Physical and chemical changes in the ocean over basin-wide zones and decadal or longer time-scales: perspectives on current and future conditions - Ocean Sustainability in the 21st Century - page 94

biological one, resulting in an overall positive feedback to climate change. Continued de-
velopment depends on the future ocean circulation for which model projections show a
large range, meaning great uncertainty.
As of 1994 (Sabine et al ., 2004 ) , over 50% of the anthropogenic carbon taken up by
the ocean was confined to the top 400 m, and was undetectable in much of the deep ocean.
Ittakesdecadesandcenturiesforthecarbontotransferintodeepoceanwaters.Thedeepest
penetration is found in the North Atlantic and the Antarctic waters related to the deep water
formations in these areas. At low latitudes, oxygen concentrations have been decreasing in
the thermocline 100-1000 m since the 1960s (Stramma et al ., 2008 ).
As carbon dioxide enters the ocean from the atmosphere, it partitions into dissolved
carbon dioxide gas, bicarbonate, and carbonate ions. The relative proportions of these three
components of 'dissolved inorganic carbon' (DIC) are determined by the pH. As more
anthropogenic carbon dioxide enters the ocean, the pH decreases (i.e. the ocean becomes
moreacidic), whichincreases thefraction ofdissolved carbondioxideandreduces thefrac-
tion of DIC existing as carbonate ions. This means that the partial pressure of carbon diox-
ide in the ocean increases, thus opposing the influx of carbon dioxide from the atmosphere
relative to what it would be before the pH started to decrease. The second effect, reduced
carbonate ion, means that the waters become less saturated with respect to carbonate ion,
making it more difficult for organisms with calcium carbonate skeletal structures to form
calcium carbonate and increase or maintain their skeletal structures.
Currently the surface layers of the ocean are saturated with calcite and aragonite, the
most common forms of calcium carbonate in marine organisms, but under-saturated below
the saturation horizon, starting at depths varying from about 200 m in parts of the high lat-
itudes and the Indian Ocean, to 3500 m in the Atlantic Ocean. Calcium carbonate dissolves
when it sinks below the saturation horizon. Because of the addition of anthropogenic car-
bon dioxide to the oceans, the saturation horizon has become shallower, especially at high
latitudes (e.g. Feely et al ., 2004 ) . Reduced alkalinity due to sea ice melt water may con-
tribute to the decrease in carbonate saturation state, evidenced in part of the Arctic Basin