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Biological Skeletal Carbonate Records Changes in Major-Ion Chemistry of Paleo-Oceans

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Biological Skeletal Carbonate Records Changes in Major-Ion Chemistry of Paleo-Oceans Commentary Biological skeletal carbonate records changes in major-ion chemistry of paleo-oceans Isabel P. Montan˜ez* Department of Geology, University of California, Davis, CA 95616 he history of the chemical evolution of clusion evidence for secular change in the A secular trend in marine potash Tseawater is of first-order importance major-ion chemistry of seawater over the evaporites (salt deposits characterized by given its fundamental role in a broad last 550 million years (13), argue strongly potassium chloride and͞or potassium sul- spectrum of geologic, geochemical, and for the concurrent trends in nonskeletal fate minerals) that is in-phase with that of paleontologic phenomena. A growing and skeletal chemical precipitates as a Phanerozoic nonskeletal carbonates (Fig. body of evidence for a more dynamic testament of the magnitude to which the 1) may provide further support for the role evolution of seawater chemistry than pre- basic seawater signal can be changed. of seawater Mg͞Ca ratio as the primary viously considered has been building over These results also have far-reaching influence on the mineralogy and compo- the past two decades. Central to this body implications for the role of seawater sition of these marine chemical precipi- of evidence are oscillating global trends, chemistry in influencing the biologi- tates (2). This reflects that changes in on a 100- to 200- cal composition of other environmental parameters that can million-year time reefs and the evolu- control the carbonate mineralogy, which scale, in the miner- tionary changes in precipitates from seawater, such as pCO2, alogy of marine The degree to which the basic biomineralization 2Ϫ [CO3 ], and temperature (8, 20, 21), carbonate cements seawater signal has varied over through time. should have little or no effect on the (1), late-stage salts During the Pha- the past 550 million years is mineralogy of marine evaporites. Varia- in marine evapor- nerozoic, there have tions in the rate of seawater cycling an issue that has been ites (2), and cal- been two periods of through midocean ridges in response to cifying organisms strongly debated. aragonite-inhibiting changes in the production of oceanic crust that are interpreted episodes (referred have been proposed as a viable mecha- to record secular to as ‘‘calcite seas’’) nism for driving secular variation in the change in seawater alternating with seawater Mg͞Ca ratio (2, 22). In this three aragonite-facilitating episodes chemistry (3, 4). The degree to which the model, it is suggested that even small (‘‘aragonite seas’’) (Fig. 1). The resulting basic seawater signal has varied over the variations (Ͻ25%) in the hydrothermal mineral trend is synchronous with long- past 550 million years is, however, an issue brine flux at midocean ridges may have a that has been strongly debated in the term sea-floor spreading rates, eustasy, plutonic emplacement, and volcanism (1), profound effect on the composition of literature (e.g., refs. 5–9). This primarily seawater. Modeled changes in the seawa- reflects the arguably equivocal nature of as well as climate cycles (14), leading to ͞ the long-held consensus that secular ter Mg Ca ratio (Fig. 1) are within the the evidence used to infer secular changes range of experimentally derived molar in seawater chemistry and the mecha- changes in marine nonskeletal carbonate mineralogy are mechanistically linked to ratios (1 to 5.5) shown to influence car- nisms for driving that change. Recon- bonate mineralogy, and are synchronous structions of seawater chemical composi- these processes, and thus ultimately a function of plate tectonics (refs. 9, 15, and with periods of aragonite and calcite seas. tion based on marine inorganic precipitate The hypothesis that seawater Mg͞Ca and fossil proxy records may be compro- 16, and references within). It has been suggested that the specific link is the ratio has been the primary influence on mised by the influence of various environ- Mg͞Ca molar ratio of seawater (2, 17, 18). the mineralogy of marine chemical pre- mental factors, and by the geochemical This strongly debated hypothesis is, in cipitates is, however, controversial given insults of diagenesis on their mineralogy part, founded in the experimental evi- that it requires substantial change in the and chemical composition (5, 10, 11). dence for the significant influence of the Mg͞Ca ratio of paleo-oceans, for which Complimentary modeling efforts that fo- solution Mg͞Ca molar ratio on carbon- there is no direct evidence. It has been cus on constraining the mechanism(s) for ate–mineral precipitation (8, †). In aque- argued that the effect of change in sea- driving substantial chemical change in ous ClϪ solutions typical of modern trop- floor spreading rate on the major-ion seawater are challenged by the uncertain- ical seawater, low-Mg calcite (Ͻ4 mol % chemistry of seawater, based on recon- ties associated with defining model input ͞ MgCO3) precipitates at Mg Ca ratios of structions of these rates from the geologic parameters (2, 5, 9). Յ1, whereas high-Mg calcite (Ͼ4 mol % record, may be of significantly lower mag- et al. In a recent issue of PNAS, Stanley MgCO3) precipitates at ratios above 1 and nitude than necessary to impart changes in (12) provide compelling, experimentally up to the present-day value of 5.2. Arago- the mineralogy of the scale observed in derived evidence that the skeletal carbon- nite, an orthorhombic polymorph of the Phanerozoic (5, 7). Moreover, mineral ate of biologically simple marine organ- CaCO3, precipitates along with high-Mg isms, analogous to inorganic marine car- calcite at solution Mg͞Ca ratios above 1.5 bonate, has the potential to record to 2. The mole % MgCO3 incorporated in See companion article on page 15323 in issue 24 of volume 99. faithfully changes in the major-ion chem- high-Mg calcite increases linearly with *E-mail: [email protected]. ͞ istry of paleo-oceans. Their results, in both ambient Mg Ca molar ratio and tem- †Fuchtbauer, H. & Hardie, L. A. (1976) Geol. Soc. Am. Abstr. concert with recently published fluid in- perature (8, 19, †). Prog. 8, 877 (abstr.). 15852–15854 ͉ PNAS ͉ December 10, 2002 ͉ vol. 99 ͉ no. 25 www.pnas.org͞cgi͞doi͞10.1073͞pnas.262659599 Downloaded by guest on September 28, 2021 organisms as an environmental dipstick of seawater Mg͞Ca ratios. Recently, Lowenstein et al. (13) docu- mented systematic changes in the chemis- try of evaporated seawater occluded within primary fluid inclusions of Pha- nerozoic marine halites (i.e., late-stage evaporites), thus providing the most de- finitive evidence to date for substantial change in the major-ion chemistry of sea- water over the past 550 million years. In particular, the empirically measured and modeled Mg͞Ca ratios overlap the range in ratios that is predicted by models of change in hydrothermal brine flux driven by changes in ocean crust production (Fig. 1). Moreover, these results indicate that the seawater Mg͞Ca ratio likely fluctu- ated throughout the late Precambrian and Phanerozoic within the range (1 to Ͼ4) necessary to impose secular variations in the mineralogy of marine chemical pre- cipitates. This finding appears to be cor- roborated by the in-phase relationship be- tween their reported seawater Mg͞Ca ratios (and Naϩ contents) and the ob- served fluctuations in the primary miner- alogy of marine inorganic chemical pre- cipitates and skeletal carbonates (Fig. 1). Stanley et al. (12) build on this evidence to test whether the fingerprint of system- atic fluctuation in paleo-seawater Mg͞Ca ratios is recorded in the skeletal mineral- ogy and Mg content of calcifying marine organisms. They document through the growth of extant coralline algae in syn- COMMENTARY thetic seawater, over a range of Mg͞Ca Fig. 1. Temporal relationship between secular fluctuations in nonskeletal carbonate (aragonite and molar ratios, that the ambient Mg͞Ca calcite seas) (1), climate cycles (14), late-stage marine potash evaporites (2), and the skeletal mineralogy ratio influences the skeletal mineralogy of of major sediment producers and reef builders (3, 4) throughout the Phanerozoic. Also shown is predicted some simple organisms, thus mimicking secular variation in seawater Mg͞Ca molar ratios based on changes in the hydrothermal brine flux at ͞ the pattern for nonskeletal precipitation. midocean ridges (blue line) (2). Measured and estimated Mg Ca mole ratios of paleo-seawater derived Significantly, their results indicate that the from fluid-inclusions in marine halites (vertical bars) are superimposed on the predicted seawater Mg͞Ca trend (13). Note that minor adjustments were made to account for differences in time-scales used between genus Amphiroa, which in today’s arago- references. nitic oceans produces a high-Mg calcite skeleton, will precipitate low-Mg calcite when grown in ambient seawater with a proxies of inferred major changes in sea- in this skeletal carbonate trend with that Late Cretaceous Mg͞Ca ratio of 1 to 2.3. water Mg͞Ca ratios may record the mod- defined by nonskeletal carbonates (and AMg͞Ca ratio of 1 has been shown ification of seawater by basinal- to global- evaporites) led them to propose that the experimentally to induce precipitation of scale processes, such as the dolomitization influence of seawater Mg͞Ca ratio on low-Mg calcite over other carbonate poly- of large expansive carbonate platforms carbonate-mineral precipitation could be morphs (8, †). during periods of high sea level, rather extended to the skeletal mineralogy of Three species of algae were grown in artificial seawater differing only in their than secular variation in the basic sea- dominant reef-builders and sediment pro- ͞ water signal (23). ducers through time. If this hypothesis Mg Ca molar ratios (from 1 to 5.8). All three species incorporated Mg contents in In the late 1990s, Stanley and Hardie (3, proves to be correct, then it has significant proportion to the ambient Mg͞Ca ratio, 4) recognized secular fluctuations in the paleoecologic and paleobiologic implica- thus indicating that their calcification was skeletal mineralogy of anatomically sim- tions.
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