THE UPS AND DOWNS OF EARLY SILURIAN BIODIVERSITY: NEW INSIGHTS FROM THALLIUM ISOTOPIC RECORDS OF SOUTH CHINA AND BALTOSCANDIA

In the early Silurian (Llandovery Epoch) biodiversity was low immediately following the Late Ordovician Mass Extinction. After the first 2 to 3 Myr of the Llandovery, marine fauna began to recover in what is called the Rhuddanian-Aeronian Radiation Event. In the late Aeronian stage of the Silurian, this diversification was paused by the Sandvika/sedgwikii Event, a period of extensive biotic turnover among major marine groups such as graptolites and brachiopods. The prevalence of organic-rich shales and evidence for globally reducing conditions at the Ordovician-Silurian boundary indicates that marine (de)oxygenation may have played a significant role in the biodiversity changes throughout the Llandovery. Previously there had been little global evidence to support this hypothesis, but now with the application of the novel Tl redox proxy to marine shales covering the entire Rhuddanian-Aeronian interval, we can begin to unravel the impacts of global (de)oxygenation of marine bottom waters on early Silurian marine biodiversity.

Geochemical analyses of organic-rich shales from drill cores in the Baltic (Aizpute-41) and South China (Lu-205) indicate bottom waters were locally reducing at both locations during the early Rhuddanian through late Aeronian. Authigenic seawater Tl isotopes are recorded under such conditions and track the degree of global Mn-oxide burial. When fewer Mn-oxides are being buried, the Tl isotopic ratio is further from the modern well-oxygenated marine value, indicating more reducing conditions globally.

Exciting new data from these drill cores suggests that the biotic low in the Rhuddanian is contemporaneous with much less Mn-oxide burial than in modern oceans, indicating global oceans were more reducing. During the Aeronian, Mn-oxides became more abundant, indicating global oceans were less reducing than before. This change is contemporaneous with both the radiation and extinction events. The Aeronian period also appears to experience more dynamic redox conditions than the Rhuddanian, suggesting that the marine redox and climate history in the early Silurian is more complex than previously understood.