Crawford Lake Consumers: Water Column and Palynological Studies

Despite their important role in lake ecosystems, the fossil record of consumers has been underutilized compared to the remains of algae and plants in paleoenvironmental studies. Cladocerans, chironomids, and testate amoebae were found in palynological preparations of sediments throughout Crawford Lake (a unique meromictic lake in Ontario, Canada), but rotifer lorica and cysts of aloricate ciliates were only preserved in seasonally laminated sediments in the monimolimnion of this lake, demonstrating the exceptional preservation potential in this portion of the lake water column. Relatively diverse assemblages of consumer palynomorphs were associated with anthropogenic impact on this lake, and the annual chronological resolution afforded by varves allowed these to be related to historic events in the small watershed, the most notable being the operation of a lumber mill on the south shore of the lake, and to archeological and pollen evidence of several phases of agricultural settlement between the 13th and 15th centuries. Lower diversity of consumer palynomorphs between the Iroquoian and Euro-Canadian settlement phases (i.e., late 15th through early 19th centuries) mainly reflects the sharp decline in most rotifer taxa and the cladoceran Bosmina longirostris, but the persistence of the rotifers Keratella hiemalis and Kellicottia longispina is evidence that the lake ecosystem did not return to pre-human impact conditions after abandonment of the Iroquoian settlement. Understanding how the trophic level of consumers responded to natural and anthropogenic stressors relied heavily on rarely preserved rotifer lorica, but the observation that the cladoceran B. longirostris tended to thrive relative to the typically more common Daphnia at times of cultural eutrophication may have broader application in palynological studies of lakes. Contrary to long-standing assumption, the exceptional preservation of organic-walled microfossils in undisturbed seasonal laminae in the deep basin of Crawford Lake cannot be explained by anoxia. Observations of seasonal migration of zooplankton to and from the mixolimnion in conjunction with instrumental measurements of dissolved oxygen, temperature, and conductivity in the water column almost monthly from October 2019 through September 2020 demonstrated that this meromictic lake is uncharacteristically well-oxygenated below the chemocline. Instead, exceptional preservation is attributed to the lack of bioturbation and the suppression of bacterial decomposition in the cold, nearly brackish, highly alkaline bottom waters devoid of benthos larger than ostracods able to migrate into the deep basin via interstitial waters. The annual resolution possible in sediments deposited in a typically well-oxygenated setting is an attractive feature in the search for a potential GSSP to define the Anthropocene Epoch using plutonium from fallout of thermonuclear testing as a primary marker

The varved succession of Crawford Lake, Milton, Ontario, Canada as a candidate Global boundary Stratotype Section and Point for the Anthropocene series

An annually laminated succession in Crawford Lake, Ontario, Canada is proposed as the Global boundary Stratotype Section and Point (GSSP) for the Anthropocene as a series/epoch with a base dated at 1950 CE. Varve couplets of organic matter capped by calcite precipitated each summer in alkaline surface waters reflect environmental change at global to local scales. Spheroidal carbonaceous particles and nitrogen isotopes record an increase in fossil fuel combustion in the early 1950s, coinciding with fallout from nuclear and thermonuclear testing—239+240Pu and 14C:12C, the latter more than compensating for the effects of old carbon in this dolomitic basin. Rapid industrial expansion in the North American Great Lakes region led to enhanced leaching of terrigenous elements by acid precipitation during the Great Acceleration, and calcite precipitation was reduced, producing thin calcite laminae around the GSSP that is marked by a sharp decline in elm pollen (Dutch Elm disease). The lack of bioturbation in well-oxygenated bottom waters, supported by the absence of fossil pigments from obligately anaerobic purple sulfur bacteria, is attributed to elevated salinities and high alkalinity below the chemocline. This aerobic depositional environment, unusual in a meromictic lake, inhibits the mobilization of 239Pu, the proposed primary stratigraphic guide for the Anthropocene

The varved succession of Crawford Lake, Milton, Ontario, Canada as a candidate Global Boundary Stratotype Section and point for the Anthropocene series

An annually laminated succession in Crawford Lake, Ontario, Canada is proposed for the Global boundary Stratotype Section and Point (GSSP) to define the Anthropocene as a series/epoch with a base dated at 1950 CE. Varve couplets of organic matter capped by calcite precipitated each summer in alkaline surface waters reflect environmental change at global to local scales. Spheroidal carbonaceous particles and nitrogen isotopes record an increase in fossil fuel combustion in the early 1950s, coinciding with early fallout from nuclear and thermonuclear testing - 239+240Pu and 14C: 12C, the latter more than compensating for the effects of old carbon in this dolomitic basin. Rapid industrial expansion in the North American Great Lakes region led to enhanced leaching of terrigenous elements by acid precipitation during the Great Acceleration, and calcite precipitation was reduced, producing thin calcite laminae around the GSSP that is marked by a sharp decline in elm pollen (Dutch Elm disease). The lack of bioturbation in well-oxygenated bottom waters, supported by the absence of fossil pigments from obligately anaerobic purple sulfur bacteria, is attributed to elevated salinities and high alkalinity below the chemocline. This aerobic depositional environment, highly unusual in a meromictic lake, inhibits the mobilization of Pu, the proposed primary stratigraphic guide for the Anthropocene. </p