Ironstone as a record of early Paleozoic ocean instability

Abstract:

It is widely accepted that late Proterozoic to early Paleozoic ventilation of the deep oceans paved the way for the fully oxygenated Phanerozoic Earth and the proliferation of multicellular life. Reinvestigation of early Paleozoic ironstone supports a more nuanced picture of ocean oxygenation and biologic evolution.

Ironstone is a Phanerozoic marine biochemical sedimentary rock enriched in syndepositional Fe that is concentrated in Ordovician-Devonian successions. Detailed study of the sedimentology, stratigraphy, petrography, and geochemistry of Ordovician ironstone in Canada (Bell Island and Wabana gps), Spain (Luarca, Castillejo, and Fombuena fms), the U.S.A. (Neda Fm.), and Wales (Ogwen Gp) as well as Silurian ironstone in the eastern U.S.A. (Clinton Gp) permits reconstruction of ironstone genesis and its linkage to the biosphere with much greater fidelity than has been previously possible.

This holistic dataset supports an emerging model of early Paleozoic oceans that were not yet fully ventilated, allowing hydrothermally derived ferruginous seawater to pool in the deep ocean. Upwelling of ferruginous waters into shallower environments produced ironstone, which share some attributes of the large, upwelling-related Paleoproterozoic iron formations. The expansion of anoxic shelf environments is also interpreted to have contributed to major Paleozoic biotic crises such as the Late Ordovician mass extinction and the Silurian Ireviken Event. The drawdown of toxic metals during ironstone deposition likely played an important role in the biotic recoveries that followed.

Such conditions are supported by published sulfur isotope data and mark a transitional state between the largely anoxic Precambrian and well oxygenated Phanerozoic oceans. Thus, ironstone is not just a geologic curio but is the direct product of climatic and oceanographic fluctuations that were intimately intertwined with a dynamic Paleozoic biosphere.