Fool’s Gold and the Jurassic Coast’s Missing Molluscs

Iron pyrite, better known as fool’s gold, is a mineral with a superficial resemblance to gold, and can be found in the fossils and rocks of the Jurassic Coast, in particular around Charmouth.  Some of the most beautiful ammonites are those pyrite fossils that have been polished up to shine like jewels.

Fool’s gold has an interesting history. In fact it played a role in the colonisation of the USA. In the 16th century European explorers to North America shipped back fool’s gold to Europe as a means of raising expedition funds. During Elizabeth I’s reign, tons of American pyrite were sold as gold, whilst it was also used as bait to lure immigrants to the colony.  This article by David Rickard, Emeritus Professor at Cardiff University, tells you more.

Pyrite however, is prone to decay, as the Jurassic Coast Collection Development Officer Chris Reedman describes below.  Pyrite ammonites can be polished to perfection but sooner or later, through exposing them to oxygen, they will disintegrate into ‘a pile of sulphur-smelling dust’.  This was a risk for the 16th century fraudsters selling American ‘gold’; sooner or later they would be found out. Whilst pyrite might have been stable enough to survive the transportation and selling process, it would inevitably decay.  Moreover, pyrite and gold have fundamental differences. As Chris explains, “Gold is fairly malleable (you can bite it and leave a mark) whereas pyrite is much harder; gold melts quite comfortably whereas pyrite is highly flammable.”  One assumes those tricksters came up with some duplicitous solutions and explanations, before vanishing into thin air.

Below Chris talks about the role of pyrite in the fossilisation process.

Uncovering the Jurassic Coast’s Missing Molluscs

By Chris Reedman

Taphonomy is the science behind fossilisation and describes the chemical, physical and biological processes that affect an organism from the moment of death until its discovery. Taphonomy can include scavenging and decay, transport, burial and mineral replacement. It’s important to remember that not all creatures survive the fossilisation process – the fossil record is biased towards animals with hard parts such as shells, bones and teeth.

Scientists study taphonomy as a tool to reconstruct ancient diagenetic (burial) and environmental conditions.  There are many complicated geochemical techniques that can contribute to our understanding of Jurassic sea settings , but these can be time-consuming, expensive and involve a lot of complicated maths! By far the easiest and most reliable method is to study the rocks and fossils themselves. Uniformitarianism is a useful concept here and asserts that we can expect natural processes, happening in our environment today, to have happened, in similar settings, throughout geological time. For example, where we see abundant shelly fossils – bivalves, brachiopods, corals and crinoids – in the fossil record, we interpret these as well-lit, nutrient-rich warm waters, home to an abundant community, like the modern day Bahamas.

My PhD looked at a taphonomic process known as the ‘Missing Mollusc’ effect originally described by scientists at Cardiff University. This describes the selective dissolution (chemical destruction) of different groups of shells in varying marine environments. Now, this all might sound a little complicated, but it becomes a little easier if we think of it in more practical terms.

The Jurassic Tethys Sea was a warm, nutrient-rich environment, home to a wide range of different flora and fauna. The survival of those creatures, as fossils, depends on the conditions of the seafloor which is in turn influenced by the availability of oxygen.

In well-oxygenated conditions, marine life can flourish – shells and burrowing organisms are abundant as are free-swimming ammonites and ichthyosaurs. When an animal dies, and its body falls to the seafloor, its soft parts are consumed by scavengers and tiny bacteria, which produces acidity – this is bad for fossilisation. The hard parts, however, are left behind, and may, in turn, become fossils. Shelly fauna are different in that their shells are hard, and resistant to decay, but vulnerable to chemical processes, specifically acidity at the seafloor. In acidic, oxygenated conditions, shelly fossils are often dissolved and leave no trace in the fossil record.

In anoxic or restricted conditions, the lack of oxygen at the sediment surface means that no bacteria can survive. Without bacteria to produce acidity, the chances of shelly fauna becoming fossils increases massively. Similarly, the chances of soft parts, like skin and muscle, preserving in the fossil record is much more likely.

But how do we study those fossils that have disappeared from the fossil record? The shelly fossils that have dissolved… the Missing Molluscs.

In the Jurassic mudrocks of Dorset, pyritisation provides a unique glimpse into the fossil record – one that preserves a true representation of the original faunal community, without the destructive influence of taphonomic processes.

When we look at pyritisation in the fossil record, particularly on the Jurassic Coast, we think of the abundant, well-preserved ammonite moulds found loose among the shingle on Charmouth Beach – but seldom do people spare a thought for the processes that formed these exceptional little fossils in such large numbers!

In the rocks around Charmouth, low oxygen levels mean that the shells of ammonites become the nucleus for pyritisation. Aerobic (air-breathing) bacteria consume all of the soft parts (tentacles, muscles, skin and organs) and rapidly use all the oxygen from within the shell. In a curious paradox, the quicker they consume the organic matter, the quicker they run out of oxygen. When no oxygen remains, pyrite production can begin.

Pyritisation is the product of a bacterial process known as sulphate reduction; these anaerobic (non-air breathing) bacteria are interesting because they do not need oxygen to survive. Formation of sedimentary pyrite requires organic matter, sulphate and iron and turns it into pyrite (FeS₂); this usually happens following burial, within the sediment column. This replacement happens early, before destruction or chemical dissolution of the shell, and can provide us with a rare insight into the true diversity and abundance of shelly fauna.

Pyritisation provides a much truer representation of the original abundance of shells in the fossil record. These bivalves are juveniles (spatfall) and are likely no more than a few weeks old. Swathes of these young shells were killed off rapidly and accumulated on the seafloor. Early pyritisation of these fossils has preserved evidence of these events that would otherwise be lost if normal taphonomic processes were allowed to destroy these shells

In order to learn more about the pyritisation of ammonite fossils, we need to look at how these minerals replace the internal mould. The best way to do this is to cut and polish the fossil to take a look at what’s inside. (We only do this on common fossils and never on any fossils that are scientifically important or rare.)

Pyritisation is a paradoxical process. The processes that preserve an exceptional array of fossils and provide a rare insight into the true ecological diversity of the Jurassic, are the very same processes that are destroying these fossils. Sadly, pyrite fossils from the Jurassic Coast are prone to decay.

This decay can reduce the fossil to nothing more than a pile of sulphur-smelling dust. This is because pyrite forms in anoxic (no oxygen) conditions, and as soon as we expose them to oxygen on the beach or in our homes, this oxygen attacks the pyrite and begins to break it down. One such example is shown below.

 

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