Peatlands as high-resolution sedimentary archives of Holocene tephrostratigraphy on Pico Island (Azores): preliminary results

Water-laden sedimentary archives, such as marine and lacustrine sequences, have revolutionised the reconstruction of eruptive histories, as they usually hold a richer and more continuous tephra record when compared to subaerial environments. Here, we explore peatlands as another sedimentary archive from which highly detailed tephrostratigraphies can be obtained. Peat sequences have the advantage of being logistically easier and cheaper to access than other water-laden sequences, and more readily amenable to radiocarbon dating than terrestrial sequences. Pico Island in the Azores Archipelago provides an ideal laboratory to test the potential of peatlands as high-resolution sedimentary archives. The island is characterised by numerous basaltic monogenetic cones, yet its tephrostratigraphy remains poorly constrained, as such eruptions typically generate small tephra dispersals, and the resulting deposits are difficult to date. Taking advantage of the ubiquitous peatlands found in the Pico central uplands (above ~600 m altitude), a coring campaign was carried out in July 2025. Eight peatlands were cored using a Russian corer and a UWITEC® piston corer installed on a platform raft. Peatland basins were surveyed using a DJI Mavic 2 drone to produce high-resolution georeferenced digital surface models. Recovered cores were opened and logged at the University of the Azores, where peat, lacustrine, and volcanic facies (tephra horizons) were identified. Loss on ignition (LOI) was determined throughout the sedimentary sequences, and their bases were radiocarbon dated. Here, we present the stratigraphic sequences of the four main peatlands: Caiado and Barreira cone craters, and Peixinho and Lavandeira inter-cone depressions. All four stratigraphic sequences contain numerous tephra horizons, ranging in thickness from less than 1 mm up to several tens of centimetres. The thickest tephra horizons are found at the sites located in the eastern sector (Peixinho and Caiado), whereas thinner horizons predominate at the western sites (Lavandeira and Barreira). Most tephra horizons correspond to primary fall deposits, with only a minor portion of reworked materials. Radiocarbon dating reveals maximum sequence ages of 8608-8514 cal yr BP (Caiado), 6558-6399 cal yr BP (Lavandeira), 5588-5474 cal yr BP (Barreira), and 2181-2046 cal yr BP (Peixinho). The bases of the Barreira and Caiado sequences consist of weathered subaerial scoria deposits, interpreted as pre-lacustrine substrate. Lower LOI values, typically found in the lower part of the sequences, suggest initial lacustrine conditions, whereas higher LOI values in the upper part of the records indicate the transition to peatland. Ongoing work will focus on systematic radiocarbon dating below primary tephra horizons and geochemical characterisation of volcanic glass shards, enabling a high-resolution temporal and spatial reconstruction of Pico’s Holocene eruptive history. This work was supported by Fundação para a Ciência e a Tecnologia (FCT) through project ExTRAP (https://doi.org/10.54499/2023.12382.PEX).