Climatological distribution of aragonite saturation state in the global oceans

Abstract Aragonite saturation state (Ω arag ) in surface and subsurface waters of the global oceans was calculated from up‐to‐date (through the year of 2012) ocean station dissolved inorganic carbon (DIC) and total alkalinity (TA) data. Surface Ω arag in the open ocean was always supersaturated (Ω > 1), ranging between 1.1 and 4.2. It was above 2.0 (2.0–4.2) between 40°N and 40°S but decreased toward higher latitude to below 1.5 in polar areas. The influences of water temperature on the TA/DIC ratio, combined with the temperature effects on inorganic carbon equilibrium and apparent solubility product ( K ′ sp ), explain the latitudinal differences in surface Ω arag . Vertically, Ω arag was highest in the surface mixed layer. Higher hydrostatic pressure, lower water temperature, and more CO 2 buildup from biological activity in the absence of air‐sea gas exchange helped maintain lower Ω arag in the deep ocean. Below the thermocline, aerobic decomposition of organic matter along the pathway of global thermohaline circulation played an important role in controlling Ω arag distributions. Seasonally, surface Ω arag above 30° latitudes was about 0.06 to 0.55 higher during warmer months than during colder months in the open‐ocean waters of both hemispheres. Decadal changes of Ω arag in the Atlantic and Pacific Oceans showed that Ω arag in waters shallower than 100 m depth decreased by 0.10 ± 0.09 (−0.40 ± 0.37% yr −1 ) on average from the decade spanning 1989–1998 to the decade spanning 1998–2010.