One hundred and eighty million years ago in the Toarcian Stage of the Lower Jurassic Period, the Earth was very different from the world we know today. The continents were all clumped together in a supercontinent called Pangaea, which was just beginning to split apart. Sea level was approximately 100m higher than at present, such that much of Britain (including Yorkshire) lay beneath shallow seas.
At this time, the Earth’s oceans were depleted in dissolved oxygen. The chain of events that caused this are complex, but can be traced back to a major volcanic event (Fig. 1). The eruption of the Karoo-Ferrar Large Igneous Province (LIP) spewed lava over what is now southern Africa and released vast amounts of carbon dioxide into the atmosphere. Just as happens now, the carbon dioxide resulted in global warming, which, in turn, had several effects on the oceans:
- Seawater became deficient in dissolved oxygen, because oxygen solubility decreases with increased temperature.
- Plankton thrived as a result of the warmer temperatures and increased nutrient supply, using up even more dissolved oxygen.
- Oceanic circulation was decreased, reducing the supply of cold oxygenated water to the oceanic basins.
- Warmer water released the green-house gas methane from the ocean floor, further accelerating global warming.
The result was the formation of a layer of water that was deficient in oxygen throughout the Earth’s oceans. Its existence was first postulated in the 1970s by Schlanger and Jenkyns (1976) to explain a similar phenomenon occurring later in the Cretaceous Period (85 to 105 million years ago), which resulted in the global deposition of mudrocks (‘black shales’) rich in organic matter. The term Oceanic Anoxic Event (OAE) was coined to describe such a phenomenon.
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