TIMING VASCULAR PLANTS TERRESTRIAL ORIGINS WITH DETRITAL ZIRCON U-PB DATES: IMPLICATIONS FOR EARTH'S EARLY PALEOZOIC BIOGEOCHEMICAL CYCLES

The evolution of vascular plants like Cooksonia likely increased weathering rates, soil development, and modified Earth's biogeochemical cycles (e.g., atmospheric oxygenation and the carbon cycle), which influenced climate and terrestrial ecosystems during the Early Paleozoic. These changes may have had an impact on global events such as the Great Ordovician Biodiversification Event and the Late Ordovician Mass Extinction. Although molecular studies and cryptospores (i.e., nonvascular plants) indicate that land plants had evolved by the Middle Ordovician (458.2-469.4 Ma), the earliest tracheophytes (vascular plants with conducting tissue and root hairs), such as the Cooksonia genus, did not appear in the macrofossil record until the Middle Silurian (~430 Ma). The timing of Cooksonia's terrestrial emergence remains elusive due to significant gaps in the fossil record between macrofossils (Cooksonia) and microfossils (trilete spores) only dated with biostratigraphy prompting us to employ U-Pb dating of sedimentary units.

The earliest unequivocal macrofossils of vascular land plants are found in the Devilsbit Mountain (S. Ireland), followed by Capel Horeb and Cwm Graig Ddu (Wales), with an age ranging from Wenlock (Sheinwoodian ~430 Ma) to Ludlow (Ludfordian ~422 Ma) based on graptolite biostratigraphy, invertebrates, and spores. In this study, we conducted U-Pb dating on hundreds of zircons from a series of five sandstone/siltstone samples that enclose the Cooksonia macrofossils from all three localities using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Chemical Abrasion Laser Ablation Inductively Coupled Plasma Mass Spectrometry (CA-LA-ICP-MS). Based on our ²³⁸U-²⁰⁶Pb LA-ICP-MS preliminary data, the Cwm Graig Ddu locality yields a youngest mode weighted mean (YMWM) date of 425±3 Ma consistent with its assessed Gorstian age. However, the Capel Horeb locality presents a YMWM date of 434±5 Ma preceding its assessed Ludfordian age, and older than the earliest Sheinwoodian age Cooksonia macrofossil from the Devilsbit Mountain of S. Ireland. Our ²³⁸U-²⁰⁶Pb dataset compares annealed vs. un-annealed unknown and standards zircon grains, distinct maximum depositional age approaches, and LA-ICP-MS and CA-LA-ICP-MS methods to constrain the age of the earliest terrestrial Cooksonia macrofossils from the UK and Ireland.