Prehistorical record of cultural eutrophication from Crawford Lake, Canada

The sharp rise in Ambrosia (ragweed pollen) at ~23 cm is dated 1830-1840 CE by varve counting, supported by several AMS dates.

“The sharp rise in Ambrosia (ragweed pollen) at ~23 cm (Figure 10), dated 1830–1840 CE by varve counting, supported by several AMS dates (Ekdahl et al., 2004; McAndrews, 1994) is an additional constraint.”

The dominance of Asterionella formosa since 1980 CE defines a major diatom assemblage change that resembles environments during the Medieval Warm Period.

“The dominance of Asterionella formosa since 1980 CE (Figures 1 and 17) defines a major diatom assemblage change that resembles environments during the Medieval Warm Period (Ekdahl et al., 2004).”

Pollen analysis identified the rise in Ambrosia at 23 cm, dated 1830-1840 CE at Crawford Lake, which reflects colonial land disturbance.

“Pollen analysis conducted at Brock University identified the rise in Ambrosia (ragweed pollen) at 23 cm, dated 1830–1840 CE at Crawford Lake (Ekdahl et al., 2004), which reflects colonial land disturbance.”

Direct observation of freeze cores collected since the early 1970s and high-resolution AMS 14C chronology confirms that these represent annual couplets through the Canadian zone.

“Direct observation of freeze cores collected since the early 1970s and high-resolution AMS 14C chronology confirms that these represent annual couplets through the Canadian zone (Ekdahl et al., 2004), allowing varves from the past 150 years to ensure accurate and precise correlation between freeze cores.”

Increases in non-planktonic diatom taxa drive a major change in diatom assemblages at ~1942 and elevated concentrations of Asterionella formosa characterize diatom assemblages since ~1980 CE.

“Increases in non-planktonic diatom taxa drive a major change in diatom assemblages at ~1942 and elevated concentrations of Asterionella formosa characterize diatom assemblages since ~1980 CE (Ekdahl et al., 2004; Gushulak et al., 2022; Figures 1 and 17).”

Varves from the past 150 years ensure accurate and precise correlation between freeze cores.

“Direct observation of freeze cores collected since the early 1970s and high-resolution AMS 14C chronology confirms that these represent annual couplets through the Canadian zone (Ekdahl et al., 2004), allowing varves from the past 150 years to ensure accurate and precise correlation between freeze cores.”

The mean sedimentation rate through the Canadian Zone in gravity core CL-19 (1.48 mm/year) is lower than the estimate of nearly 2 mm/year in a freeze core studied by Ekdahl et al. (2004).

“This is lower than the mean accumulation rate (MAR) of 1.82 mm/year (or 5.49 years/cm) estimated between 1870 and 2000 CE in freeze core CRA19-2FT-B2 (see Supplemental Table S1 and Figure S12) or the estimate of nearly 2 mm/year in a freeze core studied by Ekdahl et al. (2004).”

These sedimentation rates are higher than in lower parts of the succession in the deep basin of Crawford Lake; Ekdahl et al. (2004) estimated accumulation rates <0.4 mm/year between 1486 and 1867 CE, and ~0.5 mm/year was estimated for the post-Indigenous Agriculture zone based on the pollen record of core CL-19.

“While these sedimentation rates are very low compared to most lakes, they are higher than in lower parts of the succession in the deep basin of Crawford Lake; Ekdahl et al. (2004) estimated accumulation rates <0.4 mm/year between 1486 and 1867 CE, and ~ 0.5 mm/year was estimated for the post-Indigenous Agriculture zone based on the pollen record of core CL-19 (Figure 11a).”