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the 2019-2020 fires boosted the Southern Ocean’s carbon pump

A study led by theInstitute for Marine and Antarctic Studies (IMAS) identified a strengthening of the Southern Ocean carbon pump following the major fires of 2019-2020 in Australia. It also illustrates the possibility of removing CO2 from the atmosphere through the massive application of iron to nutrient-deficient waters. The results were recently published in the journal Geophysical Research Letters.

The wildfires that ravaged Australia in 2019-2020 released a staggering amount of carbon dioxide (CO2), estimated at some 720 million tonnes. This effect, which retroacts on global warming by amplifying it, has however been tempered by phytoplankton blooms linked to iron deposits in the Pacific portion of the Southern Ocean. By acting on the biological carbon pump, the fires have therefore indirectly favored its absorption by the ocean.

Iron fertilization and ocean carbon uptake

Among the many elements contained in smoke, there are nutrients such as iron or zinc. When carried by the winds, these particles precipitate over ocean areas that are poor in nutrient compounds, they promote phytoplankton blooms, also called algal blooms. This is what happened following the Australian fires of 2019-2020 as reported by recent works.

The Southern Ocean plays an essential role in the global carbon cycle, it is responsible for nearly half of the annual carbon transfer from surface waters to the abyss “, explains Jakob Weis, lead author of the study. ” Phytoplankton play a key role in this transfer through a process called the ‘biological carbon pump’ which captures and transports carbon to the depths by precipitating ocean plants and animals. “.

Deposition of black carbon (a) and dust (c) in the Pacific sector of the Southern Ocean between September 2019 and September 2020. Excesses relative to climatology appear in red and deficits in blue. Accumulation of black carbon (b) and dust (d) deposits on the study area. Credits: Jakob Weis & coll. 2022.

Normally, the Southern Ocean is deficient in nutrients and cannot accommodate a large quantity of phytoplankton. The biological carbon pump therefore does not work at full speed, or even does not work at all. However, when the smoke from the fires passed over the waters off Australia, iron was deposited and fertilized an area larger than the island continent. Until now, the consequences of this brutal stimulation of the biological pump were uncertain.

We used satellite observations to study this phenomenon and found that phytoplankton cells became richer in pigments and more efficient in their photosynthesis », Details the researcher. ” Just like land plants, phytoplankton absorb CO2 and produce oxygen and when this process is more efficient, so is the biological carbon pump. “.

An unexpected persistence of algal blooms

If present, the excess phytoplankton activity persisted six to nine months after the deposition of the iron particles, far exceeding what had been observed until now. The blooms measured in the case of artificial fertilization or following volcanic eruptions only lasted a few weeks. The researchers explain this persistence by the amount of iron deposited, high enough to allow effective recycling by organisms, and its high bioavailability due to the temperatures and acidity encountered during its passage through the plumes.

The iron that powers the bloom comes from recycling that occurs when iron is released back into the water upon the death of a phytoplankton cell, to be reabsorbed by new cells “, explains Zanna Chase, co-author of the study. “ Efflorescence’s ability to reuse its own iron for so long was likely due to its vast size, which slowed the loss of recycled iron at the edges of the efflorescence “. This observation should provoke renewed interest in the possibility of artificially increasing the ocean carbon sink to combat global warming.

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