New interpretations of Great Salt Lake ooids and of ancient non-skeletal carbonate mineralogy

Earlier interpretations of textural alteration affecting Great Salt Lake ooids have greatly influenced concepts of ooid diagenesis. Scanning electron microscope study shows, however, that the coarse radial aragonite rays are depositional, that no recrystallization of pellet cores has occurred, and that Great Salt Lake ooids have not suffered noticeable diagenesis. As suggested by Kahle (1974), radial texture in ancient calcitic ooids is probably mainly original, not diagenetic. Retention of such fine textures has been attributed to organic matter (since found to be equivalent in modern skeletal and non-skeletal grains) or to paramorphic replacement (proposed for non-skeletal grains whose original aragonite mineralogy was only inferred from modern analogs). Pleistocene ooids known to have been aragonite alter like aragonite shells to coarse neomorphic calcite, often with aragonite relics. The striking uniformity of that coarse texture in neomorphic calcite replacing known skeletal aragonites throughout the geologic record has been noted for over 100 years. In contrast, Mississippian ooids retain fine texture as do calcite layers of coexisting gastropods, but unlike the strongly altered aragonite layers of these same gastropods. Therefore, inferences of original aragonitic mineralogy of ancient non-skeletal carbonate grains (including muds) which are now calcite but retain fine texture appear unwarranted, as do assumptions of differential diagenetic behaviour of ancient aragonitic skeletal and non-skeletal grains. Accordingly, modern depositional environments of marine ooids and carbonate muds must be rejected as chemically unrepresentative of comparable ancient environments. It is inferred that ancient non-skeletal carbonates were originally predominantly or exclusively calcite because of an earlier lower oceanic Mg/Ca ratio (<2/1) which altered progressively to values favouring aragonite (modern Mg/Ca value = 5/1). Major influencing factors are: selective removal of calcium by planktonic foraminifers and coccolithophorids since Jurassic-Cretaceous time and by abundant younger, Mg-poor aragonite skeletons and an erratic trend toward decreasing dolomite formation (decreasing removal of oceanic Mg). The change to aragonite dominance over calcite for non-skeletal carbonates was probably during early to middle Cenozoic time.     

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

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.     

Formation of low‐magnesium calcite at cold seeps in an aragonite sea

This study investigates the conditions of occurrence and petrographic characteristics of low‐Mg calcite (LMC) from cold seeps of the Gulf of Mexico at a water depth of 2340 m. Such LMC mineral phases should precipitate in calcite seas rather than today's aragonite sea. The ¹³C‐depleted carbonates formed as a consequence of anaerobic oxidation of hydrocarbons in shallow subsurface cold seep environments. The occurrence of LMC may result from brine fluid flows. Brines are relatively Ca²⁺‐enriched and Mg²⁺‐depleted (Mg/Ca mole ratio <0.7) relative to seawater, where the Mg/Ca mole ratio is ~5, which drives high‐Mg calcite and aragonite precipitation. The dissolution of aragonitic mollusk shells, grains and cements was observed. Aerobic oxidation of hydrocarbons and H₂S is the most likely mechanism to explain carbonate dissolution. These findings have important implications for understanding the occurrence of LMC in deep water marine settings and consequently their counterparts in the geological record.     

Pleistocene speleothems of Mallorca: implications for palaeoclimate and carbonate diagenesis in mixing zones

The Pleistocene speleothems of Sa Bassa Blanca cave, Mallorca, are excellent indicators of palaeoclimate variations, and are samples that allow evaluation of the products and processes of mixing‐zone diagenesis in an open‐water cave system. Integrated stratigraphic, petrographic and geochemical data from a horizontal core of speleothem identified two main origins for speleothem precipitates: meteoric‐marine mixing zone and meteoric‐vadose zone. Mixing‐zone precipitates formed at and just below the water–air interface of cave pools during interglacial times, when the cave was flooded as a result of highstand sea‐level. Mixing‐zone precipitates include bladed and dendritic high‐Mg calcite, microporous‐bladed calcite with variable Mg content, and acicular aragonite; their presence suggests that calcium‐carbonate cementation is significant in the studied mixing‐zone system. Fluid inclusion salinities, δ¹³C and δ¹⁸O compositions of the mixing‐zone precipitates suggest that mixing ratio was not the primary control on whether precipitation or dissolution occurred, rather, the proximity to the water table and degassing of CO₂ at the interface, were the major controls on precipitation. Thus, simple two‐end‐member mixing models may apply only in mixing zones well below the water table. Meteoric‐vadose speleothems include calcite and high‐Mg calcite with columnar and bladed morphologies. Vadose speleothems precipitated during glacial stages when sea level was lower than present. Progressive increase in δ¹³C and δ¹⁸O of the vadose speleothems resulted from cooling temperatures and more positive seawater δ¹⁸O associated with glacial buildup. Such covariation could be considered as a valid alternative to models predicting invariant δ¹⁸O and highly variable δ¹³C in meteoric calcite. Glacio‐eustatic oscillations of sea‐level are recorded as alternating vadose and mixing‐zone speleothems. Short‐term climatic variations are recorded as alternating aragonite and calcite speleothems precipitated in the mixing zone. Fluid‐inclusion and stable‐isotope data suggest that aragonite, as opposed to calcite, precipitated during times of reduced meteoric recharge.     

Calcitic, aragonitic and mixed calcitic-aragonitic ooids from the mid-Proterozoic Belt Supergroup, Montana

A great variety of ooid types occurs within the Siyeh and Snowslip Formations of the mid-Proterozoic Belt Supergroup, Montana. Cortical layers are inferred to have been composed either of calcite in a radial-concentric or radial-with-dark-rays fabric or, aragonite in a radial or concentric fabric. The calcitic cortical layers record their original fabrics but the originally aragonitic cortical layers have been replaced by calcite in a range of textures and by quartz and dolomite. Some formerly aragonitic cortical layers are replaced by calcite spar which contains relics of the original cortical structure. Others consist of calcite spar without inclusions, or columnar calcite which grew radially from the nucleus, commonly a calcitic ooid. Some ooids were wholly composed of calcite, others were of aragonite, but two phase ooids were common, mostly consisting of an inner calcitic part and an outer aragonitic part. Probable microdolomite inclusions suggest a high Mg content of the calcitic cortical layers. The depositional environment of these oolites was probably analogous to Baffin Bay, Texas, where a similar range of ooid types is forming today.     

华北寒武纪苗岭世晚期δ13C演化及生态环境特征

豫北和鲁西地区寒武系苗岭统上部δ¹³C演化表现为下降趋势,芙蓉统下部δ¹³C呈上升趋势,并表现出显著正漂移,这次δ¹³C正漂移出现在三叶虫Chuangia带内,可与世界各地芙蓉统排碧阶的δ¹³C正漂移(SPICE)对比。δ¹³C演化趋势与三叶虫、牙形石、浮游植物的繁盛与萧条表现出一定的耦合关系。苗岭世晚期三叶虫大规模绝灭时期,δ¹³C呈逐渐降低趋势,苗岭世末期新的三叶虫科大量出现时期,δ¹³C呈上升趋势。另外,海平面升降对δ¹³C演化具有明显的影响,海侵时期沉积的碳酸盐岩其δ¹³C呈逐渐增大趋势,高水位早期沉积的碳酸盐岩具有较高的δ¹³C值,高水位晚期或海平面下降期沉积的碳酸盐岩的δ¹³C呈逐渐下降趋势。海平面变化是导致生态环境变化、生物群落演化、碳同位素组成演化的关键因素。     

Fabrics,facies control and diagenesis of lacustrine ooids and associated grains from the Upper Triassic,southwest England

Petrographic analysis of ooids from the Upper Triassic (Mercia Mudstone Group) of southwest England provides an opportunity to assess in detail the origins, transport pathways and diagenesis of an ancient oolite. The Clevedon Oolite is dolomitized and contains a variety of dissolved ooids (oomoulds) and associated grains. The oomoulds occur in well‐sorted, planar and cross‐stratified grainstones, packstones, sandstones and conglomerates associated with shoreface, intershoal, foreshore, beachrock and littoral strandplain deposits. The ooids grew in suspension in the shoreface zone and developed a radial aragonite microstructure. The ooids grew to 0.80 mm in diameter, after which they fractured or ceased growing. Broken grains deposited on or near mobile shoals were rapidly recoated, while other grains, deposited in less agitated, intershoal and lower foreshore areas, were micritized or microbially bound into grapestone aggregates. Locally peloids, intraclasts, quartz grains and micritized grains from intershoal areas supplied nuclei for ooids on nearby shoals. Grains deposited in foreshore areas were rapidly cemented into beachrock and reworked into conglomerates. Soon after deposition, the ooids were subjected to widespread aragonite dissolution followed by dolomitization. The lack of pre‐dolomitization calcite, together with the abundance of early (pre‐compaction) dolospar cements and fabric‐selective dolomitization of micritic fabrics, suggest aragonite dissolution by dolomitizing fluids. Copyright © 2002 John Wiley & Sons, Ltd.     

Annual trace element cycles in calcite–aragonite speleothems: evidence of drought in the western Mediterranean 1200–1100 yr BP

Each of two calcitic stalagmites from Grotte de Clamouse, Herault, southern France, displays a discrete aragonite layer dated at around 1100 yr BP. The layer of fanning aragonite ray crystals is immediately preceded by calcite with Mg and Sr compositions that are uniquely high for the past 3 kyr. Trace element compositions close to the boundary between original aragonite and calcite are consistent with quasi‐equilibrium partitioning of trace elements between the phases. Study of modern dripwaters demonstrates that pronounced covariation of Mg/Ca and Sr/Ca ratios in dripwater occurs owing to large amounts of calcite precipitation upflow of the drips that fed the stalagmites. Trace element to Ca ratios are enhanced during seasonally dry periods. Ion microprobe data demonstrate a pronounced covariation of trace elements, including Mg and Sr in calcite, and Sr, U and Ba in aragonite. The mean peak spacing is close to the long‐term mean of annual growth rates determined by differences in U‐series ages and so the trace element peaks are interpreted as annual. The trace element chemistry of the stalagmites on annual to inter‐annual scales thus directly reflects the amounts of prior calcite precipitation, interpreted as an index of aridity. The longer‐term context is a multi‐decadal period of aridity (1200–1100 yr BP) possibly correlated with an analogous episode in Central America. The arid period culminated in the nucleation of aragonite, but within a decade was followed by a return to precursor conditions. Copyright © 2005 John Wiley & Sons, Ltd.     

鲕粒原生矿物识别及对海水化学成分变化的指示意义

鲕粒是碳酸盐沉积过程中一类非常特殊的颗粒类型, 为研究当时的沉积背景、水动力条件、气候环境, 甚至储层特征提供了重要线索。然而, 鲕粒的矿物组成及控制因素问题, 长期受到忽视。组成鲕粒的原生矿物类型在地质历史时期呈周期性变化, 在显生宙表现为三个以文石和高镁方解石占主导的时期以及两个以低镁方解石占主导的时期, 这也被称作“文石海”和“方解石海”时期。原生矿物的组成, 制约着鲕粒的纹层结构、保存程度以及成岩特征, 还蕴含着海水化学成分变化的线索。鲕粒原生矿物识别主要依据:①原生纹层结构;②保存程度;③微量元素浓度, 尤其是Sr-Mg的浓度。文石质鲕粒受文石不稳定性的影响, 原生结构保存程度较差;一般保存有典型的文石残余纹层结构(例如砖砌结构、溶解变形结构以及偏心结构等);在封闭成岩环境下原生矿物为文石质的鲕粒Sr浓度往往大于2 000 ppm;纹层结构主要为切线状(占主导)和放射状。方解石质鲕粒包括低镁方解石和高镁方解石两种类型:低镁方解石为稳定矿物, 原生结构一般保存良好。尽管高镁方解石也为亚稳定矿物, 但成岩转换后的保存程度好于文石。两者Sr含量一般均低于1 000 ppm, Mg含量一般在0~20 mol % MgCO₃(两者以4 mol % MgCO₃为界)。高镁方解石受成岩作用影响, 在纹层中往往保留有微粒白云石包裹体;海相地层中保存的方解石质鲕粒为放射状或同心-放射状结构。另外还存在一类由两种矿物共同构成的双矿物鲕粒, 可以通过分析两类纹层在结构和保存特征上的差异进行区分。鲕粒原生矿物成分随时间的波动变化受到海水化学条件, 尤其是Mg/Ca比值, 大气二氧化碳分压以及碳酸盐饱和度的控制。Mg/Ca比值的波动决定着鲕粒原生矿物类型的长期变化规律。一些突发性事件可能会扰动(区域)短时间尺度下鲕粒原生矿物的组成, 造成鲕粒原生矿物的转换。通过研究碳酸盐鲕粒原生矿物特征以及控制因素进而了解海水的化学特征, 是独立于古生物学和地球化学分析之外的一种较为可靠的沉积学方法。     

海水化学演化对生物矿化的影响综述

显生宙非骨屑碳酸盐矿物经历了文石海和方解石海的交替,主要造礁生物和沉积物生产者的骨骼矿物与非骨屑碳酸盐矿物具有同步变化的趋势。这种长期的变化趋势可以用海水化学Mg/Ca摩尔比的变化来解释。流体包裹体、同位素和微量元素等证据也证实了海水化学在地质历史中经历过剧烈的变化。虽然生物诱导矿化和生物控制矿化的相对重要性一直存在争议,但古生物地层记录和人工海水养殖实验结果都表明,海水化学演化对生物矿化有重要的影响,体现在造礁生物群落的兴衰、生物起源时对骨骼矿物类型的选择以及微生物碳酸盐岩在地质历史中的分布等。这些为研究前寒武纪海水化学演化、古气候和古环境的重建、同位素地层对比以及碳酸盐的沉积和成岩等问题提供了新的思路。     

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

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.     

光合作用生物膜诱发的放射鲕粒:以江苏徐州贾旺剖面寒武系苗岭统张夏组为例*

几年来针对巴哈马现代文石鲕粒的持续性研究表明,微生物和细胞外聚合物质(EPS)在鲕粒的形成和发育中起着关键而重要的作用,从而产生了一个重要的认识,即: 鲕粒可以看作是“纹层状的有机沉积构造”并遵循着微生物岩体系的一些形成特征。但是,鲕粒30亿年的发育历史、多样化的产出环境、特征性的矿物构成和各种各样的沉积组构,确实赋予了鲕粒生长和形成机理的复杂性和神秘性,因为鲕粒在何处而且如何形成、以及鲕粒究竟记录着何种生物与非生物过程的许多问题还存在剧烈争论。来自于江苏徐州贾旺剖面苗岭统张夏组上部鲕粒滩相灰岩,由较为典型的方解石放射鲕粒所组成,表现出放射状、放射—同心状和泥晶质的沉积组构,而且在鲕粒核心、鲕粒皮层以及在鲕粒间的不规则团块或凝块的暗色泥晶质构成中高密度地保存着精美的葛万菌(Girvanella)化石,进一步表明了这些暗色泥晶构成代表着较为特征的光合作用生物膜,从而提供了一个苗岭世方解石海中放射鲕粒形成较为直接的微生物证据,以及与光合作用生物膜之间复杂的成因联系,因为葛万菌是相对较为肯定地类比于近代钙化织线菌(Plectonema)的丝状蓝细菌化石,尽管还可类比于现代的伪枝菌(Scytonema)。虽然形成放射状鲕粒皮层的放射纤维状方解石的沉淀作用确实不能解释为直接的微生物沉淀作用的结果,但是,这些放射鲕粒确实表现出光合作用生物膜诱发、滋养并促进了放射纤维状方解石皮层增生作用的重要证据,为拓宽“鲕粒谜”的阐释提供了一个较为重要的典型实例,而且还成为寒武纪苗岭世方解石海与后生动物辐射相耦合的蓝细菌繁荣的重要证据。     

A comparison of amorphous calcium carbonate crystallization in aqueous solutions of MgCl2 and MgSO4: implications for paleo-ocean chemistry

Mineralogy and Petrology - Based on the terminology of “aragonite seas” and “calcite seas”, whether different Mg sources could affect the mineralogy of carbonate sediments...     

Shallow-marine carbonate cementation in Holocene segments of the calcifying green alga Halimeda

Early-diagenetic cementation of tropical carbonates results from the combination of numerous physico-chemical and biological processes. In the marine phreatic environment it represents an essential mechanism for the development and stabilization of carbonate platforms. However, diagenetic cements that developed early in the marine phreatic environment are likely to become obliterated during later stages of meteoric or burial diagenesis. When lithified sediment samples are studied, this complicates the recognition of processes involved in early cementation, and their geological implications. In this contribution, a petrographic microfacies analysis of Holocene Halimeda segments collected on a coral island in the Spermonde Archipelago, Indonesia, is presented. Through electron microscopical analyses of polished samples, this study shows that segments are characterized by intragranular cementation of fibrous aragonite, equant High-Mg calcite (3.9 to 7.2 Mol% Mg), bladed Low-Mg calcite (0.4 to 1.0 Mol% Mg) and mini-micritic Low-Mg calcite (3.2 to 3.3 Mol% Mg). The co-existence and consecutive development of fibrous aragonite and equant High-Mg calcite results initially from the flow of oversaturated seawater along the aragonite template of the Halimeda skeleton, followed by an adjustment of cement mineralogy towards High-Mg calcite as a result of reduced permeability and fluid flow rates in the pores. Growth of bladed Low-Mg calcite cements on top of etched substrates of equant High-Mg calcite is explained by shifts in pore water pH and alkalinity through microbial sulphate reduction. Microbial activity appears to be the main trigger for the precipitation of mini-micritic Low-Mg calcite as well, based on the presumable detection of an extracellular polymeric matrix during an early stage of mini-micrite Low-Mg calcite cement precipitation. Radiocarbon analyses of five Halimeda segments furthermore indicate that virtually complete intragranular cementation in the marine phreatic environment with thermodynamically/kinetically controlled aragonite and High-Mg calcite takes place in about 100 years. Collectively, this study shows that early-diagenetic cements are highly diverse and provides new quantitative constraints on the rate of diagenetic cementation in tropical carbonate factories.     

The impact of solution chemistry on Mytilus edulis calcite and aragonite

Magnesium/calcium, Sr/Ca, and Na/Ca atom ratios were determined in the calcite and aragonite regions of Mytilus edulis shells which were grown in semi-artificial ‘seawater’ solutions having varying Mg/Ca, Sr/Ca, and Na/Ca ratios. These ratios were measured by instrumental neutron activation, atomic absorption, and electron microprobe analytical techniques. Strontium/calcium ratios in both calcite and aragonite were linearly proportional to solution Sr/Ca ratios. Magnesium/calcium ratios in calcite increased exponentially when solution Mg/Ca ratios were raised above the normal seawater ratio; whereas in aragonite, Mg/Ca ratios increased linearly with increases in solution Mg/Ca ratios. Sodium/calcium and sulfur/calcium ratios in calcite covaried with Mg/Ga solution ratios. Conversely, in aragonite, Na/Ca ratios varied linearly with solution Na/Ca ratios.Magnesium is known to inhibit calcite precipitation at its normal seawater concentration. We infer from the results of the work reported here that Mytilus edulis controls the Mg activity of the outer extrapallial fluid, thus facilitating the precipitation of calcitic shell. Increases in sulfur content suggest that changes in shell organic matrix content occur as a result of environmental stress. Certain increases in Mg content may also be correlated to stress. Sodium/calcium variations, and their absolute amounts in calcite and aragonite, are best explained by assuming that a substantial amount of Na is adsorbed on the calcium carbonate crystal surface. Strontium/calcium ratios show more promise than either Mg/Ca or Na/Ca ratios as seawater paleochemistry indicators, because the Sr/Ca distribution coefficients for both aragonite and calcite are independent of seawater Ca and Sr concentrations.     

Microbial biomineralization processes forming modern Ca:Mg carbonate stromatolites

Modern Ca:Mg carbonate stromatolites form in association with the microbial mat in the hypersaline coastal lagoon, Lagoa Vermelha (Brazil). The stromatolites, although showing diversified fabrics characterized by thin or crude lamination and/or thrombolitic clotting, exhibit a pervasive peloidal microfabric. The peloidal texture consists of dark, micritic aggregates of very high‐Mg calcite and/or Ca dolomite formed by an iso‐oriented assemblage of sub‐micron trigonal polyhedrons and organic matter. Limpid acicular crystals of aragonite arranged in spherulites surround these aggregates. Unlike the aragonite crystals, organic matter is present consistently in the dark, micritic carbonate comprising the peloids. This organic matter is observed as sub‐micron flat and filamentous mucus‐like structures inside the interspaces of the high‐Mg calcite and Ca dolomite crystals and is interpreted as the remains of degraded extracellular polymeric substances. Moreover, many fossilized bacterial cells are associated strictly with both carbonate phases. These cells consist mainly of 0·2 to 4 μm in diameter, sub‐spherical, rod‐like and filamentous forms, isolated or in colony‐like clusters. The co‐existence of fossil extracellular polymeric substances and bacterial bodies, associated with the polyhedrons of Ca:Mg carbonate, implies that the organic matter and microbial metabolism played a fundamental role in the precipitation of the minerals that form the peloids. By contrast, the lack of extracellular polymeric substances in the aragonitic phase indicates an additional precipitation mechanism. The complex processes that induce mineral precipitation in the modern Lagoa Vermelha microbial mat appear to be recorded in the studied lithified stromatolites. Sub‐micron polyhedral crystal formation of high‐Mg calcite and/or Ca dolomite results from the coalescence of carbonate nanoglobules around degraded organic matter nuclei. Sub‐micron polyhedral crystals aggregate to form larger ovoidal crystals that constitute peloids. Subsequent precipitation of aragonitic spherulites around peloids occurs as micro‐environmental water conditions around the peloids change.     

Mg fractionation in crustose coralline algae: Geochemical, biological, and sedimentological implications of secular variation in the Mg/Ca ratio of seawater

The Mg/Ca ratio of seawater has varied significantly throughout the Phanerozoic Eon, primarily as a function of the rate of ocean crust production. Specimens of the crustose coralline alga Neogoniolithon sp. were grown in artificial seawaters encompassing the range of Mg/Ca ratios shown to have existed throughout the Phanerozoic. Significantly, the coralline algae’s skeletal Mg/Ca ratio varied in lockstep with the Mg/Ca ratio of the artificial seawater. Specimens grown in seawater treatments formulated with identical Mg/Ca ratios but differing absolute concentrations of Mg and Ca exhibited no significant differences in skeletal Mg/Ca ratios, thereby emphasizing the importance of the ambient Mg/Ca ratio, and not the absolute concentration of Mg, in determining the Mg/Ca ratio of coralline algal calcite. Specimens grown in seawater of the lowest molar Mg/Ca ratio (mMg/Ca = 1.0) actually changed their skeletal mineralogy from high-Mg (skeletal mMg/Ca > 0.04) to low-Mg calcite (skeletal mMg/Ca < 0.04), suggesting that ancient calcitic red algae, which exhibit morphologies and modes of calcification comparable to Neogoniolithon sp., would have produced low-Mg calcite from the middle Cambrian to middle Mississippian and during the middle to Late Cretaceous, when oceanic mMg/Ca approached unity. By influencing the original Mg content of carbonate facies in which these algae have been ubiquitous, this condition has significant implications for the geochemistry and diagenesis of algal limestones throughout most of the Phanerozoic. The crustose coralline algae’s precipitation of high-Mg calcite from seawater that favors the abiotic precipitation of aragonite indicates that these algae dictate the precipitation of the calcitic polymorph of CaCO₃. However, the algae’s nearly abiotic pattern of Mg fractionation in their skeletal calcite suggests that their biomineralogical control is limited to polymorph specification and is generally ineffectual in the regulation of skeletal Mg incorporation. Therefore, the Mg/Ca ratio of well-preserved fossils of crustose coralline algae, when corrected for the effect of seawater temperature, may be an archive of oceanic Mg/Ca throughout the Phanerozoic. Magnesium fractionation algorithms that model algal skeletal Mg/Ca as a function of seawater Mg/Ca and temperature are presented herein. The results of this study support the empirical fossil evidence that secular variation of oceanic Mg/Ca has caused the mineralogy and skeletal chemistry of many calcifying marine organisms to change significantly over geologic time.     

Calcite cementation during bacterial manganese, iron and sulphate reduction in Jurassic shallow marine carbonates

Faunally restricted argillaceous wackestones from the Middle Jurassic of eastern England contain evidence of early diagenetic skeletal aragonite dissolution and stabilization of the carbonate matrix, closely followed by precipitation of zoned calcite cements, and precipitation of pyrite. Distinctive cathodoluminescence and trace element trends through the authigenic calcites, their negative δ¹³C compositions and the location of pyrite in the paragenetic sequence indicate that calcite precipitation took place during sequential bacterial Mn, Fe and sulphate reduction. Calcite δ¹⁸O values are compatible with cementation from essentially marine pore fluids, although compositions vary owing to minor contamination with ¹⁸O-depleted ‘late’cements. Mg and Sr concentrations in the calcites are lower than those in recent marine calcite cements. This may be a result of kinetic factors associated with the shallow burial cementation microenvironments. Bicarbonate for sustained precipitation of the authigenic calcites was derived largely from aragonite remobilization, augmented by that produced through anaerobic organic matter oxidation in the metal and sulphate reduction environments. Aragonite dissolution is thought to have been induced by acidity generated during aerobic bacterial oxidation of organic matter. Distinction of post-oxic metal reduction and anoxic sulphate reduction diagenetic environments in modern carbonate sediments is uncommon outside pelagic settings, and early bacterially mediated diagenesis in modern platform carbonates is associated with extensive carbonate dissolution. High detrital Fe contents of the Jurassic sediments, and their restricted depositional environment, were probably the critical factors promoting early cementation. These precipitates constitute a unique example of calcite authigenesis in shallow water limestones during bacterial Mn and Fe reduction.     

室温常压下 Ca2+-Mg2+-HCO31--H2O 体系的试验研究

世界上很多海域中的现代碳酸盐沉积正在形成,如巴哈马滩、美国的佛罗里达湾、古巴的巴塔诺湾、中东的波斯湾等地,以及丹麦的某些近海地区。产于这些海域中的现代碳酸盐沉积物,其矿物组成主要是文石,其次为镁方解石,纯方解石较少,现代白云石沉积更为少见。我国的现代碳酸盐沉积见于南海诸岛及海南岛的沿海地带。作者曾利用粉晶照相鉴定了采自我国南海二十余种珊瑚、瓣鳃类、腹足类、有孔虫等现代海相生物壳体的物相,发现除有孔虫和海胆壳是由镁方解石构成外,其余生物壳体皆由文石构成。     

Diagenesis of early Permian high‐latitude limestones,Lower Parmeener Supergroup,Tasmania

The Darlington (Sakmarian) and Berriedale (Artinskian) Limestones are neritic deposits that accumulated in high‐latitude environments along the south‐eastern margin of Pangea in what is now Tasmania. These rocks underwent a series of diagenetic processes that began in the marine palaeoenvironment, continued during rapid burial and were profoundly modified by alteration associated with the intrusion of Mesozoic igneous rocks. Marine diagenesis was important but contradictory; although dissolution took place, there was also coeval precipitation of fibrous calcite cement, phosphate and glauconite, as well as calcitization of aragonite shells. These processes are interpreted as having been promoted by mixing of shelf and upwelling deep ocean waters and enabled by microbial degradation of organic matter. In contrast to warm‐water carbonates where meteoric diagenesis is important, the Darlington and Berriedale Limestones were largely unaffected by meteoric diagenesis. Only minor dissolution and local cementation took place in this diagenetic environment, although mechanical compaction was ubiquitous. Correlation with burial history curves indicates that chemical compaction became important as burial depths exceeded 150 m, promoting precipitation of extensive ferroan calcite. This effect resulted from burial by rapidly deposited, overlying, thick, late Permian and Triassic terrestrial sediments. This diagenetic pathway was, however, complicated by the subsequent intrusion of massive Mesozoic diabases and associated silicifying diagenetic fluids. Finally, fractures most probably connected with Cretaceous uplift were filled with late‐stage non‐ferroan calcite cement. This study suggests that both carbonate dissolution and precipitation occur in high‐latitude marine palaeoenvironments and, therefore, the cold‐water diagenetic realm is not always destructive in terms of diagenesis. Furthermore, it appears that for the early Permian of southern Pangea at least, there was no real difference in the diagenetic pathways taken by cool‐water and cold‐water carbonates.