Density-induced settling of tephra through organic lake sediments

Tephras are often used in paleolimnology and other stratigraphic applications as a chronostratigraphic marker. Where analytical errors in radiocarbon or other dating methods make precise comparison between sites difficult, tephras provide an absolute stratigraphic reference that can be used to assess the relative ages of events across a region. Applications of tephrochronology typically make the assumption that a tephra is deposited at what was the top of the stratigraphic sequence at the time of deposition, and that the contact between the tephra and underlying sediments is an isochron. This paper presents evidence from two lakes in western Canada which suggest that tephras may be very mobile within the sedimentary column, particularly in low-density organic lake sediments. Analysis of sediment cores from Copper Lake, Alberta, suggest that the 6730 BP Mazama tephra moved down-core by the equivalent of more than 3000 years. A vertical exposure of the Holocene sediments from Doal Lake, Yukon Territory, reveals that the 1200 BP White River (WR) tephra, found throughout the southern Yukon, moved down-sequence through organic lake sediments, to a stratum dating to approximately 10,000 BP, creating conformable tephra beds at that level. This type of stratigraphic displacement has important consequences for the use of tephrochronology in lakes and other soft-sediment environments and underscores the importance of properly identifying tephras and critically assessing their stratigraphic context within a lake core.