GSA TODAY • 1999 Section Meetings Rocky Mountain, P

Vol. 9, No. 2 February 1999

INSIDE GSA TODAY • 1999 Section Meetings Rocky Mountain, p. 17 A Publication of the Geological Society of America North-Central, p. 23

Hypercalcification: Paleontology Links Plate Tectonics and Geochemistry to Sedimentology Steven M. Stanley Lawrence A. Hardie Morton K. Blaustein Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218

ABSTRACT During the Phanerozoic Eon, the mineralogies of nonskeletal marine cements and oolites have oscillated on a 100–200 m.y. scale between aragonite ± high-Mg calcite (aragonite seas) and low-Mg calcite (calcite seas). Oscilla- tions in the carbonate mineralogy of dominant reef-building and sediment- producing organisms are in harmony with the oscillations for nonskeletal carbonates. These oscillations, together with synchronous oscillations in the mineralogy of marine potash evaporites, can be explained by secular varia- An aragonitic brain coral, Diploria strigosa, of late Pleistocene age, from the Cockburn Town tion in the Mg/Ca ratio of seawater fossil coral reef, San Salvador Island, Bahamas. This reef formed during the most driven by changes in the spreading rates recent interval of aragontie seas. Photo by Al Curran, Smith College. along midocean ridges. The temporal patterns for biocalcification have come to light through a focus on (1) simple taxa that exert relatively weak control over the milieu in which they secrete their skeletons, and (2) taxa that hyper- calcify—i.e., secrete massive skeletons or are exceptionally productive, for example, in forming voluminous chalk deposits. Most major reef-building and sediment-producing taxa belong to both of these categories. It appears that the Mg/Ca ratio of seawater has not only controlled Phanerozoic oscillations in hypercalcification by simple taxa, such as calcareous nannoplankton, sponges, and bryozoans, but has strongly influenced their skeletal evolution.

Figure 1. Effect of changes in the rate of seafloor spreading (ocean crust production) on global sea INTRODUCTION level, the flux of MOR hydrothermal brine, and the chemistry of seawater as predicted by Spencer and Following an era of specialization Hardie (1990) and Hardie (1996). A—high-spreading-rate conditions; B—low-spreading-rate conditions. in the earth sciences, many conceptual Red arrows—MOR brine paths (thicknesses of the arrows are proportional to the brine flux but not to scale). Gray arrows proportional to spreading rates (not to scale). advances are now emerging through inter- disciplinary research. The flow of earth materials through chemical cycles, for example, links diverse scientific fields, as mineralogy of dominant reef-building and In a seminal study of oolites and early do sequences of causal relationships that sediment-producing organisms can be marine cements, Sandberg (1983) showed connect noncyclical physical, chemical, linked to shifts in seawater chemistry that nonskeletal carbonate precipitation and biological phenomena. We have con- controlled by changes in global spreading cluded that Phanerozoic oscillations in the rates along mid-ocean ridges (Fig. 1). Hypercalcification continued on p. 2 IN THIS ISSUE GSA TODAY February Vol. 9, No. 2 1999 Hypercalcification: Paleontology Links Washington Report ...... 12 Plate tectonics and Geochemistry to GSA On the Web ...... 12 GSA TODAY (ISSN 1052-5173) is published monthly Sedimentology ...... 1 by The Geological Society of America, Inc., with offices at 3300 GSAF Update ...... 14 Penrose Place, Boulder, Colorado. 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Nonmembers & Institutions: Free with paid subscription to both GSA Bulletin and Geology, otherwise $50 for U.S., Canada, and Mexico; $60 else- Hypercalcification continued from p. 1 low-magnesium calcite to precipitate where. Contact Subscription Services. Single copies may be requested from Publication Sales. Also available on an instead of aragonite. annual CD-ROM, (together with GSA Bulletin, Geology, GSA in Phanerozoic seas has oscillated between It has long been recognized that Data Repository, and an Electronic Retrospective Index to aragonite and calcite (Fig. 2). It has been changes in the Mg/Ca ratio of seawater journal articles from 1972); $89 to GSA Members, others call GSA Subscription Services for prices and details. Claims: widely held that relatively low levels of can dictate whether calcite or aragonite For nonreceipt or for damaged copies, members contact atmospheric pCO2 have produced “arago- precipitates from seawater. Experiments Membership Services; all others contact Subscription Ser- nite seas,” while relatively high levels have demonstrating this relationship (Fücht- vices. Claims are honored for one year; please allow suffi- cient delivery time for overseas copies, up to six months. produced “calcite seas” (Wilkinson and bauer and Hardie, 1976, 1980), which are Algeo, 1989; Mackenzie and Morse, 1992). in accord with data for natural saline lakes STAFF: Prepared from contributions from the GSA staff and membership. Calculations using the computer program (Müller et al., 1972), indicate that, at 25 °C Executive Director: Donald M. Davidson, Jr. PHRQPITZ (Plummer et al., 1988) show, and present seawater ionic strength and Science Editors: Suzanne M. Kay, Department of however, that pCO2 is not a viable control atmospheric pCO2, a ratio for Mg/Ca of Geological Sciences, Cornell University, Ithaca, NY 14853; Molly F. Miller, Department of Geology, Box 117-B, Vanderbilt (Stanley and Hardie, 1998). Seawater of ~2 separates a regime of calcite precipita- University, Nashville, TN 37235 modern composition would be supersatu- tion from a regime of aragonite ± high-Mg Forum Editor: Bruce F. Molnia, U.S. Geological Survey, rated with respect to calcite and undersat- calcite precipitation (Fig. 3). MS 917, National Center, Reston, VA 22092 Director of Publications: Peggy S. Lehr urated with respect to aragonite only for Spencer and Hardie (1990) introduced Managing Editor: Faith Rogers a narrow range of pCO2, within which all a quantitative model for calculating the Assistant Editor: Vanessa Carney Production Manager: Jon Olsen values are more than an order of magni- chemistry of ancient seawater based on Production Editor and Coordinator: Joan E. Manly tude greater than that of the present—a the premise that the composition of mod- Graphics Production: Joan E. Manly, Leatha L. Flowers level almost certainly not attained during ern seawater results primarily from the ADVERTISING: Classifieds and display: contact Ann the Cretaceous interval of calcite seas mixing of average riverwater (a Ca-HCO3 Crawford, (303) 447-2020; fax 303-447-1133; acrawford@ (Berner, 1994). In fact, such high levels water) and mid-ocean ridge (MOR) geosociety.org. of pCO2 would cause aragonitic shells of hydrothermal brines (Na-Ca-Cl waters). Issues of this publication are available as electronic Acrobat organisms to begin dissolving immediately This model predicts that relatively minor files for free download from GSA’s Web Site, http://www. geosociety.org. They can be viewed and printed on various after their secretion. Furthermore, experi- changes in the flux of MOR hydrothermal personal computer operating systems: MSDOS, MSWin- ments show that for present-day seawater brines would change the Mg/Ca, Na/K, dows, Macintosh, and Unix, using the appropriate Acrobat at 25 °C (Mg/Ca mole ratio = 5.17), raising and Cl/SO ratios in seawater enough to reader. Readers are available, free, from Adobe Corporation: 4 –4.5 –1.0 http://www.adobe.com/acrobat/readstep.html. pCO2 from 10 to 10 atm simply low- drastically alter the primary mineralogy This publication is included on GSA’s annual ers the MgCO3 content of precipitated cal- of nonskeletal marine carbonates and CD-ROM, GSA Journals on Compact Disc. cite from 12 to 7 mol% (Burton and Wal- evaporites. Call GSA Publication Sales for details. 50% Total Recoverd Fiber ter, 1991); it does not cause Printed in U.S.A. using pure soy inks. 10% Postconsumer

2 GSA TODAY, February 1999 CALL FOR NOMINATIONS POSITION AVAILABLE History of EXECUTIVE DIRECTOR Geology Award The Geological Society of America (GSA) is seeking an individual to assume the duties of Executive Director as early as June 1, 1999. The selected individual will: The History of Geology Award is pre- • Work with the Executive Committee and Council of GSA to implement a newly sented annually to an individual who adopted Strategic Plan. Work closely with GSA Foundation Board and staff to coordi- has made contributions of fundamental nate and promote the directions and policies of the GSA Council. importance to our understanding of • Maintain collaborative relationships with representatives of other national and interna- the history of the geological sciences. tional geoscience societies and organizations and actively pursue joint ventures that Outstanding contributions might enhance the financial and scholarly status of the Society. include publication of papers or books • Lead the GSA Headquarters staff of 65 persons. Articulate the vision and mission of the of distinction that contribute new and Society to staff and members through teamwork and collaborative efforts. Staff activi- profound insight into the history of ties include membership services; meetings; publications; education, public policy, and geology (based either on original outreach; marketing and strategic communications; financial services; and information research or on a synthesis of existing technology. knowledge); discovery of and making The new Director will work at the GSA headquarters in Boulder, Colorado, and will hold a available to scholars of rare resource position with competitive compensation and benefits. materials; providing comprehensive bibliographic surveys; editing a themat- ically integrated collection of articles; REQUIREMENTS organizing meetings and symposia that • Provide leadership to diverse groups such as committees, associated societies, staff, and generate interest in the history of geol- volunteers through collaborative teamwork. ogy; innovative research into original • Commitment to strong interpersonal communication among and between staff, mem- sources; creative interpretations of data; bers, and volunteers. Proven record of motivating staff to develop new sources of rev- translations of key materials; and enue and to use technology to improve efficiencies. exceptional services to the GSA History • Master’s degree in geosciences; Ph.D. preferred. Extensive management experience of Geology Division. and achievements in the areas of accrual budgeting, financial planning and invest- ments, and personnel management. Additional preparation in education, marketing, To nominate a deserving individual, and/or business management encouraged. send a letter of nomination that • Commitment to geoscience research, public outreach and education programs, and describes the contributions that war- scholarly publishing. rant the award, along with supporting • Familiarity with marketing and public relations. materials, including the candidate’s • Demonstrated familiarity with the geoscience community and GSA programs. curriculum vitae to the chair of the award committee, Gerard V. Middle- Submit a resume, the names and addresses of three references, and a letter describing ton, Dept. of Geology, McMaster your interest in the position to: University, 1280 West Main, Hamilton, Executive Director Search Committee ON L8S 4M1, Canada (middleto@ The Geological Society of America mcmaster.ca; fax 905-522-3141). All P.O. Box 9140 nominations are valid for three years. Boulder, CO 80301-9140 Nomination deadline: April 1, 1999. The deadline for applications or nominations is March 15, 1999.

Mid-ocean ridges act as huge rock- first-order sea-level curves as a proxy for Na-Mg-K-Cl-SO4 brines, respectively, fluid ion exchange systems for Ca2+ and the record of ocean crust production dur- which lie on either side of a fundamental 2+ 2+ Mg , Ca being released to the fluid and ing the Phanerozoic Eon, Hardie (1996) chemical divide, the “CaSO4 divide” Mg2+ being consumed by the rock in the employed the Spencer-Hardie model to (Hardie and Eugster, 1970). Because both 2+ 2– conversion of oceanic basalts to green- predict the mineralogies of nonskeletal Mg and SO4 are extracted from seawa- stones and amphibolites by interaction marine carbonate ooids and cements as a ter at mid-ocean ridges, whereas Ca2+ and with hot seawater (Fig. 1). Low spreading function of secular changes in the Mg/Ca K+ are released, MOR hydrothermal brines rates (= low hydrothermal brine fluxes; ratio of seawater. The results are in close are of the calcium chloride type. There- Baker et al., 1995) should lead to elevated agreement with Sandberg’s (1983) periods fore, during periods of high spreading Mg/Ca mole ratios in seawater of the open of aragonite seas and calcite seas, as shown rates, the elevated fluxes of MOR brine oceans; if this ratio rose above ~2 for in Figure 2 (see also Hardie, 1996, Fig. 5). will drive seawater toward a calcium chlo- warm surface seawater, then aragonite ± Potash evaporites in the geological ride composition that on evaporative con- high-Mg calcite would precipitate instead record fall into two main chemical groups: centration would produce KCl evaporites. of low-Mg calcite (Fig. 2), as occurs in (1) KCl evaporites characterized by potas- On the other hand, low spreading rates today’s oceans. Conversely, high spreading sium chloride salts such as sylvite (KCl) would push seawater toward MgSO4- rates (= high hydrothermal brine fluxes; and an absence of magnesium sulfate salts, enriched composition and precipitation Baker et al., 1995) should lower the and (2) MgSO4 evaporites characterized by of MgSO4-type evaporites. Calculations Mg/Ca mole ratio in seawater; if this ratio magnesium sulfate salts such as kieserite based on the Spencer-Hardie model pre- dropped below ~2, then low-Mg calcite (MgSO4 ·H2O) (Hardie, 1996). These two dict that KCl evaporites should have pre- would precipitate instead of aragonite ± evaporite types precipitate from two very cipitated from calcite seas and MgSO4 high-Mg calcite (as predicted, for example, different parent brines, Na-Ca-Mg-K-Cl for the Cretaceous Period; Fig. 2). Using brines (“calcium chloride” brines) and Hypercalcification continued on p. 4

GSA TODAY, February 1999 3 Hypercalcification continued from p. 3 dominant mineralogy of marine biocalci- tigate the effects of seawater chemistry fiers. Although such a trend has not been on biocalcification (Stanley and Hardie, evaporites from aragonite seas. The rock apparent to more recent workers (Lowen- 1998). First, we focused on what we call record confirms this correspondence stam and Weiner, 1989, p. 237), Wilkin- hypercalcifying tropical taxa. Forming one (Fig. 2; see also Hardie, 1996, Fig. 5). The son’s data led Mackenzie and Agegian subset of this group are species that have fact that the Spencer-Hardie and Hardie (1989, p. 20) to conclude that “the oscilla- secreted unusually massive skeletons for models successfully predict the Phanero- tory trend seen in non-skeletal carbonate the higher taxa to which they belong; zoic history of two different families of components … is not clearly apparent stony bryozoans of the Paleozoic are an nonskeletal minerals precipitated from in the mineralogy of fossil organisms.” example. A second, overlapping subset seawater—carbonates and potash evap- Wilkinson’s data also dissuaded Van de of hypercalcifiers includes species whose orites—makes a strong case that both Poel and Schlager (1994) from claiming populations engage in rampant carbonate models are fundamentally valid. such a correspondence, although their sur- production. Reef builders and major sedi- vey of bioclasts in Mesozoic and Cenozoic ment producers fall within this category. STRATEGIES FOR UNCOVERING carbonate rocks indicated maxima for As a second strategy, we hypothesized TRENDS IN BIOMINERALIZATION aragonitic components in Triassic and that relatively unsophisticated carbonate late Cenozoic strata. secretors—ones that exert weak control From sparse data, Wilkinson (1979) Rather than conducting a general sur- over the chemical milieu in which they proposed a unidirectional Phanerozoic vey, we adopted a double strategy to inves- secrete their skeletons—are strongly influ- trend from calcite to aragonite for the enced by the Mg/Ca ratio and temperature of seawater. Significant here is the observation that magnesium increases with temperature in the skeletons of modern marine organisms, as in nonskeletal marine carbonates, but partition coefficients vary among taxa and the effect of temperature is inversely related to biological complexity (Chave, 1954). Employing these two strategies, we uncovered a strong correspondence between the mineralogy of biologically simple hypercalcifying taxa and that of nonskeletal carbonates from Ordovician time to the present (Fig. 2). We conclude that although these taxa, including reef builders, need not have secreted skeletons that were in thermodynamic equilibrium with seawater, their skeletal productivity has been strongly influenced by the ambient Mg/Ca ratio. The taxa that yield this pattern share one deficiency: they are unable to remodel their skeletons through resorption during their ontogeny. It appears that the inability to remodel is linked to unsophisticated modes of biocalcification that also result in reliance on favorable seawater chemistry. Foraminifera, though otherwise simple organisms, are sophisti- cated skeletal secretors, which employ an “almost unparalleled” variety of basic modes of mineralization (Lowenstam and Weiner, 1989, p. 670) and also have the ability to remodel their tests; there is no apparent overall temporal relationship between their predominant mineralogy and the Mg/Ca ratio of seawater. Reef builders are probably heavily influenced by the ambient Mg/Ca ratio for two rea- sons. First, they must meet the basic demands of hypercalcification. Second, nearly all organisms able to flourish in the severe competitive battle for space within reef-building communities have succeeded through vegetative or colonial growth. Thus, these organisms have characteristi- cally been simple forms, such as algae, Figure 2. Correspondence between secular oscillations for the carbonate mineralogy of dominant sponges, and corals, which are unsophisti- hypercalcifying marine taxa, the mineralogy of marine evaporites and nonskeletal carbonates, and the Mg/Ca ratio and absolute concentration of calcium (Ca) in seawater as calculated by Hardie (1996). The cated carbonate secretors. boundary separating the nonskeletal nucleation fields of low-magnesium calcite (< 4 mol% MgCO3), As described below, application of our which we will term calcite, and high-Mg calcite (> 4 mol% MgCO3) and aragonite is shown as a hori- strategies to the geologic record provides zontal line at Mg/Ca = 2 (after Stanley and Hardie, 1998). explanations for many previously puzzling

4 GSA TODAY, February 1999 ern type (scleractinians) joined them as high taxonomic diversity during Early dominant reef builders (Stanley, 1988). Cretaceous time but had failed to form The only discrepancy between the massive chalk deposits. Following the set- mineralogy of dominant reef builders and back of calcareous nannoplankton by the that of nonskeletal carbonates is for Late terminal Cretaceous extinction, massive Jurassic and Early Cretaceous time, when chalk deposition resumed in early Pale- scleractinian corals persisted as major reef ocene time. Then, as the Mg/Ca ratio builders. The continued success of this of seawater rose toward the aragonitic aragonitic group probably resulted from domain, widespread deposition of massive two circumstances. First, the Mg/Ca ratio chalk ceased, and it failed to resume even remained near the calcite-aragonite during the exceptionally warm Eocene boundary during this interval. Second, interval, when epicontinental seas were the high absolute concentration of Ca2+ widespread. during this interval may have promoted The attribution of extensive chalk nonequilibrium precipitation of all forms deposition to changes in seawater chem- of calcium carbonate. In mid-Cretaceous istry gains support from the observation time, when the Mg/Ca ratio descended to that an increase in the concentration of its lowest Phanerozoic level according to dissolved Ca2+ enhances calcification by the calculations of Hardie (1996), corals calcareous nannoplankton in the labora- relinquished to rudists their role as domi- tory (Blackwelder et al., 1976). Additional nant reef builders (Scott, 1984). As bivalve support comes from two puzzling tempo- mollusks, rudists were probably not ral patterns for coccoliths (Houghton, strongly influenced by seawater chemistry. 1991). One of these is a polyphyletic Figure 3. Experimentally determined nucleation decline in the mean size of coccoliths fields of low-Mg calcite (red), high-Mg calcite + Thus, although shells of radiolitids, the most successful reef-building rudists of the during the Cenozoic Era. The result was aragonite (blue), and aragonite (green) in MgCl2- CaCl2-Na2CO3-H2O solutions at 28 ˚C, atmo- Late Cretaceous, contained more calcite thinner calcitic encrustation of cells. spheric pCO2 and 1 atm total pressure (1976 than aragonite (Kauffman and Johnson, The second pattern pertains to the genus unpublished data of Füchtbauer and Hardie). 1988), it is most reasonable to view the Discoaster. Coccoliths of this genus were Red symbol—calcite with MgCO3 content up rudists as beneficiaries of the decline solid, circular shields early in the Ceno- to 4 mol%; half-solid red square—calcite with of aragonitic reef-building corals that zoic, but as the era progressed, they be- MgCO3 content >4, <6 mol%. MgCO3 content of high-Mg calcite in the blue symbol field increases occurred when the Mg/Ca ratio dropped. came increasingly diminished in volume systematically with increase in the Mg/Ca ratio in Support for this interpretation comes from by marginal embayments. By the time the aqueous solution (see Füchtbauer and Hardie, the previously unexplained failure of Discoaster became extinct in the Pliocene, 1976, 1980). Black star—modern seawater (SW). corals to build large reefs for more than all of its members secreted spindly, star- 30 m.y. after the disappearance of the rud- shaped coccoliths that covered only a ists at the end of the Cretaceous Period. small fraction of the cell surface (Fig. 4). Scleractinian corals existed in considerable These trends for coccoliths can be viewed phenomena in the history of reef building diversity early in the Cenozoic Era, but as amounting to evolutionary osteoporosis, and sediment production and in the evo- produced only small, inconspicuous bio- caused by an increase in the Mg/Ca ratio lution of calcareous taxa. herms, even during the extremely warm of seawater that was accompanied by a de- Eocene interval. Not until early in the cline in the concentration of Ca2+ (Fig. 2). DOMINANT REEF BUILDERS Oligocene did corals begin to produce The Mg/Ca model also provides an Before Late Ordovician time, reefs massive reefs throughout the world (Frost, explanation for patterns of hypercalcifica- were built by taxonomically problematical 1977), despite the fact that warm seas had tion for green algae. Aragonitic codiaceans taxa of uncertain mineralogy. For this rea- contracted toward the equator (Zachos (especially Halimeda) produce vast quanti- son, we began our analysis of the mineral- et al., 1994). At this time, the Mg/Ca ratio ties of carbonate sediment today, and ogy of reef builders with the reef commu- of seawater was rising far into the arago- dasycladaceans were so productive during nity that flourished from Late Ordovician nitic regime (Fig. 2). Aragonite II that they have been regarded to Late Devonian time in the calcite sea We have provided elsewhere a more as the Halimeda of the Triassic (Elliott, designated Calcite I (Fig. 2). In accordance detailed picture of the correspondence 1984). On the other hand, the massive, with our hypothesis, this community was between the mineralogy of major reef calcitic receptaculitids were significant dominated by calcitic taxa: stromato- builders and nonskeletal marine carbon- sediment producers throughout Calcite I. poroid sponges and several groups of ates (Stanley and Hardie, 1998). Aspects of calcitic corals (Oliver and Coates, 1987). the pattern we have described were noted EVOLUTIONARY TRENDS The Late Devonian mass extinction by Van de Poel and Schlager (1994) and The Cenozoic evolutionary trend decimated the calcitic reef community. Hallock (1997). toward weakly calcified nannoplankton Late in the Mississippian Period, the species appears to reflect the influence Mg/Ca ratio of seawater shifted far into DOMINANT SEDIMENT of seawater chemistry (Fig. 4). Among the aragonitic regime (Aragonite II), and PRODUCERS cheilostome bryozoans, changes in the new reef-building communities of arago- The widespread deposition of massive Mg/Ca ratio appear to have influenced nite and high-Mg calcitic algae and chalk during Late Cretaceous time is the evolution of skeletal mineralogy. The sponges emerged (Fig. 2). Members of another phenomenon of hypercalcifica- cheilostomes originated as a calcitic group these communities formed the enormous tion that has long defied explanation but during the Cretaceous Period (Calcite II), Horseshoe Atoll of central Texas and the can be accounted for by a change in sea- although a few species of one subgroup Permian reef complex of west Texas. Arag- water chemistry: it coincided with the secreted a combination of calcite and aragonitic members of the same and similar interval during which the Mg/Ca ratio was onite (Boardman and Cheetham, 1987). taxa emerged as the reef-building com- at its lowest level during the past 500 m.y. Fully aragonitic species did not arise until munity of the Middle Triassic, and in Late (Fig. 2). Calcareous nannoplankton— Triassic time aragonitic corals of the mod- potential chalk producers—had attained Hypercalcification continued on p. 6

GSA TODAY, February 1999 5 reefs. If, long after forming, reef carbonate Figure 4. becomes metamorphosed during orogene- Stratigraphic ranges to the atmosphere of typical species of sis, it will release CO2 Discoaster, a genus (Berner, 1994). The flux of oxidized carbon of calcareous nanno- to reef carbonate reservoirs would have plankton whose calcitic been reduced at times, such as the early skeletal elements Cenozoic, when an unfavorable Mg/Ca underwent a striking ratio suppressed reef building. Carbonate net evolutionary trend during the sharp storage in reefs increased dramatically Cenozoic rise in the early in Oligocene time, following the Mg/Ca ratio of sea- shift from calcite to aragonite seas. At water. Species with about the same time, through reduced heavy, shield-shaped productivity during the dramatic rise in coccoliths gave the Mg/Ca ratio of seawater, nannoplank- way to species with delicate, star-shaped ton began contributing progressively less coccoliths. (Modified carbonate to the deep sea for possible from Houghton, future subduction and release of CO2. 1991.) Thus, during the past 30 m.y. or so, the increasing Mg/Ca ratio of seawater has influenced the relative proportions of total carbonate that have accumulated in the Hypercalcification continued from p. 5 tial to optimize their skeletal structures deep sea and in shallow-water reefs, where without inhibition by the Mg/Ca ratio of the mean residence time for oxidized the Eocene Epoch, when the Mg/Ca ratio seawater. Ammonoids, which secreted thin carbon is much longer. of seawater had risen markedly. Today shells within which the gas pressure was most cheilostome species secrete high-Mg about 1 atm, must have been served well ACKNOWLEDGMENTS calcite, but many tropical species secrete by the relatively great bending strength aragonite (Rucker and Carver, 1969); this of their nacreous aragonite. Ammonoids We thank Ronald E. Martin and distribution corresponds to the tempera- should therefore have benefited from their George D. Stanley for helpful reviews of ture pattern for nonskeletal precipitation ability to secrete aragonite readily even in our manuscript. of carbonates in laboratory experiments calcite seas. Thus, although being rela- (Morse et al., 1997). tively independent of the ambient Mg/Ca REFERENCES CITED Several workers have noted that the ratio leaves a taxon such as the Ammo- Baker, E. T., German, C. R., and Elderfield, H., 1995, mineralogy of calcareous sponges has fre- noidea unable to benefit from a favorable Hydrothermal plumes over spreading-center axes: quently coincided with that of nonskeletal ratio, all else being equal, this indepen- Global distributions and geophysical inferences, in Humphries, S., et al., eds., Seafloor hydrothermal carbonates during the Phanerozoic (Reit- dence confers long-term ecologic stability. systems: Physical, chemical, biological and geological ner, 1987; Gautret and Cuif, 1989; Wood, Conversely, although unsophisticated car- interactions: American Geophysical Union 1991). Our survey suggests that calcareous bonate secretors, such as algae, sponges, Monograph 91, p. 47–71. sponges have in fact been at the mercy of and corals, automatically benefit from the Berner, R. A., 1994, GEOCARB II: A revised model of atmospheric CO2 over Phanerozoic time: American seawater chemistry throughout their his- presence of a favorable Mg/Ca ratio, these Journal of Science, v. 294, p. 56–91. tory (Fig. 2). During the Cretaceous, for forms also suffer severe declines when Blackwelder, P. L., Weiss, R. E. and Wilbur, K. M., 1976, example, all of them appear to have been the ratio shifts to an unfavorable domain; Effects of calcium, strontium, and magnesium on the calcitic, but all present-day representatives they tend to follow a “boom-or-bust” coccolithophorid Cricosphaera (Hymenomonas) carterae. secrete aragonite, high-Mg calcite, or a pattern of productivity—and perhaps also 1. Calcification: Marine Biology, v. 34, p. 11–16. combination of these minerals (Hartman, taxonomic diversity—in the course of Boardman, R. S., and Cheetham, A. H., 1987, Phylum Bryozoa, in Boardman, R. S. and Rowell, A. H., eds., 1980). geologic time. Fossil invertebrates: Palo Alto, California, Blackwell, Martin (1995) has noted that some We think it likely that the Mg con- p. 497–549. suborders of foraminifera originated with centration in skeletons of taxa that secrete Burton, E. A. and Walter, L. M., 1991, The effects of skeletal mineralogies corresponding to high-Mg calcite has been positively corre- PCO2 and temperature on magnesium incorporation those of nonskeletal carbonates, although, lated with the Mg/Ca ratio of seawater in calcite in seawater and MgCl2-CaCl2 solutions: Geochimica et Cosmochimica Acta, v. 55, p. 777–785. as we have already noted, there does not back through Phanerozoic time, just as Chave, K. E., 1954, Aspects of the biogeochemistry of appear to be a strong temporal correlation today the Mg concentration in calcite magnesium 1. Calcareous marine organisms: Journal between the Mg/Ca ratio of seawater and skeletons increases with increasing ocean of Geology, v. 62, p. 266–283. the mineralogy of highly productive temperature (Chave, 1954). Experimental Elliott, G. F., 1984, Climatic tolerance in some foraminifera. growth of modern species under varying aragonitic green algae of the post-Palaeozoic: Palaeo- ambient Mg and Ca concentrations could geography, Palaeoclimatology, Palaeoecology, v. 48, p. 163–169. DISCUSSION shed light on this possibility by demonstrating lability in skeletal mineralogy for Frost, S. H., 1977, Miocene to Holocene evolution of The fact that we have been able to Caribbean Province reef-building corals: Proceedings, individual organisms. Study of the abun- connect many previously problematical Third International Coral Reef Symposium, Miami, dance of exsolved microdolomite and of p. 353–355. phenomena with a single causal explana- trace elements in fossils may also expand Füchtbauer, H., and Hardie, L. A., 1976, Experimentally tion gives credence to the Mg/Ca model. our knowledge of skeletal Mg concentra- determined homogeneous distribution coefficients for The results have additional biologic and precipitated magnesian calcites: Application to marine tions for extinct taxa. geologic implications, some of which sug- carbonate cements: Geological Society of America Changes in carbonate productivity Abstracts with Programs, v. 8, p. 877. gest promising avenues for future research. resulting from shifts in the Mg/Ca ratio For many taxa, degree of ecologic Füchtbauer, H., and Hardie, L. A., 1980, Comparison of of seawater must have affected the carbon experimental and natural magnesian calcites: Bochum, stability exhibited over tens or hundreds cycle significantly in the course of geo- Germany, International Association of Sedimentolo- of millions of years seems to reflect the gists, p. 167–169. logic time. Today, a large proportion of degree to which seawater chemistry has Gautret, P., and Cuif, J.-P., 1989, Les démosponges carbonate and bicarbonate ions entering influenced skeletal mineralogy. Sophisti- calcifiées des bioherms du Jurassique supérieur du the ocean are incorporated into organic cated carbonate secretors have the poten-

6 GSA TODAY, February 1999 CALL FOR NOMINATIONS REMINDERS Memorial Preprints DISTINGUISHED SERVICE AWARD The GSA Distinguished Service Award recognizes individuals The following memorial preprints are now avail- for exceptional service to the Society. GSA Members, Fellows, able, free of charge, by writing to GSA, P.O. Box 9140, Associates, or, in exceptional circumstances, GSA employees may Boulder, CO 80301. be nominated for consideration. Any GSA member or employee may make a nomination for the award. Awardees are selected by Arthur A. Baker Robert M. Kosanke the Executive Committee, and all selections are ratified by the J. David Love Aureal T. Cross, Jack A. Council. Deadline for nominations for 1999 is MARCH 1, 1999. Simon, Tom L. Phillips Frederick Betz, Jr. JOHN C. FRYE ENVIRONMENTAL GEOLOGY AWARD In cooperation with the Association of American State Geolo- Rhodes W. Fairbridge Robert Luscher gists (AASG), GSA makes an annual award for the best paper on McMaster environmental geology published either by GSA or by one of the Thomas E. Bolton John A. Peck state geological surveys. The award is a $1,000 cash prize from the Godfrey S. Nowlan, endowment income of the GSA Foundation’s John C. Frye Memo- Charles H. Smith John Elliott Nafe rial Fund. Jack E. Oliver The paper must be selected from GSA or state geological sur- Robert Sinclair Dietz vey publications; it must be selected from those published during Steve Koppes John Mason Parker III the preceding three full calendar years; and the nomination must Stephen G. Conrad include a paragraph stating the pertinence of the paper. John Van Nostrand Nominated papers must establish an environmental problem Dorr II John Evans Riddell or need, provide substantive information on the basic geology or Jacob E. Gair Richard C. Schmidt geologic process pertinent to the problem, relate the geology to the problem or need, suggest solutions or provide appropriate Charles Lum Drake Robert H. Shaver land-use recommendations based on the geology, present the Bruce B. Hanshaw Donald E. Hattin, information in a manner that is understandable and directly Carl B. Rexroad usable by geologists, and address the environmental need or Melvin (Mel) Friedman resolve the problem. It is preferred that the paper be directly John M. Logan, Thomas A. Simpson, Sr. applicable by informed laypersons (e.g., planners, engineers). David W. Stearns Philip E. LaMoreaux, Deadline for nominations for 1999 is MARCH 1, 1999. Everett Smith, James D. Hume Charles W. Copeland NATIONAL AWARDS Charles E. Stearns The deadline is April 30, 1999, for submitting nominations for these four awards: William T. Pecora Award, National Medal of Science, Vannevar Bush Award, Alan T. Waterman Award.

sud Tunisien (Oxfordien de la région de Tataouine): Morse, J. W., Wang, Q., and Tsio, M. Y., 1997, Spencer, R. J., and Hardie, L. A., 1990, Control of sea- Geobios, v. 22, p. 49–63. Influences of temperature and Mg:Ca ratio on CaCO3 water composition by mixing of river waters and mid- precipitates from seawater: Geology, v. 25, p. 85–87. ocean ridge hydrothermal brines, in Spencer, R. J., and Hallock, P., 1997, Reefs and reef limestones in earth his- Chou, I.-M., eds., Fluid-mineral interactions: A tribute tory, in Birkeland, P., ed., Life and death of coral reefs: Müller, G., Irion, G., and Förstner, U., 1972, Formation to H. P. Eugster: Geochemical Society Special Publica- New York, Chapman and Hall, p. 13–42. and diagenesis of inorganic Ca-Mg carbonates in the tion 19, p. 409–419. lacustrine environment: Naturwissenschaften, v. 59, Hardie, L. A., 1996, Secular variation in seawater chem- p. 158–164. Stanley, G. D., 1988, The history of Mesozoic reef com- istry: An explanation for the coupled secular variation munities: A three-step process: Palaios, v. 3, p. 170–183. in the mineralogies of marine limestones and potash Oliver, W. A., and Coates, A. G., 1987, Phylum evaporites over the past 600 m.y.: Geology, v. 24, Cnidaria, in Boardman, R. S., and Rowell, A. H., eds., Stanley, S. M., and Hardie, L. A., 1998, Secular oscilla- p. 279–283. Fossil invertebrates: Palo Alto, California, Blackwell, tions in the carbonate mineralogy of reef-building and p. 140–193. sediment-producing organisms driven by tectonically Hardie, L. A., and Eugster, H. P., 1970, The evolution of forced shifts in seawater chemistry: Palaeogeography, closed-basin brines: Mineralogical Society of America Plummer, L. N., Parkhurst, D. L., Fleming, G. W., and Palaeoclimatology, Palaeoecology v. 144, p. 3–19. Special Publication 3, p. 273–290. Dunkle, S. A., 1988, A computer program incorporating Pitzer’s equations for calculation of geochemical Van de Poel, H. M., and Schlager, W., 1994, Variations Hartman, W. D., 1980, Morphology of the skeleton: reactions: U.S. Geological Survey Water-Resources in Mesozoic-Cenozoic skeletal carbonate mineralogy: Sedimenta (University of Miami) VIII, p. 193–214. Investigations Report 88-4153, 310 p. Geologie en Mijnbouw, v. 73, p. 31-51. Houghton, S. D., 1991, Calcareous nannofossils, in Reitner, J., 1987, A new calcitic sphinctozoan sponge Wilkinson, B. H., 1979, Biomineralization, paleocean- Riding, R., ed., Calcareous algae and stromatolites: belonging to the Demospongiae from the Cassian For- ography, and the evolution of calcareous marine Berlin and Heidelberg, Springer-Verlag, p. 217–266. mation (Lower Carnian; Dolomites, northern Italy) organisms: Geology, v. 7, p. 524–527. Kauffman, E. G., and Johnson, C. C., 1988, The mor- and its phylogenetic relationship: Geobios, v. 20, Wilkinson, B. H., and Algeo, T. J., 1989, Sedimentary phological and ecological evolution of Middle and p. 571–589. carbonate record of calcium-magnesium cycling: Upper Cretaceous reef-building rudistids: Palaios, v. 3, Rucker, J. B., and Carver, R. E., 1969, A survey of the American Journal of Science, v. 289, p. 1158–1194. p. 194–216. carbonate mineralogy of cheilostome Bryozoa: Journal Wood, R. A., 1991, Problematic reef-building sponges, Lowenstam, H. A. and Weiner, S., 1989, On biomineral- of Paleontology, v. 43, p. 791–799. in Simonetta, A. M., and Morris, S. C., eds., The early ization: New York, Oxford University Press, 324 p. Sandberg, P. A., 1983, An oscillating trend in Phanero- evolution of Metazoa and the significance of problem- Mackenzie, F. T., and Agegian, C. R., 1989, Biomineral- zoic nonskeletal carbonate mineralogy: Nature, v. 305, atic taxa: Cambridge, UK, Cambridge University Press, ization and tentative links to plate tectonics, in Crick, p. 19–22. p. 113–124. R. E., ed., Origin, evolution, and modern aspects of Sandberg, P. A., 1985, Nonskeletal aragonite and pCO Zachos, J. C., Stott, L. C., and Lohmann, K. C., 1994, biomineralization in plants and animals: New York, 2 in the Phanerozoic and Proterozoic, in Sundquist, E. T. Evolution of early Cenozoic marine temperatures: Plenum Press, p. 11–27. and Broecker, W. S., eds., The carbon cycle and atmo- Paleoceanography, v. 9, p. 353–387. Mackenzie, F. T., and Morse, J. W., 1992, Sedimentary spheric CO : Natural variations Archean to present: 2 Manuscript received November 15, 1998; carbonates through Phanerozoic time: Geochimica et American Geophysical Union Monograph 32, accepted December 7, 1998 ■ Cosmochimica Acta, v. 56, p. 3281–3295. p. 585–594. Martin, R. E., 1995, Cyclical and secular variation in Scott, R. W., 1984, Evolution of Early Cretaceous reefs microfossil biomineralization: Clues to the biogeo- in the Gulf of Mexico: Paleontographica Americana, chemical evolution of Phanerozoic oceans: Global and v. 54, p. 406–412. Planetary Change, v. 11, p. 1–23.

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