LIMNO-CHEMICAL MYSTERY RESOLVED FOR CRAWFORD LAKE, ONTARIO, CANADA; GSSP CANDIDATE FOR THE ANTHROPOCENE EPOCH

Crawford Lake is a unique meromictic lake located near the edge of the Niagara Escarpment. The 2.4 Ha, 24 m-deep lake represents a sinkhole in the Silurian Lockport Group formed through karstic dissolution, and further eroded by glacial meltwater. The lake’s bathymetry and wind-protected shores impede complete seasonal turnover, thus maintaining density stratification and separation of the water mass at the chemocline. Chemical and isotopic analyses describe a dense oxygenated groundwater-fed monimolimnion water mass below a less dense mixolimnion. At a depth of 15 m, conductivity increases from ~600 µS/cm to 2850 µS/cm, alkalinity from ~300 to ~800 mg/L, calcium and magnesium from ~65 and ~30 mg/L to ~130, and ~40 mg/L respectively, with additional spikes in potassium and nutrients. The deep basin’s water chemistry resembles that of the surrounding groundwater, with comparably negative δ²H and δ¹⁸O values reflecting inflow of groundwater but lack of surface meteoric water circulation.

The complex hydrology facilitated the accumulation of annually laminated couplets, with light-coloured calcite precipitating in the mixolimnion during warmer temperatures, whereas the dark-coloured layers represent an influx of organics from biologic productivity. Previous research attributed preservation of these varves to a lack of bioturbation caused by bottom water anoxia. However, physio-chemical testing showed that Crawford Lake maintains oxic conditions throughout the water column and year. The supply of dissolved oxygen in the groundwater through the hydraulically conductive Gasport Formation was sufficient to allow aerobic respiration through most of the last millennium but macrobenthos was prevented from colonizing the highly alkaline monimolimnion. The undisturbed varves are a record of Indigenous and European settlements spanning the last millennium, while recently, gaining attention as a potential GSSP for the Anthropocene Epoch.