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1.
硅藻氧同位素已日益成为陆相古气候重建的一种重要手段。文章在简述这一领域已取得的基本认识基础上,着重介绍了近年来的重要进展。主要进展包括: 1)在硅藻的分离纯化方面,规范了重液分离法和新的重力差异流体分离法;2)完善了分步氟化法,创生了高温碳还原法的氧同位素制取技术;3)通过实验培养和天然湖泊监测实验,证实了硅藻氧同位素与温度的分馏平衡关系;4)湖泊硅藻氧同位素可以反映古温度、气候干旱事件和大气降水来源变化。同时,对目前硅藻氧同位素在湖泊沉积古气候研究中存在的主要问题作了讨论和展望。  相似文献   

2.
Chemical analyses of dissolved silica in the shallow groundwater of the lower part of the Salado River drainage basin indicate that silica values averaged 60 ppm. The groundwaters are oversaturated in relation to quartz, Na-plagioclase, K-feldspar, and the weathering of quartz and aluminosilicates appear to have little control on silica concentrations in solution. Groundwater is undersaturated with respect to amorphous silica present in the loessic sediments, and these sediments are specially important in the control of the groundwater composition. The sources of amorphous silica are volcanic glass shards and biogenic silica derived from plant (silicophytoliths, diatom frustules) or animal remains (sponge spicules) also present in the Pampean loess. Silicophytoliths and diatoms have also been reported in A soil horizon samples. The dissolution of amorphous silica most likely controls the high dissolved silica concentrations in groundwater.  相似文献   

3.
Reactivity of biogenic silica: Surface versus bulk charge density   总被引：2，自引：0，他引：2  
Acid-base titrations were carried out at three different ionic strengths (0.01, 0.1 and 0.7 M NaCl) on a range of marine and continental biosiliceous materials. The large variability in electrical charging behavior of the various materials is consistent with the existence of two pools of ionizable groups, one on the outer surface of and the other within the silica particles. The relative amounts of internal and external silanols were estimated by fitting a two-site complexation model to excess proton versus pH curves obtained at the different ionic strengths. For fresh diatom frustules and phytoliths, as well as recently deposited biosiliceous sediments, the abundance of internal silanols was of the same order of magnitude as, or exceeded, that of silanols on the external surface. Older biosiliceous materials exhibited lower proportions of internal groups, while a decrease in the relative amount of internal silanols was also observed for diatom frustules artificially aged in seawater. The existence of internal ionizable functional groups explains measured charge densities of biogenic silicas that largely exceed the theoretical site density of silica surfaces. Variations in the relative abundance of internal versus surface silanols further explain the non-uniform dependence of electrical charging on ionic strength, the lack of correlation between total charge density and dissolution kinetics, and the variable 950 cm−1 peak intensity in the infrared spectra of biogenic silicas. Dissolution rates correlate positively with the external charge, rather than the total charge build-up, as expected if dissolution only involves the removal of silicate units from the external surfaces of the particles. The progressive reduction with time of the internal to external silanol concentration ratio represents one of the mechanisms altering the material properties that affect the recycling and preservation of biogenic silica in earth surface environments.  相似文献   

4.
Long-chain polyamines (LCPAs) constitute a new family of natural organic compounds that have recently been isolated and characterized from the biosilicified cell walls of diatom cultures. To date, diatom-specific polyamines have not been investigated from the marine environment and their fate in the environment is entirely unknown. Here, we report a series of LCPAs in a diatom frustule-rich sediment core (TNO57-13 PC4), originating from the Atlantic sector of the Southern Ocean and spanning from the Holocene to the Last Glacial Maximum (LGM). Liquid chromatography with electrospray ionization mass spectrometry (LC-ESI-MS) revealed a complex mixture of linear polyamines with at least 28 individual molecular species. Ion trap mass fragmentation studies, combined with high resolution Time of Flight (TOF) mass spectrometry showed that the polyamine pool consisted of a series of N-methylated propylamine compounds attached to a putrescine moiety, with individual LCPAs varying in chain length and degree of methylation. The structural similarity between LCPAs extracted from the diatom-rich sediment core and those extracted from the frustules of cultured diatoms suggests that sedimentary LCPAs are derived from diatom frustules. We hypothesize that these intrinsically labile organic molecular fossils are protected from diagenesis by encapsulation within the frustule. These compounds constitute a new class of biomarkers that could potentially be indicators of diatom species distribution. Isotopic analysis of LCPAs could be used to improve age models for sediment cores that lack calcium carbonate and to improve current interpretations of diatom-based paleoproxies, including diatom-bound nitrogen isotopes.  相似文献   

5.
Comparison of X-ray diffraction spectra of marine diatom frustules and suspended clay minerals suggests the presence of a distinctive smectite in diatoms. The frustules contained 1.5% Al2O3 and were partly covered with particles of 0.02 μm in diameter. The source of Al and the fate of diatoms in the sediment are briefly discussed.  相似文献   

6.
With the TEM and physical gas adsorption techniques, porous properties of nano-ribriform silica （MLD： 92.73%） from natural chrysotile are studied in this paper. The results indicate that porous nano-fibriform silica results from brucite octahedral sheets of nature chrysotile dissolved completely and Si-O tetrahedral sheets collapsed by acid leaching. Its length is at a micron or nanometer scale. There are two types of pores： pores among neighboring fibers and pores in nanoriber. These pores （less than 6.5 nm in diameter, mostly 2.1 nm and 3.8 nm） all belong to mesopores. The pores in fibers consist of those among SiO2 particles, those among aggregates, remnant nanotubes and capillary tubes. Nanoribriform silica proves better than the traditional silica as a carrier of catalyzer and a filler for reinforce rubber and plastics.  相似文献   

7.
A Tertiary non-marine stratigraphic sequence composed of carbonates(limestone),siliceous carbonates,coaly layers overlain by pyroclastic rocks and lavas,outcrops in the Gavatha area of northwestern Lesvos Island.Pure earbonates eonsist almost completely of calcite,the siliceous carbonate sediments of quartz,opal-CT and calcite,the shales of quartz,opal CT, K-feldspar,smecite-illite and ealcite,and the coaly layers of organic matter,quartz,opal-CT,feldspars and pyrite,Geochemical data indicate that smectite-illite,feldspars and associated elements(La,Zr,Y,Ba,Ce)are the products of alteration of volcanic rocks in a subtropical area A combination of sources in suggested for the formation of silica polymorphs:(a) biogenic or non-biogenic silica(opal-A) that was originally present in the form of diatiom frustules of in the form of inorganically prccipitated silica;(b)transformation o opall-A to opal-CT and quartz opal-C from alteration of volcanic glass of intercalated tuffites and overlying volcanics;and(c)opal-CT deposited primarily from hydrothermal solutions.  相似文献   

8.
Euhedral quartz and albite crystals are common in Devonian (Givetian-Frasnian) shallow-marine shelf carbonates from the Belgian Ardennes. Several features such as morphology, the presence of carbonate inclusions, inversion temperatures and occurrence in the insoluble residues of stylolitic surfaces indicate that these crystals have developed authigenically. Oxygen isotope ratios point to an intermediate deep burial realm of origin at temperatures of 60–90°C. The predominance of illite and the almost total absence of smectite clay minerals is interpreted as an indication that illitization produced the silica needed for authigenesis. The mineral composition of inclusions indicates that the carbonate host rock must have consisted of low-Mg calcite already at the time of authigenesis. These inclusions represent an earlier diagenetic stage than the present carbonate rock, since they were protected from further diagenetic alteration by the surrounding quartz. The calcite inclusions display a higher Sr/Ca and Mg/Ca ratio than the carbonate host-rock. Because neomorphic diagenesis of the carbonate continued after silicate authigenesis, the contents of Mg and Sr in the calcite of the host carbonate are even lower. The authigenic feldspar mineralogy seems to be determined by the composition of the host-sediment.  相似文献   

9.
Recently, the increase in dissolved concentration in the batch dissolution of various salts or sucrose has been successfully modelled with three equations, one a cubic in time. However, from three separate earlier investigations with ocean sediments and phytoplankton frustules, there is residual suspicion that biogenic silica does not follow this behaviour. This paper shows that the Shrinking Object Model applies to the dissolution of sieved silica gel particles, as well as to a sample of unsieved, freeze-dried frustules of Odentella sp. Silica gel, being readily available in quantities that can be sieved, is a useful surrogate for biogenic silica in allowing problems of experimental design to be overcome. The dissolutions covered three possible analytic integrations that arise from the model: an exponential for approach to saturation with excess solid silica; the approach to near saturation with either a slight excess or deficiency of silica; dissolution at high under-saturation. Good agreement was found between experimental results and mathematical modelling. The paper provides template calculations by which future raw results can be parameterized. Nevertheless, the reasons for non-linear kinetics reported in earlier work have not been identified, and so controversy over non-linear dissolution kinetics is enhanced. Stirring regime was found to be important with silica gel dissolution, and so biogenic silica dissolution is therefore likely to be ‘transport limited’ at low stirring rate. Accordingly, all archived and future data should be scrutinized for stirring effects before being applied to the oceanic environment. A rigorous test for determining whether a substance’s dissolution deviates from the model is recommended as a preliminary to any future dissolutions, whether in batch or with the chemo-stat. A fixed amount of frustule sample is added to a series of buffered mixtures containing increasing background silicic acid concentrations. Absence of any problem is marked by a linear plot between the increase in silicic concentration accruing over a fixed reaction period and that of the background silicic acid. A novel mathematical proof is provided to justify the test’s use. The reasons for the earlier deviations from expected behaviour of, for example, oceanic sediments, are discussed. Lastly, the paper provides a novel approach to the dissolution of a population of particles of mixed sizes which will probably find ready future application in oceanography.  相似文献   

10.
An empirical calibration for the oxygen isotope fractionation between biogenic silica and water was determined for diatom frustules sampled from living diatom communities in the Jemez Mountains of northern New Mexico, USA. Over a temperature range from 5.1 to 37.8 °C, the silica-water fractionation is defined by the equation 1000 ln α(silica-water) = 2.39(±0.13) × 106T−2 + 4.23(±1.49). This relationship is in close agreement with other published silica-water fractionation factors for laboratory cultured diatom samples; however, it is as much as 8‰ lower than equilibrium quartz-water fractionations and 3-4‰ lower than observed silica-water fractionations in diatomaceous silica collected from sediment traps and sediment cores. There are three possible explanations for the disparate silica-water fractionation factors observed in diatom silica: (1) silica does not precipitate in equilibrium with ambient water, (2) silica does precipitate in equilibrium with ambient water, but the silica-water fractionation factor for diatom silica is considerably less than the equilibrium fractionation factor for quartz-water, or (3) silica precipitation is influenced by a ‘vital’ effect, where the δ18O value of the water inside the diatom cell walls is lower than the δ18O values of ambient water.Post-mortem loss of organic material results in an alteration or ‘maturation’ of diatom silica in which silica reequilibrates with a silica-water fractionation closer to the equilibrium quartz-water fractionation. Alteration is likely to occur rapidly after the diatom frustule loses its organic coating, either as it settles through the water column or at the sediment-water interface; δ18O values recorded by paleo-diatom silica therefore do not record growing conditions but more likely record conditions at the sediment-water interface. In the case of lacustrine environments, where the bottom water remains at a nearly constant 4 °C, the reequilibration of diatom silica with bottom conditions could reduce or remove the conflating effects of temperature on δ18O values recorded by paleo-diatom silica and provide direct information on the δ18O value of the lake water.  相似文献   

11.
 The biogenic Si concentration in a sediment can be determined as the non-detrital Si concentration by normalization of the total Si concentration with Al. This procedure is based upon the assumptions that (1) that Al exists predominantly in detrital, i.e. minerogenic, particles and (2) that biogenic Si (mainly diatom frustules) is the dominating non-detrital Si phase. This paper focuses on the reasons for the variations of the non-detrital Si concentration in these lake sediments. Data from three lakes are presented, representing three principally different cases regarding the Si concentration. The processes controlling the concentrations of detrital and non-detrital Si are discussed. Received: 10 June 2000 · Accepted: 25 November 2000  相似文献   

12.
The siliceous skeletal remains of diatoms are a primary component of sediments, covering vast areas of the sea floor. We used high performance liquid chromatography–electrospray ionization–tandem mass spectrometry (HPLC–ESI–MSn) under high resolution time-of-flight MS (TOF-MS) conditions, as well as nuclear magnetic resonance spectroscopy (NMR) and Fourier transform infrared spectroscopy (FTIR), to investigate the distribution and molecular diversity of long chain polyamines (LCPAs) entrapped in sedimentary diatom biosilica in sediments from the Indian sector of the Southern Ocean, the Bering Sea and the Northeast Pacific. This revealed the existence of complex polyamine populations with both known and unknown structural features, including the presence of ketone and carboxylic acid groups in the LCPA backbone, functional groups that likely modulate the silica precipitating properties of LCPAs in the environment and which may constitute points of initial silica deposition (Wallace, A.F., De Yoreso, J.J., Dove, P.M., 2009. Kinetics of silica nucleation on carboxy- and amine-terminated surfaces: insights for biomineralization. Journal of the American Chemical Society 131, 5244–5250). Although various LCPA species overlapped between samples (putrescine-based LCPAs with various degrees of methylation and N-methyl propylamine repeat units were ubiquitous in all samples), clear regional differences were shown in composition and structural characteristics, likely reflecting the various diatom species contributing to the total LCPA pool for each sedimentary environment. We further show, using scanning electron microscopy and energy dispersive X-ray analysis (SEM–EDX) that LCPAs purified from diatom skeletal remains rapidly re-direct the formation of silica nanospheres in vitro, and become embedded in the precipitated silicified matrix, forming an LCPA–silica composite material. The results show that aliphatic polyamines produced by diatoms for morphogenesis of their cell walls are widespread in sediments where the fossil frustules are preserved. These newly assigned components constitute useful biological markers of diatom input to sediments.  相似文献   

13.
Mineralogical, morphological and isotopic (RbSr and KAr) determinations were made on some detrital smectites of Palaeocene and Cenomanian ages from D.S.D.P. drillings in the Atlantic Ocean. These minerals are not inert in their depositional environment; authigenic laths grow on detrital sheets with sharp borders. This authigenesis could occur slightly after deposition in a closed system, for some of these smectites. It has been tentatively quantified by the RbSr and KAr isotopic methods, which seem also well suited to evaluate the chemical extent of this authigenesis. At least, no preferential loss of 40Ar vs. 87Sr could be detected in the minerals, even in those which are smaller than 0.2 μm.  相似文献   

14.
Diatom ooze and diatomaceous mudstone overlie terrigenous mudstone beds at Leg 19 Deep Sea Drilling Project sites. The diatomaceous units are 300-725 m thick but most commonly are about 600 m. Diagenesis of diatom frustules follows a predictable series of physical and chemical changes that are related primarily to temperature (depth of burial and local geothermal gradient). During the first 300-400 m of burial frustules are fragmented and undergo mild dissolution. By 600 m dissolution of opal-A (biogenic silica) is widespread. Silica reprecipitates abundantly as inorganic opal-A between 600 and 700 m sub-bottom depth. Inorganic opal-A is rapidly transformed by crystal growth to opal-CT. The result is formation of silica cemented mudstone and porcelanite beds. A regional acoustic reflector (called the bottom-simulating reflector, or BSR) occurs near 600 m depth in the sections. This acoustic event marks the upper surface where silicification (cementation) is active. In Bering Sea deposits, opal-A is transformed to opal-CT at temperatures between 35° and 50°C. This temperature range corresponds to a sub-bottom depth of about 600 m and is the area where silicification is most active. Thus, the BSR represents an isothermal surface; the temperature it records is that required to transform opal-A to opal-CT. Deposition of at least 500 m of diatomaceous sediment was required before the temperature at the base of the diatomaceous section was appropriate (35°-50°C) for silica diagenesis to occur. Accordingly, silica diagenesis did not begin until Pleistocene time. Once silicification began, in response to sediment accumulation during the Quaternary, the diagenetic front (the BSR) moved upsection in pace with the upward migrating thermal boundary. X-ray diffractograms and SEM photographs show three silica phases, biogenic opal-A, inorganic opal-A’, and opal-CT. These have crystallite sizes of 11-16 A, 20-27 A, and 40-81 A, respectively, normal to 101. The d(101) reflection of opal-CT decreases with depth of burial at DSDP Site 192. This occurs by solid-state ordering and requires at least 700 m of burial. Most clinoptilolite in Leg 19 cores forms from the diagenesis of siliceous debris rather than from the alteration of volcanic debris as is commonly reported.  相似文献   

15.
We studied the dissolved silica cycle in the water column of the North basin of Lake Lugano, Switzerland/Italy. Lake Lugano is a meromictic, eutrophic lake, permanently stratified below 100-m depth. A one-box model was used to calculate a silica mass-balance over 1993, based on various lake measurements, such as sediment traps, sediment cores, water analysis and biota countings. We found that the North basin of Lake Lugano is at steady state as far as dissolved silica is concerned. The primary source of dissolved silica in the lake is river input (about 80%), with diffusion from bottom sediments and groundwater input also playing a role. Atmospheric input is negligible. The main export of dissolved silica occurs via biogenic uptake by diatoms and final burial of their frustules in the bottom sediment. Loss of dissolved silica through the lake outflow only represents 15% of the total output. Of the total amount of Si exported to the lake bottom through diatom sinking, less than 20% is re-supplied to the surface water by diffusion. Thus, the lake acts as an important permanent sink for silica. The long residence time of dissolved silica in the lake (7 years) is related to the strong physical stratification of the lake. Only about 10% of the standing stock are available to phytoplankton uptake.  相似文献   

16.
The Anatolian Tertiary and especially the Neogene have some geological series with analcime as the principal rock-forming mineral. Sedimentation basins receiving material in suspension and in solution permit the authigenesis of analcime dolomite, calcite and opal(C-T). Analcimes formed by hydrothermal alteration on the ocean floor, after the pillow lava and diabasic dyke complexes of ophiolites, can form a source of detrital analcime when they are on the continental medium. Continental alteration of the pillow lavas and diabasic dykes permits the formation of solutions suitable in composition for analcime authigenesis. Analcimes formed in these environments are particularly related to the silica content of these media.Principal clay minerals formed with analcimic formations are smectites and (14s—14c) mixed layers. Illite, chlorite and corrensite are also present in smaller quantities.  相似文献   

17.
Nanometer-size (<50 nm) precipitates of amorphous silica globules were observed in laboratory systems containing nontronite NAu-1, Shewanella oneidensis strain MR-1, and lean aqueous media. Their formation was attributed to the release of polysilicic acids at the expense of dissolving NAu-1, and subsequent polymerization and stabilization mediated by biomolecules. Rapid (<24 h) silica globule formation was confirmed in the immediate vicinity of bacterial cells and extracellular polymeric substances in all experimental systems that contained bacteria, whether the bacteria were respiring dissolved O2 or Fe(III) originating from NAu-1, and whether the bacteria were viable or heat-killed. Silica globules were not observed in bacteria- and biomolecule-free systems. Thermodynamic calculations using disilicic acid, rather than monomeric silica, as the primary aqueous silica species suggest that the systems may have been supersaturated with respect to amorphous silica even though they appeared to be undersaturated if all aqueous silica was assumed to be monomeric H4SiO4. The predominant aqueous silica species in the experimental systems was likely polysilicic acids because aqueous silica was continuously supplied from the concurrent dissolution of aluminosilicate. Further polymerization and globule formation may have been driven by the presence of polyamines, a group of biologically produced compounds that are known to drive amorphous silica precipitation in diatom frustules. Globules were likely to be positively charged in our systems due to chemisorption of organic polycations onto silica surfaces that would have been otherwise negatively charged. We propose the following steps for the formation of nanometer-size silica globules in our experimental systems: (i) continuous supply of polysilicic acids due to NAu-1 dissolution; (ii) polysilicic acid polymerization to form <50 nm silica globules and subsequent stabilization mediated by microbially produced polyamines; (iii) charge reversal due to chemisorption of organic polycations; and (iv) electrostatic attraction of positively charged silica globules to net negatively charged bacterial cells. Rapid, biogenic precipitation of silica may be common in soil and sediment systems that appear to be undersaturated with respect to amorphous Si.  相似文献   

18.
Nodular cherts can provide a window on the original sediment composition, diagenetic history and biota of their host rock because of their low susceptibility to further diagenetic alteration. The majority of Phanerozoic cherts formed by the intraformational redistribution of biogenic silica, particularly siliceous sponge spicules, radiolarian tests and diatom frustules. In the absence of a biogenic silica source, Precambrian cherts necessarily had to have had a different origin than Phanerozoic cherts. The Mesoproterozoic Belt Supergroup in Glacier National Park contains a variety of chert types, including silicified oolites and stromatolites, which have similar microtextures and paragenesis to Phanerozoic cherts, despite their different origins. Much of the silicification in the Belt Supergroup occurred after the onset of intergranular compaction, but before the main episode of dolomitization. The Belt Supergroup cherts probably had an opal-CT precursor, in the same manner as many Phanerozoic cherts. Although it is likely that Precambrian seas had higher silica concentrations than at present because of the absence of silica-secreting organisms, no evidence was observed that would suggest that high dissolved silica concentrations in the Belt sea had a significant widespread effect on silicification. The rarity of microfossils in Belt Supergroup cherts indicates that early silicification, if it occurred, was exceptional and restricted to localized environments. The similarity of microtextures in cherts of different ages is evidence that the silicification process is largely controlled by host carbonate composition and dissolved silica concentration during diagenesis, regardless of the source of silica.  相似文献   

19.
This work reports on a concerted study of diatom-water interfaces for two marine planktonic (Thalassiosira weissflogii= TW, Skeletonema costatum= SC) and two freshwater periphytic species (Achnanthidium minutissimum= AMIN, Navicula minima= NMIN). Proton surface adsorption was measured at 25°C, pH of 3 to 11 and ionic strength of 0.001 to 1.0 M via potentiometric titration using a limited residence time reactor. Electrophoretic mobility of living cells and their frustules was measured as a function of pH and ionic strength. Information on the chemical composition and molecular structure of diatoms surfaces was obtained using FT-IR (in situ attenuated total reflectance) and X-ray Photoelectron Spectroscopy (XPS). The surface area of living cells and their frustules in aqueous solutions was quantified using Small Angle X-ray Scattering Spectroscopy (SAXS).These observations allowed us to identify the nature and to determine the concentration of the major surface functional groups (carboxyl, amine and silanol) responsible for the amphoteric behavior of cell surfaces in aqueous solutions. Taking into account the relative proportion of surface sites inferred from XPS and FT-IR measurements, a surface complexation model of diatom-solution interfaces was generated on the basis of surface titration results. The cell-normalized ratios of the three major surface sites {>COOH}: {>NH3}: {>SiOH} are 1:1:0.1, 1:10:0, 1:1:0.4 and 1:1:0.3 for TW, SC, AMIN and NMIN, respectively. The total amount of proton/hydroxyl active surface sites for investigated species ranges from 1 (NMIN) to 9 (SC) mmol/g dry weight. Normalization of these site densities to the area of siliceous skeleton yields values between 0.3 (NMIN) and 0.9 mmol/m2 (SC) which are an order of magnitude higher than corresponding values for organic-free frustules or amorphous silica. This suggests that the amphoteric properties and possibly the affinity for metal adsorption of diatom cultures are essentially controlled by the 3-D organic layers covering the silica frustule.  相似文献   

20.
Two parallel drilling cores, BDP96-1 (200 m), and BDP96-2 (100 m), have been taken from top of the underwater Akademichesky Ridge in Lake Baikal and dated by paleomagnetic techniques. These cores are part of the Baikal Drilling Project (Kuzmin et al., 1997a, b). Diatom analysis of the first 100 m of these sediments reveals that they store a 2.5 Myr high-resolution continuous record of the palaeoclimates of East Siberia. Sediments belonging to warmer climates have a high content of diatom algae frustules, whereas those belonging to global glaciations are diatom-barren. The record of Upper Pleistocene (500 kyr BP) strongly correlates with the oceanic SPECMAP curve and contains distinct 100, 41, 23, and 19 kyr periods, as revealed by Fourier analysis. Diversity of diatom species was small in Pliocene. Abrupt and frequent changes of the climate in Pleistocene resulted in frequent dramatic changes in the diatom communities. The extant dominating endemics, Cyclotella minuta and Aulacoseira baicalensis, became important in Lake Baikal 760 kyr and less than 120 kyr BP, respectively.  相似文献   

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