CARBONATE STRATIGRAPHY OF LAKE SUPERIOR GLACIAL VARVES

Total inorganic carbon analyses, X-ray diffraction measurements, Echosounder surveys, and varve chronologies are used to help correlate the late glacial stratigraphy between multiple sediment cores from Lake Superior. The results suggest that carbonate variations within the gray varves correlate across the basin. Following retreat of the Superior lobe from the Marquette ice margin, a sequence of proglacial red varves was deposited. The deposition of red varves was superceded by gray varves when the ice sheet retreated to the northern lake margin, allowing meltwater derived from both the ice front and Lake Agassiz to overflow into the basin. We believe that the transition from red to gray varves, which occurs everywhere but the far northeastern basin, is temporally correlative. Red varve carbonate content is characterized by both high values and high interannual variability. We attribute this to pronounced interannual variations in ice wasting, non-homogenous mixing within the proglacial lake, and flux dependent, post-depositional dissolution of carbonate. The transition to gray varves is characterized by marked decreases in sedimentation rates and total carbonate. Carbonate content increases up section within the gray varves, reaching two subtle peaks, estimated to have occurred around 9100 BP and 8300 ¹⁴C BP. Interannual fluctuations in carbonate within the gray varves are less pronounced, suggesting that the system is well mixed. Annual correlations between gray varves throughout the basin may be possible. Following cessation of gray varve deposition, low sedimentation rates prevent the accumulation of carbonate in Lake Superior.