A re-evaluation of aragonite versus calcite seas

Adabi, Mohammad Hossein

doi:10.1007/bf03178476

Cited by 26 publications

(17 citation statements)

References 65 publications

(113 reference statements)

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“…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.…”

supporting

confidence: 92%

RESEARCH FOCUS: Fuzzy seas

Kiessling

2015

Geology

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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...

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supporting

confidence: 92%

RESEARCH FOCUS: Fuzzy seas

Kiessling

2015

Geology

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“…So the record in ridge crest CaCO 3 is more or less consistent with the Hardie [1996] reconstruction. But as already mentioned, other explanations for the mineralogy changes have been proposed [ Adabi , 2004].…”

Section: Evidence From Calcites From Ridge Flank Basaltsmentioning

confidence: 99%

What do we know about the evolution of Mg to Ca ratios in seawater?

Broecker

2011

Paleoceanography

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[1] Although most reconstructions of the evolution of the Mg to Ca ratio in seawater conclude that it has increased during the course of the Cenozoic, they disagree widely regarding the magnitude of this change. On the basis of fluid inclusion and CaCO 3 mineralogy observations, the increase was at least threefold. On the basis of Mg content of foraminifera shells it was only a factor of 1.7. A recently published reconstruction based on the Mg content of calcite fillings of voids in ridge flank basalts lends support to the conclusion that the change was severalfold. But as it is very difficult to come up with a plausible geologic scenario which could account for such a large change, we lean toward the smaller estimate based on the magnesium content of foraminifera shells.Citation: Broecker, W., and J. Yu (2011), What do we know about the evolution of Mg to Ca ratios in seawater?, Paleoceanography, 26, PA3203,

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“…Furthermore, it has been argued that Mg/Ca ratio is only one of several factors that control calcite versus aragonite seas. Alternative explanations for global changes in carbonate mineralogy were proposed, including (i) temperature influence (Burton & Walter, 1987;Adabi, 2004), (ii) effect of pCO 2 and subsequent changes in pH and alkalinity (Sandberg, 1983;Wilkinson et al, 1984;Lee & Morse, 2010), and (iii) the variations in SO 2À 4 concentration (Bots et al, 2011). However, despite these complications, changes in seawater Mg/Ca ratio are generally thought to have been the major driving force behind the observed patterns in Phanerozoic carbonate mineralogy.…”

Section: Introductionmentioning

confidence: 99%

Micro‐ to nanostructure and geochemistry of extant crinoidal echinoderm skeletons

et al. 2012

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This paper reports the results of micro- to nanostructural and geochemical analyses of calcitic skeletons from extant deep-sea stalked crinoids. Fine-scale (SEM, FESEM, AFM) observations show that the crinoid skeleton is composed of carbonate nanograins, about 20-100 nm in diameter, which are partly separated by what appears to be a few nm thick organic layers. Sub-micrometre-scale geochemical mapping of crinoid ossicles using a NanoSIMS ion microprobe, combined with synchrotron high-spatial-resolution X-ray micro-fluorescence (μ-XRF) maps and X-ray absorption near-edge structure spectroscopy (XANES) show that high Mg concentration in the central region of the stereom bars correlates with the distribution of S-sulphate, which is often associated with sulphated polysaccharides in biocarbonates. These data are consistent with biomineralization models suggesting a close association between organic components (including sulphated polysaccharides) and Mg ions. Additionally, geochemical analyses (NanoSIMS, energy dispersive spectroscopy) reveal that significant variations in Mg occur at many levels: within a single stereom trabecula, within a single ossicle and within a skeleton of a single animal. Together, these data suggest that physiological factors play an important role in controlling Mg content in crinoid skeletons and that great care should be taken when using their skeletons to reconstruct, for example, palaeotemperatures and Mg/Ca palaeo-variations of the ocean.

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A re-evaluation of aragonite versus calcite seas

Cited by 26 publications

References 65 publications

RESEARCH FOCUS: Fuzzy seas

RESEARCH FOCUS: Fuzzy seas

What do we know about the evolution of Mg to Ca ratios in seawater?

Micro‐ to nanostructure and geochemistry of extant crinoidal echinoderm skeletons

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