Introduction 11 Carbonate Chemistry

Oceans have the capacity to absorb large amounts of Carbon dioxide (CO2) because CO2 dissolves and reacts in seawater to form bicarbonate (HCO3) and protons (H+). About a quarter to a third of the CO2 emitted into the

Climate Change: Observed Impacts on Planet Earth

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atmosphere from the burning of fossil fuels, cement manufacturing and land use changes has been absorbed by the oceans [1]. Over thousands of years, the changes in pH have been buffered by bases, such as carbonate ions (CO3 ). However, the rate at which CO2 is currently being absorbed into the oceans is too rapid to be buffered sufficiently to prevent substantial changes in ocean pH and CO32 . As a consequence, the relative seawater concentrations of CO2, HCO3 , CO3 and pH have been altered. Since pre-industrial times the oceans pH has decreased by a global average of 0.1 (compare Fig. 1a and b). The

Pre-industrial

Pre-industrial

Ocean Data

FIGURE 1 (a) Estimated pre industrial (1700s) sea surface pH and (b) present day (1990s) sea surface pH, both mapped using data from the Global Ocean Data Analysis Project [5] and World Ocean Atlas climatologies; however, in the absence of estimated pre industrial fields of temperature and salinity 1990s fields were used (although these contain a small signal from global warming). Note that GLODAP climatology is missing data in certain oceanic provinces (areas left white) including the Arctic Ocean, the Caribbean Sea, the Mediterranean Sea and the Malay Archipelago.

FIGURE 1 (a) Estimated pre industrial (1700s) sea surface pH and (b) present day (1990s) sea surface pH, both mapped using data from the Global Ocean Data Analysis Project [5] and World Ocean Atlas climatologies; however, in the absence of estimated pre industrial fields of temperature and salinity 1990s fields were used (although these contain a small signal from global warming). Note that GLODAP climatology is missing data in certain oceanic provinces (areas left white) including the Arctic Ocean, the Caribbean Sea, the Mediterranean Sea and the Malay Archipelago.

Continued

Future

Future

FIGURE 1 Cont'd (c) Predicted pH across the world's oceans for yr 2100 using the SOC model, which was part of the OCMIP 2 project [6] and used the IS92a CO2 scenario. Note that the pH scale is different in (c). Courtesy of Andrew Yool (National Oceanography Centre, Southampton).

Intergovernmental Panel on Climate Change (IPCC) [2], using IS92 CO2 emissions scenario, predicts that the pH of the surface ocean will decrease by as much as 0.4 by the year 2100 (Fig. 1c) and 0.77 by 2300 [3]. It will take tens of thousands of years for these changes in ocean chemistry to be buffered through neutralisation by calcium carbonate sediments and the level at which the ocean pH will eventually stabilise will be lower than it currently is [4].

The CO3 concentration directly influences the saturation, and consequently the rate of dissolution, of calcium carbonate (CaCO3) minerals in the ocean. The saturation state (O) is used to express the degree of CaCO3 saturation in seawater:

O = [Ca2+][CO2 ]/Kp where K*p is the solubility product for CaCO3 and [Ca2+] and [CO3 ] are the in situ calcium and carbonate concentrations, respectively. When O > 1, seawater is super-saturated with respect to mineral CaCO3 and the larger this value the more suitable the environment will be for organisms that produce CaCO3 (shells, liths and skeletons). When O < 1, seawater is under-saturated and corrosive to CaCO3. Currently, the vast majority of the surface ocean is super-saturated with respect to CaCO3. The depth at which O = 1 is known as the saturation horizon. The three main mineral forms of CaCO3, in order of least soluble to most soluble, are calcite, aragonite and magnesium-calcite. Therefore, each mineral form has different saturation state profiles and saturation horizons with the aragonite saturation horizon (ASH) shallower than the calcite saturation horizon (CSH). Due to differences in ocean properties (salinity, temperature and pressure) both vary with latitude and ocean basin. The Southern Ocean has the lowest O, with Oaragonite currently reaching below 1.5. The depth of the ASH is 600 m or less in the North Pacific but can be over 2000 m deep in the North Atlantic. Increasing atmospheric CO2 will cause O to decrease, as has already been occurring since pre-industrial times [6].

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