Skeletal Mineralogy Research Articles

SKELETAL CARBONATE MINERALOGY OF BRYOZOANS FROM CHILE: AN INDEPENDENT CHECK OF PHYLOGENETIC PATTERNS

Abstract Bryozoa exhibit some broad phylogenetic patterns in skeletal carbonate mineralogy that have previously been elucidated based on material from Australasia, Europe, North America, South Africa, and Antarctica. A new dataset from 19 locations in the Patagonian fiord region of Chile adds 93 specimens and 23 species to the mineralogical information on this highly variable phylum, as well as providing a comparison with generalizations made to date. Most species, genera, and families from Chile are of similar mineralogy to their worldwide counterparts, but a few are surprising. Chilean Membranipora isabellana and Diastopora sp. are both 100% aragonite despite belonging to taxa that are dominantly calcitic. Adeonella patagonica and Adeonella sp. are the only species exhibiting bimineral skeletons, from 6 to 100% calcite, a much greater range than was previously known for this genus and the Adeonellidae. Skeletal mineralogy has been tested in only 95 of the 323 families in the Bryozoa. We are, nevertheles...

Controls on carbonate skeletal mineralogy: Global CO2 evolution and mass extinctions

A quantitative compilation of carbonate skeletal mineralogy through the Phanerozoic shows a progressive replacement of low-Mg calcite by aragonite. This general trend overrides the subsidiary trend of greenhouse intervals favoring biogenic low-Mg calcite mineralogies (calcite seas), and icehouse intervals facilitating aragonite + high-Mg calcite mineralogies (aragonite seas). The replacement of low-Mg calcite by aragonite was, however, achieved episodically at mass extinction intervals. In particular, the end-Permian extinction both preferentially removed species bearing “unfavorable” low-Mg calcite, and allowed the selective radiation of biota with “favorable” aragonite. This demonstrates the importance of incumbency in the evolution of skeletal mineralogy. We suggest that the broad increase of aragonitic biota has been controlled by changes in atmospheric carbon dioxide partial pressure ( p CO2) via carbonate mineral kinetics. Through the Phanerozoic, broadly decreasing p CO2 levels led to decreasing total alkalinity and dissolved inorganic carbon, and increasing oceanic pH. Superimposed upon this general trend, there are cyclic episodes of relatively high p CO2 and saturation state combined with a lower ratio of magnesium to calcium ions in seawater driven by the relatively slow changes in mid-ocean ridge expansion rates. Mass extinction events, many of which may have been caused by rapid global changes in temperature and/or p CO2, represent major intervals of turnover.

The incertae sedis Carpathoporella Dragastan, 1995, from the Lower Cretaceous of Albania: skeletal elements (sclerites, internodes/branches, holdfasts) of colonial octocorals

The incertae sedis Carpathoporella Dragastan, 1995, reported from the Lower Cretaceous of the Western Tethyan domain, is usually interpreted as remains of calcareous algae (Dasycladales or Characeae). New thin-section material from the Aptian of Albania sheds light not only on its biogenic nature but also on the morphological variability of this taxon. In fact, Carpathoporella represents the debris of colonial, bushy, most likely gorgonid octocorals with tuberculated spheroids that may be fused at least near the basal root-like holdfast. Colony branching originates from longitudinally grooved calcareous branches or internodes. Possible relationships to other Upper Cretaceous to Palaeogene genera are discussed and a revised critical inventory of Cretaceous octocorals is presented. Due to the evidenced morphological features, Carpathoporella could either represent an ancestral isidid octocoral of the order Alcyonacea such as Moltkia Steenstrup or, due to the likely primary aragonitic skeletal mineralogy, a representative of Epiphaxum Lonsdale of the order Helioporacea. Due to morphological analogies, the new combination Carpathoporella elliotti (Radoicic) is proposed. In any case, the Lower Cretaceous record from Tethyan peri-reefal shallow-water carbonates is highlighted since numerous skeletal findings of fossil gorgonid Octocorallia were so far only known from Upper Cretaceous and younger strata of outer shelf environments of the boreal realm. The origin of deep-water Upper Cretaceous octocorals from Lower Cretaceous shallow-water taxa such as Carpathoporella is proposed as a possible further example of onshore/offshore evolutionary pattern.

Calcareous tubeworms of the Phanerozoic

Morphological similarities indicate that Palaeozoic problematic tubeworms, e.g. tentaculitids, cornulitids, microconchids, trypanoporids, Anticalyptraea, and Tymbochoos, form a monophyletic group. This group may also include hederelloids. Members of this group share affinities with lophophorates and their evolution could have partly been driven by predation. The extinction of Palaeozoic tubeworms in the Middle Jurassic was possibly at least partly caused by the ecological pressure by serpulid and sabellid polychaetes. The input of Palaeozoic tubeworms to the general ocean biocalcification system may have been smaller in the Ordovician to Jurassic than that of calcareous polychaetes in the Late Triassic to Recent. There seems to have been some correlation between the aragonite-calcite seas and the skeletal mineralogy of Triassic-Recent polychaete tubeworms.

Seeing changes in a changing sea

The ratio of magnesium to calcium in sea water is thought to have influenced the skeletal mineralogy of certain marine calcifiers throughout the Phanerozoic eon. A fresh look at old data suggests that mass extinctions may have also played a role.

Phanerozoic trends in skeletal mineralogy driven by mass extinctions

Changes in ocean chemistry that favoured the precipitation of aragonite or calcite are thought to have influenced the skeletal mineralogy of marine calcifyers. An investigation of the original skeletal mineralogy of large numbers of marine taxa suggests that the selective recovery of marine organisms from mass extinctions has a much greater influence on the overall percentage of aragonitic organisms than the Mg/Ca ratio of the oceans. Marine calcifying organisms that produce sediments and build reefs generally have skeletons and shells that are composed of either aragonite or calcite. Long-term changes in the estimated Mg/Ca ratios of sea water tend to correspond to changes in the prevailing mineralogy of these creatures1,2. High Mg/Ca ratios are expected to favour the spread of aragonitic organisms, whereas calcitic taxa are thought to benefit from low Mg/Ca ratios3,4,5,6. Here we test these patterns throughout the Phanerozoic eon and assess the relative impacts of changing ocean chemistry and mass extinctions on the evolutionary success of calcifying organisms. We find that mass extinctions are more important in regulating long-term patterns of skeletal mineralogy than the Mg/Ca ratios of the global oceans. Furthermore, selective recovery from mass extinctions is usually more important than selective extinction, in driving the Phanerozoic pattern of skeletal mineralogy. But even in the recovery phase there is no clear connection between changes in the dominance of aragonite or calcite and the Mg/Ca ratio of the oceans, thus providing further evidence for the complexity of biotic recoveries7.

Eve of biomineralization: Controls on skeletal mineralogy

Research Article| December 01, 2008 Eve of biomineralization: Controls on skeletal mineralogy Andrey Yu. Zhuravlev; Andrey Yu. Zhuravlev * 11Área y Museo de Paleontología, Departamento de Ciencias de la Tierra, Facultad de Ciencias, Universidad de Zaragoza, c/Pedro Cerbuna, E-50009, Zaragoza, Spain *E-mail: ayzhur@mail.ru Search for other works by this author on: GSW Google Scholar Rachel A. Wood Rachel A. Wood 22School of GeoSciences, University of Edinburgh, King's Buildings, West Mains Road, Edinburgh EH9 3JW, UK, and Edinburgh Collaborative of Subsurface Science and Engineering, Edinburgh EH9 3JW, UK Search for other works by this author on: GSW Google Scholar Author and Article Information Andrey Yu. Zhuravlev * 11Área y Museo de Paleontología, Departamento de Ciencias de la Tierra, Facultad de Ciencias, Universidad de Zaragoza, c/Pedro Cerbuna, E-50009, Zaragoza, Spain Rachel A. Wood 22School of GeoSciences, University of Edinburgh, King's Buildings, West Mains Road, Edinburgh EH9 3JW, UK, and Edinburgh Collaborative of Subsurface Science and Engineering, Edinburgh EH9 3JW, UK *E-mail: ayzhur@mail.ru Publisher: Geological Society of America Received: 14 May 2008 Revision Received: 12 Aug 2008 Accepted: 18 Aug 2008 First Online: 02 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2008 Geological Society of America Geology (2008) 36 (12): 923–926. https://doi.org/10.1130/G25094A.1 Article history Received: 14 May 2008 Revision Received: 12 Aug 2008 Accepted: 18 Aug 2008 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Andrey Yu. Zhuravlev, Rachel A. Wood; Eve of biomineralization: Controls on skeletal mineralogy. Geology 2008;; 36 (12): 923–926. doi: https://doi.org/10.1130/G25094A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Carbonate mineralogies have oscillated between aragonite and calcite seas through geological time, proposed to be due mainly to secular variation in the magnesium/calcium ratio driven by changing rates of ocean crust production. A quantitative compilation of inorganic and biominerals from the onset of biomineralization (late Ediacaran–Middle Ordovician) reveals a correspondence between seawater chemistry and the first adopted mineralogy of skeletal clades. Ediacaran–Tommotian skeletons and inorganic precipitates were composed exclusively of aragonite or high-Mg calcite, but these were replaced by low-Mg calcite mineralogies during the early Atdabanian, implying the onset of a calcite sea. This transition is empirically constrained by fluid inclusion data. Late Atbadanian–Botoman inorganic precipitates returned to aragonite, with high-Mg calcite echinoderms and solitary tabulaconids and massive aragonitic tabulaconids originating during this interval. Middle Cambrian–Ordovician inorganic precipitates were low-Mg calcite, and the Ordovician radiation in skeletal expression was due mostly to groups with low-Mg calcite mineralogies. These short-lived transitions can be most parsimoniously explained by minor oscillations of mMg:Ca around ~2 during this period, possibly combined with the progressive onset of greenhouse conditions during the mid-Late Cambrian. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Seawater Chemistry and Early Carbonate Biomineralization

The first appearances of aragonite and calcite skeletons in 18 animal clades that independently evolved mineralization during the late Ediacaran through the Ordovician (approximately 550 to 444 million years ago) correspond to intervals when seawater chemistry favored aragonite and calcite precipitation, respectively. Skeletal mineralogies rarely changed once skeletons evolved, despite subsequent changes in seawater chemistry. Thus, the selection of carbonate skeletal minerals appears to have been dictated by seawater chemistry at the time a clade first acquired its mineralized skeleton.

Skeletal mineralogy of bryozoans: Taxonomic and temporal patterns

Skeletal carbonate mineralogy of 1183 specimens of marine bryozoans from the literature was examined for phylogenetic patterns in order to elucidate the effects of bryozoan mineralogy on geochemical and paleoenvironmental analysis. Colonies are composed of calcite (66% of specimens), aragonite (17% of specimens) or various mixtures of the two (17% specimens) (phylum mean = 72.9 wt.% calcite, n = 1051). When calcite is present, it ranges from 0.0 to 13.7 wt.% MgCO 3 (mean = 5.0 wt.% MgCO 3, n = 873). Most (61%) calcitic specimens are formed of intermediate-Mg calcite (4 to 8 wt.% MgCO 3), others (28%) of low-Mg calcite (0 to 4 wt.% MgCO 3), and few of high-Mg calcite (> 8 wt.% MgCO 3). The phylum occupies at least 63% of the theoretical mineralogical “space” available to biomineralisation. Most of this variation occurs in the class Gymnolaemata, order Cheilostomata, suborder Neocheilostomata. Fossil and Recent stenolaemate taxa are generally low- to intermediate-Mg calcite (mean = 99.7 wt.% calcite, 2.6 wt.% MgCO 3, 17% of available biomineral space). Variability among families is related in a general way to first appearance datum: families younger than 100 Ma display greater mineralogical complexity than older ones. The cheilostome infraorder Flustrina includes unusual free-living aragonitic families, dual-calcite skeletons (mainly low-Mg calcite, but with secondary high-Mg calcite), and some genera with considerable mineralogical variability. Families (e.g., Membraniporidae and Phidoloporidae) and species (e.g., Schizoporella unicornis) with the highest degree of variability have potential for environmental correlations with mineralogy, paleoenvironmental interpretation, and possibly molecular investigation for potential cryptic species. Stenolaemate families, genera and species with low variability, on the other hand, are well-suited for geochemical work such as stable isotope analysis. Variability in the skeletal mineralogy of bryozoans suggests that they may be useful in geochemical, phylogenetic, and paleoenvironmental studies, with careful choice of study material.

Influence of seawater chemistry on biomineralization throughout phanerozoic time: Paleontological and experimental evidence

Although some organisms exercise considerable control over their biomineralization, seawater chemistry has affected skeletal secretion by many taxa. Secular changes in the magnesium / calcium ratio and absolute concentration of calcium in seawater, driven by changes in rates of deep-sea igneous activity, have influenced the precipitation of nonskeletal carbonates: low-Mg calcite forms when the ambient Mg / Ca molar ratio is < 1, high-Mg calcite forms when the ratio is 1–2, and high-Mg calcite and aragonite form when the ratio is above 2. Reef builders and other simple organisms that are highly productive biomineralizers have tended to respond to changes in seawater chemistry in ways that mirror patterns for nonskelatal carbonates. Also, changes in the concentration of silicic acid in seawater have affected the ability of organisms to secrete siliceous skeletons. In laboratory experiments, organisms that secrete high-Mg calcite in the modern aragonite sea incorporate progressively less Mg in their skeletons with a reduction in the ambient Mg / Ca ratio, producing low-Mg calcite in “Cretaceous” seawater (Mg / Ca molar ratio = 1.0). Because algae that liberate CO 2 through calcification use it in their photosynthesis, an increase in the ambient Mg / Ca ratio results in accelerated aragonite secretion and overall growth for codiacean algae, and a decrease in the Mg / Ca ratio results in greatly accelerated growth rates for calcitic coccolithophores. Controlled experiments show that the increased concentration of Ca that accompanies a reduction of the ambient Mg / Ca ratio also accelerates coccolithophore population growth. Coccolithophores' production of vast chalk deposits in Late Cretaceous time can be attributed to the low Mg / Ca ratio and high Ca concentration in ambient seawater. The high Mg / Ca ratio and low Ca concentration in modern seawater apparently limit population growth for the large majority of modern coccolithophore species: ones that fail to respond to nitrate, phosphate or iron fertilization and are confined to oligotrophic waters. Presumably the low Mg / Ca ratio of ambient seawater was at least partly responsible for reduced reef-building by scleractinian corals in Late Cretaceous time. Some taxa have secreted more robust skeletons when seawater chemistry has favored their skeletal mineralogy. Strong intrinsic control of biomineralization can buffer a taxon against secular changes in seawater chemistry. Mollusks, for example, evolved the ability to severely limit the incorporation of Mg in their skeletal calcite in seawaters with Mg / Ca ratios as high as that of the present, but not in seawaters with still higher ratios. The ability to exclude Mg is useful because Mg reduces the rate of step growth of calcite crystals. On the other hand, labile skeletal mineralogy has permitted some taxa to respond to secular changes in the Mg / Ca ratio of seawater via phenotypic or evolutionary shifts of skeletal mineralogy. Sponges and bryozoans have apparently undergone evolutionary shifts of this kind polyphyletically. Increased incorporation of Mg in skeletal calcite with secular increases in the concentration of Mg in seawater has had little effect on seawater chemistry. In contrast, removal of Si by diatoms beginning in late Mesozoic time lowered the concentration of silicic acid in seawater, forcing siliceous sponges to secrete less robust skeletons.

Mg fractionation in crustose coralline algae: Geochemical, biological, and sedimentological implications of secular variation in the Mg/Ca ratio of seawater

The Mg/Ca ratio of seawater has varied significantly throughout the Phanerozoic Eon, primarily as a function of the rate of ocean crust production. Specimens of the crustose coralline alga Neogoniolithon sp. were grown in artificial seawaters encompassing the range of Mg/Ca ratios shown to have existed throughout the Phanerozoic. Significantly, the coralline algae’s skeletal Mg/Ca ratio varied in lockstep with the Mg/Ca ratio of the artificial seawater. Specimens grown in seawater treatments formulated with identical Mg/Ca ratios but differing absolute concentrations of Mg and Ca exhibited no significant differences in skeletal Mg/Ca ratios, thereby emphasizing the importance of the ambient Mg/Ca ratio, and not the absolute concentration of Mg, in determining the Mg/Ca ratio of coralline algal calcite. Specimens grown in seawater of the lowest molar Mg/Ca ratio ( mMg/Ca = 1.0) actually changed their skeletal mineralogy from high-Mg (skeletal mMg/Ca > 0.04) to low-Mg calcite (skeletal mMg/Ca < 0.04), suggesting that ancient calcitic red algae, which exhibit morphologies and modes of calcification comparable to Neogoniolithon sp., would have produced low-Mg calcite from the middle Cambrian to middle Mississippian and during the middle to Late Cretaceous, when oceanic mMg/Ca approached unity. By influencing the original Mg content of carbonate facies in which these algae have been ubiquitous, this condition has significant implications for the geochemistry and diagenesis of algal limestones throughout most of the Phanerozoic. The crustose coralline algae’s precipitation of high-Mg calcite from seawater that favors the abiotic precipitation of aragonite indicates that these algae dictate the precipitation of the calcitic polymorph of CaCO 3. However, the algae’s nearly abiotic pattern of Mg fractionation in their skeletal calcite suggests that their biomineralogical control is limited to polymorph specification and is generally ineffectual in the regulation of skeletal Mg incorporation. Therefore, the Mg/Ca ratio of well-preserved fossils of crustose coralline algae, when corrected for the effect of seawater temperature, may be an archive of oceanic Mg/Ca throughout the Phanerozoic. Magnesium fractionation algorithms that model algal skeletal Mg/Ca as a function of seawater Mg/Ca and temperature are presented herein. The results of this study support the empirical fossil evidence that secular variation of oceanic Mg/Ca has caused the mineralogy and skeletal chemistry of many calcifying marine organisms to change significantly over geologic time.

Calcite precipitation and dissolution of biogenic aragonite in shallow Ordovician calcite seas

The Ordovician was a time of extensive and pervasive low-magnesium calcite (LMC) precipitation on shallow marine sea floors. The evidence comes from field study (extensive hardgrounds and other early cementation fabrics in shallow-water carbonate sequences) and petrography (large volumes of marine calcite cement in grainstones). Contemporaneous sea-floor events, particularly relationships with boring and encrusting organisms and reworking in sequences of intraformational conglomerates, confirm the early timing of such LMC cementation, and also of widespread associated aragonite dissolution. Local evidence points to the dissolved aragonite as a significant source of the calcite cement. This scenario, and the fabrics that provide the evidence for it, are likely to be pointers to other times in the stratigraphic record when LMC was the predominant shallow marine precipitate (Calcite Sea times). The combination of rapid calcite precipitation and aragonite dissolution at a time early in the Phanerozoic when many major invertebrate groups were becoming established may have acted as an influence on the evolution of both their skeletal mineralogy and their ecology.

A re-evaluation of aragonite versus calcite seas

Some workers have argued that the mineralogy of ancient carbonates may have been different from that of modern sediments, with calcite being considered the dominant mineral during the Ordovician, Devonian-mid Carboniferous, and Jurassic-Cretaceous to Early/Middle Cenozoic (e.g. Sandberg 1983; Wilkinson and Algeo 1989). Variation in carbonate mineralogy has been related to the position of global sea level (emergent or submergent modes, Wilkinson et al. 1985), change in rates of seafloor spreading (e.g. Mackenzie and Pigott 1981; Hardie 1996),PCO 2 level (e.g. Sandberg 1985; Mackenzie and Morse 1992; Hallock 1997) and Mg/Ca ratios related to spreading rate (e.g. Stanley and Hardie 1998). However, other researchers suggest that the assumption of change of original carbonate mineralogy through time needs to be re-evaluated in the light of mineralogical change that is related to water temperature or latitudes (e.g. Nelson 1988). Evaluation of Ordovician (Arenig to Ashgill) Gordon Group carbonates of Tasmania (Australia), based on petrographic (e.g. abundantChlorozoan biota, and oomold texture) and geochemical criteria (such as high Sr/Na ratios) indicated that aragonite (not calcite) was the predominant mineral in these warm water, subtropical carbonates (Rao 1990). Petrographic (e.g. presence or absence of aragonite relicts, abundant acicular to fibrous isopachous marine cement, presence of diffuse laminae and a number of spalled ooids) and geochemical evidences (such as elevated Sr) in the Upper Jurassic Mozduran limestone, in Kopet-Dagh Basin in northeast Iran, showed variation in carbonate mineralogy, in spite of similar atmosphericPCO 2 level, global sea-level and tectonic setting. This is evidenced by aragonite occurring in the shallowest part of the basin (Adabi and Rao 1991) and mainly calcite with some aragonite forming in the relatively deeper water (below wave base) (Adabi 1997). Carbonate mineralogy in Recent shallow marine carbonates, and in experimental studies, varies with seawater temperature. In the Recent, aragonite is the predominant mineral in warm, shallow marine carbonates and calcite the dominant mineral in marine cool water carbonates (James and Clarke 1997). Therefore, variations in carbonate mineralogy in the Mozduran limestone are attributed to seawater temperature assuming invariant seawater chemistry prevailed in the Upper Jurassic. Several Jurassic examples show variations in ooid carbonate mineralogy, such as the Upper Jurassic Smackover oolite of the Gulf Coast region (southern Arkansas and northern Louisiana) and Upper Jurassic ooids in the Purbeck limestones of Swiss and French Jura. These results are not in agreement with the concept of a “calcite sea” during the Ordovician and the Upper Jurassic periods. Very recently, Westphal and Munnecke (2003) showed that in spite of the tendency of abiotic precipitates (Sandberg 1983) and skeletal mineralogy (Stanley and Hardie 1999) to follow the general trend of calcite seas and aragonite seas, organisms with calcite and aragonite mineralogy coexisted throughout the Phanerozoic. They have examined the temporal and spatial distribution of limestone-marl alternations in Ordovician, Jurassic and Cretaceous (times of calcite seas). Limestone-marl alternations are most abundant in settings that favored aragonite production and accumulation analogous to the modern environment (Westphal and Munnecke 2003). If the above observations confirmed, the proposed secular variation in Phanerozoic carbonate mineralogy needs to be re-evaluated.

Mineralogical Variation in Shells of the Blackfoot Abalone, Haliotis iris (Mollusca: Gastropoda: Haliotidae), in Southern New Zealand

The New Zealand blackfoot abalone, Haliotis iris Gmelin, is among the few gastropods that precipitate both calcite and aragonite in their shells. The location, composition, and thickness of these mineral layers may affect color, luster, and strength of the shell, which is locally important in jewelry manufacture. Skeletal mineralogy and shell structure of H. iris from three southern New Zealand locations were determined using X-ray diffractometry, scanning electron micrography, and mineral staining. In H. iris an outer calcitic layer is separated from an inner aragonitic surface by both calcified and noncalcified organic layers running longitudinally through the shell. Skeletal mineralogy within individual shells varies from 29 to 98% aragonite, with older shell having significantly higher aragonite content than young sections. Variation within populations ranges from 40 to 98% aragonite, and among three populations from 34 to 98% aragonite. Shell thickness, too, varies within individual shells from 0.2 to 4.2 mm, with a significant positive relationship with age. Withinpopulation variation in shell thickness ranges from 2.1 to 5.4 mm, with no significant difference in shell thickness variation among populations. The high degree of variability within and among individual shells suggests that it is essential to test replicate samples from individual mollusk shells, especially when they have complex bimineral structure.

Effects of early sea-floor processes on the taphonomy of temperate shelf skeletal carbonate deposits

Cool-water shelf carbonates differ from tropical carbonates in their sources, modes, and rates of deposition, geochemistry, and diagenesis. Inorganic precipitation, marine cementation, and sediment accumulation rates are absent or slow in cool waters, so that temperate carbonates remain longer at or near the sea bed. Early sea-floor processes, occurring between biogenic calcification and ultimate deposition, thus take on an important role, and there is the potential for considerable taphonomic loss of skeletal information into the fossilised record of cool-water carbonate deposits. The physical breakdown processes of dissociation, breakage, and abrasion are mediated mainly by hydraulic regime, and are always destructive. Impact damage reduces the size of grains, removes structure and therefore information, and ultimately may transform skeletal material into anonymous particles. Abrasion is highly selective amongst and within taxa, their skeletal form and structure strongly influencing resistance to mechanical breakdown. Dissolution and precipitation are the end-members of a two-way chemical equilibrium operating in sea water. In cool waters, inorganic precipitation is rare. There is conflicting opinion about the importance of diagenetic dissolution of carbonate skeletons on the temperate sea floor, but test maceration and early loss of aragonite in particular are reported. Dissolution may relate to undersaturated acidic pore waters generated locally by a combination of microbial metabolisation of organic matter, strong bioturbation, and oxidation of solid phase sulphides immediately beneath the sea floor in otherwise very slowly accumulating skeletal deposits. Laboratory experiments demonstrate that surface-to-volume ratio and skeletal mineralogy are both important in determining skeletal resistance to dissolution. Biological processes on the sea floor include encrustation and bioerosion. Encrustation, a constructive process, may be periodic or seasonal, and can be reversed. It produces both information and material. Bioerosion, in contrast, is destructive and permanent. In temperate areas bioerosion may destroy even very large shells during their long residence at the sea floor, on the order of hundreds to thousands of years. Overall, processes on the temperate sea floor may combine to destroy more carbonate than they produce, and the preservation potential of temperate shelf carbonate into the rock record may be significantly affected. Where preservation does occur in such a destructive regime, the effects of early sea-floor processes will be key determinants of the deposit, resulting in a “taphofacies” characteristic of temperate shelf carbonate sediments.

Low-magnesium calcite produced by coralline algae in seawater of Late Cretaceous composition.

Shifts in the MgCa ratio of seawater driven by changes in midocean ridge spreading rates have produced oscillations in the mineralogy of nonskeletal carbonate precipitates from seawater on time scales of 10(8) years. Since Cambrian time, skeletal mineralogies of anatomically simple organisms functioning as major reef builders or producers of shallow marine limestones have generally corresponded in mineral composition to nonskeletal precipitates. Here we report on experiments showing that the ambient MgCa ratio actually governs the skeletal mineralogy of some simple organisms. In modern seas, coralline algae produce skeletons of high-Mg calcite (>4 mol % MgCO(3)). We grew three species of these algae in artificial seawaters having three different MgCa ratios. All of the species incorporated amounts of Mg into their skeletons in proportion to the ambient MgCa ratio, mimicking the pattern for nonskeletal precipitation. Thus, the algae calcified as if they were simply inducing precipitation from seawater through their consumption of CO(2) for photosynthesis; presumably organic templates specify the calcite crystal structure of their skeletons. In artificial seawater with the low MgCa ratio of Late Cretaceous seas, the algae in our experiments produced low-Mg calcite (<4 mol % MgCO(3)), the carbonate mineral formed by nonskeletal precipitation in those ancient seas. Our results suggest that many taxa that produce high-Mg calcite today produced low-Mg calcite in Late Cretaceous seas.

Skeletal microstructure, geochemistry, and organic remnants in Cretaceous scleractinian corals: Santonian Gosau Beds of Gosau, Austria

Extremely well-preserved specimens of the species Rennensismilia complanata and Aulosmilia cuneiformis occur in Santonian (Upper Cretaceous) strata of the Lower Gosau beds, near Gosau, Austria. Two of these, here reported, have aragonitic skeletal mineralogy and skeletal structures that are typical for their families, and, in addition, show distribution of trace elements (Sr and Mg above all) that confirm the biogenic origin of structures observed. R. complanata also has proteinaceous matrix preserved within its skeleton, which, seen here for the first time in electron micrographs of living or fossil corals, forms sheaths of organic matrix surrounding bundles of skeletal crystallites. Matrix is most abundant along the axial plane of septa, which also is the first-formed part of each septum. Although A. cuneiformis lacks observable organic matrix materials, its skeletal structure and its distribution and amount of trace elements are analogous to that seen in R. complanata and also in modern corals.

Secular oscillations in the carbonate mineralogy of reef-building and sediment-producing organisms driven by tectonically forced shifts in seawater chemistry

The primary mineralogy of oolites and early marine carbonate cements led Sandberg [Nature 305 (1983), 19–22] to divide the Phanerozoic Eon into three intervals of `aragonite seas' and two intervals of `calcite seas'. Hardie [Geology 24 (1996), 279–283] has shown that these oscillations, together with synchronous oscillations in the mineralogy of marine potash evaporites, can be explained by secular shifts in the Mg/Ca ratio of seawater driven by changes in spreading rates along mid-ocean ridges. The Hardie model also predicts that high-Mg calcite should precipitate along with aragonite, as it does in today's aragonite sea. We have uncovered oscillations in the carbonate mineralogy of hypercalcifying organisms (ones that have produced massive skeletons, large reefs, or voluminous bodies of sediment) that correspond to Sandberg's aragonite seas and calcite seas and that are predicted by the Hardie model. Particular groups of corals, sponges, and algae appear to have been dominant reef builders only when favored by an appropriate Mg/Ca ratio in seawater. In early and middle Paleozoic calcite seas (Calcite I), reefs were dominated by calcitic tabulate, heliolitid, and rugose corals and calcitic stromatoporoids. In contrast, during the period of late Paleozoic–early Mesozoic aragonite seas (Aragonite II), aragonitic groups of sponges, scleractinian corals, and phylloid algae, as well as high-Mg calcitic red algae, were principal reef builders. During Late Cretaceous time, at the acme of Calcite II, massive rudists displaced aragonitic hermatypic corals. In today's aragonite sea (Aragonite III) scleractinian corals are again dominant reef builders, along with high-Mg calcitic coralline algae. Major sediment-producing algae exhibit temporal patterns similar to those of reef builders. Calcitic receptaculitids flourished during Calcite I, whereas aragonitic dasycladaceans did not become dominant rock formers until Aragonite II. During Calcite II, calcitic nannoplankton formed massive coccolith chalks in warm shallow seas of the Late Cretaceous, after the Mg/Ca ratio of seawater had reached a very low value and calcium concentration, a very high value. As the Mg/Ca ratio of seawater rose and calcium concentration fell during the Cenozoic Era, individual coccoliths, on average, became less massive and encrusted cells less thickly. By Pliocene time, during Aragonite III, the prominent genus Discoaster secreted only narrow-rayed coccoliths that covered less than 25% of the cell surface. Also during Aragonite III, the aragonitic green alga Halimeda emerged as the dominant skeletal sediment producer in reef tracts. The influence of seawater chemistry on skeletal secretion appears to have been especially strong for morphologically simple taxa that exert relatively weak control over their own calcification. Such groups include algae, sponges, corals, and bryozoans. Morphological simplicity also permits these groups to adopt vegetative or colonial modes of growth that confer success in competition for space on reefs. This linkage, in addition to the basic chemical demands of hypercalcification, has given the Mg/Ca ratio of seawater strong control over the success of individual reef-building taxa. More generally, this ratio appears to have strongly influenced evolutionary changes in the skeletal mineralogy of sponges and cheilostome bryozoans throughout their history. We conclude that throughout Phanerozoic time a chain of causation has extended from mid-ocean ridge processes, via seawater chemistry, to the mineralogical and biological composition of reef communities and bioclastic carbonate deposits.

Skeletal carbonate mineralogy of New Zealand bryozoans

On many cool-water carbonate shelves, both modern and ancient, bryozoans are the dominant sediment contributor. Because skeletal carbonate mineralogy can be related to both taxonomic and environmental controls, and has important implications for understanding the geochemistry and diagenesis in carbonate deposits, it is highly relevant to develop a comprehensive database for bryozoan skeletal mineralogy. This X-ray diffraction study of modern New Zealand bryozoans more than doubles the published data available on bryozoan mineralogy and allows for statistical analysis of variability in mineralogy, particularly the hitherto unknown background variation within individuals and within populations. Such a database allows evaluation of differences among populations, taxa, and growth forms. The 412 analyses of 49 bryozoan species fall into four mineralogical categories: (a) 61% are single calcite, which ranges widely from low to high wt% MgCO 3 varieties; (b) 6% (in the genera Cellaria and Macropora) comprise dual calcites, a dominant one low in Mg ( X=1.8 wt% MgCO 3) and the other much higher ( X=9.0 wt% MgCO 3); (c) 27% are mainly calcite with some aragonite; and (d) 6% are mainly aragonite with some calcite. The bimineralic calcite–aragonite skeletons can arise from different processes, including biologically mediated changes in precipitation and diagenetic alteration following death. There is small but significant variation in Mg content in calcite both within colonies (Mg increasing with age) and within populations, for a total of within-species variance of 0.526. We found, however, more variation among species in both calcite/aragonite ratio and Mg content than that within species. Whereas some genera and families are reasonably consistent mineralogically (e.g., low-Mg calcite cyclostomes such as Telopora buski), others are highly variable (e.g., Chaperia acanthina). Among the higher taxa, cyclostomes tend to be entirely or mainly calcite with a low to moderate range of MgCO 3 ( X=2.1 wt%), whereas cheilostomes are far more variable, with anascans ( X=4.1 wt%) having a lower MgCO 3 content than ascophorans ( X=5.6 wt%). All dual-calcite species are anascans in the Calloporoidea. The proportions of aragonite and of Mg in calcite are greater in evolutionarily younger taxa, at least at the higher levels. In relation to growth forms, erect rigid varieties are calcitic except for several robust-branching genera such as Adeonellopsis. Encrusting forms, particularly unilaminar sheets, are highly variable and contain most of the bimineralic calcite–aragonite species. Erect flexible forms are either single or dual calcite, and extremely variable in Mg content. Free-living or vagrant forms are usually aragonitic. In relation to latitude and temperature variation, few species show any consistent or significant mineralogical trends, although Schizoporella unicornis deposits increasing amounts of aragonite at lower (warmer) latitudes. Mecynoecia purpurascens includes more (but still low) Mg in its calcite skeleton with increasing latitude, a reversal of the Mg–temperature relationship noted in several other skeletal carbonate producers.

Origins and relationships of Paleozoic coral groups and the origin of the Scleractinia

Two major groups of corals have essentially continuous records from the Early Ordovician (Tabulata) and Middle Ordovician (Rugosa) to the end of the Permian. A third major group, the living Scleractinia, range from Middle Triassic to Holocene. Additional groups have shorter ranges within the Paleozoic. The origins and relationships of these groups have been discussed for over 100 years. Relations between the Rugosa and Scleractinia have attracted the greatest interest because of their morphologic similarities and the time sequence. Arguments involve the significance of serial versus cyclic septal insertion, calcitic versus aragonitic skeletal mineralogy, and the time gap between the last rugosans and first scleractinians (there are no known Lower Triassic corals). Discussions of relationships among the various Paleozoic groups are commonly based on detailed morphological comparisons because of their overlapping stratigraphic ranges.Recent work on the living corals and anemones supports a closer relationship between groups than is suggested by placing them in different orders or suborders. The paleontological record of “anemones” is slight, but it is reasonable to assume that one or more groups of skeletonless zoantharians persisted through long parts of the Phanerozoic. It is suggested that the major groups of zoantharian corals originated through the development of skeletons in various anemone groups at several different times.