Carbonate Shell Material Research Articles

A punctuated Late Ordovician and early Silurian deglaciation and transgression: Evidence from the subsurface of northern Saudi Arabia

ABSTRACT The lower section of the lower Silurian Qusaiba Member, Qalibah Formation, is characterized by regionally developed organic-rich shales that have sourced many of the large Paleozoic petroleum systems of Saudi Arabia. In northern Saudi Arabia, these high–total organic carbon (TOC) horizons are being assessed for their unconventional shale-gas potential. The initial phase of exploration drilling, which resulted in a quadrupling of the number of penetrations in northern Saudi Arabia, had the dual purpose of (1) assessing the high-TOC horizons as an unconventional resource play and (2) acquiring the fundamental data required to understand the geologic development of the zones of interest within the lower Qusaiba. The availability of numerous new cores from across northern Saudi Arabia enabled an extensive refinement of the existing biostratigraphy and enhanced integration between graptolite and palynomorph biozonation systems. In cores from the study area, four distinct sedimentary facies are recognized, (1) pyritic siltstone, (2) black mudstone, (3) black chert, and (4) gray shale, representing distinct paleoenvironmental conditions related to the stepped latest Ordovician and early Silurian Gondwanan deglaciation. The failure of the Gondwanan ice sheet was not a simple, short-lived, consistent melting and associated flooding of a flat continental shelf. This study highlights the complex interplay of sea-floor topography, ocean currents, sediment supply, and variations in the rate of melting of the ice sheet. With the associated rising ambient temperatures there are (1) increasing clay concentrations associated with intensifying chemical weathering of the exposed land mass and (2) progressive lowering of the carbonate compensation depth as water temperatures rise, enabling the preservation of carbonate shell material.

The Application of Long-Lived Bivalve Sclerochronology in Environmental Baseline Monitoring

Assessments of the impact of construction, operation and removal of large infrastructures and other human activities on the marine environment are limited because they do not fully quantify the background baseline conditions and relevant scales of natural variability. Baselines as defined in Environmental Impact Assessments typically reflect the status of the environment and its variability drawn from published literature and augmented with some short term site specific characterization. Consequently, it can be difficult to determine whether a change in the environment subsequent to industrial activity is within or outside the range of natural background variability representative of an area over decades or centuries. An innovative approach that shows some promise in overcoming the limitations of traditional baseline monitoring methodology involves the analysis of shell material (sclerochronology) from molluscs living upon or within the seabed in potentially affected areas. Bivalves especially can be effective biomonitors of their environment over a wide range of spatial and temporal scales. A rapidly expanding body of research has established that numerous characteristics of the environment can be reflected in morphological and geochemical properties of the carbonate shell material in bivalve shells, as well as in functional responses such as growth rates. In addition, the annual banding pattern in shells can provide an absolute chronometer of environmental variability and/or industrial effects. Further, some species of very long-lived bivalves can be crossdated back in time, like trees, by comparing the annual banding patterns in their shells. It is therefore feasible to develop extended timeseries of certain marine environmental variables that can provide important insights into long temporal scales of baseline variability. We review recent innovative work on the shell structure, morphology and geochemistry of bivalves and conclude that they have substantial potential for use as monitors of environmental variability and the effects of pollutants and disturbance.

Microencapsulation of phase change materials with binary cores and calcium carbonate shell for thermal energy storage

Novel microencapsulated phase change materials with calcium carbonate shell and paraffin-based binary cores were prepared via self-assembly method. The phase change temperature of the microcapsules could be adjusted from 25 to 50°C by changing the weight ratio of binary cores. FTIR confirmed that the calcium carbonate shell material was successfully fabricated upon the paraffin-based binary cores. SEM and TEM demonstrated the microcapsules had a spherical morphology and compact surfaces with diameter ranging from 1 to 5μm. DSC results indicated that the binary cores content of microcapsules was in a range of 55.7–59.4%. TGA showed that mass loss of the microcapsules was between 5% and 28% when heated to 400°C. Infrared imager exhibited the binary cores microcapsules possessed excellent temperature-regulated properties. Besides, the thermal conductivity of the microcapsules was significantly enhanced due to the presence of highly thermally conductive calcium carbonate shell. Since the low costs and outstanding properties, it exhibits a good prospect in the thermal energy storage and thermal management.

Strontium isotope stratigraphy of the Gabbs Formation (Nevada): implications for global Norian–Rhaetian correlations and faunal turnover

The Luning and Gabbs formations in west-central Nevada, USA represent a Late Triassic shallow marine sedimentary succession with global significance (the Gabbs Formation was a candidate for the basal Jurassic GSSP). Typically, the Norian–Rhaetian stage boundary is placed at the contact between the formations, and the Rhaetian–Hettangian boundary (the Triassic–Jurassic boundary) is within the Muller Canyon Member of the Gabbs Formation. However, the use of different biostratigraphical index-species schemes in Norian–Rhaetian successions between Tethys and Panthalassa, the two largest ocean basins at the time, makes precise correlation problematic. Here, we compare 87Sr/86Sr measurements of well-preserved carbonate shell material from Nevada to the well-known and biostratigraphically constrained 87Sr/86Sr record from Tethys, where a negative excursion in 87Sr/86Sr is noted across the Norian–Rhaetian boundary. Our new 87Sr/86Sr data from the Luning and Gabbs formations reveal a comparable trend, with a sharp drop in 87Sr/86Sr within the Nun Mine Member of the Gabbs Formation, suggesting the position of the Norian–Rhaetian boundary is higher in the succession, and not between the Luning and Gabbs formations as previously defined. Relating the stage boundary using global isotopic signals is a useful tool for biostratigraphical correlation of successions between Tethys and Panthalassa, and for estimating the rate of faunal turnover at the Norian–Rhaetian stage boundary in comparison with the succeeding Late Triassic mass extinction. If correct, this biostratigraphical–chemostratigraphical correlation suggests that the current index groups for the Panthalassic stage boundary should be changed.

Talexirhynchia, a new rhynchonellid genus from the Jurassic Ethiopian Province of Jordan

A new genus and species of a rhynchonellide brachiopod from the Jurassic of Jordan, Talexirhynchia kadishi gen. et sp. nov., is described. The specimens were collected from the Mughanniyya Formation (Callovian) of Wadi Zarqa from alternating claystones, siltstones, and marly limestones with minor dolomite, dolomitic limestone, and coquinas that represent the upper part of the Jurassic sequence in Jordan. The environment of deposition was neritic; food supply and light were unlikely to have been limiting factors. The specimens are related to Ethiopian-Somali taxa and are consistent with the endemism that characterizes the rhynchonellide brachiopod faunas of the Jurassic Ethiopian Province. Specimens of Talexirhynchia lived with the umbo in an upright position directed toward the seafloor or with the dorsal valve slightly above the ventral valve. Juveniles were attached to the seafloor by the pedicle; carbonate shell material as well as other debris scattered on a limy substrate, such as shells and rocks, could have served as an attachment site for juveniles. With increasing growth, the loss of the pedicle and a semi-infaunal position resulted in an increasingly incurved ventral umbo that concealed the foramen.

Barnacle growth rate on artificial substrate in the Salton Sea, California

The Salton Sea is one of the few saline, inland lakes in the world with a population of barnacles, Balanus amphitrite. It is also one of California’s most impaired water bodies due to excessive nutrient loading which leads to phytoplankton blooms and low dissolved oxygen. Currently, B. amphitrite growth is limited due to lack of hard substrate in and around the Sea. We have hypothesized that artificial substrate could support the growth of B. amphitrite and their filter-feeding would lead to improved water quality. Periodic harvesting of the barnacles would result in the permanent removal of nitrogen and phosphorus from the Sea. A 44-day in-situ experiment was carried out in the Salton Sea to assess the rate of barnacle growth and phosphorus and nitrogen sequestration on burlap sheets suspended vertically from a floating line. Burlap panels were collected weekly and the barnacles analyzed for Ca, total-P, inorganic-P, total-N, total-C, CaCO3, and organic matter content. After 44 days of growth, the barnacle mats weighed 7.4 kg m−2 on a dry weight basis, with 80% of the mass as shell material. The nutrient sequestration was 9.4 g P m−2 and 100 g N m−2. Approximately half of the P was inorganic and appears to be coprecipitated with the calcium carbonate shell material. Results indicate that harvesting barnacles grown on artificial substrate in the Salton Sea would not be an effective method for removing N or P from the lake because of the relative proportions of shell material and organic material.

Sequestration of Carbonate Shell Material in Coastal Dunes on the Gulf of California (Baja California Sur, Mexico)

Atmospheric and oceanographic conditions stimulate high productivity of marine organisms in the Gulf of California that are bulk producers of calcium carbonate. Due to prevailing winter winds, north-facing beaches receive vast amounts of shell debris derived from offshore clam banks above the 50-m isobath. As shells undergo mechanical abrasion, the smaller particles of carbonate material are transferred from beaches by the wind and sequestered in coastal dunes. This study reports on two dune fields from the midriff zone of the Gulf. Sieve analysis is used to describe the grain size and sorting characteristics of seven samples from dunes at Cerro El Gallo near Mulegé and 12 samples from dunes farther south near San Nicolás. Dune sediments also were impregnated with epoxy to simulate rock samples from which thin sections were made to determine composition and relative abundances of constituent grains. The dunes at Cerro El Gallo are carbonate poor (5–15%) compared with those near San Nicolás (26–51%) and possible factors contributing to regional variation are explored. Another part of this study appraises the fecundity necessary to produce any carbonate fraction integrated by coastal dunes from seashells. One of the region's more abundant clams, Megapitaria squalida, was used as a model to estimate the number of individuals of a given size and age class required to generate a cubic meter of pure carbonate sand. Satellite images of the dune fields at El Gallo and San Nicolás were viewed to map surface coverage. With input on the thickness of dune deposits and their composition, it is possible to roughly estimate the number of mature Megapitaria equal to the carbonate fraction of these dunes. The method may be applied to all coastal dunes on the Gulf of California and could be used to assess a part of the region's overall carbon budget heretofore unappreciated.

A geochemical framework for the application of stable sulphur isotopes to fossil pyritization

Pyrite formation is associated with fossils where the organic material decays by sulphate reduction, or where the carbonate skeleton acts either as a nucleation substrate or induces iron sulphide precipitation by dissolution. These requirements define specific combinations of porewater chemistry and saturation state with respect to carbonates and iron sulphides. A review of modern marine sediments suggests that near-surface porewaters are always over-saturated, or close to saturation, with iron sulphides but are alternately rich in either dissolved iron or dissolved sulphide. However, significant variations in carbonate saturation state may occur, with porewaters being undersaturated in the early stages of sulphate reduction, but subsequently becoming oversaturated until the later stages of methanogenesis. Here undersaturation may return if there is a substantial input of carbon dioxide, and if little alkalinity has arisen from iron reduction. Organic matter pyritization requires that soft-tissue material decays by sulphate reduction to release dissolved sulphide, with dissolved iron supplied by the surrounding porewaters. The modern sediment studies therefore suggest that organic matter pyritization can occur only during the earliest stages of sulphate reduction, where there are iron-rich porewaters which are saturated with iron sulphides. Pyrite replacement of carbonate shell material requires porewater undersaturation with respect to carbonates and analogy with modern sediments suggests that these conditions occur either during the earliest stages of sulphate reduction, or when methanogenesis produces undersaturation. Between these periods porewaters are saturated with respect to carbonates and pyrite of different morphologies can precipitate on carbonate shells. Sulphur isotope data provide support for these associations of porewater chemistry and style of fossil pyritization. The best-described examples are for soft tissue pyritization in Beecher's Trilobite Bed (Ordovician) and the Hünsruck Slate (Devonian), where the isotopic data are consistent with rapid, early pyritization in iron-rich porewaters. More data are needed on shell pyritization with the range of isotopic compositions indicating formation through all stages of diagenesis.

Geochemistry of dissolved inorganic carbon in a Coastal Plain aquifer. 2. Modeling carbon sources, sinks, and δ13C evolution

Stable isotope data for dissolved inorganic carbon (DIC), carbonate shell material and cements, and microbial CO 2 were combined with organic and inorganic chemical data from aquifer and confining-bed pore waters to construct geochemical reaction models along a flowpath in the Black Creek aquifer of South Carolina. Carbon-isotope fractionation between DIC and precipitating cements was treated as a Rayleigh distillation process. Organic matter oxidation was coupled to microbial fermentation and sulfate reduction. All reaction models reproduced the observed chemical and isotopic compositions of final waters. However, model 1, in which all sources of carbon and electron-acceptors were assumed to be internal to the aquifer, was invalidated owing to the large ratio of fermentation CO 2 to respiration CO 2 predicted by the model (5–49) compared with measured ratios (two or less). In model 2, this ratio was reduced by assuming that confining beds adjacent to the aquifer act as sources of dissolved organic carbon and sulfate. This assumption was based on measured high concentrations of dissolved organic acids and sulfate in confining-bed pore waters (60–100 μM and 100–380 μM, respectively) relative to aquifer pore waters (from less than 30 μM and 2–80 μM, respectively). Sodium was chosen as the companion ion to organic-acid and sulfate transport from confining beds because it is the predominant cation in confining-bed pore waters. As a result, excessive amounts of Na-for-Ca ion exchange and calcite precipitation (three to four times more cement than observed in the aquifer) were required by model 2 to achieve mass and isotope balance of final water. For this reason, model 2 was invalidated. Agreement between model-predicted and measured amounts of carbonate cement and ratios of fermentation CO 2 to respiration CO 2 were obtained in a reaction model that assumed confining beds act as sources of DIC, as well as organic acids and sulfate. This assumption was supported by measured high concentrations of DIC in confining beds (2.6–2.7 mM). Results from this study show that geochemical models of confined aquifer systems must incorporate the effects of adjacent confining beds to reproduce observed groundwater chemistry accurately.

Quantitative analysis of trace elements in carbonates using laser ablation inductively coupled plasma mass spectrometry

Laser ablation inductively coupled plasma mass spectrometry has been applied to the analysis of carbonate materials. Multi-element synthetic standards, prepared both as pressed powders and fused glass discs, were used for calibration. The elements Mg, Mn, Sr, Ba and Pb were added to the pressed powder standards and these elements, together with U, were added to the fused glass standards. Calibration graphs, which are linear over at least three orders of magnitude, were produced using both types of standard but the fused glass discs gave better precision. The accuracy of the technique was evaluated using reference materials. Acceptable values were obtained using the pressed powders [e.g. BCS 393 (limestone) certified values of 905 ppm (Mg), 77 ppm (Mn) and 160 ppm (Sr)], but better accuracy was achieved with fused glass discs [e.g., BCS 393; 957 ppm (Mg), 79.6 ppm (Mn) and 167 ppm (Sr)]. The technique is applied to the analysis of carbonate shell material and demonstrates its potential in environmental monitoring.

Stable Carbon Isotopes of HCO3 in the Aquia Aquifer, Maryland: Evidence for an Isotopically Heavy Source of CO2

ABSTRACTConcentrations of HCO3 and δ13C values of dissolved inorganic carbon change along the hydrologic gradient of the Aquia aquifer. In the outcrop area, meteoric recharge rapidly dissolves carbonate shell material (δ13C ∼ 0.0 per mil) in the presence of soil‐gas CO2 (δ13C∼–26 per mil). HCO3 concentrations in this area range from 150‐200 mg/1 and δ13C values of dissolved HCO3 are approximately –13.0 per mil. Concentrations of HCO3 decrease gradually (to about 130 mg/1) and δ13C values become slightly heavier (to about –11.4 per mil) as water flows away from the outcrop area. These trends reflect progressive dissolution of metastable carbonate shell material and subsequent precipitation of secondary calcite cement. At about 40 miles downgradient in the flow system, HCO3 concentrations increase sharply (to about 400 mg/1), and δ13C values become significantly heavier (δ13C ∼–6.2 per mil). The observed 1:1 molar ratio of Na+ to HCO‐3 in this downgradient water shows that dissolution of isotopically heavy carbonate shell material cannot by itself explain the observed δ13C trend. Rather, the Na:HCO‐3 ratio demonstrates that shell material dissolution must take place in the presence of CO2. Because the aquifer in this area is closed to both soil‐gas and atmospheric CC>2, an aquifer‐generated source of isotopically heavy (δ13C ∼–5.3 per mil) CO2 is implied. Possible sources of aquifer‐generated CO2 include bacterially‐mediated fermentation of lignitic aquifer materials and bacterially‐mediated methanogenesis.

Groundwater geochemistry and calcite cementation of the aquia aquifer in southern Maryland

The Aquia aquifer in southern Maryland exhibits three regions that are characterized by distinctly different water composition. Region I is about 26 mi (42 km) wide and is parallel to the outcrop area. Water in region I has relatively high calcium (∼30 mg/l), magnesium (∼10 mg/l), and bicarbonate (∼150 mg/l) concentrations and relatively low sodium (∼2 mg/l) concentrations. Region II, parallel to and downgradient from region I, is about 24 mi (38 km) wide. Water in region II is characterized by constant bicarbonate, increasing sodium, decreasing calcium, and decreasing magnesium concentrations. Region III is downgradient from region II and is characterized by low calcium (∼2 mg/l), low magnesium (∼1 mg/l), high bicarbonate (∼300 mg/l), and high sodium (∼100 mg/l) concentrations. The major lithologic constituents of the Aquia aquifer are quartz sand (∼55%), glauconite (∼30%), and carbonate shell material (∼8%). Heavy minerals, clay minerals, and finely disseminated lignite make up about 2% of the aquifer material. Calcite cementation is commonly observed in and near the outcrop area of the Aquia. Stoichiometric chemical models were constructed based on this lithology to describe the water chemistry of each region. These models are (region I) (Ca1−x, Mgx)CO3(s) + CO2(g) + H2O(1) ⇌ (1 − x)Ca(aq)2+ + xMg(aq)2+ + 2HCO3(aq)−, (region II) (1 − x)Ca(aq)2+ + xMg(aq)2+ + Na2 · glauconite(ad) ⇌ Ca(1−x)Mgx · glauconite(ad) + 2Na(aq)+, and (region III) (Ca(1−x)Mgx)CO3(s) + CO2(g)) + H2O(1)) + Na2 · glauconite(ad) ⇌ 2Na(aq)+ + 2HCO3(aq) +Ca(1−x)Mgx · glauconite(ad). The applicability of these models was verified by comparing predicted and observed molar ratios of dissolved species using linear regression techniques. In all cases, close matches were found between predicted and measured curves. Mass balance calculations using the verified chemical models predict a spatial distribution of calcite cementation that closely matches the observed distribution in the Aquia aquifer. This suggests that the observed calcite cementation is postdepositional in origin and has resulted from reaction of groundwater with aquifer material.

Trace Elements in Ancient Atlantic Deep-sea Sediments

The deep-sea clays of the Pacific contain, on average, higher abundances of trace elements such as Mn, Ni, Co, Cu, and Zn than do those of the Atlantic1,2. But these abundances have been obtained from samples at the surface, or in the upper few metres, of the sediment column. Further, direct comparisons between sediments of the two oceans are complicated because in many parts of the Atlantic the sediments are rich in carbonate shell material. The Deep Sea Drilling Program has made possible the collection of samples down the whole length of the sediment column, and has led to the discovery of zeolitic clays at depth under the surface of the present Atlantic.

THE HYDROGRAPHY AND SEDIMENTS OF THE GULF OF VENEZUELA1

It is the object of this paper to examine the sediments of the Gulf of Venezuela and to relate them to the hydrography. The hydrography is, therefore, described in some detail. New data support the general conclusions of Redfield (1955) that the Gulf of Venezuela and Calabozo Bay operate as two estuarine cells driven by the trade winds and by outflow from Lake Maracaibo. New data also show that water enters the Outer Gulf from thc northwest and flows at mid‐depths towards the southeast in response to upwelling taking place off the coast of Paraguaná. Sediment parameters that reflect circulation best are shells of pelagic foraminifera, sand distribution, and silt clay distribution. Of unknown importance is the distribution of resistant fecal pellets and the distribution of carbonate (shell material). The distribution of foraminifera indicated that water does not enter Calabozo Bay over the central part of the sill but rather along both coasts. The silt clay distribution reflects an outflow of water from Calabozo Bay into the Outer Gulf over the sill, a flow that had been first seen in the hydrographic stations.Rainfall data showed the great dependence of water character in the Gulf of Venezuela on rainfall and outflow from Lake Maracaibo. Hydrographic stations made in 1958 following a protracted dry period showed differences in salinity of 1–3‰ between stations occupied in 1954 that were taken following 2 months of heavy rainfall.