Extracting a primary Holocene crytoptephra record from Pupuke maar sediments, Auckland, New Zealand

Abstract

Post-depositional reworking of volcanic glass shards is a common problem in tephrochronological studies. Distinguishing between primary and reworked tephra is challenging, particularly in cryptotephra investigations that depend on identifying peaks of glass shard concentrations in sediment sequences rather than discrete macroscopic tephra layers. Here, we investigated macro- and cryptotephra in sediments of Lake Pupuke, New Zealand, to identify primary from reworked tephra and thus refine the record of tephra-fall events in the region over the last ca. 9 cal ka. Geochemical fingerprinting, stratigraphic correlation, 14 C ages and X-ray fluorescence and magnetic susceptibility core scanning were employed to filter out reworked tephra and thus establish a new Holocene tephrochronological framework for the region. Five primary macrotephra layers and 18 primary cryptotephra were identified. Confidence levels are attached to the newly identified cryptotephra, and geochemical characterization of well-dated rhyolite tephra was used to establish an associated age model. Our results suggest that Holocene tephra fall from both local and distal volcanoes affected Auckland more frequently than previously documented, with a possible average frequency of at least once every 424 years. Furthermore, the study demonstrates the importance of identifying the extent of post-depositional reworking to establish the veracity of the cryptotephra record.