Detection of Late Pleistocene tephras and cryptotephras using major element chemistry of glass shards from Chikyu C9001C cores, NW Pacific Ocean

Abstract

We reinvestigated tephra and cryptotephra stratigraphy of the Late Pleistocene deep-sea C9001C cores collected during the D/V Chikyu shakedown cruise. We identified 26 glass shard concentration horizons (spikes) corresponding to four previously reported tephras (G1, G2, G3, and G4 from top to bottom). We newly found three visible tephras and 19 nonvisible cryptotephras (G0.0-0.9, G1.1-1.4, G2.1-2.5, and G3.1-3.3). We newly correlated spike G0.2 with the Towada-Hachinohe (To-H: MIS 1/2, 15 ka), G0.5 with the Towada-Biscuit 2 (To-BP2: MIS 2, 18 ka), G1.1 with the Komagatake-i (39 ka), G1.2 with the Kutcharo-Shoro (MIS 3, 40 ka), and G3.3 with the Sambe-Kisuki (MIS 5c, 100 ka) tephras. Spikes G2.2, G2.3, and G2.4 may correlate with the Towada tephra series (seven correlative candidates), spike G3.1 with an Ontake volcano tephra (MIS 5b-5c), and spike G3.2 with the Towada-Castera or Towada-Aosuji (both MIS 5b-5d) tephra. We classified spike G0.1 as reworked materials of the underlying spike G0.2 (To-H), and the combinations of spikes G0.3 and G0.4 and of G0.8 and G0.9 as the result of repeated reworking of the underlying spikes G0.5 (To-BP2) and G1 (To-Of), respectively. We also classified spikes G1.3 and G1.4 as reworked mixtures of Shikotsu-1 tephra with glass shards from other tephras. The upward decrease of the glass shard populations above spikes G3 and G4 is interpreted as indicating immediate reworking of the tephras after initial emplacement. Whether or not a marine tephra is preserved depends on depositional and post-depositional processes and not simply on the eruptive volume or the distance from source. Nevertheless, cryptotephras recognized by glass shard concentration horizons in marine cores can provide additional datum planes and contribute to marine tephrostratigraphy.