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2 Effects of Ocean Acidification on the Chemistry of Seawater
Pages 23-44

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From page 23...
... This chapter reviews the current knowledge regarding the chemical changes brought about by the increasing CO2 -- labeled collectively as ocean acidification -- in the past, the present, and the future. It first discusses the principal processes that control the acidbase chemistry of seawater and the cycling of carbon in the ocean.
From page 24...
... A fraction of the particulate organic matter sinks below the surface where it is also decomposed, causing verti cal variations in the concentrations of inorganic carbon species and pH. The net result is a characteristic maximum in CO2 concentration and minima in pH and CO32­ concentration around 500 to 1,000 meters depth 1The proper notation for carbon dioxide gas is CO2.g; carbon dioxide dissolved in water is CO2.aq.
From page 25...
... For reasons discussed below, the vertical distribution of pH in the ocean varies with geographi cal location, particularly as a function of latitude; this is illustrated in the NorthSouth transect for the Pacific Ocean in Figure 2.2b. Another important process affecting the acidbase chemistry of sea water is the production of calcium carbonate (CaCO3)
From page 26...
... and constant total boric acid using constants from Dickson et al.
From page 27...
... (a) Vertical profiles typical of the midNorth Pacific showing variations of several seawater chemical parameters with depth.
From page 28...
... . Precipitation of CaCO3 at the surface lowers the ambient pH, while its dissolution at depth increases it, partially compensating for the inverse effects of the photosynthetic reduction of CO2 that raises pH in surface waters and lowers pH in deeper waters as CO2 is regenerated by metabolic oxidation.
From page 29...
... which shows CO 2 concen tration as a function of depth in a NorthSouth transect across the North Pacific Ocean. Upwelling around the equator increases CO2 concentration near the surface at low latitudes compared to values in mid latitudes.
From page 30...
... . The decrease in carbonate ion concentration, CO32­, that results from ocean acidification will lead to reduced rates of calcification, along with the a shoaling of the saturation horizons for calcium carbonate minerals to shallower depths, and a change in the marine calcium carbonate cycle.
From page 31...
... Calculations are made for constant alkalinity using constants from Dickson et al.
From page 32...
... . 2.2.1 Projections for Surface Waters Because the relationship between atmospheric CO2 and seawater car bonate chemistry is well understood, it is a simple matter to calculate the variations in average pH and inorganic carbon species concentrations in
From page 33...
... . Figure 2.6 shows the results of actual measurements of surface sea water chemistry at a station near Hawaii between 1998 and 2008.
From page 34...
... An illustration of the time lag between surface and deep ocean acidification is shown in Figure 2.8; according to these simple calculations, under a "businessas usual" scenario of CO2 emissions, it will take about 500 years longer for a 0.3 unit decrease to occur in deep waters compared to surface waters (Caldeira and Wickett, 2003)
From page 35...
... CHEMISTRYOFSEAWATER Figure 2-6 R01733
From page 36...
... Figure 2-7 R01733 uneditable bitmapped image
From page 37...
... 2.2.3 Projections for Coastal Waters The acidbase chemistry of coastal waters is much more complex than that of open ocean surface and deep waters. It is affected by fresh water and atmospheric inputs, the supply of both organic matter and algal nutrients from land, and processes in the underlying sediments.
From page 38...
... In both river dominated and upwelling dominated coastal regions, future trends in seawater carbon chemistry may also depend strongly on climate change that influences wind patterns, upwelling and river flow. In shallow waters, sediment dissolution can partly buffer acid inputs (Andersson et al., 2003; Thomas et al., 2009)
From page 39...
... CHEMISTRYOFSEAWATER FIgURE 2.9 Distribution of the depths of the undersaturated water (aragonite saturation < 1.0; pH < 7.75) on the continental shelf of western North America from Queen Charlotte Sound, Canada, to San Gregorio Baja California Sur, Mexico.
From page 40...
... Values for years 1765 and 1994 were computed from the global gridded data product GLODAP (Key et Figure al., 2004) , whereas the saturation state 2-102050 and 2100 are the median of for years 13 ocean general circulation models R01733 forced under the IPCC's IS92a "businessas usual" CO2 emission scenario (Orr et bitmapped uneditable al., 2005)
From page 41...
... . Under current rates of CO2 emissions, models project that surface waters of the Southern Ocean, the Arctic Ocean, and parts of the subarctic Pacific will become undersaturated with respect to aragonite by the end of this century, and, in some regions, as early as 2023 (Orr et al., 2005; Steinacher et al., 2009)
From page 42...
... Management strategies designed to sequester CO2 in the ocean could potentially exacerbate ocean acidification in intermediate or deep waters. Iron fertilization of surface waters has been suggested as a potential approach for boosting primary production in regions that are ironlimited, thus increasing the export of organic carbon to the subsurface as dis cussed in the next chapter (Boyd et al., 2007)
From page 43...
... But such mitigation strategies might be feasible on a local or regional scale.


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