Low-magnesium Calcite Research Articles

High-Magnesium Calcite Dissolution in Tropical Continental Shelf Sediments Controlled by Ocean Acidification

Increases in atmospheric CO2 cause the oceanic surface water to continuously acidify, which has multiple and profound impacts on coastal and continental shelf environments. Here we present the carbonate mineral composition in surface sediments from a range of continental shelf seabed environments and their current and predicted stability under ocean acidifying conditions. Samples come from the following four tropical Australian regions: (1) Capricorn Reef (southern end of the Great Barrier Reef), (2) the Great Barrier Reef Lagoon, (3) Torres Strait, and (4) the eastern Joseph Bonaparte Gulf. Beyond the near-shore zone, these regions typically have a carbonate content in surface sediments of 80 wt % or more. The abundance of high-magnesium calcites (HMC) dominates over aragonite (Arag) and low-magnesium calcite (LMC) and constitutes between 36% and 50% of all carbonate. HMC, with a magnesium content larger than 8-12 mol %, is more soluble than both Arag and LMC, and the solubility of HMC positively correlates with its magnesium concentration. From the solubility data of Plummer and Mackenzie ( Am. J. Sci. 1974 , 274 , 61 - 83 ), 95% of HMC in the four regions is presently in metastable equilibrium relative to global mean tropical sea surface water. HMC is predicted to become destabilized in the four regions between 2040 and 2080 AD, with typical HMC decline rates between 2% and 5% per year. The range of respective estimated carbonate dissolution rates is expected to exceed current continental shelf carbonate accumulation rates, leading to net dissolution of carbonate during the period of HMC decline. In a geological context, the decline in HMC in tropical continental shelf environments is a global event triggered by reaching below-equilibrium conditions. The characteristic change in carbonate mineral composition in continental shelf sediments will serve as a geological marker for the proposed Anthropocene Epoch.

Characterization and origin of permeability–porosity heterogeneity in shallow-marine carbonates: From core scale to 3D reservoir dimension (Middle Jurassic, Paris Basin, France)

Nuclear magnetic resonance (NMR), stable isotope geochemistry of micro-sampled cores, NMR well-logs and 3D modeling are used to investigate the carbonate permeability-porosity heterogeneity along 230 m-thick limestones of the Paris Basin. Despite the global low porosity and permeability of the limestones, two aquifers units with porosity greater than 15% were identified. These two aquifers are very different in terms of pore through radii and NMR signal. The first one (A1: Aquifer 1) is a 7 m-thick mudstone unit, dominated by extended microporosity with pore throat radii of 0.25 μm to 0.3 μm. The second one (A2: Aquifer 2) is a 15 m-thick oolitic grainstone units showing macropores reaching 100 μm and pore throat radii of 32 μm. From core descriptions and wireline logs on 26 wells, a 3D static geological model is build. The fine tuning of permeability calculations from NMR logs realized along 12 of the wells, allows porosity and permeability heterogeneity to be distributed within a 3D model at the reservoir scale (area of about 2000 km2) which match the flow behavior illustrated by well tests.Associated with early meteoric calcite cements and poorly developed burial blocky calcite cements, the porous and permeable intervals may be predicted in two stratigraphic and diagenetic considerations. Firstly, the syn-sedimentary meteoric dissolution or neomorphism of the initial high magnesium calcite and aragonite particles or clasts into low magnesium calcite particles or cements prevented most mechanical and chemical compaction during the first steps of burial. Secondly, the regional stratigraphic architecture reveals the presence of local permeability barriers, which prevented Early Cretaceous lateral meteoric water circulation and the associated burial calcite cementation.

The Effect of Climate on the Chemical Composition of the Coral Reefs around Kubbar Island—Kuwait

Thick recent carbonate sediments cover all the intertidal and subtidal zones consist of mainly different shell and coral reef debris. The chemical investigations of the coral fragments were done on controlled bases, which depend on the coral size and seasons, in order to elucidate the effect of weather parameters on the coral reefs. Geochemical investigation indicated that corals gathered from high-tide mark are mainly contaminated by copper, cobalt, and chromium and the corals gathered from low-tide mark are contaminated by lead and zinc. All the analyzed coral samples are contaminated by copper. The major contamination sights by heavy metals are related to the weather pattern around the islands. Calcium concentration in all the corals indicates a low-Magnesium calcite or aragonite composition, except for some coral samples which were gathered during October from the high-tide mark. Trace element concentration levels show high concentrations at sights of low-wave action around the island during the various seasons. Chemical analysis on the coral derbies indicates compositional variations regarding calcium and magnesium mole% between the seasons and at different locations and it is dependant on wind direction and water salinity. Some localities are iron rich others are aluminum and potassium rich. Generally, aluminum, iron, and potassium occur in lower quantities than the other elements. Therefore, all the chemical data regarding trace elements indicate relationship to weather and environmental parameters.

Quaternary coral reefs of the Red Sea coast, Egypt: diagenetic sequence, isotopes and trace metals contamination

This study focuses on the diagenetic sequence under marine and meteoric conditions as well as isotopes and trace metals contamination in Quseir and Gebel Zeit areas along the Egyptian Red Sea coast through a series of modern and fossil corals, Porites lutea and Favites pentagona. The diagenetic sequence begins with deposition of thin fringes of syntaxial aragonite and micritic high-magnesian calcite in the modern corals to completely altered Porites and partially altered Favites to low-magnesium calcite in the oldest Pleistocene unit. Average δ18O and δ13C values of Pleistocene corals in the two studied areas were lower than those of modern corals. Values of modern corals and lower fossil unit indicated coralline limestone, while those of middle and upper fossil units indicated fresh water influences. Average values of trace metals in modern corals were higher than those of Pleistocene counterpart except for Mn. Modern coral samples recorded enrichment in the average values of Pb, Zn, and Mn at Quseir area and enrichment in Co, Cu, and Ni at Gebel Zeit area. This may be attributed mostly to different tourist activities, landfill due to increase urbanization and nearby of Quseir area from the old phosphate harbor at El Hamrawin area, as well as oil exploration and production activities in the Gulf of Suez area. Also, results indicated that most samples of Porites have high concentration of trace metals than in Favites, especially in Cu, Zn, Mn, and Pb. This may due to high amounts of intergranular porosity and high total surface area of Porites in contrast to Favites.

Mineralogical and petrophysical characterization of stromatolite from Salgada lagoon, Campos dos Goytacazes (RJ), Brazil

Approximately 60% of oil and 40% of gas world reserves are in carbonate reservoirs. These reservoirs are heterogeneous with a textural variety and typically fractured, leading to low recovery factors and also complex relationships between rock properties and geophysical data (Vanorio et al, 2008). Studies of these reservoirs have become very important in Brazil after 2005, when Petrobras announced a large carbonate reserve in the pre-salt region of Santos basin. The reservoir characterization requests a better understanding of the relationship between geology, petrophysical and acoustic properties. In this work, we made a mineralogical and petrophysical characterization of a stromatolite from Salgada lagoon. Measurements of X-ray diffraction (XRD) and scanning electron microscopy (SEM) showed that the stromatolite is composed mainly by high-magnesian calcite (HMC), with a small amount of aragonite and quartz. The thin section observation revealed the presence of skeletal material (shells), quartz grains and some regions that suggest recrystallization of high-magnesian calcite into low-magnesian calcite (LMC). The porosity of this sample is approximately 20%, as measured with a gas porosimeter, and these pores have different shapes and sizes, as observed with a polarizing petrographic microscope. Using a rock physical system it was possible to measure the traveltime of P wave and thus determine the velocity, which is slightly higher than 3000 m/s.

Fungi, bacteria and soil pH: the oxalate–carbonate pathway as a model for metabolic interaction

The oxalate-carbonate pathway involves the oxidation of calcium oxalate to low-magnesium calcite and represents a potential long-term terrestrial sink for atmospheric CO(2). In this pathway, bacterial oxalate degradation is associated with a strong local alkalinization and subsequent carbonate precipitation. In order to test whether this process occurs in soil, the role of bacteria, fungi and calcium oxalate amendments was studied using microcosms. In a model system with sterile soil amended with laboratory cultures of oxalotrophic bacteria and fungi, the addition of calcium oxalate induced a distinct pH shift and led to the final precipitation of calcite. However, the simultaneous presence of bacteria and fungi was essential to drive this pH shift. Growth of both oxalotrophic bacteria and fungi was confirmed by qPCR on the frc (oxalotrophic bacteria) and 16S rRNA genes, and the quantification of ergosterol (active fungal biomass) respectively. The experiment was replicated in microcosms with non-sterilized soil. In this case, the bacterial and fungal contribution to oxalate degradation was evaluated by treatments with specific biocides (cycloheximide and bronopol). Results showed that the autochthonous microflora oxidized calcium oxalate and induced a significant soil alkalinization. Moreover, data confirmed the results from the model soil showing that bacteria are essentially responsible for the pH shift, but require the presence of fungi for their oxalotrophic activity. The combined results highlight that the interaction between bacteria and fungi is essential to drive metabolic processes in complex environments such as soil.

First note on Holocene coquinite on Thrace (Black Sea) coast of Turkey

In this paper, preliminary results concerning the nature, subsurface structure and age of carbonate-cemented coquinite extending along a 1.5-km long and 40-m wide loose coquina beach on the Thracian (west Black Sea) coast of Turkey are presented. An Electrical Resistivity Tomography (ERT) survey showed that the coquinite has a maximum thickness of about 2m and comprises seaward dipping buried 20m-wide slabs under beach materials at the backshore. Cemented by low-magnesian calcite, the coquinite contains various bivalvia, benthic foraminifera, calcareous nannofossils or coccoliths and quartz sands. Diagenesis of the coquinite occurred as the result of sequential cementation, starting with the precipitation of micritic envelops typical of a marine phreatic zone. This early stage was followed by pore-lining cements and bladed rims composed of equal-sized subhedral and anhedral crystals of calcite, suggesting evidence of meteoric phreatic and meteoric vadose environments, respectively. Calibrated values from radiocarbon dating of four bulk samples of bivalvia revealed that the coquinite shells were deposited between 3730 and 2850years BP. During that period the sea-level was similar to the present at an early stage then dropped to −2m due to Phanagorian regression (between 3200 and 2200 BP), which is evidenced by consecutive cementation patterns of distinctive origin.

Miocene shallow-marine cold seep carbonate in fold-and-thrust Western Foothills, SW Taiwan

A mound-shaped authigenic carbonate buildup (50m wide and 5m high) occurred in the middle part of the Late Miocene shallow marine succession (the Hunghuatzu Formation) exposed along the Nantzuhsien River, SW Taiwan. The carbonate concretions are classified into four types based on morphology, which appeared to develop in upward sequence with vague boundaries: (1) flat-pipe shaped nodules: highly dolomitic small pipes or nodules subparallel to the host strata, (2) bulb-shaped nodules: discrete or combined small mesoclots with subspherical to irregular shapes, (3) mushroom-shaped concretions: large mushroom-like or irregular shaped blocks with chimney or vent structures, and (4) carbonated layers with sedimentary structures. These concretions are mainly composed of dolomite micrites, quartz, feldspar, muscovite, and clay minerals, ubiquitously interweaved with blackish brown colored low magnesium calcite (LMC) veins. Low δ13CVPDB values (−51.8‰ to −29.8‰) of all types of the carbonates reveal that the carbonates were mainly derived from anaerobic oxidation of hydrocarbons. Many samples are also 18O-depleted (δ18OVPDB: −11.5‰ to 1.00‰) due to the mixture with isotopically light LMC veins, likely precipitated from meteoric waters. There are numerous chemosymbiotic bivalve fossils (genus Monitilora? diameter 3–4cm) in growth position just above the carbonate buildup. According to the geological and isotopic signatures, the Hunghuatzu carbonates are related to a cold seep pseudobioherm, which formed in a very shallow marine environment like tidal-flat in a passive margin setting. Exposures of the Hunghuatzu pseudobioherm may provide clues for tracking and comparing the evolution history of gas reservoir systems and hints of active tectonics of the SW Taiwan region.

Distribution and mineralogy of carbonate sediments on Antarctic shelves

We analyzed 214 new core-top samples for their CaCO3 content from shelves all around Antarctica in order to understand their distribution and contribution to the marine carbon cycle. The distribution of sedimentary CaCO3 on the Antarctic shelves is connected to environmental parameters where we considered water depth, width of the shelf, sea-ice coverage and primary production. While CaCO3 contents of surface sediments are usually low, high (>15%) CaCO3 contents occur at shallow water depths (150–200m) on the narrow shelves of the eastern Weddell Sea and at a depth range of 600–900m on the broader and deeper shelves of the Amundsen, Bellingshausen and western Weddell Seas. Regions with high primary production, such as the Ross Sea and the western Antarctic Peninsula region, have generally low CaCO3 contents in the surface sediments.The predominant mineral phase of CaCO3 on the Antarctic shelves is low-magnesium calcite. With respect to ocean acidification, our findings suggest that dissolution of carbonates in Antarctic shelf sediments may be an important negative feedback only after the onset of calcite undersaturation on the Antarctic shelves.Macrozoobenthic CaCO3 standing stocks do not increase the CaCO3 budget significantly as they are two orders of magnitude lower than the budget of the sediments.This first circumpolar compilation of Antarctic shelf carbonate data does not claim to be complete. Future studies are encouraged and needed to fill data gaps especially in the under-sampled southwest Pacific and Indian Ocean sectors of the Southern Ocean.

Beachrocks record Holocene events, including natural disasters

Rocks that form by cementation of sediments in intertidal parts of beaches are known as beachrocks. Modern beachrocks are restricted to warm climatic belts between latitudes 35EN and 35ES. Pottery fragments and debris from humans (including World War II relics such as shell casings) affirm that cementation in the rocks is still occurring today. The rate of cementation of sedimentary particles into beachrocks is very rapid. The cement between the particles may include aragonite, high-magnesian calcite, and/or low-magnesian calcite. All beachrocks sampled worldwide, except samples from Java, Indonesia, and Bardawil (Pelusium, Nile Delta), Egypt, have similar δ 13C and δ 18O signatures ranging from +0.1 to +5.2 and from +2.2 to −3.5, respectively. By contrast, beachrock from Java produced δ13C‰PDB −18.0 and δ 18O‰PDB −14.8, and from Bardawil (Pelusium, Nile Delta) δ13C‰PDB −39.3 to −3.3 and δ 18O‰PDB 3.8 to −36.6. The Krakatoa explosion on Java in Indonesia of August 27, 1883 has been designated as “the day the world exploded,” and was involved in its beachrock formation. The carbonate beachrock samples of dolomite from Bardawil yielded anomalous negative δ 13C values that suggest the carbon was derived from methane, which had formed at the expense of original gypsum and dolomite. Bacteria converted methane to CO2, thus making isotopically light bicarbonate available for precipitation. The timing (3,300–3,500 bp) appears to be related to the Exodus event as described in Jewish tradition.

Carbonate and organic matter sedimentation and isotopic signatures in Lake Chungará, Chilean Altiplano, during the last 12.3 kyr

Sediments in lakes in the Andean volcanic setting are often made up of diatomaceous ooze together with volcaniclastics and small amounts of carbonates. Despite their scarcity, carbonates along with organic matter provide significant paleoenvironmental information about lake systems. This study focuses on the carbonates in Lake Chungará, their morphologies, distribution and origin deduced from the isotopic markers. These markers reflected changes in the water and the biomass between the onset of the Holocene and around 9.6 cal kyr BP. These changes are marked by general increases in TOC, TN, and TN-δ 15N AIR, and by fluctuating values of TOC-δ 13C VPDB in its sediments and are probably related to major shifts in the lake surface/volume associated with rises in lake level. An increase in salinity around 10 cal kyr BP is thought to be linked to a short dry period, giving rise to the onset of carbonate production. The mid-Holocene arid period between 7.3 and 3.5 cal ka BP, with a maximum of aridity around 6.0 cal kyr BP, was deduced from δ 18O VPDB values in the endogenic carbonates. These results match the reconstructions in Lake Titicaca based on benthic diatoms and paleoshore levels. Offshore sediments mainly consist of a diatomaceous ooze, laminated in the lower half (Unit 1), and banded-massive with tephra layers in the upper half of the sequence (Unit 2). TOC-δ 13C VPDB and the C/N ratio confirm that phytoplankton was the main source of organic matter in these sediments. Shallower sediments (units 3 to 5) developed in platform and littoral settings, providing evidence of subaqueous macrophytes and, to a lesser extent, land plants. Carbonate content ranges between 0.1 and 6 wt.% in offshore settings (30 to 40 m water depth) and reaches the maximum values in the lower part of Unit 2. Carbonate minerals (low magnesium calcite and minor amounts of high magnesium calcite and aragonite) are scarce and are arranged in mm-thick layers, commonly forming cm-thick levels or bioclasts. Carbonate layers are made up of euhedral-to-subhedral spindle-shaped calcite crystals and, to a lesser extent, aragonite needles, all in the μm range. Aragonite spheroids coexist in littoral sediments with other carbonate shapes and charophyte remains, where carbonate reaches locally up to 20 wt.%. CO 2 photosynthetic depletions related to seasonal phytoplankton blooms were responsible for the high frequency deposition of mm-thick carbonate layers. The average values for δ 13C VPDB in lake water, plankton and sediments of Lake Chungará (as organic matter or as carbonate) are around 15‰ higher than commonly reported values in other lakes. This 13C enrichment is attributed to carbon assimilation from a DIC affected by methanogenesis, in which HCO 3 − is the dominant species. The δ 13C VPDB and δ 18O VPDB variations and their covariation in endogenic carbonates suggest that lake water volume and lake level increased along the Holocene.

Late Pleistocene and Holocene cool‐water carbonates of the Western Mediterranean Sea

AbstractPost‐glacial, neritic cool‐water carbonates of the Western Mediterranean Sea were examined by means of hydroacoustic data, sediment surface sampling and vibrocoring to unravel geometries and to reconstruct sedimentary evolution in response to the last sea‐level rise. The analysed areas, located on the Alboran Ridge, in the Bay of Oran, and at the southern shelf of the island of Mallorca, are microtidal and bathed by oligotrophic to weakly mesotrophic waters. Seasonal water temperature varies between 13 °C and 27 °C. Echosounder profiles show that the Bay of Oran and the southern shelf of Mallorca are distally steepened ramps, while the Alboran Ridge forms a steep‐flanked rugged plateau around the Alboran Island. In the three areas, an up to 10 m thick post‐glacial sediment cover overlies an unconformity. In Oran and Mallorca, stacked lowstand wedges occur in water depths of 120 to 130 m. On the Alboran Ridge and in the Bay of Oran, highstand wedges occur at 35 to 40 m. Up to 5 m long cores of upper Pleistocene to Holocene successions were recovered in water depths between 40 and 81 m. Deposits contain more than 80% carbonate, with mixed carbonate‐volcaniclastics in the lower part of some cores in Alboran. The carbonates consist of up to 53% of aragonite and up to 83% of high magnesium calcite. Radiocarbon dating of bivalve shells, coralline algae and serpulid tubes indicates that deposits are as old as 12 400 cal yr bp. The carbonate factories in the three areas are dominated mostly by red algae, but some intervals in the cores are richer in bivalves. A facies rich in the gastropod Turritella, reflecting elevated surface productivity, is restricted to the Mallorca Shelf. Rhodoliths occur at the sediment surface in most areas at water depths shallower than 70 m; they form a 10 to 20 cm thick veneer overlying rhodolith‐poor bioclastic sediments which, nonetheless, contain abundant red algal debris. This rhodolith layer has been developing for the past 800 to 1000 years. Similar layers at different positions in the cores are interpreted as reflecting in situ growth of rhodoliths at times of reduced net sedimentation. Sedimentary successions in the cores record the post‐glacial sea‐level rise and the degree of sediment exposure to bottom currents. Deepening‐upward trends in the successions are either reflected by shallow to deep facies transitions or by a corresponding change of depth‐indicative red algae. There are only weak downcore variations of carbonate mineralogy, which indicate that no dissolution or high magnesium to low magnesium calcite neomorphism occurs in the shallow subsurface. These new data support the approach of using the Recent facies distribution for interpretation of past cool‐water, low‐energy, microtidal carbonate depositional systems. Hydroacoustic data show that previous Pleistocene transgressive and highstand inner ramp deposits and wedges were removed during sea‐level lowstands and accumulated downslope as stacked lowstand wedges; this suggests that, under conditions of high‐amplitude sea‐level fluctuations, the stratigraphic record of similar cool‐water carbonates may be biased.

Diagenesis of fossil ostracods: Implications for stable isotope based palaeoenvironmental reconstruction

A protocol for assessing the diagenetic alteration of ostracod carapaces for use in palaeoenvironmental reconstruction is proposed, based on comparison between fossil (Carboniferous) and recent specimens. Recent ostracods have a carapace ultrastructure of fine (< 0.5 μm diameter) crystals of primary low-magnesium calcite. The degree of recrystallisation in fossil ostracods is assessed based on the carapace ultrastructure and geochemistry. Six diagenetic stages are identified: 1, replacement of the original finely crystalline calcite carapace with neomorphic calcite; 2, framboidal and euhedral pyrite growth; 3, ferroan calcite carapace replacement and cavity (void) filling cement; 4, ferroan dolomite carapace replacement and void fills; 5, siderite growth; and 6, sphalerite and barite mineralisation. Stage 1 occurs during early shallow burial, stages 2–3 soon after, and stages 4–6 during much later burial (possibly hydrothermal) alteration. Ostracods and macrofossils exhibiting diagenetic stages 1–5 were analysed for oxygen and carbon isotope ratios. The oxygen isotope compositions clearly reflect the diagenetic stages. The most negative oxygen isotopes (δ 18O − 12.9‰) represent the precipitation of ferroan dolomite (Stage 4), and the most positive values of the neomorphic calcite (δ 18O − 0.9‰) represent Stage 1, and are within the predicted range of Carboniferous seawater (δ 18O − 4‰ to 0‰). The diagenetic protocol established here is specific to Palaeozoic fossil ostracods from carbonate and clastic sediments that were subject to shallow burial diagenesis and later hydrothermal alteration. Nevertheless, the approach could be used to assess the utility of fossil ostracods for palaeoenvironmental interpretation from a wide range of geological settings.

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.

Improved AMS14C Dating of Shell Carbonates Using High-Precision X-Ray Diffraction and a Novel Density Separation Protocol (Cards)

One critical variable in the successful application of radiocarbon dating is the effective removal of carbonaceous contaminants. In the case of marine carbonates, contamination appears usually in the form of secondary low-magnesium calcite, the stable polymorph of calcium carbonate and byproduct of the post-mortem recrystallization or replacement of the autochthonous phase, originally in the form of high-magnesium calcite or aragonite. Depending on the nature of the depositional environment, the secondary phase may be contemporary in age with the original shell carbonate and may have even been derived from it by dissolution-recrystallization processes, or can be an exogenous contaminant of younger or older age. The limited ability of current pretreatment protocols to detect and remove the secondary mineralogical phases prior to dating carbonates has been one of the reasons marine shell and coral14C determinations are often difficult to validate in terms of their reliability. We have developed a new pretreatment protocol designed to achieve greater reliability and accuracy in the dating of this material. The method entails 2 steps. The first one involves the improved detection and quantification of secondary calcite in aragonite using X-ray diffraction, at a precision of ∼0.1% and ∼0.8%, respectively. Next, where this is required, a novel density separation step using non-toxic heavy liquids (CarDS) is applied to the diagenetic sample. This enables the clear separation of calcite and aragonite, with only the latter kept for dating. We have applied the new steps, screening and separation, on standard and archaeological examples and our initial results suggest that it is successful and reproducible. In this paper, we describe the method and initial results.

Palaeoenvironmental significance of carbon- and oxygen-isotope stratigraphy of marine Triassic–Jurassic boundary sections in SW Britain

Abstract: Carbon-isotope stratigraphy is a useful tool for stratigraphic correlation, especially for strata deposited during major perturbations of the carbon cycle that affected the marine, terrestrial and atmospheric reservoirs. For the Triassic–Jurassic boundary, effectively defined by a first-order mass extinction, major fluctuations in carbon-isotope values have been well documented, but these datasets have generally been derived from bulk-rock samples. Hence, the extent to which features of the isotopic curve reflect diagenetic alteration or changing proportions of constituent materials is unconstrained. Here, carbon- and oxygen-isotope data are presented from well-preserved oyster shells ( Liostrea ) comprising low-magnesium calcite, a mineral species relatively resistant to diagenetic alteration. Samples were obtained from Lavernock Point, Glamorgan, Wales, a coastal section close to a candidate stratotype for the base of the Jurassic at St Audrie's Bay, Somerset, England. The carbon-isotope signature from St Audrie's Bay, previously defined on the basis of analysis of bulk organic matter, is confirmed by our new data. Major features are (1) the upper part of an ’initial' negative isotope excursion in the lowest part of the section, followed by (2) a pronounced positive excursion, and (3) an extended ’main' negative isotope excursion in the highest part of the section. The data confirm that the carbon-isotope stratigraphy previously documented from bulk organic matter in SW England records the chemical composition of the contemporaneous seawater. Bulk carbonates sampled over the same interval near Lyme Regis, England, show similar trends to those from oyster calcite in the lower part of the study section, but there are more 13 C-depleted values up-section. These lower values probably result from an admixture of primary and diagenetic carbonate. Palaeotemperatures calculated from oxygen-isotope values from Lavernock Point oyster shells are relatively cool at the beginning of the positive carbon-isotope excursion, and increased by up to 10 °C during the main negative carbon-isotope excursion. The new results are compatible with the view that positive carbon-isotope excursions correspond to times of low atmospheric carbon dioxide content, whereas negative carbon-isotope excursions correspond to times of high atmospheric carbon dioxide content, as is also found to be the case during the Early Jurassic (Toarcian) Oceanic Anoxic Event. The Mg/Ca and Sr/Ca ratios and δ 18 O of investigated Liostrea hisingeri show no correlation, supporting data from modern bivalves that indicate that incorporation of Mg and Sr is controlled mainly by factors other than temperature.

Giant Holocene Freshwater Microbialites, Laguna Bacalar, Quintana Roo, Mexico

AbstractWith more than 10 km of total length, Holocene microbialites in Laguna Bacalar, Mexico, belong to the largest freshwater microbialite occurrences. Microbialites include domes, ledges and oncolites. Domal forms can grow to diameters and heights of 3 m. Microbialites are composed of low magnesium calcite which is, to a large extent, precipitated due to the metabolic activity of the cyanobacteria Homeothrix and Leptolyngbya, and associated diatoms. Photosynthesis removes carbon dioxide and triggers carbonate precipitation. Also, an elevated carbonate concentration in lagoon waters, derived from dissolution of Cenozoic limestone in a karst system, supports carbonate precipitation. Trapping and binding of detrital grains is also observed, but is not as common as precipitation. Bacalar microbialites are largely thrombolitic, however, stromatolitic sections occur as well. The bulk of Bacalar microbialites probably formed in the Late Holocene (ca 1 kyr BP until present). According to 14C dating, microbialites accreted 9 to 8 cal kyr BP; however, these ages may be too old as a result of a strong hard water effect. This effect is seen in 14C ages of living bivalve and gastropod mollusc shells from Bacalar Lagoon, which are 8 to 7 cal kyr BP. The modern associated fauna of microbialites is characterized by low diversity and high abundance of the bivalve mollusc Dreissena sp. and the gastropod Pomacea sp. The abundant grazing gastropods presumably hamper modern microbialite formation. A comparison of Bacalar microbialites with other modern microbialite occurrences worldwide shows only a few patterns: sizes, shapes, microbial taxa, mineralogy, type of accretion and settings including water properties of microbialite occurrences exhibit high variability. A trend may be seen in the grazing metazoa, which are rare to absent in the marine and brackish examples, but apparently present in all the freshwater occurrences of microbialites. Also, freshwater examples are usually characterized by elevated concentrations of carbonate and/or calcium ions in the surrounding waters.

Characterization of various stages of calcitization in Porites sp corals from uplifted reefs — Case studies from New Caledonia, Vanuatu, and Futuna (South-West Pacific)

This study focuses on the processes of calcitization under meteoric conditions based on observations and analyses of a series of subaerial fossil corals of the genus Porites collected from emerged Holocene and Pleistocene reefs in the Pacific Ocean. We can establish two types of low-magnesian calcite (LMC) precipitates according to their textural characteristics after their transformation from the initial aragonitic skeleton to the calcitized corals. In the first variety (LMC1), the calcite contains remnants of the original exoskeleton structure ( in situ insoluble organic tissues, pieces of aragonite needles) while in the second variety (LMC2) — filling the primary inter-skeletal macro-pores — such relicts are not present. Such textural segregation is faithfully reflected in the elemental data on some parts of calcitized corals. LMC1 is characterized by Mg 2+, Sr 2+, and Mn 2+ compositions inherited from the aragonite precursor, indicating a formation in a semi-closed intra-skeleton diagenetic environment in disequilibrium with the meteoric bulk aquifer water. LMC2 has a chemistry indicative of equilibrium with ambient bulk meteoric water. These chemical characteristics can be likened to a two-fold mechanism: a fine-scale process — neomorphism — typified by the concomitant dissolution of intra-fabric aragonite and re-precipitation of the LMC1 resulting from the ‘biogenic’ skeleton and marine waters, followed by the cementation of the inter-fabric macro-voids by an allochthonous subaerial and meteoric LMC2. In the other parts of calcitized corals, LMC1 and LMC2 have similar trace element contents in Mg 2+ and Sr 2+. The chemical data are consistent with formation by partial skeletal dissolution followed eventually by cementation of primary voids (LMC2) and secondary voids (LMC1) by calcite in equilibrium with meteoric bulk aquifer water. These two mechanisms of the calcitization of skeletal aragonite mainly differ in the space and time length-scales of the intervening transport step, and are commonly found juxtaposed in the same thin section. Taking into account such a heterogeneity is a real challenge for the development of predictive models assessing the effects of diagenetic products on paleoclimatic reconstructions.

Behavior of carbonate-associated sulfate during meteoric diagenesis and implications for the sulfur isotope paleoproxy

Although carbonate-associated sulfate (CAS) is used widely as a proxy for the sulfur isotope composition of ancient seawater, little is known about the effects of diagenesis on retention of primary δ 34S signals. Our case study of the Key Largo Limestone, Pleistocene, Florida, is the first systematic assessment of the impact of meteoric diagenesis on CAS properties. Geochemical and petrographic data show that meteoric diagenesis has affected the exposed coralline facies to varying degrees, yielding differences now expressed as sharp reaction fronts between primary and secondary carbonate minerals within individual coral heads. Specifically, analyses across high-resolution transects in the Key Largo Limestone show that concentrations of strontium and sodium decrease across the recrystallization front from original aragonite to meteoric low-magnesium calcite by factors of roughly 5 and 10, respectively. Predictably, δ 18O values decrease across these same fronts. The δ 13C relationships are more complex, with the most depleted values observed in the latest-formed calcite. Such trends likely reflect carbon isotope buffering capacity that decreased as reaction progressed, as well as protracted development of soil profiles and the associated terrestrial biomass and thus depleted δ 13C during sea-level lowstand. Conversely, δ 34S values of CAS vary within a narrow ‘buffered’ range from 20.6 to 22.6‰ (compared to 20.8–22.0‰ of coeval Pleistocene seawater) across the same mineralogical transition, despite sulfate concentrations that drop in the diagenetic calcite by an average factor of 12. Collectively, these data point to robust preservation of primary δ 34S for carbonates that have experienced intense meteoric diagenesis, which is encouraging news for those using the isotopic composition of CAS as a paleoceanographic proxy. At the same time, the vulnerability of CAS concentrations to diagenetic resetting is clear.

Nature and origin of carbonate particles along a transect on the western margin of Great Bahama Bank (Middle Miocene): sedimentary processes and depositional model

Abstract During ODP Leg 166, Middle Miocene sediments were collected along the western margin of the Great Bahama Bank (GBB) at four sites, distributed along a proximal-distal transect. Site 1006 is located in the basin, Site 1007 at the toe-of-slope and Sites 1003 and 1005 on the mid and upper slope. The carbonate slope deposits of GBB consist of periplatform oozes, an admixture of platform-derived aragonite and high-magnesium calcite particles, and pelagic low-magnesium calcite. An assessment of carbonate sedimentation is made in order to estimate the proportion of platform-derived versus pelagic components. The originality of this study is based on the application of a separation method giving access to homogeneous granulometric fractions, ranging from 63 to 3 μm in grain diameter. This method, associated with a multidisciplinary approach (micropaleontology, optical and electronic microscopy, mineralogy – X-ray diffractometry, and trace elements geochemistry – strontium and magnesium), allows the quantification and characterization of different kinds of carbonate particles. In Miocene sediments, three types of particles display a biogenic structure: planktonic foraminifera, calcareous nannofossils and fragments of neritic organisms. Two types of particles do not exhibit any structures that allow for a determination of their origin. Based on their size and their mineralogy, we have distinguished macroparticles (calcite and dolomite) and microparticles (calcite and aragonite). The detailed study of the composition of the separated fractions highlights major differences in carbonate ooze composition between the different sites along the transect. The unlithified samples of Sites 1006 and 1007 are dominated by pelagic components (planktonic foraminifera and calcareous nannofossils) and contain aragonite microparticles. In contrast, lithified sediments of Sites 1003 and 1005 (and 1007) are characterized by higher proportions of neritic debris and particles without biologic structure, the pelagic phase is impoverished and aragonite needles are absent. The origin of particles without biological structure has been demonstrated by their spatial distribution and by mineralogical as well as geochemical criteria. The rhombohedral calcitic microparticles mainly occur at slope sites. Their high magnesium contents support a formation on the bank implying an allochthonous origin. This suggests that calcitic microparticles correspond to the bank micrite exported towards slope environments. Rhomboedral calcitic macroparticles, which fill sediment voids, originate from in situ transformations of metastable carbonates due to pore fluids, which preferentially flow in the slope permeable sediments. Two sedimentary sources are proposed: 1) an autochthonous pelagic source with low-magnesium calcitic components and 2) an allochthonous neritic source with both aragonitic and calcitic components. Export and depositional processes can be differentiated for the allochthonous particles. The neritic debris and calcitic microparticles in slope Sites 1003 and 1005 (and less frequently in Site 1007) were probably exported by gravity currents whereas the aragonite microparticles, due to their shape and granulometry, were perhaps transported as suspension and deposited at the distal Sites 1006 and 1007.