Aragonite stromatolitic buildups from Santorini (Aegean Sea,Greece): Geochemical and palaeontological constraints of the caldera palaeoenvironment prior to the Minoan eruption (ca 3600 yr bp)

The pyroclastic deposits of the Minoan eruption (ca 3600 yr bp ) in Santorini contain abundant xenoliths. Most of these deposits are calcareous blocks of laminated‐botryoidal, stromatolite‐like buildups that formed in the shallow waters of the flooded pre‐Minoan caldera; they consist of (i) light laminae, of fibrous aragonite arranged perpendicular to layering, and (ii) dark laminae, with calcified filaments of probable biological origin. These microstructures are absent in the light laminae, suggesting a predominant inorganic precipitation of aragonite on substrates probably colonized by microbes. Internal cavities contain loose skeletal grains (molluscs, ostracods, foraminifera and diatoms) that comprise taxa typical of shallow marine and/or lagoon environments. Most of these forms are typical of warm water environments, although no typical taxa from hydrothermal vents have been observed. Past gasohydrothermal venting is recorded by the occurrence of barite, pyrolusite and pyrite traces. The most striking features of the stable isotopic data set are: (i) an overall wide range in δ¹³C_PDB (0·16 to 12·97‰) with a narrower variation for δ¹⁸O_PDB (?0·23 to 4·33‰); and (ii) a relatively uniform isotopic composition for the fibrous aragonite (δ¹³C = 12·40 ± 0·43‰ and δ¹⁸O = 2·42 ± 0·77‰, n = 21). The δ¹³C and δ¹⁸O values from molluscs and ostracods display a covariant trend, which reflects a mixing between sea water and a fluid influenced by volcano‐hydrothermal activity. Accordingly, ⁸⁷Sr/⁸⁶Sr from the studied carbonates (0·708758 to 0·709011 in fibrous aragonite and 0·708920 to 0·708991 in molluscs) suggests that the aragonite buildups developed in sea water under the influence of a hydrothermal/volcanic source. Significant differences in trace elements have been detected between the fibrous aragonite and modern marine aragonite cements. The caldera water from which the fibrous aragonite crusts formed received an input from a volcano‐hydrothermal system, probably producing diffuse venting of volcanogenic CO₂ gas and of a fluid enriched in Ca, Mn and Ba, and depleted in Mg and probably in Sr.     

Origin of aragonite in a continental saline seep,north Queensland

Aragonite, low‐magnesian calcite, gypsum and halite were identified by X‐ray diffraction and electron microbeam techniques in mineral precipitates near a salt seep 50 km southwest of Charters Towers in north Queensland. The chemistry of water from the creek and from the groundwater at the salt seep shows that Mg:Ca ratios are greater than or equal to 1.5 throughout the year. The formation of halite and gypsum is due to evaporative concentration of the water at the seep and that of the carbonates, in particular aragonite, is probably due to a combination of evaporation and photosynthetic activity by diatoms.     

The fine structure of check rings in the otolith of the New Zealand orange roughy (Hoplostethus atlanticus)

Heating otoliths of the orange roughy (Hoplostethus atlanticus; Trachichthyidae) by the break‐and‐burn method causes a crystal transformation from aragonite to calcite. However, the change does not affect the legibility of the check rings. Orange roughy otolith check rings fall into two classes: a diffuse deposit (that is not visible in the break‐and‐burn method, but may have an annual periodicity) and structural check rings (which appear as fine lines on the surface, but as dark bands in the body of the otolith). Structural check ring deposition is irregular, but fortuitous aggregations of structural check rings might be mistaken for conventional annual check rings.     

Quantitative aspects of Mn-activated cathodoluminescence of natural and synthetic aragonite

The cathodoluminescence analyses of the trigonal carbonates calcite and dolomite have been applied intensively in sedimentary petrology for a long time and the properties of these minerals are well-known, but much less attention has been paid to aragonite. In this study, the cathodoluminescence behaviour and the trace element composition of natural and synthetic aragonite have been studied employing trace element analyses (proton induced X-ray emission) and luminescence spectroscopy. Aragonite doped with Mn²⁺ has been synthesized in a NH₄⁺–Mg²⁺–Ca²⁺–Cl⁻ solution in contact with a CO₂–H₂O–NH₃ atmosphere. The low effective distribution coefficients indicate a rapid growth of the crystals of millimetre size which occurred within hours or days. The natural aragonite samples contain Mn, Fe and Sr in different concentrations. The Mn-bearing aragonites exhibit a bright green luminescence which is caused by a strong emission band at 575 nm with a half-width of about 84 nm. The luminescence intensity shows a strong positive correlation with Mn in aragonite when Fe and Mn do not exceed 2000 p.p.m. The intensity is depressed if the concentration of these elements exceeds the critical values. In the shell of a recent Unio sp., the luminescence intensity deviates from the linear correlation, although the trace element contents are not too high; this is probably an effect of quenching by organic material between the crystallites of the biogenic aragonite.     

木兰山蓝片岩中两种文石的产出特征及其动力学意义

微米－纳米尺度显微超显微研究表明，木兰山蓝片岩中存在微米级粒间文石和亚微米级文石包裹体，它们是木兰山蓝片岩快速折返动力学机制的证据。并指出木兰山蓝片岩区硬柱石和绿纤石等Ca-Al硅酸盐矿物的缺失与本区变质流体中CO2浓度较高和（或）岩石中含方解石／文石有关。     

In situ impedance spectroscopy on pyrophyllite and CaCO<Subscript>3</Subscript> at high pressure and temperature: phase transformations and kinetics of atomistic transport

In situ impedance spectroscopy in laboratory experiments at high pressure and temperature can provide crucial quantitative information on properties of rock materials at depth as well as on physical and chemical processes occurring in the deep Earth. We developed an experimental setup for in situ electrical impedance measurements in a piston-cylinder apparatus and applied it to study the kinetics of charge carriers and phase transformations in pyrophyllite and CaCO₃ aggregates. From comparison with previous studies, we found that absolute values of electrical conductivity and pressure–temperature conditions for dehydration reactions in pyrophyllite and phase transformations in CaCO₃ can be accurately determined using our setup. Dehydration of pyrophyllite significantly enhances the transport kinetics and the effect is more pronounced under undrained conditions than under drained conditions. When dehydroxylation and decomposition temperatures for pyrophyllite under undrained and drained conditions are combined, they appear independent of pressure rather than increasing with pressure as previously suggested. Electrical conductivity of CaCO₃ varies with impurity content and grain size, and is most likely controlled by diffusion of oxygen along wet grain boundaries. When applied to the Earth, the results on pyrophyllite suggest that the increase in electrical conductivity in rocks that undergo dehydration should be taken into account in interpreting magnetotelluric surveys of regions with anomalously high conductivity found above subducting plates. The results on CaCO₃ indicate that grain boundary transport controls the electrical conductivity in fine-grained calcite rocks; hence calcite mylonites may be detected using magnetotelluric methods. Order–disorder transformations, such as occurring in calcite, possibly affect the physical properties of rocks (e.g., rheology) by changing the kinetics of atomistic transport processes.     

Anaerobic microbial activity affects earliest diagenetic pathways of bivalve shells

The earliest diagenetic post‐mortem exposure of biogenic carbonates at the sea floor and in the uppermost sediment column results in the colonization of hard‐part surfaces by bacterial communities. Some of the metabolic redox processes related to these communities have the potential to alter carbonate shell properties, and hence affect earliest diagenetic pathways with significant consequences for archive data. During a three‐month in vitro study, shell subsamples of the ocean quahog Arctica islandica (Linnaeus, 1767) were incubated in natural anoxic sediment slurries and bacterial culture medium of the heterotrophic Shewanella sediminis HAW ‐EB 3. Bulk analyses of the liquid media from the Shewanella sediminis incubation revealed an over ten‐fold increase in total alkalinity, dissolved inorganic carbon and Ω_Aragonite, and the alteration of the Mg/Ca, Mg/Sr and Sr/Ca ratios relative to control incubations without cultures. Ion ratios were most affected in the incubation with anoxic sediment, depicting a 25% decrease in Mg/Ca relative to the control. Shell sample surfaces that were exposed to both incubations displayed visible surface dissolution features, and an 8 wt% loss in calcium content. No such alteration features were detected in control shells. Apparently, alteration of shell carbonate properties was induced by microbially driven decomposition of shell intercrystalline organic constituents and subsequent opening of pathways for pore fluid–crystal exchange. This study illustrates the potential influence of benthic bacterial metabolism on biogenic carbonate archives during the initial stages of diagenetic alteration within a relatively short experimental duration of only three months. These results suggest that foremost the biological effect of bacterial cation adsorption on divalent cation ratios has the potential to complicate proxy interpretation. Results shown here highlight the necessity to consider bacterial metabolic activities in marine sediments for the interpretation of palaeo‐environmental proxies from shell carbonate archives.     

德令哈盆地尕海湖DG03孔岩芯矿物组合与古环境变化

对气候敏感区德令哈盆地尕海湖DG03孔岩芯的矿物学分析表明,DG03孔岩芯的矿物主要包括石英、斜长石、微斜长石、绿泥石、伊利石、方解石、文石、白云石、石膏及石盐等。岩芯记录了尕海湖约11 ka以来（AMS 14C年龄）的古环境变化。根据矿物、碳酸盐含量及岩性的变化,整个岩芯可划分为三个部分:Ⅰ. 约11 ka～10 ka BP,晚冰期末期;Ⅱ. 10 ka～4 ka BP,全新世早中期;Ⅲ.4 ka至今,全新世晚期。矿物组合表明,自晚冰期以来,尕海湖先后经历了湖泊演化的逆向和正向演化阶段,即尕海湖先后经历了晚冰期的干冷气候,早全新世的暖干气候,中全新世的暖湿气候以及晚全新世以来的逐渐干冷的气候。岩性分析还表明,尕海湖沉积环境复杂,存在多种微相沉积。     

Origin of blocky aragonite cement in Cenozoic glaciomarine sediments,McMurdo Sound,Antarctica

Inorganic aragonite occurs in a wide spectrum of depositional environments and its precipitation is controlled by complex physio-chemical factors. This study investigates diagenetic conditions that led to aragonite cement precipitation in Cenozoic glaciomarine deposits of McMurdo Sound, Antarctica. A total of 42 sandstones that host intergranular cement were collected from the CIROS-1 core, located proximal to the terminus of Ferrar Glacier. Standard petrography, Raman spectroscopy and electron microprobe analysis reveal a prominent aragonite cement phase that occurs as a pore-filling blocky fabric throughout the core. Oxygen isotope compositions (δ¹⁸O = −30·0 to −8·6‰ Vienna Pee-Dee Belemnite) and clumped isotope temperatures (TΔ₄₇ = 13·1 to 31·5°C) determined from the aragonite cements provide precise constraints on isotopic compositions (δ¹⁸O_w) of the parent fluid, which mostly range from −10·8 to −7·2‰ Vienna Standard Mean Ocean Water. The fluid δ¹⁸O_w values are consistent with those of pore water, previously identified as cryogenic brine in the nearby AND-2A core. Petrographic and geochemical data suggest that aragonite cement in the CIROS-1 core precipitated from a similar brine. The brine likely formed and infiltrated sediments in flooded glacial valleys along the western margin of McMurdo Sound during the middle Miocene Climatic Transition, and subsequently flowed basinward in the subsurface. Consequently, the brine forms as a longstanding subsurface fluid that has saturated Cenozoic sediments below southern McMurdo Sound since at least the middle Miocene. Aragonite cementation in the CIROS-1 core is interpreted to reflect its proximal position to sites of brine formation and greater likelihood of experiencing brines with sustained high carbonate saturation states and Mg/Ca ratios. This unusual occurrence expands the range of known natural occurrences of aragonite cement. Given the potential for cryogenic brine formation in glaciomarine settings, blocky aragonite, as the end member of the spectrum of aragonite cement morphology, may be more widespread in glaciomarine sediments than currently thought.     

黄土-古土壤序列14C年代学研究进展

^14C年龄的可靠性在于测年物质的可靠。木质样品被认为是最可靠的测年物质,但在黄土序列中不易发现。最常用的测年物质是有机质,其含量低于2％,并受到农业施肥和现代植物根系渗透的影响,造成^14C年龄偏年轻,本实验室采用新的前处理法能有效地分离年轻污染物。在稳定沉积的古土壤中,孢粉可作为可靠的测年物质。黄土中的蜗牛吸收了不同放射性比度的^14C,如果用于测年能影响年龄的准确性,通常挑选蜗牛文石进行测年。加速器质谱测年技术的运用使得黄土序列^14C年代研究更快捷、更灵敏,但研究重点仍集中在可靠测年物质的选择和提取方法上。