Origin of Compositional Diversity of Marine Tephra during the Late Middle Pleistocene B-KY1 Baekdusan Volcanic Eruption

: 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 20EEZ-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 20EEZ-1 core. The di ﬀ erence 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.

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). A previous study reported the stratigraphic positions of Toya (ca. 106 ka) and Aso-1 (ca. 255 ka) tephras in the ODP 794A core [16,29]. We analyzed the major elements and backscattered electron images (BSEIs) of the Toya, B-KY1, and Aso-1 tephras, and sedimentary facies related to millennial scale paleoclimatic fluctuation or eustatic sea level change records [23,25,26,29]. The major element data provide a geochemical fingerprint for the tephras, and BSEIs allow their textural characteristics to be determined, reflecting the explosive eruption and long-range transportation distance from source volcanoes. Precise stratigraphic correlation with chronologic marine events was used to refine the age of B-KY1 tephra. The findings of this study will contribute to reconstruction of the eruption history of Baekdusan during the late Middle Pleistocene.
Studies using marine tephras in the East Sea have reconstructed the explosive volcanic history of Baekdusan ( [1,2,16,[18][19][20]23]; their distribution patterns were influenced by eruption intensity and the strength of westerly winds [18,21]. The preservation of tephra layers within marine sediments in is controlled by prevailing bottom currents and bioturbation by benthic organisms. Bottom water circulation and the oxygen level are higher at highstand than at lowstand in the East Sea [26]; thus, marine tephras deposited during lowstand ("dark laminated deposits") are better preserved [18,19].

Materials and Methods
Between the Toya (ca. 106 ka) and Aso-1 (ca. 255 ka) tephras, which have been identified stratigraphically, a visible B-KY1 tephra layer was identified in the ODP 794A core ( Figure 3). The major elements (SiO 2 , TiO 2 , Al 2 O 3 , FeO*, MnO, MgO, CaO, Na 2 O, and K 2 O) of the three tephras were identified using an electron microprobe analyzer (JXA8900; JEOL Co.) at the Geological Survey of Japan (GSJ), National Institute of Advanced Industrial Science and Technology (AIST). Subsamples of fine-grained hemipelagic particles were removed from tephra samples by submersion in an ultrasonic bath. Dried tephra grains were mounted on glass using epoxy resin, which solidified after 3 days at room temperature. Thin sections were polished using increasingly fine diamond paste (300 mesh to 1 µm). Electron microprobe analyses were performed at an acceleration voltage of 15 kV, beam current of 12 nA, and beam size of 10 µm. The peak counting time of all major elements was 10 s, and that for the background was 5 s. Prior to analysis of the three tephras, the microprobe was calibrated to major elements using GSJ standard samples and well-known working standards (sub-alkali AT and alkali U-Oki tephras). Geochemical elements of B-KY1 tephra were plotted against well-known Aso-4, Toya, Aso-1, B-J, B-Sado, B-Ym, B-KY1 (20EEZ-1 core), B-KY2, B-Og tephras using major elements normalized to 100 wt.%. BSEIs of tephras were used to measure grain size and textural characteristics based on the classification of glass shards [39,40]. The grain size and constituent composition of the tephra were used to characterize the explosiveness, transport distance, and type of eruption of the source volcanoes. Photographs and X-radiographs of the ODP 794A and 20EEZ-1 cores were interpreted by comparing the sedimentary facies to records of East Sea eustatic sea-level variation [41]. Distinct sedimentary facies related to the B-KY1 tephra were interpreted for stratigraphic correlation and age model construction. Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 16

Late Pleistocene Toya Tephra
Stratigraphically identified Toya tephra was found within thick-bedded, light-colored bioturbated mud (LBM) at an interval of 428.2-429.3 cm below the seafloor (cmbsf; 1H-4, 14.2-15.3 cm) in the ODP 794A core ( Figure 4B). Gray Toya tephra mainly consisted of pumice shards, with minor bubble-wall shards and phenocrysts ( Figure 5A; Table 1). The maximum grain sizes of the pumice shards, bubble-wall shards, and phenocrysts ranged from 90 to 120 μm, and the degree of vesicularity and stretching of the pumice shards ranged from low to moderate ( Figure 5A; Table 1).

Discussion
In this study, we characterized the B-KY1 tephra between the Toya (ca. 106 ka) and Aso-1 (ca. 255 ka) tephras in the ODP 794A core, which was collected from the Japan Basin of the East Sea/Japan Sea. Stratigraphically, the B-KY1 tephra is located between underlying DLM and overlying LBM ( Figure 4C), in exactly the same stratigraphic position as the B-KY1 tephra that was first identified in the 20EEZ-1 core from the Kita-Yamato Trough ( In the B-KY1 tephra, high SiO 2 content was correlated with less Al 2 O 3 , CaO, and Na 2 O, and slightly more FeO* ( Figure 7C). The elevated FeO* content remains open to interpretation. However, the geochemical characters of B-KY1 tephra in the ODP 794A core distinguished it from Aso-4 (88 ka; Figure 5D), Toya, and Aso-1 tephras, which erupted from Japanese volcanoes ( Figure 6). The B-KY1 tephras from the ODP 794A and 20EEZ-1 cores showed different textural characteristics (Table 1). To the south of Baekdusan, mixtures of coarse-grained pumice shards and bubble-wall shards were transported to the 20EEZ-1 coring site of the Kita-Yamato Trough. In the B-KY1 tephra layer of the ODP 794A core, relatively fine-grained bubble-wall shards were dominantly transported to the north of Baekdusan, perhaps reflecting changing wind patterns during the late Middle Pleistocene B-KY1 Baekdusan volcanic eruption. Shifts in wind direction can influence the distribution of tephra during its long-range transportation by explosive volcanic eruption [44]. Density separation due to greater distance from Baekdusan may also have contributed to the distribution pattern of B-KY1 tephra in the East Sea. Previous studies have concluded that explosive volcanic eruptions of Baekdusan are caused by comendite or comenditic trachyte magma [1,2,8,18], whereas B-KY1 tephra in the ODP 794A core originated from comendite, comenditic trachyte, and pantelleritic magmas ( Figure 8B). The high SiO 2 content (>72 wt.%) of B-KY1 tephra in the ODP 794A core was correlated with lower Al 2 O 3 content (9.17-11.86 wt.%), indicating the presence of pantelleritic magma. However, B-KY1 tephra in the 20EEZ-1 core was composed of comendite and comenditic trachyte magma, without pantelleritic magma ( Figure 8B). This wide range of magma types may be the results of a compositionally zoned magma chamber [44,45] or magma mixing/mingling [44,46,47]. Pumices in the Upper Pollara eruption in Italy showed a heterogeneous SiO 2 composition (61-76 wt.%) due to mixing/mingling of andesite and rhyolite [46]. Even magma mixing between two andesite magmas can cause light and dark volcanic glass, the former having greater SiO 2 , K 2 O, and FeO* content, and less CaO and Al 2 O 3 [47]. Recently, zircon isochron ages from Holocene pyroclastic deposits on Baekdusan suggested the occurrence of episodic magma generation at 110-130 ka and > 230 ka [11,27,28]. This indicates subsequent mixing of comendite and comenditic trachyte magma with injected pantelleritic magma during the late Middle Pleistocene Baekdusan volcanic eruption.  In the present study, we compared sedimentary facies against eustatic sea level fluctuation records to refine the eruption age of B-KY1 tephra. X-radiographs of DLMs in the 20EEZ-1 core reflect euxinic bottom water conditions during lowstand (Figure 9). The B-KY1 tephra directly overlies DLM sedimentary facies, with a sharp lower boundary. Overlying a pyritized LBM (pLBM) has been interpreted to indicate deglaciation, consistent with restricted bottom water circulation [26]. SKP-IV tephra was found within LBM in both cores examined in this study, 20EEZ-1 and ODP 794A ( Figure  9), suggesting that the B-KY1 tephra was synchronously deposited in marine sediment at the start of In the present study, we compared sedimentary facies against eustatic sea level fluctuation records to refine the eruption age of B-KY1 tephra. X-radiographs of DLMs in the 20EEZ-1 core reflect euxinic bottom water conditions during lowstand ( Figure 9). The B-KY1 tephra directly overlies DLM sedimentary facies, with a sharp lower boundary. Overlying a pyritized LBM (pLBM) has been interpreted to indicate deglaciation, consistent with restricted bottom water circulation [26]. SKP-IV tephra was found within LBM in both cores examined in this study, 20EEZ-1 and ODP 794A (Figure 9), suggesting that the B-KY1 tephra was synchronously deposited in marine sediment at the start of the penultimate deglaciation at ca. 135 ka. Further marine tephrochronological studies should facilitate further reconstruction of the explosive eruption history of Baekdusan. Bidirectional research linking Baekdusan pyroclastic deposits and distal marine tephras may improve our understanding of explosive volcanic eruptions and their climatic implications.

Conclusions
The B-KY1 tephra, located between the Toya (ca. 106 ka) and Aso-1 (ca. 255 ka) tephras in the ODP 794A core from the Japan Basin of the East Sea, was transported about 900 km from Baekdusan at ca. 135 ka. The stratigraphic position of the B-KY1 tephra correlates exactly with the first B-KY1 tephra identified in the 20EEZ-1 core from the Kita-Yamato Trough. The B-KY1 tephra is characterized by diverse comendite, comenditic trachyte, and pantelleritic magmas, reflecting magma generation during late Middle Pleistocene Baekdusan volcanic eruptions. Differences in geochemical and textural characteristics between the B-KY1 tephras from the ODP 794A and 20EEZ-1 cores indicate that wind directional shift influenced tephra distribution during its long-range transportation. Plinian explosive volcanic eruptions from Baekdusan may have occurred frequently during the effusive cone construction stage during the Pliestocene. Further research linking proximal terrestrial pyroclastic deposits and distal marine sediment tephras should allow us to reconstruct the explosive volcanic history of Baekdusan.