techniq ues (Factor Analysis, End-member Analysis); were used to illustrate how par tial mass flux signatures can be assigned to two different types of ore lod es (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 independen tly check Pb-210 determinations under conditions of variable sediment mass flux and to demonstrate constant excess Pb-210 delivery to sediments in the presence of massive slime clay lending. The results suggest assumptions of Pb-210 dating may apply under conditions where sediment accumulation is hi ghly variable.