mystery solved: Why ocean’s carbon budget plummets beyond the twilight zone

Helping fill a gap in the understanding of the biological carbon pump – a major climate regulator – a new study shows that fragmentation of large organic particles into small ones accounts for roughly half of particle loss in the ocean, making it perhaps the most important process controlling the sequestration of sinking organic carbon in the oceans. These results, gathered using leading-edge robotic sensors, can help inform and improve global climate studies, say Aditya Nayak and Michael Twardowski in a related Perspective. Sinking organic particles like plankton and detritus sequester 15 to 30% of carbon from the surface to the seafloor, playing a critical role in lowering atmospheric carbon dioxide concentration. However, of the billions of metric tons of carbon absorbed by the ocean’s surface, only about 10 to 30% reaches beyond the mesopelagic zone, also called the “twilight zone” (a 200- to 1000-meters deep transitional layer between the ocean’s surface layers and the abyss). To explore different hypotheses for this sudden loss requires calculating sinking rates of different sized particles, which has been difficult given less than 5% of over 3,000 data-collecting floats in the oceans are equipped with sensors that can characterize particle concentration. Here, Nathan Briggs and colleagues analyzed “pulses” of rapidly sinking large particle aggregates between 2013 and 2016, using 25 autonomous profiling floats distributed over the North Atlantic and the Southern Ocean. These floats were equipped with advanced sensors that detect scattering of light and chlorophyll fluorescence to identify sinking particles. The researchers found that elevated levels of large particles were accompanied by increases in small particle concentration. Without fragmentation, smaller particle concentrations would be decreasing because of their aggregation to form larger particles, so an increase in small particle concentrations supports the claim that fragmentation is indeed occurring. Fragmentation rates varied between the Atlantic and Southern oceans, which merits further investigation.

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