ACID-CARBONATE SYSTEM
6.1 Initial Considerations
In this section we investigate the changes in the CO2-carbonic acid system of
seawater as coupled to atmospheric CO2 variations and weathering inputs from
land during the Phanerozoic Eon based mainly on modelling efforts . We recog- nise the fact that modelling has its drawbacks as expressed in the phrase “garbage in, garbage out”, but modelling constrained by both observational data and experimental information is in our opinion the most robust approach to under- standing the behaviour of complex systems like the Earth’s exogenic system or
ecosphere, including its ocean and atmosphere . For Fred these efforts in modelling
the Earth’s exosphere were initially a delightful collaboration, among others, on
two papers between him and his colleagues Bob Garrels and Abraham Lerman .
The first paper (Lerman et al ., 1975), using the phosphorus cycle as an example,
set up the mathematical methodology for the modelling of geochemical cycles .
The second paper was published in the American Scientist (Garrels et al ., 1976)
and considered the controls of atmospheric CO2 and O2 on the multi-million year
geologic time scale . Interestingly because of the generalised nature of the journal in which the paper was published, it was not a widely disseminated paper and a number of the ideas were not picked up on until later .
At the completion of the Garrels et al. (1976) paper involving the global
cycle of CO2 and O2, Fred diverted his attention somewhat to global models of
the behaviour of a variety of trace elements in the atmosphere and of the minor
elements aluminum and lithium in the ocean (e.g., Mackenzie et al ., 1979; Lantzy
and Mackenzie, 1979; Stoffyn and Mackenzie, 1982; Egli-Stoffyn and Mackenzie,
1984) . However at the same time, he encouraged his Ph .D . student, John Pigott,
to investigate the original mineralogy of ooids through the Phaerozoic Eon in the anticipation that their mineralogy might be a clue to the evolution of the
Earth’s ocean and atmosphere . This hypothesis was based on Philip Sandberg’s
initial comparison of the differences in the original mineralogy and fabric of Pleistocene and Recent Great Salt Lake ooids with those of Mississippian age
(1975) . John painstakingly looked at several hundred petrographic thin sections
of various Phanerozic carbonate rock units from a number of sedimentary basins . His work led to the dataset for the original depositional mineralogy of ooids
depicted in Figure 4 .9 . The results of this work were published in the Journal
of the Geological Society of London (Mackenzie and Pigott, 1981) and included a
model interpretation of the data that involved two alternating cycles of originally
calcite and aragonite/magnesian calcite mineralogies . John and Fred concluded
that these cycles were ultimately driven by global tectonic changes and labeled the cycles accordingly submergent and emergent tectonic cycles . The former were
hypothesised to be times of high plate accretion rates, high sea levels, and high
atmospheric CO2 levels; the latter the opposite . The former become the calcite
seas and the latter the aragonite seas of Sandberg (Sandberg, 1983) . This work
initiated Fred’s next stage in modelling the Earth’s exosphere discussed in further
sections of this article .
In part constrained by the observational and experimental data discussed previously and other considerations, numerous models have been constructed to
estimate changes in atmospheric carbon dioxide partial pressure (pCO2) during
the Phanerozoic Eon (e.g., Berner et al ., 1983; Mora et al ., 1996; Pearson and
Palmer, 2000; Wallmann, 2001; Nordt et al ., 2002; Demicco et al ., 2003; Edmond
and Huh, 2003; Hansen and Wallmann, 2003; Royer et al ., 2004; Haworth et al .,
2005; Guidry et al . 2007; Mackenzie et al ., 2008; Arvidson et al ., 2006, 2011; Berner,
2004, 2006; Royer, 2006; Li and Elderfield, 2013) . Estimates of maximum atmos-
pheric pCO2 values exceed 6,000 matm or more than 20 times the preindustrial
modern value at times (Fig . 6 .1) . For modern seawater with a salinity of 35‰
and a total alkalinity (TA) of 2 .3 mmol kg-1 at 0 oC and one atmosphere pres-
sure, seawater in equilibrium with this atmospheric CO2 concentration would
be undersaturated with respect to both calcite (Wc = 0 .54) and aragonite (Wa =
0 .36) at these conditions . It would have a pH of about 7 on the seawater pH scale
and a dissolved inorganic carbon content of 2 .44 mmol kg-1 . Equilibrium with
aragonite would be obtained only at an alkalinity of about 4 .0 mmol kg-1 . For
comparison, it should be noted that modern seawater composition of S = 35‰ at
25 oC and when at equilibrium with the atmosphere (pCO
2 = 390 matm) is about
4 times supersaturated with respect to aragonite and 6 times supersaturated with respect to calcite .
There also have been many model-based papers attempting to estimate
the impact of variable pCO2 values on seawater saturation state with respect to
carbonate minerals and the CO2-carbonic acid system parameters, such as pH,
during all or parts of the Phanerozoic Eon (e.g., Arvidson et al ., 2000, 2006, 2011;
Zeebe, 2001; Tyrrell and Zeebe, 2004; Hönish and Hemming, 2005; Riding and
Liang, 2005a,b; Locklair and Lerman, 2005; Guidry et al., 2007; Mackenzie et al .,
2008, 2011) . Seawater DIC, based on the MAGic (Mackenzie, Arvidson, Guidry
interactive cycles, see below for more detailed discussion of the model) model
estimates of Arvidson et al . (2006), may have undergone large relative changes
of up to about five times the current value of roughly 2 .2 mmol kg-1 (Fig . 6 .1) .
A significant amount, but not all of this variation, is believed to be the result of large changes in the partial pressure of atmospheric carbon dioxide . It should be
kept in mind that MAGic seawater chemistry is calculated using a one-box global
ocean reservoir with no proviso for geographical or depth variations in ocean chemistry . Also as we will see later, the depositional flux of dolomite to the sea floor significantly affects the DIC content of seawater through Phanerozoic time and this flux is problematic .
I n t he i n it ia l modelling runs using
t he M AGic model ,
Arvidson et al . (2006) also
attempted to estimate the saturation state of the global ocean with respect to calcite, aragonite, and dolomite . The results are shown in Figure 6 .2 as the modelled seawater ion activity product (IAP) to solubility product
(Ksp) ratio for aragonite
and dolomite (note this ratio for calcite is 1 .51 times that of aragonite at
25 oC) . Although absolute
values should be taken with caution, two impor- tant conclusions can be made from this prelimi- nary work: (1) during much of the Phanerozoic Eon, the saturation state of seawater with respect to carbonate minerals varied significantly, and (2) although the changes in calcite and aragonite saturation state must inherently track each other, dolomite does not always track aragonite saturation state because of other confounding factors that influence its saturation state such as the Mg concentration of seawater .
In a later synthesis section of this article, MAGic model results for the atmos-
phere, ocean, and sediment compositional changes during the Phanerozoic will be discussed in more detail .
Based on the observation that the calcite compensation depth (CCD), i.e.
the depth at which calcite is no longer found in the sediments, in the ocean has varied relatively little during the past 100 million years, an alternative modelling approach for a portion of the Phanerozoic Eon was used by Tyrrell and Zeebe (2004) to conclude that for this interval of time, the saturation state of seawater
with respect to CaCO3 has been close to constant . Based on this conclusion, the
carbonate ion concentration was estimated to have more than quadrupled since the Cretaceous Period . Surface ocean pH was calculated to have increased in a close to linear fashion from about 7 .5 in the Mesozoic to the modern value of 8 .2 . Model reconstructions of long-term changes in Phanerozoic mean ocean surface
Figure 6.1 Changes in dissolved inorganic carbon (DIC, heavy purple line; Arvidson et al., 2006) and
pCO2 during the Phanerozoic Eon (heavy dark
blue line from MAGic, Arvidson et al., 2006; thinner light green line from GEOCARB III, Berner and Kothavala, 2001).
pH have also been calcu- lated at 20 million year intervals by Ridgwell
(2005; Kump et al ., 2009;
Zeebe and Ridg well, 2011) .
Ba s ed on t he considerations described in this section, Table 6 .1 has been constructed for the estimated composi- tion of seawater at times of what appear to be the most extreme differences from modern seawater . The times chosen were the Albian Stage of the Cretaceous Period and the Ordovician Period (calcite-dolomite seas) . The data for the Albian
for K, Mg, Ca, and SO4
concent rat ions were primarily obtained from
Timofeeff et al . (2006) . The composition of the seawater was initially set at the Cl
concentration of modern seawater and the charge was balanced using Na+ . The
pCO2 estimate of Arvidson et al . (2006) was used to calculate the DIC concentra-
tion for their estimate of a supersaturation with respect to aragonite (Ωarag) of
10, resulting in a value of ~9 mmol kg-1 of DIC which is close to their estimate of
10 mmol kg-1 . The calculations were based on a Pitzer equation-based program
(e.g., Pitzer, 1973, 1975) . The calculations were also carried out for DIC for a Ωarag
= 1 . pH values were also calculated and are in reasonable agreement with the estimates of Zeebe (2001) for seawater pH during the Cretaceous .
The same general approach was taken with Ordovician seawater compo-
sition except Mg, Ca, and SO4 concentrations were estimated from the data of
Horita et al . (2002) . K and Cl concentrations of modern seawater were used and
the charge balance was obtained using Na+ . The pCO
2 estimate of Arvidson
et al. (2006) was used to calculate the DIC concentration for their estimate of a
supersaturation with respect to aragonite (Ωarag) of 10, resulting in a value of ~7
mmol kg-1 DIC which is lower than their estimate of ~10 mmol kg-1 . Discussions
of the details of the estimated composition of seawater for calcite-dolomite seas as given in the table are left for a further section .
Figure 6.2 The variation in the saturation state of seawater (IAP/Ksp) in equilibrium with the
atmosphere with respect to aragonite and dolomite. The saturation state with respect to calcite is 1.51 times that of aragonite (modi- fied from Arvidson et al., 2006).
Table 6.1
Estimated composition and related parameters of “calcite-dolomite seas” seawater for the Albian Stage of the Cretaceous Period and the Ordovician Period. Ion concentrations and DIC are mmol kg-1. pCO2 values are in matm.
See text for sources and discussion. Note that DIC and pH were calculated at both a Ωarag of 10 and 1 (in parentheses).
Albian (~106 Ma) Ordovician (~425 Ma)
Parameter Value % Mod SW Value % Mod SW
Na+ 425 86 443 90 K+ 11 100 11 100 Mg2+ 42 58 38 73 Ca2+ 36 214 28 272 Cl- 565 100 565 100 SO42- 9 39 7 25 DIC 9 (1) 409 (46) 7 (2) 318 (91) pCO2 2800 1000 5300 1893 pH 8.0 (7.2) Δ= 0.2 (1.0) 7.8 (7.1) Δ= 0.4 (1.1) Ω arag 10 (1) 2.5x (0.25x) 10 (1) 2.5x (0.25x) Mg/Ca 1.2 1.4