EARLY TO MID HOLOCENE CLIMATIC AND ENVIRONMENTAL CHANGE IN SOUTHWESTERN ONTARIO, CANADA INFERRED FROM STABLE ISOTOPES OF ORGANIC MATTER, MARL AND COEXISTING LACUSTRINE SHELLY FAUNA

A better understanding of the climatic and environmental conditions in southwestern Ontario from ~10,500 to 5,400 cal BP is being obtained using proxies from multiple 5.25m long sediment cores from a marl-peat accumulation located near St. Agatha, Ontario. We are using the oxygen and carbon isotopic compositions of molluscs and marl, and the carbon and nitrogen contents and isotopic compositions of terrestrial organic matter (OM) (needles) from the marl to reconstruct paleoclimate for this locality. The OM has C/N of 24 to 35 and C isotopic compositions ranging from –27.3 to –24.3‰ (VPDB). Four genera of fresh water molluscs (Pisidium sp., Valvata sp., Fossaria sp. and Gyraulus sp.) have aragonite oxygen isotopic compositions of +20.4 to +23.6‰ (VSMOW) and carbon isotopic compositions of –9.4 to –6.3‰ (VPDB). The marl (calcite) ranges in oxygen and carbon isotopic compositions from +18.6 to +21.8‰ and –4.5 to –2.4‰, respectively. The C and O isotopic compositions are strongly correlated.

The C/N and C isotopic variations of the OM reflect its C3 terrestrial origin and changing water stress over the depositional period of the marl. The O isotopic compositions of the marl and coeval shelly fauna provide internally coherent records of water isotopic composition, and hence hydrology and temperature over the same time period. The marl oxygen and carbon isotopic compositions demonstrate a cyclical variation of ~±1‰ and ±1.5‰, respectively, over much of the sequence. A larger increase in both oxygen and carbon isotopic compositions occurs near the upper contact with peat, as infilling of the lake progressed. Large differences in C isotopic composition between the marl and molluscs indicate different reservoirs of dissolved inorganic carbon (DIC) during their formation. Aquatic photosynthetic activity likely caused carbon-13 enrichment of shallow DIC during the period of marl precipitation. The molluscs are variably affected by C-isotope vital effects during shell formation. Contributions of re-mineralized carbon from decay of organic matter have also affected the carbon isotopic composition of shelly fauna, especially benthic species. Overall, the carbonate C isotopic compositions represent a dynamic response to variations in DIC sources and lacustrine productivity over the time period represented by marl formation.