Seawater Carbonate Saturation State Research Articles

Girvanella fossils from the Phanerozoic: Distribution, evolution and controlling factors

Girvanella is one of the common genera of cyanobacteria that plays a monumental role in the evolution of life on Earth and the formation of microbialites. Based on a detailed search in the literature of Girvanella fossils, we have compiled a global database of Girvanella fossils and revealed the evolution of Girvanella fossils throughout the Phanerozoic. Four species, Girvanella kasakiensis, Girvanella problematica, Girvanella wetheredii, and Girvanella staminea, are recognized and described. These data show that Girvanella fossils have well-defined temporal distribution during the Paleozoic Era, have a significant temporal gap in the Mesozoic Era, and have only been recorded sporadically in the Cenozoic Era. They were relatively abundant during the Cambrian Epoch 2–Early Ordovician, Late Ordovician, Late Devonian–Mississippian, and tended to lesser degrees during the Silurian–Early Devonian, Lopingian Epoch–Middle Jurassic, and Cretaceous–Present day. Furthermore, the evolution of the abundance and diversity of Girvanella fossils was fundamentally consistent and showed episodical declining during the Phanerozoic. To further explore these relationships, we thoroughly compared them with environmental factors such as seawater carbonate saturation state, Ca2+ concentration, pH values, and atmospheric partial pressure of carbon dioxide (pCO2). This study indicates that seawater carbonate saturation state and Ca2+ concentration are major controls on secular patterns of the abundance and diversity of Girvanella fossils, together with the secondary factors of pH values and pCO2. Considering the long history of Girvanella fossils, their abundance and diversity offer the potential to assist the interpretation of the long-term evolution of marine and atmosphere components during the Phanerozoic.

Implications of giant ooids for the carbonate chemistry of Early Triassic seawater

Abstract Lower Triassic limestones contain giant ooids (>2 mm) along with other precipitated carbonate textures more typical of Precambrian strata. These features appear to have resulted from changes in seawater chemistry associated with the end-Permian mass extinction, but quantifying the carbonate chemistry of Early Triassic seawater has remained challenging. To constrain seawater carbonate saturation state, dissolved inorganic carbon, alkalinity, and pH, we applied a physicochemical model of ooid formation constrained by new size data on Lower Triassic ooids from south China, finding that the Triassic giant ooids require a higher carbonate saturation state than typifies modern sites of ooid formation. Model calculations indicate that Early Triassic oceans were at least seven times supersaturated with respect to aragonite and calcite. When combined with independent constraints on atmospheric pCO2 and oceanic [Ca2+], these findings require that Early Triassic oceans had more than twice the modern levels of dissolved inorganic carbon and alkalinity and a pH near 7.6. Such conditions may have played a role in inhibiting the recovery of skeletal animals and algae during Early Triassic time.

Interactions between sediment production and transport in the geometry of carbonate platforms: Insights from forward modeling of the Great Bank of Guizhou (Early to Middle Triassic), south China

Carbonate platforms grow through the precipitation, transport, and final deposition of carbonate sediment out of seawater. Quantifying the relative contributions of initial production versus subsequent transport in determining the growth rates and geometries of platforms remains a significant challenge. In this study, stratigraphic forward modeling is used to quantify the roles of sediment production, transport, and deposition during each growth stage of a Permian-Triassic carbonate platform with a complex growth history. Parameter optimization and sensitivity analysis show that, within the range of reasonable values, the morphology of the platform is most sensitive to sediment transport, moderately sensitive to maximum carbonate production rate, and least sensitive to the productivity-depth curve. The ramp-to-high relief, steep-sloped platform transition during Early Triassic time can be explained by any factor that limits sediment transport from shallow water areas of high production to the slope and basin. Reefs may play a role in limiting sediment transport on many platforms but other processes, such as early marine cementation or carbonate production along the slope, may be equally capable of yielding this shift in platform geometry. In this particular case, early lithification of ooid and skeletal shoals on the platform margin, perhaps facilitated by unusually high carbonate saturation state of seawater, may have inhibited sediment transport into the basin prior to the development of a reef on the platform margin. Later, Anisian progradation of the platform margin can be explained by the development of a slope factory rather than requiring increased sediment transport from the platform top. The development of an escarpment margin in the Ladinian is mainly influenced by accommodation in the slope profile created by antecedent Anisian topography. A general implication from the model results is that the growth of steep-sloped carbonate platforms lacking slope factories may often be limited by transport of sediment from the platform top to accommodation on the slope rather than by the intrinsic production capacity of the platform top factory.

Effects of low pH and feeding on calcification rates of the cold-water coral Desmophyllum dianthus.

Cold-Water Corals (CWCs), and most marine calcifiers, are especially threatened by ocean acidification (OA) and the decrease in the carbonate saturation state of seawater. The vulnerability of these organisms, however, also involves other global stressors like warming, deoxygenation or changes in sea surface productivity and, hence, food supply via the downward transport of organic matter to the deep ocean. This study examined the response of the CWC Desmophyllum dianthus to low pH under different feeding regimes through a long-term incubation experiment. For this experiment, 152 polyps were incubated at pH 8.1, 7.8, 7.5 and 7.2 and two feeding regimes for 14 months. Mean calcification rates over the entire duration of the experiment ranged between −0.3 and 0.3 mg CaCO3 g−1d−1. Polyps incubated at pH 7.2 were the most affected and 30% mortality was observed in this treatment. In addition, many of the surviving polyps at pH 7.2 showed negative calcification rates indicating that, in the long term, CWCs may have difficulty thriving in such aragonite undersaturated waters. The feeding regime had a significant effect on skeletal growth of corals, with high feeding frequency resulting in more positive and variable calcification rates. This was especially evident in corals reared at pH 7.5 (ΩA = 0.8) compared to the low frequency feeding treatment. Early life-stages, which are essential for the recruitment and maintenance of coral communities and their associated biodiversity, were revealed to be at highest risk. Overall, this study demonstrates the vulnerability of D. dianthus corals to low pH and low food availability. Future projected pH decreases and related changes in zooplankton communities may potentially compromise the viability of CWC populations.

Decreasing carbonate load of seagrass leaves with increasing latitude

Seagrass meadows play a significant role in the formation of carbonate sediments, serving as a substrate for carbonate-producing epiphyte communities. The magnitude of the epiphyte load depends on plant structural and physiological parameters, related to the time available for epiphyte colonization. Yet, the carbonate accumulation is likely to also depend on the carbonate saturation state of seawater (Ω) that tends to decrease as latitude increases due to decreasing temperature and salinity. A decrease in carbonate accumulation with increasing latitude has already been demonstrated for other carbonate producing communities. The aim of this study was to assess whether there was any correlation between latitude and the epiphyte carbonate load and net carbonate production rate on seagrass leaves. Shoots from 8 different meadows of the Zostera genus distributed across a broad latitudinal range (27 °S to up to 64 °N) were sampled along with measurements of temperature and Ω. The Ω within meadows significantly decreased as latitude increased and temperature decreased. The mean carbonate content and load on seagrass leaves ranged from 17% DW to 36% DW and 0.4–2.3 mg CO3 cm−2, respectively, and the associated mean carbonate net production rate varied from 0.007 to 0.9 mg CO3 cm−2 d-1. Mean carbonate load and net production rates decreased from subtropical and tropical, warmer regions towards subpolar latitudes, consistent with the decrease in Ω. These results point to a latitudinal variation in the contribution of seagrass to the accumulation of carbonates in their sediments which affect important processes occurring in seagrass meadows, such as nutrient cycling, carbon sequestration and sediment accretion.

Detecting the calcium carbonate saturation state under the stress of ocean acidification using saturometry technique

CO2-induced ocean acidification lowers the degree of carbonate saturation state of the seawater, which affects calcification of marine organisms and influences the marine carbonate cycle, thus have negative impacts on the entire marine biogeochemical system. This study seeks to develop a rapid technique to detect carbonate saturation state of seawater based on conductivity changes. A series of batch and flow-through experiments were conducted using various CaCO3 materials. Results show that the conductivity ratios of seawater with and without carbonate addition increase generally with decreasing carbonate saturation states (Ω). The relationship between conductivity ratio and log10Ω apparently follows a linear trend when Ω < 1. It suggests that conductivity measurements can be used to indicate carbonate saturation state of seawater. It is expected to be deployed on CTD instrument to produce depth profiles of seawater carbonate saturation state and will be of great help to future studies on ocean acidification.

Boron isotope systematics of cultured brachiopods: Response to acidification, vital effects and implications for palaeo-pH reconstruction

CO2-induced ocean acidification and associated decrease of seawater carbonate saturation state contributed to multiple environmental crises in Earth’s history, and currently poses a major threat for marine calcifying organisms. Owing to their high abundance and good preservation in the Phanerozoic geological record, brachiopods present an advantageous taxon of marine calcifiers for palaeo-proxy applications as well as studies on biological mechanism to cope with environmental change. To investigate the geochemical and physiological responses of brachiopods to prolonged low-pH conditions we cultured Magellania venosa, Terebratella dorsata and Pajaudina atlantica under controlled experimental settings over a period of more than two years. Our experiments demonstrate that brachiopods form their calcite shells under strong biological control, which enables them to survive and grow under low-pH conditions and even in seawater strongly undersaturated with respect to calcite (pH = 7.35, Ωcal = 0.6). Using boron isotope (δ11B) systematics including MC-ICP-MS as well as SIMS analyses, validated against in vivo microelectrode measurements, we show that this resilience is achieved by strict regulation of the calcifying fluid pH between the epithelial mantle and the shell. We provide a culture-based δ11B−pH calibration, which as a result of the internal pH regulatory mechanisms deviates from the inorganic borate ion to pH relationship, but confirms a clear yet subtle pH dependency for brachiopods. At a micro-scale level, the incorporation of 11B appears to be principally driven by a physiological gradient across the shell, where the δ11B values of the innermost calcite record the internal calcifying fluid pH while the composition of the outermost layers is also influenced by seawater pH. These findings are of consequence to studies on biomineralisation processes, physiological adaptations as well as past climate reconstructions.

The enigma of rare Quaternary oolites in the Indian and Pacific Oceans: A result of global oceanographic physicochemical conditions or a sampling bias?

Marine ooids are iconic indicators of shallow seawater carbonate saturation state, and their formation has traditionally been ascribed to physicochemical processes. The Indo-Pacific stands out as a region devoid of oolites, particularly during the Quaternary: the “ooid enigma”. Here we present results from recent coring by the International Ocean Discovery Program (IODP Expedition 356) off west Australia that shows that ooid horizons are common in Pleistocene strata up to 730,000 years old. Extensive “ooid factories” were created due to the presence of long-lived tidally influenced flat–topped tropical platforms suitable for intermittent ooid accretion over hundreds to thousands of years during highstands and times of lower sea level. This work suggests marine ooids may actually be more common in Indo-Pacific than previously reported. Past global ocean alkalinity was elevated during Pleistocene glacial periods and continental climate was generally more arid in the Indo-Pacific region compared to interglacials and the Holocene. Therefore, increased aridity associated with higher alkalinity conditions during the glacials facilitated ooid precipitation on adjacent tropical carbonate platforms particularly offshore from arid Australia. This confluence of factors suggests that more “ooid factories” may be encountered by further coring Indo-Pacific regions with Pleistocene flat long-lived carbonate shelves. However, Indo-Pacific Quaternary ooid occurrences outside Australia are rare, suggesting that the Northwest Shelf may be a unique archive of this non-skeletal precipitate. Further investigations into the petrography and geochemistry of pre-Holocene ooid occurrences will provide insights into their origin and the relative role of biotic, physicochemical and other factors in their formation.

Calcified rivulariaceans from the Ordovician of the Tarim Basin, Northwest China, Phanerozoic lagoonal examples, and possible controlling factors

A distinctive association of rivulariacean-like calcified microfossils is recognized in back-reef lagoon facies on the Bachu-Tazhong Platform in the Lianglitag Formation (Katian Stage, Upper Ordovician) of the Tarim Basin, based on investigation of 4500 thin sections from 35 well drill cores. The genera include Hedstroemia, Ortonella, Zonotrichites, Cayeuxia, and Garwoodia, most of which have features comparable with present-day calcified cyanobacteria such as Rivularia, Calothrix and Dichothrix (Rivulariaceae, Nostocales). A similar association is present in lagoonal and other restricted nearshore shallow-marine carbonate environments during much of the Paleozoic and Mesozoic. This suggests the sustained presence of a rivulariacean-dominated cyanobacterial association characteristic of back-reef/lagoonal environments. At the present-day, uncalcified Rivularia, Calothrix and Dichothrix remain common in back-reef, lagoon, mangrove-swamp, rocky shore, salt-marsh, and saline lake environments. The ability of these cyanobacteria to grow in environments low in inorganic nitrate and phosphate could help to explain this distribution. Cenozoic decline in marine calcified rivulariaceans is attributed to global reduction of seawater carbonate saturation state. The Phanerozoic record of calcified rivulariacean cyanobacteria appears to sensitively reflect long-term variations in the carbonate and nutrient chemistry of marine environments.

Late Jurassic Epiphyton-like cyanobacteria: Indicators of long-term episodic variation in marine bioinduced microbial calcification?

Epiphytaceans occur in Late Jurassic shallow-marine reef limestones at several localities in the Carpathian Mountains of Romania. These calcified dendritic microfossils are well-preserved in sparry calcite as sub-millimetric radial clusters of narrow well-defined filaments, 10–30μm in diameter that show dichotomous branching and consist of dense dark micrite, locally with tubiform structure. Epiphytaceans are widely interpreted as photosynthetic algae or bacteria, but their precise affinities remain elusive and the group may be heterogeneous. These Late Jurassic examples most closely resemble Cambrian Tubomorphophyton and Late Devonian Paraepiphyton. We interpret them to be calcified cyanobacterial sheaths. Post-Devonian records of epiphytaceans are extremely scarce. The Kimmeridgian–Tithonian specimens reported here represent one of the youngest known occurrences of epiphytaceans. Their highly sporadic geological distribution resembles that of marine calcified cyanobacteria, which show Phanerozoic abundance peaks in the Early Paleozoic, Late Devonian–Mississippian, and Late Jurassic–Early Cretaceous. We propose that Kimmeridgian–Tithonian epiphytacean cyanobacteria reflect environmental conditions that favored bioinduced calcification, in particular elevated seawater carbonate saturation state.

COVARIANCE OF MICROFOSSIL ASSEMBLAGES AND MICROBIALITE TEXTURES ACROSS AN UPPER MESOPROTEROZOIC CARBONATE PLATFORM

Early diagenetic chert nodules and beds in the upper Mesoproterozoic Angmaat (formerly Society Cliffs) Formation, Baffin and Bylot islands, preserve microfossils and primary petrofabrics that record microbial mat deposition and lithification across a range of peritidal carbonate environments. Five distinct microfossil assemblages document the distribution of mat-building and mat-dwelling populations across a gradient from restricted, frequently exposed flats to more persistently subaqueous environments. Mats built primarily by thin filamentous or coccoidal cyanobacteria give way to a series of more robust forms that show increasing assemblage diversity with decreasing evidence of subaerial exposure. Distinct fabric elements are associated with each microbial assemblage, and aspects of these petrofabrics are recognizably preserved within unsilicified carbonate in the same beds. These include some features that are distinctly geologic in nature (e.g., seafloor cements) and others that reflect microbial growth and decomposition (e.g., tufted microbialites). A particularly distinctive, micronodular fabric is here interpreted as carbonate infilling of primary voids within microbial mat structures. Such structures mark the co-occurrence of cyanobacterial photosynthesis that produced oxygen gas, filamentous mat builders that imparted the coherence necessary to trap gas bubbles, elevated carbonate saturation required to preserve void fabrics via penecontemporaneous cementation, and a relative paucity of detrital sediment that would have inhibited mat growth. Petrofabrics preserved in Angmaat samples are widespread in upper Paleoproterozoic and Mesoproterozoic carbonate successions but are rare thereafter, perhaps recording, at least in part, the declining carbonate saturation state of seawater. Covariation of microfossil assemblages with petrofabrics in both silicified and unsilicified portions of carbonate beds supports hypotheses that link stromatolite microstructure to the composition and diversity of mat communities.

The influence of temperature and seawater carbonate saturation state on 13C-18O bond ordering in bivalve mollusks

The influence of temperature and seawater carbonate saturation state on 13C-18O bond ordering in bivalve mollusks

Marine invertebrate skeleton size varies with latitude, temperature and carbonate saturation: implications for global change and ocean acidification.

There is great concern over the future effects of ocean acidification on marine organisms, especially for skeletal calcification, yet little is known of natural variation in skeleton size and composition across the globe, and this is a prerequisite for identifying factors currently controlling skeleton mass and thickness. Here, taxonomically controlled latitudinal variations in shell morphology and composition were investigated in bivalve and gastropod molluscs, brachiopods, and echinoids. Total inorganic content, a proxy for skeletal CaCO3 , decreased with latitude, decreasing seawater temperature, and decreasing seawater carbonate saturation state (for CaCO3 as calcite (Ωcal )) in all taxa. Shell mass decreased with latitude in molluscs and shell inorganic content decreased with latitude in buccinid gastropods. Shell thickness decreased with latitude in buccinid gastropods (excepting the Australian temperate buccinid) and echinoids, but not brachiopods and laternulid clams. In the latter, the polar species had the thickest shell. There was no latitudinal trend in shell thickness within brachiopods. The variation in trends in shell thickness by taxon suggests that in some circumstances ecological factors may override latitudinal trends. Latitudinal gradients may produce effects similar to those of future CO2 -driven ocean acidification on CaCO3 saturation state. Responses to latitudinal trends in temperature and saturation state may therefore be useful in informing predictions of organism responses to ocean acidification over long-term adaptive timescales.

Effects of ocean acidification on calcification of symbiont-bearing reef foraminifers

Abstract. Ocean acidification (decreases in carbonate ion concentration and pH) in response to rising atmospheric pCO2 is generally expected to reduce rates of calcification by reef calcifying organisms, with potentially severe implications for coral reef ecosystems. Large, algal symbiont-bearing benthic foraminifers, which are important primary and carbonate producers in coral reefs, produce high-Mg calcite shells, whose solubility can exceed that of aragonite produced by corals, making them the "first responder" in coral reefs to the decreasing carbonate saturation state of seawater. Here we report results of culture experiments performed to assess the effects of ongoing ocean acidification on the calcification of symbiont-bearing reef foraminifers using a high-precision pCO2 control system. Living clone individuals of three foraminiferal species (Baculogypsina sphaerulata, Calcarina gaudichaudii, and Amphisorus hemprichii) were subjected to seawater at five pCO2 levels from 260 to 970 μatm. Cultured individuals were maintained for about 12 weeks in an indoor flow-through system under constant water temperature, light intensity, and photoperiod. After the experiments, the shell diameter and weight of each cultured specimen were measured. Net calcification of B. sphaerulata and C. gaudichaudii, which secrete a hyaline shell and host diatom symbionts, increased under intermediate levels of pCO2 (580 and/or 770 μatm) and decreased at a higher pCO2 level (970 μatm). Net calcification of A. hemprichii, which secretes a porcelaneous shell and hosts dinoflagellate symbionts, tended to decrease at elevated pCO2. Observed different responses between hyaline and porcelaneous species are possibly caused by the relative importance of elevated pCO2, which induces CO2 fertilization effects by algal symbionts, versus associated changes in seawater carbonate chemistry, which decreases a carbonate concentration. Our findings suggest that ongoing ocean acidification might favor symbiont-bearing reef foraminifers with hyaline shells at intermediate pCO2 levels (580 to 770 μatm) but be unfavorable to those with either hyaline or porcelaneous shells at higher pCO2 levels (near 1000 μatm).

Procedures for measurement of carbonate ion concentrations in seawater by direct spectrophotometric observations of Pb(II) complexation

The health of coral reefs and calcareous plankton is strongly influenced by the carbonate saturation state of seawater. Calculations of carbonate saturation states currently require measurements of two CO 2 system parameters, such as pH and total dissolved carbon, plus thermodynamic calculations that relate carbonate ion concentrations to directly measured parameters. In this work we report novel procedures for direct measurements of carbonate ion concentrations and saturation states in seawater. Measurements are obtained via ultraviolet spectroscopic observations of Pb(II) spectra as the relative concentrations of PbCO 3 0 and an ensemble of lead chloride complexes vary in response to dissolved CO 3 2−. Measurement precision is enhanced by parameterization in terms of absorbance ratios. The PbCO 3 0 stability constant, and Pb(II) molar absorbance ratios in seawater, were determined at 25 °C over a range of salinity between 36 and 20. The procedures described in this work are well suited to measurements throughout the normal range of carbonate ion concentrations in the oceans. Rapid equilibration rates for Pb(II) carbonate complexation make the procedures described in this work well suited to rapid direct analysis in situ.

Cyanobacterial calcification, carbon dioxide concentrating mechanisms, and Proterozoic–Cambrian changes in atmospheric composition

ABSTRACTPhotosynthetic uptake of inorganic carbon can raise the pH adjacent to cyanobacterial cells, promoting CaCO3 precipitation. This effect is enhanced by CO2 concentrating mechanisms that actively transport into cells for carbon fixation. CO2 concentrating mechanisms presumably developed in response to atmospheric decrease in CO2 and increase in O2 over geological timescales. In present‐day cyanobacteria, CO2 concentrating mechanisms are induced when the atmospheric partial pressure of CO2 (pCO2) falls below ∼0.4%. Reduction in pCO2 during the Proterozoic may have had two successive effects on cyanobacterial calcification. First, fall in pCO2 below ∼1% (33 times present atmospheric level, PAL) resulted in lower dissolved inorganic carbon (DIC) concentrations that reduced pH buffering sufficiently for isolated CaCO3 crystals to begin to nucleate adjacent to cyanobacterial cells. As a result, blooms of planktic cyanobacteria induced precipitated ‘whitings’ of carbonate mud in the water column whose sedimentary accumulation began to dominate carbonate platforms ∼1400–1300 Ma. Second, fall in pCO2 below ∼0.4% (10 PAL) induced CO2‐concentrating mechanisms that further increased pH rise adjacent to cells and promoted in vivo cyanobacterial sheath calcification. Crossing of this second threshold is indicated in the fossil record by the appearance of Girvanella 750–700 Ma. Coeval acquisition of CO2 concentrating mechanisms by planktic cyanobacteria further stimulated whiting production. These inferences, that pCO2 fell below ∼1%∼1400–1300 Ma and below ∼0.4% 750–700 Ma, are consistent with empirical and modelled palaeo‐atmosphere estimates. Development of CO2 concentrating mechanisms was probably temporarily slowed by global cooling ∼700–570 Ma that favoured diffusive entry of CO2 into cells. Lower levels of temperature and DIC at this time would have reduced seawater carbonate saturation state, also hindering cyanobacterial calcification. It is suggested that as Earth emerged from ‘Snowball’ glaciations in the late Neoproterozoic, global warming and O2 rise reactivated the development of CO2 concentrating mechanisms. At the same time, rising levels of temperature, calcium ions and DIC increased seawater carbonate saturation state, stimulating widespread cyanobacterial in vivo sheath calcification in the Early Cambrian. This biocalcification event promoted rapid widespread development of calcified cyanobacterial reefs and transformed benthic microbial carbonate fabrics.

Monomineralic carbonate ooid types in the Triassic sediments from Northwestern Bulgaria

Oolitic rocks occur in almost all lithostratigraphic units from the Iskur Carbonate Group (Lower – Upper Triassic) in NW Bulgaria. Primarily monomineralic ooids with homogenous cortices belong to several fabric types: micritic/microsparitic, brickwork, sparry relic, blocky, radial fibrous, radial sparry, shrunken and mottled. In addition, ooids with compound cortical fabrics were distinguished: blockymicritic, micritic-radial and radial-brickwork. Cracked, spalled, broken, abraded, distorted and pitted types were differentiated according to outer deformed shape. Apart from the single individuals polyooids were found in some rocks. The primary ooid fabrics were modified by various isochemical diagenetic processes including microbial micritization, compaction, aragonite transformation, recrystallization, dissolution and cementation. These proceeded in the marine-phreatic, meteoric-phreatic and deep-burial connate diagenetic environments. Only one specific ooid association shows relation with a particular depositional setting. At the very base of the Lower Triassic peritidal sequence (Mogila Fm) mottled and radialbrickwork ooids coexist with bimineralic individuals and broken shapes. The formation of such association is attributed to hypersaline conditions, as this interpretation is based on other sedimentological data and analogy with similar occurrences from the geological record. The original mineralogy of some ooid types was inferred mainly on the basis of petrographic and geochemical data, but actualistic evidence was considered as well. The precipitation of aragonite and high-Mg calcite ooids was proved. Their stratigraphic occurrence conforms well to the global distribution of carbonate ooids through the Triassic period. Thus aragonite ooid precipitation was predominant during Late Olenekian – Middle Anisian times. The main control was high carbonate saturation state of seawater promoting higher precipitation rates for aragonite due to kinetic reasons. In this particular case, the high degree of seawater carbonate saturation was probably further promoted by an increased salinity reflecting intense evaporation in an arid peritidal setting. The contrastingly large predominance of calcitic ooids in the Upper Anisian – Lower Carnian sediments corresponds to the secular increase of calcite abiotic precipititation during the Middle Triassic epoch. This change was related to the appearance of global highstand, more intense seafloor spreading, warmer climate, higher CO2 levels, and consequently, to low carbonate saturation state of seawater favouring calcite precipitation. In this particular case, the seawater reached normal salinity from the Middle Anisian onwards, thus probably further promoting calcite ooid formation. Locally controlled short-term shifts in the carbonate precipitation also took place in the epicontinental Triassic sea. This is inferred from the coexistence or interbedding of primary aragonitic and calcitic ooids, as well as presence of bimineralic ooid cortices. It is suggested that subtle temperature and/or salinity changes ultimately controlled the carbonate saturation state of seawater. Unlike the established trend of primary ooid mineralogy, marine phreatic cements in the studied Triassic rocks were predominantly composed of high-Mg calcite. This discrepancy is generally explained by the lower supply of carbonate ions into interparticle pores, which favoured calcite cementation, although poor preservation of marine aragonite cements is another possibility. Further evidence on the abiotic carbonate precipitation might be obtained from investigation of the limestone micritic matrix.

CO2 partial pressure controls the calcification rate of a coral community

SummaryPrevious studies have demonstrated that coral and algal calcification is tightly regulated by the calcium carbonate saturation state of seawater. This parameter is likely to decrease in response to the increase of dissolved CO2 resulting from the global increase of the partial pressure of atmospheric CO2. We have investigated the response of a coral reef community dominated by scleractinian corals, but also including other calcifying organisms such as calcareous algae, crustaceans, gastropods and echinoderms, and kept in an open‐top mesocosm. Seawater pCO2 was modified by manipulating the pCO2 of air used to bubble the mesocosm. The aragonite saturation state (Ωarag) of the seawater in the mesocosm varied between 1.3 and 5.4. Community calcification decreased as a function of increasing pCO2 and decreasing Ωarag. This result is in agreement with previous data collected on scleractinian corals, coralline algae and in a reef mesocosm, even though some of these studies did not manipulate CO2 directly. Our data suggest that the rate of calcification during the last glacial maximum might have been 114% of the preindustrial rate. Moreover, using the average emission scenario (IS92a) of the Intergovernmental Panel on Climate Change, we predict that the calcification rate of scleractinian‐dominated communities may decrease by 21% between the pre‐industrial period (year 1880) and the time at which pCO2 will double (year 2065).