Shallow- and deep-ocean Fe cycling and redox evolution across the Pliensbachian–Toarcian boundary and Toarcian Oceanic Anoxic Event in Panthalassa

Abstract

The late Pliensbachian to early Toarcian was characterized by major climatic and environmental changes, encompassing the early Toarcian Oceanic Anoxic Event (T-OAE, or Jenkyns Event, ∼183 Ma) and the preceding Pliensbachian–Toarcian boundary event (Pl/To). Information on seawater redox conditions through this time interval has thus far come mainly from European sections deposited in hydrographically restricted basins, and hence our understanding of the redox evolution of the open ocean (and in particular Panthalassa – the largest ocean to have existed) is limited. Here, we present high-resolution Fe-speciation and redox-sensitive trace metal data from two Panthalassic Ocean sections across the Pl/To and the T-OAE intervals, one deposited in deep water (paleo-water depth >∼2.7 km) and the other on a shallow margin (paleo-water depth likely <∼50 m). Data from the deep-water open-ocean site indicate anoxic-ferruginous conditions from the late Pliensbachian to the onset of the T-OAE, with a rather fluctuating redox state alternating between oxic and anoxic/euxinic conditions across the Pl/To boundary. At least intermittent bottom-water euxinia characterized the T-OAE, followed by a subsequent transition toward more oxygenated conditions. By contrast, trace metal data from the shallow margin site indicate that oxygenated to possibly suboxic conditions prevailed. However, elevated highly reactive iron contents, dominated by Fe (oxyhydr)oxides, characterize this shallow-water site. These observations suggest that upwelling, driven in part by increased sea level and prevailing winds from the open ocean, brought anoxic-ferruginous waters onto the shelf, whereupon Fe2+ oxidation was initiated in oxic shallow waters.