COMBINED LAKE SEDIMENT CARBON CONTENT AND BULK ORGANIC CARBON ISOTOPE MEASUREMENTS PROVIDE A MEANS TO DECIPHER A 1000-YEAR FLOOD AND DROUGHT HISTORY OF THE RED RIVER BASIN, MANITOBA, CANADA

In 1999 a 1.5 m core (~ 1000 year record) was retrieved from the south basin of Lake Winnipeg for the purpose of reconstructing Red River basin flood and drought frequencies. This was accomplished by measuring the carbon content and isotopic composition of the sediment bulk organics (d13Cbulk organic) at 1 cm core intervals. Increases in the Dissolved Inorganic Carbon concentration [DIC] of the south basin during flood episodes and elevated demands on [DIC] during dry/drought periods resulted in distinct sediment d13Cbulk organic signatures that have been linked to historical floods and droughts in the Red River basin. Flood related increases in [DIC] result in non-limiting conditions for phytoplankton. This translates into anomalously depleted sediment d13Cbulk organic signatures that correspond to the following historical floods 1997, 1950, 1882, 1861, 1852, and 1826. Pre-historical depleted d13Cbulk organic signatures suggest that there has been an increase in the occurrence major floods per century between 1000 and 1900 AD. Further, by accounting for the increased sedimentation rates during the 20th century portion of the core and diagenetic loss of organics during the lower core portion, it can be shown that Red River flood discharge rates were significantly higher during the earlier record. One flood, reconstructed to have occurred ~ 1170 may have produced a 15 to 20 % higher discharge than the largest historically known flood in 1826. Additionally, increased water residence times in the south basin during dry/drought periods amplify demand on [DIC] resulting in enriched sediment d13Cbulk organic values that correspond to historically dry/drought periods during the 1930s and early 1800s. Pre-historical enriched carbon isotope signatures suggest an increased occurrence of dry/drought periods in the Red River basin during the Little Ice Age.