946 eruption of Paektu Mountain

The 946 eruption of Paektu Mountain, on the boundaries of China/Korea, was one of the most powerful in recorded history and is classified as a VEI-7 event. The eruption resulted in a brief period of significant climate change in China. The age of eruption has not been well constrained, but a possible date is A.D. 946.^[1] The 946 eruption of Paektu Mountain has been dubbed the "Millennium eruption" or the "Tianchi eruption", and erupted about 100–120 km³ (24 to 28.8 cubic miles) tephra.^[2][3] The eruption began with a strong Plinian column, and ended with voluminous pyroclastic flows. An average of 5 cm of Plinian ashfall and coignimbrite ashfall covered about 1.5 million km² of the Sea of Japan and northern Japan.^[2] This ash layer has been named the "Baegdusan-Tomakomai ash"(B-Tm). It probably occurred in the winter of A.D. 946.^[4] This was one of the largest and most violent eruptions in the last 5000 years along with the Hatepe eruption of Lake Taupo at around 180 AD and the 1815 eruption of Tambora.

Age

The age of the eruption has not been well constrained. The earliest radiocarbon ages of the carbonized woods yielded three different ages: 1,050 ± 70 B.P., 1,120 ± 70 B.P., and 1410 ± 80 B.P.^[4]

History of 14C wiggle-matching dating

In 1996, Dunlap reported a high-precision wiggle-matching age determined at the University of Arizona as 1039 ± 18 AD(2σ).^[5] However, in 1998, Liu reported a 14C measurements from the center to the edge of the wood, followed by fitting with a high-accuracy tree ring calibrating curve, the obtained age of the Millennium eruption was determined to be 1215 ± 15 AD.^[6] In 2000, Horn reported another wiggle-matched radiocarbon dating with an AMS-mass spectrometer, and the interval of highest probability is 969 +24/-15 AD(945–984 AD; 2σ), which is widely used. In the 2000s, at least 5 of high-precision 14C wiggle-matching ages had been reported: 930–943 AD, 926 ± 10 AD, 945–960 AD, 931 ± 10 AD, and 946 ± 6 AD.^{[1][7][8][9][10]}

Recent 14C wiggle-matching dating

Xu et al.,(2013)^[1] reported 27 best wiggle-match datings from a single partially charred 264-year-old tree, which is 946 ± 3 AD(1σ). Yin et al.,(2012) also reported 82 best wiggle-matched AMS 14C ages of samples from four carbonized logs, which is 938/939. However, the result of Xu et al. (2013)^[1] used a “regional 14C offset” in their ages to decrease the error, and their new date was obtained from the longer tree-ring sequence with the higher analytical precision of ±25 14C years, on a 260-year tree-ring sequence that covers three consecutive wiggles around A.D. 910, A.D. 785, and A.D. 730. Since longer dated tree-ring sequence, finer sample resolution, and higher 14C analytical precision all facilitate more and tighter tie-points for better WM dating. The new date is believed to represent yet the best high-accuracy and high-precision 14C WM chronology for the Millennium eruption.^[1] Xu's wood samples were cut from a tree growing in the area about 24 km from the vent of Changbaishan volcano, it is not clear if volcanic CO2 emission before the eruption could affect the samples and produce ages that are slightly too old.^[1] The best WM dates for the Millennium eruption use the outliers-removed subset of the original 14C measurements and also account for the effect of possible regional 14C offset, and yielded two nearly identical WM ages of A.D. 945 ± 3 and A.D. 947 ± 3, where overall and combined agreement indices of the models reach their highest values.^[1] Therefore, the average of these two WM ages (A.D. 946 ± 3) represents the best modeled WM age for the Millennium eruption.^[1]

History record

The book «高丽史» (History of Goryeo) describe "是歳天鼓鳴赦" and "定宗元年天鼓鳴" (thunders from the heaven drum) in 946 A.D. Also, the book «興福寺年代記» (Heungboksa Temple History) record "十月七日夜白灰散如雪" (3 November, white ash rain as snowing) on 3 Nov, 946.^[4] The thunders may relate to the Millennium eruption, and white ash rain may relate to B-Tm ash.^[4] Three months later, on 7 February (947 A.D.), "十四日，空中有聲如雷鳴" (drum thunders) and "正月十四日庚子，此日空中有声，如雷" (drum thunders) were recorded in «貞信公記» and «日本紀略».^[4] Another similar record is on 19 Feb, 944, in «日本紀略» "廿三日丙申，子刻，振動，聲在上".^[4] Based on the historical record, the eruption may have started on February 944 or November 946, reaching a climax in February 947.^[4]

Ice-core

Sun et al.,(2013)^[11] found volcanic glass in Greenland, which could well have originated in the Millennium eruption magma(rhyolite and trachyte). The age of volcanic glass layer is 939/940 A.D. However, Sigl et al.,(2015)^[12] found out that ice-core chronologies are 7 years offset, and the Millennium eruption glass layer should be in 946/947 A.D. This conclusion is consistent with wiggle-matching dating and history records.

Eruption volume

The eruption volume has not been well constrained, from 70 to 160 km³. Machida et al.,(1990)^[13] roughly estimated the proximal volume(include ignimbrite and Plinian fall) no more than 20 km³, and the volume of distal B-Tm ashfall attains more than 50 km³. A low total bulk volume estimate is 70 km³. Horn and Schmincke (2000)^[2] used an exponential method for minimum area/thickness and maximum area/thickness to obtain the volume of the Plinian ashfall as 82 ± 17 km³, and used the area-thickness method for ignimbrite to obtain 14.9 ± 2.6 km³. The total bulk volume estimate was 96 ± 19 km³. Liu et al.,(1998)^[3] also used same method with Horn and Schimincke to calculate the volume of Plinian ashfall, and obtained the similar value of 83 km³. However, Liu used a different area-thickness value for ignimbrite. Liu assumed a distribution of ignimbrite within 40 km of the caldera, and an average ignimbrite thickness of 7.47m, yielding a volume of ignimbrite of 37.5 km³. The total bulk volume from this is 120 km³. Guo et al.,(2001)^[14] used the exponential method estimate that volume of ashfall is 135.2 ± 7.8 km³. But Guo assumed the geometry of ignimbrite is a cone, and the volume of ignimbrite could be 20.1 km³. Guo also calculated the volume of valley-ignimbrite, because in a valley the thickness of ignimbrite could be 80 m. Then, the total bulk volume is 161.6 ± 7.8 km³. However, 100–120 km³ has been widely used.^[15]

Eruption dynamics

Base on the sequence of pyroclastic, the eruption began with pumice and ash falls, and then an eruption column collapse formed ignimbrite. The column collapse probably was a pulsing collapse, because the ignimbrite and pumice fall deposits are interleaved. Machida et al.,(1990)^[13] divided the Millennium eruption into 4 stages: Baegdu Plinian pumice fall, Changbai pyroclastic flow, Yuanchi tephra falls, and Baishan pyroclastic flow. But Baishan pyroclastic flow may related to post-caldera activity (?A.D.1668 eruption?).^[15] More recent study indicate that eruption include 2 stages: Plinian pumice fall and unwelded ignimbrite.^[2][3]

Plinian stage

This stage formed a large area of white comenditic pumice and ash. The Plinian eruption column reached about 36 km in height.^[3] B-Tm ash and "white ash rain" may related to this stage.^[4] Based on variations of grain-size and thickness of pumice, the Plinian stage can divided into 3 parts: early period, climax, and later period.^[3]

Early period

In a Plinian pumice-fall section, the grain-size of pumice is reversely graded (coarse pumice on bottom and fine pumice on top). The variation of pumice size shows a major fluctuation in eruption column height during this Plinian event. Base on distribution of maximum lithics clasts, in the early eruption, eruption column probably reached 28 km (H_B=20 km), and mass discharge rate attain 10⁸ kg/s (10⁵ m³/s). Early period may released 1.88–5.63 × 10¹⁹ joule, and eruption might lasted for 33.5–115.5 hours.^[3]

Climax period

Based on the distribution of crosswind of maximum lithics clasts, the top of eruption column might have reached 36 km (H_B=25 km), with a mass discharge rate of around 3.6 × 10⁸ kg/s (3.6 × 10⁵ m³/s). The distribution of downwind of maximum lithics clasts showed the wind direction at the time to be SE120°, and the wind speed to be 30 m/s. The height of eruption column (H_B=25 km), the water content of magma (1–2%), and the temperature of magma (1000 k) indicate that radius of the eruption vent was 200 m. The climax of the eruption may released 4.18–12.43 × 10¹⁹ joule, and the eruption might lasted for 35–104 hours.^[3]

Later period

This period eruption formed the upper part of the Plinian pumice fall, which is the fine pumice. The later Plinian pumice fall and pyroclastic flow occurred simultaneously, because some sections show that pumice fall and ignimbrite are interleaved. Combining the grain-size of pumice and thickness of pumice fall, the height of eruption column of later period was no higher than 14 km (H_B=10 km), and the mass discharge rate was 5 × 10⁶ kg/s (5 × 10³ m³/s). The later period may released 8.76–26.16 × 10¹⁷ joule for Plinian eruption and keeping eruption column.^[3]

Ignimbrite stages

In many sections, a large grey ground-surge under an ignimbrite sheet, which might be from the front part of pyroclastic flow, and the unwelded ignimbrite always underlie a large ash-cloud surge. Ignimbrite was deposited within a 40 km radius of the caldera, at an average thickness of 7.47 m. In many valleys, the thickness of ignimbrite may be 70–80 m. The Changbaishan ignimbrite has a low aspect ratio of 1.87 × 10⁻⁴. The speed of the initial pyroclastic flow might have been 170 m/s (610 km/h), and 50 m/s (180 km/h) at 50 km away from the caldera.^[3]

Duration

The vent radius and H2O content of magma indicate that the average volume discharge rate of the Plinian eruption and ignimbrite was 1–3 × 10⁵ m³/s (1–3 × 10⁸ kg/s). A total bulk ejecta of 120 km³ was derived from bulk volumes of pumice fall and ignimbrite of 83 km³ and 37.5 km³, respectively. The ignimbrite-forming eruption may lasted one and a half day to four days (35–104 hours), while the Plinian eruption may lasted three days to nine and a half days (77–230 hours). The total duration of the eruption may be four and a half days to fourteen days (111–333 hours).^[3]

Volatile

Plinian volcanic eruptions can inject a large amount of volatiles and aerosols into the atmosphere, leading to climate and environment changes.^[14] Chlorine concentrations in the peralkaline from the Millennium eruption were postulated to have reached up to 2% and an average of 0.44%. The Millennium eruption was thus thought to have emitted an enormous mass of volatiles into the stratosphere, potentially resulting in a major climatic impact.^[2]

Cl

McCurry used an electron microprobe to analyze the volatile in glass inclusion of feldspar.. McCurry concluded that Millennium eruption may have released 2000 Mt Cl.^[16] Liu used chromatography to analysed the average of volatile of 5 whole-rock samples, and the contents of halogen is 0.08%–0.11%.^[16] A more recent and more detailed study by Horn and Schmincke (2000)^[2] used an ion probe to analyze the average of volatile in 6 of matrix glass and 19 melt inclusions, and the average of content of Cl in melt inclusions and matrix glass were found to be 0.4762% and 0.3853%, respectively. Horn and Schmincke concluded that the Millennium eruption may have released 45 ± 10 Mt of Cl. Another author, Guo,^[14] who studies petrology and geochemistry, shows the average of contents of Cl in melt inclusions and matrix glass to be 0.45% and 0.33%, respectively.^[14] They concluded that Millennium eruption may have released 109.88 Mt of Cl, and 15.82 into the stratosphere.^[14] The Cl contents in the melt inclusions are similar to those of Mayor Island, and higher than those of Tambora (0.211%), Krakatau (0.238%) and Pinatubo (0.88–0.106%).^[2][14] The large difference of results between Guo and Horn is because Guo used higher volume and density of magma.

SO2

Liu used chromatography to analyze the average of volatile of 5 pumice and obsidian, finding the contents of S to be 0.0415%, and Liu assumed the degassing efficiency factor of S is 0.3. Liu estimated that the Millennium eruption may released 40 Mt of SO2.^[16] However, Horn and Schimincke^[2] calculated that only 20% of the sulfur in the magma had been degassed, because 80% of all analyses of inclusions and matrix fall below the detection limit of ion probe. The results of average contents of S in 19 of inclusions are 0.0455%, Horn assumed the contents of S in matrix glass are 0.025% because 250 ppm is detection limit of ion probe.^[2] They concluded that the total SO2 released from eruption was only 4 ± 1.2 Mt, but Horn suggests that may be excess sulfur accumulated in vapor phase.^[2] Guo calculated the average contents of S in 9 glass inclusions and 1 matrix glass are 0.03% and 0.017%, respectively. The results of Guo are 23.14 Mt of SO2 released from eruption, and 3.33 Mt of SO2 input to stratosphere.^[14] The S contents in glass inclusions show the reverse correlation with SiO2 concentrations, indicating that S solubility in magma is controlled by magma differentiation process because of occurrence of the S-rich fluid inclusions.^[14]

F

Liu used chromatography to analyze the average of volatiles of 5 pumice and obsidian, finding the fluorine content to be 0.0158–0.0481%. Horn and Schimincke used an ion probe to find an average fluorine content in inclusions of 0.4294%, but F concentrations in matrix glass show a significant bimodal distribution into F-rich (0.3992% F) and F-poor (0.2431% F).^[2] In order not to over-estimate syn-eruptive F loss, they considered this bimodal distribution of F for calculating the volatile difference between matrix glass and melt inclusions (4300 ppm F). The volatile loss is approximately 300 ppm F for melt inclusion and F-rich matrix glass (64% proportion of the comenditic magma), whereas it is 1900 ppm F for melt inclusion and F-poor matrix glass (36% proportion of the comenditic magma). Horn concluded that 42 ± 11 Mt of F was released by the eruption.^[2] Guo, based on less samples (9 inclusions and 3 matrix glass), calculated that F contents in inclusions and matrix glass are 0.42% and 0.21%, respectively.^[14] Guo concluded that 196.8 Mt of F was released from eruption, with 28.34 Mt of F injected into the stratosphere.^[14] With magma evolving, halogen contents increase irregularly, parallel to the increase of SiO2 concentrations in glass inclusions^[14] The large difference of results between Guo and Horn is because Guo used a higher volume and density of magma, and higher difference contents between matrix glass and inclusions.

Vapor phase

Sulfur is not strongly enriched during differentiation, in contrast to water, chlorine, and fluorine. The reason could be pre- or syn-eruptive degassing of a separate vapor phase, such as that postulated for the Pinatubo and Redoubt eruptions. The ultimate source for the excess volatile observed during the 1991 Pinatubo eruption is assumed to be sulfur-rich basaltic magmas underlying, and syn-eruptively intruded, into the overlying felsic magmas. The sulfur-rich trachytic and trachyandesitic magmas which underlay the rhyolitic magma at Changbaishan may have been a possible source for excess sulfur accumulation. If this scenario is realistic, clearcut proxies for the environmental impact of the eruption would be expected.^[2][14] Millennium eruption magmas are predominantly phenocryst-poor (≤ 3 vol%) comendites plus a volumetrically minor late-stage, more phenocryst-rich (10-20 vol%) trachyte. Sizeable (100-500 µm diameter) glassy but bubble-bearing melt inclusions are widespread in anorthoclase and hedenbergite phenocrysts, as well as in rarer quartz and fayalite phenocrysts. Comparing the relative enrichments of incompatible volatile and non-volatile elements in melt inclusions along a liquid line of descent shows decreasing volatile/Zr ratios suggesting the partitioning of volatiles into a fluid phase. This suggests that current gas-yield estimates (Horn & Schminke, 2000^[2]) for the Millennium eruption, based on the petrologic method (difference in volatiles between melt inclusions and matrix glass), could be severe underestimates.^[17]

Climate effects

The Millennium eruption was thus thought to have emitted an enormous mass of volatiles into the stratosphere, potentially resulting in a major climatic impact, but more recent study indicate that the Millennium eruption of Changbaishan volcano might have limited regional climatic effects, rather than global or hemispheric impact as implied by its magnitude.^{[1][2][11][12]} However, there are some temperature and atmospheric anomalies in A.D. 945–948 which may relate to the Millennium eruption.^[18][19]

References