Every year, the cross-shelf transport of carbon-rich particles from the Barents and Kara Seas could bind up to 3.6 million metric tons of CO2 in the Arctic deep sea for millennia. In this region alone, a previously unknown transport route uses the biological carbon pump and ocean currents to absorb atmospheric CO2 on the scale of Iceland’s total annual emissions, as researchers from the Alfred Wegener Institute and partner institutes report in the current issue of the journal Nature Geoscience.
Compared to other oceans, the biological productivity of the central Arctic Ocean is limited, since sunlight is often in short supply — either due to the Polar Night or to sea-ice cover — and the available nutrient sources are scarce. Consequently, microalgae (phytoplankton) in the upper water layers have access to less energy than their counterparts in other waters. As such, the surprise was great when, on the expedition ARCTIC2018 in August and September 2018 on board the Russian research vessel Akademik Tryoshnikov, large quantities of particulate — i.e., stored in plant remains — carbon were discovered in the Nansen Basin of the central Arctic. Subsequent analyses revealed a body of water with large amounts of particulate carbon to depths of up to two kilometres, composed of bottom water from the Barents Sea. The latter is produced when sea ice forms in winter, then cold and heavy water sinks, and subsequently flows from the shallow coastal shelf down the continental slope and into the deep Artic Basin.
“Based on our measurements, we calculated that through this water-mass transport, more than 2,000 metric tons of carbon flow into the Arctic deep sea per day, the equivalent of 8,500 metric tons of atmospheric CO2. Extrapolated to the total annual amount revealed even 13.6 million metric tons of CO2, which is on the same scale as Iceland’s total annual emissions,” explains Dr Andreas Rogge, first author of the Nature Geoscience study and an oceanographer at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). This plume of carbon-rich water spans from the Barents- and Kara Sea shelf to roughly 1,000 kilometres into the Arctic Basin. In light of this newly discovered mechanism, the Barents Sea — already known to be the most productive marginal sea in the Arctic — would appear to effectively remove roughly 30 percent more carbon from the atmosphere than previously believed. Moreover, model-based simulations determined that the outflow manifests in seasonal pulses, since in the Arctic’s coastal seas, the absorption of CO2 by phytoplankton only takes place in summer.
Understanding transport and transformation processes within the carbon cycle is essential to creating global carbon dioxide budgets and therefore also projections for global warming. On the ocean’s surface, single-celled algae absorb CO2 from the atmosphere and sink towards the deep sea when aged out. Once carbon bound in this manner reaches the deep water, it stays there until overturning currents bring the water back to the ocean’s surface, which takes several thousand years in the Arctic. And if the carbon is deposited in deep-sea sediments, it can even be trapped there for millions of years, as only volcanic activity can release it. This process, also known as the biological carbon pump, can remove carbon from the atmosphere for long periods of time and represents a vital sink in our planet’s carbon cycle. The process also represents a food source for the local deep sea fauna like sea stars, sponges and worms. What percentage of the carbon is actually absorbed by the ecosystem is something only further research can tell us.
The polar shelf seas harbour other largely unexplored regions in which bottom water is formed and flows into the deep sea. As such, it can be assumed that the global influence of this mechanism as a carbon sink is actually much greater. “However, due to the ongoing global warming, less ice and therefore less bottom water is formed. At the same time more light and nutrients are available for the phytoplankton, allowing more CO2 to be bound. Accordingly, it’s currently impossible to predict how this carbon sink will develop, and the identification of potential tipping points urgently calls for additional research,” says Andreas Rogge.
Kwa mfano, kila mwaka, usafirishaji wa chembechembe zenye kaboni kutoka Bahari ya Barents na Bahari ya Kara inaweza kuweka hadi tani milioni 3.6 za CO2 katika bahari ya Arctic kwa maelfu ya miaka. "Kupitia njia ya usafirishaji isiyojulikana hapo awali, bomba la kaboni ya kibiolojia na mkondo wa bahari hutumia CO2 ya angahewa kwa kiwango cha uzalishaji wa jumla wa kila mwaka wa Iceland, kama watafiti kutoka Taasisi ya Alfred Wegener na taasisi za washirika wanaripoti katika toleo la sasa la jarida la ""Nature Geoscience.""" Ikilinganishwa na bahari nyingine, uzalishaji wa kibiolojia wa Bahari ya Aktiki ya kati ni mdogo, kwani mwanga wa jua mara nyingi ni mdogo - ama kwa sababu ya Usiku wa Polar au kwa kifuniko cha barafu ya bahari - na vyanzo vya virutubisho vinavyopatikana ni vichache. Kwa hiyo, microalgae (phytoplankton) katika tabaka za juu za maji hupata nishati kidogo kuliko wenzao katika maji mengine. Katika safari ya Arctic 2018 iliyofanyika mnamo Agosti na Septemba 2018 kwenye meli ya utafiti ya Urusi ya Akademik Tryushnikov, kiasi kikubwa cha chembechembe za kaboni zilizowekwa katika mabaki ya mimea iligunduliwa katika Bonde la Nansen la Arctic ya kati. Uchunguzi uliofanywa baadaye ulifunua maji yenye kiasi kikubwa cha kaboni katika kina cha kilomita mbili, ambayo ni maji ya chini ya bahari ya Barents. Mvua ya barafu hutokea wakati barafu ya bahari inapofanyizwa wakati wa majira ya baridi kali, kisha maji baridi na mazito huanguka, na baadaye hutiririka kutoka kwenye rafu ya pwani ya chini chini ya mteremko wa bara na kuingia ndani ya Bonde la Arctic. "Kwa kuzingatia vipimo vyetu, tulihesabu kwamba kupitia usafirishaji huu wa wingi wa maji, zaidi ya tani 2,000 za kaboni hutiririka ndani ya bahari ya Arctic kila siku, sawa na tani 8,500 za CO2 ya angahewa. """ "Kutokana na utafiti huo, ""kiasi cha asilimia ya CO2 iliyotolewa kila mwaka ni milioni 13.6,"" anaeleza Andreas Rogge, mwandishi wa kwanza wa utafiti huo na mtaalamu wa bahari katika Taasisi ya Alfred Wegener, Kituo cha Helmholtz cha Utafiti wa Polar na Bahari." Mto huo wenye kaboni nyingi unaenea kutoka Bahari ya Barents na Bahari ya Kara hadi karibu kilomita 1,000 ndani ya Bonde la Aktiki. Kwa kuzingatia utaratibu huu mpya, Bahari ya Barents, ambayo tayari inajulikana kuwa bahari ya pembeni yenye uzalishaji zaidi katika Arctic, inaonekana kuondoa kwa ufanisi karibu asilimia 30 zaidi ya kaboni kutoka angahewa kuliko ilivyodhaniwa hapo awali. Kwa kuongezea, simulation za mfano zinaonyesha kuwa mtiririko wa CO2 unaonekana katika msimu wa msimu, kwani katika bahari za pwani za Arctic, kunyonya kwa phytoplankton hufanyika tu wakati wa majira ya joto. Kuelewa usafirishaji na mabadiliko ya mchakato ndani ya mzunguko wa kaboni ni muhimu kwa kuunda bajeti ya kimataifa ya kaboni dioksidi na kwa hivyo pia makadirio ya ongezeko la joto duniani. Katika bahari, mwani wa chembe moja huchukua CO2 kutoka angahewa na kuzama kuelekea bahari ya kina kirefu wakati wa kuzeeka. Baada ya kaboni iliyounganishwa kwa njia hii kufikia maji ya kina kirefu, inabaki hapo hadi mikondo ya kugeuka ikirudisha maji juu ya uso wa bahari, ambayo inachukua maelfu kadhaa ya miaka katika Aktiki. Na ikiwa kaboni imewekwa katika matope ya bahari ya kina kirefu, inaweza hata kukamatwa huko kwa mamilioni ya miaka, kwani shughuli za volkano tu zinaweza kuifungua. Utaratibu huu, unaojulikana pia kama pampu ya kaboni ya kibiolojia, unaweza kuondoa kaboni kutoka angahewa kwa vipindi virefu vya wakati na inawakilisha sinki muhimu katika mzunguko wa kaboni ya sayari yetu. Utaratibu huo pia huwakilisha chanzo cha chakula kwa wanyama wa ndani wa bahari kama nyota za bahari, sifongo na minyoo. Ni utafiti zaidi tu unaoweza kutuambia ni asilimia ngapi ya kaboni inayomezwa na mazingira. Bahari za barafu za barafu zina maeneo mengine ambayo hayajachunguzwa sana ambapo maji ya chini hufanyizwa na kuenea ndani ya bahari. Kama vile, inaweza kudhaniwa kwamba ushawishi wa kimataifa wa utaratibu huu kama sink kaboni ni kweli kubwa zaidi. Hata hivyo, kwa sababu ya ongezeko la joto duniani, barafu kidogo na kwa hiyo maji kidogo ya chini hufanyizwa. Wakati huo huo, mwanga na virutubisho vingi vinapatikana kwa phytoplankton, na hivyo CO2 nyingi huweza kushikamana. Kwa hiyo, kwa sasa haiwezekani kutabiri jinsi sinki hii ya kaboni itakavyoendelea, na utambulisho wa pointi za uwezekano wa haraka inahitaji utafiti wa ziada, "anasema Andreas Rogge.