Blue carbon sequestration: An unintended driving force for marine pollution

Global climate change driven by anthropogenic CO2 emissions has intensified efforts to achieve carbon neutrality by 2050. Blue carbon ecosystems, including mangroves, seagrasses, tidal flats, and offshore systems, collectively serve as critical sinks that store 26% of global CO2 emissions from 2014 to 2023. In recognition of their potential in mitigating climate impacts, blue carbon sequestration has been deemed a critical natural solution to achieve negative emissions and carbon neutrality by scientists and policymakers. To this end, a new UN ocean decade program on marine negative emission technologies (NETs) has been proposed: the Global Ocean Negative Carbon Emissions Program (ONCE, https://www.global-once.org), which encompasses ocean alkalization, fertilization, coastal wetland restoration, and sustainable mariculture. Although most of these activities remain conceptual, model-based, or pilot studies, these approaches are expected to become nonnegligible anthropogenic intervention forces in marine biogeochemical cycles.

Synergistic pollutant biogeochemical cycling with marine NETs

Considering that most marine pollutants can be considered anthropogenic organic carbon, carbon sequestration processes in marine NETs show a high degree of overlap with multiphase transport and the biogeochemical cycling and biological responses of marine pollutants.

Ocean fertilization (OF) technology, typically implemented in the open ocean (offshore), aims to accelerate phytoplankton growth (atmospheric carbon fixation) and biological carbon pumps (BCPs) (vertical carbon transport to the deep sea) through the addition of micronutrients such as iron, nitrogen, or phosphorus. The BCP sequesters atmospheric CO2 and releases particulate organic carbon (POC) as well as dissolved organic carbon (DOC) into seawater, where the microbial carbon pump involved can transform DOC into recalcitrant DOC (RDOC). Notably, POC and RDOC, which include structural polymers such as polysaccharides, humic substances, and carboxyl-rich alicyclic molecules, have a strong ability to bind organic pollutants and metal ions. Pollutants absorbed or aggregated by POC and RDOC can remain in the water column for long periods of time and thereby be simultaneously sequestered from surface water, which is referred to as the “pollutant biological pump.” Therefore, the increased anti-biodegradable forms of POC and RDOC in OF deployment not only serve as a significant long-term carbon sequestration mechanism but also strongly enhance the air-to-water flux and vertical transport of pollutants.

Ocean alkalinity enhancement (OAE), another offshore intervention technology, aims to store atmospheric CO2 in the form of carbonate and bicarbonate in the ocean via the addition of alkaline minerals. OAEs can modify the physicochemical conditions of seawater, thereby affecting the speciation and bioavailability of marine pollutants. For example, calcite carbonate can serve as an effective ballast to sink low-density plastic debris to ocean sediments.4 In addition, the major minerals used in OAEs, such as limestone, olivine, brucite, forsterite, or calcite, have very high metal contents (e.g., Ca, Mg, Fe, Ni, Cr, and other heavy metals). In the OAE, the annual mining, grinding, distribution, and deployment of gigatons of rock are accompanied by an additional and effective land-to-sea transport route for metal ions.