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.     

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.     

Mollusc and brachiopod skeletal hard parts: Intricate archives of their marine environment

The biogenic carbonate hard parts of fossil bivalves, cephalopods and brachiopods are among the most widely exploited marine archives of Phanerozoic environmental and climate dynamics research. The advent of novel analytical tools has led many workers to explore non‐traditional geochemical and petrographic proxies, and work performed in neighbouring disciplines sheds light on the complex biomineralization strategies applied by these organisms. These considerations form a strong motivation to review the potential and problems related to the compilation and interpretation of proxy data from bivalve, cephalopod and brachiopod hard parts from the viewpoint of the sedimentologist and palaeoceanographer. Specific focus is on the complex biomineralization pathways of a given dissolved ion or food particle from its aquatic environment via the digestion and biomineralization apparatus in molluscs and brachiopods and its incorporation into a biomineral. Given that molluscs and brachiopods do not secrete their hard parts from seawater but rather from their mantle and periostracum, this paper evaluates differences and similarities of seawater versus that of body fluids. Cephalopods, bivalves and brachiopods exert a strong biological control on biomineralization that, to some degree, may buffer their shell geochemistry against secular changes in seawater chemistry. Disordered (amorphous) calcium carbonate precursor phases, later transformed to crystalline biominerals, may be significant in carbonate archive research due to expected geochemical offset relative to the direct precipitation of stable phases. A reasonable level of understanding of the related mechanisms is thus crucial for those who use these skeletal hard parts as archives of the palaeo‐environment. The impact of what is commonly referred to as ‘biological factors’ on the geochemistry of mollusc and brachiopod hard parts is explored for conventional isotope systems such as carbon, oxygen, strontium and traditionally used element to calcium ratios. In particular, the often used δ¹³C_carb or the Mg/Ca and Sr/Ca elemental proxies are fraught with problems. An interesting new research field represents the analysis, calibration and application of non‐traditional proxies to mollusc and brachiopod hard parts. Examples include the carbonate clumped isotope (Δ₄₇) approach and the analysis of the isotopes of Ca, Mg, N, Li, S or element to Ca ratios such as Li/Ca or B/Ca and rare earth elements. Based on considerations discussed here, a series of “do's and don'ts” in mollusc and brachiopod archive research are proposed and suggestions for future work are presented. In essence, the suggestions proposed here include experimental work (also field experiments) making use of recent archive organisms or, where possible, a reasonable recent analogue in the case of extinct groups. Moreover, the detailed understanding of the architecture of mollusc and brachiopod hard parts and their ultra‐structures must guide sampling strategies for geochemical analyses. Where feasible, a detailed understanding of the diagenetic pathways and the application of multi‐proxy and multi‐archive approaches should form the foundation of fossil carbonate archive research. The uncritical compilation of large data sets from various carbonate‐shelled organisms collected at different locations is not encouraged.     

Response of the coccolithophores Emiliania huxleyi and Coccolithus braarudii to changing seawater Mg and Ca concentrations: Mg/Ca, Sr/Ca ratios and δCa, δMg of coccolith calcite

Calcium and magnesium concentrations in seawater have varied over geological time scales. On short time scales, variations in the major ion composition of seawater influences coccolithophorid physiology and the chemistry of biogenically produced coccoliths. Validation of those changes via controlled laboratory experiments is a crucial step in applying coccolithophorid based paleoproxies for the reconstruction of past environmental conditions. Therefore, we examined the response of two species of coccolithophores, Emiliania huxleyi and Coccolithus braarudii, to changes in the seawater Mg/Ca ratio (≈0.5 to 10 mol/mol) by either manipulating the magnesium or calcium concentration under controlled laboratory conditions. Concurrently, seawater Sr/Ca ratios were also modified (≈2 to 40 mmol/mol), while keeping salinity constant at 35. The physiological response was monitored by measurements of the cell growth rate as well as the production rates of particulate inorganic and organic carbon, and chlorophyll a. Additionally, coccolithophorid calcite was analyzed for its elemental composition (Sr/Ca and Mg/Ca) as well as isotope fractionation of calcium and magnesium (Δ^44/40Ca and Δ^26/24Mg). Our results reveal that physiological rates were substantially influenced by changes in seawater calcium rather than magnesium concentration within the range estimated to have occurred over the past 250 million years when coccolithophores appear in the fossil record. All physiological rates of E. huxleyi decreased at a calcium concentration above 25 mmol L⁻¹, whereas C. braarudii displayed a higher tolerance to increased seawater calcium concentrations. Partition coefficient of Sr was calculated as 0.36 ± 0.04 (±2σ) independent of species. Partition coefficient of Mg²⁺ increased with increasing seawater Ca²⁺ concentrations in both coccolithophore species. Calcium isotope fractionation was constant at 1.1 ± 0.1‰ (±2σ) and not altered by changes in seawater Mg/Ca ratio. There is a well-defined inverse linear relationship between calcium isotope fractionation and partition coefficient of Sr²⁺ in all experiments, suggesting similar controls on both proxies in the investigated species. Magnesium isotope ratios were relatively stable for seawater Mg/Ca ratios ranging from 1 to 5, with a higher degree of fractionation in Emiliania huxleyi (by ≈0.2‰ in Δ^26/24Mg). Although Mg/Ca ratios in the calcite of coccolithophores and foraminifera are similar, the former have considerably higher Δ^26/24Mg (by >+3‰), presumably due to differences in calcification mechanisms between the two taxa. These observations suggest, a physiological control over magnesium elemental and isotopic fractionation during the process of calcification in coccolithophores.     

Interindividual variability and ontogenetic effects on Mg and Sr incorporation in the planktonic foraminifer Globigerinoides sacculifer

In order to investigate the interindividual and ontogenetic effects on Mg and Sr incorporation, magnesium/calcium (Mg/Ca) and strontium/calcium (Sr/Ca) ratios of cultured planktonic foraminifera have been determined. Specimens of Globigerinoides sacculifer were grown under controlled physical and chemical seawater conditions in the laboratory. By using this approach, we minimised the effect of potential environmental variability on Mg/Ca and Sr/Ca ratios. Whereas temperature is the overriding control of Mg/Ca ratios, the interindividual variability observed in the Mg/Ca values contributes 2-3 °C to the apparent temperature variance. Interindividual variability in Sr/Ca ratios is much smaller than that observed in Mg/Ca values. The variability due to ontogeny corresponds to −0.43 mmol/mol of Mg/Ca ratio per chamber added. This translates into an apparent decrease of ∼1 °C in Mg/Ca-based temperature per ontogenetic (chamber) stage. No significant ontogenetic effect is observed on Sr incorporation. We conclude that the presence of a significant ontogenetic effect on Mg incorporation can potentially offset Mg/Ca-based temperature reconstructions. We propose two new empirical Mg/Ca-temperature equation based on Mg/Ca measurements of the last four ontogenetic (chamber) stages and whole foraminiferal test: Mg/Ca = (0.55(±0.03) − 0.0002(±4 × 10⁻⁵) MSD) e^0.089T and, Mg/Ca = (0.55(±0.03) − 0.0001(±2 × 10⁻⁵) MSD) e^0.089T, respectively, where MSD corresponds to the maximum shell diameter of the individual.     

A strong temperature effect on U/Ca in planktonic foraminiferal carbonates

Three planktonic foraminiferal species Globigerina bulloides, Neogloboquadrina pachyderma (d), and Globorotalia inflata collected from core-tops spanning 35° to 65°N in the North Atlantic were used for U/Ca and Mg/Ca and foraminiferal shell weight analyses. Except for U/Ca in G. bulloides calcified under warm conditions (>∼13 °C), U/Ca ratios in all three studied species increase with decreasing latitude and show strong positive correlations with Mg/Ca ratios. A dissolution effect on planktonic U/Ca is suggested by decreased shell weight and U/Ca and Mg/Ca ratios for shells from very deep water depth (>4.4 km) along the latitudinal transect. G. bulloides from down core samples in the North Atlantic show low U/Ca ratios during the last glacial and high ratios during the Holocene, similar to the Mg/Ca evolution trend. In general, our data indicate that the U incorporation into planktonic foraminiferal carbonates is strongly influenced by calcification temperature, although U/Ca in G. bulloides may be affected by seawater carbonate ion concentration under warm conditions and/or other factors.     

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.     

青藏高原中部色林错中晚全新世以来古气候变化特征

色林错位于青藏高原中部印度季风和西风环流的过渡地带,同时受西风环流和印度季风系统控制,是研究二者进退变化特征的理想场所。本文利用色林错SL-1钻孔中介形虫Limnocythere inopinata的丰度及其壳体微量元素Mg/Ca和Mn/Ca比值重建了色林错5.3 ka BP以来的古气候环境变化特征。5.3～2.9 ka BP,L.inopinata丰度较小,壳体的低Mg/Ca比值和高Mn/Ca比值表明此阶段气候偏冷湿;2.9～1.8 ka BP,L.inopinata丰度较前一阶段增加,壳体Mg/Ca比值略有增长但仍为低值表明气温虽然有所回升但仍然较低,Mn/Ca比值较前一阶段明显降低,指示湖泊水位下降;1.8 ka BP至今,L.inopinata丰度达到最大,壳体的高Mg/Ca和Mn/Ca比值指示湖泊温度和水位均呈显著的上升趋势。通过与西风区、过渡区以及印度季风区其他湖泊的环境沉积记录对比,本文认为青藏高原中部地区在中全新世晚期主要受西风环流影响,气温较低,西风带来大量水汽使得湖面呈扩张趋势;而到晚全新世西风环流逐渐北撤,色林错受季风影响更大,季风带来的降水和气温升高导致的冰川融...     

Stable isotope profiles (Ca,O, C) through modern brachiopod shells of T. septentrionalis and G. vitreus: Implications for calcium isotope paleo-ocean chemistry

Brachiopod shells are widely used as an archive to reconstruct elemental and isotopic composition of seawater. Studies, focused on oxygen and carbon isotopes over the last decades, are increasingly extending to the emerging calcium isotope system. To date, only little attention has been paid to test the reliability of fossil brachiopods on their modern counterparts.In this context, the present study investigates two modern brachiopods, Terebratulina septentrionalis (eastern Canada, 5–30 m depth, 7.1 °C seasonal temperature variation, two-layer shell) and Gryphus vitreus (northern Mediterranean, 200 m depth, constant all-year round temperature, three-layer shell). Both species were sampled along the ontogenetic growth direction and calcium, oxygen, and carbon isotopes as well as elemental concentration were measured. Calcium isotopes were analyzed on TIMS. The elemental composition was analyzed by LA-ICP-MS and ICP-AES.The results indicate an intra-specimen δ^44/40Ca variation ranging from 0.16 to 0.33‰, pointing to a fairly homogenous distribution of calcium isotopes in brachiopod shells. However, in the light of the suggested 0.7‰ increase in calcium isotopes over the Phanerozoic such intra-specimen variations constrain ocean reconstruction. δ^44/40Ca values of T. septentrionalis do not seem to be affected by growth rate. Calcium isotopic values of G. vitreus are heavy in the central part of the shell and trend towards lighter values in peripheral areas approaching the maximum isotopic composition of T. septentrionalis. The maximum inter-species δ^44/40Ca difference of 0.62‰ between T. septentrionalis and G. vitreus indicates that care should be taken when using different taxa, species with different strontium content or brachiopods with specialized shell structure, such as G. vitreus, for ocean water reconstruction in terms of Ca isotopic composition. T. septentrionalis may record Ca isotopic fractionation related to seasonal seawater temperature variations in its shell but this is difficult to resolve at the current analytical precision. Average δ¹⁸O-derived temperatures of the two investigated species are close to on-site measured temperatures.     

Towards a better understanding of magnesium-isotope ratios from marine skeletal carbonates

This study presents magnesium stable-isotope compositions of various biogenic carbonates of several marine calcifying organisms and an algae species, seawater samples collected from the western Dutch Wadden Sea, and reference materials. The aim of this study is to explore the influence of mineralogy, taxonomy and environmental factors (e.g., seawater isotopic composition, temperature, salinity) on magnesium-isotopic (δ²⁶Mg) ratios of skeletal carbonates. Using high-precision multi-collector inductively coupled plasma mass spectrometry, we observed that the magnesium-isotopic composition of seawater from the semi-enclosed Dutch Wadden Sea is identical to that of open marine seawater. We further found that a considerable component of the observed variability in δ²⁶Mg values of marine skeletal carbonates can be attributed to differences in mineralogy. Furthermore, magnesium-isotope fractionation is species-dependent, with all skeletal carbonates being isotopically lighter than seawater. While δ²⁶Mg values of skeletal aragonite and high-magnesium calcite of coralline red algae indicate the absence or negligibility of metabolic influences, the δ²⁶Mg values of echinoids, brachiopods and bivalves likely result from a taxon-specific level of control on Mg-isotope incorporation during biocalcification. Moreover, no resolvable salinity and temperature effect were observed for coralline red algae and echinoids. In contrast, Mg-isotope data of bivalves yield ambiguous results, which require further validation. The data presented here, point to a limited use of Mg isotopes as temperature proxy, but highlight the method’s potential as tracer of seawater chemistry through Earth’s history.     

Calcium-isotope fractionation in selected modern and ancient marine carbonates

The calcium-isotope composition (δ^44/42Ca) was analyzed in modern, Cretaceous and Carboniferous marine skeletal carbonates as well as in bioclasts, non-skeletal components, and diagenetic cements of Cretaceous and Carboniferous limestones. In order to gain insight in Ca²⁺_aq-CaCO₃-isotope fractionation mechanisms in marine carbonates, splits of samples were analyzed for Sr, Mg, Fe, and Mn concentrations and for their oxygen and carbon isotopic composition. Biological carbonates generally have lower δ^44/42Ca values than inorganic marine cements, and there appears to be no fractionation between seawater and marine inorganic calcite. A kinetic isotope effect related to precipitation rate is considered to control the overall discrimination against ⁴⁴Ca in biological carbonates when compared to inorganic precipitates. This is supported by a well-defined correlation of the δ^44/42Ca values with Sr concentrations in Cretaceous limestones that contain biological carbonates at various stages of marine diagenetic alteration. No significant temperature dependence of Ca-isotope fractionation was found in shells of Cretaceous rudist bivalves that have recorded large seasonal temperature variations as derived from δ¹⁸O values and Mg concentrations. The reconstruction of secular variations in the δ^44/42Ca value of seawater from well preserved skeletal calcite is compromised by a broad range of variation found in both modern and Cretaceous biological carbonates, independent of chemical composition or mineralogy. Despite these variations that may be due to still unidentified biological fractionation mechanisms, the δ^44/42Ca values of Cretaceous skeletal calcite suggest that the δ^44/42Ca value of Cretaceous seawater was 0.3-0.4‰ lower than that of the modern ocean.     

Mg/Ca, Sr/Ca, and stable isotopes in modern and Holocene Protothaca staminea shells from a northern California coastal upwelling region

This study explores the potential of intertidal Protothaca staminea shells as high-resolution geochemical archives of environmental change in a coastal upwelling region. Mg/Ca and Sr/Ca ratios were analyzed by excimer laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) at sub-weekly temporal resolution in shells growing ∼1 mm per month. Growth patterns of a modern P. staminea shell from Humboldt Bay, California, collected in December 1999 made it possible to infer a lifespan from 1993 to 1998. Growth hiatuses in the shell may have excluded records of extreme events. Mg/Ca ratios appeared to be partly controlled by water temperature; the correlation coefficient between temperature and Mg/Ca was r = 0.71 in one of four growth increments. Significant year-to-year differences in the sensitivity of Mg/Ca to temperature in P. staminea could not be explained, however. Sr/Ca ratios appeared to be more closely related to shell growth rate. Oxygen isotopes, measured at 2-week temporal resolution in the same shell, did not show a clear relation to local temperature in summer, possibly because temperatures were higher and less variable at the King Salmon mudflat, where the shell was collected, than in the main channel of Humboldt Bay, where water properties were monitored. Negative shell δ¹³C values (<−0.5‰) marked spring and summer coastal upwelling events.The Mg contents of P. staminea midden shells dated to ∼3 ka and ∼9 ka were significantly lower than in the modern shell. This may have resulted from degradation of a Mg-rich shell organic matrix and precluded quantitative interpretation of the older high-resolution records. Elevated δ¹³C values in the ∼3 ka shell suggested that the individual grew in highly productive or stratified environment, such as a shallow coastal embayment or lagoon.     

Evolution of saline waters and brines in the Benue-Trough,Nigeria

Hydrogeochemical assessment of 40 saline waters and brines from 20 locations within the lower (southern) and middle regions of the Benue-Trough, Nigeria are presented and discussed in terms of genesis of the primary salinity and subsequent hydrochemical evolution. The total dissolved ions range from 5263 to 88,800 mg/L and 5148 to 47,145 mg/L in the lower and middle region, respectively.The saline waters and brines are characteristically Na–Cl type enriched in Ca and Sr on the one hand and depleted in Mg and SO₄ on the other, relative to the seawater evaporation trend. Ionic ratios, Na–Cl–Br systematic and divalent cations suggest two likely sources of primary salinity: a fossil seawater source and dissolution of halite. However, water–rock interaction involving Mg uptake by clay minerals and possibly dolomitization during diagenesis appear to be responsible for further modification of the primary chemistry. A conceptualized hydrogeological/flow model for the brines is presented.     

Application of calcite Mg partitioning functions to the reconstruction of paleocean Mg/Ca

Calcite Mg/Ca is usually assumed to vary linearly with solution Mg/Ca, that a constant partition coefficient describes the relationship between these two ratios. Numerous published empirical datasets suggests that this relationship is better described by a power function. We provide a compilation of these literature data for biotic and abiotic calcite in the form of Calcite Mg/Ca = F(Solution Mg/Ca)^H, where F and H are empirically determined fitting parameters describing the slope and deviation from linearity, respectively, of the function. This is equivalent to Freundlich sorption behavior controlling Mg incorporation in calcite. Using a power function, instead of a partition coefficient, lowers Phanerozoic seawater Mg/Ca estimates based on echinoderm skeletal material by, on average, 0.5 mol/mol from previous estimates.These functions can also be used to model the primary skeletal calcite Mg/Ca of numerous calcite phases through geologic time. Such modeling suggests that the Mg/Ca of all calcite precipitated from seawater has varied through the Phanerozoic in response to changing seawater Mg/Ca and that the overall range in Mg/Ca measured among various calcite phases would be greatest when seawater Mg/Ca was also high (e.g., “aragonite seas”) and lowest when seawater Mg/Ca was low (e.g., “calcite seas”). It follows that, during times of “calcite seas” when the seawater Mg/Ca is presumed to have been lower, deposition of calcite with low Mg contents would have resulted in a depressed drive for diagenetic stabilization of shelfal carbonate and, in turn, lead to greater preservation of crystal and skeletal microfabrics and primary chemistries in biotic and abiotic calcites.     

In-situ Microanalysis of Mg/Ca Ratio in Planktonic Foraminiferal Shells from the Indonesian Seas

Laser Ablation-Inductively Coupled Plasma-Mass Spectrometer (LA-ICP-MS) provides an opportunity for us to perform in-situ microanalysis for depth profiling of elements and trace elements/Calcium ratios in calcite foraminiferal shells. In this study, Globigerinoides ruber shells were investigated with a total of 173 ablating positions from a total of 11 core top sediment samples that retrieved from the Indonesian Seas. The results showed changing compositions of Magnesium and Calcium from the inside to the outside surface for both of intra-test and inter-test. Subsequently, Mg/Ca ratios from the LA-ICP-MS microanalysis were compared with Mg/Ca ratios based on regular analyses, suggesting that LA-ICP-MS microanalysis may provide a comparatively safe and convenient way for investigating Mg/Ca ratio of planktonic foraminiferal shells with considerable reliability and accuracy.     

Li/Ca ratios of ostracod shells at Lake Qinghai, NE Tibetan Plateau, China: a potential temperature indicator

Acquiring continuous and high-resolution natural records in recent 2,000?years is the hot issue in the palaeoclimate research. Recent studies revealed that Li/Ca and Mg/Li ratio of carbonate is a potential tracer of past environmental changes in the oceans, and is seldom applied for palaeoclimate reconstruction in lakes. To make full understanding of the potential of Li/Ca ratios of carbonates in lakes, Li/Ca ratios of monospecific ostracod shell Eucypris inflate with similar size in the lacustrine sediment core from Lake Qinghai, NE Tibetan Plateau have been analysed for the first time. Single species can effectively avoid the interspecies effects. In combination with the sedimentation rate that existed, data derived from ²¹⁰Pb and ¹³⁷Cs, downcore variations of Li/Ca ratios of ostracod shells during the past 800?years in Lake Qinghai have been reconstructed successfully. By comparing Li/Ca ratios and temperature inferred from meteorological records and tree ring widths in Dulan and Qilianshan in adjacent regions, Li/Ca ratios of ostracod shells negatively correlate with temperature. Higher temperature corresponds with lower Li/Ca ratios, and vice versa, indicating that Li/Ca ratio of ostracod shells is an effective indicator for temperature variations. Therefore, ongoing in-depth investigation using Li/Ca ratios of carbonates in more lakes to further reveal its palaeotemperature implications would be deserved.     

杭州湾及其邻近海域沉积物中Fe、Mn、Ca、Mg的地球化学研究

本文通过对杭州湾及其邻近海域29个站位的表层沉积物化学、粘土矿物及碎屑矿物资料的分析,详细研究了开放型海湾沉积物中Fe、Mn、Ca、Mg元素地球化学特征。研究表明,1)与同类型海湾相比,杭州湾显示高Fe、Mn、Mg低Ca的特点。表明物质来源丰富,而生物作用较弱;2)Fe、Mn主要来自长江和钱塘江径流搬运,受粘土控制。Mg主要来自粘土对海水中Mg2+的吸附,同时受上覆水盐度的影响;3)北区元素间关系明显的比南区强烈,表明南区物质来源较北区复杂;4)主断面沉积物中Mg/Ca由河口向海洋增加,并与有机碳呈明显的正相关,显示杭州湾及邻近陆架区可能发生着原始碳酸盐（钙）白云岩化的反应。     

The control of Phanerozoic atmosphere and seawater composition by basalt–seawater exchange reactions

We present results from a long term geochemical cycling model, with a focus on the sensitivity of atmospheric carbon dioxide, oxygen, and the major element composition of seawater to seafloor spreading rates. This model incorporates rock weathering, basalt–seawater exchange reactions, and the formation and destruction of chemical sediments and organic matter. Hydrothermal reactions between seafloor and seawater involving calcium, magnesium, sodium, potassium, sulfate and carbon are the high temperature counterparts to low temperature redox, weathering, precipitation and diagenetic reactions. A major source of uncertainty is the extent to which these exchange fluxes are controlled by seafloor spreading rate. In addition, the return fluxes of these components to the atmospheric and primary silicate reservoirs reflect not only the overall rates of subduction and metamorphism, but the distribution of the overlying sedimentary burden and authigenic minerals formed during basalt alteration as well. In particular, we show how the stoichiometry of exchange fluxes (Mg/Ca and SO₄/Ca) may buffer atmospheric CO₂ and O₂ concentrations.     

表生环境中镁同位素的地球化学循环

近些年表生环境中镁同位素分馏取得了一系列重要研究进展,这些新认识为深入理解表生环境中镁同位素地球化学循环奠定了基础。表生环境中镁同位素的地球化学循环主要涉及风化、河流搬运、碳酸盐沉淀、水岩反应等重要地质过程。风化过程中镁同位素发生显著分馏,硅酸盐风化产物中富集重的镁同位素,轻的镁同位素易进入水体。河流搬运过程中,镁同位素不发生分馏,但外源输入可能影响水体的镁同位素组成。河水汇入海洋后,碳酸盐沉淀过程可导致轻的镁同位素以碳酸盐的形式从海水中移出。在海底高温水岩反应过程中,海水中绝大多数的镁(80%~87%)都进入岩石,循环后的热液可能富集轻的镁同位素。海底低温水岩反应过程中海水的镁可以进入岩石并形成次生矿物,此过程的镁同位素分馏主要与次生矿物的形成有关。此外,海水中的镁易与黏土矿物发生交换反应,此过程黏土矿物倾向于吸附轻的镁同位素。总之,在表生环境中上地壳的镁(δ26Mg约为-0.22‰)经历风化作用、河流搬运、海洋贮存,最终以碳酸盐岩(δ26Mg一般小于-1‰)或与玄武岩发生反应的形式重新回到岩石圈。     

I/(Ca+Mg)作为指示碳酸盐沉积氧化还原条件的重要指标: 研究进展与问题评述*

海水氧化还原条件显著影响真核生物的起源与早期演化,但以往有关早期海水氧化还原条件研究的对象,主要依赖相对深水的细粒碎屑岩沉积(如黑色页岩),而对真核生物集中分布的浅水环境中的碳酸盐岩关注不够且手段缺乏。这显著制约了对真核生物起源与早期演化机理的认识。近年来,有学者提出碳酸盐岩的I/(Ca+Mg)值可作为反映海洋氧化还原条件的重要指标,并将其广泛应用于海相碳酸盐岩的古氧相研究中。该指标的提出主要基于对现代海洋碘组分的观测以及室内方解石合成实验结果: 观测结果表明,海水中的碘主要以氧化态(IO₃^-)和还原态(I^-)2种形式存在,随着氧含量的下降(如在氧极小带),氧化态的碘被逐步转换为还原态的碘,且海水中的IO₃^-浓度与海水氧含量大体呈正相关。实验研究证明,IO₃^-可按一定的分配系数进入到碳酸盐矿物晶格中,但I^-则不能。由于IO₃^-/I^-的还原势能与O₂/H₂O的还原势能接近,因此I/(Ca+Mg)值是最早响应海洋氧含量下降的指标之一,可用于表征深时(如前寒武纪)次氧化环境中表层海水的氧含量波动。此外,学者们也尝试建立I/(Ca+Mg)值与氧含量之间的半定量关系,如I/(Ca+Mg)值大于0和2.5μmol/mol这两个临界值所对应的海水氧含量。结合大量现代缺氧水体和氧极小带中碘组分与溶解氧浓度相关关系的研究,作者提出I/(Ca+Mg)=1.5μmol/mol为重要的临界值之一,可用于限定初级生产力在表层海水中所能产生的最大氧浓度值(~10 μM),并能进一步区分海水和大气的氧化。此外,对I/(Ca+Mg)值的应用进展及潜在问题进行评述,并对可能的发展方向进行展望。