Redrawing the history of volcanic eruptions The Bayesian method draws a more precise timeline

The volcano of La Palma in the Spanish Canary Islands has been active ever since its eruption on September 19. Large-scale volcanic eruptions release large amounts of volcanic gas and ash, which can have a serious impact on the Earth's climate and human activities.

For scientists, volcanic ash is an excellent object of study and is the key information to understand the evolution of the Earth system. Recently, Chen Xuanyu, postdoctoral fellow of the State Key Laboratory of Isotope Geochemistry and the Center for Excellence in Deep Earth Sciences, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, and Xu Yigang, academician of the Chinese Academy of Sciences, in cooperation with British scholars, used Bayesian statistical methods to study the eruption age of two major volcanic ashes, and made new progress in the study of young volcanic eruption dating, which was published in Quaternary Geology and Chronology.

Determining the age of volcanic ash is significant

Volcanic ash refers to eruptive debris less than 2 mm in diameter formed by an explosive eruption of a volcano. During explosive volcanic activity, the surrounding rock and magma are blown up into fine particles to form volcanic ash. Volcanic ash erupts from the crater into the atmosphere, is transported by the atmosphere and then settles into various environments, and the whole process takes a very short time, usually only a few years or so. This is almost instantaneous on geological time scales, so in various geological records, volcanic ash is a high-precision absolute time marker layer.

"Volcanic ash chronology is the science of determining geological age using volcanic ash layers in geological records, and the research content involves the chemical composition of volcanic ash, formation location, spatial distribution, and eruption age." Chen Xuanyu said that volcanic ash chronology can help us solve some important scientific problems, such as the genesis link between large volcanic eruptions and climate change, human evolution, and regional differences in rapid climate change events.

It is worth mentioning that absolute time is an important parameter in earth science research, and only by obtaining the accurate age of each geological event can the causal relationship between related events (such as volcanic eruptions and climate change) and understand the complete evolutionary history of the earth.

One of the core principles of volcanic ash chronology is that the same ash layer found at different sites has a consistent age, so it is crucial to determine the accurate eruption age of the volcanic ash marker layer, which can be used for the dating of various geological, paleoenvironmental and archaeological records. The subjects of study were Ko-g and Ma-f~j volcanic ash, which are important time marker layers in northern Japan, from the largest Pliny eruptions of the Holocene of Hokkaido's Komagatake and Mashu volcanoes, respectively.

"Although past studies have carried out a large amount of radiocarbon (14C) dating of the above volcanic ash, the results of different studies have varied greatly, and there is no consensus on the age of eruption of volcanic ash." Chen Xuanyu pointed out that in past studies, there were also problems such as sampling deviation, possible sample contamination and 14C age correction, which had a greater impact on the results of a single 14C dating.

In this study, the researchers used Bayesian statistical modeling methods to study the age of volcanic eruptions. After a comprehensive analysis of all near- and distant-source chronological and stratigraphic information, the study provided the most accurate and accurate age estimates for Ko-g ash and the most accurate age estimates for Ma-f~j ash, confirming that the two large eruptions occurred about 6600 and 7500 years ago, respectively.

There are two main types of ash dating methods

Estimating the timing of volcanic eruptions is actually a dating study of the eruption products. Volcanic ash is the product of volcanic explosive eruptions, and its dating generally includes two types of methods: direct dating and indirect dating.

Direct dating uses primary minerals or glass in volcanic ash to determine the age of volcanic ash, and the methods are mainly radioactive dating methods, such as argon-argon method, uranium system method, light emission method, etc. Indirect dating uses foreign material wrapped in or wrapped in volcanic ash to indirectly determine the age of volcanic ash, mainly by radiometric dating and incremental methods. For example, the 14C dating of carbonized wood in volcanic eruptive debris or organic matter in the soil under volcanic ash belongs to the radiometric dating method, while the age of volcanic ash obtained by striated chronology and ice core chronology belongs to the incremental method.

"Indirect dating also includes age simulations of sedimentary sequences containing volcanic ash, and this study falls into this category." Chen Xuanyu said.

According to reports, this study used the OxCal software of the University of Oxford in the United Kingdom to establish a stage model to analyze the near source 14C age of volcanic ash. Depending on their relative stratigraphic relationship with the ash layer, the samples were incorporated into different sedimentary stages, such as pre-eruption, co-eruption, and post-eruption phases. In the process of correcting the age of the sample, the stage model also takes into account the chronological constraints of the sample stratigraphic position on the volcanic ash. Chen Xuanyuan told reporters: "This is more accurate than the previous study, the age of the samples taken from the middle and below the volcanic ash (in the adjacent strata) was considered to be the age of the eruption." ”

Since Ko-g and Ma-f~j volcanic ash are identified in distant lakes, and the corresponding lake sedimentary records have a large number of 14C dating data, this makes it possible to further analyze all near and far sources using Bayesian statistical methods. Using lake 14C chronology and stratigraphic data, the researchers constructed a formal "sedimentation" model that uses the Poisson process to simulate lake sediment formation. Based on the levels of volcanic ash in the sedimentary record, the sedimentary model cross-references the stage model of the above two layers of volcanic ash at the corresponding location, enabling simultaneous analysis of all available chronological and stratigraphic information related to volcanic ash.

It helps to refine the Holocene volcanic ash stratigraphic framework in East Asia

The chronological modeling method used in this study is a statistical method based on Bayesian analysis.

"The Bayesian analysis method is a method of synthesizing prior information about unknown parameters with sample information, deriving posterior information based on Bayesian formulas, and then inferring unknown parameters based on posterior information. One of the advantages of this approach is its efficiency when dealing with very complex problems. According to Chen Xuanyu, for East Asian volcanic ash research, the past estimates of volcanic eruption age were usually based on a single 14C dating result, but due to sampling deviations, possible sample contamination and 14C age correction, the error of a single sample dating result was large, and the results obtained by different studies could vary by thousands of years.

"The error of the dating results is mainly related to two factors. On the one hand, the errors inherent in various dating methods, such as the errors measured by the instrument, and the errors generated during the 14C age correction process, such errors are difficult to avoid; on the other hand, there may be deviations or sample contamination in the sampling process, etc., such errors belong to the errors artificially introduced in the sample processing process, which can usually be avoided. Chen Xuanyu said.

Using Bayesian statistical methods to build models, the study synthesized all available chronological information related to volcanic ash from multiple sites, while also incorporating stratigraphic information into the model. This method of comprehensively analyzing large amounts of information has greatly improved the accuracy of estimating the age of volcanic eruptions. In the study, the accuracy of the estimation of the age of the eruption was also improved compared to previous studies.

In the international volcanology community, scholars have realized that Bayesian statistical methods can provide more accurate and accurate results for estimating the age of volcanic eruptions, and this method is increasingly being applied to volcanic ash chronology. The products of large explosive volcanic eruptions, due to their wide distribution, are often found and dated in different records, so there is a lot of stratigraphic and age information about such volcanic ash in the literature, and Bayesian statistical methods can be used to comprehensively analyze this information and optimize the age of volcanic eruptions.

"This method is currently more used in Europe and the United States, but very little in Asia." Chen Xuanyu said that the study is an example of using Bayesian statistical methods to optimize volcanic ash age, and the results clarify the past debate about volcanic eruption age, enhance the role of corresponding volcanic ash as a dating tool, and provide key information for improving the Holocene volcanic ash strata framework in East Asia. (Reporter Ye Qing Correspondent Deng Tulian)

Source: Science and Technology Daily