Origins and environmental impacts of distal rhyolitic tephra layers on Chatham Island, New Zealand

Abstract

Chatham Island, ~700 km east of mainland New Zealand, preserves several macroscopic tephra beds, sourced from the central Taupō Volcanic Zone (TVZ) some 850 km away. These layers, often enclosed within peat, provide insights into the eruptive characteristics, timing and paleoenvironmental impacts of large explosive rhyolitic eruptions in a distal setting. We report the nature, origins and impacts of these tephras by documenting bulk and centimetric-scale stratigraphic changes in glass shard geochemistry and millimetric-scale changes in pollen assemblages in the enclosing peats. Three layers are investigated: the ~25.5 ka Kawakawa–Ōruanui Tephra (KOT), the ~345 ka Rangitawa Tephra and one newly discovered tephra. The KOT shows no stratigraphic variability in glass composition throughout ~15 cm thickness. The Rangitawa Tephra contains three glass populations that were deposited sequentially throughout the eruption and further link this tephra to the Whakamaru supereruption. The new tephra has two glass populations and is matched to the ~322 ka Matahina caldera-forming eruption. High-resolution sampling of peat bracketing the KOT and Rangitawa Tephra reveals marked changes in local vegetation following the deposition of both tephras. The nature of the changes varies between events and can be explained by differing paleoenvironmental settings, climate and/or tephra thickness.