HYDROLOGIC CHANGES IN THE MONIMOLIMNION OF CRAWFORD LAKE (ONTARIO, CANADA) LINKED WITH NEARBY INDIGENOUS SETTLEMENT

The presence of okenone, a pigment produced by obligately anaerobic purple sulfur bacteria, in the sediment record of Crawford Lake suggests anoxic conditions in the lake’s monimolimnion prior to the early 16^th century. The disappearance of okenone from the sediment record coincides with the end of the Indigenous Agricultural Zone (IAZ), ca. 1300–1500 CE, when Ontario Woodland Tradition (OWT) Indigenous people inhabited longhouses on thick glacial drift, practicing Three Sisters agriculture in the northwestern part of Crawford Lake’s small catchment (0.9 km²). A sharp drop in okenone ca. 1100 CE correlates with increased Mn, indicating changes in the redox conditions of the monimolimnion. At the same time, elevated relative abundances of non-arboreal pollen and microcharcoal, along with elevated counts of terrigenous elements (e.g., Si, K, Ti, Fe), record nearby anthropogenic impact, consistent with archaeological evidence of the first OWT settlement above the Niagara Escarpment beginning at ca. 1030 CE. Shifting redox conditions may be attributed to land disturbance, including the controlled use of fire to clear forests for agriculture. This land clearing may have led to increased flooding, flushing sediment from the karst aquifers that transect Crawford Lake, increasing the flow of highly oxygenated groundwater to the monimolimnion. Comparison of surface soil samples from the catchment with an anomalous terrigenous unit capping the IAZ (high in Rb, Fe, and Zr) reveals close similarity with floodplain sediments of Crawford Lake’s inlet stream, suggesting the disappearance of okenone and onset of complete monimolimnetic oxygenation may be related to a particularly severe flood in the early 16^th century. This flood, together with the onset of cold, dry, Little Ice Age conditions, may have led to the abandonment of the Crawford Lake site.