Plastiglomerate takeover!

In the search for a golden spike, ranging from SCPs and atomic bomb radionuclides, to anthropogenic soils and industrial CO₂ peaks, could a new type of "plastic stone" signal the start of the Anthropocene? This "stone" aka "plastiglomerate" is a hybrid mix of melted plastics, beach sediment, basaltic lava, and organic debris (Corcoran et al 2014). How does plastic become merged with rock, you ask? Well, the bits of plastic debris are essentially melted onto rocks mostly through beach campfires, but possibly also lava flows and forest fires (Herreria 2014). As a result, this relatively dense plastiglomerate is not easily transported by wind or water, and therefore has the potential to be buried and preserved, unlike plastic floating around in the oceans (Corcoran et al 2014). Could this partially plastic rock become a global stratigraphic marker for the Anthropocene?

This poor oystercatcher died from starvation when its beak
became stuck in a plastic cap (Source) 

Plastics are a completely human-made material used for consumer and non-consumer goods (Barnes et al 2009). Unless you've been living in a cave with your eyes and ears taped shut, I'm sure you will be aware that these durable, lightweight plastics are becoming quite the environmental problem (e.g. recent BBC article discussing the thousands of pink Vanish bottles washing up on the Cornish coast). With inefficient waste disposal steadily increasing, plastic debris washing up along shorelines and water catchments have caused fatal consequences for freshwater and marine species (Derraik 2002). Plastics are literally choking our planet. But it isn't just the sheer volume of plastics in the environment which make them a concern, it is the fact they do not degrade. Over long periods of time, plastics just break down into smaller and smaller pieces. Though awful news for the wildlife who ingest or get tangled up in debris, and more generally for the environment, this is good news for a potential Anthropocene golden spike as preservation is a key feature of a GSSP.

The Great Pacific Garbage Patch - the oceanic source of much
plastic debris washed up on shores worldwide (Source).

A recent study by Patricia Corcoran et al (2014) discovered these plastiglomerates on one of the world's dirtiest beaches - Kamilo Beach in Hawaii. Despite being distant from the nearest town and miles away from other land, the pristine looking sea is washing up a seemingly endless supply of our planet's plastic waste (tyres, hats, bottles, fishing gear, pellets). Most of this material comes from the North Pacific Subtropical Gyre aka the "Great Pacific Garbage Patch" (Herreria 2014) due to anticyclonic surface ocean currents. If you took a stroll along this beach (though I don't recommend it), over 50% of the material underneath your feet would be tiny plastic debris, with an appearance of multi-coloured sand particles. Even if you started digging up the material (maybe you're trying to build a plastic sandcastle?), you would see nothing but vast depths of plastic. It seems this Hawaiian beach is slowly becoming entirely plastic.

An example of a plastiglomerate containing plastic pellets
and wood chips (Source: Patricia Corcoran)

I mentioned earlier that a potential facilitator of plastiglomerates on Kamilo Beach is molten lava flow. However, this has been dismissed by Corcoran et al (2014) as recorded locations of flowing lava do not coincide with the position of the beach. Therefore, it seems this "plastic rock" is of pure anthropogenic formation (Corcoran et al 2014), making it a distinct marker for human pollution, and a suitable match for the Anthropocene concept. Even if plastiglomerates are formed elsewhere through non-anthropogenic sources (e.g. lava flows and forest fires), the artificial plastics still signify an important identifier of human activity. I also personally think the plastiglomerates act as a cultural signifier of the Anthropocene, demonstrating that humans are literally morphing their way into nature.

The burning question with the use of these plastiglomerates as a geological marker is whether they will preserve long enough into the future to be easily detected. In a recent interview in Science (Chen 2014), geophysicist Douglas Jerolmack agrees the new rocks would make a good marker for the Anthropocene, but doubts their endurance in the fossil record with extreme temperatures in Earth's depths. In the same article, Zalasiewicz, head of the AWG, gives the nod of approval for plastiglomerates as a potential Anthropocene marker, providing they preserve well enough in the geologic record. However, Gibbard (who we've met briefly in my other blog posts e.g. here) sets a seed of doubt in plastiglomerate use, as he is concerned the plastics could revert back to oil depending on burial conditions (Chen 2014). Zalasiewicz and Corcoran disagree with Gibbard, arguing the plastiglomerates may instead be preserved as a "thin carbon film", similar to fossil preservation, that could even leave an imprint of a flattened plastic bottle (Chen 2014)! Lots of important questions about the utility of this "plastic rock" as a marker for the Anthropocene are in the process of being answered by Patricia Corcoran. Corcoran is (as we speak) undertaking experiments testing the heat and pressure that plastiglomerates can withstand (Jazvac, in Valentine 2015).

(Source)

Though much more research is needed into these newly discovered stones of the Anthropocene, for now they are an interesting finding which highlights a fascinating, yet worrying signature of human pollution on the planet. In regards to a date these plastiglomerates would be globally detectable and thus a date of onset for the Anthropocene, I highly suspect it will be another case of a diachronous signal.