scholarly journals Long-term experimental diagenesis of aragonitic biocarbonates: from organic matter loss to abiogenic calcite formation

2021 ◽  
Author(s):  
Pablo Forjanes ◽  
María Simonet Roda ◽  
Martina Greiner ◽  
Erika Griesshaber ◽  
Nelson A. Lagos ◽  
...  
Keyword(s):  
Hydrothermal Alteration ◽  
Electron Backscatter Diffraction ◽  
Arctica Islandica ◽  
Secondary Porosity ◽  
Prismatic Layer ◽  
Diagenetic Alteration ◽  
Geochemical Proxies ◽  
Hard Tissues ◽  
Organic Matter Loss

Abstract. Carbonate biological hard tissues are valuable archives of environmental information. However, this information can be blurred or even completely lost as hard tissues undergo diagenetic alteration. This is more likely to occur in aragonitic skeletons because bioaragonite commonly transforms into calcite during diagenesis. For reliably using aragonitic skeletons as geochemical proxies, it is necessary to understand in depth the diagenetic alteration processes that they undergo. Several works have recently investigated the hydrothermal alteration of aragonitic hard tissues during short term experiments at high temperatures (T > 160 °C). In this study, we conduct long term (4 and 6 months) hydrothermal alteration experiments at 80 °C using burial-like fluids. We document and evaluate the changes undergone by the outer and inner layers of Arctica islandica shell, the prismatic and nacreous layers of Haliotis ovina shell, and the skeleton of Porites sp. combining a variety of analytical tools (X-ray diffraction, thermogravimetry analysis, laser confocal microscopy, scanning electron microscopy, electron backscatter diffraction and atomic force microscopy). We demonstrate that this approach is the most adequate to trace subtle, diagenetic alteration-related changes in aragonitic biocarbonates. Furthermore, we unveil that the diagenetic alteration of aragonitic hard tissues is a complex multi-step process where major changes occur even at the low temperature used in this study and well before any aragonite into calcite transformation takes place. Alteration starts with biopolymer decomposition and concomitant generation of secondary porosity. These processes are followed by abiogenic aragonite precipitation that partially or totally obliterates the secondary porosity. Only afterwards any transformation of aragonite into calcite takes place. The kinetics of the alteration is highly dependent on primary microstructural features of the aragonitic biomineral. While the skeleton of Porites sp. remains virtually unaltered within the time spam of the experiments, Haliotis ovina nacre undergoes extensive abiogenic aragonite precipitation, the outer and inner layers of Arctica islandica shell are significantly affected by aragonite transformation into calcite and this transformations extensive in the case of the prismatic layer of Haliotis ovina shell. Our results suggest that most aragonitic fossil archives may be overprinted, even those free of clear diagenetic alteration signs. This finding may have major implications for the use of these archives as geochemical proxies.

scholarly journals Hydrothermal alteration of aragonitic biocarbonates: assessment of micro- and nanostructural dissolution–reprecipitation and constraints of diagenetic overprint from quantitative statistical grain-area analysis

2018 ◽  
Vol 15 (24) ◽  
pp. 7451-7484 ◽  
Author(s):  
Laura A. Casella ◽  
Sixin He ◽  
Erika Griesshaber ◽  
Lourdes Fernández-Díaz ◽  
Martina Greiner ◽  
...  
Keyword(s):  
Hydrothermal Alteration ◽  
Structural Changes ◽  
Geochemical Data ◽  
Present Contribution ◽  
Secondary Porosity ◽  
Biogenic Carbonate ◽  
Carbonate Formation ◽  
Hard Tissues ◽  
Primary Porosity ◽  
Carbonate Materials

Abstract. The assessment of diagenetic overprint on microstructural and geochemical data gained from fossil archives is of fundamental importance for understanding palaeoenvironments. The correct reconstruction of past environmental dynamics is only possible when pristine skeletons are unequivocally distinguished from altered skeletal elements. Our previous studies show (i) that replacement of biogenic carbonate by inorganic calcite occurs via an interface-coupled dissolution–reprecipitation mechanism. (ii) A comprehensive understanding of alteration of the biogenic skeleton is only given when structural changes are assessed on both, the micrometre as well as on the nanometre scale.In the present contribution we investigate experimental hydrothermal alteration of six different modern biogenic carbonate materials to (i) assess their potential for withstanding diagenetic overprint and to (ii) find characteristics for the preservation of their microstructure in the fossil record. Experiments were performed at 175 °C with a 100 mM NaCl + 10 mM MgCl2 alteration solution and lasted for up to 35 days. For each type of microstructure we (i) examine the evolution of biogenic carbonate replacement by inorganic calcite, (ii) highlight different stages of inorganic carbonate formation, (iii) explore microstructural changes at different degrees of alteration, and (iv) perform a statistical evaluation of microstructural data to highlight changes in crystallite size between the pristine and the altered skeletons.We find that alteration from biogenic aragonite to inorganic calcite proceeds along pathways where the fluid enters the material. It is fastest in hard tissues with an existing primary porosity and a biopolymer fabric within the skeleton that consists of a network of fibrils. The slowest alteration kinetics occurs when biogenic nacreous aragonite is replaced by inorganic calcite, irrespective of the mode of assembly of nacre tablets. For all investigated biogenic carbonates we distinguish the following intermediate stages of alteration: (i) decomposition of biopolymers and the associated formation of secondary porosity, (ii) homoepitactic overgrowth with preservation of the original phase leading to amalgamation of neighbouring mineral units (i.e. recrystallization by grain growth eliminating grain boundaries), (iii) deletion of the original microstructure, however, at first, under retention of the original mineralogical phase, and (iv) replacement of both, the pristine microstructure and original phase with the newly formed abiogenic product.At the alteration front we find between newly formed calcite and reworked biogenic aragonite the formation of metastable Mg-rich carbonates with a calcite-type structure and compositions ranging from dolomitic to about 80 mol % magnesite. This high-Mg calcite seam shifts with the alteration front when the latter is displaced within the unaltered biogenic aragonite. For all investigated biocarbonate hard tissues we observe the destruction of the microstructure first, and, in a second step, the replacement of the original with the newly formed phase.

Precise Temporal Correlation of Holocene Mollusk Shells Using Sclerochronology

2000 ◽  
Vol 53 (2) ◽  
pp. 236-246 ◽  
Author(s):  
Thomas M. Marchitto ◽  
Glenn A. Jones ◽  
Glenn A. Goodfriend ◽  
Christopher R. Weidman
Keyword(s):  
Shell Growth ◽  
Morphological Characteristics ◽  
Temporal Correlation ◽  
Band Width ◽  
Georges Bank ◽  
Arctica Islandica ◽  
Mollusk Shells ◽  
Aspartic Acid Racemization ◽  
Geochemical Proxies ◽  
Growth Bands

AbstractAnnual growth bands of mollusk shells record several types of paleoenvironmental information, including geochemical proxies for water properties and morphological characteristics of growth and mortality. Sclerochronology, the marine counterpart of dendrochronology, offers a way to link individual shells together to form long continuous records of such parameters. It also allows for precise dating of recent shells and identification of contemporaneous fossil individuals. The longevity of the ocean quahog Arctica islandica (commonly >100 yr) makes this species well suited for sclerochronology. Band width records of contemporaneous A. islandica specimens from the same region exhibit high correlations (ρ = 0.60–0.80 for spans of ≥30 bands), indicating some common environmental influences on shell growth. By adopting several strict criteria, fossil (dead-collected) shells can be linked into composite sclerochronologies. A seven-shell 154-yr chronology was constructed for Georges Bank using three live-collected and four dead-collected shells. Band width matching indicates that the dead-collected individuals died in A.D. 1950, 1971, 1978, and 1989. Sclerochronological age assignments were verified using aspartic acid racemization dating. Construction of a 1000-yr sclerochronology is judged to be feasible using the described methods.

Paleocommunity information retrieval vs. shell accumulation mode in Paleozoic carbonates: examples from the Lebanon Limestone (Middle Ordovician), Tennessee, U.S.A.

1992 ◽  
Vol 6 ◽  
pp. 81-81
Author(s):  
David J. Davies ◽  
Molly F. Miller
Keyword(s):  
Information Retrieval ◽  
High Energy ◽  
Sediment Surface ◽  
Time Averaging ◽  
Short Term ◽  
Diagenetic Alteration ◽  
Shell Beds ◽  
Carbonate Shell ◽  
Shell Bed

Compared to their terrigenous counterparts, carbonate shell accumulations have until recently been relatively little studied to determine either descriptive or genetic classifications of shell bed types, the preservation potential of each type, or their relative ability to preserve community-level information. A partial classification of Paleozoic carbonate shell-rich soft sediment accumulations is proposed using sedimentation patterns in the Lebanon limestone of the Stones River Group. Paleoecological information preserved therein is then contrasted by shell bed type. The Lebanon represents typical Ordovician shallow to moderate subtidal carbonate shelf deposits in outcrops flanking the Nashville Dome and peritidal deposits in the Sequatchie Anticline of Eastern Tennessee; shell beds alternate with shell poor sediments (micrites, wackestones and diagenetically enhanced dolomites and clay-rich partings).None of the analyzed shell beds was strictly biological in origin; most are sedimentological although >10% are combined sedimentological/diagenetic. While the majority are single simple shell beds, >20% are amalgamated. All are thin (1 shell to 15 cm) stringers that pinch and swell showing poor lateral continuity (outcrop scale, tens to hundreds of meters) likely enhanced by burial dissolution. These shell beds differ greatly in fabric (packing/sorting), clast composition, taphonomic signature, and intensity of time averaging; thus community information retrieval is biased in predictable patterns. Virtually no shell beds show common shell dissolution or encrustation from long-term sediment surface exposure or hardground formation. Five major categories of accumulation are herein proposed using a DESCRIPTIVE, non-genetic terminology modified from previous works of DJD, as well as a Genetic interpretation for each. These are easily distinguished in the field and are also discriminated by Q-mode cluster analysis.Categories include, in decreasing frequency of occurrence: 1. SHELL GRAVELS; Storm/“event” beds: Sharp bases; poorly sorted coarse basal bioclasts and/or intraclasts, often with no preferred orientation; clasts fine upward to comminuted shell material and micrite. Horizontal platy brachiopods often cap the beds. High diversity and a wide range in shell alteration is represented, from whole unaltered brachiopods to minor abraded fragments, indicating extreme time averaging and poor resolution of short-term community dynamics. 2. COMMINUTED SHELLY LS; Current/ripple concentrations: Small tidal channel fill and discrete ripple trough accumulations are composed of cross-stratified bioclastic deposits with local concentrations of rip-ups. Beds are not graded; typically clasts are abraded, rounded and concordant with cross-beds. Intense time averaging and mixing of discrete communities is inferred due to continual reworking in these background deposits. 3. SHELL/CEMENT LS; Early cementation beds: Intense early diagenetic alteration is inferred due to red discoloration and rapid intergranular cementation; some beds show diagenetic micritic rinds. Beds may be brecciated and show deep burial stylolitization cutting bioclasts and cement. They may represent zones of preferred early cementation rather than a change in shell accumulation rate. Many shells from some beds show little postmortem alteration; these units may preserve much of the original community structure. 4. DENSE SHELL PAVEMENTS; Subtidal surficial pavements: Single layers of shells, commonly concave down, overlie mudstones/wackestones with no basal erosion. No obrution deposits were noted. Bioclasts are typically disarticulated and reoriented, but are not substantially abraded, broken, or dissolved. Diversity is low. Only minor temporal and lateral community mixing with small environmental fluctuation is indicated. 5. VERTICALLY IMBRICATE SHELLY LS; High energy beach zones: Platy whole and major fragments of brachiopods are deposited in low diversity, high angle imbricate beds. Less postmortem reworking and time averaging is evident compared to types 1 and 2.Thus, the most common (physically reworked) shell bed types show the most intense loss of short-term paleocommunity information. There are surprisingly few insitu community pavements or obligate long-term accumulations. This pattern differs from some described Ordovician carbonates, which may contain common community beds or hardgrounds/hiatal accumulations. This implies a relatively low rate of net sediment accumulation on a shallow, periodically wave swept shelf, and no major flooding surfaces or other indications of significant sea level change. Delineation of the sequence stratigraphic position of these carbonates is enhanced from this type of integrated community/biostratinomic analysis.

scholarly journals Oxygen isotope composition of the Phanerozoic ocean and a possible solution to the dolomite problem

2018 ◽  
Vol 115 (26) ◽  
pp. 6602-6607 ◽  
Author(s):  
Uri Ryb ◽  
John M. Eiler
Keyword(s):  
Oxygen Isotope ◽  
Isotope Exchange ◽  
Isotope Composition ◽  
Temporal Variations ◽  
Oxygen Isotope Exchange ◽  
Diagenetic Alteration ◽  
Isotope Studies ◽  
Dolomite Problem ◽  
Clumped Isotope

The18O/16O of calcite fossils increased by ∼8‰ between the Cambrian and present. It has long been controversial whether this change reflects evolution in the δ18O of seawater, or a decrease in ocean temperatures, or greater extents of diagenesis of older strata. Here, we present measurements of the oxygen and ‟clumped” isotope compositions of Phanerozoic dolomites and compare these data with published oxygen isotope studies of carbonate rocks. We show that the δ18O values of dolomites and calcite fossils of similar age overlap one another, suggesting they are controlled by similar processes. Clumped isotope measurements of Cambrian to Pleistocene dolomites imply crystallization temperatures of 15–158 °C and parent waters having δ18OVSMOWvalues from −2 to +12‰. These data are consistent with dolomitization through sediment/rock reaction with seawater and diagenetically modified seawater, over timescales of 100 My, and suggest that, like dolomite, temporal variations of the calcite fossil δ18O record are largely driven by diagenetic alteration. We find no evidence that Phanerozoic seawater was significantly lower in δ18O than preglacial Cenozoic seawater. Thus, the fluxes of oxygen–isotope exchange associated with weathering and hydrothermal alteration reactions have remained stable throughout the Phanerozoic, despite major tectonic, climatic and biologic perturbations. This stability implies that a long-term feedback exists between the global rates of seafloor spreading and weathering. We note that massive dolomites have crystallized in pre-Cenozoic units at temperatures >40 °C. Since Cenozoic platforms generally have not reached such conditions, their thermal immaturity could explain their paucity of dolomites.

scholarly journals The Effect of Simultaneous Si and Ti/Mo Alloying on High-Temperature Strength of Fe3Al-Based Iron Aluminides

Molecules ◽  
2020 ◽  
Vol 25 (18) ◽  
pp. 4268
Author(s):  
Věra Vodičková ◽  
Martin Švec ◽  
Pavel Hanus ◽  
Pavel Novák ◽  
Antonín Záděra ◽  
...  
Keyword(s):  
Phase Composition ◽  
High Temperature ◽  
Yield Stress ◽  
Electron Backscatter Diffraction ◽  
Xrd Analysis ◽  
Iron Aluminides ◽  
High Temperature Strength ◽  
X Ray Diffraction ◽  
Backscatter Diffraction

The effect of phase composition and morphology on high-temperature strength in the compression of Fe-Al-Si-based iron aluminides manufactured by casting was investigated. The structure and high-temperature strength in the compression of three alloys—Fe28Al5Si, Fe28Al5Si2Mo, and Fe28Al5Si2Ti—were studied. Long-term (at 800 °C for 100 h) annealing was performed for the achievement of structural stability. The phase composition and grain size of alloys were primarily described by means of scanning electron microscopy equipped with energy dispersive analysis and Electron Backscatter Diffraction (EBSD). The phase composition was verified by X-ray diffraction (XRD) analysis. The effect of Mo and Ti addition as well as the effect of long-term annealing on high-temperature yield stress in compression were investigated. Both additives—Mo and Ti—affected the yield stress values positively. Long-term annealing of Fe28Al5Si-X iron aluminide alloyed with Mo and Ti deteriorates yield stress values slightly due to grain coarsening.

scholarly journals Molecular taphonomy of animal and plant cuticles: selective preservation and diagenesis

1999 ◽  
Vol 354 (1379) ◽  
pp. 7-17 ◽  
Author(s):  
Derek E. G. Briggs
Keyword(s):  
Decay Resistance ◽  
Major Influence ◽  
Partial Explanation ◽  
Diagenetic Alteration ◽  
Plant Cuticles ◽  
Selective Preservation ◽  
Almost All ◽  
Long Term Preservation

The nature of organic material and the environment in which it is deposited exert a major influence on the extent to which biomacromolecules are preserved in the fossil record. The role of these factors is explored with a particular focus on the cuticle of arthropods and leaves. Preservation of the original chemistry of arthropod cuticles is favoured by their thickness and degree of sclerotization, and the presence of biominerals. Decay and burial in terrestrial as opposed to marine, and anoxic rather than oxygenated conditions, likewise appear to enhance preservation. The most important factor in the long–term preservation of the chemistry of both animal and plant cuticles, however, is diagenetic alteration to an aliphatic composition. This occurs even in amber, which encapsulates the fossil, eliminating almost all external factors. Some plants contain an original decay–resistant macromolecular aliphatic component but this is not the case in arthropods. It appears that the aliphatic components of many plant as well as animal fossils may be the result of diagenetic polymerization. Selective preservation as a result of decay resistance may explain the initial survival of organic materials in sediments, but in many cases longer–term preservation relies on chemical changes. Selective preservation is only a partial explanation for the origin of kerogen.

Role of fracturing and regional tectonic structures on secondary porosity generation in a CO2 storage plant: Hontomin pilot-plant (Spain)

2020 ◽  
Author(s):  
Adrià Ramos ◽  
José F. Mediato ◽  
Raúl Pérez-López ◽  
Miguel A. Rodríguez-Pascua ◽  
Roberto Martínez-Orío ◽  
...  
Keyword(s):  
Pilot Plant ◽  
Lateral Displacement ◽  
Co2 Storage ◽  
Fracture Pattern ◽  
Test Facility ◽  
Fracture System ◽  
Secondary Porosity ◽  
Wide Range ◽  
The Impact

<p>The long-term managing from the geological hazard point of view of the Hontomín onshore pilot-plant for CO<sub>2 </sub>storage, located in Spain and recognized as the first and only key-test facility in Europe, is one of the main objectives stated in the ENOS European project. This project is led and funded by the European Network of Excellence on the Geological Storage of CO<sub>2</sub> (CO<sub>2</sub>GeoNet).</p><p>The complex geological emplacement of the Hontomín Carbon capture and storage plant is considered rather relevant to analyse the impact of fracturing and both local and regional strain field on the reservoir parameters. The reservoir of Hontomín pilot-plant is formed by highly fractured Middle Jurassic dolomites with associated secondary porosity. This parameter is one of the main concerns when managing CO<sub>2</sub> storage and monitoring.</p><p>In order to characterize the fracture pattern and its implications on a proper CO<sub>2</sub> monitoring, we characterized the surface structural elements through the study area and their relationship with fractures affecting the reservoir porosity. The methodology followed in this work is mainly based on detailed geological mapping (field work complimented with orthophoto analysis), adding missing information from previous works. This analysis does not increase the cost for long-term monitoring, given that they are low-cost and the results are acquired in a few months.</p><p>The main structural trend in the study area, concerning faults with a wide range of displacement and metric to decametric folds, follows a regional E-W orientation. On the other hand, fractures show two main sets of trends, from NW-SE to NE-SW.</p><p>This fracturing pattern, considered as a conjugate fracture system, corresponds to the tectonic stress recorded in both Mesozoic and Cenozoic sedimentary successions where the Hontomín pilot-plant is placed. Riddle faults formed from a nearby regional right-lateral strike slip fault (Ubierna Fault) are the responsible structures for the fracture system affecting the area and the reservoir. Moreover, this fracturing pattern is in agreement with local to regional active tectonic field from Cenozoic times to present-day, when the Ubierna Fault recorded its maximum right-lateral displacement (15 km).</p><p>Secondary porosity within the reservoir can be produced from this fracture pattern, highly increasing the permeable migration paths for CO<sub>2</sub> migration after the injection. Therefore, we state that a combination between fracture analysis and structural and tectonic study, should be considered as mandatory in the monitoring phases of the CO<sub>2</sub> plume, during and after injection operations.</p>

Crystal chemistry of clinker relicts from aged cementitious materials

2014 ◽  
Vol 47 (5) ◽  
pp. 1626-1637 ◽  
Author(s):  
Michele Secco ◽  
Luca Peruzzo ◽  
Laurie Palasse ◽  
Gilberto Artioli ◽  
Alberto Viani ◽  
...  
Keyword(s):  
Electron Backscatter Diffraction ◽  
Microstructural Analysis ◽  
Chemical Properties ◽  
Cementitious Materials ◽  
Operating Conditions ◽  
General Tendency ◽  
Hydration Kinetics ◽  
Definition Of ◽  
Backscatter Diffraction

Despite the general tendency to consider Portland cement virtually fully hydrated within a few years, the occurrence of non-reacted clinker phases in cementitious materials that are several decades old is rather common. In this work, the integration of microstructural analysis by scanning electron microscopy and quantitative microchemical and micromineralogical characterization techniques, such as electron microprobe analysis and electron backscatter diffraction, allowed the definition of the crystal-chemical properties of partially hydrated cement residuals within different types of aged cementitious materials. The results on several clinker relicts show that the calcium silicate phases are transformed systematically and pervasively by hydration reactions, whereas the aluminate and ferrite phases do frequently persist in the anhydrous state. These relict phases may be distinguished through their peculiar chemical, mineralogical and textural features. These observations raise interesting questions concerning the long-term hydration kinetics of clinker phases and the durability behaviour of cementitious materials in operating conditions.

scholarly journals Experimental diagenesis: insights into aragonite to calcite transformation of <i>Arctica islandica</i> shells by hydrothermal treatment

2017 ◽  
Vol 14 (6) ◽  
pp. 1461-1492 ◽  
Author(s):  
Laura A. Casella ◽  
Erika Griesshaber ◽  
Xiaofei Yin ◽  
Andreas Ziegler ◽  
Vasileios Mavromatis ◽  
...  
Keyword(s):  
Hydrothermal Alteration ◽  
Fossil Record ◽  
Marine Invertebrates ◽  
Structural Features ◽  
Geochemical Data ◽  
Replacement Reaction ◽  
Bulk Analysis ◽  
Arctica Islandica ◽  
Experimental Conditions ◽  
Living Organisms

Abstract. Biomineralised hard parts form the most important physical fossil record of past environmental conditions. However, living organisms are not in thermodynamic equilibrium with their environment and create local chemical compartments within their bodies where physiologic processes such as biomineralisation take place. In generating their mineralised hard parts, most marine invertebrates produce metastable aragonite rather than the stable polymorph of CaCO3, calcite. After death of the organism the physiological conditions, which were present during biomineralisation, are not sustained any further and the system moves toward inorganic equilibrium with the surrounding inorganic geological system. Thus, during diagenesis the original biogenic structure of aragonitic tissue disappears and is replaced by inorganic structural features. In order to understand the diagenetic replacement of biogenic aragonite to non-biogenic calcite, we subjected Arctica islandica mollusc shells to hydrothermal alteration experiments. Experimental conditions were between 100 and 175 °C, with the main focus on 100 and 175 °C, reaction durations between 1 and 84 days, and alteration fluids simulating meteoric and burial waters, respectively. Detailed microstructural and geochemical data were collected for samples altered at 100 °C (and at 0.1 MPa pressure) for 28 days and for samples altered at 175 °C (and at 0.9 MPa pressure) for 7 and 84 days. During hydrothermal alteration at 100 °C for 28 days most but not the entire biopolymer matrix was destroyed, while shell aragonite and its characteristic microstructure was largely preserved. In all experiments up to 174 °C, there are no signs of a replacement reaction of shell aragonite to calcite in X-ray diffraction bulk analysis. At 175 °C the replacement reaction started after a dormant time of 4 days, and the original shell microstructure was almost completely overprinted by the aragonite to calcite replacement reaction after 10 days. Newly formed calcite nucleated at locations which were in contact with the fluid, at the shell surface, in the open pore system, and along growth lines. In the experiments with fluids simulating meteoric water, calcite crystals reached sizes up to 200 µm, while in the experiments with Mg-containing fluids the calcite crystals reached sizes up to 1 mm after 7 days of alteration. Aragonite is metastable at all applied conditions. Only a small bulk thermodynamic driving force exists for the transition to calcite. We attribute the sluggish replacement reaction to the inhibition of calcite nucleation in the temperature window from ca. 50 to ca. 170 °C or, additionally, to the presence of magnesium. Correspondingly, in Mg2+-bearing solutions the newly formed calcite crystals are larger than in Mg2+-free solutions. Overall, the aragonite–calcite transition occurs via an interface-coupled dissolution–reprecipitation mechanism, which preserves morphologies down to the sub-micrometre scale and induces porosity in the newly formed phase. The absence of aragonite replacement by calcite at temperatures lower than 175 °C contributes to explaining why aragonitic or bimineralic shells and skeletons have a good potential of preservation and a complete fossil record.

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