Redrawing the history of volcanic eruptions
Bayesian method to draw a more accurate timeline
Our reporter Ye Qing, correspondent Deng Tulian
Volcanic ash chronology can help us solve some important scientific problems, such as the genetic links between large-scale volcanic eruptions and climate change, human evolution, and regional differences in rapid climate change events.
The volcano of La Palma, Canary Islands, Spain, has continued to be active since its eruption on September 19.
Large-scale volcanic eruptions will release a large amount of volcanic gas and volcanic ash, which may have a serious impact on the earth’s climate and human activities.
For scientists, volcanic ash is an excellent research object and the key information for understanding the evolution of the earth system.
Recently, Chen Xuanyu, a postdoctoral fellow at the State Key Laboratory of Isotope Geochemistry at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences and the Center for Excellence in Deep Earth Sciences, and Xu Yigang, an academician of the Chinese Academy of Sciences, collaborated with British scholars to study the eruption age of two volcanic ash using Bayesian statistics. , Made new progress in the dating of young volcanic eruptions, and related research was published in "Quaternary Geological Chronology".
Determining the age of volcanic ash is of great significance
Volcanic ash refers to eruptive debris with a diameter of less than 2 mm formed by explosive volcanic eruptions.
In explosive volcanic activity, surrounding rock and magma are broken into fine particles to form volcanic ash.
Volcanic ash erupts from the crater into the atmosphere, is transported by the atmosphere and then sinks into various environments. The entire process takes a very short time, usually only a few years.
From the perspective of geological time scale, this is almost instantaneous, so in various geological records, volcanic ash is a high-precision absolute time marker layer.
"The study of volcanic ash chronology is the science of using the volcanic ash layer in the geological record to determine the geological age. The research content involves the chemical composition, stratum location, spatial distribution, and eruption age of volcanic ash." Chen Xuanyu said that volcanic ash chronology can help us solve the problem. Some important scientific issues, such as the genetic links between large-scale 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. Only by obtaining the exact age of each geological event, can we judge the causality of related events (such as volcanic eruptions and climate change) and understand the complete evolution history of the earth. .
One of the core principles of volcanic ash chronology is that the same volcanic ash layer found in different locations has the same age. Therefore, it is very important to determine the accurate eruption age of the volcanic ash marker layer. It can be used for dating various geological, paleoenvironmental and archaeological records.
The selected research objects this time are Ko-g and Ma-f~j volcanic ash, which are important time markers in northern Japan and come from the largest Pliny-style eruptions in the Holocene from Hokkaido Komagatake and Mashu volcanoes, respectively.
"Although past studies have carried out a lot of radiocarbon (14C) dating of the above-mentioned volcanic ash, the results of different studies are quite different, and there is no consensus on the eruption age of the volcanic ash." Chen Xuanyu pointed out that there are still existing studies in the past. Sampling deviation, possible sample contamination, 14C age correction and other issues, these issues have a greater impact on a single 14C dating result.
In this study, the researchers used Bayesian statistical modeling methods to study the age of volcanic eruptions.
After comprehensively analyzing all near-source and far-source chronology and stratigraphic information, this research provides the most accurate and most accurate age estimation for Ko-g volcanic ash, and the most accurate age estimation for Ma-f~j volcanic ash. Age estimation confirms that the two large-scale eruptions above occurred approximately 6,600 and 7,500 years ago, respectively.
There are two general categories of volcanic ash dating methods
The estimation of volcanic eruption time is actually a dating study of eruption products.
Volcanic ash is the product of explosive volcanic 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. The main methods are radioactive dating methods, such as argon-argon method, uranium-based method, and luminescence method.
Indirect dating uses volcanic ash wrapped or foreign matter wrapped in volcanic ash to indirectly determine the age of volcanic ash. The main methods are radioactive dating and incremental methods.
For example, the 14C dating of the carbonized wood in the volcanic eruption debris or the organic matter in the soil under the volcanic ash belongs to the radioactive dating method, while the volcanic ash age obtained by laminar dating and ice core dating is an incremental method.
"The indirect dating method also includes the age simulation of sedimentary sequences containing volcanic ash, and this research 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.
Based on the relative stratigraphic relationship between the samples and the volcanic ash layer, the samples are included in different depositional stages, such as pre-eruption, co-eruption, and post-eruption stages.
In the process of correcting the age of the sample, the stage model also takes into account the chronological constraints of the volcanic ash caused by the sample stratigraphic position.
Chen Xuanyu told reporters: "This is more accurate than the age of samples collected in and below the volcanic ash (in the immediate strata) in previous studies.
Because Ko-g and Ma-f~j volcanic ash are identified in distant lakes, and the corresponding lake sediment records have a large amount of 14C dating data, this makes it possible to use Bayesian statistical methods to further analyze all near and far source information.
Researchers used lake 14C chronology and stratigraphic data to construct a formal "sediment" model, which uses the Poisson process to simulate the formation of lake sediments.
According to the position of the volcanic ash in the sedimentary record, the deposition model cross-references the two-layer volcanic ash stage model in the corresponding position, realizing the simultaneous analysis of all available chronological and stratigraphic information related to the volcanic ash.
Help perfect the Holocene volcanic ash stratigraphic framework of East Asia
The chronological modeling method used in this study is a statistical method based on Bayesian analysis.
"The Bayesian analysis method is to integrate prior information about unknown parameters with sample information, obtain posterior information according to the Bayesian formula, and then infer unknown parameters based on the posterior information. One of the advantages of this method is in Efficient in dealing with very complex problems.” According to Chen Xuanyu, for East Asian volcanic ash research, the past estimation of volcanic eruption age was usually based on a single 14C dating result, but due to sampling bias, possible sample contamination, and 14C age correction, etc. The dating results of a single sample have large errors, and the results obtained from different studies can differ by thousands of years.
"The error of the dating results is mainly related to two factors. On the one hand, there are inherent errors of various dating methods, such as the errors of instrument measurement, and the errors generated in the 14C age correction process. Such errors are more difficult to avoid; on the other hand, On the one hand, there may be deviations or sample contamination in the sampling process. Such errors are artificially introduced in the sample processing process and can usually be avoided." Chen Xuanyu said.
This study uses Bayesian statistical methods to build a model, comprehensively analyzes all available chronological information related to volcanic ash from multiple locations, and also incorporates stratigraphic information into the model.
This type of comprehensive analysis of a large amount of information has greatly improved the accuracy of estimating the age of volcanic eruptions.
In the study, the accuracy of eruption age estimation is also improved compared to previous studies.
In the international volcanology community, scholars have realized that the Bayesian statistical method can provide more accurate and precise results for estimating the age of volcanic eruptions, and this method has been increasingly applied to the study of volcanic ash chronology.
The products of large explosive volcanic eruptions are widely distributed and are often found and dated in different records. Therefore, there are many information on the stratigraphy and age of such volcanic ash in the literature, and Bayesian statistical methods can be used to comprehensively analyze this information and Optimize the age of volcanic eruption.
"This method is currently widely used in Europe and the United States, but rarely in Asia." Chen Xuanyu said that the study is an example of using Bayesian statistical methods to optimize the age of volcanic ash. The controversy has enhanced the role of the corresponding volcanic ash as a dating tool, and provided key information for improving the Holocene volcanic ash stratigraphic framework in East Asia.