Abstract
Biogeochemical cycling in the Southern Ocean (SO) plays a key role in the global sea‐air CO2balance and in the ocean anthropogenic carbon inventory (Ito et al., 2010; Khatiwala et al., 2009; Sarmiento et al., 2004). Some previous studies suggest a decreasing trend in the Southern Ocean carbon sink (Le Quéré et al., 2007; Lovenduski et al., 2007; Wetzel et al., 2005). We investigate the interannual and decadal variations in sea‐air CO2flux and phytoplankton production in the SO with hindcast simulations by an ocean biogeochemical model. Decreasing trends in sinking POC and primary production are found from 1979 to 2003, concurrent with a decreasing trend in carbon uptake from the atmosphere. Simulations show substantial interannual and decadal variability in productivity. The sea‐air CO2flux is significantly correlated with sinking POC, especially in high productivity regions of the Southern Ocean. Both mixed layer depths and iron concentrations are important to the long‐term trends in production and phytoplankton community structure. Sea ice cover also plays an important role at high latitudes. Variability in dust deposition in recent decades has little influence on total SO productivity and carbon uptake, however, there are regional impacts near dust source regions. Accurately representing mixed layer depths and their impacts on phytoplankton light stress are critical for understanding how climate change impacts SO ecosystems and biogeochemistry.
Highlights
[2] The Southern Ocean (SO) has been recognized as a key region for the global carbon cycle, accounting for $40% of the anthropogenic CO2 sink [Gruber et al, 2009; Khatiwala et al, 2009; Sabine et al, 2004]
It was suggested that dust storms in the Australian sector of the SO had resulted in significant elevations of phytoplankton abundance and a strong CO2 drawdown was associated with the dust events [Gabric et al, 2010]
Other studies highlighted the importance of dust deposition in phytoplankton carbon fixation and interannual variations in air-sea CO2 fluxes in the SO [Cassar et al, 2007; Patra et al, 2007]
Summary
[2] The Southern Ocean (SO) has been recognized as a key region for the global carbon cycle, accounting for $40% of the anthropogenic CO2 sink [Gruber et al, 2009; Khatiwala et al, 2009; Sabine et al, 2004]. Many observations have shown interannual, and regional variability in phytoplankton community composition, biomass and productivity [Arrigo et al, 2008; Prézelin et al, 2004; Smith and Comiso, 2008] Some of this variability was related to physical forcing, such as changes in the SAM and sea ice dynamics [Arrigo et al, 2008; Smith and Comiso, 2008; Vernet et al, 2008]. Recent modeling results suggested that Southern Ocean biogeochemistry was more heavily influenced by sedimentary iron sources than dust deposition [Moore and Braucher, 2008; Tagliabue et al, 2010]. Reconstructed dust deposition in the second half of the 20th century [Mahowald et al, 2010] is used to investigate dust impacts on the SO
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