Whitby Jet and the Toarcian Oceanic Anoxic Event

Arthur Speed (UK) 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: Fig. 1. Volcanism during the eruption of the Karoo-Ferrar LIP may have triggered the Toarcian Oceanic Anoxic Event (Ulrich, 1983). 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 … Read More

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Jurassic Coast (or is it?) with the Geologists’ Association

Mervyn Jones (UK) Since 2012, the Geologists’ Association (GA) has put on annual field trips to the Dorset coast led by Prof John CW Cope (of the National Museum Wales), who is author of the definitive Field Guide No 22. The second edition was published in April 2016 (Geology of the Dorset Coast (2nd ed)). In fact, the trips were started to celebrate the publication of the first edition of the guide. The Dorset Coast is often equated with the ‘Jurassic Coast’ when, in fact, the geology stretches from the topmost Triassic, near the Devon border, through Jurassic and Cretaceous successions, to Eocene deposits at Studland. For this and other reasons, it attracts amateur geologists in large numbers. John’s guide provides essential information including descriptions of the succession and practical guidance about access. What’s missing are the entertaining stories that John Cope can provide and the context provided by exploring inland a bit. Day 1 – Saturday (1 October) For our fifth field meeting, we met up in Lyme Regis (in the car park next to the newly-restored house originally owned by John Fowles – see below) – a town to stir the heart of any geologist. Our mission for the weekend was to look at the unconformity below the Cretaceous, as it oversteps the older Jurassic and Triassic strata progressively in a westerly direction. En route, we observed the instability of the cliffs and suffered the same ourselves, as we scrambled over the boulders and shingle. On this occasion, … Read More

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