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.     

Geochemical implications of induced changes in C13 fractionation by blue-green algae

The photosynthetic fractionation of carbon isotopes by blue-green algae in laboratory culture is dependent in a non-linear fashion on the CO₂ concentration in the feed gas. For the three species tested, the minimum fractionation occurred at a CO₂ concentration of 0.2% in air and was approximately zero for the two marine species tested. Enrichment of C¹² in the reduced carbon is not an inevitable result of photosynthetic carbon fixation. Temperature and pH had no detectable effect on fractionation. The maximum fractionation observed in the laboratory cultures or in recent blue-green algal mats was 18‰. Differences in the isotope ratio of coexisting oxidized and reduced carbon in Precambrian stromatolites are as great as 31‰. Present carbon isotopic evidence is not consistent with the idea that blue-green algae were major contributors to the organic matter in Precambrian sediments.     

重庆地区石笋记录中Mg/Ca比值及古气候意义

在重庆梁天湾洞石笋LT14 31.9~15.02ka.B.P.古气候重建成果已发表的此基础上,本文选取Mg/Ca比值这一新的替代指标,探讨其在重庆地区独特的水热组合模式下的古气候指示意义,从而进一步探讨了该时段的气候变化.研究结果发现,重庆地区Mg/Ca比值与碳同位素的正相关关系较好(R2=0.60),可能与碳酸钙溶液在到达洞顶之前提前沉淀或滴率减小机制有关,它间接响应了降水的变化.运用Mg/Ca值变化对古气候的重建结果与以前的研究基本相符,说明Mg/Ca比值可以作为古降水的指标,但需同碳氧稳定同位素结合起来讨论,以便相互验证,提高古环境解译的准确性.     

Environmental and biological controls on elemental (Mg/Ca, Sr/Ca and Mn/Ca) ratios in shells of the king scallop Pecten maximus

The relationship between potential elemental proxies (Mg/Ca, Sr/Ca and Mn/Ca ratios) and environmental factors was investigated for the bivalve Pecten maximus in a detailed field study undertaken in the Menai Strait, Wales, U.K. An age model constructed for each shell by comparison of measured and predicted oxygen-isotope ratios allowed comparison on a calendar time scale of shell elemental data with environmental variables, as well as estimation of shell growth rates. The seasonal variation of shell Mn/Ca ratios followed a similar pattern to one previously described for dissolved Mn²⁺ in the Menai Strait, although further calibration work is needed to validate such a relationship. Shell Sr/Ca ratios unexpectedly were found to co-vary most significantly with calcification temperature, whilst shell Mg/Ca ratios were the next most significant control. The temporal variation in the factors that control shell Sr/Ca ratios strongly suggest the former observation most likely to be the result of a secondary influence on shell Sr/Ca ratios by kinetic effects, the latter driven by seasonal variation in shell growth rate that is in turn influenced in part by seawater temperature. P. maximus shell Mg/Ca ratio to calcification temperature relationships exhibit an inverse correlation during autumn to early spring (October to March-April) and a positive correlation from late spring through summer (May-June to September). No clear explanation is evident for the former trend, but the similarity of the records from the three shells analysed indicate that it is a real signal and not a spurious observation. These observations confirm that application of the Mg/Ca proxy in P. maximus shells remains problematic, even for seasonal or absolute temperature reconstructions. For the range of calcification temperatures of 5-19 °C, our shell Mg/Ca ratios in P. maximus are approximately one-fourth those in inorganic calcite, half those in the bivalve Pinna nobilis, twice those in the bivalve Mytilus trossulus, and four to five times higher than Mg/Ca ratios in planktonic and benthonic foraminifera. Our findings further support observations that Mg/Ca ratios in bivalve shell calcite are an unreliable temperature proxy, as well as substantial taxon- and species-specific variation in Mg incorporation into bivalves and other calcifying organisms, with profound implications for the application of this geochemical proxy to the bivalve fossil record.     

Variations of the Ca/Ca ratio in seawater during the past 24 million years: evidence from δCa and δO values of Miocene phosphates

The Ca isotope variation of 11 Miocene and Pleistocene, authigenic, marine phosphates is rather small compared to the corresponding variation in δ¹⁸O values. The δ⁴⁴Ca values are not correlated with the δ¹⁸O values and, therefore, they are not temperature controlled. It is likely that the δ⁴⁴Ca values of the phosphorites reflect the variation in the δ⁴⁴Ca values of paleo-seawater but, in contrast to Sr, not the isotopic composition of seawater itself. Furthermore, Ca and Sr isotopic compositions are decoupled with decreasing stratigraphic age of the phosphate peloids with Sr isotopic compositions changing to more radiogenic values while Ca isotopic compositions remain rather stable. All samples have δ⁴⁴Ca values below present-day seawater values, suggesting that phosphate formation discriminates against heavy Ca isotopes.Phosphorites and carbonaceous sediments have a similar Ca isotopic variation during the Miocene. A systematic and more or less constant shift between marine carbonates and phosphates is observed: the phosphate samples are slightly less enriched in ⁴⁰Ca compared to carbonates. This shift has been related to a mineral-dependent kinetic mass fractionation during precipitation from seawater. The rather stable δ⁴⁴Ca value for the 19 to 9 Ma old phosphorites points to a constant δ⁴⁴Ca fractionation of about 1.1 between seawater and phosphorites during the past and suggests steady-state conditions for the Mid-Miocene seawater (sedimentation flux equals erosion flux).     

Proximal to distal sedimentological variation in a linear turbidite trough: implications for the fan model

Determination of geographically dependent sedimentological variation (‘proximality’) in ancient flysch deposits formed an important part of early turbidite studies. Attempts to quantify this variation highlighted anomalies which were neatly resolved by application of vertical sequence analysis and the use of fan models. However, there are many turbidite formations, such as the Lower Cretaceous Cumberland Bay Formation (CBF) of South Georgia, which cannot be described in terms of existing fan models but show strong proximal to distal sedimentological changes. The CBF is a thick sequence of volcaniclastic sandstone turbidites deposited in a linear back-arc basin, principally by currents flowing WNW, parallel to the basin margin. Four lithofacies associations are recognized on the basis of sandstone/shale ratio. The two finergrained associations are constant in character across the CBF outcrop. In the coarse-grained associations there is a change in character WNW, down the palaeocurrent direction. This is brought out by decreasing sandstone bed thickness and percentage amalgamation, but these changes are not always marked or consistent. In contrast, the internal character of the sandstone beds changes strongly, with a marked proportional increase in T_b and T_c divisions within the bed downcurrent. The evidence suggests that the system was aggradational rather than progradational: tectonic control of the basin margins prevented major migration of the depositional system, and most areas remained in the same position relative to source through time. Comparison of the CBF with other turbidite formations suggests two end-member states which will produce radically different vertical sequences. Progradational systems will produce strong vertical facies changes, where beds deposited in distal environments are overlain by beds deposited in environments progressively nearer source, however there will be no lateral change in the character of any particular facies type. In contrast in aggradational systems the major sedimentological variation will be lateral rather than vertical.     

The impact of salinity on the Mg/Ca and Sr/Ca ratio in the benthic foraminifera Ammonia tepida: Results from culture experiments

Over the last decade, sea surface temperature (SST) reconstructed from the Mg/Ca ratio of foraminiferal calcite has increasingly been used, in combination with the δ¹⁸O signal measured on the same material, to calculate the δ¹⁸O_w, a proxy for sea surface salinity (SSS). A number of studies, however, have shown that the Mg/Ca ratio is also sensitive to other parameters, such as pH or , and salinity. To increase the reliability of foraminiferal Mg/Ca ratios as temperature proxies, these effects should be quantified in isolation. Individuals of the benthic foraminifera Ammonia tepida were cultured at three different salinities (20, 33 and 40 psu) and two temperatures (10-15 °C). The Mg/Ca and Sr/Ca ratios of newly formed calcite were analyzed by Laser Ablation ICP-MS and demonstrate that the Mg concentration in A. tepida is overall relatively low (mean value per experimental condition between 0.5 and 1.3 mmol/mol) when compared to other foraminiferal species, Sr being similar to other foraminiferal species. The Mg and Sr incorporation are both enhanced with increasing temperatures. However, the temperature dependency for Sr disappears when the distribution factor D_Sr is plotted as a function of calcite saturation state (Ω). This suggests that a kinetic process related to Ω is responsible for the observed dependency of Sr incorporation on sea water temperature. The inferred relative increase in D_Mg per unit salinity is 2.8% at 10 °C and 3.3% at 15 °C, for the salinity interval 20-40 psu. This implies that a salinity increase of 2 psu results in enhanced Mg incorporation equivalent to 1 °C temperature increase. The D_Sr increase per unit salinity is 0.8% at 10 °C and 1.3% at 15 °C, for the salinity interval 20-40 psu.     

南海北部甲烷渗漏的沉积地球化学指标(Sr/Ca和Mg/Ca)识别

自生碳酸盐矿物是揭示甲烷渗漏过程及其周期性变化的重要指标。为了方便、快速地识别出自生碳酸盐矿物,在探讨了前人建立的端元组分模型(文石、高镁方解石、生物成因方解石和碎屑)在我国南海北部适用性的基础上,利用沉积物全样中Sr/Ca和Mg/Ca值计算了南海北部神狐海域两支沉积物柱状样中自生碳酸盐矿物的质量分数,计算结果表明,在不同沉积深度均有含量不等的自生碳酸盐矿物。这一结果被X射线衍射结果和沉积物中存在晶形完好的自生高镁方解石和文石矿物所验证,表明利用前人建立的端元组分模型计算的结果具有可靠性。自生碳酸盐矿物的出现显示该海域深受甲烷渗漏作用影响。利用地球化学指标(Sr/Ca和Mg/Ca)获得的自生碳酸盐矿物含量垂向变化显示该区域甲烷渗漏具有强弱交替的周期变化。运用端元组分模型来获取沉积物柱状样中自生碳酸盐矿物的高分辨率剖面,从而识别甲烷渗漏信息非常方便,在大范围寻找甲烷渗漏和天然气水合物方面具有较大潜力。     

Geniculate coralline algae from the Neogene-Quaternary sediments in and around Porbandar,southwest coast of India

The present paper documents nine geniculate coralline algal species, namely Arthrocardia cretacica, Calliarthron antiquum, Corallina hayasaki, C. marshallensis, C. prisca, C. typica, Jania mengaudi, J. sripadaraoi and J. vetus from Neogene-Quaternary sediments of Porbandar area, Saurahtra, Gujarat. Out of nine species, six species namely Arthrocardia cretacica, Calliarthron antiquum, Corallina hayasaki, C. marshallensis, C. prisca and C. typica occur in the Dwarka Formation (lower-middle Miocene), one species Jania sripadaraoi occurs in the Adatiana Member of the Miliolite Formation (early middle-late Pleistocene) and two species namely, Jania mengaudi and J. vetus occur in the Porbandar Calcarenite Member of the Chaya Formation (late Pleistocene-late Holocene). These species indicate that the Dwarka Formation and the Adatiana Member of the Miliolite Formation were deposited in tropical marine environment with moderate to low energy conditions at depths ranging from intertidal to 60 m, and that the Porbandar Calcarenite Member of the Chaya Formation was deposited in a tropical marine environment under moderate to low energy conditions at depths ranging from 40 m to 60 m.     

Geochemical variation within the northern Ryukyu Arc: magma source compositions and geodynamic implications

The major and trace element and Pb–Sr–Nd isotopic compositions of Quaternary mafic lavas from the northern Ryukyu arc provide insights into the nature of the mantle wedge and its tectonic evolution. Beneath the volcanic front in the northern part of the arc, the subducted slab of the Philippine Sea Plate bends sharply and steepens at a depth of ∼80 km. Lavas from the volcanic front have high abundances of large ion lithophile elements and light rare earth elements relative to the high field strength elements, consistent with the result of fluid enrichment processes related to dehydration of the subducting slab. New Pb isotopic data identify two distinct asthenospheric domains in the mantle wedge beneath the south Kyushu and northern Ryukyu arc, which, in a parallel with data from the Lau Basin, appear to reflect mantle with affinities to Indian and Pacific-type mid-ocean ridge basalt (MORB). Indian Ocean MORB-type mantle, contaminated with subducted Ryukyu sediments can account for the variation of lavas erupted on south Kyushu, and probably in the middle Okinawa Trough. In contrast, magmas of the northern Ryukyu volcanic front appear to be derived from sources of Pacific MORB-type mantle contaminated with a sedimentary component. Along-arc variation in the northern Ryukyus reflects increasing involvement of a sedimentary component to the south. Compositions of alkalic basalts from the south Kyushu back-arc resemble intraplate-type basalts erupted in NW Kyushu since ∼12 Ma. We propose that the bending of the subducted slab was either caused by or resulted in lateral migration of asthenospheric mantle, yielding Indian Ocean-type characteristics from a mantle upwelling zone beneath NW Kyushu and the East China Sea. This model also accounts for (1) extensional counter-clockwise crustal rotation (∼4–2 Ma), (2) voluminous andesite volcanism (∼2 Ma), and (3) the recent distinctive felsic magmatism in the south Kyushu region. Received: 30 November 1999 / Accepted: 20 July 2000     

Non-conservative behaviour of molybdenum in coastal waters: Coupling geochemical, biological, and sedimentological processes

Non-conservative behaviour of dissolved Mo was observed during specific time periods in the water column of the Wadden Sea of NW Germany. In July 2005 dissolved Mo declined within 36 h from a level only slightly below seawater (82 nM) to a minimum value of 30 nM, whereas in August 2002 dissolved Mo revealed a tidal cyclicity with maximum values up to 158 nM at low tide. In contrast, cruises in August 2003 and 2004 displayed an almost conservative behaviour of Mo. The decrease in dissolved Mo during July 2005 and elevated values in August 2002 were accompanied by Mo enrichments on aggregates in the water column of the Wadden Sea. Along with Mo, dissolved Mn showed unusual concentration patterns in July 2005, with values distinctly below the common summer level (by a factor of five). A direct relation between the loss of Mo and scavenging by freshly formed MnO_x phases could not be inferred from our data because both metals revealed inverse patterns. Parallel to decreasing dissolved Mo concentrations dissolved Mn showed an increasing trend while particulate Mn decreased. Such finding is compatible with the formation of oxygen-depleted zones in aggregates, which provide suitable conditions for the rapid fixation of Mo and parallel release of Mn by chemically and/or microbially mediated processes. Our assumption is supported by biological (e.g. number of aggregate-associated bacteria) and sedimentological (e.g. aggregate abundance and size) parameters. The production of organic components (e.g. TEP) during breakdown of an algae bloom in July 2005 led to the formation of larger Mo-enriched aggregates, thus depleting the water column in dissolved Mo. After deposition on and incorporation into sandy tidal flats these aggregates are rapidly decomposed by microbial activity. Pore water profiles document that during microbial decomposition of these aggregates, substantial amounts of Mo are released and may replenish and even enrich Mo in the open water column. We postulate a conceptual model for the observed non-conservative behaviour of Mo in coastal waters, which is based on the tight coupling of geochemical, biological, and sedimentological processes.     

Influence of natural organic acids on the Mg/Ca ratio in the bottom sediments of highly mineralized lakes

Thermodynamic modeling of equilibria in the system water–rock–organic acids was used to study the influence of organic acids on Ca and Mg redistribution between a solution and a solid phase in connection with the use of calcites of variable composition Ca_xMg_1–xCO₃ as indicators of paleoclimatic environments. In the thermodynamic model, high-molecular humic substances (fulvic + humic acids) were represented by a set of independent metal-binding centers. Therefore, their number was preset based on the given density of proton- or metal-binding sites. The numerical implementation of several geochemical situations involving the dissolution/deposition of calcites with different Mg contents showed that the main effect of fulvic and humic acids is the acidification of solutions and the reduction of carbonate stability. Although humic substances can play an important role in fixing Ca and Mg and removing them from solution, their actual concentrations in natural media (<<1 g/L) do not cause significant changes in the composition of Ca_xMg_1–xCO₃ phases. On the other hand, there is quantitative evidence that variations in the Mg/Ca ratio in a solution and a solid phase are significantly influenced by the evaporative concentration of Mg-oversaturated solutions, alkalization/acidification during their evolution, or CO₂ content variations owing to changes in climate and lake activity.     

Heterogeneous Mg isotopic composition of the early Carboniferous limestone: implications for carbonate as a seawater archive

Carbonate precipitation and hydrothermal reaction are the two major processes that remove Mg from seawater. Mg isotopes are significantly (up to 5‰) fractionated during carbonate precipitation by preferential incorporation of ²⁴Mg, while hydrothermal reactions are associated with negligible Mg isotope fractionation by preferential sequestration of ²⁶Mg. Thus, the marine Mg cycle could be reflected by seawater Mg isotopic composition (δ²⁶Mg_sw), which might be recorded in marine carbonate. However, carbonates are both texturally and compositionally heterogeneous, and it is unclear which carbonate component is the most reliable for reconstructing δ²⁶Mg_sw. In this study, we measured Mg isotopic compositions of limestone samples collected from the early Carboniferous Huangjin Formation in South China. Based on petrographic studies, four carbonate components were recognized: micrite, marine cement, brachiopod shell, and mixture. The four components had distinct δ²⁶Mg: (1) micrite samples ranged from ?2.86‰ to ?2.97‰; (2) pure marine cements varied from ?3.40‰ to ?3.54‰, while impure cement samples containing small amount of Rugosa coral skeletons showed a wider range (?3.27‰ to ?3.75‰); (3) values for the mixture component were ?3.17‰ and ?3.49‰; and (4) brachiopod shells ranged from ?2.20‰ to ?3.07‰, with the thickened hinge area enriched in ²⁴Mg. Due to having multiple carbonate sources, neither the micrite nor the mixture component could be used to reconstruct δ²⁶Mg_sw. In addition, the marine cement was homogenous in Mg isotopes, but lacking the fractionation by inorganic carbonate precipitation that is prerequisite for the accurate determination of δ²⁶Mg_sw. Furthermore, brachiopod shells had heterogeneous C and Mg isotopes, suggesting a significant vital effect during growth. Overall, the heterogeneous δ²⁶Mg of the Huangjin limestone makes it difficult to reconstruct δ²⁶Mg_sw using bulk carbonate/calcareous sediments. Finally, δ²⁶Mg_sw was only slightly affected by the faunal composition of carbonate-secreting organisms, even though biogenic carbonate accounts for more than 90% of marine carbonate production in Phanerozoic oceans and there is a wide range (0.2‰–4.8‰) of fractionation during biogenic carbonate formation.     

Geochemical fractionation of manganese in the Riogrande II reservoir, Antioquia, Colombia

The Riogrande II reservoir in Colombia has a total storage capacity of 240 million m³ and lies 2,270 m above sea level. The reservoir is used for power generation, water supply and environmental improvement. Dissolved manganese (Mn) is removed from reservoir water dedicated to domestic use by purification processes. Removal of Mn, however, poses a major challenge to purification processes and warrants the study of ways to naturally reduce dissolved Mn levels in the reservoir. The source of Mn within the reservoir is not well understood, however, presumably arises from sediment mobilization initiated by variation in pH, redox potential (ORP or Eh), dissolved oxygen (O₂) and ionic strength conditions. This study investigated conditions within the reservoir to further understand Mn transfer from the sediment into the water column. O₂, pH, oxidation–reduction potential (ORP or Eh), organic matter content and electric conductivity were measured in water samples and sediment from the reservoir. Sequential extraction (SE) procedures were used to test the specific effects exerted by each of these conditions on Mn mobilization from the sediments. The European Community Bureau of Reference (BCR) sequential extraction procedure was used to quantify metals in sediment (referred to as the BCR extraction below). Statistical analysis of geochemical data from water samples (both water column and sediment pore water) and sediments demonstrated the conditions under which Mn can be released from sediments into the water column. The results indicated a primarily oxic water column and anoxic reducing conditions in the sediment (ORP or Eh ≤ ?80 mV). The pH of water in contact with bottom sediments varied from 7.6 to 6.8. The pH of sedimentary pore water varied from 6.8 to 4.7. The sediments contained significant amounts of organic matter (20 %). Chemical extractions showed that the exchangeable fraction contained over 50 % of the total Mn within sediments. Microscopic analysis using scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS) indicated that Mn does not occur within well-crystallized mineral phases in the Riogrande II sediments. A large proportion of Mn exists instead as material adsorbed onto the surfaces of recently deposited sediment particles. Bacterial oxidation of organic matter may cause the observed anoxic conditions at the bottom of the reservoir. Mineralization of organic matter therefore contributes to reducing conditions within the sediments. Mobilization of Mn from the sediment into the water column may result from reductive dissolution of this fraction. Manganese release by this mechanism diminishes the water quality of the Riogrande II reservoir and warrants further study.     

Catastrophic environmental transition at the Permian-Triassic Neo-Tethyan margin of Gondwanaland: Geochemical,isotopic and sedimentological evidence in the Spiti Valley,India

Interpreting global consequences of the Permian-Triassic (P-Tr) extinction requires examination across paleogeographic realms of Pangaea. The Spiti Valley in India, remnant of the peri-Gondwanan shelf, preserved trails of this environmental catastrophe in the Neo-Tethys Ocean. We document new sedimentological observations and high-resolution trace element concentrations and carbon, oxygen, lead isotope data across the P-Tr boundary in Spiti. Framboidal pyrites, fossils and laminated lithology of the Late Permian shales indicate deeper anoxic depositional environment while δ¹³C_org excursions of 2.4‰ and 3.1‰ in Atargu and Guling outcrops, respectively, identify the P-Tr transition across a clayey, partly gypsiferous ferruginous layer. Sedimentological similarities of this layer to other Neo-Tethyan sections from Transcaucasia and Iran indicate subaqueous oxidation of shallow marine sediments on a regional scale. Light Rare Earth Element – enriched Late Permian shales with conspicuous Ce–Eu anomalies reflect their source from the adjacent Panjal Trap basalts (ca. 289 Ma) of Kashmir. Continental crustal Nb–Ta and Zr–Hf anomalies appear at the P-Tr boundary sediments, and prevail through the overlying Early Triassic carbonates. Original Pb isotope ratios, along with an increasing Pb abundance closer to the P-Tr boundary, distinguish the volcanic source of the Late Permian shales from the continental crustal siliciclastic signature of the Early Triassic carbonates. Our δ¹³C_org, trace element and Pb isotope record from Spiti indicate catastrophic changes in sediment sources and facies, with effects on carbon cycle and are consistent with an abrupt episode of marine regression and erosional forcing, also observed elsewhere along northern Gondwanaland. Simultaneous eruption of Siberian volcanics and bolide impacts in Parana basin of Brazil and elsewhere implicating impact-triggered volcanism, left catastrophic regional-global imprints on sea level, climate, marine anoxia and tectonic stability that connected the P-Tr crisis across terrestrial and marine realms worlwide.     

洞穴次生碳酸盐沉积的Mg/Ca与Sr/Ca比值研究进展——兼论洞穴次生沉积物Mg/Ca与Sr/Ca的影响机制

微量元素是岩溶洞穴沉积中非常重要的一类古气候环境替代指标,为近20年来国内外的一个研究热点。总结前人的研究,主要取得了以下一些重要认识:（1）洞穴上覆土壤 和围岩是洞穴次生碳酸盐沉积Mg、Sr的主要来源;（2）Mg/Ca与Sr/Ca能够指示气候环境变化,但需结合其它指标综合考虑。（3）洞穴次生碳酸盐沉积Mg/Ca与Sr/Ca受多种气候环境因素（包括土壤和围岩的组成和性质、水-岩相互作用、先期碳酸盐沉积、分配系数等）影响,其古气候环境指示意义具有多解性;（4）矿物结晶作用对Mg/Ca与Sr/Ca有一定的影响,特别是文石在向方解石转变的过程中容易丢失Mg、Sr,此外,杂质的混入也将抑制Mg、Sr进入方解石,从而引起洞穴次生碳酸盐沉积Mg/Ca与Sr/Ca比值的变化。今后应进一步加强对石笋中这些微量元素的影响机制研究,尤其是对一些影响因素与微量元素含量变化之间的定量关系进行探讨。      

Controls on Sr/Ca and Mg/Ca in scleractinian corals: The effects of Ca-ATPase and transcellular Ca channels on skeletal chemistry

The Sr/Ca of aragonitic coral skeletons is a commonly used palaeothermometer. However skeletal Sr/Ca is typically dominated by weekly-monthly oscillations which do not reflect temperature or seawater composition and the origins of which are currently unknown. To test the impact of transcellular Ca²⁺ transport processes on skeletal Sr/Ca, colonies of the branching coral, Pocillopora damicornis, were cultured in the presence of inhibitors of Ca-ATPase (ruthenium red) and Ca channels (verapamil hydrochloride). The photosynthesis, respiration and calcification rates of the colonies were monitored throughout the experiment. The skeleton deposited in the presence of the inhibitors was identified (by ⁴²Ca spike) and analysed for Sr/Ca and Mg/Ca by secondary ion mass spectrometry. The Sr/Ca of the aragonite deposited in the presence of either of the inhibitors was not significantly different from that of the solvent (dimethyl sulfoxide) control, although the coral calcification rate was reduced by up to 66% and 73% in the ruthenium red and verapamil treatments, respectively. The typical precision (95% confidence limits) of mean Sr/Ca determinations within any treatment was <±1% and differences in skeletal Sr/Ca between treatments were correspondingly small. Either Ca-ATPase and Ca channels transport Sr²⁺ and Ca²⁺ in virtually the same ratio in which they are present in seawater or transcellular processes contribute little Ca²⁺ to the skeleton and most Ca is derived from seawater transported directly to the calcification site. Variations in the activities of Ca-ATPase and Ca-channels are not responsible for the weekly-monthly Sr/Ca oscillations observed in skeletal chronologies, assuming that the specificities of Ca transcellular transport processes are similar between coral genera.     

Formation and compositional variation of phlogopites in the Horoman peridotite complex,Hokkaido, northern Japan: implications for origin and fractionation of metasomatic fluids in the upper mantle

Harzburgite and lherzolite tectonites from the Horoman peridotite complex, Hokkaido, northern Japan, contain variable amounts of secondary phlogopite and amphibole. Phlogopite-rich veinlets parallel to the foliation planes usually cut olivine-rich parts of the rocks; single-grained interstitial phlogopites are usually associated with orthopyroxene grains. Amphiboles are disseminated in rocks or sometimes occur in the phlogopite-rich veinlets. Within individual veinlets, phlogopites show extensive inter-grain variations in K/(K + Na) ratio (0.96–0.75), generally decreasing from the central (usually the thickest) part towards the marginal parts of veinlets. In contrast, Ti contents are nearly constant in Ti-poor veins or decrease slightly with decreasing K/(K + Na) in T-rich veins. Variation of Ti in phlogopites is very large (0.1–6.8 wt%) and is inversely correlated with Mg/(Mg + Fe^*) (Fe^*, total iron) atomic ratios, which vary from 0.96 to 0.88. Intra-vein variation of phlogopite chemistry (especially K/(K + Na) ratio) could be achieved by in situ fractional crystallization of trapped fluids; variation of Ti, however, cannot be explained by in situ fractionation of the fluids, indicating various Ti contents of the parent fluids. It is suggested that fluids responsible for the formation of the Horoman phlogopites and amphiboles were magmatic volatiles successively released from evolving alkali basaltic magmas. Individual fluids trapped within peridotites were fractionated, precipitating phlogopites successively poorer in K. When the fluids became rich enough in Na, amphiboles co-precipitated with phlogopites. Similar fractional crystallization of phlogopites and amphiboles is expected in the upper mantle on a larger scale if fluids move upwards. This process may control, at least partly, a lateral K/Na distribution in the upper mantle; K and Na may be concentrated in deeper and shallower parts, respectively.     

浮游有孔虫壳体Mg/Ca值--SST的替代性指标

通过对东北印度洋区MD77181和MD81349两支岩心中Globigerinoides sacculifer壳体极为精细的清洗和溶样后,由ICP—MS测试获得了近260ka以来Mg／Ca值．结果表明Mg／Ca值与δ^18O值变化趋势较为吻合,随温度的增加而升高,浮游有孔虫G．sacculifer壳体Mg／Ca值是海洋表层水温(SST)良好的替代性指标．运用函数式Mg／Ca=0．4717exp(0．0825*SST)计算表明,包括印度洋在内的中、低纬地区,LGM时期的SST与全新世的温差大于最初CLIMAP推测的2℃,很可能达到4℃．