RESEARCH FOCUS: Fuzzy seas

Abstract

The more we learn about something, the more complex it tends to become. This is nicely demonstrated by the example of calcite-aragonite seas. Secular changes in the prevalent mineralogy of abiotic calcium carbonate precipitates have long been known, since Sandberg's (1983) discovery of Phanerozoic oscillations. Abiotic carbonates (e.g., oolites, cements) precipitate in equilibrium with ambient water, thus their composition may inform us about the chemical state of seawater. Three periods of aragonite seas alternating with two of calcite seas from the latest Precambrian to today have been recognized. Sandberg was wrong about the driver-he favored CO 2 levels over Mg/Ca-but he got the pattern right. The hypothesis that the Mg/Ca ratio in seawater was a dominant control, not only of the mineralogy of abiotic precipitates (Hardie, 1996) but perhaps also of the prevalence of sediment-producing calcifiers and reef builders (Stanley and Hardie, 1998), has led to renewed interest in these trends in the late 1990s. Morse et al. (1997) showed experimentally that temperature may be important, but most authors embraced tectonically driven fluctuations in Mg/Ca as prominent, largely ignoring temperature. Balthasar and Cusack (2015, p. 99 in this issue of Geology) use an advanced experimental approach to revisit the temperature-versus-Mg/Ca dependence in precipitation of abiotic calcium carbonate phases. Surprisingly, and in contradiction of previous work, they find that aragonite and calcite co-exist over a wide range of experimental conditions. There is a gradual change in the proportion of mineral phases with variation in Mg/Ca and temperature, rather than a threshold at which mineral phases change from one to the other. Also surprising is the weak influence of CaCO 3 saturation state and pCO 2 . The calcite content in experimental precipitates increases at lower temperatures and Mg/Ca, but boundary conditions for pure calcite seas are so extreme that they are unlikely to have been met during the past 540 m.y. For example, aragonite proportions <1% would be expected at Mg/Ca of 1 and temperatures of <15 °C. Such low Mg/Ca may have occurred in extreme "calcite seas," but the low temperatures were unlikely in the Phanerozoic tropics. Balthasar and Cusack emphasize that substantial geographic variation in aragonite/calcite proportions must have existed, especially in calcite seas. This agrees with common observations of aragonitic ooids in tropical calcite seas (Prasada Rao, 1990;Adabi, 2004). Pure aragonite seas are more plausible given the experimental results. Abiogenic aragonite thus would more commonly occur in calcite seas than calcite in aragonite seas. Sandberg (1983) realized that aragonite and calcite may co-occur under aragonite-facilitating conditions, but probably underestimated their overlap. Balthasar and Cusack's results, combined with differences between Mg/Ca models and uncertainties in paleoclimates, make the classical concept of calcite and aragonite seas fuzzy: there is little black and white, but a lot o...

Bimineralic Middle Triassic ooids from Hydra Island: Diagenetic pathways and implications for ancient seawater geochemistry

“…18-30°C; Higuchi et al, 2017), it is believed that the narrow range of temperatures seen over the Middle Triassic (26-27°C; Song et al, 2019) was a subordinate control on these short-lived chemical changes. Aragonite proportions of <1% would be expected at a Mg/Ca ratio of 1 and temperatures of <15°C (Kiessling, 2015), whereas a ratio of less than 0.4 at 25°C is necessary for calcite to form (Morse et al, 1997). Accordingly, the dominant distribution of investigated bimineralic ooids during the Lower to Middle Anisian (Middle Triassic) is attributed to high Mg/Ca seawater ratios, principally controlled by low hydrothermal brine fluxes, which caused higher precipitation rates for aragonite and high-Mg calcite relative to calcite.…”

Temperature dependence of aragonite and calcite skeleton formation by a scleractinian coral in low mMg/Ca seawater

“…As in a previous report on inorganic precipitation (Balthasar and Cusack, 2015), polymorphism of CaCO 3 by corals in low mMg/Ca seawater is shown here to be dependent upon seawater temperature. Kiessling's (2015) linear regression model to estimate temperature (T, in °C) from known percentages of abiogenic aragonite (A) and seawater mMg/Ca (R 2 = 0.73), when converted to determine aragonite percentages [i.e., A = (T + 8.5 × mMg/Ca -26.6)/0.18], shows that most of the present results were above the line of theoretical inorganic precipitation of aragonite (Fig. 2).…”

“…However, the biological responses of skeletal organisms to environmental changes during the Phanerozoic Eon are still unknown; there have been no experimental studies on the effects of synergistic changes in both temperature and mMg/Ca using skeletal organisms. As suggested by Kiessling (2015), studies on the skeletal formation of CaCO 3 during manipulation of both temperature and mMg/Ca are required. Here we report the temperature dependence of aragonite and calcite formation by scleractinian corals in low mMg/Ca seawater.…”