The development of a Holocene cryptotephra framework in northwestern North America

Abstract

This thesis contributes to the development of a cryptotephra framework in northwestern North America, providing the initial data for the construction of a regional cryptotephra framework where key cryptotephra layers (those which are widely distributed, easily correlated, and have well constrained chronological control) are identified for use as chronostratigraphic tools. Three aims are outlined: addressing geographical gaps for cryptotephra records in northwestern Canada; defining regionally significant cryptotephra beds that can be used to improve local and regional scale chronologies; and demonstrating the potential for cryptotephra as isochrons in the area. Previous knowledge of cryptotephra in the area and any tephra with potential to be transported and preserved distally is summarised, including reference geochemical major element glass datasets and Bayesian age modelling of quality-assessed bounding dates. Previously published data is reviewed and supplemented using new data produced at the University of Alberta. This represents an important summary and evaluation of tephra data for nineteen beds from Alaska and the Yukon. A new cryptotephra record covering ~10,400 cal yr BP is presented from a soligenic peatland in north-central Yukon. Seventy-five tephra peaks are identified during the Holocene, and detailed discussion, including modelled ages and geochemical major element glass datasets, is given for fifty-two samples representing sixty-four geochemical populations. A total of thirty-four tephra identified have a combination of significant glass concentrations, distinctive geochemical data, useful stratigraphic positions/timings, and potential correlation to other known records over a wide area. These beds and are important contributions to the regional tephrostratigraphy. Chronological data for three methods commonly used to date peatland archives (14C, 210Pb and cryptotephra) are compared at six sites in northern and central Alberta. Consistent offsets between 14C and 210Pb dates are identified at five of these sites before the 1960s. Historical cryptotephra, reported here for the first time in Alberta, are used as an independent check of these methods. They also contribute complementary data to ecological interpretations of changes in peat accumulation rate (e.g. local fires, permafrost development, water availability) that are included as boundaries in the final age-depth models. Bayesian modelling of the resulting data using the P_Sequence function of OxCal v4.2 produces improved age models at four of the sites. All data are synthesised in Chapter 5 and a comparative cryptotephra stratigraphy is produced using the records published across North America. Appendices are included here in the text; supplementary Tables and Figures are deposited as digital files through Dataverse. Supplementary data for this thesis can be found here: https://borealisdata.ca/dataverse/cryptotephra