Alkalinity and dissolution of calcium carbonate particles in the ocean
The figure below shows the depth at which calcite (top) and aragonite (bottom) among calcium carbonate particles become saturated (【Ca2+】×【CO32-】/ Ksp = 1) according to the carbon cycle model. Shallower than this depth, calcium carbonate particles will not dissolve because they are undersaturated. In the North Atlantic, calcite is undersaturated (dissolves) at depths below 4500 m, indicating that it is not dissolved in deep water down to about 4000 m depth. In the North Pacific, calcite is undersaturated (dissolved) at depths below 500 m.
Top：Figure 1 of A. Yool, E. E. Popova, and T. R. Anderson, Geosci. Model Dev. Discuss., 6, 1259–1365, 2013., www.geosci-model-dev-discuss.net/6/1259/2013/ doi:10.5194/gmdd-6-1259-2013, © Author(s) 2013. CC Attribution 3.0 License.
Bottom：Figure 1 of R. Gangstø, M. Gehlen1, B. Schneider, L. Bopp, O. Aumont, and F. Joos, Biogeosciences, 5, 1057–1072, 2008, www.biogeosciences.net/5/1057/2008/, © Author(s) 2008. , This figure is distributed under the Creative Commons Attribution 3.0 License.
General Characteristics
As shown in the figure above, Ksp values also vary depending on the crystal structure of calcium carbonate (calcite or aragonite). It is known that the more robust calcite (shells of coccolithophorids and foraminifera) does not dissolve in the bottom waters of the North Atlantic and that undersaturated areas extend from the South Atlantic bottom waters. Calcite, on the other hand, is dissolved in South Atlantic bottom waters (depths below 4000 m), South Pacific deep waters (depths below 3500 m), and North Pacific intermediate waters (depths below 800 m).
If it is aragonite (the skeleton of pteropods and corals), it will dissolve in North Atlantic deep waters. One reason why there is an increase in alkalinity from the North Atlantic Deep to the South Atlantic Deep is due to the dissolution of aragonite. The Atlantic Ocean has a central ridge, along which the depth of calcite saturation is shallower. The reason may be that many organic particles reach the relatively shallow seafloor, and organic decomposition in the sediments lowers the pH, leading to increased dissolution of calcium carbonate particles (this interpretation includes estimation, and we will update the content when we have more solid information).
Furthermore, in the northern Bering Sea, which is the outlet of the North Pacific, on the continental shelf (about 200 m depth) and on the continental shelf of the Chukchi Sea in the Arctic (depths shallower than 100 m), even surface water is undersaturated with aragonite. In shelf waters where aragonite can originally dissolve, increased atmospheric carbon dioxide and freshwater inflow※ could jeopardize the survival of aragonite-bearing pteropods (swimming mollusks). The effects of "ocean acidification" in the Arctic region "ocean acidification" in the Arctic region” have been the focus of much attention in recent years.
※ An increase in anthropogenic carbon dioxide will decrease the pH of seawater and lower CO32- concentrations. It is also assumed that as warming increases due to increased anthropogenic carbon dioxide, freshwater inflow (glaciers and sea ice melting) will increase at high latitudes. Higher freshwater ratios will result in lower Ca2+ and CO32- concentrations. The double punch of these effects is thought to accelerate the dissolution of aragonite shells.