J. Great Lakes Res. 25(4):697–720 Internat. Assoc. Great Lakes Res., 1999 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 W. Charles Kerfoot *,1 and John A. Robbins 2 1 Lake Superior Ecosystem Research Center and Department of Biological Sciences Michigan Technological University Houghton, Michigan 49931 2 NOAA Great Lakes Environmental Research Laboratory 2205 Commonwealth Blvd. Ann Arbor, Michigan 48105 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 exam- ple, 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, multi- variate techniques are capable of deciphering original source signals. Here non-destructive neutron acti- vation analysis was utilized to construct elemental flux and concentration profiles, then multivariate tech- niques (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 (ero- sional) sedimentation. Temporal patterns were verified through archived company discharge records. Also exploited were the varve-like deposition of slime clays to independently check 210 Pb determinations under conditions of variable sediment mass flux and to demonstrate constant excess 210 Pb delivery to sed- iments in the presence of massive slime clay loading. The results suggest assumptions of 210 Pb dating may apply under conditions where sediment accumulation is highly variable. INDEX WORDS: Metals, Cu, Lake Superior, 210 Pb, 137 Cs, Principle Component Analysis. 697 INTRODUCTION Of all the Laurentian Great Lakes, Lake Superior contains the strongest development of a separate coastal regime, chemically and biologically distinct from cooler offshore waters. Erosion of metal-rich ore bodies around Lake Superior, either through high-energy shoreline scour or through stream dis- charge, results in sediments naturally enriched in Cu, Zn, and other elements relative to concentra- tions characteristic of the lower Great Lakes (Mudroch et al. 1988). In addition, turn-of-the-cen- tury anthropogenic inputs, largely along the shore- line, greatly accelerated metal transport and cycling in the basin (Nussman 1965, Kemp et al. 1978, Kerfoot et al. 1994). Earlier work on sediment cores from the Keweenaw Waterway region of Lake Superior revealed extensive varve-like slime clay layers associated with mining discharges. Realizing the potential for a high-resolution long-term record of mining impacts, a combination of laminae counts and radioisotope dating techniques was employed to contrast Cu concentration and flux profiles in Portage Lake (Kerfoot et al. 1994). Evidence was presented that copper-rich sediments extend east- ward from the peninsula, along the track of the Ke- weenaw Current, suggesting the strong current *Corresponding author. E-mail: wkerfoot@mtu.edu