Some of the basic underlying concepts about calcite-dolomite versus aragonite seas and seawater composition stem from a long series of experimental studies of calcite and aragonite nucleation and growth kinetics (for the most recent
review of studies relevant to the marine environment see Morse et al ., 2007,
which contains ~340 references) . Here we discuss some of the history of those experimental results . These results are discussed in more detail and expanded on in Section 7 . A central point is the strong inhibitory influence of dissolved Mg on calcite nucleation and growth which has been recognised for nearly 100 years (Leitmeier, 1909) and was extensively investigated from the mid-1950s to the
early-1970s (e.g., Monoghan and Lytle, 1956; Lippmann, 1960; Cloud, 1962; Curl,
1962; Simkiss, 1964; Taft and Harbaugh, 1964; Fyfe and Bischoff, 1965; Pytko- wicz, 1965, 1973; Chave and Suess, 1967; Bischoff, 1968; Bischoff and Fyfe, 1968; Berner, 1971; Wollast, 1971) . A similar influence of Mg on aragonite nucleation and precipitation was not observed by Berner (1975) . This led to several papers in the 1980s and 1990s arguing that the Mg to Ca ratio during times of calcite seas must be considerably lower (close to 1) than the present ratio of about 5 (see previous discussion) .
Experimental studies of calcite precipitation kinetics during the 1980s
demonstrated not only the influence of the Mg/Ca ratio but also that of SO4
on calcite precipitation
kinetics (Reddy et al ., 1981;
Mucci and Morse, 1983; Walter, 1986; Fig . 5 .1) . The difference in reaction rates caused by a change
from no SO4 to a concen-
tration close to that of seawater was similar to the influence of changing the Mg to Ca ratio from 1 to 5 . Walter (1986) also found that such a change in sulphate concentration at a Mg/Ca ratio of 1 had a much greater inhibitory influence on the aragonite precipitation rate than on the calcite rate causing a reversal in the reaction rate at a given calcium and carbonate ion activity
Figure 5.1 Influence of the dissolved Mg/Ca ratio and dissolved sulphate on the precipitation kinetics of calcite at ~25 oC and 1 atm. Ω is
the saturation state with respect to calcite. Based on the data of Reddy et al., 1981; Mucci and Morse, 1983; and Walter, 1986 (after Morse and Mackenzie, 1990).
product . It is interesting to note that this major influence by sulphate has attracted little attention in the literature on calcite-dolomite versus aragonite seas until
recently (Bots et al ., 2011) .
It has been widely observed that aragonite and calcite with significant Mg contents are dominant forms of calcium carbonate in warm tropical to subtrop- ical waters, but in cooler high latitude waters and at depth in the ocean, the Mg content becomes gradually lower and calcite of lower Mg content becomes
increasingly dominant (e.g., Chave, 1954; Lees and Butler, 1972; Schlager and
James, 1978; Nelson, 1988; Opdyke and Wilkinson, 1990; Rao and Jayawardane,
1994;Videtich, 1985) . These trends are due to both increasing pressure with
increasing depth, and most importantly decreasing temperature with depth and higher latitude, exerting some control on the formation and the preservation of calcium carbonate . Consequently, the combination of major ion seawater compo- sition, aqueous carbonate chemistry, temperature, and pressure appear to influ-
ence carbonate mineralogy . Morse et al . (1997) experimentally investigated the
combined influences of seawater Mg/Ca ratio and temperature on abiotic calcium carbonate precipitation . They found (Fig . 5 .2) that as temperature decreased, the Mg/Ca ratio at which calcite could precipitate instead of aragonite increased
such that at the current seawater Mg/Ca ratio and below about 8 oC, only calcite
would precipitate . A major influence of temperature and solution chemistry on the morphology of the aragonite precipitated was also observed .
Although the Mg content in both abiotic and biotic carbonate precipitates appears to decrease with increasing geographic latitude and ocean depth, the exact mechanism(s) controlling the Mg content is poorly understood . Marine calcifiers depositing aragonite contain almost no or very little magnesium (<1 mol%) and the same is true for open ocean calcite producers, which are mostly represented by pelagic calcifiers including coccolithophorids and foraminifera . Among organ- isms depositing Mg calcite of various compositions, ranging from a few mol% Mg to as much as 30 mol%, there are distinct differences between different species . Clearly, there is a strong taxonomic control on the magnesium content of calcitic skeletons (Chave, 1954), but the observed latitudinal trends suggest a direct or indirect control by environmental parameters such as aqueous carbonate chemistry, temperature, and light . One hypothesis suggests that variations in organism growth rates, which depend on environmental conditions and also are a function of the availability of food and nutrients, may explain the observed
latitudinal trend in skeletal Mg content (Moberly 1968; Mackenzie et al ., 1983) .
Fred encouraged his and Keith Chave’s Ph .D . student Kitty Agegian to investigate
this problem by some carefully performed mesocosm experiments in which the
coralline algal Porolithon gardineri was grown in a flowing seawater mesocosm
chamber subjected to differing temperatures and carbonate mineral saturation state conditions . These experimental manipulations confirmed that growth rate
may be important in determining the Mg content of the Porolithon gardieneri
but temperature and saturation state also have an effect . (e.g., Agegian, 1985;
Mackenzie and Agegian, 1989) . More recently Ries (2011) reached an opposite conclusion with respect to growth rate and its effect on the Mg content of coralline
algae . These variables are not necessarily mutually independent in nature, and it is not always easy to separate the influence of one variable from another on the Mg content of a biotic Mg-calcite phase . There is obviously still a strong need for some cleverly designed experiments that would elucidate the mechanistic controls on the Mg content in calcareous shells and skeletons .
Im ag e c ou rt es y o f C hi p C la rk , S m ith so ni an In st itu tio n ( ca ta lo g n um be r 1 04 90 8- 00 ; p ho to nu m be r: 9 7-35 09 6) . Im ag e c re dit : V ol ke r B et z.
Figure 5.2 (top) Influences of seawater dissolved Mg/Ca ratio and temperature on the mineralogy of the calcium carbonate nucleated (after Morse et al., 2007; based on Morse et al., 1997). Blue dots are aragonite; yellow dots calcite; green dots indicate first calcite precipitated followed by some later aragonite due to increasing Mg to Ca ratio with extent of precipitation. (bottom) Photographs of a calcite twined crystal from Deming, New Mexico, USA (left) and an aragonite specimen from Auvergne, France (right).
In Section 7 we use the experiments described here and others to comple- ment the observational evidence discussed above and modelling efforts below (Section 6) to arrive at a tentative picture of the evolution of the ocean- atmosphere- carbonate sediment system during Phanerozoic time .