EARLY PALEOZOIC OCEANS ON THE EDGE OF ANOXIA

Atmospheric and oceanic redox conditions exert significant influence on the evolution and diversification of marine life throughout Earth’s history. Despite progress in reconstructing atmospheric and oceanic oxygen (O₂) levels from geological and geochemical records, significant gaps persist in comprehending the factors governing their evolution and its interactions with the biosphere. A mechanistic, quantitative framework integrating biogeochemical cycles of carbon, nitrogen, phosphorus, oxygen, and sulfur is essential for a comprehensive understanding of the redox evolution of Earth’s atmosphere and oceans.

Here, a novel biogeochemical model is employed to reconstruct the secular evolution of atmospheric and oceanic chemistry during the early-mid Paleozoic. The core of the model is a biogeochemical model, CANOPS-GRB, which incorporates a suite of biogeochemical processes controlling the redox state of the ocean-atmosphere system. The model has been improved by coupling with global carbon cycle and energy balance models. This enables us to simulate the secular evolution of climate and atmospheric (CO₂, CH₄, and O₂) and oceanic chemistry (DIC, ALK, O₂, PO₄, NO₃, NH₃, SO₄, H₂S, and CH₄) simultaneously.

Standard model runs indicate that during the early-mid Paleozoic, atmospheric O₂ levels were low (0.3-0.5 present atmospheric level), with widespread hypoxic to anoxic conditions in the ocean interior, consistent with previous arguments. Crucially, the model highlights the role of redox-sensitive phosphorus recycling from sediments in stabilizing atmospheric O₂ levels. This stabilization mechanism maintains atmospheric O₂ levels so that the ocean is near the boundary between oxic and anoxic conditions, termed the “edge of anoxia (EoA)”. Furthermore, our model demonstrates that the emergence of land plants played a pivotal role in decoupling the Earth system from the EoA, resulting in the establishment of a widely oxygenated ocean.

Although the position of the EoA in terms of atmospheric O₂ levels is influenced by various environmental factors (e.g., continental shelf area), this study provides mechanistic insights into the evolution of atmospheric and oceanic redox states during the early-mid Paleozoic, contributing to the understanding of the drivers behind these dynamics and their impact on the evolution and diversification of life.