Continental shelf pump

In oceanic biogeochemistry, the continental shelf pump is proposed to operate in the shallow waters of the continental shelves, acting as a mechanism to transport carbon (as either dissolved or particulate material) from surface waters to the interior of the adjacent deep ocean.^[1]

Overview

Originally formulated by Tsunogai et al. (1999),^[1] the pump is believed to occur where the solubility and biological pumps interact with a local hydrography that feeds dense water from the shelf floor into sub-surface (at least subthermocline) waters in the neighbouring deep ocean. Tsunogai et al.'s (1999)^[1] original work focused on the East China Sea, and the observation that, averaged over the year, its surface waters represented a sink for carbon dioxide. This observation was combined with others of the distribution of dissolved carbonate and alkalinity and explained as follows :

• the shallowness of the continental shelf restricts convection of cooling water
• as a consequence, cooling is greater for continental shelf waters than for neighbouring open ocean waters
• this leads to the production of relatively cool and dense water on the shelf
• the cooler waters promote the solubility pump and lead to an increased storage of dissolved inorganic carbon
• this extra carbon storage is augmented by the increased biological production characteristic of shelves^[2]
• the dense, carbon-rich shelf waters sink to the shelf floor and enter the sub-surface layer of the open ocean via isopycnal mixing

Significance

Based on their measurements of the CO₂ flux over the East China Sea (35 g C m⁻² y⁻¹), Tsunogai et al. (1999)^[1] estimated that the continental shelf pump could be responsible for an air-to-sea flux of approximately 1 Gt C y⁻¹ over the world's shelf areas. Given that observational^[3] and modelling^[4] of anthropogenic emissions of CO₂ estimates suggest that the ocean is currently responsible for the uptake of approximately 2 Gt C y⁻¹, and that these estimates are poor for the shelf regions, the continental shelf pump may play an important role in the ocean's carbon cycle.

One caveat to this calculation is that the original work was concerned with the hydrography of the East China Sea, where cooling plays the dominant role in the formation of dense shelf water, and that this mechanism may not apply in other regions. However, it has been suggested^[5] that other processes may drive the pump under different climatic conditions. For instance, in polar regions, the formation of sea-ice results in the extrusion of salt that may increase seawater density. Similarly, in tropical regions, evaporation may increase local salinity and seawater density.

The strong sink of CO₂ at temperate latitudes reported by Tsunogai et al. (1999)^[1] was later confirmed in the Gulf of Biscay,^[6] the Middle Atlantic Bight^[7] and the North Sea.^[8] On the other hand, in the sub-tropical South Atlantic Bight reported a source of CO₂ to the atmosphere.^[9]

Recently, work^[10][11] has compiled and scaled available data on CO₂ fluxes in coastal environments, and shown that globally marginal seas act as a significant CO₂ sink (-1.6 mol C m⁻² y⁻¹; -0.45 Gt C y⁻¹) in agreement with previous estimates. However, the global sink of CO₂ in marginal seas could be almost fully compensated by the emission of CO₂ (+11.1 mol C m⁻² y⁻¹; +0.40 Gt C y⁻¹) from the ensemble of near-shore coastal ecosystems, mostly related to the emission of CO₂ from estuaries (0.34 Gt C y⁻¹).

An interesting application of this work has been examining the impact of sea level rise over the last de-glacial transition on the global carbon cycle. During the last glacial maximum sea level was some 120 m lower than today. As sea level rose the surface area of the shelf seas grew and in consequence the strength of the shelf sea pump should increase. Rippeth et al (2008) examine the consequences of this sea level rise on the strength of the shelf sea pump.

Rippeth TP, Scourse JD, Uehara, K (2008). Impact of sea-level rise over the last deglacial transition on the strength of the continental shelf CO(2) pump, GEOPHYSICAL RESEARCH LETTERS, 35(24), L24604 DOI: 10.1029/2008GL035880

1. ^ ^{a b c d e} Tsunogai, S.; Watanabe, S.; Sato, T. (1999). "Is there a "continental shelf pump" for the absorption of atmospheric CO₂". Tellus, Ser. B 51 (3): 701–712. doi:10.1034/j.1600-0889.1999.t01-2-00010.x. 
2. ^ Wollast, R. (1998). Evaluation and comparison of the global carbon cycle in the coastal zone and in the open ocean, p. 213-252. In K. H. Brink and A. R. Robinson (eds.), The Global Coastal Ocean. John Wiley & Sons.
3. ^ Takahashi, T.; Sutherland, S. C.; Sweeney, C.; Poisson, A.; Metzl, N.; Tilbrook, B.; Bates, N.; Wanninkhof, R. et al. (2002). "Global sea-air CO₂ flux based on climatological surface ocean pCO₂, and seasonal biological and temperature effects". Deep-Sea Res. Pt. II 49 (9–10): 1601–1622. doi:10.1016/S0967-0645(02)00003-6. http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VGC-452W7KK-2&_user=10&_coverDate=12%2F31%2F2002&_rdoc=2&_fmt=summary&_orig=browse&_srch=doc-info(%23toc%236035%232002%23999509990%231%23FLA%23display%23Volume)&_cdi=6035&_sort=d&_docanchor=&view=c&_ct=19&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=1eb682d5552c8d58911ddf6e2012da02. 
4. ^ Orr, J. C.; Maier-Reimer, E.; Mikolajewicz, U.; Monfray, P.; Sarmiento, J. L.; Toggweiler, J. R.; Taylor, N. K.; Palmer, J. et al. (2001). "Estimates of anthropogenic carbon uptake from four three-dimensional global ocean models". Global Biogeochem. Cycles 15: 43–60. Bibcode 2001GBioC..15...43O. doi:10.1029/2000GB001273. 
5. ^ Yool, A.; Fasham, M. J. R. (2001). "An examination of the "continental shelf pump" in an open ocean general circulation model". Global Biogeochem. Cycles 15 (4): 831–844. Bibcode 2001GBioC..15..831Y. doi:10.1029/2000GB001359. 
6. ^ Frankignoulle, M.; Borges, A. V. (2001). "European continental shelf as a significant sink for atmospheric carbon dioxide". Global Biogeochemical Cycles 15: 569–576. 
7. ^ DeGrandpre, M. D.; Olbu, G. J.; Beatty, C. M.; Hammar, T. R. (2002). "Air-sea CO₂ fluxes on the US Middle Atlantic Bight". Deep-Sea Research Part II 49 (20): 4355–4367. doi:10.1016/S0967-0645(02)00122-4. 
8. ^ Thomas, H.; Bozec, Y.; Elkalay, K.; Baar, H. J. W. De (2004). "Enhanced open ocean storage of CO₂ from shelf sea pumping". Science 304 (5673): 1005–1008. doi:10.1126/science.1095491. PMID 15143279. 
9. ^ Cai, Wei-Jun (2003). "The role of marsh-dominated heterotrophic continental margins in transport of CO₂ between the atmosphere, the land-sea interface and the ocean". Geophysical Research Letters 30 (16). Bibcode 2003GeoRL..30pOCE3C. doi:10.1029/2003GL017633. 
10. ^ Borges, A. V. (2005). "Do we have enough pieces of the jigsaw to integrate CO₂ fluxes in the Coastal Ocean?". Estuaries 28: 3–27. doi:10.1007/BF02732750. 
11. ^ Borges A. V., B. Delille and M. Frankignoulle (2005). Budgeting sinks and sources of CO₂ in the coastal ocean: Diversity of ecosystems counts, Geophysical Research Letters 32, L14601, doi:10.1029/2005GL023053.

• Biological pump
• Ocean acidification
• Solubility pump