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 chemistry 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 seawater 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 precipitates. This finding appears to be cor- roborated by the in-phase relationship between their reported seawater Mg͞Ca ratios (and Naϩ contents) and the observed fluctuations in the primary mineralogy of marine inorganic chemical precipitates and skeletal carbonates (Fig. 1). Stanley et al. (12) build on this evidence to test whether the fingerprint of systematic fluctuation in paleo-seawater Mg͞Ca ratios is recorded in the skeletal mineralogy 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. Previous workers, however, have induced from seawater simply through ple organisms such as algae, sponges and attributed secular change in the skeletal their photosynthetic consumption of CO2. corals. Their observations indicate that in mineralogy of microfossils and other an- Intriguingly, algae that were initially calcite seas, reefs were dominated by cal- atomically simple taxa to the complex grown in a solution with a modern sea- citic corals and stromatoporoids, whereas interplay of several physicochemical fac- water Mg͞Ca ratio or greater (5.8), and aragonitic sponges, corals and algae, as tors, as well as to possible vital or kinetic subsequently introduced to a solution with well as high-Mg calcitic red algae, were the effects (refs. 15 and 16, and references an ambient Mg͞Ca molar ratio of 2.5, major reef builders in aragonite seas (Fig. within). This finding implies that there incorporated progressively less Mg within 1). Aragonitic scleractinian corals and could be significant uncertainty associated several days until stabilizing at percent- high-Mg calcitic coralline algae dominate with using the skeletal mineralogy and ages close to those produced by nonskel- in today’s aragonite sea. The concurrence composition of fossil biologically simple etal precipitation. The lack of aragonite

Montan˜ez PNAS ͉ December 10, 2002 ͉ vol. 99 ͉ no. 25 ͉ 15853 Downloaded by guest on September 28, 2021 precipitation argues for an organic tem- increasing Mg͞Ca ratios during the failure of early Cenozoic aragonitic corals plate specificity for calcite (see ref. 24). Cenozoic. to have reestablished themselves as build- Thus, it seems likely that the influence The degree of ecologic stability of reef ers of massive reefs with the disappear- of seawater chemistry on the skeletal se- building organisms and, thus, the biologic ance of rudists from the reef ecologic cretion of biologically simple organisms composition of reefs through time may be niche despite considerable diversity in such as algae, sponges, corals, and bryo- a fickle function of the Mg͞Ca ratio of scleractinian corals at that time. zoans is a first-order influence on the paleo-seawater through its influence on The influence of fluctuating seawater ͞ observed Phanerozoic trends in biominer- skeletal mineralogy (3, 4). Biologically Mg Ca ratios on the carbonate productiv- alization, marine carbonate sedimenta- simple taxa have dominated reefs ity of major sediment producers (e.g., tion, and reef composition (3, 4) (Fig. 1). throughout Phanerozoic time, given that calcareous nannoplankton for deep-water The skeletons of morphologically simple their adaptive growth modes have pro- chalks) and the biologic composition of taxa that consist of high-Mg calcite today, vided them with a competitive edge in the shallow-water reef builders has major im- probably formed as low-Mg calcite in cal- reef ecosystem. However, the inability of plications for global C cycling and atmo- cite seas. Stanley et al. (12) suggest that major reef builders in the Phanerozoic to spheric pCO2 levels. This reflects the po- remodel their skeletons through resorp- tential of seawater Mg͞Ca ratios to impart the Mg-content of skeletal calcite of even tion during their ontogeny may have made significant changes in the partitioning of anatomically more complex taxa, such as them susceptible to the whims of fluctu- skeletal carbonate of varying mineralogy echinoderms and calcitic bryozoa, may ating seawater chemistry. Changes in their into shallow- and deep-water reservoirs. have been lower than present-day equiv- ͞ rates of calcification (i.e., hypercalcifica- Lastly, the change in seawater chemistry alents because of the influence of Mg Ca tion during periods of favorable Mg͞Ca over geologic time implied by the recent ratio on carbonate mineral precipitation. ratio and reduced skeletal productivity results of the studies of Stanley et al. (12) It now seems likely that the driving force during unfavorable periods), and, thus, and Lowenstein et al. (13) is substantially for evolutionary changes in the biominer- the rise and demise of Phanerozoic reef greater than previously contended. This alization of cheilostome bryozoans and builders was guided, at least in part, by necessitates a reconsideration of our cur- calcareous sponges was secular change in secular changes in the major-ion content rent working view of the geochemical cy- ͞ seawater Mg Ca ratios (3, 4, 25). The of the paleo-oceans. This model may pro- cles of Ca and Mg. There is little doubt enigmatic evolutionary trend in progres- vide an explanation for two coupled but that the debate over the nature of the sive reduction of calcification (i.e., less puzzling phenomena (4, 27, 28). The first chemical evolution of seawater will con- robust morphologies or progressively less is the decline in aragonitic corals as reef tinue but these studies develop an argu- shell volume) of calcareous nannoplank- builders in Late Cretaceous calcite seas ment of unprecedented strength for a ton over the past 50 million years (26) coincident with the rise of predominantly chemically dynamic ocean over the past appears to be a consequence of the rapidly low-Mg calcitic rudists. The second is the half billion years of Earth history.

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