Shengchao Yang
School of Earth Sciences and Engineering, Nanjing University

Volcanism has a significant influence on the environmental and biological evolution in geological history, usually recorded by volcanic ash layers (bentonites). However, most volcanic eruption materials were dispersed and mixed with sediments as cryptotephra (invisible volcanic ash layers), unrecognisable by the naked eye, making the role of volcanism in major geological events obscure. Therefore, identification of cryptotephra and reconstruction of the volcanic activities are critical for understanding its influence on the environmental and biological evolution during major geological periods.

At present, volcanism identification methods are classified into the following three aspects: First, empirical visual recognition, a method useful for describing visible bentonites, but it is not suitable for the thin ash layers. Second, mineral composition and morphological characteristics, a method used to distinguish dispersed volcanic shards from aeolian minerals within ice cores, but it is difficult to apply to sediments due to the complex composition of sediments and the diagenesis. Third, special elements and their related isotopic signatures, such as Hg and Δ199Hg. However, volcanism is not the only cause of Hg anomalies, and the mismatching between the isotopic signatures and volcanic ash occurrence suggest this method requires further assessment. Is there a widely comparable indicator that identifies the volcanic material and thus indicates volcanism? Via analysis and correlation of the sources, diagenetic processes, and geochemical features of bentonites, we established a set of new geochemical fingerprints for cryptotephra identification within shales.

Our results demonstrate that Zr, Hf, Zr/Cr, Zr/Al2O3, Cr/Al2O3, V/Al2O3, Ni/Al2O3, SiO2/Al2O3, and K2O/Rb were relatively reliable geochemical fingerprints of volcanic material input within shales, and useful in reconstructing the volcanic activities in the Lower Yangtze region during the O/S transition. These fingerprints indicate two stages of intensive volcanisms, during the middle–late Katian and at the Hirnantian/Rhuddanian transition, which were tightly coupled with the rapid biodiversity decline during middle- late Katian and the second pulse of the Late Ordovician mass extinction (LOME), respectively. The relationship between intensive volcanism and the two pulses of the LOME further supports a volcanic stressor for the biotic crises (Figure 1).

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