Ocean acidification studies and the uncertainties relevance on measurements of marine carbonate system properties.

Abstract

The global ocean has a key role on the Earth's climate system. It possesses a direct connection with the atmospheric gases, including the greenhouses, allowing exchanges between those compartments and oceanic storage of carbon. Through the years, this exchange of gases occurred based on gas equilibrium between ocean and atmosphere. After the Industrial Revolution, human activities have increased the emissions of greenhouse gases, mainly carbon dioxide (CO2), which changed the atmospheric concentration from ~280 ppm of CO2 to values as high as 391 ppm between c.a. 1750 and 2011 (Ciais et al., 2013). Recently, the measured CO2 atmospheric values are ranging near or above 400 ppm, as recorded by the Mauna Loa observatory, in Hawaii (daily CO2 measurements information available on www.scripps.ucsd.edu). A regional study in the south-southeast Brazilian continental shelf agrees with this value, which has measured an average of 396.7±2.5 ppm in the atmosphere during the spring of October 2014 (Kerr et al., 2016). This enhancement is reflected in the ocean, which has absorbed about 25% to 30% of the anthropogenic atmospheric CO2 emissions (Sabine and Tanhua, 2010; Le Quére et al., 2016). The CO2 uptake by the oceans directly affects the seawater chemistry and marine biogeochemical processes, impacting both the ecosystems and their respective biota (Doney et al., 2009).

Atmospheric CO2 can be transferred to seawater through the air-sea interface, where it dissolves and reacts with H2O forming the carbonic acid (H2CO3). The H2CO3 is a weak acid and promptly suffers two dissociation processes that release protons (H+): the first one originates bicarbonate ion (HCO3-); and the second one, carbonate ion (CO32-) (Figure 1). The term total dissolved inorganic carbon (DIC) is the sum of the three main inorganic forms of CO2 in seawater (i.e., CO2*, HCO3- and CO32-; the CO2* represents the sum of CO2(aq) and H2CO3 because the latter is rapidly dissociated in seawater), which can be also referred, in some works, as CT, TIC, TCO2 or ∑CO2 (Zeebe, 2012). The equilibrium system of the DIC species on seawater is known as the marine carbonate system and is the main responsible to drive the buffer capacity of the ocean. Thus, the chemical species of the carbonate system in seawater changes to equilibrate the H+ content, leaving the ocean with an average pH around 8.2 units (Zeebe and Wolf-Gladrow, 2001).