Without bitumen there would be no Whitby Jet, but why? The definition of jet is a ‘drifted wood, with a secondary impregnation of bitumen from its surrounding sedimentary environment’ (Stach, Murchison et al. 1982). Now, this may seem a little fanciful at first glance but on consideration, jet does seem to share many physical properties with solidified bitumen, or asphalt as it is commonly referred to in the U.S.

If we think about road surfacing, the situation in which most modern humans encounter a material like asphalt or bitumen in a liquid form (although this road surfacing material is usually artificially created), it is indeed jet-black, has a glossy lustre and smells rather lovely, just like the aroma we sometimes smell when cutting jet. When it cools it sets hard and looks remarkably jet-like.

We know that in the case of Whitby jet the bitumen with which it is impregnated is derived from the Mulgrave Shales, the ‘Jet Rock’ as it used to be called. This bitumen occurs in these rocks by the same processes that generated the hydrocarbons that give us North Sea Oil, in fact bitumen is actually a sticky, black, highly viscous liquid or semi-solid form of petroleum. In the Whitby stratigraphy we don’t often see much evidence of this oil or bitumen at the surface, we need to venture underground into the abandoned Victorian jet mines to see the bitumen which occurs as sticky black coverings on the walls of the mine tunnels and adits.

There are places on earth today however where we can study the effects of bitumen both in the fossil record, and observe it being generated in real time.

The La Brea Tar Pits in Los Angeles have fascinated scientists and visitors for over a century, and today, this area is the only actively excavated Ice Age fossil site found in an urban location in the world. Over the last 50,000 years, a diverse range of animals, plants, and insects were trapped in sticky bitumen, which preserved them in amazing detail ranging in size from huge mammoths and sloths to ‘microfossils’, the tiny remains of plants and animals that give us clues about how ancient ecosystems and climates changed. The name La Brea is the Spanish term for pitch or tar, however the black gooey substance bubbling to the surface is actually bitumen. A large petroleum reservoir called the Salt Lake Oil Field is located below the surface at La Brea. The oil was formed from marine plankton deposited in an ocean basin during the Miocene Epoch (5-25 million years ago). The petroleum has been gradually migrating to the surface, either along a fault zones or along steeply dipping, porous sedimentary rock layers.

The iconic Lake Pit, located in front of the La Brea Museum, is a remnant from asphalt mining operations in the late 1800s. Rain and groundwater has collected above the bubbling asphalt, creating a small lake. Methane also bubbles to the surface giving a distinctive sulphurous odour, with the addition of a family of mammoths soon to become trapped in the “tar.”

During the last 50,000 years, the La Brea tar pits have trapped and preserved animals and plants that lived in the surrounding area.

The museum housed within the Hancock Park is dedicated to these finds of the tar pits. The researchers there have recovered more than one million bones since 1906, representing over 231 species of vertebrates. In addition, 159 species of plants and 234 species of invertebrates have been identified. However, it is estimated that the collections at La Brea Tar Pits contain about three million items. Once removed the artefacts are brought into the museum labs where they are cleaned and catalogued prior to further research.

Many of the finds are then recreated within the museum galleries such as this magnificent Columbian mammoth photo’d below.

The museum also houses a number of artefacts which document Native American usage of bitumen in the local area. To someone such as myself who researches human interaction with hydrocarbons over the millennia these objects are fascinating. The natural bitumen was exploited for its waterproofing properties by smearing it on woven baskets.

The bitumen was also used as an adhesive to attach flints to arrow shafts and hooks to fishing lines and poles, and for decorative applications, such as this steatite vessel (fig.10 below) decorated with olivella beads inlaid into the bitumen to secure them.

Quartz crystal points have also been discovered embedded in bitumen, likely to hold them in place so that they could be used for scoring softer materials such as jet, shell and turquoise in jewellery making.

Whereas at La Brea, the bitumen was certainly responsible for the demise of many creatures and aided their remarkable preservation, the process is very different to that observed in the formation of jet. Jet is formed by jetonisation, a process which is poorly understood at present. Jetonisation in the Whitby area is however impossible without bitumen as an agent, but the story is way more complicated than one of simple impregnation. I will cover jetonisation in future blogs but in the meantime check out https://tarpits.org for the latest research on the Ice Age fossils of La Brea.

References

Stach, E., et al. (1982). Stach’s textbook of coal petrology, Borntraeger Berlin.