MICRO-XRF GEOCHEMICAL AND MICROPALEONTOLOGICAL INVESTIGATION OF HYDROLOGIC CHANGES IN THE KARSTIC AQUIFERS OF CRAWFORD LAKE (ONTARIO, CANADA) AND POSSIBLE LINKAGES WITH INDIGENOUS SETTLEMENT ABANDONMENT

Crawford Lake is a small (2 ha), deep, meromictic lake in Milton, Ontario, Canada whose varved sediments have been proposed to mark the base of the Anthropocene epoch. Unlike most meromictic lakes, Crawford Lake’s bottom waters do not experience anoxia due to the flow of highly oxygenated groundwater through the karstic bedrock aquifers that transect the basin. The sudden oxygenation of the lake’s monimolimnion is marked in cores by the last occurrence of the fossil pigment okenone, which is produced by obligately anaerobic purple sulfur bacteria, during the late 15^th c. AD, coinciding with abandonment of an Indigenous settlement that had existed near the lake for > 200 years. It is possible that the event that caused this major hydrological change in Crawford Lake also led to the site abandonment. Micro-XRF geochemical analysis of core CL-4 shows an anomalous spike in terrigenous elements (Fe, Rb, and Zr) within a distinctive, 0.5cm thick reddish clay unit at the top of the Indigenous Agricultural Zone (IAZ), which is defined by the appearance of cultigen pollen and spores of their pathogens. The elemental composition of the anomalous clay unit differs from the signature of increased terrigenous elements that occurs during the rest of the IAZ, which is anthropogenically sourced from the sandy Wentworth Till plain on which the Indigenous settlement was built. This suggests a different source for the anomalous clay unit, possibly an intense storm/flood event, caused in part by the onset of cooler, wetter conditions during the Little Ice Age. This event may have caused a surge of water through the karstic aquifers strong enough to mobilize relatively impermeable clay sediments that had been obstructing groundwater flow to the lake, depositing the anomalous clay unit and enhancing groundwater recharge, leading to the oxygenation of the monimolimnion. This hypothesis will be further tested by micro-XRF and micropaleontologic analysis of lake cores and surficial till deposits sampled within the catchment area to determine sediment sources and the possible origin of the anomalous clay unit.