New geochemical identification fingerprints of volcanism during the Ordovician-Silurian transition and its implications for biological and environmental evolution

Volcanism had been an important factor in several geological events, 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. Via analysis and correlation of the sources, diagenetic processes, and geochemical features of bentonites, this study established a set of new geochemical fingerprints for cryptotephra identification within shales, reconstructed the volcanic activities in the Lower Yangtze region during the O/S transition, and discussed the impact of volcanism on the Late Ordovician mass extinction (LOME) and related climate events. The main findings are: (1) 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. (2) The prolonged volcanic eruptions were sustained between the visible volcanic ash layers and characterised by two stages namely intensive volcanism, during the middle–late Katian and at the Hirnantian/Rhuddanian transition, and much weaker volcanism, during the very late Katian to early Hirnantian. (3) The intensive volcanism identified in middle–late Katian was tightly coupled with rapid biodiversity decline, and the prolonged (3–4 Ma) volcanic activities could have continuously affected the ecosystem, ultimately causing the first pulse of the LOME. In addition, the identified strong volcanic activities at the Hirnantian/Rhuddanian boundary were coincident with the second pulse of the LOME. The relationship between intensive volcanism and the two pulses of the LOME further supports a volcanic stressor for the biotic crises. (4) Volcanism was not only an important factor for marine productivity in geological history but also a nonnegligible mechanism for the Hirnantian glaciation. The methods and the geochemical fingerprints proposed in this study can serve as references for volcanism reconstruction and its environmental implications in other geological periods.