Award: OCE-2127669

Plastics are accumulating in ocean surface waters. Like natural organic matter, plastics are carbon-based. Before this project started, we conducted a synthesis of data from studies of plastics and studies of natural organic carbon. Data for plastics at sea and natural organic carbon at sea are collected by different scientific communities, using different methods. Comparing data from these two communities suggested plastic-carbon may now rival concentrations of natural organic carbon at the surface of the ocean. To determine if this is true, we conducted an interdisciplinary study combining analysis of plastic-carbon and natural organic carbon for the same samples. We did this in the North Atlantic Subtropical gyre, one of the areas where plastics accumulate at sea, to test our Hypothesis that plastic-carbon is now a significant fraction of total organic carbon at the sea surface. Motivations for this work included 1) the geochemical question of whether geoscientists now need to consider how plastic-carbon may mislead them in the inferences they make based on the chemistry and isotopic signatures of organic carbon samples, 2) whether we need to consider plastics as a component and modulator of the ocean carbon cycle, and 3) whether plastics are impacting the ecology and health of the surface ocean. Through this work we developed new methods for the quantification of plastics and delivered new understanding about the amount of plastic at sea.

 

Bulk radiocarbon and stable carbon isotopic data for particulate organic carbon revealed that surface ocean particulate organic carbon is radiocarbon depleted. Plastics are overwhelmingly made from fossil fuels. As such, they are devoid of radiocarbon or “radiocarbon dead”. An isotope mixing model using the bulk particulate organic carbon data and the assumption of a modern biogenic particulate organic carbon pool and radiocarbon dead plastic carbon pool suggests that five to twenty seven percent of surface ocean particulate organic carbon may be plastic carbon. In addition to analyzing bulk particulate organic carbon isotopes, samples collected on cellulose acetate filters were digested to isolate plastics. We developed novel methods for the complete digestion of cellulose acetate filters and other biogenic materials, while preserving larger microplastics. To quantify plastic carbon in the digested samples we used both chemical quantification via pyrolysis gas chromatography mass spectrometry and radiocarbon isotope analysis. Both methods indicated plastics were present in surface samples. The amount of plastic measured by each method correlated, suggesting general agreement between the methods about the trends in plastic carbon concentrations across our samples. However, the chemical method indicated much higher concentrations than the radiocarbon data. Together, these different methods all indicate that plastics are a significant fraction of open ocean carbon. However, further work is required to understand the source of the disagreement between the methods to reach a more robust assessment of the true concentration of plastics in ocean waters.

 

This work supported: a postdoc, Lixin Zhu, who is applying for faculty positions in China; three PhD students: Erin Tuttle, who is now a tenure track Professor at Assumption University, and Andresa Lima and Nicole Vandale who are now in their 3^rd years at Northeastern. The PIs disseminated outcomes to broad audiences, including high school, undergraduate, and graduate students; the scientific community outside of geosciences; environmental and other public groups; and policy experts and policy makers. This work provides novel methods useful to measure plastics in any setting (e.g., human tissue, drinking water) and new understanding about plastics at sea that can be used to design actions to mitigate global plastic pollution. 

 

Last Modified: 01/07/2025
Modified by: Aron Stubbins