Calcite and aragonite seas and the de novo acquisition of carbonate skeletons

A longstanding question in paleontology has been the influence of calcite and aragonite seas on the evolution of carbonate skeletons. An earlier study based on 21 taxa that evolved skeletons during the Ediacaran through Ordovician suggested that carbonate skeletal mineralogy is determined by seawater chemistry at the time skeletons first evolve in a clade. Here I test this hypothesis using an expanded dataset comprising 40 well‐defined animal taxa that evolved skeletons de novo in the last 600 Myr. Of the 37 taxa whose mineralogy is known with some confidence, 25 acquired mineralogies that matched seawater chemistry of the time, whereas only two taxa acquired non‐matching mineralogies. (Ten appeared during times when seawater chemistry is not well constrained.) The results suggest that calcite and aragonite seas do have a strong influence on carbonate skeletal mineralogy, however, this appears to be true only at the time mineralized skeletons first evolve. Few taxa switch mineralogies (from calcite to aragonite or vice versa) despite subsequent changes in seawater chemistry, and those that do switch do not appear to do so in response to changing aragonite–calcite seas. This suggests that there may be evolutionary constraints on skeletal mineralogy, and that although there may be increased costs associated with producing a mineralogy not favored by seawater, the costs of switching mineralogies are even greater.     

Calcite Formation in Soft Coral Sclerites Is Determined by a Single Reactive Extracellular Protein

Calcium carbonate exists in two main forms, calcite and aragonite, in the skeletons of marine organisms. The primary mineralogy of marine carbonates has changed over the history of the earth depending on the magnesium/calcium ratio in seawater during the periods of the so-called “calcite and aragonite seas.” Organisms that prefer certain mineralogy appear to flourish when their preferred mineralogy is favored by seawater chemistry. However, this rule is not without exceptions. For example, some octocorals produce calcite despite living in an aragonite sea. Here, we address the unresolved question of how organisms such as soft corals are able to form calcitic skeletal elements in an aragonite sea. We show that an extracellular protein called ECMP-67 isolated from soft coral sclerites induces calcite formation in vitro even when the composition of the calcifying solution favors aragonite precipitation. Structural details of both the surface and the interior of single crystals generated upon interaction with ECMP-67 were analyzed with an apertureless-type near-field IR microscope with high spatial resolution. The results show that this protein is the main determining factor for driving the production of calcite instead of aragonite in the biocalcification process and that –OH, secondary structures (e.g. α-helices and amides), and other necessary chemical groups are distributed over the center of the calcite crystals. Using an atomic force microscope, we also explored how this extracellular protein significantly affects the molecular-scale kinetics of crystal formation. We anticipate that a more thorough investigation of the proteinaceous skeleton content of different calcite-producing marine organisms will reveal similar components that determine the mineralogy of the organisms. These findings have significant implications for future models of the crystal structure of calcite in nature.     

Carbonate mineralogy of a tropical bryozoan biota and its vulnerability to ocean acidification

Decreasing pH levels in the world’s oceans are widely recognized as a threat to marine life. Bryozoans are among several phyla that produce calcium carbonate skeletons potentially affected by ocean acidification (OA). Depending on species, bryozoan skeletons can consist of calcite, aragonite or have a bimineralic combination of these two minerals. Aragonite is generally more soluble in seawater than calcite, making aragonitic species more vulnerable to OA. Here, for the first time we use Raman spectroscopy to determine the mineral composition of a tropical bryozoan biota. Compared with bryozoan biotas from higher latitudes in which calcite predominates, aragonite was found to occur in a much higher proportion of the 22 cheilostome bryozoan species collected from the shorelines of Penang and Langkawi in Malaysia, where 46% of species are calcitic, 41% aragonitic and 13% bimineralic. All but one of the aragonitic or bimineralic species belong to the ascophorans, whereas calcitic skeletons characterized most of the anascans, many of which are primitive ‘weedy’ malacostegines. These results suggest a relatively high vulnerability of tropical bryozoan faunas to OA, with the weedier taxa likely to be least impacted.     

Effects of Organic Matter on Solubilities and Crystal Form of Carbonates

The results of a study of the role of organic compounds in theformation of carlxmate crystals in marine biological systemsare reported. In an increasing concentration of certain organiccompounds which complex calcium ions, the proportion of aragonitedecreases and that of calcite increases. In increasing concentrationsof magnesium ions the proportion of aragonite increases andthat of calcite and vaterite decreases. When the influence oforganic compounds is greater or smaller than that of magnesiumions, only calcite or only aragonite is formed, respectively.Organic compounds forming a strong complex with calcium ionscause the formation of magnesium-rich calcite, and with an increasein temperature and the concentration of magnesium ions, themagnesium carbonate content of precipitated magnesian calciteincreases. When the influence of organic compounds is almostequivalent to that of magnesium ions, in increasing or decreasingtemperatures, the proportion of calcite decreases or increases,respectively, and the proportion of aragonite increases or decreases,respectively. The concentration of magnesium ions in the bodyfluids of marine calcareous organisms seems to differ littlefrom that of other organisms, and seems to be similar to thatof sea water. Only the presence of certain organic compoundsbrings about the formation of the carbonate crystals observedin marine biological systems. The very important role of organicmatter in the formation of crystals found in skeletal carbonatesis emphasized.     

Calcite and aragonite distributions in the skeletons of bimineralic bryozoans as revealed by Raman spectroscopy

Abstract. Bryozoans are among a diverse range of invertebrates capable of secreting calcium carbonate skeletons. Relatively little is known about biomineralization in bryozoans, despite the importance of understanding biomineralization processes for nanotechnology and the threats imposed by ocean acidification on organisms having calcareous skeletons. Ten species of cheilostome bryozoans that are reported to have bimineralic skeletons of calcite and aragonite are studied here using Raman spectroscopy. This technique allowed identification of the two mineral phases at submicron spatial resolution, allowing the distributions of calcite and aragonite within bryozoan skeletons to be determined with unprecedented precision. Confirming previous findings based on the use of chemical stains, most of the bimineralic species analyzed exhibited a calcitic skeletal framework, composed of basal, vertical, and inner frontal walls, having aragonite deposited subsequently onto the outer surfaces of the frontal walls. In one species ( Odontionella cyclops ), aragonite formed the superstructure above the autozooids, and in two others, traces of aragonite were detected on the undersides of the frontal shields. Using Raman spectroscopy, it was possible for the first time to determine the mineralogy of small-scale structures, including orificial rims, condyles and hinge teeth, avicularian pivotal bars and rostra, and ascopore rims and sieve plates. Even when surrounded by aragonitic frontal shields, these structures were found typically to be calcitic, the two exceptions being the aragonitic avicularia of Stylopoma inchoans and O. cyclops . Unexpectedly, the first-formed part of the basal wall at the distalmost growing edge of Pentapora foliacea was found to consist mainly of aragonite. This may point to a precursory phase of biomineralization comparable with the unusual mineralogies identified previously in the earliest-formed skeletons of members of some other invertebrate phyla.     

Shell mineralogical trends in epifaunal Mesozoic bivalves and their relationship to seawater chemistry and atmospheric carbon dioxide concentration

The Late Triassic-Early Jurassic change from aragonite- to calcite-facilitating conditions in the oceans, which was caused by a decrease of the Mg²⁺/Ca²⁺ ratio of seawater in combination with an increase of the partial pressure of carbon dioxide, also affected the shell mineralogy of epifaunal bivalves. In the “calcite sea” of the Jurassic and Cretaceous, the most diverse and abundant families of epifaunal bivalves had largely calcitic shells. Some of them, such as the Inoceramidae, acquired this shell mineralogy earlier in Earth's history but did not significantly diversify until the onset of “calcite sea” conditions. Others, however, replaced aragonite by calcite in their shell at the beginning of the Jurassic, as shown for the Ostreidae, Gryphaeidae, Pectinidae, Plicatulidae, and Buchiidae. In these families, replacement of aragonite by calcite took place in the middle and inner layer of the shell and was not associated with changes in morphology and life habit. It is therefore proposed that lower metabolic costs rather than higher resistance against dissolution or advantageous physical properties triggered the calcite expansion in their shells. This explanation fits well the observation that clades of thin-shelled bivalves were less affected by the change of seawater chemistry. Thick-shelled clades, by contrast, may suffer a severe decline in diversity until they adapt their shell mineralogy, as demonstrated by the Hippuritoida: The diversity of the Megalodontoidea, which failed to adapt their shell mineralogy to “calcite sea” conditions, dramatically decreased at the end of the Triassic, whereas their descendents became dominant carbonate producers during the Late Mesozoic after they acquired a calcitic outer shell layer in the Late Jurassic. These examples indicate that changes in the seawater chemistry and in the partial pressure of carbon dioxide are factors that influence the diversity of carbonate-secreting animals, and, as in the case of the decline of the Megalodontoidea, may contribute to mass extinctions.     

Perspectives in coral reef hydrodynamics

Knowledge of skeletogenesis in scleractinian corals is central to reconstructing past ocean and climate histories, assessing and counteracting future climate and ocean acidification impacts upon coral reefs, and determining the taxonomy and evolutionary path of the Scleractinia. To better understand skeletogenesis and mineralogy in extant scleractinian corals, we have investigated the nature of the initial calcium carbonate skeleton deposited by newly settling coral recruits. Settling Acropora millepora larvae were sampled daily for 10 days from initial attachment, and the carbonate mineralogy of their newly deposited skeletons was investigated. Bulk analyses using Raman and infrared spectroscopic methods revealed that the skeletons were predominantly comprised of aragonite, with no evidence of calcite or an amorphous precursor phase, although presence of the latter cannot be discounted. Sensitive selected area electron diffraction analyses of sub-micron areas of skeletal regions further consolidated these data. These findings help to address the uncertainty surrounding reported differences in carbonate mineralogy between larval and adult extant coral skeletons by indicating that skeletons of new coral recruits share the same aragonitic mineralogy as those of their mature counterparts. In this respect, we can expect that skeletogenesis in both larval and mature growth stages of scleractinian corals will be similarly affected by ocean acidification and predicted environmental changes.     

Mineralogy of Arctic bryozoan skeletons in a global context

Bryozoans are major carbonate producers in some ancient and Recent benthic environments, including parts of the Arctic Ocean. Seventy-six species of bryozoans from within the Arctic Circle have been studied using XRD to determine their carbonate mineralogies and the Mg content of the calcite. The majority of species were found to be calcitic, only four having bimineralic skeletons that combined calcite and aragonite, and none being entirely aragonitic. In almost all species, the calcite was of the low- (<4 mol% MgCO₃) or intermediate-Mg (4–11.99 mol% MgCO₃) varieties. Previous regional studies of bryozoan biomineralogy have found higher proportions of bimineralic and/or aragonitic species in New Zealand and the Mediterranean, with a greater number of calcitic species employing intermediate- and high-Mg calcite. The Antarctic bryozoan fauna, however, has a similar mineralogical composition to the Arctic. The lesser solubility of low-Mg calcite compared to both Mg calcite and aragonite in cold polar waters is most likely responsible for this latitudinal pattern. However, it is unknown to what extent environmental factors drive the pattern directly through eliciting an ecophenotypic response from the bryozoans concerned or the pattern reflects genetic adaptations by particular bryozoan clades.     

Not all aragonitic molluscs are missing: taphonomy and significance of a unique shelly lagerstätte from the Jurassic of SW Britain

The Blue Lias Formation at Lyme Regis (Dorset, UK) includes an exceptional pavement of abundant large ammonites that accumulated during a period of profound sedimentary condensation. Ammonites were originally composed of aragonite, an unstable polymorph of calcium carbonate, and such fossils are typically prone to dissolution; the occurrence of a rich association of aragonitic shells in a condensed bed is highly unusual. Aragonite dissolution occurs when pore‐water pH is reduced by the oxidization of hydrogen sulphide close to the sediment‐water interface. Evidence suggests that, in this case, the oxygen concentrations in the overlying water column were low during deposition. This inhibited the oxidation of sulphides and the associated lowering of pH, allowing aragonite to survive long enough for the shell to be neomorphosed to calcite. The loss of aragonite impacts upon estimates of past biodiversity and carbonate accumulation rates. The preservational model presented here implies that diagenetic loss of aragonite will be greatest in those areas where dysoxic‐anoxic sediment lies beneath an oxic waterbody but least where the sediment and overlying water are oxygen depleted. Unfortunately, this implies that preservational bias through aragonite loss will be greatest in those biotopes which are typically most diverse and least where biodiversity is lowest due to oxygen restriction.     

Variations in primary aragonite, calcite, and clay in fine-grained calcareous rhythmites of Cambrian to Jurassic age— an environmental archive?

Limestone-marl alternations represent a common type of fine-grained calcareous rhythmites during the entire Phanerozoic. Their diagenetic overprint, however, obliterates their value for palaeoenvironmental interpretations. The original mineralogical composition of the carbonate fraction (aragonite, high-Mg calcite, low-Mg calcite) would potentially yield important information on palaeoenvironmental conditions: for example shallow-water carbonate factories are usually characterised by extensive aragonite production, whereas pelagic carbonate production is dominated by calcitic organisms. Therefore, a reconstruction of the pre-diagenetic mineralogical composition of limestone-marl precursors would be desirable. A particularly conspicuous attribute of fine-grained calcareous rhythmites is the intercalation of two rock types that have undergone two entirely different diagenetic pathways (“differential diagenesis”). As indicated by earlier petrography work, in the interlayers selective aragonite dissolution has taken place, and the dissolved aragonite provided the cement for the limestones. Primary aragonite usually is not preserved in diagenetically mature fine-grained limestones. However, in a recently published paper a method is proposed to quantify the primary mineralogical composition of the precursor sediments of a fine-grained calcareous rhythmite. Here we apply this method to several published data sets from sections of Cambrian to Jurassic age. We try to answer the following questions: Where does the aragonite come from, especially during times of “calcite seas”? What is the impact of the enhanced pelagic carbonate production since the Late Jurassic on the formation of limestone-marl alternations? How much dissolved aragonite is lost to sea water during early marine burial diagenesis, i.e. how closed is the diagenetic system? As demonstrated for the five examples shown here, the new method for reconstructing primary mineralogy potentially provides insight into ancient depositional environments, surface productivity, and ocean chemistry.     

Non-skeletal carbonate production and stromatolite growth within a pleistocene deep ocean (Last glacial maximum, Red Sea)

Summary Quasi modern non-skeletal carbonates and stromatolites occur from the Red Sea shelf, proximal to the reef systems and continuous into the axial basin (512 to 2704 metres below present sea level). The lithified carbonates are intermixed with carbonate ooze forming a hard layer of 50 to 90 cm thickness. Decimetric platey fragments of lithified carbonate exhibit planar to columnar stromatolitic growth forms, lumpy microbial fabrics and internal brecciation features. The ultrastructures of lithified carbonates are formed by cryptocrystalline carbonate, mainly as aragonite spherulites, Mg-calcite peloids, anhedral mosaics of Mg-calcite and aragonite. Biogenic overgrowth, intercalations of calciturbidites and the type of interlocking relationships in the crystalline fabrics all indicate precipitation at the sea floor or close by. Morphological and mineralogical similarity with previously published microfabrics indicate microbial mediation of carbonate precipitation. The hard layer was formed during the last glacial maximum (LGM, approx. 23ka to 13ka), when pelagic sedimentation rates droped to a minimum due to high surface water salinities. Precipitation of the excess calcium carbonate in the deep ocean was strongly enhanced because the Hanish sill at the southern end of the Red Sea blocked input of cold bottom waters which in turn caused bottom water with high temperatures and salinities (>50 ‰), and low oxygen content. The scenario of a starved greenhouse-type ocean for the Red Sea during the LGM compliments models for the origin of ancient (Mesozoic and Palaeozoic) deepwater stromatolites.     

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.     

New constraints on the last aragonite–calcite sea transition from early Jurassic ooids

Calcite and aragonite seas are commonly distinguished based on the prevailing primary mineralogy of ooids and carbonate cements over time. Secular oscillations of these seas are usually attributed to changes in ocean chemistry and paleoclimate. While the veracity of such oscillations has been verified by independent data and modeling approaches, the timing of the transition from one ocean state to the other remains poorly resolved. Here, the timing of the last aragonite–calcite sea transition is estimated by assessing the preservation of Early Jurassic ooids from the Trento Platform in northern Italy. Point counting of ooid-bearing limestones from four distinct stratigraphic levels provides a contrasting pattern: Hettangian and Sinemurian ooids are all poorly preserved and were probably predominantly originally aragonitic, whereas Pliensbachian and Toarcian ooids are excellently preserved, suggesting a primary calcitic mineralogy. Although calcitic ooids may have already been common in the Late Triassic, it is proposed that the last aragonite–calcite sea transition occurred in the Early Jurassic between the Sinemurian and Pliensbachian, at least in this subtropical region. Therefore, the selective extinction of aragonite-secreting organisms at the end-Triassic mass extinction cannot be attributed to secular changes in ocean chemistry.     

Geochemical environments of continental shelf-upper slope sediments in the northern Gulf of Mexico

Geochemical environments were characterized for 14 sites along the northern Gulf of Mexico continental shelf and upper slope, in an effort to examine the relationship between sediment geochemistry and carbonate shell taphonomy in a long-term study—Shelf and Slope Experimental Taphonomy Initiative (SSETI). Three groups of environments of preservation (seep, near-seep, and shelf-and-slope) were identified based on their geochemical characteristics (i.e., oxygen uptake rate and penetration depth, pore-water saturation states, and carbonate dissolution fluxes). Diffusive oxygen uptake rate increased in the order of shelf-and-slope, near-seep, and seep, although carbonate dissolution flux did not show significant correlation with O₂ flux, presumably due to non-diffusive behavior at some sites. Using pore-water saturation indices with respect to aragonite and calcite and sedimentation rates, we defined a semi-quantitative parameter, carbonate dissolution index (CDI), to predict carbonate preservation potential during the taphonomic processes. Our limited database suggests that both the seep and the shelf-and-slope sediments may have higher carbonate preservation potential than the near-seep sediments.     

On the potential for ocean acidification to be a general cause of ancient reef crises

Anthropogenic rise in the carbon dioxide concentration in the atmosphere leads to global warming and acidification of the oceans. Ocean acidification (OA) is harmful to many organisms but especially to those that build massive skeletons of calcium carbonate, such as reef corals. Here, we test the recent suggestion that OA leads not only to declining calcification of reef corals and reduced growth rates of reefs but may also have been a trigger of ancient reef crises and mass extinctions in the sea. We analyse the fossil record of biogenic reefs and marine organisms to (1) assess the timing and intensity of ancient reef crises, (2) check which reef crises were concurrent with inferred pulses of carbon dioxide concentrations and (3) evaluate the correlation between reef crises and mass extinctions and their selectivity in terms of inferred physiological buffering. We conclude that four of five global metazoan reef crises in the last 500 Myr were probably at least partially governed by OA and rapid global warming. However, only two of the big five mass extinctions show geological evidence of OA.     

Rapid warming and salinity changes in the Gulf of Maine alter surface ocean carbonate parameters and hide ocean acidification

A profound warming event in the Gulf of Maine during the last decade has caused sea surface temperatures to rise to levels exceeding any earlier observations recorded in the region over the last 150 years. This event dramatically affected CO₂ solubility and, in turn, the status of the sea surface carbonate system. When combined with the concomitant increase in sea surface salinity and assumed rapid equilibration of carbon dioxide across the air sea interface, thermodynamic forcing partially mitigated the effects of ocean acidification for pH, while raising the saturation index of aragonite (\(\varOmega_{AR}\)) by an average of 0.14 U. Although the recent event is categorically extreme, we find that carbonate system parameters also respond to interannual and decadal variability in temperature and salinity, and that such phenomena can mask the expression of ocean acidification caused by increasing atmospheric carbon dioxide. An analysis of a 34-year salinity and SST time series (1981–2014) shows instances of 5–10 years anomalies in temperature and salinity that perturb the carbonate system to an extent greater than that expected from ocean acidification. Because such conditions are not uncommon in our time series, it is critical to understand processes controlling the carbonate system and how ecosystems with calcifying organisms respond to its rapidly changing conditions. It is also imperative that regional and global models used to estimate carbonate system trends carefully resolve variations in the physical processes that control CO₂ concentrations in the surface ocean on timescales from episodic events to decades and longer.     

Carbonate crystal growth controlled by interfacial interactions of artificial cell membranes

Morphology of carbonate crystals grown on the surface of artificial cell membranes was controlled by changing the interfacial chemistry. For octadecyltriethoxysilane (OTE) films with terminal methyl groups interacting little with an aqueous calcium carbonate solution, calcite (104) crystals were formed. Polymerized pentacosadiynoic acid (PDA) films with terminal carboxylic acid groups induced deposition of calcite (012) crystals aligned along with each other within a polymer domain. On the other hand, stearyl alcohol (StOH) films with terminal hydroxyl groups induced deposition of aragonite crystals. When PDA was mixed with StOH, the 8∶1 PDA∶StOH (molar ratio) film produced dominating calcite (012) crystals without any crystal alignment, and the 4∶1 mixture film produced minor calcite (012) crystals and major aragonite crystals. For the 2∶1, 1∶1, 1∶2, and 1∶4 mixture films, aragonite crystals were dominating. Hence, it is found that the chemical composition at the interface plays a very important role in controlling the morphology of deposited carbonate crystals.     

Miocene methane‐derived carbonates from southwestern Washington,USA and a model for silicification at seeps

Kuechler, R.R., Birgel, D, Kiel, S, Freiwald, A, Goedert, J.L., Thiel, V & Peckmann, J. 2011: Miocene methane‐derived carbonates from southwestern Washington, USA and a model for silicification at seeps. Lethaia, Vol. 45, pp. 259–273. Exotic limestone masses with silicified fossils, enclosed within deep‐water marine siliciclastic sediments of the Early to Middle Miocene Astoria Formation, are exposed along the north shore of the Columbia River in southwestern Washington, USA. Samples from four localities were studied to clarify the origin and diagenesis of these limestone deposits. The bioturbated and reworked limestones contain a faunal assemblage resembling that of modern and Cenozoic deep‐water methane‐seeps. Five phases make up the paragenetic sequence: (1) micrite and microspar; (2) fibrous, banded and botryoidal aragonite cement, partially replaced by silica or recrystallized to calcite; (3) yellow calcite; (4) quartz replacing carbonate phases and quartz cement; and (5) equant calcite spar and pseudospar. Layers of pyrite frequently separate different carbonate phases and generations, indicating periods of corrosion. Negative δ¹³C_carbonate values as low as ?37.6‰ V‐PDB reveal an uptake of methane‐derived carbon. In other cases, δ¹³C_carbonate values as high as 7.1‰ point to a residual, ¹³C‐enriched carbon pool affected by methanogenesis. Lipid biomarkers include ¹³C‐depleted, archaeal 2,6,10,15,19‐pentamethylicosane (PMI; δ¹³C: ?128‰), crocetane and phytane, as well as various iso‐ and anteiso‐carbon chains, most likely derived from sulphate‐reducing bacteria. The biomarker inventory proves that the majority of the carbonates formed as a consequence of sulphate‐dependent anaerobic oxidation of methane. Silicification of fossils and early diagenetic carbonate cements as well as the precipitation of quartz cement – also observed in other methane‐seep limestones enclosed in sediments with abundant diatoms or radiolarians – is a consequence of a preceding increase of alkalinity due to anaerobic oxidation of methane, inducing the dissolution of silica skeletons. Once anaerobic oxidation of methane has ceased, the pH drops again and silica phases can precipitate. □Biomarkers, carbonates, isotopes, methane, Miocene, silicification, Washington.     

Quaternary dune carbonates from the Mediterranean Coast of Egypt: Petrography and diagenesis

Summary The Quaternary carbonates of the Mediterranean coast of Egypt between Alexandria and Salum appear as parallel limestone ridges rising up to 100 m above sea level. These ridges are dominated by dunal carbonates which differ not only in their primary composition but also by distinct grades of meteoric water diagenesis. Oolitic facies dominates the younger aeolianites of the first and second ridges. Bioclastic facies with abundant coralline algae, benthonic foraminifers, molluscs, echinoderms and intraclasts represents the major rock type in the older aeolianites. Features of meteoric water diagenesis include precipitation of increasing amounts of avoid-filling low Mg-calcite spar, dissolution of aragonite and stabilization of aragonite and high Mg-calcite to low Mg-calcite. Aeolianites below paleosol horizons contain abundant calcrete cements, micritized fossils and detrital grains which are commonly corroded and replaced by calcite. Three stages of progressive meteoric diagenesis are recognised in the Egyptian Quaternary aeolianites. In stage 1 minor precipitation of low Mg-calcite occurs at the grain boundaries. Stage 2 is marked by partial dissolution of aragonite, partial loss of high Mg-calcite and precipitation of low Mg-calcite in some pore spaces. In stage 3, most of the remaining pores are occluded by cementation. At the end of stage 3, Mg is removed from high Mg-calcite but some aragonite still persists. The early vadose cements are represented by miniscus, bridge and pendant cements. The phreatic cements were precipitated as bladed spar in the isopachous rims and equant spar in the intergranular and mouldic porosity. The late vadose cements are represented by micritic cements that were related to calcrete formation. Elemental behaviour during meteoric water diagenesis indicates that it leads to a gradual decrease in bulk Sr along with Na in progressively altered aeolianites. Mn distribution is controlled by the carbonate mineralogy (aragonite versus calcite) as well as the proximity of the aeolianites to the overlying paleosol horizons.     

Perlinhibin, a cysteine-, histidine-, and arginine-rich miniprotein from abalone (Haliotis laevigata) nacre, inhibits in vitro calcium carbonate crystallization

We have isolated a 4.785 Da protein from the nacreous layer of the sea snail Haliotis laevigata (greenlip abalone) shell after demineralization with acetic acid. The sequence of 41 amino acids was determined by Edman degradation supported by mass spectrometry. The most abundant amino acids were cysteine (19.5%), histidine (17%), and arginine (14.6%). The positively charged amino acids were almost counterbalanced by negatively charged ones resulting in a calculated isoelectric point of 7.86. Atomic-force microscopy studies of the interaction of the protein with calcite surfaces in supersaturated calcium carbonate solution or calcium chloride solution showed that the protein bound specifically to calcite steps, inhibiting further crystal growth at these sites in carbonate solution and preventing crystal dissolution when carbonate was substituted with chloride. Therefore this protein was named perlinhibin. X-ray diffraction investigation of the crystal after atomic-force microscopy growth experiments showed that the formation of aragonite was induced on the calcite substrate around holes caused by perlinhibin crystal-growth inhibition. The strong interaction of the protein with calcium carbonate was also shown by vapor diffusion crystallization. In the presence of the protein, the crystal surfaces were covered with holes due to protein binding and local inhibition of crystal growth. In addition to perlinhibin, we isolated and sequenced a perlinhibin-related protein, indicating that perlinhibin may be a member of a family of closely related proteins.