Constraining peatland environmental change: exploiting the emerging eastern North American crypto-tephrostratigraphic record

The relatively recent advances in tephrochronology have led to the investigation of non-visible (crypto) tephra horizons in sediment distal from volcanic origins. Such studies have been predominantly centered on western Europe; however the potential of North American cryptotephras is rapidly emerging. This not only facilitates the construction of more robust chronologies in areas thought to be outside the scope of this technique, but also the provision of isochrons enhances comparisons of environmental spatial synchronicity across sites and regions. Four tephrostratigraphies across a transect of peatlands in Maine, Nova Scotia and south-western Newfoundland have been constructed. A total of 18 horizons were detected over the last ca. 4000 years, the preliminary geochemical analysis of which suggest that all constrained eruptions originate from the Cascade Range and Alaska ca. 5000-6000 km to the west of the sites. These results complement the one existing record from eastern Newfoundland (Pyne-O'Donnell et al. 2012), facilitating the extension of the late Holocene crypto-tephrostratigraphic framework for the eastern seaboard of North America. Peatlands are considered to be ideal archives for preserving tephrostratigraphies since cryptotephra horizons are often present in discrete layers, thought to represent primary airfall. Such preservation in this setting is critiqued here, assisted by radiocarbon measurements. The tephra horizons are used as pinning-points between records to address the wider aim of the study: to examine the terrestrial manifestations of late Holocene climatic change across an eastern North American climatic gradient. Of particular interest are the temporal and spatial characteristics of changes in peatland accumulation and reconstructed water table depth during the most dominant late Holocene climatic perturbations: the Medieval Climatic Anomaly and the Little Ice Age. This time period is constrained by the most dominant eruption, White River Ash (AD ~843), which is present in all sites and the subsequent Mount St Helens eruption (AD ~1482) which is restricted to the more northerly records; suggesting the geographical limit of the tephra in this region. Changes in bulk density, organic matter content, carbon content, plant macrofossil and testate amoebae records investigate the relationship between peat accumulation and changes in the moisture balance. Results reveal differences in the sensitivity of the four peatlands to environmental change across the transect and to different climatic forcing throughout the last 2000 years. The identification of these differences is facilitated by the improved temporal precision; highlighting the role that crypo-tephrochronology plays in enhancing palaeoenvironmental reconstructions and understanding of the environmental history of eastern North America.