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Fuzzy Seas S RESEARCH FOCUS RESEARCH FOCUS: Fuzzy seas Wolfgang Kiessling* GeoZentrum Nordbayern, University of Erlangen-Nürnberg, Erlangen 91054, Germany The more we learn about something, the more complex it tends to calcite-aragonite co-exist in experiments, (Mg/Ca: 0.5–2.5; Balthasar and become. This is nicely demonstrated by the example of calcite-aragonite Cusack, their figure 1), we can derive a multiple linear regression model seas. Secular changes in the prevalent mineralogy of abiotic calcium to estimate temperature (T in °C) at a known percentage of abiogenic ara- carbonate precipitates have long been known, since Sandberg’s (1983) gonite (A) and seawater Mg/Ca: discovery of Phanerozoic oscillations. Abiotic carbonates (e.g., oolites, cements) precipitate in equilibrium with ambient water, thus their compo- T =+26.6 0.18*A – 8.5*Mg/Ca. (1) sition may inform us about the chemical state of seawater. Three periods of aragonite seas alternating with two of calcite seas from the latest Pre- The adjusted R2 for this regression is 0.73 (p < 0.001), sufficient to cambrian to today have been recognized. Sandberg was wrong about the justify more experimental work. Given the restrictions on Mg/Ca in the driver—he favored CO2 levels over Mg/Ca—but he got the pattern right. model, the proxy would mostly be applicable to calcite seas; that is, from The hypothesis that the Mg/Ca ratio in seawater was a dominant con- Cambrian Stage 2 to the Mississippian, and from the Late Jurassic to the trol, not only of the mineralogy of abiotic precipitates (Hardie, 1996) but Paleogene. Although Mg/Ca varies among models, fluid inclusions and perhaps also of the prevalence of sediment-producing calcifiers and reef other geochemical proxies provide independent evidence (Lowenstein et builders (Stanley and Hardie, 1998), has led to renewed interest in these al., 2001; Siemann, 2003), so that ambient Mg/Ca may soon be better trends in the late 1990s. Morse et al. (1997) showed experimentally that constrained than traditional “global” temperature. Quantifying the propor- temperature may be important, but most authors embraced tectonically tions of original aragonite and calcite in ancient samples is challenging, driven fluctuations in Mg/Ca as prominent, largely ignoring temperature. but criteria appear robust (Sandberg, 1983; Wilkinson et al., 1985). Balthasar and Cusack (2015, p. 99 in this issue of Geology) use an Can such fuzzy seas exert control on biomineralization and the evolu- advanced experimental approach to revisit the temperature-versus-Mg/Ca tionary success of organisms (Stanley and Hardie, 1998; Hautmann, 2006; dependence in precipitation of abiotic calcium carbonate phases. Surpris- Porter, 2007, 2010)? Mg/Ca influences the shell composition of marine ingly, and in contradiction of previous work, they find that aragonite and invertebrates (Ries, 2005, 2010; Ries et al., 2006), but these experiments calcite co-exist over a wide range of experimental conditions. There is kept temperature constant, so that we do not know the effects of changing a gradual change in the proportion of mineral phases with variation in temperature on skeletal mineralogy. If the results for abiotic carbonates Mg/Ca and temperature, rather than a threshold at which mineral phases are applicable to skeletal organisms, aragonitic organisms could thrive in change from one to the other. Also surprising is the weak influence of calcite seas, at least in the tropics and subtropics, whereas calcitic organ- CaCO3 saturation state and pCO2. The calcite content in experimental pre- isms in aragonite seas might have a harder time, for which there is some cipitates increases at lower temperatures and Mg/Ca, but boundary condi- evidence. Clades originating or acquiring skeletons in aragonite seas all tions for pure calcite seas are so extreme that they are unlikely to have had an original aragonitic mineralogy, but some aragonitic clades first been met during the past 540 m.y. For example, aragonite proportions appeared in calcite seas (Porter, 2010). This asymmetry may be stronger <1% would be expected at Mg/Ca of 1 and temperatures of <15 °C. Such than Porter acknowledged, as more aragonitic taxa than shown emerged low Mg/Ca may have occurred in extreme “calcite seas,” but the low tem- in the early Paleozoic calcite sea (Balthasar et al., 2011). We would also peratures were unlikely in the Phanerozoic tropics. Balthasar and Cusack expect a latitudinal gradient in skeletal mineralogy, with more calcite at emphasize that substantial geographic variation in aragonite/calcite pro- high latitudes. Indeed, modern and ancient cool-water carbonates are char- portions must have existed, especially in calcite seas. This agrees with acterized by predominantly calcitic skeletons (Nelson, 1988), whereas common observations of aragonitic ooids in tropical calcite seas (Prasada todays’ tropics are dominated by aragonitic organisms. Rao, 1990; Adabi, 2004). Pure aragonite seas are more plausible given The overall match between skeletal mineralogy and inferred oceanic the experimental results. Abiogenic aragonite thus would more commonly state, however, is not as good as often claimed, even when climate change is occur in calcite seas than calcite in aragonite seas. taken into account. More substantial changes in mineralogical proportions Sandberg (1983) realized that aragonite and calcite may co-occur are observed across mass extinction episodes than over the transition from under aragonite-facilitating conditions, but probably underestimated their one oceanic state to another (Kiessling et al., 2008). Aragonitic taxa increased overlap. Balthasar and Cusack’s results, combined with differences between in abundance after the Permian-Triassic mass extinction in an aragonite sea Mg/Ca models and uncertainties in paleoclimates, make the classical con- during substantial warming (Sun et al., 2012), in line with Balthasar and cept of calcite and aragonite seas fuzzy: there is little black and white, but Cusack’s findings. However, aragonitic taxa decreased in the warming ara- a lot of gray. gonite seas across the end-Triassic mass extinction, and increased in the The surprisingly strong impact of temperature on mineral phases calcite sea across the Cretaceous-Paleogene (K/Pg) boundary. According to might be developed into a paleotemperature proxy in deep time, if Mg/Ca Equation 1, and assuming Mg/Ca of 1.5, it would require a temperature in seawater and the proportion of aragonite in abiotic carbonates become increase of 15 °C to explain the 9% increase of epifaunal aragonitic taxa better constrained. Stable oxygen isotopes are still our major source for across the K/Pg (Kiessling et al., 2008), a time without dramatic long-term paleotemperature estimates, but have problems due to diagenetic impacts climate change (Kemp et al., 2014). Finally, factors independent of Mg/Ca (Pearson et al., 2007), and lack of knowledge of the oxygen isotopic may govern the fate of skeletal clades. For example, the Cretaceous coral- composition of seawater, which is dependent on continental ice volume to-rudist transition in reefal habitats was emphasized by Stanley and Hardie and may have changed over the Phanerozoic (Veizer et al., 1997). Where (1998) as forced by decreasing Mg/Ca (see also Ries et al., 2006). Stanley and Hardie argued that the bimineralic, dominantly calcitic rudist bivalves *E-mail: [email protected] did not directly benefit from the lowered Mg/Ca, but the aragonitic corals GEOLOGY, February 2015; v. 43; no. 2; p. 191–192 | doi:10.1130/focus022015.1 GEOLOGY© 2015 Geological | Volume Society 43 | ofNumber America. 2 For| www.gsapubs.org permission to copy, contact [email protected]. 191 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/43/2/191/3548234/191.pdf by guest on 02 October 2021 could not compete successfully in the calcite sea and thus declined relative Lowenstein, T.K., Timofeeff, M.N., Brennan, S.T., Hardie, L.A., and Demicco, to rudists. However, the rising middle Cretaceous temperatures (Wilson and R.V., 2001, Oscillations in Phanerozoic seawater chemistry: evidence from Norris, 2001; Forster et al., 2007) should have facilitated growth of the cor- fluid inclusions: Science, v. 294, p. 1086–1088, doi:10.1126/science.1064280. Morse, J.W., Wang, Q., and Tsio, M.Y., 1997, Influences of temperature and als, but they obviously did not. Warming may have imposed physiological Mg:Ca ratio on CaCO3 precipitates from seawater: Geology, v. 25, p. 85–87, stress on corals, whereas rudists were perhaps better adapted to warming, doi:10.1130/0091-7613(1997)025<0085:IOTAMC>2.3.CO;2. thus gaining competitive advantage. In summary, the experimental results Nelson, C.S., 1988, An introductory perspective on non-tropical shelf carbonates: for abiogenic calcium carbonate phases may not be directly transferable to Sedimentary Geology, v. 60, p. 3–12, doi:10.1016/0037-0738(88)90108-X. Pearson, P.N., van Dongen, B.E., Nicholas, C.J., Pancost, R.D., Schouten, S., organisms because physiology plays in. Singano, J.M., and Wade, B.S., 2007, Stable warm tropical climate through Resolving fuzzy-sea scenarios can be achieved in many ways. Most the Eocene Epoch: Geology, v. 35, p. 211–214, doi:10.1130/G23175A.1. importantly, calcite, aragonite, and gray states must be better constrained in Porter, S.M., 2007, Seawater chemistry and early carbonate biomineralization: time and space. Little progress has been made quantifying the proportional Science, v. 316, p. 1302, doi:10.1126/science.1137284. mineral abundance in oolites and cements since the mid-1980s. New obser- Porter, S.M., 2010, Calcite and aragonite seas and the de novo acquisition of car- bonate skeletons: Geobiology, v. 8, p. 256–277, doi:10.1111/j.1472-4669 vations need to be quantified more rigorously using point counting and, ide- .2010 .00246.x. ally, compiled in an online database, recording the stratigraphic as well as Prasada Rao, C., 1990, Petrography, trace elements and oxygen and carbon iso- the (paleo-)geographic setting.
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