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Biogenic Silica (biogenic + silica)
Selected AbstractsIsotopes in Biogenic Silica (IBiS),JOURNAL OF QUATERNARY SCIENCE, Issue 4 2008Melanie J. Leng This special issue of the Journal of Quaternary Science comprises a selection of papers from the third meeting of a series on ,ISOtopes in PALaeoenvironmental reconstruction' (ISOPAL), themed around Isotopes and Biogenic silica (IBiS). The meeting was held at the British Geological Survey (Nottingham, UK) in April 2007, and consisted of a series of presentations giving methodological approaches to using isotopes in biogenic silica but also specific examples of the application, the majority of which are presented in this issue. © Natural Environment Research Council (NERC) copyright 2008. Reproduced with the permission of NERC. Published by John Wiley & Sons Ltd. [source] UNCOUPLING OF SILICON COMPARED WITH CARBON AND NITROGEN METABOLISMS AND THE ROLE OF THE CELL CYCLE IN CONTINUOUS CULTURES OF THALASSIOSIRA PSEUDONANA (BACILLARIOPHYCEAE) UNDER LIGHT, NITROGEN, AND PHOSPHORUS CONTROL1JOURNAL OF PHYCOLOGY, Issue 5 2002Pascal Claquin The elemental composition and the cell cycle stages of the marine diatom Thalassiosira pseudonana Hasle and Heimdal were studied in continuous cultures over a range of different light- (E), nitrogen- (N), and phosphorus- (P) limited growth rates. In all growth conditions investigated, the decrease in the growth rate was linked with a higher relative contribution of the G2+M phase. The other phases of the cell cycle, G1 and S, showed different patterns, depending on the type of limitation. All experiments showed a highly significant increase in the amount of biogenic silica per cell and per cell surface with decreasing growth rates. At low growth rates, the G2+M elongation allowed an increase of the silicification of the cells. This pattern could be explained by the major uptake of silicon during the G2+M phase and by the independence of this process on the requirements of the other elements. This was illustrated by the elemental ratios Si/C and Si/N that increased from 2- to 6-fold, depending of the type of limitation, whereas the C/N ratio decreased by 10% (E limitation) or increased by 50% (P limitation). The variations of the ratios clearly demonstrate the uncoupling of the Si metabolism compared with the C and N metabolisms. This uncoupling enabled us to explain that in any of the growth condition investigated, the silicification of the cells increased at low growth rates, whereas carbon and nitrogen cellular content are differently regulated, depending of the growth conditions. [source] Nanoscale uniformity of pore architecture in diatomaceous silica: a combined small and wide angle x-ray scattering studyJOURNAL OF PHYCOLOGY, Issue 1 2000Engel G. Vrieling Combined small and wide angle X-ray scattering (SAXS and WAXS) analysis was applied to purified biogenic silica of cultured diatom frustules and of natural populations sampled on marine tidal flats. The overall WAXS patterns did not reveal crystalline phases (WAXS domain between 0.07 to 0.5 nm) in this biogenic silica, which is in line with previous reports on the amorphous character of the SiO2 matrix of diatom frustules. One exception was the silica of the pennate species Cylindrotheca fusiformis Reimann et Lewin, which revealed wide peaks in the WAXS spectra. These peaks either indicate the presence of a yet unknown crystalline phase with a repetitive distance (d -value ,0.06 nm) or are caused by the ordering of the fibrous silica fragments; numerous girdle bands. The SAXS spectra revealed the size range of pores (diameter d between 3.0 and 65 nm), the presence of distinct pores (slope transitions), and structure factors (oscillation of the spectra). All slopes varied in the range of ,4.0 to ,2.5, with two clear common regions among species: d < 10 nm (slopes ,4, denoted as region I and also called the Porod region), and 10.0 < d < 40.0 nm (slopes ,2.9 to ,3.8, denoted as region II). The existence of these common regions suggests the presence of comparable form (region I) and structure (region II) factors, respectively the shape of the primary building units of the silica and the geometry of the pores. Contrast variation experiments using dibromomethane to fill pores in the SiO2 matrix showed that scattering was caused by pores rather than silica particles. Electron microscopic analysis confirmed the presence of circular, elliptical, and rectangular pores ranging in size from 3 to 65 nm, determining the structure factor. The fine architecture (length/width ratio of pore diameters) and distribution of the pores, however, seemed to be influenced by environmental factors, such as the salinity of and additions of AlCl3 to the growth medium. The results indicate that diatoms deposit silica with pores <50 nm in size and are highly homologous with respect to geometry. Consequently, it is suggested that in diatoms, whether pennate or centric, the formation of silica at a nanoscale level is a uniform process. [source] Isotopes in Biogenic Silica (IBiS),JOURNAL OF QUATERNARY SCIENCE, Issue 4 2008Melanie J. Leng This special issue of the Journal of Quaternary Science comprises a selection of papers from the third meeting of a series on ,ISOtopes in PALaeoenvironmental reconstruction' (ISOPAL), themed around Isotopes and Biogenic silica (IBiS). The meeting was held at the British Geological Survey (Nottingham, UK) in April 2007, and consisted of a series of presentations giving methodological approaches to using isotopes in biogenic silica but also specific examples of the application, the majority of which are presented in this issue. © Natural Environment Research Council (NERC) copyright 2008. Reproduced with the permission of NERC. Published by John Wiley & Sons Ltd. [source] Combined oxygen and silicon isotope analysis of biogenic silica,JOURNAL OF QUATERNARY SCIENCE, Issue 4 2008Melanie J. Leng Abstract There is increasing interest in the use of biogenic silica O and Si isotope ratios to understand climate and environmental processes. Virtually all of the fairly substantial body of literature deals with either oxygen or silicon. This is partly because measurement of oxygen isotope composition is done using either vacuum dehydration, isotope exchange or stepped fluorination techniques, while increasingly researchers are turning to multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) for Si isotope analysis, even though Si isotope analysis can be done using fluorination methods used for the oxygen isotope measurements. Here we describe a procedure for simultaneous determination of isotopic abundances of oxygen and silicon from the same aliquot of biogenic silica. Pure silica is dissociated into O and Si compounds using a fluorination technique, in which reaction with bromine pentafluoride (BrF5) produces oxygen (O2, subsequently converted to CO2), silicon tetrafluoride (SiF4) and other fluorine by-products (e.g. BrF3). These compounds are cryogenically separated using cold traps. Yields for oxygen and silicon recovery are 70,80% for biogenic silica depending on the nature of the hydrous layer and 97,99% for pure quartz. Reproducibility of the oxygen isotopic composition is ca. 0.3, and silicon between 0.06,0.12,. © Natural Environment Research Council (NERC) copyright 2008. Reproduced with the permission of NERC. Published by John Wiley & Sons, Ltd. [source] Differential compaction due to the irregular topology of a diagenetic reaction boundary: a new mechanism for the formation of polygonal faultsBASIN RESEARCH, Issue 3 2009R. J. Davies ABSTRACT We propose a new mechanism for the formation of some polygonal fault arrays. Seismically imaged opal-A (biogenic silica) to opal-CT (cristobalite and tridymite) diagenetic boundaries from two regions offshore of Norway have developed regular wavelength patterns. The pattern consists of cell-shaped elevations that are 200,2600 m wide and up to 200 m high, separated by troughs. The cells represent regions that undergo diagenesis at shallower burial depths, earlier than adjacent areas. The chemical change leads to mechanical compaction and porosity reduction; therefore subsidence occurs above the cells in the overburden. Roughly circular depressions form above the cells, and a network of folds form above inter-cell areas. Networks of normal faults form on the crests and margins of the folds as a result of flexure during the folding. The progressive lateral growth of the cells causes the depressions to widen and intervening folds to narrow resulting in new differential compaction-induced faults to form with variable strike orientations. Lateral and vertical growth of cells leads to cells conjoining and the re-establishment of a uniform planar reaction boundary. This novel but simple mechanism can explain some polygonal fault arrays that form above opal-A to opal-CT reaction boundaries and in these settings the mechanism should be considered in addition to syneresis, density inversion or low coefficients of residual friction which are the most commonly cited drivers for polygonal fault systems. [source] A fossilized Opal A to Opal C/T transformation on the northeast Atlantic margin: support for a significantly elevated Palaeogeothermal gradient during the Neogene?BASIN RESEARCH, Issue 4 2002R. J. Davies ABSTRACT Rock samples , collected from a recent deep-water exploration well drilled in the Faeroe-Shetland Channel, northwest of the UK , confirm that a distinctive high-amplitude seismic reflector that cross-cuts the Upper Palaeogene and Neogene succession and covers an area of 10 000 km2 is an example of a fossilized Opal A to Opal C/T (Cristobalite/Tridymite) transition. Analysis of these rock fragments tied to an extensive two-dimensional and three-dimensional seismic database constrains the time at which the boundary was fossilized and in addition reveals the unusual geometrical characteristics of a relict bottom-simulating reflector. The diagenetic transformation of biogenic silica (Opal A) to Opal C/T is predominantly temperature-controlled and requires sediments that contain biogenic silica. The reflector (termed as Horizon E) probably initially represented a biosiliceous ooze or a siltstone that contained a component of biogenic silica that underwent transformation as the diagenetic front migrated upsection during burial. The parallelism it shows with a shallower early Pliocene reflector and its apparent upsection migration during a compressional episode in the basin indicate that it was active during the middle and late Miocene and ceased activity during the early Pliocene when there was between 200 and 400 m of overburden. The present-day burial depth of the boundary is ca. 700 m and the temperature at the inactive diagenetic front at the well location is 24 °C. Given these temperature and depth constraints, we hypothesize that even if this is an example of a relatively low-temperature Opal A to Opal C/T transformation, a temporarily elevated geothermal gradient of ca. 60 °C km,1 would have been required to initiate and arrest upsection migration of the boundary during the middle and late Miocene. Factors such as climatic deterioration and the onset of cold deep-water circulation are likely to only have had a contributory role in arresting the upward migration of the boundary. [source] Rapid Holocene climate changes in the North Atlantic: evidence from lake sediments from the Faroe IslandsBOREAS, Issue 1 2006CAMILLA S. ANDRESEN Holocene records from two lakes on the Faroe Islands were investigated to determine regional climatic variability: the fairly wind-exposed Lake Starvatn on Streymoy and the more sheltered Lake Lykkjuvötn on Sandoy. Sediment cores were analysed for content of biogenic silica, organic carbon and clastic material, and magnetic susceptibility. In addition, a new qualitative proxy for past lake ice cover and wind activity was developed using the flux of clastic grains that are larger than 255 ,m. Both long-term and short-term climatic developments were similar between the two lakes, suggesting a response to a regional climate signal. The long-term climate development is characterized by early Holocene rapid warming followed by Holocene climatic optimum conditions ending around 8300 cal. yr BP. A more open landscape as evidenced from increased sand grain influx in the period 8300,7200 cal. yr BP could reflect the aftermath of the 8200 cal. yr BP event, although the event itself is not recognized in either of the two lake records. From around 7200 cal. yr BP the mid-Holocene climate deterioration is observed and from 4200 cal. yr BP the climate deteriorated further with increased amplitude of centennial cooling episodes. [source] | ||||||||||