STABLE ISOTOPE PALEOLIMNOLOGY OF BARRY LAKE, SOUTHEASTERN ONTARIO, CANADA SINCE AD ~1300

We have examined multiple proxies from two gravity cores (each 41 cm) collected from Barry Lake, a small kettle lake (0.16 km²) located 32 km east of Peterborough, Ontario, Canada. We have measured the C and N isotopic compositions of organic matter (OM), and C and O isotopic compositions of authigenic calcite (marl) and aragonite from gastropod and clam shells. These data have been used to reconstruct climatic and environmental conditions over the last 1000 years. We also measured modern (2014) water and dissolved inorganic carbon (DIC) isotopic compositions, and physical and chemical properties of Barry Lake, as well as modern molluscs, to provide a modern baseline. We have used archival climate records from AD 1866 to 2011 to help calibrate the results. An age/depth model was established for the gravity cores using Pb-210 dating, a radiocarbon date and historical records of settlement.

Most proxies show significant variations during time periods typically associated with climatic events such as Medieval Warm Period (MWP) and Little Ice Age (LIA). We calculate a 1‰ increase (–7 to –6‰) in the O isotopic composition of lake water, as estimated from abundant authigenic calcite precipitated between AD ~1270 and 1350. We associate this change with warming and reduced precipitation during the MWP. The O isotopic results for the shelly fauna are coherent with those from the marl; further, the combined C isotopic data demonstrate depth-related and seasonal variations in oxygenation and DIC isotopic composition consistent with those of the modern system. A similar evaluation suggests a ~2‰ decrease (–6 to –8‰) in lake water O isotopic composition between AD ~1615 and 1850, based on authigenic calcite, which is also much less abundant. This change is consistent with the lower temperatures anticipated for the LIA. A sharp increase in magnetic susceptibility of the sediments marks European settlement at AD ~1830. A slow decrease in C/N and increasing C isotopic composition of bulk organic matter (–31 to –29‰) since AD ~1850 are consistent with increasing lake productivity following European settlement. Lake water O isotopic composition has remained constant (~–8‰, based on authigenic calcite) over the last 150 years, suggesting a period of stable climate, which correlates well with archival temperature and precipitation records.