Calcite Seas Research Articles

Controlling effects of paleo-climate and paleo-ocean on formation of carbonate reservoirs

Abstract The controlling effects of paleo-climate and paleo-ocean on the formation of marine carbonate reservoirs were studied taking key carbonate strata in the Tarim, Sichuan and Ordos Basins as examples. Dry heat/aridity and moist heat/moisture are the two distinctive types of paleo-climate as far as carbonate sedimentation and reservoir formation are concerned. The geochemistry of the Phanerozoic ocean experienced cyclic variation, with calcite sea and aragonite sea occurring alternately. Carbonate exploration practice in the Tarim, Sichuan and Ordos Basins shows that the features of carbonate reservoir are evidently controlled by paleo-climate and paleo-ocean environment. The aragonite sea under dry heat/arid climate is most favorable for the formation of porous dolomite reservoirs. The calcite sea under dry heat/arid climate is favorable for the formation of porous dolomite reservoirs with evaporite moldic pores. The calcite sea and aragonite sea under moist heat/moist climate may only lead to the formation of cavernous carbonate reservoir. Paleo-climate and paleo-ocean environment exert great influence on carbonate sedimentation and its diagenesis, and ultimately determine the porosity features and types of carbonate reservoirs.

Component-specific petrographic and geochemical characterization of fine-grained carbonates along Carboniferous and Jurassic platform-to-basin transects

Fine-grained carbonates are present throughout much of the geological record and are widely used as geochemical archives, even though their origin and diagenetic pathways remain poorly understood. Here, petrographical and geochemical properties of granulometrically separated component spectra of marine mudstones sampled along two proximal-to-distal transects (Carboniferous of Spain and Jurassic of Morocco) are documented. These settings represent end members in terms of platform geometry, steep flanked versus gentle ramp, and the aragonite versus calcite sea mode. The data from Spain reveal a bimodal organization of microcrystalline carbonate isotope values from platform top and slope and toe-of-slope settings. The data from Morocco lack a clear spatial and bathymetrical pattern. The significance of the complex, site-specific biological and physico-chemical parameters is emphasized. Mudstones have been separated in granulometric fractions of 8–5, 5–3 and <3μm respectively, and resulting particle classes are described and interpreted in terms of their origin and diagenetic pathways. Fine-grained carbonate particles from both sites show remarkably similar size and crystallographic features. Their isotopic composition reflects the volumetrically proportion and component-specific geochemical signature of each particle class. Decreasing particle size classes are characterized by decreasing isotope values. This might be due to an enhanced diagenetic reactivity of the finest micritic particles to diagenetic processes. This implies that stratigraphic differences in mean fine carbonate grain sizes may trigger shifts in isotope values. Mean bulk and mean component-specific isotope ratios from the two case settings differ by about 0.5‰ for carbon and 0.7‰ for oxygen. The results shown here are of general significance for those concerned with fine-grained carbonates-based chemostratigraphy and environmental analysis.

Formation of low‐magnesium calcite at cold seeps in an aragonite sea

AbstractThis study investigates the conditions of occurrence and petrographic characteristics of low‐Mg calcite (LMC) from cold seeps of the Gulf of Mexico at a water depth of 2340 m. Such LMC mineral phases should precipitate in calcite seas rather than today's aragonite sea. The 13C‐depleted carbonates formed as a consequence of anaerobic oxidation of hydrocarbons in shallow subsurface cold seep environments. The occurrence of LMC may result from brine fluid flows. Brines are relatively Ca2+‐enriched and Mg2+‐depleted (Mg/Ca mole ratio &lt;0.7) relative to seawater, where the Mg/Ca mole ratio is ~5, which drives high‐Mg calcite and aragonite precipitation. The dissolution of aragonitic mollusk shells, grains and cements was observed. Aerobic oxidation of hydrocarbons and H2S is the most likely mechanism to explain carbonate dissolution. These findings have important implications for understanding the occurrence of LMC in deep water marine settings and consequently their counterparts in the geological record.

Seawater chemistry driven by supercontinent assembly, breakup, and dispersal

Global oceans are known to have alternated between aragonite and calcite seas. These oscillations refl ect changes in the Mg/Ca ratios of seawater that control biomineralization and the composition of marine carbonates, and are thought to be caused mainly by the time dependence of crustal accretion at mid-ocean ridge crests and the associated high-temperature mid-ocean ridge fl uid fl ux. Here we use global ocean basin reconstructions to demonstrate that the fl uctuations in hydrothermal ocean inputs are instead caused by the gradual growth and destruction of mid-ocean ridges and their relatively cool fl anks during long-term tectonic cycles, thus linking ocean chemistry to off-ridge low-temperature hydrothermal exchange. Early Jurassic aragonite seas were a consequence of supercontinent stability and a minimum in mid-ocean ridge length and global basalt alteration. The breakup of Pangea resulted in a gradual doubling in ridge length and a 50% increase in the ridge fl ank area, leading to an enhanced volume of basalt to be altered. The associated increase in the total global hydrothermal fl uid fl ux by as much as 65%, peaking at 120 Ma, led to lowered seawater Mg/Ca ratios and marine hypercalcifi cation from 140 to 35 Ma. A return to aragonite seas with preferential aragonite and high-Mg calcite precipitation was driven by pronounced continental dispersal, leading to progressive subduction of ridges and their fl anks along the Pacifi c rim.

Microhabitat complexity—an example from Middle Devonian bryozoan-rich sediments in the Blankenheim Syncline (northern Eifel, Rheinisches Schiefergebirge)

A Middle Devonian section in the Blankenheim Syncline (northern Eifel, Rheinisches Schiefergebirge) contains a rather diverse fauna dominated by bryozoans, brachiopods, corals and other taxa, such as calcimicrobes. The section covers sediments of the Junkerberg Formation and the transition to the Freilingen Formation. The transition between the Junkerberg and Freilingen formations coincides with the base of the otomari Event Interval. The section displays an overall transgressive trend in shallow subtidal and deeper subtidal environments below fair-weather wave base. In contrast to the general transgressive trend, one horizon is interpreted to represent a regressive trend of local origin. Sedimentation rates were quite low, as indicated by the abundant occurrence of suspension-feeding organisms. According to Standard Microfacies Types (SMF), the carbonates represent mainly wackestones and floatstones and can be assigned to SMF 5 and SMF 8, respectively. Complex fossil communities in some horizons suggest excellent living conditions during deposition. Large abundances of calcifying taxa, such as Girvanella, occur in some layers, particularly at the base of the Junkerberg Formation. Other encrusting/calcifying organisms are present in various layers, suggesting deposition during supersaturation of CaCO3 minerals in a calcite sea in a low-energy environment. The studied bryozoan fauna is very diverse throughout the entire section, erect branching colonies being dominant. Five new taxa are described: the trepostomes Trachytoechus globosus sp. n., Multihemiphragma minor sp. n. and Eridotrypella multaeformis sp. n., and the rhabdomesine cryptostomes Isostylus vulgaris n. sp. and Bigeyella prima n. sp. Lenaporidae fam. n. is erected for the genera Lenapora and Bigeyella.

Relic aragonite from Ordovician–Silurian brachiopods: Implications for the evolution of calcification

Understanding the influence of aragonite/calcite sea conditions on the evolution of biocalcification relies strongly on the correct interpretation of the original composition of calcareous taxa. Aragonite dissolves or inverts into calcite over geological time, and its preservation is currently unknown to predate the Pennsylvanian. Here we present direct evidence for the common occurrence of relic aragonite in Ordovician and Silurian trimerellid brachiopods, thereby extending the known range of aragonite preservation by more than 130 million years. Together with associated hypercalcifying taxa of putatively original aragonite or high-magnesium calcite composition and considerations of the temperature dependence of aragonite and calcite precipitation, our results suggest that the evolution of aragonite biomineralization might have presented an adaptive advantage in shallow marine tropical waters of calcite seas. A targeted search for Paleozoic aragonite should both resolve the original composition of consistently recrystallized taxa and enable the reassessment of the aragonite/calcite sea paradigm in a paleoenvironmental context.

Chronology of early Cambrian biomineralization

AbstractData on the first appearances of major animal groups with mineralized skeletons on the Siberian Platform and worldwide are revised and summarized herein with references to an improved carbon isotope stratigraphy and radiometric dating in order to reconstruct the Cambrian radiation (popularly known as the ‘Cambrian explosion’) with a higher precision and provide a basis for the definition of Cambrian Stages 2 to 4. The Lophotrochozoa and, probably, Chaetognatha were first among protostomians to achieve biomineralization during the Terreneuvian Epoch, mainly the Fortunian Age. Fast evolutionary radiation within the Lophotrochozoa was followed by radiation of the sclerotized and biomineralized Ecdysozoa during Stage 3. The first mineralized skeletons of the Deuterostomia, represented by echinoderms, appeared in the middle of Cambrian Stage 3. The fossil record of sponges and cnidarians suggests that they acquired biomineralized skeletons in the late Neoproterozoic, but diversification of both definite sponges and cnidarians was in parallel to that of bilaterians. The distribution of calcium carbonate skeletal mineralogies from the upper Ediacaran to lower Cambrian reflects fluctuations in the global ocean chemistry and shows that the Cambrian radiation occurred mainly during a time of aragonite and high-magnesium calcite seas.

Seawater chemistry and biomineralization: did trepostome bryozoans become hypercalcified in the ‘calcite sea’ of the Ordovician?

Secular variations in the proportion of Mg and Ca ions in seawater during the Phanerozoic have driven alternations between calcite seas (Mg:Ca 2). There is mounting evidence that these changes in seawater chemistry have impacted the evolution of marine organisms constructing calcareous skeletons, favouring calcite as the CaCO3 biomineral during times of calcite seas but aragonite during times of aragonite seas. It has been suggested that some organisms became hypercalcified when the mineralogy of their skeletons matched seawater type. This paper tests the proposal that calcitic trepostome bryozoans (‘stony bryozoans’) became hypercalcified in the calcite sea of the Ordovician. Data on two independent hypercalcification proxies—the diameter of branches, and exozonal wall thickness—have been compiled from the literature for ramose trepostome species from the Ordovician (calcite sea), Devonian (calcite sea) and Permian (aragonite sea). No significant difference was found in branch diameter between the calcite and aragonite sea periods, whereas wall thickness was found to be greater in the Permian than in the Ordovician and Devonian, counter to expectations. Either these two parameters are inadequate as proxies for hypercalcification or, more likely, trepostomes did not become hypercalcified in the calcite sea of the Early Palaeozoic, probably because they exerted a higher degree of control over their biomineralization than some other groups such as corals.

The role of SO4 in the switch from calcite to aragonite seas

Research Article| April 01, 2011 The role of SO4 in the switch from calcite to aragonite seas P. Bots; P. Bots 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK Search for other works by this author on: GSW Google Scholar L.G. Benning; L.G. Benning 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK Search for other works by this author on: GSW Google Scholar R.E.M. Rickaby; R.E.M. Rickaby 2Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK Search for other works by this author on: GSW Google Scholar S. Shaw S. Shaw 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK Search for other works by this author on: GSW Google Scholar Author and Article Information P. Bots 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK L.G. Benning 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK R.E.M. Rickaby 2Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK S. Shaw 1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK Publisher: Geological Society of America Received: 28 Jul 2010 Revision Received: 13 Oct 2010 Accepted: 08 Nov 2010 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2011 Geological Society of America Geology (2011) 39 (4): 331–334. https://doi.org/10.1130/G31619.1 Article history Received: 28 Jul 2010 Revision Received: 13 Oct 2010 Accepted: 08 Nov 2010 First Online: 09 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 P. Bots, L.G. Benning, R.E.M. Rickaby, S. Shaw; The role of SO4 in the switch from calcite to aragonite seas. Geology 2011;; 39 (4): 331–334. doi: https://doi.org/10.1130/G31619.1 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 Throughout the Phanerozoic, the primary inorganic marine calcium carbonate mineralogy oscillated between calcite and aragonite, reflecting changes in seawater chemistry. These variations in seawater composition also appear to have influenced the evolution of calcifying organisms. However, the processes controlling these mineralogical and biological changes are poorly constrained. Previous work has focused mainly on the Mg/Ca ratio in seawater as the primary driver. Here, we examine the role of dissolved SO4 in these processes by performing controlled laboratory precipitation experiments and geochemical modeling of evaporite formation. We show that an increase in dissolved SO4 decreases the Mg/Ca ratio at which calcite is destabilized and aragonite becomes the dominant CaCO3 polymorph. Our data suggest that the Mg/Ca and SO4 thresholds for formation of calcite seas are significantly lower than previous estimates and are mutually dependent. This shows that our understanding of Phanerozoic changes in seawater chemistry and the models relating this to primary CaCO3 mineralogy need reevaluation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Crystallographic Control of the Hydrothermal Conversion of Calcitic Sea Urchin Spine (Paracentrotus lividus) into Apatite

We analyzed the crystallographic relationships during hydrothermal conversion of a calcitic sea urchin spine into apatite. We identified a pseudomorphic mineral replacement mechanism involving a superficial dissolution of calcite and a subsequent overgrowth of oriented carbonated hydroxylapatite (HA) nanocrystals. Cross-section images of these converted spines show that the dimensions of the HA crystals increase the further they are from the outer surface. This replacement process is favored by an increase in porosity, which enables both fluid and mass to be transported by diffusion, thereby allowing the replacement reaction to progress toward the interior of the spine. These recrystallization reactions take place on the surface of the calcite single crystal, which acts as a substrate for the epitaxial nucleation of HA crystals. The epitaxial relationship observed between the parent calcite crystal and the newly formed apatite crystals can be defined as (0001) apatite//(011̅8) calcite and [10.0] apatite//[4̅4.1] calcite. The apatite crystals are related by the 3-fold axis arising from the trigonal symmetry of the parent calcite crystal. There is therefore a strong structural control which favors the conversion of calcite into apatite. This process coexists with the formation of apatite crystals which are not structurally related to the calcite crystal and which may precipitate within the porosity of the material. The analysis of crystallographic relationships is a fundamental step toward understanding mineral replacement reactions, which can be used for the synthesis of artificial materials with predefined shapes and microstructural characteristics, a technique that may have interesting technological applications.

Review: geological and experimental evidence for secular variation in seawater Mg/Ca (calcite-aragonite seas) and its effects on marine biological calcification

Abstract. Synchronized transitions in the polymorph mineralogy of the major reef-building and sediment-producing calcareous marine organisms and abiotic CaCO3 precipitates (ooids, marine cements) throughout Phanerozoic time are believed to have been caused by tectonically induced variations in the Mg/Ca ratio of seawater (molar Mg/Ca&gt;2="aragonite seas", &lt;2="calcite seas"). Here, I assess the geological evidence in support of secular variation in seawater Mg/Ca and its effects on marine calcifiers, and review a series of recent experiments that investigate the effects of seawater Mg/Ca (1.0–5.2) on extant representatives of calcifying taxa that have experienced variations in this ionic ratio of seawater throughout the geologic past. Secular variation in seawater Mg/Ca is supported by synchronized secular variations in (1) the ionic composition of fluid inclusions in primary marine halite, (2) the mineralogies of late stage marine evaporites, abiogenic carbonates, and reef- and sediment-forming marine calcifiers, (3) the Mg/Ca ratios of fossil echinoderms, molluscs, rugose corals, and abiogenic carbonates, (4) global rates of tectonism that drive the exchange of Mg2+ and Ca2+ along zones of ocean crust production, and (5) additional proxies of seawater Mg/Ca including Sr/Mg ratios of abiogenic carbonates, Sr/Ca ratios of biogenic carbonates, and Br concentrations in marine halite. Laboratory experiments have revealed that aragonite-secreting bryopsidalean algae and scleractinian corals and calcite-secreting coccolithophores exhibit higher rates of calcification and growth in experimental seawaters formulated with seawater Mg/Ca ratios that favor their skeletal mineral. These results support the assertion that seawater Mg/Ca played an important role in determining which hypercalcifying marine organisms were the major reef-builders and sediment-producers throughout Earth history. The observation that primary production increased along with calcification within the bryopsidalean and coccolithophorid algae in mineralogically favorable seawater is consistent with the hypothesis that calcification promotes photosynthesis within some species of these algae through the liberation of CO2. The experiments also revealed that aragonite-secreting bryopsidalean algae and scleractinian corals, and bacterial biofilms that secrete a mixture of aragonite and high Mg calcite, began secreting an increased proportion of their calcium carbonate as the calcite polymorph in the lower-Mg/Ca experimental seawaters. Furthermore, the Mg/Ca ratio of calcite secreted by the coccolithophores, coralline red algae, reef-dwelling animals (crustacea, urchins, calcareous tube worms), bacterial biofilms, scleractinian corals, and bryopsidalean algae declined with reductions in seawater Mg/Ca. Notably, Mg fractionation in autotrophic organisms was more strongly influenced by changes in seawater Mg/Ca than in heterotrophic organisms, a probable consequence of autotrophic organisms inducing a less controlled mode of calcification simply through the removal of CO2 via photosynthesis. These results indicate that biomineralogical control can be partially overridden by ambient seawater Mg/Ca and suggest that modern aragonite-secreting organisms may have secreted a mixture of aragonite and low Mg calcite, and that modern high Mg calcite-secreting organisms probably secreted low Mg calcite, in calcite seas of the past. These effects of seawater Mg/Ca on the polymorph mineralogy and calcite Mg/Ca ratio of calcareous skeletons should be accounted for in thermal-chemical reconstructions of seawater that are based upon skeletal Mg/Ca. Lastly, by identifying how marine calcifiers respond to changes in seawater Mg/Ca and absolute Ca2+ concentration, this work should enhance our interpretation of parallel studies investigating the effects of anthropogenic CO2-induced ocean acidification on marine calcification.

Application of calcite Mg partitioning functions to the reconstruction of paleocean Mg/Ca

Calcite Mg/Ca is usually assumed to vary linearly with solution Mg/Ca, that a constant partition coefficient describes the relationship between these two ratios. Numerous published empirical datasets suggests that this relationship is better described by a power function. We provide a compilation of these literature data for biotic and abiotic calcite in the form of Calcite Mg/Ca = F(Solution Mg/Ca) H , where F and H are empirically determined fitting parameters describing the slope and deviation from linearity, respectively, of the function. This is equivalent to Freundlich sorption behavior controlling Mg incorporation in calcite. Using a power function, instead of a partition coefficient, lowers Phanerozoic seawater Mg/Ca estimates based on echinoderm skeletal material by, on average, 0.5 mol/mol from previous estimates. These functions can also be used to model the primary skeletal calcite Mg/Ca of numerous calcite phases through geologic time. Such modeling suggests that the Mg/Ca of all calcite precipitated from seawater has varied through the Phanerozoic in response to changing seawater Mg/Ca and that the overall range in Mg/Ca measured among various calcite phases would be greatest when seawater Mg/Ca was also high (e.g., “aragonite seas”) and lowest when seawater Mg/Ca was low (e.g., “calcite seas”). It follows that, during times of “calcite seas” when the seawater Mg/Ca is presumed to have been lower, deposition of calcite with low Mg contents would have resulted in a depressed drive for diagenetic stabilization of shelfal carbonate and, in turn, lead to greater preservation of crystal and skeletal microfabrics and primary chemistries in biotic and abiotic calcites.

Microporous and tight limestones in the Urgonian Formation (late Hauterivian to early Aptian) of the French Jura Mountains: Focus on the factors controlling the formation of microporous facies

Microporous and tight limestones, with contrasting porosity and permeability values directly related to the microfabric of the micritic matrix, outcrop in the Urgonian Formation of the French Jura Mountains. This study investigates the factors controlling the differentiation between the microporous and tight facies, and proposes a diagenetic model for the development and preservation of the microporosity in these limestones. The petrophysical properties are not related to the depositional texture, the petrographical content or the mineralogical composition. However, the tight layers contain indications of emersion (e.g.: bird eyes, keystone vugs, and desiccation cracks). The sedimentation in very shallow conditions up to emersion is confirmed by the covariant more positive values of oxygen and carbon isotopes. The microporous intervals systematically occur a few meters below the tight layers affected by emersion. This position strongly suggests the importance of meteoric water input rapidly after sedimentation in the differentiation between tight and microporous limestones. The diagenetic model proposed for the development and preservation of the microporous facies involves partial early cementation of the interstitial mud, mainly composed of low-Mg calcite crystals (sedimentation during a calcite sea period), inside a meteoric phreatic lens by in situ dissolution–reprecipitation processes (“hybrid Ostwald ripening”). This early cementation partly preserves the original microfabric and intercrystalline microporosity and allows the carbonate sediment to resist compaction during burial. The identification of the conditions favorable to the development of microporosity in these Urgonian limestones may improve the knowledge and modeling of some microporous carbonate reservoir rocks.

Influences of alkalinity and pCO2 on CaCO3 nucleation from estimated Cretaceous composition seawater representative of “calcite seas”

Research Article| February 01, 2010 Influences of alkalinity and pCO2 on CaCO3 nucleation from estimated Cretaceous composition seawater representative of “calcite seas” Janie Lee; Janie Lee * Department of Oceanography, Texas A&M University, College Station, Texas 77843, USA *E-mail: jlee@ocean.tamu.edu. Search for other works by this author on: GSW Google Scholar John W. Morse John W. Morse Department of Oceanography, Texas A&M University, College Station, Texas 77843, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Janie Lee * Department of Oceanography, Texas A&M University, College Station, Texas 77843, USA John W. Morse Department of Oceanography, Texas A&M University, College Station, Texas 77843, USA *E-mail: jlee@ocean.tamu.edu. Publisher: Geological Society of America Received: 17 Jul 2009 Revision Received: 20 Aug 2009 Accepted: 21 Aug 2009 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2010 Geological Society of America Geology (2010) 38 (2): 115–118. https://doi.org/10.1130/G30537.1 Article history Received: 17 Jul 2009 Revision Received: 20 Aug 2009 Accepted: 21 Aug 2009 First Online: 09 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 Janie Lee, John W. Morse; Influences of alkalinity and pCO2 on CaCO3 nucleation from estimated Cretaceous composition seawater representative of “calcite seas”. Geology 2010;; 38 (2): 115–118. doi: https://doi.org/10.1130/G30537.1 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 Experiments were conducted on CaCO3 nucleation in seawater of estimated Cretaceous Period “calcite sea” (i.e., low-magnesium calcite being the primary inorganic marine calcium carbonate precipitate) composition to determine the influences of alkalinity and pCO2 on calcite versus aragonite formation. The pH was continually monitored throughout the experiments and it was used, in combination with the initial alkalinity, to calculate the pCO2 and saturation state of aragonite and calcite at the time of nucleation. The morphology and mineralogy of the precipitates were determined using scanning electron microscopy and X-ray diffraction analyses. It was observed that the initial alkalinity affected greatly the nucleation pCO2 and the CaCO3 polymorph that is precipitated. In seawater with Mg2+/Ca2+ = 1.2, ∼10 mM alkalinity, and a pCO2 <2500 μatm, calcite is the initial polymorph that nucleates. Aragonite nucleates when the pCO2 is >2500 μatm. Seawater with Mg2+/Ca2+ = 1.2 and a wide range of initial alkalinities (5–50 mM) produces variable results. Seawater with Mg2+/Ca2+ = 1.7 produces only aragonite at lower alkalinities (<∼11 mM), but calcite is formed when the alkalinity is >∼18 mM. These results demonstrate the need to also consider the effects of alkalinity and pCO2 in the critical Mg2+/Ca2+ ratio region of ∼1–2 for calcite seas versus “aragonite seas” (in which the primary inorganic carbonate precipitates are aragonite and high-magnesium calcite) at various times throughout the Phanerozoic Eon and to use reasonable estimated values of alkalinity and pCO2 in experimental studies. The results of this study indicate that it may have been possible for CaCO3 to commonly nucleate directly from Cretaceous seawater due to elevated calcium and alkalinity concentrations, even though atmospheric pCO2 was higher. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Increased Production of Calcite and Slower Growth for the Major Sediment-Producing Alga Halimeda as the Mg/Ca Ratio of Seawater is Lowered to a "Calcite Sea" Level

Abstract Laboratory experiments were conducted to subject the calcareous alga Halimeda, a prolific producer of aragonite in the modern ocean, to a variety of ambient Mg/Ca ratios. These experiments were based on three predictions: (1) that Halimeda would calcify at a lower rate when the ambient Mg/Ca ratio was reduced from its high level in modern seawater (5.2); (2) that this alga would grow more slowly under these conditions because its calcification yields as a byproduct CO2, which is available for photosynthesis; and (3) that the calcite content of Halimeda’s skeleton would increase as a percentage of total CaCO3. As predicted, reduction of the Mg/Ca ratio of ambient artificial seawater from its modern level to estimated early Oligocene and early Eocene levels (2.5 and 1.5, respectively) resulted in progressively lower rates of calcification, production of organic matter, and linear growth for Halimeda incrassata colonies. The ambient Mg/Ca ratio exerted a stronger influence over the rate of calcification than [Ca2+], which in the geologic past has risen in seawater whenever the ambient Mg/Ca ratio has declined. Thus, although elevation of [Ca2+] at such times would have promoted the calcification and growth of Halimeda, the concomitant decline of the Mg/Ca ratio would have had a stronger negative effect. In addition, the percentage of skeletal material consisting of calcite rose from 8% in modern-day seawater to 16% in artificial early Oligocene seawater and 46% in artificial early Eocene seawater. Magnesium was incorporated into this calcite as a function of the ambient Mg/Ca ratio with a fractionation pattern differing only slightly from that of nonskeletal calcite. Halimeda exerts less biological control over its biomineralization than the related genera Penicillus and Udotea, which in previous experiments produced only 22–25% of skeletal CaCO3 as calcite in seawater having a very low Mg/Ca ratio (1.0). Nonetheless, it is likely that many taxa of green algae that produce little or no calcite today would have produced a substantial proportion of skeletal low-Mg CaCO3 as calcite in ancient seas, such as those of the Cretaceous, whose Mg/Ca ratios were close to unity.

New data on molluscs and their shell microstructures from the Middle Cambrian Gowers Formation, Australia

Numerous new cases of preserved shell micro- structure were discovered in molluscs from the Middle Cam- brian Gowers Formation (Ptychagnostus atavus ⁄Peronopsis opimus Zone, Floran Stage) in the Georgina Basin, Australia. The new data provide further evidence that, by the Middle Cambrian, molluscan shell microstructures were diverse, and many molluscs had a complex shell with multiple types of shell microstructure. In addition, many new occurrences of laminar microstructures are described herein. For many, the nature of these laminar microstructures is not known, but in three species the microstructure is foliated calcite, and in at least two the microstructure is more likely to have been cal- citic semi-nacre, a type of microstructure known in brachio- pods and bryozoans but unknown in modern molluscs. This commonality among these three closely related lophotroch- ozoans underscores a similar mechanism of biomineraliza- tion. Moreover, these observations suggest a prevalence of calcite-shelled lineages among molluscs from the Middle Cambrian, a time of calcite seas. In addition, the broad occurrence of laminar, nacre-like microstructures in many of these fossils reveals how widespread these strong (fracture- resistant) microstructures were in Middle Cambrian mol- luscs. Additionally, a few specimens of Yochelcionella preserve imprints of a bilaterally symmetrical pair of muscle scars. New taxa described here include Corystos thorntoniensis gen. et sp. nov., Yochelcionella snorkorum sp. nov., Yochelcionella saginata sp. nov., and Anhuiconus? agrenon sp. nov.

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.

SHALLOW‐MARINE MICROPOROUS CARBONATE RESERVOIR ROCKS IN THE MIDDLE EAST: RELATIONSHIP WITH SEAWATER Mg/Ca RATIO AND EUSTATIC SEA LEVEL

The formation of shallow‐marine microporous carbonate reservoir rocks remains poorly understood in spite of their economic importance, particularly in the Middle East. In this paper, we investigate relationships between the stratigraphic occurrence of these carbonates in the Middle East and (i) the evolution of the Mg/Ca ratio in seawater; and (ii) cyclic variations in relative sea‐level.An inventory of carbonate formations in the Middle East was compiled for three geological time intervals characterised by different seawater chemistries: the Late Carboniferous to Triassic (aragonite seas); the Cretaceous (calcite seas); and the Cenozoic (transitional from calcite to aragonite seas). For each time interval, carbonate formations described as microporous have been listed.During the Cretaceous calcite sea, eleven microporous carbonate formations were deposited in the Middle East. However, no microporous carbonates were formed during the Late Carboniferous to Triassic, a time of aragonite seas. During the Cenozoic, four of the five microporous carbonate formations recorded were deposited before the transition from calcite to aragonite seas. Thus, these shallow‐marine microporous carbonates appear to have developed from precursor muds which were mainly composed of low‐Mg calcite crystals. Moreover, during the Cretaceous and the Cenozoic, microporous carbonate formations in the Middle East were generally associated with major transgressions and highstands of relative sea level.The relatively high stability of low‐Mg calcite muds may explain why shallow‐marine microporous carbonates formed during time intervals with calcite seas. In contrast to muds composed of aragonite or high‐Mg calcite crystals, the original microfabric (including intercrystalline microporosity) of low‐Mg calcite muds can partly survive moderate diagenesis.

Effects of secular variation in seawater Mg/Ca ratio (calcite–aragonite seas) on CaCO3 sediment production by the calcareous algae Halimeda, Penicillus and Udotea– evidence from recent experiments and the geological record

AbstractIndependent lines of geological evidence suggest that fluctuations in the Mg/Ca ratio of seawater between 1.0 and 5.2 have caused the oceans to alternate between favouring the precipitation of the aragonite and high‐Mg calcite polymorphs of calcium carbonate (mMg/Ca &gt; 2; aragonite seas) and the low‐Mg calcite polymorph (mMg/Ca &lt; 2; calcite seas) throughout Phanerozoic time. The rise of aragonite‐secreting bryopsidalean algae as major producers of carbonate sediments in middle Palaeogene time, a role that they maintained through to the present, has been attributed to a transition from calcite‐to‐aragonite seas in early Cenozoic time. Recent experiments on the modern, carbonate‐sediment‐producing bryopsidales Halimeda, Penicillus and Udotea reveal that their rates of calcification, linear extension and primary production decline when reared in experimental calcite seawaters (mMg/Ca &lt; 2). These normally aragonite‐secreting algae also began producing at least one‐quarter of their CaCO3 as calcite under calcite sea conditions, indicating that their biomineralogical control can be partially overridden by ambient seawater chemistry. The observation that primary production and linear extension declined along with calcification in the mineralogically unfavourable seawater suggests that photosynthesis within these algae is enhanced by calcification via liberation of CO2 and/or H+. Thus, the reduced fitness of these algae associated with their low rates of calcification in calcite seas may have been exacerbated by concomitant reductions in tissue mass and algal height.

Secular Fluctuations in Palaeozoic and Mesozoic Reef-Forming Organisms During Greenhouse Periods: Geobiological Interrelations and Consequences

Reefs have formed recurrently in the Phanerozoic and occupied a central place in the development of Earth's complex interactions; as such, reefs are essentially a microcosm of the geobiological system. The long-term waxing and waning of reefs was governed not only by the evolution of organisms themselves, but also by cyclical marine conditions (e.g., calcitic sea vs. aragonitic sea; greenhouse vs. icehouse; oligotrophy vs. eutrophy; and carbonate saturation states) that led to the replacement of dominant kinds of taxa. Viewed in perspective, Palaeozoic to Mesozoic reefs changed fundamentally, particularly during greenhouse periods, due to the shifts in marine environmental regimes and ensuing improvements. Metazoan reefs formed temporarily immediately after the earliest Cambrian biomineralization and subsequently became dominant after the Ordovician radiations, as a consequence of networked geobiological interrelationships that were triggered by the invasion of biota onto land. Major extinctions were also triggers for the subsequent (re)diversifications of preexisting, potential reef builders, not necessarily accompanied by new higher taxa, and for the replacement of dominant organisms, whether reef builders or not. Given that certain regimes persisted, progressive, internal modifications and improvements also accelerated in accordance with niche exploitations that are clearly evidenced by changes in the overall growth morphologies of reef builders. Reefs as a microcosm should be further deciphered in light of how both skeletal organisms and microbes mutually responded to ambient, changing habitat conditions, and how sequential contributors prevailed in turn under given conditions at various spatio-temporal scales. Such studies would elucidate the evolution of geobiological constituents and their interrelationships with the underlying background, and more importantly, irreversibly progressive changes in Earth history.