Ocean Anoxic Event 1a (OAE 1a)

Context

A recent study has refined the timing and duration of Ocean Anoxic Event 1a (OAE 1a)

About Ocean Anoxic Event 1a (OAE 1a)

• OAE 1a, also known as Selli Event, occurred during the Early Cretaceous Period (approximately 119.5 million years ago).
• It was a significant interval when Earth’s oceans experienced severe oxygen depletion, disrupting marine ecosystems and altering carbon cycling.
• Causes:
  • Massive Volcanic Eruptions:
    • Triggered by eruptions such as those from the Ontong Java Nui complex.
    • Released large amounts of CO₂ into the atmosphere, leading to global warming.
  • Oceanic Anoxia:
    • Increased temperatures and nutrient influx caused stratification, reducing oxygen circulation in deep ocean layers.
    • Created anoxic marine basins, devoid of oxygen.

Impacts:

• Marine Ecosystems:
  • Decreased oxygen levels led to widespread extinction, particularly of plankton and shell-forming marine organisms.
  • Favored the survival of specialized anaerobic microbes and fungi.
• Geological Records:
  • Formation of black shales (organic carbon-rich layers) due to slowed decomposition under low oxygen conditions.
  • Evidence of long-term carbon sequestration, which temporarily reduced atmospheric CO₂ levels.
• Ocean Chemistry:
  • Interaction of excess CO₂ with seawater formed carbonic acid, dissolving carbonate shells and contributing to ocean acidification. 
• Anoxic Marine Basins:
  • Regions with extremely low or absent oxygen levels, inhibiting most aerobic organisms.
  • Modern examples: Black Sea, Cariaco Basin, Orca Basin.

Highlights from Recent Study:

• Timing:
  • Initiated around 119.5 million years ago, as confirmed by isotopic analysis of volcanic tuffs from Hokkaido Island, Japan.
  • Duration: Approximately 1.1 million years.
• Volcanic Activity:
  • Linked to large-scale CO₂ emissions from volcanic complexes, exacerbating warming and anoxia.
• Modern Relevance:
  • Parallels between ancient volcanic CO₂ emissions and current anthropogenic climate change highlight potential risks.
  • Raises concerns that rapid modern warming could lead to marine ecosystem collapses or even a Holocene extinction event.

Source: Phys

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Previous Year Question

With reference to ‘Indian Ocean Dipole (IOD)’ sometimes mentioned in the news while forecasting Indian monsoon, which of the following statements is/are correct?
1. The IOD phenomenon is characterised by a difference in sea surface temperature between tropical Western Indian Ocean and tropical Eastern Pacific Ocean.
2. An IOD phenomenon can influence an El Nino’s impact on the monsoon.
Select the correct answer using the code given below:

[UPSC Civil Service Exam – 2017 Prelims]

(a) 1 only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2

Answer: (b)

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