Calcite and aragonite precipitation from seawater solutions of various salinities: Precipitation rates and overgrowth compositions     

Shaojun Zhong  Alfonso Mucci   《Chemical Geology》1989,78(3-4):283-299

The reaction rate and composition of calcite and aragonite overgrowths precipitated from seawater solutions of various salinities (i.e. S=5, 15, 25, 35, 44) were determined at 25°C and 10^−2.5-atm. CO₂ partial pressure using a constant disequilibrium seeded technique. The rate data were fitted to an empirical rate law of the form:

where n is the empirical reaction order; and k is the rate constant. Calcite precipitation rates in seawater solutions do not vary appreciably as a result of salinity variations over the range investigated, while those for aragonite show an increase in going from the higher (i.e. S=35, 44) to the lower (i.e. S=5, 15, 25) salinity range. This study also confirms previously published findings that above a given saturation state (i.e. Ω_c>/2.6) aragonite precipitates more rapidly than calcite at 25°C.

The incorporation of Sr²⁺ in aragonite and Mg²⁺ in calcite overgrowths are independent of the precipitation rate. The partition coefficient of Sr²⁺ in aragonite is approximately equal to unity and is unaffected by salinity variations between 5 and 44. However, the Mg²⁺ partition coefficient in calcite, increases with decreasing salinity of the parent seawater solutions, possibly as a result of variations in the sulfate content of the solutions and solids.

The experimental results were discussed in the context of a number of geological environments.  相似文献   

Pressure effect on magnesian calcite coating calcite and synthetic magnesian calcite seeds added to seawater in a closed system     

B. Koch  A. Disteche 《Geochimica et cosmochimica acta》1984,48(3):583-589

When pure crystalline calcite seeds are added to supersaturated seawater, precipitation results in a coating which with time equilibrates at atmospheric pressure with seawater and corresponds to a calcite containing probably only 2 or 3% of MgCO₃ (mole fraction).If synthetic crystalline magnesian calcite is added, the surface layer equilibrates not only with respect to seawater but also in relation with the crystalline sites initiating precipitation. Adding Mg_0.03Ca_0.97CO₃ results in a coating with a solubility close to that of calcite. This confirms that the surface coating on pure calcite seeds contains about 2 or 3% MgCO₃ (K'_sp = 10?6.30).The surface layer precipitated on a synthetic Mg_0.08Ca_0.92CO₃ equilibrates finally with a carbonate more soluble than calcite (K'_sp = 10?5.94) corresponding to the seeds composition.Experiments at 1000 kg cm t-2 imply that when magnesian calcites are precipitated at the surface of calcite or magnesian calcite seeds, the precipitate must be hydrated, otherwise pressure accelerated recrystallization or rearrangement with loss of Mg would thermodynamically be impossible.By changing the pressure of a seawater sample originally saturated with a solid carbonate phase, changes in pH result from the effect of pressure on the dissociation constants of carbonic acid and boric acid causing either undersaturation or supersaturation with respect to the solid. By changing pressure we can show whether precipitation, dissolution and recrystallization are reversible processes if pH is taken as criteria of reversibility.  相似文献   

The solubility of calcite in seawater solutions of various magnesium concentration, $\text{I}{}_{\mathrm{t}}\text{= 0.697}\text{m}$ at 25 °C and one atmosphere total pressure     

Alfonso Mucci  John W. Morse 《Geochimica et cosmochimica acta》1984,48(4):815-822

Stoichiometric solubility constants of calcite in initially supersaturated solutions of various magnesium to calcium concentration ratios but identical ionic strength were determined at 25°C and one atmosphere total pressure.The thermodynamic solubility constant of calcite is used with ion pairing equations to interpret the data reported in this study. Results indicate that even though magnesian calcites, rather than pure calcite, precipitate from seawater solutions containing magnesium ions, the incorporation of MgCO₃ in the calcite crystal lattice does not extensively alter the equilibrium calcium carbonate activity product.The equilibrium activity of the ionic species in solution and the composition of magnesian calcite overgrowths precipitated from solutions of similar composition are used to calculate the solubility of magnesian calcites. The values for magnesian calcite solubilities obtained by this approach are lower than those obtained from the dissolution kinetics of biogenic carbonates.  相似文献   

The role of magnesium in the crystal growth of calcite and aragonite from sea water     

R.A. Berner 《Geochimica et cosmochimica acta》1975,39(4):489-504

The seeded precipitation (crystal growth) of aragonite and calcite from sea water, magnesium-depleted sea water, and magnesium-free sea water has been studied by means of the steady-state disequilibrium initial rate method. Dissolved magnesium at sea water levels appears to have no effect on the rate of crystal growth of aragonite, but a strong retarding effect on that of calcite. By contrast, at levels less than about 5 per cent of the sea water level, Mg has little or no effect on calcite growth. Extended crystal growth on pure calcite seeds in sea water of normal Mg content resulted in the crystallization of magnesium calcite overgrowths, containing 7–10 mole % MgCO₃ in solid solution. This suggests that the rate inhibition by Mg is due to its incorporation within the calcite crystal structure during growth, which causes the resulting magnesian calcite to be considerably more soluble than pure calcite. The standard free energy of formation of 8.5 mole% Mg calcite calculated on this assumption is in good agreement with independent estimates of magnesian calcite stability.From the work of Katz (Geochim. Cosmochim. Acta37, 1563–1586, 1973), Plummer and Mackenzie (Amer. J. Sci. 273, 515–522, 1974), and the present paper, it can be predicted that the most stable calcite in Ca-Mg exchange equilibrium with sea water contains between 2 and 7 mole%MgCO₃ in solid solution. Likewise, calcites containing more than 8.5 mole% MgCO₃ are less stable, and those containing less than 8.5 mole% MgCO₃ are more stable than aragonite plus Ca and Mg in sea water.  相似文献   

Shallow subsurface diagenesis of Pleistocene periplatform ooze: northern Bahamas   总被引：2，自引：0，他引：2  

HENRY T. MULLINS  SHERWOOD W. WISE ANNE F. GARDULSKI  EDWARD J. HINCHEY  PHILIP M. MASTERS  D.I. SIEGEL 《Sedimentology》1985,32(4):473-494

Previous studies on early submarine diagenesis of periplatform carbonates have implied that these originally polymineralic (aragonite, magnesian calcite, calcite) sediments are susceptible to early diagenesis only in current-swept open seaways or where surficially exposed by erosion on the seafloor. It has also been proposed that while in the shallow subsurface, periplatform oozes retain their original mineralogy for at least 200,000–400,000 yr and remain unlithified for tens of millions of years. Evidence is reported here for extensive calcitization and selective lithification of periplatform oozes of late Pleistocene age in two piston cores collected from water depths of ～ 1,000 m north of Little Bahama Bank. It is shown that shallow (<30 m) subsurface diagenesis can significantly alter the original mineralogy of periplatform oozes to predominantly calcite in less than 440,000 yr, and that cementation by calcite can produce chalk-ooze sequences within the same time-frame. Periplatform oozes that originally contain a high percentage of bank-derived magnesian calcite appear to have a higher diagenetic potential than those originally low in magnesian calcite. Shallow subsurface calcitization and fithification greatly reduce the diagenetic potential of periplatform carbonates, and chalk-ooze sequences apparently can persist for tens of millions of years and to burial depths of at least 300 m. Shallow subsurface diagenesis, at water depths > 1,000 m, proceeds via dissolution of magnesian calcite and aragonite and reprecipitation of calcite as allochem fillings, exterior overgrowths and cement. It is speculated that density-driven ‘Kohout convection‘, where seawaters under-saturated with respect to magnesian calcite and aragonite and saturated/supersaturated with respect to calcite flow through the margins of carbonate platforms, is the primary driving mechanism for shallow subsurface diagenesis. Removal of Mg during early stages of deep seafloor and shallow subsurface diagenesis should increase the Mg content of interstitial waters which is likely to increase the ‘dolomitizing potential’ of Kohout convection fluid flow.  相似文献   

Effects of secular variation in seawater Mg/Ca ratio (calcite–aragonite seas) on CaCO₃ sediment production by the calcareous algae Halimeda, Penicillus and Udotea– evidence from recent experiments and the geological record     

Justin Baker Ries 《地学学报》2009,21(5):323-339

Independent lines of geological evidence suggest that fluctuations in the Mg/Ca ratio of seawater between 1.0 and 5.2 have caused the oceans to alternate between favouring the precipitation of the aragonite and high-Mg calcite polymorphs of calcium carbonate ( m Mg/Ca > 2; aragonite seas) and the low-Mg calcite polymorph ( m Mg/Ca < 2; calcite seas) throughout Phanerozoic time. The rise of aragonite-secreting bryopsidalean algae as major producers of carbonate sediments in middle Palaeogene time, a role that they maintained through to the present, has been attributed to a transition from calcite-to-aragonite seas in early Cenozoic time. Recent experiments on the modern, carbonate-sediment-producing bryopsidales Halimeda , Penicillus and Udotea reveal that their rates of calcification, linear extension and primary production decline when reared in experimental calcite seawaters ( m Mg/Ca < 2). These normally aragonite-secreting algae also began producing at least one-quarter of their CaCO₃ as calcite under calcite sea conditions, indicating that their biomineralogical control can be partially overridden by ambient seawater chemistry. The observation that primary production and linear extension declined along with calcification in the mineralogically unfavourable seawater suggests that photosynthesis within these algae is enhanced by calcification via liberation of CO₂ and/or H⁺. Thus, the reduced fitness of these algae associated with their low rates of calcification in calcite seas may have been exacerbated by concomitant reductions in tissue mass and algal height.  相似文献   

Sr isotopes and pore fluid chemistry in carbonate sediment of the Ontong Java Plateau: Calcite recrystallization rates and evidence for a rapid rise in seawater Mg over the last 10 million years     

Matthew S. Fantle  Donald J. DePaolo 《Geochimica et cosmochimica acta》2006,70(15):3883-3904

The ⁸⁷Sr/⁸⁶Sr ratios and Sr concentrations in sediment and pore fluids are used to evaluate the rates of calcite recrystallization at ODP Site 807A on the Ontong Java Plateau, an 800-meter thick section of carbonate ooze and chalk. A numerical model is used to evaluate the pore fluid chemistry and Sr isotopes in an accumulating section. The deduced calcite recrystallization rate is 2% per million years (%/Myr) near the top of the section and decreases systematically in older parts of the section such that the rate is close to 0.1/age (in years). The deduced recrystallization rates have important implications for the interpretation of Ca and Mg concentration profiles in the pore fluids. The effect of calcite recrystallization on pore fluid chemistry is described by the reaction length, L, which varies by element, and depends on the concentration in pore fluid and solid. When L is small compared to the thickness of the sedimentary section, the pore fluid concentration is controlled by equilibrium or steady-state exchange with the solid phase, except within a distance L of the sediment-water interface. When L is large relative to the thickness of sediment, the pore fluid concentration is mostly controlled by the boundary conditions and diffusion. The values of L for Ca, Sr, and Mg are of order 15, 150, and 1500 meters, respectively. L_Sr is derived from isotopic data and modeling, and allows us to infer the values of L_Ca and L_Mg. The small value for L_Ca indicates that pore fluid Ca concentrations, which gradually increase down section, must be equilibrium values that are maintained by solution-precipitation exchange with calcite and do not reflect Ca sources within or below the sediment column. The pore fluid Ca measurements and measured alkalinity allow us to calculate the in situ pH in the pore fluids, which decreases from 7.6 near the sediment-water interface to 7.1 ± 0.1 at 400-800 mbsf. While the calculated pH values are in agreement with some of the values measured during ODP Leg 130, most of the measurements are artifacts. The large value for L_Mg indicates that the pore fluid Mg concentrations at 807A are not controlled by calcite-fluid equilibrium but instead are determined by the changing Mg concentration of seawater during deposition, modified by aqueous diffusion in the pore fluids. We use the pore fluid Mg concentration profile at Site 807A to retrieve a global record for seawater Mg over the past 35 Myr, which shows that seawater Mg has increased rapidly over the past 10 Myr, rather than gradually over the past 60 Myr. This observation suggests that the Cenozoic rise in seawater Mg is controlled by continental weathering inputs rather than by exchange with oceanic crust. The relationship determined between reaction rate and age in silicates and carbonates is strikingly similar, which suggests that reaction affinity is not the primary determinant of silicate dissolution rates in nature.  相似文献   

The incorporation of Mg²⁺ and Sr²⁺ into calcite overgrowths: influences of growth rate and solution composition     

Alfonso Mucci  John W Morse 《Geochimica et cosmochimica acta》1983,47(2):217-233

The concentrations of Mg²⁺ and Sr²⁺ incorporated within calcite overgrowths precipitated from seawater and related solutions, determined at 25°C, were independent of the precipitation rate over approximately an order of magnitude. The saturation states used to produce this range of precipitation rates varied from 3 to 17 depending on the composition of the solution.The amount of Mg²⁺ incorporated in the overgrowths was not directly proportional to $\text{Mg}{}^{\mathrm{2+}}\text{Ca}{}^{\mathrm{2+}}$ in solution over the entire range (1–20) of ratios studied. Below a ratio of 7.5, the overgrowth was enriched in MgCO₃ relative to what is predicted by the constant distribution coefficient measured above a ratio of 7.5. This increased MgCO₃ correlates with the relative enrichment of adsorbed Mg²⁺. Above a ratio of 7.5 the concentration of MgCO₃ in the calcite overgrowths followed a classical thermodynamic behavior characterized by a constant distribution coefficient of 0.0123 (±0.008 std dev).The concentration of SrCO₃ incorporated in the overgrowths was linearly related to the MgCO₃ content of the overgrowths, and is attributed to increased solubility of SrCO₃ in calcite due to the incorporation of the smaller Mg²⁺ ions.The kinetic data indicate that the growth mechanism involves the adsorption of the cations on the surface of the calcite prior to dehydration and final incorporation. It is suggested that dehydration of cations at the surface is the rate controlling step.  相似文献   

Magnesian calcite stabilities: A reevaluation     

Lynn M. Walter 《Geochimica et cosmochimica acta》1984,48(5):1059-1069

Equilibrium constants at stoichiometric saturation with respect to various magnesian calcite compositions were measured using free-drift dissolution rate data and inverse time plots to estimate equilibrium pH. The equilibrium constants determined for two ultrasonically cleaned and annealed biogenic magnesian calcites (12 and 18 mole % MgCO₃) in CaCl₂ + MgCl₂ media at two Mg:Ca molar ratios (1:5 and 5:1) are about three times smaller than those previously reported by Plummer and Mackenzie (1974). These equilibrium constants are not affected by changes in initial pH value, solid:solution ratio, or solution Mg:Ca molar ratio when the ion activity product is expressed in the fractional exponent form. Other models for expression of the equilibrium ion activity product fail to yield consistent values in solutions of different Mg:Ca molar ratios.Experiments performed using crushed samples not ultrasonically cleaned and annealed yield equilibrium constants which vary with solid:solution ratio. Those performed at high solid:solution ratios yield values which approach those previously reported. Submicron size particles and crystal strain induced by crushing the biogenic carbonates may cause more rapid dissolution rates and, hence, overestimation of the solubility of samples not prepared so as to minimize these effects. Thus, the large range in reported solubilities of magnesium calcites may be a result of differences in sample preparation procedure.The results of these measurements shift the thermodynamic equivalence point of aragonite and magnesian calcite from 7.5 mole % MgCO₃ up to 12 mole % MgCO₃ and prompt a reassessment of models for carbonate diagenetic reactions in natural environments.  相似文献   

方解石表面Cd2+与Pb2+类质同象的AFM研究     

Teng H.Henry  赵良 《高校地质学报》2012,18(2):193-202

 借助原位液槽原子力显微镜（in situ AFM）的观察,通过Cd2+,Pb2+替代方解石最外层晶格Ca2+生长模式的实验研究, 探讨了Cd2+与Pb2+作用下方解石表面溶解与结晶行为。在液体反应槽中,分别将含不饱和Cd2+与Pb2+溶液流经方解石{101 _ 4}解理面,结果发现:（1）Cd2+的存在不影响方解石沿<4_41> 晶向台阶的溶解,而Pb2+的存在则强烈阻碍了方解石沿<441>+晶向台阶的溶解;（2）停止输入溶液含Cd2+,Pb2+溶液后,随着方解石表面与溶液达到平衡,溶解过程逐渐转变为结晶过程。结果显示在Cd2+存在时,单分子生长层具有方解石原有的定向性,而在Pb2+存在时的生长则不具任何定向性。尽管有此差异, 但（Ca,Cd）CO3 和（Ca,Pb）CO3 固溶体都受控于单分子层外延生长这一结晶机理。 　　含Cd2+和Pb2+溶液对方解石溶解动力学的作用与选择性吸附的阳离子半径大小、吸附复合体的几何形状及其结晶学取 向有关。Cd2+离子倾向于优先进入更狭小的<4_41>- 晶向的微台阶上,而Pb2+则倾向于形成扭曲的八面体络合物吸附在更开 阔的<4_41>+ 晶向台阶上。因此,Pb2+存在下方解石表面生长方向无序可认为是白铅矿和方解石结构差异的原因。  相似文献   

Cathodoluminescent bimineralic ooids from the Pleistocene of the Florida continental shelf   总被引：1，自引：0，他引：1  

R. P. MAJOR  R. B. HALLEY†  K. J. LUKAS† 《Sedimentology》1988,35(5):843-855

A bored and encrusted late Pleistocene ooid grainstone was recovered from the seafloor at a depth of approximately 40 m on the outer continental shelf of eastern Florida. Ooid cortices are dominantly bimineralic, generally consisting of inner layers of radial magnesian calcite and outer layers of tangential aragonite. Ooid nuclei are dominantly rounded cryptocrystalline grains, although quartz grains and a variety of skeletal grains also occur as nuclei. Ooids are partially cemented by blocky calcite, and interparticle porosity is partially filled by micrite. Radial cortex layers are composed of brightly cathodoluminescent magnesian calcite having a composition of approximately 12 mol% MgCO₃ and 1000 ppm strontium. The iron and manganese concentrations in radial cortex layers are generally in the range of 500–1000 ppm and 100–250 ppm, respectively. Tangential cortex layers are composed of noncathodoluminescent aragonite containing approximately 11 500 ppm strontium and less than 0.5 mol% MgCO₃. Iron concentrations in tangential cortex layers are generally in the range of 150–400 ppm, and manganese concentrations are generally below the detection limit of 100 ppm. Echinoderm skeletal fragments, which are present as accessory grains, are composed of brightly cathodoluminescent magnesian calcite. Some ooid nuclei and the thin outer edges of some blocky calcite cement are cathodoluminescent; micrite matrix and the bulk of blocky calcite cement are noncathodoluminescent. Ooids do not exhibit textural evidence of recrystallization. The ooid grainstone underwent an episode of meteoric diagenesis. but ooid cortices were not affected by the event. We propose a previously unrecognized process by which the magnesian calcite cortex layers underwent diagenetic alteration in oxygen-depleted seawater. During this diagenesis, magnesium was lost and manganese was incorporated without apparent textural alteration and without mineralogical stabilization. Thus, we Suggest that cathodoluminescence may result from diagenetic alteration on the sea-floor.  相似文献   

The major-ion composition of silurian seawater     

Sean T Brennan  Tim K Lowenstein 《Geochimica et cosmochimica acta》2002,66(15):2683-2700

One-hundred fluid inclusions in Silurian marine halite were analyzed in order to determine the major-ion composition of Silurian seawater. The samples analyzed were from three formations in the Late Silurian Michigan Basin, the A-1, A-2, and B Evaporites of the Salina Group, and one formation in the Early Silurian Canning Basin (Australia), the Mallowa Salt of the Carribuddy Group. The results indicate that the major-ion composition of Silurian seawater was not the same as present-day seawater. The Silurian ocean had lower concentrations of Mg²⁺, Na⁺, and SO₄²⁻, and much higher concentrations of Ca²⁺ relative to the ocean’s present-day composition. Furthermore, Silurian seawater had Ca²⁺ in excess of SO₄²⁻. Evaporation of Silurian seawater of the composition determined in this study produces KCl-type potash minerals that lack the MgSO₄-type late stage salts formed during the evaporation of present-day seawater. The relatively low Na⁺ concentrations in Silurian seawater support the hypothesis that oscillations in the major-ion composition of the oceans are primarily controlled by changes in the flux of mid-ocean ridge brine and riverine inputs and not global or basin-scale, seawater-driven dolomitization. The Mg²⁺/Ca²⁺ ratio of Silurian seawater was ∼1.4, and the K⁺/Ca²⁺ ratio was ∼0.3, both of which differ from the present-day counterparts of 5 and 1, respectively. Seawaters with Mg²⁺/Ca²⁺ <2 facilitate the precipitation of low-magnesian calcite (mol % Mg < 4) marine ooids and submarine carbonate cements whereas seawaters with Mg²⁺/Ca²⁺ >2 (e.g., modern seawater) facilitate the precipitation of aragonite and high-magnesian calcite. Therefore, the early Paleozoic calcite seas were likely due to the low Mg²⁺/Ca²⁺ ratio of seawater, not the pCO₂ of the Silurian atmosphere.  相似文献   

The dolomitization of CaCO₃: an experimental study at 252–295°C     

Amitai Katz  Alan Matthews 《Geochimica et cosmochimica acta》1977,41(2):297-308

The results of experiments on the hydrothermal dolomitization of calcite (between 252 and 295°C) and aragonite (at 252°C) by a 2 M CaCl₂-MgCl₂ aqueous solution are reported and discussed. Dolomitization of calcite proceeds via an intermediate high (ca. 35 mole %) magnesian calcite, whereas that of aragonite is carried out through the conversion of the reactant into a low (5.6 mole %) magnesian calcite which in turn transforms into a high (39.6 mole %) magnesian calcite. Both the intermediate phases and dolomite crystallize through a dissolution-precipitation reaction. The intermediate phases form under local equilibrium within a reaction zone surrounding the dissolving reactant grains. The volume of the reaction zone solution can be estimated from Sr²⁺ and Mg²⁺ partitioning equations. In the case of low magnesian calcite growing at the expense of aragonite at 252°C, the total volume of these zones is in the range of 2 × 10^?5 to 2 × 10^?4 1., out of 5 × 10^?3 1., the volume of the bulk solution.The apparent activation energies for the initial crystallization of high magnesian calcite and dolomite are 48 and 49 kcal/mole, respectively.Calcite transforms completely into dolomite within 100 hr at 252°C. The overall reaction time is reduced to approximately 4 hr at 295°C. The transformation of aragonite to dolomite at 252°C occurs within 24 hr. The nature of the reactant dictates the relative rates of crystallization of the intermediate phases and dolomite. With calcite as reactant, dolomite growth is faster than that of magnesian calcite; this situation is reversed when aragonite is dolomitized.Coprecipitation of Sr²⁺ with dolomite is independent of temperature (within analytical error) between 252 and 295°C. Its partitioning, with respect to calcium, between dolomite and solution results in distribution coefficients in the range of 2.31 × 10^?2 to 2.78 × 10^?2.  相似文献   

Hadean Ocean Carbonate Geochemistry     

John W. Morse  Fred T. Mackenzie 《Aquatic Geochemistry》1998,4(3-4):301-319

Relatively soon (0.2 Ga) after the Earthformed, it is likely that major oceans appeared in ahot (100°C) reducing environment where carbondioxide was probably the dominant atmospheric gas,with PCO2, values reaching perhaps in excess of 10atm. During the Hadean Eon between 4.3 and 3.8 Ga BP,major changes in the concentration of atmosphericCO2 and associated temperature changes had aprofound influence on the carbonate geochemistry ofthe Hadean Ocean. Although no rocks are known to havesurvived prior to the Archean Eon, it is stillpossible to calculate approximate values for importantseawater parameters during the Hadean Eon based onother sources of information and reasonableassumptions about processes such as weatheringreactions.Our calculations are based on a linear temperaturechange from 100°C to 70°C and logPCO2 change from 1 to -1.5 over the Hadean Eon. Over this range in temperature and P CO2, theinfluence of T is relatively small, but changes inP CO2 result in large compositional variations inthe carbonate chemistry of Hadean seawater. In theearly Hadean, seawater pH was probably about5.8 ± 0.2, DIC may have reached close to 130 mM,and alkalinity was perhaps close to 30 mM. By thelate Hadean, seawater pH probably had changed to closeto neutral (6.8), and DIC and alkalinity were closerto present-day values. Even large uncertainties inNa+ + Cl-, K+ and Mg2+concentrations produce relatively small uncertaintiesin our calculated values for the carbonic acid system. However, larger uncertainties result from reasonableranges for Ca2+ concentrations and the saturationstate of Hadean seawater with respect to calcite.Our calculations support the hypothesis that acarbonate chemistry of seawater roughly similar tothat of modern oceans could have been acquired veryearly in Earth history. If seawater composition werebuffered by reactions involving carbonates andsilicates, then the composition of late Hadean-earlyArchean seawater was not vastly different from that oftoday. Thus, by the conclusion of the Hadean Eon, ifnot before, environmental conditions at the Earth's surface, including temperature and seawatercomposition, were sufficiently equable for theevolution of life, including the Archaebacteria: theextreme halophiles and thermophiles and methanogens.Contrary to the hypothesis of an early Na-bicarbonateocean, our calculations suggest the possibility thatthe early oceans of Earth were a NaCl-dominatedaqueous solution, with somewhat higher DIC andalkalinity concentrations, higher saturation state,and the possibility of lower calcium concentrations.The time course of approach of Hadean seawater to acarbonate composition closer to that of today isdifficult to predict. It is distinctly possible thatthe concentration of calcium in seawater did not reachlevels like that of modern seawater until the latePrecambrian and thus constrained the timing of the"Big Bang" of organic evolution, the emergence of theshelled invertebrates at the beginning of thePhanerozoic.  相似文献   

The influence of temperature and seawater composition on calcite crystal growth mechanisms and kinetics: Implications for Mg incorporation in calcite lattice     

Olivier Lopez  Pierpaolo Zuddas  Damien Faivre 《Geochimica et cosmochimica acta》2009,73(2):337-6592

The composition of carbonate minerals formed in past and present oceans is assumed to be significantly controlled by temperature and seawater composition. To determine if and how temperature is kinetically responsible for the amount of Mg incorporated in calcite, we quantified the influence of temperature and specific dissolved components on the complex mechanism of calcite precipitation in seawater. A kinetic study was carried out in artificial seawater and NaCl-CaCl₂ solutions, each having a total ionic strength of 0.7 M. The constant addition technique was used to maintain [Ca²⁺] at 10.5 mmol kg⁻¹ while [] was varied to isolate the role of this variable on the precipitation rate of calcite.Our results show that the overall reaction of calcite precipitation in both seawater and NaCl-CaCl₂ solutions is dominated by the following reaction: