The Carbon Isotope Organic Geochemistry of Early Ordovician Rocks from the Annascaul Formation, County Kerry

Abstract

Ireland is well known to geologists as containing some of the thickest successions of Early Ordovician (485! 470 Ma; Walker et al. 2012) sedimentary rocks in the world. The carbon stable isotope compositions (d 13 C value) of similarly aged rocks have been reported for only very few places in the world (i.e., Argentina, southern China, and southern France), and no such analyses have been performed on the Early Ordovician, organic-rich rocks of Ireland. Here we report the d 13 C values of bulk organic material and organic isolates recovered from the Annascaul Formation of Southwestern Ireland. Members of the Annascaul Formation spanning the Early Ordovician were sampled at multiple sites within five localities on the Dingle Peninsula, County Kerry. Mean bulk organic d 13 C values for the organic matter within rocks of the Farranacarriga, Tinal, Illaunglass, Bealacoon, and Killelton Members ranged from 28.7! (Farranacarriga) to 22.1! (Illaunglass); the average d 13 C values of organics isolated from the Farranacarriga and Bealacoon Members were 29.0 and 28.4! , respectively. No statistical difference was observed between the d 13 C value of isolates and the d 13 C value of bulk sediment from which organics had been isolated (P# 0.85, Farranacarriga, n# 6; P# 0.81, Bealacoon, n# 5; paired t-test). The d 13 C values we present here agree well with the previously published d 13 C values for Early Ordovician organic carbon (average # 28.4, 25.4, 25.5! ) from Argentina, southern China, and southern France (respectively). Our new data from the Annascaul Formation are ! 3.3! higher than the average value reported for d 13 C values of organic matter of marine origin for the same period, raising the possibility that terrestrial bryophytes (or other terrestrial photosynthesizers) contributed to these Early Ordovician sediments. Further equivocal evidence is provided by the abundant organic macerals found within the Farranacarriga Member with possible vitrinite origin. The d 13 C values of palynomorphsized organic isolates from the Annascaul Formation reflect a marine origin for these organisms, consistent with their previous identification as acritarchs. Given the recognition of the earliest thalloid macrofossils and land-plant cryptospores in the Middle Ordovician sediments of the Appalachian basin (USA) and Argentina, respectively, our results highlight the Early! Middle Ordovician boundary as a potentially crucial time of terrestrial ecosystem expansion and development.