Strong Dissolution Research Articles

Microstructural Evolution and Electrochemical Properties of HRDSR AZ61-X (X = Ca, Ti) Alloys.

The microstructure and corrosion properties of as-cast AZ61 (Mg-6%Al-1%Zn) and AZ61 alloys doped with titanium and calcium and subjected to high ratio differential speed rolling were investigated. Addition of the alloying elements to the AZ61 alloy resulted in remarkable modification of the morphology and the amount of continuous β (Mg17Al12)-phase. Addition of Ti to the as-cast AZ61 alloy causes a decrease in the volume fraction (or discontinuity of the β-phase), leading to strong anodic dissolution. In contrast, addition of Ca to the as-cast AZ61 alloy is rather effective for preventing pitting corrosion. This is attributed to the formation of a semi-continuous network β-structure. The (Mg, Al)4Ca phases dispersed between the β (Mg17Al12)-phases led to continuity in the AZ61 alloy with Ca. The AZ61 and AZ61-X(Ca, Ti) alloys subjected to severe plastic deformation via high-ratio differential speed rolling possessed a nano-composite-like microstructure in which the α-Mg matrix with an ultra-fine grain was surrounded by a large number of fine β particles. These particles were either dynamically precipitated or broken at the grain boundaries, as well as in the grain interiors, by the high ratio differential speed rolling process. The corrosion resistance of the AZ61 and AZ61-X (X = Ca, Ti) alloys subjected to high ratio differential speed rolling was largely improved by the microstructural modification. The high ratio differential speed rolling process greatly influenced the texture of the Mg alloys, which significantly affected their corrosion behavior.

Methane adsorption in the low–middle-matured Neoproterozoic Xiamaling marine shale in Zhangjiakou, Hebei

ABSTRACTThe adsorption capacity of high–over-matured shale has been widely investigated, but the adsorption capacity and the main factors influencing low–middle-matured, type II kerogen-containing, and organic-rich marine shale have been rarely explored. This research conducts organic geochemistry, mineralogical composition, adsorption isotherm tests to reveal the adsorption and main influencing factors of the different geochemistry characteristics, mineralogical compositions, temperature and pressure conditions of the low–middle-matured Neoproterozoic Xiamaling marine shale in Zhangjiakou, Hebei. The investigated shale is in a low–middle maturity stage, contains type II kerogen and is rich in organic matter. The results show that the total organic carbon (TOC) content of the Xiamaling shale is positively correlated with the methane adsorption capacity, which is the most important influencing factor on the adsorptive property of shale. The methane adsorption capacity first decreases to the minimum value as the temperature reaches 360°C equivalent Ro (EqRo = 1.0%), then increases and reaches the maximum value at 620°C (EqRo = 3.28%) and finally decreases again as the temperature rises at the last simulation stage. The mineral components exhibit an insignificant influence on the methane absorbability because of organic-matter coatings. The TOC-normalised methane adsorption capacity is positively correlated with the illite–smectite and clay-mineral contents but shows no significant correlation with brittle minerals, such as quartz. Soluble organic matter and kerogen caused the methane dissolution and adsorbtion, respectively. The strong dissolution property of the soluble organic matter is the most important cause of the larger adsorption capacity of the original shale compared with that of the residual samples. Moreover, the methane adsorption capacity of the Xiamaling shale decreases with increasing temperature, and increases with pressure below the critical pressure but decreases exceeding the critical pressure.

Variation of system openness and geochemical features in overpressured sandstones of the Yinggehai Basin, offshore South China Sea

Abstract Hydrocarbon discoveries in recent years in the Upper Miocene of the Yinggehai Basin show differences in overpressure magnitude, gas and mineral components. Overpressure close to or reaching the leakage threshold has induced high system openness for one field, but the overpressure of another field remains to be safely closed. A new vision to investigate the interplay between the system openness and the gas chemical and mineralogical features were conducted. The gas components in the high openness system are rich in high maturity hydrocarbons and inorganic CO2, which both have heavier carbon isotopes. Low maturity gases with lighter carbon isotopes are dominant in the low openness system. The abnormal carbon isotope ratio between iso-butane and n-butane in the high openness system confirms that early low maturity leakage has generated abnormal compositional and isotopic fractionation. Although the sedimentation process was uniform in the two systems, the high openness system has rare K-feldspar grains and chlorite, but it is rich in illite/smectite mixed layers (I/S). Precipitation is characterized by ankerite and kaolinite in the high openness system, whereas the low openness system primarily consists of calcite. The strong dissolution of K-feldspar grains and chlorite should have released abundant potassium and silicon. The high abundances of I/S and kaolinite, which are both sensitive to potassium and the rarity of quartz overgrowths in the high openness system, imply that the leakage of potassium/silicon from the sandstones has induced a mass imbalance between dissolution and precipitation. Even the sandstone porosity was slightly increased by the mass imbalance in the high openness system. The risk of hydrocarbon expulsion during brine leakage cannot be ignored.

Ab Initio Monte Carlo Simulations of the Acidic Dissolution of Stainless Steels: Further Insights into the Mechanisms

Using Ab initio based Monte Carlo simulation techniques (AbMC), dissolution kinetics in acidic media was evaluated for different stainless steels (SS) as a function of their chemical composition, with particular attention paid to the role of transition metals series (TM). The elementary dissolution events at the atomic scale were treated as stochastic processes, from which dissolution probabilities were deduced. Two determining factors emerge as critical for the control of dissolution kinetics: the bonding energy of an alloying element in the SS matrix and its standard electrochemical potential. The first reflects the ability of an element to form stronger (Mo, Nb, Ni etc.) or weaker (Cr, Cu, Sn etc.) bonding than Fe, and the second refers to its facility to dissolve under an applied potential. High bonding energies and high standard potentials lead to a strong dissolution resistance (Mo), whereas low bonding energies and low standard potentials induce high dissolution kinetics (Sn). The intermediate situations: high-bonding energy and low standard potential (Nb) or low bonding energy and high standard potential (Cu) usually lead to moderate dissolution regimes with a strong impact on the dissolution mechanisms.

The diagenetic and tectonic significance of hydrocarbon accumulation in the southern Junggar Basin, NW China

The Jurassic petroleum system plays an increasingly important role in petroleum explorations in the southern Junggar Basin. To assess the hydrocarbon distribution and the accumulation history of the Jurassic reservoirs in this area, we carried out reservoir petrographic, fluorescence spectroscopic, and fluid inclusion petrographic analyses on the core and cutting samples from the QQ‐2 well in the Qigu Anticline. The oil accumulation has the partiality for high‐quality sandstone reservoirs that have the high porosity and permeability with values of 14–18% and (1–450) × 10−2 μm2, respectively, in the Middle Jurassic Toutunhe Formation (J2t) reservoirs. The pores are big with average diameters of 60–100 μm, and the pore throats are wide with values of 3.45–6.92 μm in the oil reservoirs. The intergranular pores survive thanks to the poor compaction and the poor cementation, and the dissolved and mould pores develop attributed to the strong dissolution in the oil reservoirs. Two episodes of oil charging and one episode of gas invasion were identified in the Middle Jurassic reservoirs in the southern Junggar Basin. The low‐maturity paleo‐oil charged the sandstone reservoirs at about 35 Ma when the paleo‐traps took shape during the Cretaceous–Eocene. A large number of faults and anticlinal traps was developed in the southern Junggar Basin at about 7 Ma, caused by the reactivation of the Tianshan Mountains in the late Cenozoic. Simultaneously, the leakage of the paleo‐oil, the accumulation of late oil, and the gas invasion all took place. Fault activities led to the leakage of paleo‐oil and helped the oil and gas accumulate in the Middle Jurassic reservoirs in the anticlinal trap shaping at about 7–8 Ma in the southern Junggar Basin.

Reservoir characteristics and controlling factors of Silurian Lower Kepingtage Formation in Tahe area, Tarim Basin, NW China

With the breakthrough of exploration in Well TP16-1, the lower Kepingtage Formation becomes a key target for petroleum exploration of deep clastic reservoir in Tahe area. In this paper we focused on the research of the reservoir characteristics and its controlling factors in two sub-member formations (S1k 1 1 and S1k 1 3 ). Based on X-ray diffraction, conventional physical properties data (porosity and permeability) and reservoir storage space data (casting thin section and scanning electron microscope), we determined that the S1k1 Formation belongs to extra-low porosity and permeability reservoir, although the upper S1k 1 3 Formation shows relative better physical characteristic than the lower S1k 1 1 Formation. The development of storage space in the study area is controlled by sedimentary microfacies, diagenesis process. Reservoirs in S1k1 Formation are mainly located in channel (S1k 1 1 sandstones) and sand flat (S1k 1 3 sandstones). The sand flat sediments with a more coarse grain size compared with the channel. In diagenesis, compaction is the major controlling factor for reducing the porosity, followed by cementation. Dissolution of diagenesis is the major controlling factor in enhancing the reservoir porosities. Compared with channel (S1k 1 1 ) sandstones, sand flat sandstones (S1k 1 3 ) have better reservoir quality for its weaker compaction, cementation and stronger dissolution. On the basis of sedimentary characteristics (grain size and subfacies), physical property (porosity and permeability) and reservoir storage space, we divide the S1k1 reservoir into three categories (I, II and III). Type I reservoir is high quality reservoir. It is mainly distributed in the south area of S1k 1 1 and S1k 1 3 reservoir. Type II is moderate reservoir. It is located in the middle of S1k 1 1 reservoir and in the north of S1k 1 3 reservoir. Type III is the poor reservoir. It is only located in the north of S1k 1 1 reservoir.

Reservoir quality, natural fractures, and gas productivity of upper Triassic Xujiahe tight gas sandstones in western Sichuan Basin, China

The main target for hydrocarbon exploration in the western Sichuan Basin in China is the tight gas sandstone reservoir of the Upper Triassic Xujiahe Formation. Sandstones of the Xujiahe Formation are characterized as tight with ultralow porosity, ultralow permeability, and intensive heterogeneity. In this study, various techniques, such as scanning electron microscopy, X-ray diffraction, cathodoluminescence microscopy, casting thin-section, and fluid inclusion thermometry, were employed to investigate the composition, sedimentary facies, diagenesis history, diagenetic intensity, natural fracture characteristics, and their impact on reservoir quality. The results showed that subaqueous distributary channels and mouth bars with low rock fragment content are favorable for pore preservation and secondary pore development. Except for the medium to strong dissolution intensity, chlorite rim growth is also important for the development of high quality reservoirs. Intraformational shear fractures and horizontal shear fractures associated with tectonic movement increase pore space and improve the capability of transmitting fluids. Differences in gas productivity could be attributed to differences in reservoir quality, which were controlled by the sedimentary environment, diagenetic intensity, and fracture characteristics. A significantly positive correlation was found between total porosity and gas productivity of the tight sandstones. The high density fracture distribution was found owing to the fault system and structural deformation zone, which resulted in high gas productivity. The research results were successfully applied to improve production efficiency in the Xujiahe tight gas sandstones, and could be applicable to other similar fields involving tight gas.

Stability limits of tin-based electrocatalyst supports

Tin-based oxides are attractive catalyst support materials considered for application in fuel cells and electrolysers. If properly doped, these oxides are relatively good conductors, assuring that ohmic drop in real applications is minimal. Corrosion of dopants, however, will lead to severe performance deterioration. The present work aims to investigate the potential dependent dissolution rates of indium tin oxide (ITO), fluorine doped tin oxide (FTO) and antimony doped tin oxide (ATO) in the broad potential window ranging from −0.6 to 3.2 VRHE in 0.1 M H2SO4 electrolyte. It is shown that in the cathodic part of the studied potential window all oxides dissolve during the electrochemical reduction of the oxide – cathodic dissolution. In case an oxidation potential is applied to the reduced electrode, metal oxidation is accompanied with additional dissolution – anodic dissolution. Additional dissolution is observed during the oxygen evolution reaction. FTO withstands anodic conditions best, while little and strong dissolution is observed for ATO and ITO, respectively. In discussion of possible corrosion mechanisms, obtained dissolution onset potentials are correlated with existing thermodynamic data.

Formation mechanism of δ-ferrite and metallurgy reaction in molten pool during press-hardened steel laser welding

Al-Si coated press-hardened steel (PHS) welding was carried out using fiber laser with Ar and air atmospheres under different atmospheres. The formation mechanism of δ-ferrite in welded seam (WS) was: the melting Al-Si coating diffused into molten pool such that the stabilization of high-temperature δ-ferrite was improved as well as the phase transformation (δ→γ) was restrained by the effect of laser rapid cooling. Al-content in δ-ferrite of WS with Ar atmospheres was more than air atmospheres, leading to less δ-ferrite. A few of oxygen and nitrogen were ionized and droplet reaction zone with strong dissolution did not occur during laser lap welding, so welding defects were not obtained. Moreover, O element reacted with Al in the molten pool to form oxides of less than 1μm (Al2O3), it did not deteriorate the properties of WS.

A kinetic model to explain the grain size and organic matter content dependence of magnetic susceptibility in transitional marine environments: A case study in Ria de Muros (NWIberia)

AbstractMagnetic minerals in marine sediments are sensitive indicators of processes such as provenance changes, climatic controls, pollution, and postdepositional geochemical changes. Magnetic susceptibility is the bulk property of the sediments most commonly used to understand the magnetic characteristics of sediments. Before conclusions can be drawn from changes in this parameter, it is important to understand what factors and to what extent control changes in magnetic susceptibility. The magnetic susceptibility of surficial sediments in the Galician Rias Baixas, in NW Spain, has been shown to covary with sediment texture and organic matter content. Downcore, the magnetic properties of these sediments experience drastic changes as a result of strong dissolution caused by early diagenesis. In this paper, we further explore the relationship between these factors and formalize the observed covariations as the result of a simple second‐order kinetic model dependent on the content of organic matter in surficial sediments in the Ria de Muros. The reanalysis of previously reported data from the Rias de Vigo and Pontevedra confirmed the validity of this model and suggested further controls such as wave climate and water depth in the rates at which magnetic susceptibility changes are controlled by organic matter content.

Diagenesis and its impact on reservoir quality for the Chang 8 oil group tight sandstone of the Yanchang Formation (upper Triassic) in southwestern Ordos basin, China

Based on the applications of thin section, cathode luminescence, scanning electron microscope and X-ray diffraction, this paper studies the reservoir characteristics, diagenesis features and their influences on the development of reservoir pores for the Chang 8 oil group of Yanchang Formation in southwestern Ordos Basin. Results show that the reservoir is characterized by low composition maturity, low structure maturity, low porosity/ultra-low porosity and ultra-low permeability which belong to tight reservoir. The reservoirs develop medium intensity of compaction, moderate to strong cementation, relatively strong dissolution, suggesting that it is in stage B of the middle diagenetic evolution. The main diagenetic types include mechanical compaction, cementation by carbonates, quartz and clay minerals, and dissolution which have directly influenced the reservoir quality. In the early diagenetic period, the rapid compaction and cementation of carbonate had rapidly destroyed the reservoir quality, while the particles lining of chlorites had protected the reservoir space. In the middle diagenetic stage A, secondary dissolution pores were formed as soluble clastic grains were dissolved by the acidic fluid with organic matters maturation. The dissolution has played a constructive role for the pore generation and preservation. In the middle diagenetic stage B, the later compaction and cementation further reduced the reservoir quality. The analysis of pore evolution demonstrates that the initial reservoir porosity is about 30.59%. 16.925% overall porosity is lost through compaction, 11.65% porosity loss is due to cementation, and 6.36% porosity was created by dissolution. Finally, the reservoir is holding 8.375% porosity in present.

Change in composition and pore structure of Longmaxi black shale during oxidative dissolution

Owing to the presence of chemically unstable compositions (e.g., clay minerals, carbonate, pyrite, and organic matter (OM)) which are closely related to shale structure failures or dissolution pores, the methods to increase gas well long-term productivity from tight shale matrix need to take into account the chemical interaction of shale with injected fluid. In this study, black shale samples obtained from Lower Silurian Longmaxi formation in Sichuan basin, China were treated with 15wt% hydrogen peroxide (H2O2) for the comprehensive understanding of the change in shale composition and the associated dissolution structures. The measurements of mass loss, total organic carbon content, and mineralogical composition showed that carbonate, reductive inorganic minerals containing ferrous iron (e.g., pyrite and chlorite), and OM in samples exhibited strong dissolution; however, the other minerals behaved in a non-reactive manner at the experimental time scale. After the oxidative treatment for 240h, a large amount of oxidation-induced fractures and dissolution pores were observed by field-emission scanning electron microscopy. The fractures mainly oriented parallel to lamination were attributed to the dissolution of OM and structural alteration of clay minerals. All the dissolution pores seemed to be strongly dependent on the loss of dolomite, pyrite, and OM. Results from high-pressure mercury intrusion and low-pressure nitrogen adsorption analysis showed that these dissolution pores ranging from 10 to 500nm in diameter exhibited a significant increase in pore volume due to the removal of interconnected pore-filling OM, while the volume of pores>1μm in size exhibited a minor increase because the micrometer-size dissolved particles appeared to be discrete or unconnected. Thus the oxidative dissolution could lead to the higher porosity and better connectivity of nanometer-size pore networks in shale samples. The induced fractures reduced the size of diffusion dominant zones in shale matrix, and the dissolution pores increased the size of gas transport pathways into fractures. These results indicate that the injection of H2O2 may play an important role in shale matrix stimulation by oxidative dissolution which is likely to improve matrix diffusivity.

Karst paleogeomorphology of the fourth Member of Sinian Dengying Formation in Gaoshiti-Moxi area, Sichuan Basin, SW China: Restoration and geological significance

Abstract Based on 3-D seismic data and drill-hole data in Moxi-Gaoshiti area, the impression seismic thickness from the top of Longwangmiao Formation, which is easier traced, to the bottom of Cambrian System was selected to characterize the karst paleogeomorphology of the fourth Member of Dengying Formation. The paleogeomorphology of the member can be further divided into three geomorphic units: karst platform, slope and superimposed slope, which had different paleohydrologic conditions, and thus different karstification intensity and reservoir quality. Among them, the superimposed slope with the strongest dissolution, has larger solution groove and cave systems, where the reservoirs with mainly honeycombed dissolved pores and cavities as storage space, are best in quality; the platform slope with weaker dissolution, has smaller solution groove and cave systems, where the reservoirs are poorer than those in the superimposed slope; the karst platform with the weakest dissolution, has piebald karst systems mainly, where the reservoirs with mainly pinhole to smaller dissolved pores and cavities as storage space, are poorer in quality on the whole, but there are some good quality reservoir bodies in local areas. The results show that, besides the highly explored scarp belt, the slope in the intra-platform with low exploration degree can also be favorable exploration area of the fourth member of Dengying Formation.

Diagenesis of unconformity and the influence on reservoir physical properties: a case study of the lower Jurassic in Xiazijie area, Junggar Basin, NW China

Diagenesis process of unconformity is very important in understanding the physical properties of the reservoirs. In this paper, based on a comprehensive analysis of well cores, thin sections, casting thin sections, scanning electron microscopy and other data, we discussed the diagenetic characteristics and the influences on reservoir physical properties of unconformity of the lower Jurassic in Xiazijie area, Junggar Basin. The results show that the semi-weathered rock layers of the unconformity had undergone a series of diagenesis, such as compaction, cementation, dissolution and disruption during burial diagenetic evolution. However, the diagenetic evolution is mainly in the stage A of middle diagenesis, and its alteration degree has become gradually weak as the distance increased from the unconformity surface. The quantitative analysis of diagenesis shows that diagenetic characteristics of semi-weathered rock layer of the unconformity are as follows: strong compaction (average optic compaction is 67.28%), middle cementation (average optic cementation rate is 42.97%) and strong dissolution (average optic dissolution porosity is 65.00%). According to their influence on reservoir physical properties, the intensity of diagenesis is sequenced in the following order: compaction > dissolution > cementation > disruption. Among these processes, compaction, dissolution and cementation can change the porosity by −26.1, 6.0 and −4.3%, respectively. Dissolution is the main controlling factor on reservoir properties of unconformity semi-weathered rock layers.

Diagenetic differences caused by the charging of natural gases with various compositions - a case study on the lower Zhuhai Formation clastic reservoirs in the WC-A sag, the Pearl River Mouth Basin

Only a limited number of comparative studies have explored the diagenetic differences of reservoirs caused by the charging of natural gases with different compositions during the migration and accumulation of oil and gas. In this study, we quantitatively assessed the diagenetic variations of the lower Zhuhai Formation reservoirs in the WC-A sag, the Pearl River Mouth Basin, as a result of natural gas charging with different compositions. The employed methods included electron microscopy, scanning electron microscopy, cathode luminescence, X-ray diffraction, chemical composition analysis of formation water, stable isotope analysis, and fluid inclusion determination. The results indicated that: (1) in the lower Zhuhai Formation reservoir that are near the large fractures, the late-charging of CO2-rich thermal fluid promoted the dissolution of minerals, and changed the porosity, permeability, pH value of fluid, and the contents of ions such as K+, Na+, Mg2+ of the reservoirs. These changes, in turn, indirectly affected the content, form, and distribution of clay minerals in the reservoirs. In addition, the secondary enlargement of quartz was enhanced. (2) The charging of CO2-rich thermal fluid caused strong dissolution and produced high contents of cements. The dissolution mainly occurred in a half open system with strong fluid activity. Consequently, the significantly decreasing of permeability was hindered and middle porosity–middle permeability reservoirs were generated. The reservoirs that are far from the large fractures, however, were mainly early charged by the hydrocarbon-rich fluid. The dissolution was weak and the generated cements were limited. In addition, the dissolution products could not be discharged in such closed diagenetic systems. As a result, the permeability of the reservoir was declined sharply and resulted in low porosity–low permeability reservoirs.

Hexavalent chromium and some trace metals in concrete from buildings of different ages in northern Italy

The content of total hexavalent chromium (CrVI), total chromium (Cr) and other metals (Fe, Mn, Ni and Co) was determined in the cement matrix of concrete present in samples collected in both old and recent buildings located in the town of Modena (northern Italy). The cement matrix of concrete was obtained by eliminating the aggregate components by means of disaggregation and was ground until it could pass through a 63-μm-sieve. In addition, samples of bagged commercial cements were considered. Metal determination was obtained by inductively coupled plasma optical emission spectroscopy after strong acid dissolution (HCl–HNO3). Total Cr(VI) determination was obtained after mild acid attack (HCl) and analysed by UV–Vis spectrophotometry. Analytical data values obtained indicate that the highest concentrations of metals were found in concrete and in bagged cement samples characterized by high Fe concentration. High Cr(VI) concentrations both in the cement matrix of concrete and in cement samples were found in total Cr-rich samples. Cr(VI) concentrations were about 20–30% of total Cr. The data obtained indicate that secondary raw materials and industrial by-product powders utilized in the modern cement industry may significantly increase Cr(VI) in concrete.

Long-term corrosion tests of Ti3SiC2 and Ti2AlC in oxygen containing LBE at temperatures up to 700 °C

Two MAX-phase materials, Ti3SiC and Ti2AlC, were tested at 550 °C, 650 °C and 700 °C up to 10 000 h in LBE (lead-bismuth-eutectic) containing of 10−6 and 10−8 wt% oxygen. It was found that secondary phases have a strong influence on corrosion effects. Ti3SiC showed a surface disintegration at 550°C/10−6 wt% oxygen after longer exposure, while a 4–7 μm thick TiO2 layer with Pb-Bi inclusions was detected on Ti2AlC. However, Ti3SiC is protected by a double layered oxide with an outer part of TiO2 and a mixed inner layer of SiO2 and TiO2 at the higher temperatures. Ti2AlC formed a TiO2 surface layer containing Al2O3. Some defects could be observed on the Ti3SiC surface in LBE containing 10−8 wt% oxygen at 550 °C and 650 °C. The secondary phases between the Ti3SiC grains showed strong oxidation at 700 °C. Due to the high Al solubility in LBE, Ti2AlC experienced strong dissolution attack after longer exposure times at 650 and 700 °C.

The fate of diatom valves in the Subantarctic and Polar Frontal Zones of the Southern Ocean: Sediment trap versus surface sediment assemblages

An array of deep ocean sediment traps was initiated in 1997 within the Australian sector of the Southern Ocean, and serviced annually for over a decade (the Subantarctic site is ongoing as part of the Southern Ocean Time Series). Here, we expand on previous findings obtained from the shallow traps (~1000m depth) in the Polar Frontal Zone (PFZ) and Subantarctic Zone (SAZ) by examining the chemical composition and diatom assemblages collected by sediment traps deployed at bathypelagic depths in the PFZ (1500m depth; years 1997–1998, 1999–2000 and 2003–2004) and SAZ (2000 and 3800m; year 1999–2000). Additionally, the diatom assemblages of the surface sediments below the traps were analyzed to document how the seasonal signal is recorded in the sedimentary record. Analysis of the changes of the BSi:POC ratios with depth at both sites confirms previous work suggesting that the decoupling between Si and C cycles in the Southern Ocean is not different from other biogeochemical provinces of the world's ocean. Comparison of the seasonal flux pattern registered by the traps indicates that the diatom assemblages in the sediments of the Antarctic Zone and PFZ mainly represent the summer period. In contrast, the assemblages found in the sediments of the SAZ are a reflection of the spring and summer months, a period characterized by a larger variability in chemical and physical parameters. The strong correlation between the POC fluxes and the relative abundance of a group of diatom species (mainly Pseudo-nitzschia and small Fragilariopsis) in the PFZ traps suggest that there is an intimate relationship between the development of these species and the POC pulses to the deepest layers of the water column in this region. Interestingly, analysis of the diatom assemblages in the surface sediments reveals that the strong dissolution in the sediment-water interface removed the signal of these “carbon sinkers” from the sedimentary record leaving the sediment assemblage enriched in heavily silicified species.

High precipitation rate in a Middle Triassic carbonate platform: Implications on the relationship between seawater saturation state and carbonate production

Abstract Three-dimensional geological modeling of the Middle Triassic Latemar carbonate platform is coupled with facies modal analysis to estimate its carbonate precipitation rate (G). The 3D model, strongly constrained by field data, encompasses a specific stratigraphic interval of the platform, bounded by two isochronous surfaces. Modal analysis of thin sections allows estimating the proportion of syndepositional vs postdepositional carbonate in the facies associations of the platform. This, together with the 3D facies distribution in the model that takes into account lateral and vertical facies variability, permits to calculate the volumes of syndepositional carbonate preserved at Latemar between the two considered isochrones. Given the peculiar characteristics of the platform, that does not show evidences of strong dissolution processes or large carbonate mass loss through export in the nearby basins, results can be used to estimate the average precipitation rate of the platform in the considered time interval. This estimate allows discussion in relation to models of ocean water saturation state (Ω) with respect to carbonates in the geological past, and comparison to the calculated precipitation rates of modern tropical coral reef ecosystems at global and reef scale. A high G value is found at Latemar and represents the first empirical confirmation that, in the Triassic, extremes in Ω may have triggered high carbonate precipitation in shallow water settings; moreover, comparison to modern reefs points to a possible common relationship that may link seawater Ω and precipitation rate in carbonate platform ecosystems through geological time.

Calculating structural and geometrical parameters by laboratory measurements and X-ray microtomography: a comparative study applied to a limestone sample before and after a dissolution experiment

Abstract. The aim of this study is to compare the structural, geometrical and transport parameters of a limestone rock sample determined by X-ray microtomography (XMT) images and laboratory experiments. Total and effective porosity, pore-size distribution, tortuosity, and effective diffusion coefficient have been estimated. Sensitivity analyses of the segmentation parameters have been performed. The limestone rock sample studied here has been characterized using both approaches before and after a reactive percolation experiment. Strong dissolution process occurred during the percolation, promoting a wormhole formation. This strong heterogeneity formed after the percolation step allows us to apply our methodology to two different samples and enhance the use of experimental techniques or XMT images depending on the rock heterogeneity. We established that for most of the parameters calculated here, the values obtained by computing XMT images are in agreement with the classical laboratory measurements. We demonstrated that the computational porosity is more informative than the laboratory measurement. We observed that pore-size distributions obtained by XMT images and laboratory experiments are slightly different but complementary. Regarding the effective diffusion coefficient, we concluded that both approaches are valuable and give similar results. Nevertheless, we concluded that computing XMT images to determine transport, geometrical, and petrophysical parameters provide similar results to those measured at the laboratory but with much shorter durations.