low-Mg Calcite Research Articles

Patterns of Element Incorporation in Calcium Carbonate Biominerals Recapitulate Phylogeny for a Diverse Range of Marine Calcifiers.

Elemental ratios in biogenic marine calcium carbonates are widely used in geobiology, environmental science, and paleoenvironmental reconstructions. It is generally accepted that the elemental abundance of biogenic marine carbonates reflects a combination of the abundance of that ion in seawater, the physical properties of seawater, the mineralogy of the biomineral, and the pathways and mechanisms of biomineralization. Here we report measurements of a suite of nine elemental ratios (Li/Ca, B/Ca, Na/Ca, Mg/Ca, Zn/Ca, Sr/Ca, Cd/Ca, Ba/Ca, and U/Ca) in 18 species of benthic marine invertebrates spanning a range of biogenic carbonate polymorph mineralogies (low-Mg calcite, high-Mg calcite, aragonite, mixed mineralogy) and of phyla (including Mollusca, Echinodermata, Arthropoda, Annelida, Cnidaria, Chlorophyta, and Rhodophyta) cultured at a single temperature (25°C) and a range of pCO2 treatments (ca. 409, 606, 903, and 2856 ppm). This dataset was used to explore various controls over elemental partitioning in biogenic marine carbonates, including species-level and biomineralization-pathway-level controls, the influence of internal pH regulation compared to external pH changes, and biocalcification responses to changes in seawater carbonate chemistry. The dataset also enables exploration of broad scale phylogenetic patterns of elemental partitioning across calcifying species, exhibiting high phylogenetic signals estimated from both uni- and multivariate analyses of the elemental ratio data (univariate: λ = 0-0.889; multivariate: λ = 0.895-0.99). Comparing partial R 2 values returned from non-phylogenetic and phylogenetic regression analyses echo the importance of and show that phylogeny explains the elemental ratio data 1.4-59 times better than mineralogy in five out of nine of the elements analyzed. Therefore, the strong associations between biomineral elemental chemistry and species relatedness suggests mechanistic controls over element incorporation rooted in the evolution of biomineralization mechanisms.

Primary characteristics of selected stalagmites from four caves located between Erdemli and Silifke (Mersin), southern Turkey—implications on their formation

This study describes the primary chacteristics of the selected stalagmites from the four caves in southern Turkey and interprets their formation via petrographic, SEM–EDX, XRD, ICP–AES and stable isotope techniques. The analytical results show that the stalagmites are composed of low Mg-calcite associated with trace to small amount of impurities such as clays. Lamination is characteristic in all the stalagmites and mainly includes sparitic and micritic growth layers. In the sparitic layers, columnar crystal fabric is common together with rare dendritic fabric. The columnar fabric consists of elongated calcite crystals and indicates slow inorganic precipitation. Whereas the dendritic fabric consists of fibrous-like calcite crystals oriented at two directions, and is related to the colonization of fungus and later their calcification indicating growth interruptions. Micritic growth layers characterize relatively rapid precipitation. The stable isotope (δ18O and δ13C) values suggest precipitation mainly from meteoric water and reflect the effect of the seawater by wash over or splash action in the stalagmite sample of Guvercinlik Cave located at sea level. The small variations in the δ18O and δ13C values are mainly controlled by amount of rainfall due to the seasonal changes and also input of seawater. Rainy periods are characterized by the relatively more negative δ18O and δ13C values and low contents of trace elements. Sea water input significantly increases the δ18O, δ13C, Na2O, MgO, Sr and Ba values.

Chromium isotope heterogeneity on a modern carbonate platform

Chromium isotope (δ53Cr) values of marine sedimentary rocks are increasingly utilised for monitoring changes in redox conditions in the Earth's ancient oceans and atmosphere. Carbonate rocks are of particular interest as they are relatively common in the sedimentary record and have been shown under laboratory conditions to be capable of recording the Cr isotope composition of precipitating fluids. However, little data from appropriate modern analogues is currently available to constrain the variability in δ53Cr values within a carbonate platform and slope system and assess the preservation potential of marine δ53Cr values during burial, lithification and diagenesis. In this paper we investigate the heterogeneity of δ53Cr values on a modern marine carbonate platform in the context of carbonate sediment deposition, mineralogy, and the influence of burial and early lithification. We present new δ53Cr values from carbonate sediments in the upper 200 m of ODP Leg 166 Site 1003 taken from the western margin of the Great Bahama Bank (GBB) and compare these data to modern carbonate sediments accumulating on the GBB platform-top. Shallow-water surface sediments record a range of heavy δ53Cr values from +0.77 to +1.07‰ while drill core samples record a larger range of δ53Cr values from +0.51 to +0.97‰. Down-core variability in δ53Cr values occurs systematically with changes in sediment composition and mineralogy, where more positive δ53Cr values occur in drill core samples that preserve proportionally more banktop-derived carbonate and less well developed diagenetic textures. These values decrease down-core where proportionally more pelagic carbonate exists, and with increasing neomorphism of primary aragonite to low-Mg calcite. The mineralogical dependence of δ53Cr values at Site 1003 appears to be related to variations in relative sea-level across the platform and the degree of lithification and diagenetic recrystallization with depth, rather than in response to changes in the δ53Cr composition of the overlying water column. As the GBB represents a reasonable modern analogue for many Phanerozoic and Precambrian carbonate successions, the preliminary framework presented here may have significant implications for the interpretation of vertical profiles of δ53Cr values in paleo-redox studies. In particular, smaller scale variability in ancient carbonate sections can be more simply explained by local variations in depositional environment and diagenetic regime rather than fluctuations in water column redox conditions.

Evolution of calcite microcrystal morphology during experimental dissolution

ABSTRACTPhanerozoic limestones are composed of low-Mg calcite microcrystals (i.e., micrite) that typically measure between 1 and 9 μm in diameter. These microcrystals, which host most of the microporosity in subsurface reservoirs, are characterized by a variety of microtextures. Despite the overwhelming consensus that calcite microcrystals are diagenetic, the origin of the various textures is widely debated. The most commonly reported texture is characterized by polyhedral and rounded calcite microcrystals, which are interpreted to form via partial dissolution of rhombic microcrystals during burial diagenesis. A proposed implication of this model is that dissolution during burial is responsible for significant porosity generation. This claim has been previously criticized based on mass-balance considerations and geochemical constrains. To explicitly test the dissolution model, a series of laboratory experiments were conducted whereby various types of calcites composed of rhombic and polyhedral microcrystals were partially dissolved under a constant degree of undersaturation, both near and far-from-equilibrium.Our results indicate that calcite crystals dissolved under far-from-equilibrium conditions develop rounded edges and corners, inter-crystal gulfs (narrow grooves or channels between adjacent crystals), and a few etch pits on crystal faces—observations consistent with the burial-dissolution hypothesis. Crystals dissolved under near-equilibrium conditions, in contrast, retain sharp edges and corners and develop ledges and pits—suggesting that dissolution occurs more selectively at high-energy sites. These observations support the longstanding understanding that far-from-equilibrium dissolution is transport-controlled, and near-equilibrium dissolution is surface-controlled. Our results also show that while the rhombic calcite crystals may develop rounded edges and corners when dissolved under far-from-equilibrium conditions the crystals themselves do not become spherical. By contrast, polyhedral crystals not only develop rounded edges and corners when dissolved under far-from-equilibrium conditions but become nearly spherical with continued dissolution. Collectively, these observations suggest that rounded calcite microcrystals more likely form from a precursor exhibiting an equant polyhedral texture, rather than from a euhedral rhombic precursor as previously proposed. Lastly, the observation that calcite crystals developed rounded edges and corners and inter-crystal gulfs after only 5% dissolution indicates that the presence of such features in natural limestones need not imply that significant porosity generation has occurred.

Chemostratigraphy of the Pliensbachian, Puesto Araya Formation (Neuquén Basin, Argentina)

In a preliminary attempt to establish an isotope stratigraphy, strontium, carbon and oxygen isotope ratios were determined from marine biogenic carbonates of Pliensbachian age, in the context of scheme of local ammonite Zones correlatable to the European Standard Zonation. Two sections, rio Atuel and arroyo Serrucho, of the mainly siliciclastic Puesto Araya Formation, Neuquen Basin, south-western Mendoza, Argentina, were studied. Specimens of the bivalve genera Weyla Bhom and Gryphaea Lamarck were selected for the isotopic determinations because of their low-Mg calcite original mineralogy and widespread presence. Scanning electron microscopy, X-ray diffraction and X-ray fluorescence spectrometry techniques were used to control the good degree of preservation of most of the biogenic material, as evidenced by pristine fabrics, 100% calcite composition and Sr, Mn and Fe concentrations. Although strontium isotope ratios are slightly scattered, it is possible to compare them with those of the Early Jurassic seawater reference curve. Carbon isotope signals show two relative maxima, correlatable with those recorded for the upper part of the Ibex Zone and the middle part of the Margaritatus Zone in various European sections, indicating the possible global significance of these events. d18O values were found to be unreliable for isotope stratigraphy, as they are largely depleted in comparison to those of coeval unaltered marine carbonates.

Facies character and geochemical signature in the late Quaternary meteoric diagenetic carbonate succession at the Xisha Islands, South China Sea

The late Quaternary shallow-water carbonates have been altered by a variety of diagenetic processes, and further influenced by high-amplitude global and regional sea level changes. This study utilizes a new borehole drilled on the Yongxing Island, Xisha Islands to investigate meteoric diagenetic alteration in the late Quaternary shallow-water carbonates. Petrographic, mineralogical, stable isotopic and elemental data provide new insights into the meteoric diagenetic processes of the reef limestone. The results show the variation in the distribution of aragonite, high-Mg calcite (HMC) and low-Mg calcite (LMC) divides the shallow-water carbonates in Core SSZK1 into three intervals, which are Unit I (31.20–55.92 m, LMC), Unit II (18.39–31.20 m, aragonite and LMC) and Unit III (upper 18.39 m of core, aragonite, LMC and HMC). Various degrees of meteoric diagenesis exist in the identified three units. The lowermost Unit I has suffered almost complete freshwater diagenesis, whereas the overlying Units II and III have undergone incompletely meteoric diagenesis. The amount of time that limestone has been in the freshwater diagenetic environment has the largest impact on the degree of meteoric diagenesis. Approximately four intact facies/water depth cycles are recognized. The cumulative depletion of elements such as strontium (Sr), sodium (Na) and sulphur (S) caused by duplicated meteoric diagenesis in the older reef sequences are distinguished from the younger reef sequences. This study provides a new record of meteoric diagenesis, which is well reflected by whole-rock mineralogy and geochemistry.

When a mid-ocean ridge encroaches a continent: Seafloor-type hydrothermal activity in Lake Asal (Afar Rift)

At the place where the submarine Aden Ridge encroaches on the African continent and interacts with the East African Rift system, two small basins form: Ghoubbet-al-Kharab and Lake Asal. Whereas Ghoubbet-al-Kharab is connected to the open ocean, Lake Asal is a typical example of oceanic “embryo”, which is defined as a system that is detached from the ocean, but has features of a marine basin with an oceanic type crust and a seawater-based water body. In order to shed light on the source of water, type of hydrothermal activity and hydrothermal deposits, and controls on the water chemistry in an oceanic “embryo”, we undertook a mineralogical-geochemical study of the lake water, hydrothermal fluids and hydrothermal carbonate deposits of Lake Asal. The geochemical analyses of lake water and hydrothermal fluids show that Lake Asal (located in an arid zone with strong evaporation and with no riverine input) is fed by seafloor-type hydrothermal fluids according to the following scenario: percolation of seawater along faults and cracks of extension in the rift, reaction of seawater with the hot basaltic rocks and hydrothermal fluid generation, discharge of the hydrothermal fluid in the Asal depression and accumulation of the Lake Asal water body. The fluid venting at the Lake Asal bottom is a mixture of 97% end-member hydrothermal fluid and 3% lake water. The calculated end-member hydrothermal fluid of this oceanic “embryo” is poorer in metals than the seafloor hydrothermal fluids of an open and evolved ocean. In addition to the seawater/rock interaction, the chemistry of Lake Asal is also controlled by evaporation leading to hyper salinity. In a hyper saline water body a number of hydrothermally supplied metals are stabilized as chloride complexes and accumulate. This results in a metal rich and mildly acidic “embryonic” ocean. Unlike an open and evolved modern ocean, the “embryonic” ocean located in an arid zone has heavy C and O isotope composition and light Zn and Fe isotope composition. Calcium isotope compositions of both types of ocean are similarly heavy. There are two genetically different sources of elements to the Lake Asal that are vertically separated: hydrothermal (lower, or bottom) and aeolian (upper, or surficial). Another important control on the lake water chemistry is the formation of carbonate spires at the lake bottom. Ca‑carbonate precipitation immobilizes substantial amount of hydrothermally supplied Ca and drives up the (Mg/Ca)mol of the lake water. Increasing (Mg/Ca)mol of the evolving lake water leads to changes in the mineralogy of spires: from low-Mg calcite to aragonite. Thus, the spire formation exerts a self-control on its mineralogy. Carbonate spire deposition affects also the Ca, Zn and Fe isotope composition of the lake water through adsorption or/and co-precipitation induced isotope fractionation.

A Review and Meta-Analysis of Potential Impacts of Ocean Acidification on Marine Calcifiers From the Southern Ocean

Understanding the vulnerability of marine calcifiers to ocean acidification is a critical issue, especially in the Southern Ocean (SO), which is likely to be the one of the first, and most severely affected regions. Since the industrial revolution, ~30% of anthropogenic CO2 has been absorbed by the global oceans. Average surface seawater pH levels have already decreased by 0.1 and are projected to decline by ~0.3 by the year 2100. This process, known as ocean acidification (OA), is shallowing the saturation horizon, which is the depth below which calcium carbonate (CaCO3) dissolves, likely increasing the vulnerability of many resident marine calcifiers to dissolution. The negative impact of OA may be seen first in species depositing more soluble CaCO3 mineral phases such as aragonite and high-Mg calcite (HMC). Ocean warming could further exacerbate the effects of OA in these particular species. Here we combine a review and a quantitative meta-analysis to provide an overview of the current state of knowledge about skeletal mineralogy of major taxonomic groups of SO marine calcifiers and to make projections about how OA might affect a broad range of SO taxa. We consider a species' geographic range, skeletal mineralogy, biological traits, and potential strategies to overcome OA. The meta-analysis of studies investigating the effects of the OA on a range of biological responses such as shell state, development and growth rate illustrates that the response variation is largely dependent on mineralogical composition. Species-specific responses due to mineralogical composition indicate that taxa with calcitic, aragonitic, and HMC skeletons, could be at greater risk to expected future carbonate chemistry alterations, and low-Mg calcite (LMC) species could be mostly resilient to these changes. Environmental and biological control on the calcification process and/or Mg content in calcite, biological traits, and physiological processes are also expected to influence species-specific responses.

In a pinch: Skeletal carbonate mineralogy of crabs (Arthropoda: Crustacea: Decapoda)

Skeletal carbonate mineralogy data from the literature was combined with new X-ray diffractometry data from New Zealand crabs, in order to elucidate mineralogical patterns related to body part, habitat, latitude, and phylogeny, with a view to understanding both the potential vulnerability and the preservation potential of crab shells. In general, crab carbonate is 100% Mg-calcite. In nine specimens from three New Zealand species (Austrohelice crassa, Charybdis japonica, and Hemigrapsus crenulatus) we found the first record of aragonite in brachyurid crab carbonate. Brachyurid crabs tested here generally produce Mg-calcite with mean of 6–7 wt% MgCO3, though families Epaltidae, Majidae, Menippidae and Oziidae produ\\ce high-Mg calcite (8–9 wt% MgCO3), while Eiphiidae and Hymenosomatidae produce low-Mg calcite (3–4 wt% MgCO3). Anomurid crabs showed no aragonite in any specimen, but varied more in Mg content, with the family Porcellanidae producing the highest mean Mg content found (9.3 wt%MgCO3). In most cases the standard deviation and range of values within species is small (SD 0–1.5, range 1–3 wt% MgCO3. There is little evidence of partitioning of Mg in skeletal elements; Mg in the claw, for example, is not significantly different than that of the carapace, legs, or abdomen, whether considered across all species, or within individuals. Latitude (as a proxy for water temperature), too, does not appear to affect Mg in crabs, though it is possible that environment (rocky vs sandy shore) does. The strong tendency for crabs to produce mid-range Mg in calcite suggests that they may be striking a balance between mechanical strengthening of calcite while limiting solubility in sea water.

High Magnesium Calcite and Dolomite composition carbonate in Amphiroa (Lithophyllaceae, Corallinales, Rhodophyta): further documentation of elevated Mg in Corallinales with climate change implications.

Species of the calcified, articulate coralline Amphiroa are key components of many shallow marine ecosystems. Understanding their mineral composition is important as their susceptibility to dissolution, due to ocean acidification, may vary with mineral composition. We studied the distribution of Mg-calcite, very high magnesium calcite (VHMC), and dolomite within Amphiroa species to elucidate their mineral properties and susceptibility to dissolution. Results revealed that the asymmetrical X-ray diffraction (XRD) pattern typical of Amphiroa globally represents high levels of VHMC and dolomite composition carbonate. The dolomite seems most likely to be disordered, but higher resolution XRD is required for confirmation. The calcified long sides of medullary cells have predominantly VHMC/dolomite and the corners have bands of VHMC/dolomite. Epithallial cell walls are low Mg-calcite, and cortical cells are low Mg-calcite with bands of VHMC. VHMC/dolomite is more stable than Mg-calcite, and this may provide a competitive advantage for Amphiroa species as seawater pH declines. Further work is required to determine the metabolic controls on VHMC/dolomite mineral formation.

Marine vs. terrestrial environments during Early Triassic deposition on the northeastern margin of the Central European Basin – a multidisciplinary study on the Middle Buntsandstein of the Bartoszyce IG 1 borehole, NE Poland

The Middle Buntsandstein Subgroup of the Lower Triassic of north-eastern Poland has been investigated in the context of possible marine ingressions into the Central European Basin. To better constrain these, palynofacies analysis and mineralogical analysis of mudstones were undertaken on samples taken from the Lidzbark and Malbork formations penetrated by the Bartoszyce IG 1 borehole, serving as stratotype section of both lithostratigraphic units. Microfacies and geochemical analyses, including C and O isotope analysis, were conducted additionally on oolitic limestones of the basal Lidzbark Formation (the lowermost Middle Buntsandstein), and the boron content was measured on all mudstone samples. Seven palynofacies types are distinguished: types 1 to 3 within the Malbork Formation and types 4 to 7 within the Lidzbark Formation. Types 1 and 2 originated on a floodplain, type 3 probably in a deltaic or barrier setting, and types 4 and 5 in a brackish lagoon or a more open basin, possibly of marine origin. Palynofacies type 6 reflects long transport and reworking, whereas palynofacies type 7 is interpreted as formed due to pedogenic processes. Clay minerals and quartz, accompanied by feldspars, calcite and dolomite are the main components of the mudstones investigated. The clay mineral association consists of illite or a mixture of illite and smectite, and chlorite. The Lidzbark Formation and the lowermost part of the Malbork Formation show less variability in mineralogical composition than the upper part of the Malbork Formation. Smectite admixtures were detected only in the upper part of the Malbork Formation (the uppermost Middle Buntsandstein). The boron content, achieved after aqua regia digestion, ranges from 70 to 121 mg/kg (96 mg/kg at average), oscillating generally around 90 mg/kg. A higher boron content, bound in silicate structure, is associated with the upper part of the Malbork Formation. All major mineralogical and geochemical changes coincide more or less with the transition from the supposed marine to the terrestrial environmental realm, interpreted from lithological and sedimentological observations within the lowermost part of the Malbork Formation. However, diagenetic alteration of the clay minerals, and of the boron content, could not be ruled out. The oolitic limestones, mainly grainstones, contain admixtures of quartz grains and rare bioclasts, the ooids nuclei being peloids or unrecognizable. The carbonates are almost exclusively composed of low-Mg calcite. A high content of Mg and the presence of small amounts of dolomite suggest that the ooids were primarily composed of high-Mg calcite and are comparable with similar Early Triassic deposits in Tethyan settings. The δ 13 C values range from –2 to +1‰ VPDB, fitting well with the known ranges of Lower Triassic marine carbonates. An observed δ 13 C depletion towards the top of the oolite-bearing part of the section may reflect a local shallowing trend that led to overall salinity decrease. A possible connection with the one of the global oceanic geochemical episodes has to be tested further. The results obtained suggest a marine origin of the lower Middle Buntsandstein deposits studied and document a terrestrial origin for the upper Malbork Formation.

Synchrotron high-resolution XRD and thermal expansion of synthetic Mg calcites

The structural parameters and the thermal behavior of a complete series of Ca–Mg carbonates synthesized at high pressure and temperature (1–1.5 GPa, 1273–1373 K) in the range 0–50 mol% MgCO3 have been investigated by in situ powder synchrotron high-resolution X-ray diffraction at ambient and up to 1073 K under self-controlled CO2 partial pressure. The crystal structures are disordered Mg calcite in the range 1–41 mol% MgCO3, and Ca dolomite at 49 mol% MgCO3. New calibration curves of the cell parameters for the Mg content and thermal expansion from ambient to 1073 K are given. Short-range structural effects of cation substitution and ordering and their thermal behavior as a function of Mg content were identified from three sets of data: the peak broadening, the cell parameter strains and the Raman band enlargements. Both intra- and inter-crystalline levels of compositional heterogeneity are identified and allow splitting the Mg calcites into two groups: low- and high-Mg calcites. The low-Mg calcites (up to 22 mol% MgCO3) are homogeneous in Mg content with short-range ordering. High-Mg calcite (up to 41 mol% MgCO3) displays domains with different local ordering configurations and similar or slightly different Mg contents, and to which is added a compositional variation between crystals, as determined by EMP, of the order of ± 0.8 mol% MgCO3. The cation ordering in Ca-rich dolomites similarly occurs in high-Mg synthetic calcites. The role of (CO3)2− group ordering is shown to be an important factor in the formation of Ca–Mg carbonates.

Mg isotopic systematics during early diagenetic aragonite-calcite transition: Insights from the Key Largo Limestone

The seawater magnesium (Mg) isotopic composition (δ26MgSW) can be applied to quantify marine Mg cycle in Earth's history. However, reconstruction of δ26MgSW by using marine carbonate has been challenged by the large range of variation in Mg isotopic composition of carbonate (δ26Mgcarb). Such variation, on the one hand, reflects, e.g. the mineralogy, temperature, and precipitation rate dependent isotopic fractionation during Ca‑carbonate precipitation. On the other hand, the pristine seawater record could be modified in carbonate diagenesis, during which less stable carbonate minerals would spontaneously convert to more stable mineral phases. In the modern ocean with high Mg/Ca (molar) ratio (~5), aragonite precipitation is favored. However, aragonite is thermodynamically less stable than low-Mg calcite at ambient surface environment, resulting in the spontaneous transition from aragonite to low-Mg calcite in the earliest stage of diagenesis (i.e. the aragonite-calcite transition). As a common process in the modern ocean, Mg isotopic fractionation in aragonite-calcite transition may provide valuable constraints on the variation of δ26Mgcarb. In this study, we measured Mg isotopic compositions of Pleistocene Key Largo Limestone (Florida), which has been experiencing the aragonite-calcite transition during early diagenesis. The Key Largo Limestone samples consist of a mixture of calcite and aragonite. δ26Mg of aragonite (δ26Mgarag) shows higher and more stable values, while calcite has lower yet more variable Mg isotopes (δ26Mgcal). We suggest a model conceptualizing a water-film attached to the mineral surface, in which the aragonite-calcite transition initiates with aragonite dissolution and the Mg exchange with external fluid can be variable, followed by Mg isotopes fractionate during low-Mg calcite precipitation. In this model, δ26Mgcal is controlled by (1) the chemical composition of water-film, which is dependent on the mineralogical composition of aragonite (i.e. Mg content and δ26Mgarag), the composition of external fluid (i.e. fresh water, seawater, or mixed fresh water and seawater), and the mixing ratio between dissolved aragonite and external fluid, and (2) the isotopic fractionation during calcite precipitation. Because the isotopic fractionation in calcite is sensitive to the rate of calcite precipitation that is dependent on the calcite saturation state of solution. Therefore, δ26Mgcal could be highly heterogeneous depending on the aragonite dissolution and calcite precipitation processes, further complicating the interpretation of carbonate data. Thus, the process-controlled Mg isotopic fractionation should be taken into consideration when reconstructing δ26MgSW by using carbonate sediments/rocks.

Structural and Elemental Analysis of the Freshwater, Low-Mg Calcite Coralline Alga Pneophyllum cetinaensis.

Coralline algae are one of the most diversified groups of red algae and represent a major component of marine benthic habitats from the poles to the tropics. This group was believed to be exclusively marine until 2016, when the first freshwater coralline algae Pneophyllum cetinaensis was discovered in the Cetina River, southern Croatia. While several studies investigated the element compositions of marine coralline algal thalli, no information is yet available for the freshwater species. Using XRD, LA-ICP-MS and nano indentation, this study presents the first living low-Mg calcite coralline algae with Mg concentrations ten times lower than is common for the average marine species. Despite the lower Mg concentrations, hardness and elastic modulus (1.71 ± 1.58 GPa and 29.7 ± 18.0 GPa, respectively) are in the same range as other marine coralline algae, possibly due to other biogenic impurities. When compared to marine species, Ba/Ca values were unusually low, even though Ba concentrations are generally higher in rivers than in seawater. These low values might be linked to different physical and chemical characteristics of the Cetina River.

Late Holocene Red Sea Coastal Evolution: Evidence from Shallow Subsurface Sedimentary Facies, North Al-Wajh, Saudi Arabia

The stratigraphy of the late Holocene shallow subsurface coastal sediments, North Al-Wajh, Saudi Arabia was investigated to reconstruct the environmental evolution of the area in relation to sea-level changes. Five shallow cores (1.1 to 2.2 m long) were collected from the intertidal flat bordering the Dumaygh Lagoon, and the mouths of wadis Antar and Haramil. The cores exhibit siliciclastic-dominated sediments, in some parts containing reworked calcareous shell fragments of bivalves, gastropods and coral remains. The sediments were analyzed for grain size, loss on ignition (LOI), CaCO3 content, and bulk mineralogy. Mineralogically, the sediments were dominated by siliciclastic minerals (quartz, plagioclase, feldspars, and hornblende) followed by low Mg-calcite (LMC), high Mg-calcite (HMC) and aragonite. In addition, traces of mica, amphiboles, clay minerals, and dolomite were recognized. There is a slight lateral variation in the relative abundance of aragonite and high Mg calcite (HMC), particularly at the base of cores. The results provide a simple scenario for the late Holocene Northern Red Sea coastal evolution. Two vertically stacked sedimentary facies were recognized from the cores; grey mud-dominated lagoonal/bayfill at the base and yellowish-brown sand-dominated intertidal flat at the top. The vertical facies stacking suggests a forestepping pattern or progradational sequence possibly attributed to the late Holocene relative sea-level fall.

Effect of removal of organic material on stable isotope ratios in skeletal carbonate from taxonomic groups with complex mineralogies.

Stable oxygen and carbon isotope ratios are one of the most accurate ways of determining environmental changes in the past, which are used to predict future environmental change. Biogenic carbonates from marine organisms are the most common source of samples for stable isotope analysis. Before they are analyzed by mass spectrometry, any organic material is traditionally removed by one of three common pretreatment methods: roasting, bleaching, or with hydrogen peroxide at various strengths and durations. This study compares δ18 O and δ13 C values in a control with no pretreatment with those from five different pretreatment methods using conventional acid digestion mass spectrometry. The objectives are to: assess the impact of the most common pretreatment methods on δ18 O and δ13 C values from (1) taxonomically underrepresented groups in previous studies, and (2) those that precipitate a wide range of biomineralogies, in the debate of whether to pretreat or not to pretreat. We analyzed the following biomineralogically complex temperate marine organisms from southern New Zealand: four species of bryozoans, four species of molluscs, two species of arthropods, and one species each of annelid, red alga, brachiopod, and echinoderm (test plates and spines treated separately). These species precipitate aragonite, High-, Intermediate-, and/or Low-Mg calcite (LMC) in their skeletons. We used linear mixed statistical models to compare the effects of the pretreatments and mineralogical composition on their δ18 O and δ13 C values. Roasting was the most effective pretreatment for the removal of organic matter and light H2 O2 the least, but the former had corresponding impacts on isotope ratios. δ18 O values were not directly affected by wt% MgCO3 , but they were significantly affected by the interaction of roasting and wt% MgCO3 . This same negative effect of roasting on species with higher wt% MgCO3 occurred in δ13 C values, but it was much more pronounced in δ18 O values. Both H2 O2 pretreatments significantly and negatively affected δ18 O values at higher wt% MgCO3 . Neither bleaching pretreatment significantly affected δ18 O values. δ13 C values were most negatively affected in skeletons with high wt% MgCO3 . There was also a strong negative roasting effect and more so at higher wt% MgCO3 . Bleaching and H2 O2 did not significantly affect δ13 C values. Based on these results, and when using skeletal carbonate of complex mineralogies, we recommend considering the abandonment of pretreatment of biogenic carbonate for stable isotope analysis due to confounded results from previous studies, difficulties with preparation, and/or the absence of significant effects of organic material on isotope ratios. If pretreatment is necessary, avoid roasting especially at higher temperatures and durations, use minimal bleaching, and in general avoid using High-Mg calcite species in O and C stable isotope studies. If bleaching is used, clearly indicate the concentration and duration of exposure.

Oxic and post-oxic chemical changes related to eogenesis and mesogenesis in a Miocene paleolake

In the Forez Basin of the French Massif Central, clay-rich alluvial deposits include a series of red- and green-colored sediments and carbonate cement that record evaporation and oxidizing/reducing conditions related to intra-continental climate during the Early to Middle Miocene. The mineralogy, chemistry and relative chronology of authigenic calcite-ankerite, ferroan dolomite, pyrite, and analcite-clinoptilolite in clay-rich sediment enabled reconstruction of a series of processes related to deposition, eogenesis, and mesogenesis. The low-Mg calcite, ankerite, and ferroan-dolomite cement, systematically associated with zeolites (analcite and clinoptilolite), represent eogenetic precipitation associated with oxic to post-oxic water with Ca–Fe–Mg carbonic, then Na–Al–Si(OH)4-rich water. Occurrence of framboidal and cubic pyrites with low-Mg calcite and analcite is related to post-oxic conditions associated with deposition, eogenesis and times of early mesogenesis. Changes in the morphology, size, and chemistry of framboidal and cubic pyrite grains were related to reducing-oxidizing cycles and to the growth of grains. Sulfur isotope measures on framboidal and cubic pyrite suggest that both morphologies are related to bacterial reduction of SO42− to H2S and HS−1. With the exception of some sample depths (0–40 m below surface), similar chemical contents (trace elements [TE], rare earth elements [REE] and platinoids) suggest a similar, constant reservoir of metal and metalloids associated with the clayey sediment and volcanic fragments. Moreover, the TE and REE chemistry of cubic pyrite, in contrast to carbonates, indicates growth of pyrite during eogenesis to mesogenesis stages. The mineralogical and chemical changes are interpreted as reflecting dissolution of iron oxyhydroxides, rather than ankerite and ferroan-dolomite. Chemical elements from dissolution of iron oxyhydroxides would have mixed with hydrogen monosulfide and contributed to the growth of cubic pyrite. The crystal size distributions of pyrite grains suggest a closed system with surface-controlled growth. Assuming those conditions, the growth time of framboidal to cubic pyrite, based on diffusion of HS−1 in the clay-rich sediments, was estimated to have been from several days to a few years, to as much as 500 years (mm-scale pyrite).

A high-resolution Middle to Late Permian paleotemperature curve reconstructed using oxygen isotopes of well-preserved brachiopod shells

Climatic warming has been widely cited as a driver for many mass extinction events in Earth history, including the middle-late Permian Guadalupian-Lopingian boundary (GLB) mass extinction and end-Permian mass extinction (EPME) investigated in this study. However, the role of warming in driving the EPME event is under debate because current paleotemperature studies indicate the rapid increase in sea-surface temperature postdate the onset of the EPME event, and a reliable paleotemperature record across the GLB event is lacking. Here, we present a high-resolution and successive δ18O record using low-Mg calcite shells of brachiopods (bLMCs) from a single stratigraphic section in central China to reconstruct paleotemperature changes from the middle to the latest Permian. After a rigorous screening for diagenesis, our data show a ∼1.0‰ decrease in δ18Ocalcite (and thus climatic warming) during the middle Capitanian, followed by a gradual ∼2.0‰ increase (climatic cooling) during the late Wuchiapingian. From the latest Wuchiapingian to the early Changhsingian, δ18O decreased gradually and reached the lowest values of -4.0‰ (and thus rapid climatic warming) immediately before the Permian-Triassic boundary. The two warming events are coeval with the onset of the Emeishan Large Igneous Province and the Siberian Traps Large Igneous Province, respectively, suggesting the two volcanic events are the plausible drivers of these climatic warming events. A close comparison of paleontological and geochemical records indicates that the warming events occurred predate the GLB and EPME events, this is in contrast with conodont δ18Oapatite data which document the rapid climatic warming lagged the onset of the extinction event. The comparison between δ18O of bLMCs and δ18O of conodonts from the same section shows that the long-term trends are similar, but, at higher temporal resolution, discrepancies are present, entailing further investigation in the near future. Our study confirms that δ18O of the brachiopod shells underwent important changes during the end of Paleozoic, highlighting that sea surface temperature is a key factor to understand the biosphere history, since it changes simultaneously with the biological crises, and that bLMCs are invaluable archive for tracking physical and chemical conditions of past oceans.

LE EVINOSPONGE: IPOTESI GENETICHE DI CEMENTI CALCITICI NELLA PIATTAFORMA LADINICA DELLE PREALPI LOMBARDE (ALPI MERIDIONALI)

The "evinosponge" are globose structures, both isolated and in anastomotic groups, whose size ranges from a few centimeters up to 3 meters. Each structure consists of several isopachous concentric crusts composed of low-Mg calcite crystals arranged in pseudofibres. Individual calcite crystals show irregular intercrystalline boundaries, deformed twins and cleavages and strong undulose extinction. Located within the medium-upper part of the ladinian Esino Limestone, the "evinosponge" are distinctive of those facies, such as open subtidal, prograding margin and slope, which are characterised by a high primary porosity and are facing the deep basins of the W engen Formation. Although the "evinosponge" are not pervasive in all the Esino Limestone platform margins, they represent up to the 70% of the whole rock in some areas. Formerly interpreted as fossil organisms (Stoppani, 1858), the "evinosponge" are here considered typical reef cements whose morphology developed through processes of dissolution and reprecipitation during early to burial diagenetic stages. The dissolution and reprecipitation processes took place in a mixed marine-meteoric water environment characterised by continuous changes in the chemical composition of the pore fluids and consequent fluctuations in carbonate saturation state. Furthemore, organisms such as the Tubiphytes and the Spongiostromata contributed to the development of the "evinosponge" acting as cement catalysts. Through the effects of perspiration and decaying organisms were also responsible for changes in the pore fluid chemistry towards undersaturation with respect to the more unstable phase of CaCO3. The cavity filling fibrous cements of the "evinosponge" underwent late diagenetic homogeneisation and crystal deformation which conferred random optical characters to the calcite pseudofibres.

The effects of diagenesis on lithium isotope ratios of shallow marine carbonates

In this study, we present new data on the δ7Li values and Li/(Ca+Mg) ratios of carbonate cores from the Great Bahama Bank (Clino, Unda), a deep water core off of the bank top (ODP Leg 166 Site 1007), and the coralline Key Largo Limestone. We use these samples to evaluate the influence of meteoric diagenesis, marine burial diagenesis, and dolomitization on the Li isotope system in carbonates. We find that recrystallization of aragonite to low-Mg calcite in the presence of meteoric fluids results in a systematic decrease of the Li/(Ca+Mg) ratio in Clino, Unda and Key Largo samples, due to the lower Li/(Ca+Mg) ratio in meteoric fluids compared to seawater. For Li isotopes, we observe that the δ7Li of meteorically altered low-Mg calcite is +22.0±3.8‰ (n=28, 1σ), which is coincidentally similar to the original aragonite-rich sediments (+22±1‰ in the Bahamas, +18±1‰ in Key Largo), but with a larger variability (from +15 to +27‰). We interpret these features as reflecting the overprinting of primary Li during meteoric alteration with a highly variable isotope signature that may be controlled by a combination of local porewater and/or global climatic conditions; in either case, meteoric diagenesis produces isotopic signatures that are unrelated to seawater composition. In contrast, marine burial diagenesis and dolomitization of Clino and Unda sediments under “fluid-buffered” conditions result in Li isotope composition that is similar (+30.2±1.5‰, n=36, 1σ) to modern seawater (+31‰). For Site 1007, the δ7Li values range between +23 permil and +31 permil. We interpret this range as reflecting a combination of varying diagenesis style (fluid to sediment-buffered) and varying contribution of calcite derived from pelagic sediments, with distinct isotopic composition due to primary mineralogy. Altogether, our results show that diagenesis does not invalidate the use of bulk carbonates for deriving Li isotope paleo-records, but the reliability of past carbonates as recorders of seawater δ7Li values will depend on carefully characterizing their diagenetic history.