A DETAILED STUDY OF SEDIMENT, PHOSPHORUS, AND MERCURY DEPOSITON IN A UNIQUE RIVERINE LAKE USING A NOVEL REPEAT CORING APPROACH

Lake Pepin is a natural riverine lake on the Upper Mississippi River, located below the river’s confluence with the Minnesota and St. Croix rivers in southern Minnesota. Upstream of the lake, the Upper Mississippi River carries a large load of suspended sediment and related environmental pollutants, severely impairing water quality. Lake Pepin is an effective trap for sediments and records the history of sediment and pollutant loads in the river dating back to before local Euro-American settlement (c. 1830). This unique lake provides one of the few places in a large agricultural riverine system where pre-Euro-American conditions can be compared to current conditions using dated sediment cores. Constructing a detailed and precise sediment chronology is critical to linking changes in land use and climate to water quality changes. Cores were collected in 1996 and dated using ²¹⁰Pb, ¹³⁷Cs and ¹⁴C. Ten sites were re-cored in 2008 and aligned to the previous cores using distinct and ubiquitous magnetic susceptibility features, allowing the new cores to be aligned with the older cores and synchronized with already-established chronologies. Varying degrees of compaction, due to higher accumulation rates in the upper end of the lake compared to lower portions, are accounted for in each core. The re-coring method also provides an independent comparison of accumulation rates that does not rely on radioisotope dating methods. Current suspended sediment loading calculated from the 2008 cores is remarkably consistent (<1%) with measured loads (1996-2008) from a gauging station just upstream of the lake. This agreement between measured loads and sediment core records confirms the accuracy of our chronology and whole-basin calculations from the ten cores in Lake Pepin. Recent sediment accumulation rates have decreased 12% from the previous decade, but remain almost an order of magnitude greater than pre-settlement rates. This similarity in magnitude of 1996 rates with 2008 rates also helps to validate the credibility of results for the first coring effort. TP loads also showed a slight decrease (8%) in the most recent decade, while Hg had the most significant drop in whole-basin accumulation rate (23%).