Plasticrusts, plastiglomerate and pyroplastic: new connections among novel plastic debris forms

Background/Question/Methods

Although coastal pollution has been intensively studied over the last 50 years, novel forms of plastic debris, including plasticrusts, plastiglomerate and pyroplastic, keep appearing worldwide. Plasticrusts are plastics encrusting rocky intertidal habitats, plastiglomerates consist of plastic melted together with sand or pebbles and pyroplastic is essentially melted plastic of rock-like appearance that can float in water. All these novel debris forms have only recently been discovered in Madeira Island (Atlantic Ocean), Hawaii (Pacific Ocean) and the United Kingdom (English Channel), respectively. Subsequent records stem from Giglio Island (Mediterranean Sea) and Japan (Seto Inland Sea). However, due to the recency of their discoveries, information on their origins and their relationships with each other is still limited. For example, plasticrusts were proposed to derive from plastic being smashed against intertidal rocks by oceanic waves. Yet, observations of this process are lacking and it is unclear from which kind of plastic the plasticrusts derive. Also, plastiglomerate and pyroplastic have both been related to campfires but it is unknown whether plastiglomerate and pyroplastic are related to each other. To address these knowledge gaps, we surveyed rocky intertidal habitats in Madeira for these novel plastic debris forms and examined our findings macro-, micro- and spectroscopically.

Results/Conclusions

We detected a green plasticrust, a green maritime rope, blue plastiglomerate and blue pyroplastic. Spectroscopic analyses revealed that all green plastics consisted of polypropylene and that all blue plastics consisted of both polyethylene and polypropylene. This shows that plasticrusts can derive from maritime ropes and that plastiglomerate and pyroplastic can derive from the same polymer types. Microscopic examinations indicated that the plasticrust was composed of melted rope fibers which suggests that high rock temperatures contribute to plasticrust formation. Macroscopic examinations confirmed that plastiglomerate sinks whereas pyroplastic floats. The plastiglomerate (which contained a pebble) and the pyroplastic had several pebble shaped clasts. These pebble clasts must have formed during plastiglomerate formation (due to incomplete pyroplastic-pebble agglutination) or plastiglomerate degradation (caused by pebble loss). Regardless of the mechanism involved, these pebble clasts unequivocally relate plastiglomerate to pyroplastic and, thereby, established the first link between these two novel plastic debris forms. Thereby, our study highlights that rocky intertidal habitats are useful study systems to examine the formation and degradation of these plastic pollutants under natural field conditions and provides a starting point to implement simple mitigation measures, such as the replacement of synthetic ropes with natural ropes, against these novel plastic debris forms.