Initial tephrochronology for Cook Islands sediments — evidence of far travelled New Zealand ash layers

Geochemically characterised tephra layers are widely used for synchronising and dating paleoenvironmental records. Advances in the detection of invisible tephra horizons have led to the ongoing development and integration of regional tephra frameworks. Although there are multiple volcanic sources that could potentially have supplied volcanic ash to the South Pacific region, paleoclimatic archives in this area currently lack tephra markers.

Here, we report the first discovery of a cryptotephra layer in the Cook Islands. Volcanic glass shards were collected by sieving and applying heavy liquid separation technique from a laminated gyttja sequence in Lake Teroto, Atiu Island. The major elements were obtained by electron microprobe analysis. Based on the geochemical data, the detected layer is attributed to the Okataina Volcanic Centre, located 3,000 km from the coring site. Radiocarbon dating below the layer narrows the potential source eruption to the Whakatāne event, which occurred 5,500 years BP (Smith et al., 2006). It is presumed that the studied tephra originates from the M-type batch of magma from the Makatiti-Tapahoro vents, which were the main source of Plinian tephra falls (Kobayashi et al., 2005).

Our findings indicate the most distal Holocene tephra from the Okataina Volcanic Centre and significantly extend the mapped dispersal of the Whakatāne eruption. The discovery of New Zealand-sourced cryptotephra in the Cook Islands also highlights the potential for further utilisation of volcanic ash in the South Pacific, contributing to the development of a regional Holocene tephrochronological lattice.