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1. The Transformation of Aragonite to Calcite in the Presence of Magnesium: Implications for Marine Diagenesis

   Hashim, M.; Kaczmarek, S.E. (September 2021, Earth and planetary science letters)

   null (Ed.)

   Magnesium (Mg) in natural waters plays a critical role in governing carbonate mineral formation, dissolution, and diagenesis. Previous laboratory experiments show that Mg can strongly inhibit direct calcite precipitation as well as aragonite to calcite diagenetic transformation. Data from natural settings, however, suggest that diagenetic calcite in most Phanerozoic limestones has formed in the shallow marine burial realm in the presence of ample Mg. Thus, the diagenetic conditions under which aragonite-rich sediments convert to calcite-rich limestones are poorly understood. Here, we present data from laboratory experiments whereby aragonite is converted to calcite at 70◦C in Mg-bearing solutions to investigate the effects of fluid:solid ratio (F:S), which varies greatly across diagenetic environments, on Mg inhibition and incorporation in calcite. Our data show that not only can the transformation of aragonite to calcite occur in solutions with higher [Mg] than previously shown possible in laboratory experiments, but that progressively lower F:S increase the rate at which aragonite stabilizes to calcite. For example, in experiments with an F:S of 0.3 mL/g, which corresponds to sediments in a closed system with 50% porosity, aragonite stabilizes to calcite in solution with [Mg]=30 mM (Mg/Ca=5.14) when an initial high degree of undersaturation with respect to aragonite is used and in a solution with [Mg]=20 (Mg/Ca=5.14) when a low degree of undersaturation is used. In contrast, aragonite does not stabilize to calcite after nearly 3000 h in experiments with an F:S of 100 mL/g, which is more typical of an open system, even in a solution with [Mg]=5 mM (Mg/Ca=5.14) regardless of the degree of undersaturation. Our results also show that the amount of Mg incorporated into calcite products increases linearly with the increase of F:S. Collectively, these observations further point to F:S as an important factor in carbonate diagenesis with broad implications. First, the observations that transformation of aragonite to calcite is inhibited at high [Mg] and F:S imply that calcite precipitation is unlikely to occur in marine diagenetic environments that are in direct hydrologic contact with seawater. This leaves aragonite dissolution as the dominant diagenetic process in these environments, which may represent an underrated source of alkalinity to the open ocean. Second, transformation from aragonite-rich sediments to the calcite-rich limestones that dominate the rock record is likely promoted by a decrease in the F:S and the development of a closed system during progressive burial. 

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2. Mg zonation and heterogeneity in low-Mg calcite microcrystals of a depositional chalk

   https://doi.org/10.2110/jsr.2020.173  

   Rinderknecht, Chanse J.; Hasiuk, Franek J.; Oborny, Stephan C. (August 2021, Journal of Sedimentary Research)

   null (Ed.)

   ABSTRACT Diagenetic low-magnesium calcite (LMC) microcrystals constitute the framework that hosts most micropores in limestone reservoirs and therefore create the storage capacity for hydrocarbons, water, and anthropogenic CO2. Limestones dominated by LMC microcrystals are also commonly used for paleoclimate reconstructions and chemostratigraphic correlations. LMC microcrystals are well known to exhibit a range of textures (e.g., granular, fitted, clustered), but there exists uncertainty with regard to how these textures form. One hypothesis is that during crystal growth, Mg is incorporated into diagenetic overgrowths (cement), where the chemical zonation and microtexture may reflect diagenetic processes. To evaluate small-scale geochemical zonation in LMC microcrystals, this study uses scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) to measure the Mg/Ca ratio across the interiors of LMC granular microcrystals from a Late Cretaceous marine chalk from the Tor Fm. (Norwegian North Sea). Mg/Ca zonation was identified in all LMC microcrystals with a diameter > 5 μm. Generally, the cores of large crystals have lower Mg/Ca (≈ 5.9 mmol/mol) and the rims have elevated Mg/Ca (≈ 13 mmol/mol). Smaller microcrystals (< 5 μm) show no resolvable zonation, but do exhibit a wide range in Mg/Ca content from 2.9 to 32.2 mmol/mol. Measured Mg/Ca values are arbitrarily divided into three populations: low Mg (average ≈ 5.9 mmol/mol), intermediate Mg (average ≈ 13.3 mmol/mol), and high Mg (average ≈ 20 mmol/mol). The observed zonation and Mg enrichment within LMC microcrystals is interpreted to reflect depositional as well as multiple diagenetic signals, such as constructive precipitation through recrystallization and pore-filling cementation. Although chalk from the Tor Fm. is dominated by granular euhedral LMC microcrystals, using SEM-EDS to find Mg/Ca heterogeneity in other LMC microcrystal textures may provide insight into the diagenetic processes that create textural variations in micropore-dominated limestones. The Mg data also more broadly suggest that there is useful, measurable diagenetic information in material that is otherwise considered homogeneous. Distinguishing between possible primary compositions and secondary cementation has implications for studies that rely on the primary chemistry of fine-grained carbonate deposits (e.g., micrite), such as paleoclimatology, Mg paleothermometry, and chemostratigraphy. 

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3. Evolution of calcite microcrystal morphology during experimental dissolution. Doi: https://doi.org/10.2110/jsr.2020.154

   Hashim, M.; Kaczmarek, S.E. (March 2021, Journal of sedimentary research)

   null (Ed.)

   Phanerozoic limestones are composed of low-Mg calcite microcrystals (i.e., micrite) that typically measure between 1 and 9 lm 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 intercrystal gulfs after only 5%dissolution indicates that the presence of such features in natural limestones need not imply that significant porosity generation has occurred. 

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4. Reconstructing paleoenvironments of the Late Cretaceous Western Interior Seaway, USA, using paired triple oxygen and carbonate clumped isotope measurements

   https://doi.org/10.1130/B37543.1  

   Wostbrock, Jordan AG; Witts, James D; Gao, Yang; Peshek, Catherine; Myers, Corinne E; Henkes, Gregory; Sharp, Zachary D (July 2024, Geological Society of America Bulletin)

   Fossiliferous carbonate concretions are commonly found in sediments deposited in the Late Cretaceous Western Interior Seaway. Although concretions are diagenetic features, well-preserved fossils from within them have been instrumental in reconstructing the temperature and δ18O value of Western Interior Seaway seawater, which is essential for accurate reconstruction of Late Cretaceous climate. Here, we constrain formation conditions of Late Campanian and early Maastrichtian carbonate concretions by combining triple oxygen isotope measurements with carbonate clumped isotope paleothermometry on different carbonate phases within the concretions. We measured both fossil skeletal aragonite and sparry calcite infill from cracks and within macrofossil voids to evaluate differences between “primary” and “altered” geochemical signals. Based on the two temperature-sensitive isotope systems of the primary fossil shell aragonite, the temperature of the Western Interior Seaway was between 20 °C and 40 °C and was likely thermally stratified during the Campanian. The reconstructed δ18Oseawater values of ∼−1‰ for Campanian Western Interior Seaway waters are similar to those expected for the open ocean during greenhouse climates, while the Maastrichtian Western Interior Seaway may have been more restricted, with a δ18Oseawater value of ∼2‰, which reflects more evaporative conditions. We reconstructed the diagenetic history of the sparry infill and altered fossils using a fluid-rock mixing model. Alteration temperature, alteration fluid δ18O value, and the initial formation temperature were calculated by applying the fluid-rock mixing model to a particle swarm optimization algorithm. We found a different range of initial formation temperatures between the Campanian (25−38 °C) and Maastrichtian (9−28 °C). We also found that alteration in the presence of light meteoric fluids (δ18O ≈ −10‰) is required to explain both the sparry infill and the altered fossil isotopic values. Based on our results, both lithification and alteration of the carbonates occurred soon after burial, and light meteoric fluids support prior findings that high-topographic relief existed on the western margin of the Western Interior Seaway during the Late Cretaceous. As one of the first studies to apply these techniques in concert and across multiple mineralogical phases within samples, our results provide important constraints on paleoenvironmental conditions in an enigmatic ocean system and will improve interpretations of the overall health of ecosystems leading into the end-Cretaceous mass extinction. 

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5. Sea level, carbonate mineralogy, and early diagenesis controlled δ13C records in Upper Ordovician carbonates

   https://doi.org/10.1130/G46861.1  

   Jones, David S.; Brothers, R. William; Crüger Ahm, Anne-Sofie; Slater, Nicholas; Higgins, John A.; Fike, David A. (December 2019, Geology)

   Abstract Stratigraphic variability in the geochemistry of sedimentary rocks provides critical data for interpreting paleoenvironmental change throughout Earth history. However, the vast majority of pre-Jurassic geochemical records derive from shallow-water carbonate platforms that may not reflect global ocean chemistry. Here, we used calcium isotope ratios (δ44Ca) in conjunction with minor-element geochemistry (Sr/Ca) and field observations to explore the links among sea-level change, carbonate mineralogy, and marine diagenesis and the expression of a globally documented interval of elevated carbon isotope ratios (δ13C; Hirnantian isotopic carbon excursion [HICE]) associated with glaciation in Upper Ordovician shallow-water carbonate strata from Anticosti Island, Canada, and the Great Basin, Nevada and Utah, USA. The HICE on Anticosti is preserved in limestones with low δ44Ca and high Sr/Ca, consistent with aragonite as a major component of primary mineralogy. Great Basin strata are characterized by lateral gradients in δ44Ca and δ13C that reflect variations in the extent of early marine diagenesis across the platform. In deep-ramp settings, deposition during synglacial sea-level lowstand and subsequent postglacial flooding increased the preservation of an aragonitic signature with elevated δ13C produced in shallow-water environments. In contrast, on the mid- and inner ramp, extensive early marine diagenesis under seawater-buffered conditions muted the magnitude of the shift in δ13C. The processes documented here provide an alternative explanation for variability in a range of geochemical proxies preserved in shallow-water carbonates at other times in Earth history, and challenge the notion that these proxies necessarily record changes in the global ocean. 

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