New Thallium isotopes from Baltica and South China reveal global redox shifts coincide with significant declines in early Silurian biodiversity

Biodiversity was low in the early Silurian (Llandovery Epoch) following the Late Ordovician Mass Extinction (LOME). After the first 1 to 2 Myr of the Rhuddanian Stage, marine biodiversity began to recover in what is called the Rhuddanian-Aeronian Radiation Event. This recovery was punctuated by several biotic turnover events, notably the Sandvika/sedgwikii Event in the late Aeronian Stage (impacting marine groups such as graptolites, brachiopods, conodonts, gastropods, trilobites, and acritarchs) and the Utilis/Valgu Event in the early Telychian Stage (affecting graptolites and conodonts). It has long been hypothesized that the warming climate during the Silurian resulted in major environmental changes that would have impacted biodiversity. The prevalence of organic-rich shales throughout much of the Llandovery and widespread anoxic bottom waters during the LOME suggest that changes in marine redox conditions may have played a significant role throughout the Llandovery. However, global redox conditions of early Silurian oceans remain poorly understood. Geochemical analyses from Baltic (Aizpute-41 and Rostanga-1) and South China (Lu-205) drill cores indicate marine bottom waters were locally anoxic during the early Rhuddanian-late Aeronian. These conditions are ideal for recording authigenic seawater Tl isotopes, a proxy that tracks the degree of Mn-oxide burial globally. Under more expansive reducing conditions, fewer Mn-oxides are buried globally, causing the seawater Tl isotopic ratio to shift positively towards source values (-2 epsilon units). Preliminary Tl isotope results show when biodiversity was low in the Rhuddanian marine bottom water values were much more positive (~-2.5) than modern oceans (-6). Secondly, during the subsequent radiation period Tl isotopes shifted rapidly several times: positively during the Sandvika/sedgwikii Event, negatively in the early Telychian, and positively during the Utillis/Valgu Event. Widespread anoxia during the Rhuddanian would have severely restricted oxygen supply for marine biota following the LOME. Later, as biota radiated more successfully, the rapid fluctuations of global anoxia likely stressed biota, resulting in the events marked by elevated extinction rates. These findings corroborate the hypothesis that throughout the Silurian environmental conditions were variable as the world transitioned from an icehouse to a greenhouse, resulting in frequent marine biodiversity changes. Additionally, this gives us another glimpse into how the changing environment likely shaped the diversification and evolution of early Phanerozoic life, a key interval in evolutionary history.