Abstract
Reliable measurements of the thermodynamics of the carbonate system are needed to better understand the CO2 system in natural waters. New measurements of the carbonic acid pK1* and pK2* in seawater have been made over a wide range of temperatures (1 to 50°C) and salinities. The commonly used CO2 constants of Mehrbach et al., (1973) were limited to salinities (19 to 43) and temperatures (2 to 35°C). They cannot be used to study estuarine or fresh waters. The results of these measured pK1* and pK2* values are in good agreement with those determined using the Miami Pitzer equations (Millero and Pierrot, 1998). The results in this dissertation can be demonstrates the validity of the model that can be used to study the carbonate system in most natural waters. The so called Miami model is presently being used to examine the effect of ocean acidification on natural waters. The boric acid effect on the dissociation constants in seawater and NaCl solutions was tested. The addition of boric acid has little or no effect on pK1* values. However, the values of pK2*, decreases with the addition of small amounts of boric acid to ASW in agreement with the work of Mojica-Prieto and Millero, (2002). The addition of larger concentrations of boric acid cause the values of pK2* to increase. These effects have been attributed to the interactions of boric acid with the carbonate ion (CO32-) in seawater (Mojica-Prieto and Millero, 2002). The addition of boric acid to NaCl solutions in contrast, caused the values of pK1* and pK2* to decrease. This has been attributed to the interactions of borate ions with Mg2+ and Ca2+ in seawater. Further measurements in Na-Mg-Cl and Na-Ca-Cl solutions are needed to prove that this is the case. The boric acid effect on the carbonate constants indicate that an increase in boric acid has no affect on pK1*, but does change the values of pK2*. At low concentrations of boric acid, pK*2 decreases, and at higher concentrations it increases. These results indicate that boric acid has some ionic interactions with the carbonate ion. Similar studies in NaCl indicate that both pK1* and pK2* decrease when boric acid is added. The differences between seawater and NaCl may be related to the interactions of Mg2+ and Ca2+ with borate anions. Further studies of NaCl with additions of MgCl2 and CaCl2 are needed to examine the effects in detail. Preliminary studies on the effect of DOC on the carbonate constants are not definitive. The change of the DOC concentration from 50 to 100 µmol kg-1 has little effect on the values of pK1* and pK2*. Dilutions of seawater with artificial seawater are complicated by changes in the concentration of boric acid. Earlier studies indicated that DOC may cause the 8 mu-mol kg-1 increase in total alkalinity of seawater needed to balance the thermodynamics of the system (Millero et al., 2002). This may be partially due to the new values for the B/Cl ratio in seawater found by Lee et al., (2010) that increases the TA by ~ 6 µmol kg-1. Further studies are needed to examine the effect of humic compounds in estuarine waters on the carbonate system. Measurements of pH or pCO2 along with TA and TCO2 can be used to separate the effect of organic ligands on TA. If DOC measurements are also made, one can relate the effect to organic ligands that can accept a proton. The cruises in the Little Bahama Banks show for the first time the active precipitation of CaCO3 (Bustos-Serrano et al., 2009). This causes measured decreases of TA, TCO2 and pH and increases in pCO2 in the whitings. This is in contrast to earlier studies on the Grand Bahama Banks where no active precipitation of CaCO3 was every found (Morse et al., 2003; Millero et al., 2005). The differences appear to be due to the movement of fresh saturated seawater from the Gulf Stream into the LBB. The Gulf Stream water enters the GBB in the winter, and the precipitation occurs on the suspended sediment over the year. Observations are needed on the Grand Bahama Banks in the winter and throughout the year to prove that this is the case.