The Plastic Anthropocene

In the previous post, I referenced growing production trends of plastic since the 1950s. This week’s blog will build upon this, to indicate how as a growing material element of modern life, and an environmental pollutant, plastic can potentially be used sensu lato as a stratigraphic marker to define the concept of Anthropocene (Zalasiewicz et al., 2016).

First things first however, what is the concept of Anthropocene?

Anthropocene is proposed as a chronostratigraphic and geochronologic unit of geological time, distinct from the previous Holocene epoch which began 11,700 years ago (Waters et al., 2016). The concept of Anthropocene provides recognition to which human activity and interaction has altered and dominated many of the intimate surface geological processes to the point whereby a new geological epoch has been fashioned (Zalasiewicz et al., 2016; Zalasiewicz et al., 2017). The Anthropocene concept was popularised in the early 2000s by Crutzen and Stoermer (2000). However, various terms have been used previously to indicate the concept,  referencing that humankind is a global agent of change, with previous terms including: Anthropozoic (Stoppani, 1873) and Noösphere (Le Roy, 1927) all of which appeared during the 19^th and early 20^th century (Zalasiewicz et al., 2010). The pervasive nature of human activity has long-term implications for Earth’s history over the next millennia, with the anticipated postponement of the next glacial maximum indicating one of the suggested long-term climate implications (Waters et al., 2014; Zalasiewicz et al., 2017).

Since the meeting of the Anthropocene Working Group (AWG) in August 2016, there has been an interim affirmation that Anthropocene is stratigraphically real, with the series based upon the mid 20^th century boundary (Steffens et al., 2015; Zalasiewicz et al., 2017). The mid 20^th century boundary corresponds with the post-World War II ‘Great Acceleration’ of the global economy, population growth and resource consumption (Steffen et al., 2015; Davis, 2015). The inception of Anthropocene could be defined through a numerical age, as part of the Global Standard Stratigraphic Age (GSSA), such as using the detonation of the first atomic bomb on 16^th July 1945 (Waters et al., 2015). However, the AWG community is more comfortable with a definition adhering to the Global boundary Stratotype Section and Point (GSSP), which specifies a physical reference point and a ‘golden spike’ locality (Zalasiewicz et al., 2017). Previous Quaternary subdivisions used signals including the Earth’s orbit and the cyclical forcing derived from climate change, along with unexpected events including volcano eruptions. The stratigraphic markers for Anthropocene require additional indicators to highlight anthropogenic impacts, despite the continued forcing used to previously recognise Quaternary subdivisions (Waters et al., 2016). A range of substantiated evidence is available to provide, through various proxies, the synchronous base required for Anthropocene to be a formal chronostratigraphic unit of geological time (Zalasiewicz et al., 2017).

Primary proxy markers include radioactive isotopes, such as plutonium 239 from the fallout of first nuclear explosion in 1945, which have a long-half-life and high particle reactivity (Waters et al., 2015). Secondary markers include fly ash particles and plastics (Rose, 2015; Zalasiewicz et al., 2016).

  

Figure 1: Various stratigraphical signatures which could indicate Anthropocene (Waters et al., 2014) 

How does plastic make a good marker?

Plastics offer an alternative secondary marker and stratigraphic indicator of the Anthropocene epoch. The cumulative total production of plastic projected for 2050, holding the ceteris paribus assumption, is an expected 40 billion tons, which is the equivalent of 6 layers of cling film around the Earth (Rochman et al., 2013). This global production countered with dispersal through physical and biological processes, has resulted in macro-plastics and micro-plastics infiltrating most terrestrial and marine environments globally, including even the deep-sea floor (Waters et al., 2016). However, the heterogeneity of plastic distribution might vary according to natural and anthropogenic factors, resulting in a greater noted accumulation within terrestrial rather than marine environments (Zalasiewicz et al., 2016). Nonetheless, the wide distribution and extensive production, since the advent of the ‘Great Acceleration’, has now warranted recent considerable attention towards treating plastic as a geological material for marking the Anthropocene strata (Davis, 2015; Waters et al., 2016). Plastics provide a partially effective stratigraphic marker for the mid-20^th century boundary, with the growing dispersal of plastics corresponding with increased commodification of resources and global economic expansion (Davis, 2015; Zalasiewicz et al., 2016). The countered relative ease of identifying plastic without the application of technology, such as to identify other stratigraphic markers including radionuclides, also helps lend support towards plastic as an applicable stratigraphic marker (Zalasiewicz et al., 2016).

In addition, plastics mostly have resistance and resilience toward microbial attacks, underlying their practicality towards providing future identifiable geochemical records, through their longevity within both terrestrial and marine environments (Waters et al., 2016). However, there is contention regarding the geological longevity of these plastic polymers, with the risk of potential modification of molecule structures, impeding future recognition over the course of geological timescales (Zalasiewicz et al., 2016).

Conclusion: 

Nonetheless, despite practical applications for stratigraphy, plastics often encounter limitations through their sparse recorded dispersal and infiltration into terrestrial and marine environments, following the Great Acceleration, with noted accumulation beginning only during the start of the 1960s and 1970s (Zalasiewicz et al., 2016). Plastics therefore, might be regarded as isochronous for stratigraphic markings and helps disqualify plastic as a primary marker, following its inability to precisely indicate the start of the Anthropocene epoch (Walters et al., 2016).  

Figure 2: Anthropogenic signatures (Waters et al., 2014)