aragonite

This digital document is a journal article from Palaeogeography, Palaeoclimatology, Palaeoecology, published by Elsevier in . The article is delivered in HTML format and is available in your Amazon.com Media Library immediately after purchase. You can view it with any web browser.

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Although some organisms exercise considerable control over their biomineralization, seawater chemistry has affected skeletal secretion by many taxa. Secular changes in the magnesium/calcium ratio and absolute concentration of calcium in seawater, driven by changes in rates of deep-sea igneous activity, have influenced the precipitation of nonskeletal carbonates: low-Mg calcite forms when the ambient Mg/Ca molar ratio is <1, high-Mg calcite forms when the ratio is 1-2, and high-Mg calcite and aragonite form when the ratio is above 2. Reef builders and other simple organisms that are highly productive biomineralizers have tended to respond to changes in seawater chemistry in ways that mirror patterns for nonskelatal carbonates. Also, changes in the concentration of silicic acid in seawater have affected the ability of organisms to secrete siliceous skeletons. In laboratory experiments, organisms that secrete high-Mg calcite in the modern aragonite sea incorporate progressively less Mg in their skeletons with a reduction in the ambient Mg/Ca ratio, producing low-Mg calcite in ''Cretaceous'' seawater (Mg/Ca molar ratio=1.0). Because algae that liberate CO"2 through calcification use it in their photosynthesis, an increase in the ambient Mg/Ca ratio results in accelerated aragonite secretion and overall growth for codiacean algae, and a decrease in the Mg/Ca ratio results in greatly accelerated growth rates for calcitic coccolithophores. Controlled experiments show that the increased concentration of Ca that accompanies a reduction of the ambient Mg/Ca ratio also accelerates coccolithophore population growth. Coccolithophores' production of vast chalk deposits in Late Cretaceous time can be attributed to the low Mg/Ca ratio and high Ca concentration in ambient seawater. The high Mg/Ca ratio and low Ca concentration in modern seawater apparently limit population growth for the large majority of modern coccolithophore species: ones that fail to respond to nitrate, phosphate or iron fertilization and are confined to oligotrophic waters. Presumably the low Mg/Ca ratio of ambient seawater was at least partly responsible for reduced reef-building by scleractinian corals in Late Cretaceous time. Some taxa have secreted more robust skeletons when seawater chemistry has favored their skeletal mineralogy. Strong intrinsic control of biomineralization can buffer a taxon against secular changes in seawater chemistry. Mollusks, for example, evolved the ability to severely limit the incorporation of Mg in their skeletal calcite in seawaters with Mg/Ca ratios as high as that of the present, but not in seawaters with still higher ratios. The ability to exclude Mg is useful because Mg reduces the rate of step growth of calcite crystals. On the other hand, labile skeletal mineralogy has permitted some taxa to respond to secular changes in the Mg/Ca ratio of seawater via phenotypic or evolutionary shifts of skeletal mineralogy. Sponges and bryozoans have apparently undergone evolutionary shifts of this kind polyphyletically. Increased incorporation of Mg in skeletal calcite with secular increases in the concentration of Mg in seawater has had little effect on seawater chemistry. In contrast, removal of Si by diatoms beginning in late Mesozoic time lowered the concentration of silicic acid in seawater, forcing siliceous sponges to secrete less robust skeletons. g in seawater has had little effect on seawater chemistry. In contrast, removal of Si by diatoms beginning in late Mesozoic time lowered the concentration of silicic acid in seawater, forcing siliceous sponges to secrete less robust skeletons.

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This digital document is an article from Revista Geolôgica de Amêrica Central, published by Universidad de Costa Rica on June 1, 2008. The length of the article is 7082 words. The page length shown above is based on a typical 300-word page. The article is delivered in HTML format and is available immediately after purchase. You can view it with any web browser.

From the author: Key words: Larger foraminifera, Miogypsina, Oligocene, Costa Rica, Carriacou, strontium isotope ratios.

Citation Details
Title: Late Oligocene larger foraminifera from Nosara (Nicoya Peninsula, Costa Rica) and indward (Carriacou, Lesser Antilles), Calibrated by [sup.87]SR/ [sup.86]SR isotope stratigraphy.(Report)
Author: Claudia Baumgartner-Mora
Publication: Revista Geolôgica de Amêrica Central (Magazine/Journal)
Date: June 1, 2008
Publisher: Universidad de Costa Rica
Issue: 38 Page: 33(20)

Article Type: Report

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This digital document is a journal article from Palaeogeography, Palaeoclimatology, Palaeoecology, published by Elsevier in . The article is delivered in HTML format and is available in your Amazon.com Media Library immediately after purchase. You can view it with any web browser.

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Two active chemoherm build-ups growing freely up into the oceanic water column, the Pinnacle and the South East-Knoll Chemoherms, have been discovered at Hydrate Ridge on the Cascadia continental margin. These microbially-mediated carbonate formations rise above the seafloor by several tens of meters and display a pinnacle-shaped morphology with steep flanks. The recovered rocks are pure carbonates dominated by aragonite. Based on fabric and mineralogic composition different varieties of authigenic aragonite can be distinguished. Detailed visual and petrographic investigations unambiguously reveal the involvement of microbes during the formation of the carbonates. The fabric of the cryptocrystalline and fibrous aragonite can be described as thrombolitic. Fossilized microbial filaments in the microcrystalline aragonite indicate the intimate relationship between microbes and carbonates. The strongly ^1^3C-depleted carbon isotope values of the samples (as low as -48.1 %% PDB) are characteristic of methane as the major carbon source for the carbonate formation. The methane-rich fluids from which the carbonates are precipitated originate most probably from a gas reservoir below the bottom-simulating reflector (BSR) and rise through fault systems. The @d^1^8O values of the aragonitic chemoherm carbonates are substantially higher (as high as 5.0 %% PDB) than the expected equilibrium value for an aragonite forming from ambient seawater (3.5 %% PDB). As a first approximation this indicates formation from glacial ocean water but other factors are considered as well. A conceptual model is presented for the precipitation of these chemoherm carbonates based on in situ observations and the detailed petrographic investigation of the carbonates. This model explains the function of the consortium of archaea and sulfate-reducing bacteria that grows on the carbonates performing anaerobic oxidation of methane (AOM) and enabling the precipitation of the chemoherms above the seafloor surrounded by oxic seawater. Beggiatoa mats growing on the surface of the chemoherms oxidize the sulfide provided by sulfate-dependent anaerobic oxidation of methane within an oxic environment. The contact between Beggiatoa and the underlying microbial consortium represents the interface between the overlying oxic water column and an anoxic micro-environment where carbonate formation takes place.

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This digital document is a journal article from Palaeogeography, Palaeoclimatology, Palaeoecology, published by Elsevier in 2005. The article is delivered in HTML format and is available in your Amazon.com Media Library immediately after purchase. You can view it with any web browser.

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On the open temperate shelf off Otago Peninsula, eastern South Island, New Zealand, in water depths ranging from 90 to 140 m, free-living motile bryozoans of the genus Otionellina survive in shifting quartzofeldspathic sands. Unlike most bryozoans, these free-living colonies characteristically precipitate skeletons of aragonite. Here, we report on skeletal carbonate mineralogy of 104 specimens of Otionellina from four different species, across the shelf, which are also in different stages of development. All were dominantly composed of aragonite, with only five specimens containing 0-15 wt.% calcite (but with an overall mean of <1 wt.%), with low to intermediate MgCO"3 content in the calcite (<5 wt.% MgCO"3). Mineralogy in Otionellina varies, but is not related to species, colony size, or environmental factors such as water temperature, salinity, water depth, and substratum. Otionellinids may lack the environmentally influenced biomineralisation pathways found in other carbonate-producing taxa. It appears that most free-living (or ''vagrant'') bryozoans are aragonitic, although some rare examples contain up to 5% calcite; there may be no free-living genera which are always entirely aragonite. While preservation of aragonitic skeletons is by no means certain, the presence of free-living bryozoans indicates a sandy outer-shelf environment and a diagenetic history that allows for preservation of aragonite.

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This digital document is an article from The Mineralogical Record, published by The Mineralogical, Inc. on January 1, 2010. The length of the article is 2139 words. The page length shown above is based on a typical 300-word page. The article is delivered in HTML format and is available immediately after purchase. You can view it with any web browser.

Citation Details
Title: The Dugupi-Maanshan antimony deposit: Weishan County, Yunnan Province, China.
Author: Chris Schroeder
Publication: The Mineralogical Record (Magazine/Journal)
Date: January 1, 2010
Publisher: The Mineralogical, Inc.
Volume: 41 Issue: 1 Page: 75(9)

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This digital document is a journal article from Earth Science Reviews, published by Elsevier in 2006. The article is delivered in HTML format and is available in your Amazon.com Media Library immediately after purchase. You can view it with any web browser.

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Skeletal carbonate mineralogy of 1183 specimens of marine bryozoans from the literature was examined for phylogenetic patterns in order to elucidate the effects of bryozoan mineralogy on geochemical and paleoenvironmental analysis. Colonies are composed of calcite (66% of specimens), aragonite (17% of specimens) or various mixtures of the two (17% specimens) (phylum mean=72.9 wt.% calcite, n=1051). When calcite is present, it ranges from 0.0 to 13.7 wt.% MgCO”3 (mean=5.0 wt.% MgCO”3, n=873). Most (61%) calcitic specimens are formed of intermediate-Mg calcite (4 to 8 wt.% MgCO”3), others (28%) of low-Mg calcite (0 to 4 wt.% MgCO”3), and few of high-Mg calcite (>8 wt.% MgCO”3). The phylum occupies at least 63% of the theoretical mineralogical ”space” available to biomineralisation. Most of this variation occurs in the class Gymnolaemata, order Cheilostomata, suborder Neocheilostomata. Fossil and Recent stenolaemate taxa are generally low- to intermediate-Mg calcite (mean=99.7 wt.% calcite, 2.6 wt.% MgCO”3, 17% of available biomineral space). Variability among families is related in a general way to first appearance datum: families younger than 100 Ma display greater mineralogical complexity than older ones. The cheilostome infraorder Flustrina includes unusual free-living aragonitic families, dual-calcite skeletons (mainly low-Mg calcite, but with secondary high-Mg calcite), and some genera with considerable mineralogical variability. Families (e.g., Membraniporidae and Phidoloporidae) and species (e.g., Schizoporella unicornis) with the highest degree of variability have potential for environmental correlations with mineralogy, paleoenvironmental interpretation, and possibly molecular investigation for potential cryptic species. Stenolaemate families, genera and species with low variability, on the other hand, are well-suited for geochemical work such as stable isotope analysis. Variability in the skeletal mineralogy of bryozoans suggests that they may be useful in geochemical, phylogenetic, and paleoenvironmental studies, with careful choice of study material.

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