Marine anoxia linked to abrupt global warming during Earth’s penultimate icehouse

Piecing together the history of carbon (C) perturbation events throughout Earth’s history has provided key insights into how the Earth system responds to abrupt warming. Previous studies, however, focused on short-term warming events that were superimposed on longer-term greenhouse climate states. Here, we present an integrated proxy (C and uranium [U] isotopes and paleo CO₂) and multicomponent modeling approach to investigate an abrupt C perturbation and global warming event (∼304 Ma) that occurred during a paleo-glacial state. We report pronounced negative C and U isotopic excursions coincident with a doubling of atmospheric CO₂ partial pressure and a biodiversity nadir. The isotopic excursions can be linked to an injection of ∼9,000 Gt of organic matter–derived C over ∼300 kyr and to near 20% of areal extent of seafloor anoxia. Earth system modeling indicates that widespread anoxic conditions can be linked to enhanced thermocline stratification and increased nutrient fluxes during this global warming within an icehouse.

Observations and climate models indicate that the dissolved oxygen inventory of the modern ocean is decreasing, with temperature-driven decline in oxygen solubility being a key driver (1). A decline in ocean dissolved oxygen, expressed as an expanded oxygen minimum zone (OMZ), will negatively impact marine ecosystems, leading to significant loss of biodiversity in the ocean (2). This is of significant concern for the world’s largest fisheries situated in the most productive areas of global oceans, as these regions are particularly susceptible to ocean deoxygenation (3, 4). Substantial uncertainty in estimating the extent of deoxygenation over the upcoming millennia, however, drives the current focus on understanding past episodes of ocean deoxygenation.Empirical constraints on the magnitude of ocean deoxygenation during climate perturbations come predominantly from the Quaternary glacial–interglacial transitions (5, 6) or early Cenozoic rapid warming events, in particular the Paleocene–Eocene Thermal Maximum (PETM) event (7, 8). The temporal scales of warming and deoxygenation of these events differ by an order of magnitude (10⁴ vs. 10⁵ y). Constraints on ocean circulation and biogeochemical cycles across warming events in the Quaternary are more robust than in Earth’s deep past (5, 9). On the other hand, Quaternary partial pressure of CO₂ (pCO₂) and temperature shifts were gradual and the overall perturbations small in magnitude (10) relative to predicted changes for the next millennia or two. Although changes in pCO₂ during the early Cenozoic warming events (foremost, the PETM) were larger in magnitude and more rapid than carbon (C) perturbations of Quaternary glacial–interglacial transitions (11, 12), they occurred under a background greenhouse climate state characterized by high baseline atmospheric pCO₂ (∼1,000 ppm). Other periods of pre-Cenozoic C perturbations (13, 14), such as the Cretaceous and Jurassic (Toarcian) ocean anoxic events (OAEs) (15, 16), and the end-Triassic (17) and the end-Permian mass extinction events (18), also occurred during background greenhouse climates (19, 20). These greenhouse OAEs have provided constraints on and insights into how to model climate change and marine redox evolution. To date, the degree of deoxygenation and spread of anoxic conditions with warming in a glacial state is relatively unexplored.Here, we provide a perspective on global warming–induced ocean deoxygenation by documenting a 10⁵-y C-perturbation event associated with widespread oceanic anoxia, superimposed on the late Paleozoic glacial climate state. This past icehouse (with main episode between ∼340 and 290 Ma) existed under atmospheric CO₂ levels comparable to that of the past few million years (21) and was a period of dynamic and widespread glaciation in the Southern Hemisphere (Gondwana). As such, it provides unique constraints on deoxygenation with warming under a background glacial state, albeit with different paleogeographic boundary and marine and ecosystem conditions compared with those of the younger greenhouse periods of C perturbation.