Reframing the carbon cycle of the subpolar Southern Ocean

MacGilchrist, Graeme A.; Garabato, Alberto C. Naveira; Brown, Peter J.; Jullion, Loïc; Bacon, Sheldon; Bakker, D. C. E.; Hoppema, Mario; Meredith, Michael P.; Torres-Valdés, Sinhue

doi:10.1126/sciadv.aav6410

Cited by 37 publications

(65 citation statements)

References 46 publications

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“…Factors expected to affect Southern Ocean species arise predominantly from three major areas, increased temperature, altered sea ice [and iceberg scour in benthic habitats; ( 34 )], and ocean acidification. The role of the cold Southern Ocean as a global atmospheric carbon sink has been highlighted, especially since the 2000s ( 35 , 36 ), exacerbating the challenge of acidification. Further factors, including salinity/freshening ( 22 , 37 , 38 ) and low oxygen levels ( 39 ), have potentially large impacts in coastal, and especially fiordic, environments ( 21 , 40 – 43 ).…”

Section: Introduction – Climate Change In Antarcticamentioning

confidence: 99%

Antarctic environmental change and biological responses

2019

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Antarctica and the surrounding Southern Ocean are facing complex environmental change. Their native biota has adapted to the region's extreme conditions over many millions of years. This unique biota is now challenged by environmental change and the direct impacts of human activity. The terrestrial biota is characterized by considerable physiological and ecological flexibility and is expected to show increases in productivity, population sizes and ranges of individual species, and community complexity. However, the establishment of non-native organisms in both terrestrial and marine ecosystems may present an even greater threat than climate change itself. In the marine environment, much more limited response flexibility means that even small levels of warming are threatening. Changing sea ice has large impacts on ecosystem processes, while ocean acidification and coastal freshening are expected to have major impacts.

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Section: Introduction – Climate Change In Antarcticamentioning

confidence: 99%

Antarctic environmental change and biological responses

2019

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“…Following the analysis by MacGilchrist et al, 2019, we compute the full‐depth export from the Adélie Land in the postcalving polynya configuration of 3 ± 2 Tg C yr −1 that represents the sum of the overturning and horizontal transports (i.e., total transport) (Figure 8b). This quantity is significantly smaller than the Weddell Sea Gyre total export of 53 ± 10 Tg C yr −1 (about 5%; MacGilchrist et al, 2019) and global particulate organic matter export across 2,000 m of 430 Tg C yr −1 (less than 1%; Honjo et al, 2008). Compared to the open‐ocean subpolar Weddell Sea Gyre, the coastal Mertz and Ninnis Polynyas produce and transport less DSW on a smaller spacial scale over narrow, shallow continental shelf and are influenced by a more modified form of CDW associated with lower concentration of TCO 2 .…”

Section: Discussionmentioning

confidence: 99%

“…Compared to the open‐ocean subpolar Weddell Sea Gyre, the coastal Mertz and Ninnis Polynyas produce and transport less DSW on a smaller spacial scale over narrow, shallow continental shelf and are influenced by a more modified form of CDW associated with lower concentration of TCO 2 . In the Adélie Land system, the high rate of winter sea ice production (and the rejection of salinity) is the major process enriching TCO 2 in the exported water rather than the local biological pump as in the Weddell Sea Gyre (MacGilchrist et al, 2019).…”

Section: Discussionmentioning

confidence: 99%

A Continental Shelf Pump for CO₂ on the Adélie Land Coast, East Antarctica

et al. 2020

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We quantify the transport of inorganic carbon from the continental shelf to the deep ocean in Dense Shelf Water (DSW) from the Mertz and Ninnis Polynyas along the Adélie Land coast in East Antarctica. For this purpose, observations of total dissolved inorganic carbon (TCO 2) from two summer hydrographic surveys in 2015 and 2017 were paired with DSW volume transport estimates derived from a coupled ocean-sea ice-ice shelf model to examine the fate of inorganic carbon in DSW from Adélie Land. Transports indicate a net outflow of 227 ± 115 Tg C yr −1 with DSW in the postglacial calving configuration of the Mertz Polynya. The greatest outflow of inorganic carbon from the shelf region was delivered through the northern boundary across the Adélie and Mertz Sills, with an additional transport westward from the Mertz Polynya. Inorganic carbon in DSW is derived primarily from inflowing TCO 2-rich modified Circumpolar Deep Water; local processes (biological productivity, air-sea exchange of CO 2 , and the addition of brine during sea ice formation) make much smaller contributions. This study proposes that DSW export serves as a continental shelf pump for CO 2 and is a pathway to sequester inorganic carbon from the shallow Antarctic continental shelf to the abyssal ocean, removing CO 2 from atmospheric exchange on the time scale of centuries. Plain Language Summary Dense waters formed on the Antarctic continental shelf flow into bottom waters that spread throughout the global ocean. These Dense Shelf Waters play an important role in transporting material from the middle and upper layers of the water column to the abyssal ocean. By combining observations and model simulations, we track the transport of dissolved CO 2 in these Dense Shelf Waters from the continental shelf to the deep ocean from the Mertz and Ninnis Polynyas in East Antarctica. We find the largest contributor of dissolved CO 2 to Dense Shelf Water formed in this region is the CO 2-rich midlayers that move onto the continental shelf from offshore. Once on the shelf, local processes in the upper water column further enrich these waters with dissolved CO 2 before their subsequent descent into bottom waters and the abyssal ocean. Our findings highlight the importance of the CO 2-rich midlayers, rather than upper ocean shelf processes, in driving the conditions for carbon sequestration in the deep ocean.

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“…Five BGC-Argo floats drifted across the open waters of the eastern Weddell Gyre (Fig. 1b), a region that was identified as a major carbon sink due to strong primary production 55 . The Kerguelen Plateau is a major obstacle on the eastward flow of the ACC.…”

Section: Methodsmentioning

confidence: 99%

Remote assessment of the fate of phytoplankton in the Southern Ocean sea-ice zone

2020

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In the Southern Ocean, large-scale phytoplankton blooms occur in open water and the sea-ice zone (SIZ). These blooms have a range of fates including physical advection, downward carbon export, or grazing. Here, we determine the magnitude, timing and spatial trends of the biogeochemical (export) and ecological (foodwebs) fates of phytoplankton, based on seven BGC-Argo floats spanning three years across the SIZ. We calculate loss terms using the production of chlorophyll-based on nitrate depletion-compared with measured chlorophyll. Export losses are estimated using conspicuous chlorophyll pulses at depth. By subtracting export losses, we calculate grazing-mediated losses. Herbivory accounts for~90% of the annually-averaged losses (169 mg C m −2 d −1), and phytodetritus POC export comprises 10%. Furthermore, export and grazing losses each exhibit distinctive seasonality captured by all floats spanning 60°S to 69°S. These similar trends reveal widespread patterns in phytoplankton fate throughout the Southern Ocean SIZ.

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Reframing the carbon cycle of the subpolar Southern Ocean

Cited by 37 publications

References 46 publications

Antarctic environmental change and biological responses

Antarctic environmental change and biological responses

A Continental Shelf Pump for CO₂ on the Adélie Land Coast, East Antarctica

Remote assessment of the fate of phytoplankton in the Southern Ocean sea-ice zone

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Reframing the carbon cycle of the subpolar Southern Ocean

Cited by 37 publications

References 46 publications

Antarctic environmental change and biological responses

Antarctic environmental change and biological responses

A Continental Shelf Pump for CO2 on the Adélie Land Coast, East Antarctica

Remote assessment of the fate of phytoplankton in the Southern Ocean sea-ice zone

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A Continental Shelf Pump for CO₂ on the Adélie Land Coast, East Antarctica