Late Quaternary paleoecology of the southern Niagara Escarpment, Ontario, Canada, a multiple proxy investigation of vegetation and climate history

Multiple proxy data from Crawford Lake and Twiss Marl Pond revealed a complete vegetation and climate history since deglaciation at $\sim$13,000 $\sp{14}$C BP. Fossil pollen and plant-macrofossils indicate that Dryas integrifolia - Alnus cf. crispa sparse tundra or periglacial desert was succeeded by Salix herbacea - Juniperus-Cyperaceae dense tundra. Afforestation started at $\sim$12,000 BP, dominated by Picea woodland until 10,000 BP. Holocene vegetation changed from Pinus-dominated forest at 10,000-7500 BP, through mixed Tsuga-hardwood forest at 7500-4800 BP, then to a Fagus-Acer hardwood forest for most of the last 4800 years. Tsuga shows non-recovery from its 4800 BP decline and a second more destructive decline at 2700 BP. Plant macrofossils and conifer stomata refined the postglacial vegetation reconstruction, especially Thuja and Juniperus history. The Younger Dryas cooling event (YD) at 10,920-10,000 $\sp{14}$C BP was indicated by a 1-3$\perthous$ negative excursion in $\delta\sp{18}$O, an increase in minerogenic matter and high concentrations of erosion-derived elements (Al, Na, K, Ti and V) from four cores at two sites. The Preboreal Oscillation (PB) at 9650 BP was indicated by Picea recurrence and a decrease (0.4$\perthous$) of $\delta\sp{18}$O. A probable pre-YD cooling event shortly before 11,000 BP was indicated by a 0.8$\perthous$ decrease of $\delta\sp{18}$O. The sequence, magnitude and timing that matched in detail with records from the Atlantic Seaboard indicate that these oscillations were an expression of global climate changes rather that local meltwater-induced cooling. Multiple data documented differential responses of natural systems. The $\delta\sp{18}$O was the first among the proxies to show the Bolling-Allerod (BOA) warmth, as early as $\sim$12,500 BP, indicating rapid response of $\delta\sp{18}$O in precipitation to climate. The Picea woodland established after peaks of $\delta\sp{18}$O and carbonate, suggesting a large response of upland vegetation. Warmth-loving aquatics also indicate BOA warmth. Increased openings of forest and accelerated soil erosion were in response to YD cooling, but there were no forest transformations, due to insensitive Picea-dominated non-ecotonal vegetation. However, the Picea-Pinus ecotonal vegetation responded to the PB event. Sediment hiatuses, detritus layers and inwashed moss layers in five cores at Crawford Lake show the lake level was low at $\sim$4800-2000 BP, caused by decreased effective moisture. The negative shift of 2.4$\perthous$ in $\delta\sp{18}$O between $\sim$5000 and 2000 BP indicates a change in $\delta\sp{18}$O of source meteoric water. In the late mid-Holocene, the isotopically-heavy moisture from the Gulf of Mexico, the major moisture source for the study area, might have been reduced, probably due to more frequent blockage by the eastward extension of dry North Pacific air with depleted $\sp{18}$O. This hypothesis implies that the $\delta\sp{18}$O of paleo-precipitation in the mid-Holocene reflected moisture-source history more strongly than paleo-temperature.