Figure 1. The solubility carbon pump. MORE
Figure 2. The biological carbon pump. MORE
Figure 3. Anthropogenic carbon in the ocean. Source: Khatiwala et al, 2013. MORE
Figure 5. Annual chlorophyll climatology. Source: ESA Ocean Colour CCI MORE
Science background
The ocean plays a pivotal role in the global carbon cycle, taking up carbon dioxide from the atmosphere and storing it in the deep ocean for centuries. We still lack a detailed understanding of the ocean's carbon cycle, and this limits our ability to predict how ocean uptake of CO2 may be affected by climate change.
Carbon uptake and storage
Two 'carbon pumps' are key to the ocean's uptake of atmospheric carbon dioxide:
- The solubility pump is driven by air-sea transfer of CO2 and the sinking of high-latitude water rich in dissolved inorganic carbon (DIC) (fig.1).
- The biological carbon pump is driven by phytoplankton production of organic carbon and the sinking and re-mineralisation of organic material at depth (fig.2).
Dissolved inorganic carbon (DIC) is the sum of inorganic carbon species in the water. Inorganic carbon species include carbon dioxide (CO2), carbonic acid (H2CO3), bicarbonate anion (HCO3-), and carbonate (CO32-). More about inorganic carbon chemistry.
These transport over 100 Gt carbon per year into the deep ocean where it remains isolated from the atmosphere for centuries. This is a major control on atmospheric CO2 levels; estimates indicate that the biological pump alone keeps atmospheric CO2 about 200 ppm lower than it would otherwise be.[1] The ocean has absorbed approximately 25% of the CO2 emitted to the atmosphere by human activities, and the North Atlantic is a major repository of this anthropogenic carbon (fig. 3).[2] Understanding oceanic carbon uptake, storage and transport is key to predicting future atmospheric CO2 levels.
P. Parekh, S. Dutkiewicz, M. J. Follows and T. Ito (2006): Atmospheric carbon dioxide in a less dusty world. Geophysical Research Letters, 33, L03610. doi:10.1029/2005GL025098
S. Khatiwala, T. Tanhua, S. M. Fletcher, M. Gerber, S. C. Doney, H. D. Graven, N. Gruber, G. A. McKinley, A. Murata, A. F. Ríos, and C. L. Sabine (2013):
Global ocean storage of anthropogenic carbon.
Biogeosciences, 10, 2169-2191.
www.biogeosciences.net/10/2169/2013/bg-10-2169-2013.html
Trichodesmium can fix nitrogen MORE
Trichodesmium bloom MORE
Role of the subtropical North Atlantic
The North Atlantic subtropical gyre (fig.4) is a sink for CO2. Its low nutrient content (dark blue) makes it less productive than the temperate and subpolar North Atlantic, but its large size means that export of organic carbon from the gyre region makes a major contribution to carbon uptake.[3] Nitrogen fixation is also likely to contribute.[4]
The long term ability of the ocean to maintain its carbon uptake is linked to the horizontal transport of carbon and nutrients in the wind-driven surface layer.[5] This also applies to variability of carbon uptake at interannual to decadal time scales. Our understanding of the links between nutrient transport, climate-scale biogeochemical variability and carbon uptake is limited, and the theories of how the gyre contributes are largely untested. [6-9]
T. Takahashi, S.C. Sutherland, R. Wanninkhof, C. Sweeney, R.A. Feely, D.W. Chipman, B. Hales, G. Friederich, F. Chavez, C. Sabine, and others. (2009): Climatological mean and decadal changes in surface ocean pCO2, and net sea-air CO2 flux over the global oceans. Deep Sea Research Part II, 56, 554-577. doi:10.1016/j.dsr2.2008.12.009
C.M. Moore, M.M. Mills, E.P. Achterberg, R.J. Geider, J. LaRoche, M.I. Lucas, E.L. McDonagh, Xi Pan, A.J. Poulton, M.J.A. Rijkenberg, D.J. Suggett, S.J. Ussher and E.M.S. Woodward (2009): Large-scale distribution of Atlantic nitrogen fixation controlled by iron availability. Nature Geoscience, 2, 867-871. doi:10.1038/ngeo667
R.G. Williams and M.J. Follows (1998): The Ekman transfer of nutrients and maintenance of new production over the North Atlantic. Deep-Sea Research I, 45, 461-489. doi:10.1016/S0967-0637(97)00094-0
N.R. Bates (2001): Interannual variability of oceanic CO2 and biogeochemical properties in the Western North Atlantic subtropical gyre
Deep-Sea Research II, 48, 1507-1528.
doi:10.1016/S0967-0645(00)00151-X
J.B. Palter, M.S. Lozier and R.T. Barber (2005): The effect of advection on the nutrient reservoir in the North Atlantic subtropical gyre. Nature, 437, 687-692. doi:10.1038/nature03969
M.W. Lomas, N.R. Bates, R.J. Johnson, A.H. Knap, D.K. Steinberg and C.A. Carlson (2013): Two decades and counting: 24-years of sustained open ocean biogeochemical measurements in the Sargasso Sea. Deep-Sea Research II, 93, 16-32. doi:10.1016/j.dsr2.2013.01.008
A. Singh, M.W. Lomas and N.R. Bates (2013): Revisiting N2 fixation in the North Atlantic Ocean: Significance of deviations from the Redfield Ratio, atmospheric deposition and climate variability. Deep-Sea Research II, 93, 148-158. doi:10.1016/j.dsr2.2013.04.008
Uncertainty of carbon budgets
Many earlier studies of anthropogenic carbon transport in the Atlantic are based on estimates derived measurements made over several weeks during research cruises that are typically repeated every five years. Hence it is not possible to determine how carbon and nutrient fluxes vary with time, or assess how this temporal variability affects flux estimates. The conclusions drawn from such limited measurements remain uncertain.
Until recently this was also the case for the meridional transports of mass, heat and salt. In the last ten years the RAPID array at 26°N has changed this.
New insights from the RAPID array at 26°N
RAPID has shown how the Atlantic overturning circulation varies on seasonal,[10] interannual,[11] and longer[12] time-scales. Combining data from Argo floats and the RAPID array across the Atlantic at 26°N has revealed variability in the transport of heat,[13] salt and freshwater - all strongly correlated with the Atlantic meridional overturning circulation (AMOC).
Kanzow, T., Cunningham, S.A., Johns, W.E., Hirschi, J.J-M., Marotzke, J., Baringer, M.O., Meinen, C.S., Chidichimo, M.P., Atkinson, C., Beal, L.M., Bryden, H.L. and Collins, J. (2010): Seasonal variability of the Atlantic meridional overturning circulation at 26.5°N Journal of Climate, 23, 5678-5698. doi:10.1175/2010JCLI3389.1
McCarthy, G., Frajka-Williams, E., Johns, W. E., Baringer, M. O., Meinen, C. S., Bryden, H. L., Rayner, D., Duchez, A., Cunningham, S. A. (2012): Observed interannual variability of the Atlantic meridional overturning circulation at 26.5°N. Geophysical Research Letters, 39, L19609. doi:10.1029/2012GL052933.
Smeed, D.A., McCarthy, G.D., Cunningham, S.A., Frajka-Williams, E., Rayner, D., Johns, W.E., Meinen, C.S., Baringer, M.O., Moat, B., Duchez, A., Bryden, H.L. (2014): Observed decline of the Atlantic meridional overturning circulation 2004-2012. Ocean Science, 10, 29-38. doi:10.5194/os-10-29-2014.
W.E. Johns, M. O. Baringer, L. M. Beal, S. A. Cunningham, T. Kanzow, H. L. Bryden, J. J. M. Hirschi, J. Marotzke, C. S. Meinen, B. Shaw, R. Curry (2011): Continuous, Array-Based Estimates of Atlantic Ocean Heat Transport at 26.5°N Journal of Climate, 24, 2429-2449. doi:10.1175/2010JCLI3997.1.
The 10-year time series from the RAPID array is changing our understanding of the AMOC. For example, an apparent slow-down identified in 2005 on the basis of five research cruises between 1957 and 2004[14] is now revealed as the result of aliasing a seasonal signal.[10] RAPID has also shown that variability in the AMOC is greater than originally expected. Heat transport at 26°N is linked to variability in the oceanic heat content to the north,[15-16] and thus to variability in European weather and climate.[17]
H.L. Bryden, H.R. Longworth and S.A. Cunningham (2005): Slowing of the Atlantic meridional overturning circulation at 25°N. Nature, 438, 655-657. doi:10.1038/nature04385
Kanzow, T., Cunningham, S.A., Johns, W.E., Hirschi, J.J-M., Marotzke, J., Baringer, M.O., Meinen, C.S., Chidichimo, M.P., Atkinson, C., Beal, L.M., Bryden, H.L. and Collins, J. (2010): Seasonal variability of the Atlantic meridional overturning circulation at 26.5°N Journal of Climate, 23, 5678-5698. doi:10.1175/2010JCLI3389.1
S.A. Cunningham, C.D. Roberts, E.Frajka-Williams, W.E. Johns, W. Hobbs, M.D. Palmer, D. Rayner, D.A. Smeed and G. McCarthy (2013):
Atlantic Meridional Overturning Circulation slowdown cooled the subtropical ocean.
Geophysical Research Letters, 40, 6202-6207.
doi:10.1002/2013GL058464
H.L. Bryden, B.A. King, G.D. McCarthy, and E.L. McDonagh (2014):
Impact of a 30% reduction in Atlantic meridional overturning during 2009-2010
Ocean Science, 10, 683-691.
doi:10.5194/os-10-683-2014
S.R. Taws, R. Marsh, N.C. Wells, J. Hirschi (2011): Re-emerging ocean temperature anomalies in late 2010 associated with a repeat negative NAO. Geophysical Research Letters, 38, L20601. doi:10.1029/2011GL048978
Links are emerging between the circulation, heat content and air-sea heat fluxes. Learning what drives the strength and variability of each will allow us understand and possibly predict extreme winter weather.[18] Similar linkages should be possible between ocean circulation and biogeochemical processes such as air-sea CO2 fluxes, the flow of nutrients, and variations in carbon uptake due to the biological carbon pump.
A. Maidens, A. Arribas, A.A. Scaife, C. MacLachlan, D. Peterson, J. Knight (2013): The Influence of Surface Forcings on Prediction of the North Atlantic Oscillation Regime of Winter 2010/11. Monthly Weather Review, 41 (11), 3801-3813. doi:10.1175/MWR-D-13-00033.1
Colony of the cynobacterium Trichodesmium
Nitrogen fixation
Nitrogen makes up 75% of the atmosphere and is an essential nutrient for plants, but most organisms cannot use its gaseous form (N2). Some marine blue-green algae (cyanobacteria) can absorb nitrogen from the atmosphere, turning it into ammonium (NH4) and nitrate (NO3) - forms readily available to plants.
Trichodesmium is one of the main nitrogen fixers. To fix the nitrogen, the bacteria use an enzyme called nitrogenase, which contains iron. The amount of iron available may be a limiting factor in the production of the enzyme. The major iron input to the tropical and subtropical Atlantic is dust blown from the Sahara Desert.
Bloom of Trichodesmium - also known as 'sea sawdust'.
Trichodesmium can form large blooms in nutrient-poor waters - usually when it has been calm for some time, and the surface temperature is over 20°C. The blooms release nitrogen and other nutrients, which then become available to other marine phytoplankton. As a result the blooms are important to the oceanic ecosystem and may contribute to an increase in overall plant productivity and carbon uptake.
Climate change is projected to increase stratification and reduce the depth of the wind-mixed surface layer. Both of these factors favour Trichodesmium blooms and may therefore increase their occurrence in the future.[1,2]
C.M. Moore, M.M. Mills, E.P. Achterberg, R.J. Geider, J. LaRoche, M.I. Lucas, E.L. McDonagh, Xi Pan, A.J. Poulton, M.J.A. Rijkenberg, D.J. Suggett, S.J. Ussher and E.M.S. Woodward (2009): Large-scale distribution of Atlantic nitrogen fixation controlled by iron availability. Nature Geoscience, 2, 867-871. doi:10.1038/ngeo667
B. Bergman, G. Sandh, S. Lin, H. Larsson and E.J. Carpenter (2013): Trichodesmium - a widespread marine cyanobacterium with unusual nitrogen fixation properties. FEMS Microbiology Reviews, 37, 1–17. doi:10.1111/j.1574-6976.2012.00352.x
Figure 1. The carbon solubility pump. Credit: NOC/V.Byfield. PDF available on request.
The carbon solubility pump
Cold, high-latitude water can hold more carbon dioxide than warmer water. If the water is under-saturated with CO2, gas molecules diffuse across the air-sea boundary, and may also enter the water from bubbles mixed down by breaking waves. In the surface ocean the gas reacts with water molecules to create carbonic acid [H2CO3], which in turn dissociates into hydrogen ions [H+] and bicarbonate ions [HC03-]. This transition allows more CO2 gas to be dissolved in the water.
The surface ocean exchanges CO2 gas with the atmosphere at relatively short time scales. Most of the anthropogenic carbon taken up by the ocean is stored in the deep ocean, where it can remain for centuries.
Deepwater formation occurs only in a few regions of the world - in the Southern Ocean, and in the subpolar North Atlantic. Here the cold, carbon-rich surface water becomes dense enough to sink into the deep ocean. North Atlantic Deep Water (NADW) moves southward at a depth of 3-5000 metres, eventually it crosses the equator, and flows into the Indian Ocean, and then the Pacific. The gradual return to the surface can take centuries.
Figure 2. The biological carbon pump. Credit: NOC/V.Byfield. PDF available on request.
The biological carbon pump (BCP)
Just like plants on land, the microscopic marine phytoplankton take up carbon dioxide [CO2] and water [H2O]from their surrounding and use energy from sunlight to turn it into glucose [C6H12] and oxygen [O2].
The glucose powers the metabolism of the plankton cell, and can be turned into other organic compounds. If enough nutrients are available the plankton will grow and multiply. Phytoplankton is the 'grass' of the sea - at the bottom of the marine food chain. Eventually the plankton and the animals that feed on them die and sink into deeper water, where they decompose.
The creating of organic carbon through photosynthesis, the sinking of organic matter and its subsequent decomposition in the deep ocean is known as the 'biological carbon pump'. It contributes to the ocean's uptake and storage of carbon dioxide, and keeps atmospheric CO2 about 200 ppm lower than it would be if the ocean were without life.
Figure 3. Global 2010 column inventories (mol m-2) of anthropogenic CO2. Credit: Adapted from
Khatiwala et al. 2013. (Author's permission pending).
Estimating ocean uptake and storage of anthropogenic carbon
Current research estimates that the ocean has taken up and stored about 45% of fossil fuel CO2 emissions over the industrial period. There is reasonably good agreement about this figure, but more uncertainty about how this carbon is distributed.
Being able to estimate the magnitude and variability of the storage and distribution of anthropogenic carbon (Cant) in the ocean is important for understanding the human impact on climate. Current estimates based on relatively sparse observations agree within their uncertainty, but there are considerable differences in the spatial distribution. Air-sea flux and interior transport of Cant are difficult to observe directly, but may be estimated by combining observations with numerical ocean circulation models. The results are highly dependent on the modeled circulation, with the spread due to different ocean models at least as large as that from the different methods used to estimate Cant from observations.
The 2013 review by Khatiwala and co-authors
highlights the importance of repeat measurements of hydrographic and biogeochemical parameters to estimate the storage of Cant in the presence of the variability in circulation.
Measurements of carbon and nutrients such as those planned by ABC will provide more frequent observations that will give a better understanding of how carbon fluxes
vary in the ocean. The data may be used to constrain ocean models and will ultimately lead to better estimates of Cant.
REFERENCE
Khatiwala, S., Tanhua, T., Mikaloff Fletcher, S., Gerber, M., Doney, S. C., Graven, H. D., Gruber, N., McKinley, G. A., Murata, A., Ríos, A. F., and Sabine, C. L. (2013): Global ocean storage of anthropogenic carbon, Biogeosciences, 10, 2169-2191, doi:10.5194/bg-10-2169-2013.
Figure 4. Schematic of the AMOC with concentrations of anthropogenic carbon concentrations at 24.5°N in 2010. ABC will deploy biogeochemical sensors (oxygen, pCO2, pH) and remote access sampling (RAS) on some of the RAPID moorings, as indicated in the figure. Credit: NOC/P.Brown & V.Byfield. [Larger version is available on request].
Estimated anthropogenic carbon in the subtropical North Atlantic
The anthrophogenic carbon concentrations shown in the figure above are calculated using the ΔC* anthropogenic carbon estimation technique [Gruber et al (1996)],
with measurements of total dissolved inorganic carbon, total alkalinity, nutrients and temperature/salinity made from the RRS Discovery at 24.5°N during January-February 2010 [Schuster et al, 2014].
These estimate has previously only been included in an assessment of increasing anthropogenic carbon in the water column, calculated using multiple techniques [Guallart et al (2015)].
REFERENCE
Schuster, U., Watson, A. J., Bakker, D. C. E., de Boer, A. M., Jones, E. M., Lee, G. A., Legge, O., Louwerse, A., Riley, J., and Scally, S.: Measurements of total alkalinity and inorganic dissolved carbon in the Atlantic Ocean and adjacent Southern Ocean between 2008 and 2010, Earth Syst. Sci. Data, 6, 175-183, doi:10.5194/essd-6-175-2014, 2014
Gruber, N., J. L. Sarmiento, and T. F. Stocker (1996), An improved method for detecting anthropogenic CO2 in the oceans, Global Biogeochemical Cycles, 10, 809-837, doi: 10.1029/96GB01608.
Guallart, E. F., et al. (2015), Trends in anthropogenic CO2 in water masses of the Subtropical North Atlantic Ocean, Progress in Oceanography, 131(0), 21-32, doi: http://dx.doi.org/10.1016/j.pocean.2014.11.006.
Figure 5. Annual chlorophyll climatology from the ESA Ocean Colour Climate Change Initiative.
Low productivity in the ocean's subtropical gyres
UNDER DEVELOPMENT