VARIABILITY OF SEDIMENT MASS ACCUMULATION RATES IN LAKE SUPERIOR

Multi-cores were recovered during the summer of 2003 from three widely separated sites – one near the Duluth, MN area (MPCA-1), one near Isle Royale (MPCA-2), and one from a deep trough near Grand Portage, MN (MPCA-3). Three sub-cores at each site were extruded at 3 mm intervals, combined at common depths, and analyzed for 137Cs, 210Pb, biogenic silica (BSi), total organic carbon (TOC), and total organic nitrogen.

Core MPCA-1 exhibits variable, but high siliciclastic, biogenic silica, and organic carbon sediment mass accumulation rates (MAR's) with depth due to its proximity to the highly erosional Wisconsin shoreline, as well as to the Nemadji and St. Louis Rivers. Organic carbon MAR's parallel siliciclastic MAR's reflecting the relatively constant % organic carbon in sediments. C/N ratios around 20 indicate a large flux of terrestrial organic carbon sometime in the early 1960's, possibly due to increased runoff from the Nemadji and St. Louis Rivers. BSi mass accumulation rates tripled over the last 25 years despite the dilutional effects of a recent increase in siliciclastic mass accumulation.

Using the Constant Rate of Supply of 210Pb age model, over the last 50 to 60 years, siliciclastic MAR's in MPCA-2 have tripled, organic carbon MAR's quadrupled, and BSi MAR's have increased by a factor of 5. MPCA-3 shows similar trends with siliciclastic MAR's increasing by a factor of 1.5 and organic carbon and BSi MAR's doubling. The rapid decrease of organic carbon and BSi MAR's with depth in these cores is most likely not an artifact of eutrophication, but from the natural variability in the lake system and rapid chemical diagenesis of freshly deposited organic matter and diatom tests in the upper 3 cm of sediment. The increase in siliciclastic sediment accumulation is possibly due to settlement or logging of the drainage basin north of Lake Superior. MPCA-3 also shows an earlier peak of siliciclastic and BSi MAR's around 1900, possibly fingerprinting a period of intense logging in this area and the subsequent transportation of phytoliths to the lake.