###### Abstract  
The focus of this study was the Baekdusan-Kita 1 (B-KY1) eruption during the late Middle Pleistocene. We identified B-KY1 tephra between the Toya and Aso-1 tephras in the ODP 794A core from the Japan Basin of the East Sea/Japan Sea. The stratigraphic position of the B-KY1 tephra correlated exactly with the first B-KY1 to be identified, in the 20 EEZ-1 core from the Kita-Yamato Trough. However, B-KY1 tephra in the ODP 794A core showed a wide range of geochemical compositions. The textural characteristics of B-KY1 tephra in the ODP 794A core was characterized by higher content of fine-grained bubble-wall shards than that of the B-KY1 tephra in the 20 EEZ-1 core. The difference in B-KY1 tephra between the two coring sites may reflect shifts in wind direction during volcanic eruptions. We refined the eruption age of the B-KY1 tephra by examining distinct sedimentary facies related to the start of the penultimate deglaciation of this region at ca. 135 ka. The findings of this study suggest that the compositional diversity of B-KY1 tephra may have been influenced by subsequent mixing of comendite and comenditic trachyte magma with injected pantelleritic magma during the late Middle Pleistocene Baekdusan volcanic eruption. B-KY1 tephra; late Middle Pleistocene; injected pantelleritic magma 20202023 June 20202028 June 2020223 June 20202028 June 2020223 June 202023 June 20202028 June 2020223 June  
collected from the East Sea using piston corers with a barrel length of \(<\)7 m. A Middle Pleistocene B-Og tephra layer [16] has been identified only in the ODP 794A long-core sediments collected at about 14 m below the seafloor (Figure 1). B-V (ca. 24.5 ka) and B-J (ca. 51.5 ka) tephra layers constructed by millennial-scale interstadial paleoclimatic fluctuation records are distinguished by their dark/light sediment patterns [18; 19; 20] (Figure 2). The distribution pattern of B-J tephra suggests that seasonal westerly winds influenced atmospheric transport of the eruption plume through the stratosphere during the eruption [18; 21]. The trajectory of the B-J eruption plume extended further southward than that of the B-Tm eruption (Figure 1). B-Sado (ca. 67.6 ka) and B-Ym (ca. 85.8 ka) tephras were identified by instrumental neutron activation analysis (INAA) of marine sediments [20; 22], revealing an invisible tephra layer of micro-sized cryptotephra (Figure 2). B-KY1 (ca. 130 ka) and B-KY2 (ca. 196 ka) tephras have been reported in marine  
Figure 1: Map of coring sites and source volcanoes in the East Sea/Japan Sea. Solid black line: trajectory of the B-Tm tephra (Adapted from [1]), Dotted red line: trajectory of the B-J tephra (Adapted from [18]), Solid black triangles: Source volcanoes in this study, Solid red circles: Coring sites in this study. Blue squares: Sampling sites in Japanese island. T: Tomakomai, OG: Oga Peninsula, LI: Lake Ichi-no-Megata, LS: Lake Suigetsu. sediments from a flat hill in the Kita-Yamato Trough [23]; these represent the southernmost visible Baekdusan tephras identified (Figure 2). Many marine cores obtained from the Ulleung Basin and South Korea Plateau in central to southern East Sea contain no visible Baekdusan tephra layers [23; 24; 25; 26]. Recently, analysis of the zircon age (U-Th) distribution of terrestrial pyroclastic deposits from the Millennium Eruption revealed multiple age components at 11.3 \(\pm\) 1.3 ka, ca. 110 ka, and 230 ka [27]. Other zircons (U-Th) from 0.3 ka pyroclastic deposits have been dated to 2.6 \(\pm\) 1.8 ka, ca. 130 ka, and >230 ka [11; 28]. Based on the zircon isochron ages of terrestrial pyroclastic deposits on Baekdusan, episodic magma generation may have occurred at 110-130 ka and ca. 230 ka during the Pleistocene. The main objective of this study was to report the identification of a newly discovered Middle Pleistocene B-KY1 tephra from ODP 794A core about 900 km from Baekdusan, which is the most northerly tephra identified to date. Other cores cannot be used to identify Middle Pleistocene Baekdusan tephras due to their short core length (<7 m).