Jet: Gemmy fossil record of Jurassic global warming. Prized since the Bronze Age, jet’s po

Jet: Gemmy fossil record of Jurassic global warming.Prized since the Bronze Age, jet’s popularity reached its peak in the Victorian era when it was extensively used in mourning jewellery and as buttons in clothing. After Albert’s death the Queen Emperess decreed jet as the only gem allowed at court for a year, starting a trend that petered out somewhat after the first world war. Fascinatingly, it also reveals to geoscientists an episode of sudden global warming in the Toarcian period of the Jurassic. The common expression ‘black as jet’ comes from the intense opaque blackness unique to this gem. Jet is somewhat similar to coal, though unlike its cousin this fossilised wood is tougher and can be easily carved (Mohs hardness 2.5-4), taking a beautiful polish, resulting in a lustrous black gem. It dsometimes has golden crystals of pyrite embedded in it. Its low density means that large pieces can be worn without the weight causing a problem. Imitiations include onyx (dyed black agate), glass (both cool to the touch, unlike jet) and various plastics such as bakelite (separated out by density). It was formed 180 million years ago, when driftwood (mostly from Auracaria, the monkey puzzle tree), became waterlogged and sank to the bottom of a muddy ammonite filled sea.It occurs in several places, though the only deposits worked commercially are in Spain and at Whitby (England, where the best quality jet occurs), where a 20 metre thick layer of dark shale contains the deposit, along with clear isotope evidence of a wild pendulum swing in our world’s climate, somewhat similar to the Paleocene-Eocene thermal maximum (see http://tinyurl.com/povhewl). This deposit has been traced Europe wide, and deposits containing the same isotopic story occur globally.Dark marine mudstones tend to signify a harsh time for life. They reflect a period of oceanic oxygen starvation, when the powerful circulatory currents that distribute heat through the globe’s oceans slows or shuts down. The anoxic conditions mean that organic matter, such as aurocaria wood or plankton, does not decompose, leaving dark organic matter behind (such rocks are the source of crude oil).At the same time the jet was forming, a marine extinction was underway, in which sea levels rose roughly 20 metres and the sea floor became oxygen starved, allowing the wood to be preserved rather than consumed by micro organisms. The oxygen isotopes in the jet stratum indicate a rapid sea temperature rise of five degrees Celsius (based on the ratio of light and heavy oxygen, in which spikes of the heavier isotope indicate a warm period, with high evaporation, which selects more of the light isotope).The carbon isotopes also spike, in this case with the light isotope, which is preferentially used by life since it takes less energy to incorporate it into bio molecules. This implies a large release from a reservoir of organic carbon, such as methane clathrates, or an interaction between magma and coal, causing the global warming and anoxic marine conditions that allowed jet to form. It took around 100,000 years for weathering and other processes to extract the carbon from the air and re deposit it as sediments. Both these spikes occur in sediments worldwide, indicating a global event.Mining has always been artisanal, and the holes in the unstable shale cliffs re-filled after the jet has been extracted. Its popularity grew in part due to the introduction of railways, which made Whitby a popular seaside tourist destination. Showcased at the Great Exhibition at Crystal Palace in 1851, it was adopted by royalty around the continent, before Albert’s death made it de rigeur amongst the fashionable British aristocracy. the oldest pice found is a carving of a damsel fly larva 10,000 years old from the German Neolithic. It has also turned up in Bronze age excavations, and the ancient Romans loved it, calling it gagat, from which the modern name was probably derived. It was exported all over the empire, often as bangles or beads. Pliny the elder thought it warded off snakes, and ancient Greeks used it, both as incense, and, mixed with wine, as a remedy against toothache.Jet has gone out of fashion, being mostly unknown now outside steampunk circles, probably due to its association with mourning in a society that has a more timorous attitude to death than those of past centuries (in which having several children die on you in your home was common, and people passed on in their beds, surrounded by their families, rather than in hospitals or nursing homes).What a series of interconnections, from floating Jurassic driftwood to Victorian mourning, with the added bonus of an incredible story from the pages of our beautiful world’s distant past.LozImage credit: Annie R.http://www.whitbyjetstore.com/content.php/info_id/364http://www.gemselect.com/other-info/about-jet.php -- source link
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