CLINTON IRONSTONE AND THE SILURIAN IREVIKEN EVENT: FERRUGINOUS UPWELLING AND PALEOZOIC OCEAN INSTABILITY

Phanerozoic ironstone is a marine, hematite-rich biochemical sedimentary rock that contains >15 wt. % Fe. Its origin and paleoceanographic implications remain enigmatic primarily because of a lack of precise modern analogues. Nevertheless, there is increasing recognition that many Paleozoic ironstones accumulated during global biotic events, suggesting that they are an underutilised archive of oceanic and biotic change through time.

Accumulation of the Silurian Clinton ironstone in the Appalachian Foreland Basin coincided with the Ireviken Event, a globally recognizable extinction and associated positive δ¹³C excursion. This provides an excellent opportunity to further elucidate the origin of ironstone and its relationship to global perturbations in the carbon cycle. Sedimentologic and stratigraphic results from the type area in New York, USA, are combined with mineralogic and geochemical data (XRD, SEM, EMPA, and in situ LA-ICP-MS) to understand ironstone paragenesis and oceanography. This unique dataset captures depositional processes at the extremes of spatial scale, permitting reconstruction of the ironstone factory with greater fidelity than previously possible.

Redox-sensitive trace elements in Fe coated grains and depositional textures imply grain growth by redox-controlled synsedimentary precipitation in the shallow subseafloor. These authigenic processes were apparently stimulated by Fe derived from anoxic ferruginous water masses. Such a relationship supports an emerging model for ironstone that relies on a basinal Fe source, as opposed to the more traditional idea of terrestrial input. Paleoclimatic reconstructions suggest ferruginous water was tapped from an upwelling zone along the Laurentian margin and transported into the Appalachian Basin.

The common occurrence of other “upwelling-related” ironstones implies that the early Paleozoic ocean was not yet completely ventilated. Periodic shifts in circulation driven by global climate and eustasy introduced toxic metalliferous waters into neritic environments through upwelling, leading to expanded shelf anoxia, local ironstone deposition, carbon cycle perturbations, and extinctions such as the Ireviken Event. These results emphasize the transitional and unsettled nature of the early Paleozoic oceans and biosphere.