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:

• 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.

The OAE triggered by the eruption of the Karoo-Ferrar LIP is known as the Toarcian–Oceanic Anoxic Event (T-OAE) because of its timing. In Yorkshire, which at that time lay under shallow seas, the oxygen deficient layer mainly affected the seafloor and sediment. Sediment washed into the sea by rivers was deposited on the seafloor. Because of the oxygen-deficient conditions at the time, any organic matter, such as the bodies of dead plankton, was not consumed by bacteria on the seafloor, resulting in the formation of mudrocks with a high total organic content (known as ‘TOC’). These rocks now comprise the Toarcian age ‘Jet Rock’ and ‘Bituminous Shales’ of the Mulgrave Shale Member of the Whitby Mudstone Formation of North Yorkshire.

During the Toarcian, conifers were the dominant tree species on land and flourished over large areas of Pangaea; other plants, such as ginkgoes, cycads and horsetail ferns were common too. When these trees died, they were swept into the nearby shallow seas by rivers, became waterlogged and sank to the seafloor. Because of the anoxic conditions, just like other organic matter, this woody material did not decompose and was covered by further sediment, which eventually was turned to rock. At the same time, the wood was compressed under the weight of overlying sediment and fossilised to form Whitby Jet.

The prevailing belief to date is that Whitby Jet forms specifically from the wood of the Araucariacae family. The modern Araucaria araucana tree, which is commonly known as the Monkey Puzzle tree or Chilean Pine, may thus be a modern analogue (Fig. 2).

However, at the recent ‘Uncovering the Yorkshire Jurassic Symposium’ in York (19 May 2018), Sarah Steele, a geologist, gemmologist and lapidary in Whitby, very convincingly demonstrated that this is simply not true. She considers jet to be derived from several Lower Jurassic tree species and thinks that it is most probably the environment of preservation that is distinctive, rather than the species of tree (see Sarah’s website: The Ebor Jetworks at  http://www.eborjetworks.co.uk/). Sarah is currently researching the origins of Whitby Jet further as part of collaborative project with jet manufacturers in Spain – and we eagerly await her results.

Whitby Jet has been used in jewellery and grave goods since the Neolithic Period and jet artefacts are found in the Bronze Age burials. More recently, Queen Victoria wore it when in mourning for Prince Albert after his death in 1871, providing a great boost to the jet industry in Victorian times (Fig. 3). Today, jet continues to form the raw material for Whitby’s jet jewellery business, and its beauty as a gemstone is still admired by one and all.

Acknowledgements

Thanks to Dr Annette McGrath for encouraging an interest in rocks of the Jurassic through my engagement in the Postgraduate Diploma in ‘The Geology of Yorkshire & Northern England’. I would also like to thank Annette for suggesting this article for publication, and for her discussions and guidance on content and writing style.

About the author and the PG Diploma

Arthur Speed is a current first year student on the Postgraduate Diploma in ‘The Geology of Yorkshire & Northern England’ at the University of York. This exciting two-year, part-time programme is run entirely online by distance learning, but also includes a residential week in York at the beginning of each of the two years, for field and class-based study.

Dr Annette McGrath told Deposits Magazine that “the PG Diploma concentrates solely upon the geological evolution of northern England, to integrate regional knowledge into the interpretation of larger-scale Earth processes and structures. Undertaking this multidisciplinary qualification will equip students with a breadth of transferable skills, including advanced research, science communication and palaeoenvironmental analysis”.

Applications are currently being considered for the new intake in September 2018, so if you would like to know more about the programme (which has recently changed name to ‘The Geology of Northern England’) or would like to apply for a place, please visit: https://www.york.ac.uk/lifelonglearning/geology/ or contact lifelonglearning@york.ac.uk or call 01904 328473 for further details.

References

Schlanger, S.O. and Jenkyns, H.C. (1976). Cretaceous oceanic anoxic events: causes and consequences. Geologie en mijnbouw, 55(3-4), 79-184.

Silver, A. (2014). USGS. A specimen of carved Whitby Jet dating from 1895. [Online]. Available at https://commons.wikimedia.org/wiki/File:Whitby-Jet.jpg [Accessed 26 May 2018].

The Ebor Jetworks (2013). What is Whitby Jet? [Online]. Available at http://www.eborjetworks.co.uk/jet.html [Accessed 26 May 2018].

Ulrich, G. E. (1983). USGS. An active volcano. [Online]. Available at https://en.wikipedia.org/wiki/Hotspot_(geology)#/media/File:Puu_Oo_cropped.jpg [Accessed 26 May 2018].

Zona, S. (2010). The Araucaria araucana tree. [Online]. Available at https://commons.wikimedia.org/wiki/File:Araucaria_araucana_by_Scott_Zona_-_002.jpg  [Accessed 26 May 2018].