Ocean Anoxic Event 1a In Cretaceous Period

Scientists have recently studied ancient rocks and fossils from Mount Ashibetsu in Japan and uncovered details about Ocean Anoxic Event 1a (OAE 1a), which occurred approximately 119.5 million years ago. This event caused a critical lack of oxygen in the oceans and led to extensive extinctions, particularly among plankton.

Ocean Anoxic Event 1a

Why In News

• Scientists have recently studied ancient rocks and fossils from Mount Ashibetsu in Japan and uncovered details about Ocean Anoxic Event 1a (OAE 1a), which occurred approximately 119.5 million years ago. This event caused a critical lack of oxygen in the oceans and led to extensive extinctions, particularly among plankton.

Causes of OAE 1a

• OAE 1a was likely triggered by massive undersea volcanic eruptions. These eruptions released vast amounts of carbon dioxide (CO2), which warmed the planet and disrupted ocean oxygen levels. The combination of high CO2 and low oxygen created a hostile environment for marine life.

• The event lasted just over 1.1 million years. It is one of the largest ocean anoxic events recorded during the Cretaceous Period. Such events can lead to biodiversity loss, as seen in the drastic reduction of plankton populations. The concept of ocean anoxic events emerged in the 1970s.
• Geologists discovered black, carbon-rich shale deposits formed under low oxygen conditions. These deposits accumulated organic matter instead of decomposing, indicating the environmental stress of past eras.

Research Methodology

• Researchers examined volcanic ash layers on Hokkaido Island. These layers solidified over time from ancient volcanic activity. By studying the crystals in the ash, scientists could date eruptions and correlate them with OAE 1a.

• They also analysed carbon and osmium isotopes to understand changes in the carbon cycle. he study linked findings from Japan with global volcanic activity. The OAE 1a event began with a rapid rise in volcanic CO2 emissions, followed by the burial of excess organic material. This aligns with eruptions from the Ontong Java Nui volcanic complex.

Impacts:

• The interaction of CO₂ and seawater formed carbonic acid, dissolving marine organisms’ shells and reducing oxygen levels.
• This resulted in the extinction of several marine species, particularly plankton, and led to the formation of black shales, organic carbon-rich layers preserved in geological records.
• Anoxic marine basins: Bodies of water, often in deep ocean regions, where oxygen levels are extremely low or absent.
• Characteristics: Favor the survival of specialized microbes and fungi while inhibiting most aerobic life forms.

• Carbon Sequestration: Low oxygen levels slow the decomposition of organic matter, aiding in long-term carbon sequestration, which helps reduce atmospheric CO₂ levels.
• Examples: Black Sea, Cariaco Basin (Caribbean Sea), and Orca Basin (Gulf of Mexico).

Some Terms

• Mount Ashibetsu: Mount Ashibetsu is located in Japan. It is known for its ancient rocks and fossils, which provide critical vital information about past environmental crises like OAE 1a.

• Ontong Java Nui: Ontong Java Nui is a massive igneous province in the Pacific Ocean. It contributed to the OAE 1a event through extensive volcanic eruptions releasing CO2.

• Hokkaido Island: Hokkaido Island is part of Japan. Researchers studied its volcanic ash layers to date ancient eruptions and understand their connection to OAE 1a and global climate shifts.