ISOTOPIC COVARIANCE AND PHASING IN AN ANNUAL-RESOLUTION 20TH CENTURY URBAN LAKE RECORD (LAKE MCCARRONS, MINNESOTA, USA)

Lakes in urban landscapes are subject to rapid changes in surface hydrology and in solute and sediment inputs. Physical stratification regime, planktonic community composition, and ionic chemistry can thus be altered over short time scales by land-use changes and even by remediation efforts. Such lakes, which may have relatively well-constrained historical records (e.g., in the midcontinental US, since European settlement circa 1860), are promising sites for improving our understanding of carbon and oxygen isotopic responses to internal and external forcing factors that in deeper time may be obscure. The sedimentary record of Lake McCarrons (Twin Cities metro area, Minnesota) shows positive covariance between C and O stable isotopes in authigenic carbonates (δ¹³C_cc and δ¹⁸O_cc) prior to ~1860 and a breakdown in that relationship under the major human impacts that followed. Trends in organic matter δ¹³C (δ¹³C_om) in the 20^th century closely follow those of other proxies that indicate eutrophication, but do not match the pattern in δ¹³C_cc , which appears partially driven by hydrology and partially by stratification. The contribution of hypolimnetic biogeochemical processes (e.g., methanogenesis) may be significant to the dissolved inorganic carbon (DIC) budget and this pool’s isotopic value (δ¹³C_DIC) in a lake that is currently oligomictic and whose bottom waters have been persistently anoxic (as demonstrated by annually laminated sediments) since at least the early 20^th century.