Cambrian sea water preserved as inclusions in marine low-magnesium calcite cement

Nature ◽  
1993 ◽  
Vol 362 (6418) ◽  
pp. 335-337 ◽  
Author(s):  
W. J. Johnson ◽  
R. H. Goldstein
Keyword(s):  
Sea Water ◽  
Low Magnesium ◽  
Magnesium Calcite ◽  
Calcite Cement

Origin and environment of formation of late diagenetic dolomite in Cretaceous/Tertiary chalk, North Sea Central Graben

1994 ◽  
Vol 131 (5) ◽  
pp. 609-617 ◽  
Author(s):  
R. G. Maliva ◽  
J. A. D. Dickson
Keyword(s):  
North Sea ◽  
Elevated Temperatures ◽  
Sea Water ◽  
Petroleum Reservoirs ◽  
Low Magnesium ◽  
The North ◽  
The United Kingdom ◽  
Magnesium Calcite ◽  
The North Sea ◽  
Dolomite Precipitation

AbstractDolomite is common in parts of the Chalk Group of North Sea petroleum reservoirs. The stratigraphic distribution, microtextures, and stable isotope ratios of dolomite vary both within and between four of the North Sea chalk fields examined in this study, the Eldfisk and Tor of the Norwegian Sector, the Dan Field of the Danish Sector, and the Machar Field of the United Kingdom Sector, indicating that the conditions favourable for dolomitization occurred at different times and places in the North Sea Central Graben. Dolomitization in all of the examined fields occurred late during diagenesis, after significant compactional grain breakage. Dolomite precipitation in the Eldfisk, Dan, and Machar fields occurred in modified sea water, which had Σ18O values between the assumed Cretaceous/Tertiary seawater value −1 and +8‰ (SMOW). The enrichment in 18O was probably the product of calcite recrystallization at elevated temperatures in a low water/rock ratio system. The timing of dolomitization varied with respect to organic diagenesis; dolomite precipitation in the Eldfisk Field coincided with bacterial methanogenesis whereas dolomite precipitation in the Machar Field probably coincided with bacterial sulphate reduction. The magnesium in the dolomite may have been derived from the neomorphism of high magnesium calcite to low magnesium calcite.

Effect of sulfate on magnesium incorporation in low-magnesium calcite

2019 ◽  
Vol 265 ◽  
pp. 505-519 ◽  
Author(s):  
Katja E. Goetschl ◽  
Bettina Purgstaller ◽  
Martin Dietzel ◽  
Vasileios Mavromatis
Keyword(s):  
Low Magnesium ◽  
Magnesium Calcite

New constraints on the formation of hydrocarbon-derived low magnesium calcite at brine seeps in the Gulf of Mexico

2020 ◽  
Vol 398 ◽  
pp. 105572 ◽  
Author(s):  
Huiwen Huang ◽  
Xudong Wang ◽  
Shanggui Gong ◽  
Nicola Krake ◽  
Meng Jin ◽  
...  
Keyword(s):  
Gulf Of Mexico ◽  
Low Magnesium ◽  
Magnesium Calcite

Geochemistry and time-series analyses of orbitally forced Upper Cretaceous marl–limestone rhythmites (Lehrte West Syncline, northern Germany)

2005 ◽  
Vol 142 (1) ◽  
pp. 31-55 ◽  
Author(s):  
BIRGIT NIEBUHR
Keyword(s):  
Clay Minerals ◽  
Low Frequency ◽  
Geochemical Data ◽  
Carbonate Content ◽  
Element Geochemistry ◽  
Low Magnesium ◽  
Magnesium Calcite ◽  
High Carbonate ◽  
Time Series Analyses ◽  
End Members

A cyclic marl–limestone succession of Middle–Late Campanian age has been investigated with respect to a Milankovitch-controlled origin of geochemical data. In general, the major element geochemistry of the marl–limestone rhythmites can be explained by a simple two-component mixing model with the end-members calcium carbonate and ‘average shale’-like material. Carbonate content varies from 55 to 90%. Non-carbonate components are clay minerals (illite, smectite) and biogenic silica from sponge spicules, as well as authigenically formed zeolites (strontian heulandite) and quartz. The redox potential suggests oxidizing conditions throughout the section. Trace element and stable isotopic data as well as SEM investigations show that the carbonate mud is mostly composed of low-magnesium calcitic tests of planktic coccolithophorids and calcareous dinoflagellate cysts (calcispheres). Diagenetic overprint results in a decrease of 2% δ18O and an increase in Mn of up to 250 ppm. However, the sediment seems to preserve most of its high Sr content compared to the primary low-magnesium calcite of co-occurring belemnite rostra. The periodicity of geochemical cycles is dominated by 413 ka and weak signals between 51 and 22.5 ka, attributable to orbital forcing. Accumulation rates within these cycles vary between 40 and 50 m/Ma. The resulting cyclic sedimentary sequence is the product of (a) changes in primary production of low-magnesium calcitic biogenic material in surface waters within the long eccentricity and the precession, demonstrated by the CaCO3 content and the Mg/Al, Mn/Al and Sr/Al ratios, and (b) fluctuations in climate and continental weathering, which changed the quality of supplied clay minerals (the illite/smectite ratio), demonstrated by the K/Al ratio. High carbonate productivity correlates with smectite-favouring weathering (semi-arid conditions, conspicuously dry and moist seasonal changes in warmer climates). Ti as the proxy indicator for the detrital terrigenous influx, as well as Rb, Si, Zr and Na, shows only low frequency signals, indicating nearly constant rates of supply throughout the more or less pure pelagic carbonate deposition of the long-lasting third-order Middle–Upper Campanian sedimentary cycle.

The Ultrastructure of Brachiopod Shells - A Mechanically Optimized Material with Hierarchical Architecture

2005 ◽  
Vol 898 ◽  
Author(s):  
Erika Griesshaber ◽  
Klemens Kelm ◽  
Angelika Sehrbrock ◽  
Reinhart Job ◽  
Wolfgang W. Schmahl ◽  
...  
Keyword(s):  
Electron Microscopy ◽  
Distribution Pattern ◽  
Vickers Microhardness ◽  
Hierarchical Architecture ◽  
Low Magnesium ◽  
Primary Layer ◽  
Transmission Electron ◽  
Secondary Layer ◽  
Magnesium Calcite ◽  
Oceanographic Conditions

AbstractBrachiopod shells consist of low-magnesium calcite and belong to one of the most intriguing species for studies of marine paleoenvironments, variations in oceanographic conditions and ocean chemistry [6, 7, 11 – 13]. We have investigated the ultrastructure together with nano- and microhardness properties of modern brachiopod shells with transmission electron microscopy (TEM), scanning electron microscopy (SEM), nanoindentation and Vickers microhardness analyses. Brachiopod shells are structured into several layers, a thin, outer, hard, protective primary layer composed of randomly oriented nanocrystalline calcite, which is followed inward towards the soft tissue of the animal by a much softer shell segment (secondary layer) built of long calcite fibres, stacked parallely into blocks. The hardness distribution pattern within the shells is non-uniform and varies on scales as small as a few tens of microns. Our results show that the hardness of this biomaterial is controlled by two predominant features: (1.) The morphological orientation of the calcite fibres (not by the crystallographic orientation of the fibres), and (2.) the amount and distribution pattern of organic material between and within the calcite crystals.

scholarly journals THE HIGH-MAGNESIUM CALCITE ORIGIN OF NUMMULITID FORAMINIFERA AND IMPLICATIONS FOR THE IDENTIFICATION OF CALCITE DIAGENESIS

Palaios ◽  
2020 ◽  
Vol 35 (10) ◽  
pp. 421-431
Author(s):  
LAURA J. COTTON ◽  
DAVID EVANS ◽  
SIMON J. BEAVINGTON-PENNEY
Keyword(s):  
Magnesium Content ◽  
Carbonate Diagenesis ◽  
X Ray Diffraction ◽  
Larger Benthic Foraminifera ◽  
X Ray ◽  
Low Magnesium ◽  
Fossil Material ◽  
Magnesium Calcite ◽  
High Magnesium Calcite ◽  
Geochemical Studies

ABSTRACT Nummulites were one of the most abundant and widespread larger benthic foraminifera of the Paleogene, however, confusion remains within the literature as to whether their original test mineralogy was high or low magnesium calcite. As the number of studies using proxies based on Nummulites and related nummulitid geochemistry increase, it is essential to have a firm understanding of test composition to assess preservation within potential samples, and to interpret results. Here we employ a combination of X-ray diffraction, Fourier transform infra-red spectroscopy, and laser ablation ICPMS to determine magnesium content across exceptionally preserved and poorly preserved fossil material as well as modern examples of nummulitids—showing conclusively a primary intermediate to high magnesium calcite composition. This composition appears to be closely related to fluctuating ocean chemistry through the Paleogene. Using these results as an indicator of preservation we examine variation in trace element data across a suite of samples, and introduce the concept of the preservagram, a method of quickly visualizing different styles of carbonate diagenesis. Understanding the original mineralogy of nummulitids and, therefore, the extent to which specimens have been diagenetically altered, is essential as larger foraminifera are increasingly used in geochemical studies.

A precise and accurate method for the quantitative determination of carbonate minerals by X-Ray diffraction using a spiking technique

1971 ◽  
Vol 38 (296) ◽  
pp. 481-487 ◽  
Author(s):  
H. A. Gunatilaka ◽  
Roger Till
Keyword(s):  
Diffraction Method ◽  
Accurate Method ◽  
X Ray Diffraction ◽  
Carbonate Minerals ◽  
X Ray ◽  
Low Magnesium ◽  
Coefficients Of Variation ◽  
Magnesium Calcite ◽  
Peak Areas ◽  
High Magnesium Calcite

SummaryA precise and accurate X-ray diffraction method has been developed whereby the weight percentages of aragonite and low- and high-magnesium calcite are determined from the integrated peak areas of spiked and unspiked samples. The spike mixture was prepared from organisms extracted from the samples to be analysed. Use of a spiking method also avoided the preparation of working curves from artificial mixtures of carbonate minerals, which may not have the same diffraction behaviour as the unknowns. A test of the precision of the method indicates the following coefficients of variation: aragonite, 1·4 %; low-magnesium calcite, 1·5 %; high-magnesium calcite, 7·8 %. A test of the accuracy of the method indicates no significant bias in any of the carbonate results, except in samples where high-magnesium calcite values are below 10 %. Quartz may also be determined by this method (coefficient of variation 23·9 %; positive bias in values greater than 10 %).

Biocrystallization models and skeletal structure of Phanerozoic corals

1996 ◽  
Vol 1 ◽  
pp. 159-185 ◽  
Author(s):  
James E. Sorauf
Keyword(s):  
Crystal Growth ◽  
Organic Matrix ◽  
Structural Motif ◽  
Skeletal Structure ◽  
Low Magnesium ◽  
Stony Corals ◽  
Fossil Corals ◽  
Magnesium Calcite ◽  
Matrix Control ◽  
Diagenetic History

Modern understanding of skeletal microstructure in fossil corals builds on knowledge of structure and biomineralization in modern corals and diagenesis of carbonate skeletons. It is agreed that the skeleton of living stony corals, the Scleractinia, is made of fibrous aragonite, with growth of biocrystals generally according to rules of crystal growth as observed in inorganic aragonite, but here controlled by organic matrix. Fossil scleractinians all apparently fit the same model of biomineralization seen in living corals, although some early taxa (Triassic) lack septal trabeculae, rod-like framework structures typical of all living and most fossil septate corals.Paleozoic corals, both septate Rugosa and non-septate Tabulata, had a skeleton of calcite, most likely low-magnesium calcite, thus had diagenetic histories differing considerably from the aragonitic Scleractinia. Agreement is lacking as to whether a single structural motif can be defined for the calcitic corals, that is, whether the Rugosa and Tabulata originally had a calcitic skeleton built of fibrous biocrystals, analogous to the scleractinians, or whether some others originally had a non-fibrous, lamellar skeletal microstructure. The disagreement hinges on whether both of these basic configurations are biogenic, or whether the latter is sometimes or always diagenetic in origin. The presence of matrix control over biomineralization in Rugosa and Tabulata is yet to be proven, but will play an important role in models for biocrystallization in these older cnidarians. Details of diagenetic history and modification of structures in these calcitic corals likewise warrant investigation to improve our ability to interpret the Paleozoic corals.

Magnesium transport in freshwater teleosts

1998 ◽  
Vol 201 (13) ◽  
pp. 1981-1990 ◽  
Author(s):  
MJ Bijvelds ◽  
JA Velden ◽  
ZI Kolar ◽  
G Flik
Keyword(s):  
Soft Tissues ◽  
Sea Water ◽  
Molecular Probes ◽  
Magnesium Transport ◽  
Terrestrial Vertebrates ◽  
Magnesium Homeostasis ◽  
Low Magnesium ◽  
Magnesium Uptake ◽  
Total Body

The magnesium handling of freshwater teleost fish is discussed, with an emphasis on the role of branchial, intestinal and renal transport. In response to the eminent threat of constant diffusive losses of minerals such as magnesium, freshwater fish have developed efficient mechanisms for magnesium homeostasis. Magnesium losses are overcome by the uptake of magnesium from the food, making the intestine an important route for magnesium uptake. Some evidence suggests that intestinal magnesium uptake in fish is a regulated, cellular process. The ambient water is an additional magnesium source for fish, implicating the gills as a secondary route for magnesium uptake. Certainly, in some species, direct uptake from the water, probably via branchial routes, ameliorates the effects of a low-magnesium diet. The hard tissues, representing over 50 % of the total body magnesium pool, form a reservoir from which magnesium can be recruited to perform its functions in the cellular metabolism of soft tissues such as muscle. In fish, as in terrestrial vertebrates, the balance of a variety of elements becomes disturbed when the magnesium homeostasis of the soft tissues is disrupted. However, fish appear to be less sensitive than terrestrial vertebrates to these perturbations. Magnesium is reabsorbed in the kidneys to minimise losses. For renal cells, part of a cellular pathway has been elucidated that would allow absorptive magnesium transport (a magnesium conductive pathway in renal brush-border membranes). In some euryhaline teleosts, the kidneys appear to switch instantaneously to rapid magnesium secretion upon magnesium loading, a response common to marine fish that are threatened by diffusive magnesium entry. This enigmatic mechanism underlies the capacity of some euryhaline species to acclimate rapidly to sea water. Despite the progress made over the last decade, much of the cellular and molecular basis of magnesium transport in the gills, intestine and kidneys remains obscure. The application of fluorescent, radioactive and molecular probes, some of which have only recently become available, may yield rapid progress in the field of magnesium research.

Sign in / Sign up

Export Citation Format

Share Document