Ancient Deep Ocean Remained Oxygen-Rich During Early Cretaceous Oceanic Anoxic Event

Throughout Earth's history, there have been multiple periods called Oceanic Anoxic Events (OAEs), where large parts of the ocean lost their oxygen. These OAEs are often linked to rapid global warming. Understanding their impact on past ecosystems and ocean currents is crucial today, as we face modern global warming. However, scientists weren't sure if one such event, known as OAE 1a, also affected the deepest parts of the open ocean. This major event in the Early Cretaceous period began 119.55 million years ago (Ma) and lasted for 1.116 million years.
To investigate this, research team studied the Tokoro Belt, distributed in northeastern Hokkaido, consists of sedimentary and igneous rocks formed on the seafloor during the Jurassic and Cretaceous periods. The Nikoro Group within the Tokoro Belt contains iron oxide and manganese oxide deposits, which were previously mined. The research team analyzed the chemical composition and osmium (Os) isotope ratios of iron and manganese deposits to understand how and when they formed.
The results showed that iron oxide deposits were formed by hydrothermal activity at mid-ocean ridges during the Callovian stage in the Middle Jurassic (165.3–161.5 Ma), while manganese deposits were formed by hydrothermal activity on oceanic islands in the Early Cretaceous (approximately 120 Ma). Furthermore, the Os isotope ratios of the iron deposits indicated that volcanic activity at mid-ocean ridges (which releases carbon dioxide) was already active before the onset of global warming that triggered the Late Jurassic OAE.
In contrast, the Os isotope ratios of the manganese deposits indicated that the manganese oxide deposits in the western Nikoro Group formed just before the onset of OAE 1a (124–119.55 Ma), while those in the southern Nikoro Group formed during OAE 1a (119.5–118.5 Ma). Since manganese oxides require highly oxic environments to precipitate, these findings demonstrate that deep-sea environments in pelagic regions remained oxygen-rich even during OAE 1a.

Figure: Iron and manganese deposits analyzed in this study

Papers
Journal: Ore Geology Reviews
Title: Origin of ferromanganese deposits in the Jurassic to Cretaceous accretionary complex: Implications
for the deep-sea environment around ocean anoxic events
Authors: Keishiro Azami, Koichiro Fujinaga, Naoto Hirano, Yasuhiro Kato
DOI: 10.1016/j.oregeorev.2025.106661