Tephra identification
The geochemical similarity of individual tephra samples can be compared using the similarity coefficient (SC) of Borchardt et al. (1972) as a discriminator (Benson et al., 1997; Hallet et al., 2001; King et al., 2001; Lee et al., 2007). When calculating SC, unit weighting was given to the oxides of Si, Al, K, Fe, and Ca; 0.25 weighting was given to the oxides of Ti, Mg, and Mn due to the low contents of these elements and the consequent potential for a minor analytical error that could result in a large relative error in their measured contents (Rieck et al., 1992; Hallet et al., 2001; King et al., 2001; Lee et al., 2007). A weighting of 0.25 was also given to Na because of its high mobility, as well as the variation in concentration due to differing analytical conditions (Madsen et al., 2002). The similarity coefficient for two identical analyses was 1.00. In this study, a SC of ≥ 0.95 between two samples was considered indicative of a correlation.
In this study, the SC was calculated for the unknown tephra and for six tephras originating from the Kyushu area, Japan (AT, Aso-4, K-Tz, Ata, Aso-3, and Kb-Ks). Based on the SC calculation, an SC of 0.95 was obtained only between the unknown tephra and the Kobayashi-Kasamori (Kb-Ks) tephra (Table 1), suggesting that they are geochemically indistinguishable from each other; thus, they are considered equivalent.
Table 1
Similarity coefficient (SC) calculations comparing the major elements of volcanic glass shards (based on the work of Borchardt et al., 1972). Data used for the SC calculation were from Machida and Arai (1992).
| JC | Aso-4 | K-Tz | Ata | Aso-3 | Kb-Ks |
JC | 1.00 | | | | | |
Aso-4 | 0.88 | 1.00 | | | | |
K-Tz | 0.88 | 0.78 | 1.00 | | | . |
Ata | 0.76 | 0.76 | 0.71 | 1.00 | | |
Aso-3 | 0.76 | 0.80 | 0.66 | 0.88 | 1.00 | |
Kb-Ks | 0.95 | 0.91 | 0.85 | 0.74 | 0.76 | 1.00 |
* JC = Jeokjung-Chogye tephra (this study), K-Tz = Kikai-Tozurahara |
The RI values of our samples showed similar results (Fig. 5). They ranged from 1.5041 to 1.5069, which is identical to the RI range of Kb-Ks tephra (1.504–1.507, Machida and Arai, 1992). Based on the results of two independent methods, the tephra samples found in the Jeokjung-Chogye Basin can be regarded as Kb-Ks tephra.
In Korea, some tephra layers originating from Japan have been reported in Pleistocene Paleosol or terrestrial sediments, such as AT and Ata (Yi et al., 1998; Lim et al., 2006, 2007; Kim et al., 2021). Kb-Ks tephra has also been reported from a marine sediment core from the East Sea (Kido et al., 2007), but our report describes the first identification in terrestrial sediments in Korea.
Kb-Ks tephra is characterized by a biotite and hornblende phenocryst assemblage. Although it has not been reliably linked to a specific eruption site, the most likely source is presumed to be the Kobayashi caldera near Kakuto (Machida and Arai, 1992). Distal ash is found in marine deposits from Kyushu to eastern Honshu as a marker-tephra within MIS 13/14, ca. 520–530 ka (Machida and Arai, 1992). The reported thickness of the ash layer is ca. 25 cm in the Kansai area and ca. 15 cm in the Kanto area; thus, it is slightly thicker than the AT tephra layer (Machida and Arai, 1992).
Geological Implications
The discovery of Kb-Ks tephra provides important information concerning the timing of the meteorite impact and deposition of the Jeokjung-Chogye Basin. Previously, Lim et al. (2021) radiocarbon-dated 15 charcoal samples to determine the depositional age of the lacustrine sediments; they obtained ages that ranged from 43,992 ± 522 to 57,049 ± 1,421 14C yr BP with stratigraphic inconsistency. Based on an inferred sedimentation rate of 2.11 mm/y, they insisted that the lake environment formed between 30,000 and 63,000 year BP; the meteorite impact event might have occurred before 63,000 year BP. However, the occurrence of Kb-Ks tephra (ca. 520 ka) found in this study suggests that the depositional age of the lacustrine deposits is much older than previously suspected. Thus, it is clear that the impact event occurred before 520 ka. Because the Kb-Ks tephra layer was found in the upper part (18.32–18.44 m) of the core, the meteorite collision was presumably much older than 520 ka. However, other radiometric dating of the impact breccia is needed to determine the exact timing of the meteorite impact. Because zircons within the impact breccia were presumably reset by high temperature during the collision, fission track dating is regarded as the most suitable technique (e.g., Kohn et al., 1995; Bigazzi and De Michele, 1996). To our knowledge, no terrestrial sediments older than 500 ka have been reported in Korea. Therefore, the finding of Kb-Ks tephra in this study indicates that lake sediments in the Jeokjung-Chogye Basin are very important for paleoclimate and paleoenvironment studies in the Korean Peninsula.
The discovery of Kb-Ks tephra also has an important meaning in terms of volcanic disasters in the Korean Peninsula. Volcanic disasters occur frequently worldwide. Recent representative eruptions that caused massive damage include Mount St. Helens in the United States (1980), Unzen in Japan (1990–1995), Pinatubo in the Philippines (1991), and Eyjafjallajökull in Iceland (2010). The Korean Peninsula has mainly been affected by volcanic eruptions in the Kyushu area, Japan. When volcanoes in the Kyushu region erupt, volcanic ash spreads eastward on westerly winds under normal atmospheric conditions (Lee et al., 2014). However, if southeasterly or easterly winds develop during a volcanic eruption in the Kyushu area, volcanic ash can move toward the Korean Peninsula and spread to the East Sea (Lee and Yun, 2011). AT and Ata volcanic ashes found in the Pleistocene terrestrial deposits in Korea support this hypothesis (Yi et al., 1998; Lim et al., 2006, 2007; Kim et al., 2021). For quantitative estimation of the damage caused by volcanic ash, the thickness of the ash layer should be measured. For the AT and Ata tephras, it is difficult to determine the exact thickness of the ash layer because AT and Ata volcanic ashes are found in Pleistocene Paleosol and marine terrace deposits, respectively (Lim et al., 2006, 2007; Kim et al., 2021). However, because the Kb-Ks tephra found in this study is preserved in lake deposits, the thickness of the ash layer could be measured; it was ca. 12 cm. Although it is difficult to confirm that this entire ash layer was primary deposits, at least several centimeters of volcanic ash fell in the southern part of the Korean Peninsula during the volcanic eruption. Such data concerning the thickness of the volcanic ash layer are very useful for quantitative prediction of the damage caused by a volcanic eruption in the Kyushu area, Japan. However, additional drilling is needed to confirm the thickness of the Kb-Ks ash layer.