Molar-tooth Carbonate Research Articles

Molar-Tooth Carbonate: An excellent proxy for reconstructing lithium isotopic composition of Precambrian seawater

Shallow water carbonates are extensively utilized as archives for reconstructing past seawater chemistry and understanding global geochemical cycles. However, their reliability is often compromised by diagenetic alteration, obscuring primary environmental signals. Molar-tooth carbonate (MTC) exhibits exceptional resistance to diagenesis compared to other carbonate minerals, making it a promising candidate for reliably reconstructing ancient seawater chemistry. This study investigates the preservation of primary lithium isotopic composition in Neoproterozoic (Tonian) MTC and its host carbonate from the Dalian area, northeastern China (ca. 980 Ma – 920 Ma). We assess the reliability of MTC in reconstructing paleo-seawater δ7Li evolution and provide a reconstruction of Tonian seawater δ7Li.Geochemical filter tests indicate that both MTC and host carbonate remain largely unaltered by meteoric and burial diagenesis. Carbonate δ7Li values exhibit a consistent decrease with stratigraphic height, with MTC displaying a narrower range of variation (4–12‰) compared to its host carbonate (1–15‰). Numerical modeling of early marine carbonate diagenesis suggests minimal impact on MTC δ7Li values, while host carbonates show varying degrees of diagenetic alteration. Based on the consistent Li isotopic offset observed between ‘fluid-buffered’ diagenetic host carbonate and MTC in our data, we deduce a fractionation factor of 4.8 ± 1.2‰ for seawater-MTC Li isotopic exchange.Reconstructed paleoseawater δ7Li values reveal a progressive decrease from 16.9 ± 1.2‰ to 8.8 ± 1.2‰ during the early Tonian period. This change is likely linked to the assembly of the Rodinia supercontinent and emplacement of large igneous province (LIP). Furthermore, mass balance modeling and Monte Carlo simulations suggest that this decline in seawater δ7Li can be attributed to the transition towards more congruent weathering during the early Tonian, facilitated by the assembly of Rodinia and associated large igneous province events.Overall, our findings underscore the potential of MTC as a reliable proxy for elucidating Precambrian seawater Li isotopic composition and understanding associated paleoenvironmental and paleoclimatic changes.

Giving Some Tooth to Precambrian Carbonates and the Tales They Tell About Ancient Oceans

AbstractEnigmatic is a word that often comes up in discussions about Proterozoic molar tooth carbonate structures (MTS). But when unusual features such as these are common in rocks of a particular age, there is almost always an important message waiting to be discovered. In this case, the observed temporal patterns for MTS likely track first‐order trends in evolving compositions in the oceans during Earth's middle history when CO2 in the atmosphere and carbonate saturation in the ocean were high but declining and oxygen (O2) in the ocean‐atmosphere system was on the rise. A new paper by Tang et al. (2023, https://doi.org/10.1029/2022JG007077) gives us a new way to think about MTS origins, and nested within their model are wide ranging implicit and explicit linkages to Earth surface evolution as life was becoming more complex in a slow march toward the world we know today.

Carbonate fabric diversity and environmental heterogeneity in the late Mesoproterozoic Era

AbstractTime-distinctive features within carbonate rocks, specifically the presence, abundance and distribution of stromatolites, molar-tooth fabric and specific morphologies of marine cement, have been identified as potential indicators of global-scale changes in the chemistry of marine environments. Recently, Cantineet al.(2019) introduced a database approach seeking to quantify spatial and temporal patterns in these carbonate features through the Precambrian. Despite the coarse temporal scale, results support earlier inferences of temporal change in carbonate sedimentation. Here, we use original field notes to dissect late Mesoproterozoic (˜1.3 to 1.0 Ga) carbonate strata at a high resolution, analyse time-distinctive carbonate fabrics within a database context and compare sedimentological patterns within this narrow time range to observations of the Proterozoic as a whole. Late Mesoproterozoic strata contain a variety of features (e.g. stromatolites, seafloor precipitates, herringbone carbonate, molar-tooth carbonate), often in close spatial and temporal proximity, that are commonly considered to be temporally restricted during the Precambrian. The spatial distribution of such features within Mesoproterozoic basins demonstrates the importance of recognizing even rare occurrences of time-distinctive facies and permits inference of environmental drivers that may have interacted to affect carbonate precipitation. Such spatial variability reflects a subtle division of Mesoproterozoic carbonate platform environments driven by globally high sea level, elevated carbonate saturation and a low-oxygen water column. The heterogeneous, mosaic nature of environments appears to be a hallmark of Mesoproterozoic carbonate sedimentation and emphasizes the importance of these basins in understanding longer-term trends in carbonate deposition.

Geochemistry of the Neoproterozoic Molar Tooth Carbonate in the Benxi Area, North China Craton: The Paleo-Ocean Environment

Wu, H.; Wang, G.H.; Ding, X.Z., and Wang, H., 2020. Geochemistry of the Neoproterozoic molar tooth carbonate in the Benxi Area, North China Craton: The paleo-ocean environment. In: Bai, X. and Zhou, H. (eds.), Advances in Water Resources, Environmental Protection, and Sustainable Development. Journal of Coastal Research, Special Issue No. 115, pp. 446-450. Coconut Creek (Florida), ISSN 0749-0208.Molar tooth carbonate is a special sedimentary rock, which has been widely found around the world, such as Europe, North America, Africa and Asia. The features and genesis of molar tooth carbonate are still the focus of the sedimentary study so far. The molar tooth carbonate has an important significant for the stratigraphic correlation, sedimentary condition and paleoenvironment. In the Benxi area, the discovery of the molar tooth carbonate in the Nanhua System has a great meaning to study the Precambrian geology of the Jiao-Liao Block, then reconstruct the Neoproterozoic paleogeography of North China Craton. In the last decade, the methods of geochemistry have been widely used to solve geological problems, for example petrogenesis, tectonic evolution, dynamics and so on. The main geochemistry method of carbonate is C-O isotope analysis, which can suggest the feature of ancient water temperature. It also can be used to identify the history of oxidation event or glaciation. There are some evidences show that molar tooth carbonate has relationship with oxidation event and glaciation. The oxidation event and glaciation can change and reflect the setting of paleo-ocean environment and then influence the diversity of marine lives.

Molar tooth carbonates and benthic methane fluxes in Proterozoic oceans.

Molar tooth structures are ptygmatically folded and microspar-filled structures common in early- and mid-Proterozoic (∼2,500–750 million years ago, Ma) subtidal successions, but extremely rare in rocks <750 Ma. Here, on the basis of Mg and S isotopes, we show that molar tooth structures may have formed within sediments where microbial sulphate reduction and methanogenesis converged. The convergence was driven by the abundant production of methyl sulphides (dimethyl sulphide and methanethiol) in euxinic or H2S-rich seawaters that were widespread in Proterozoic continental margins. In this convergence zone, methyl sulphides served as a non-competitive substrate supporting methane generation and methanethiol inhibited anaerobic oxidation of methane, resulting in the buildup of CH4, formation of degassing cracks in sediments and an increase in the benthic methane flux from sediments. Precipitation of crack-filling microspar was driven by methanogenesis-related alkalinity accumulation. Deep ocean ventilation and oxygenation around 750 Ma brought molar tooth structures to an end.

Genesis types of the Neoproterozoic Molar tooth structures in the southeastern Jilin and eastern Liaoning provinces and its research significances

Molar tooth (briefly called MT) carbonate is one of the Proterozoic carbonates with enigmatic sedimentary structure. According to the morphology of MT, it can be divided into two main genesis-morphology types, i.e., autochthonous and allochthonous, and each type can be further divided into a series of subtypes. The autochthonous MT can be divided into filamentous (MF1), ribbon, nodular and dotted type (MF4), in which the ribbon one can be subdivided into simple vertical (MF2) and complex ribbon (MF3), broken ribbon by storm in situ (MF5) and MT within nodular limestone (MF6). Allochthonous MT includes conglomerate clasts and horizontal detritus. Studying on the links between MT and the host rocks on five stratigraphic sections in the southeastern Jilin and eastern Liaoning indicates that the morphology of MT is closely related to host rocks, and seven genetic types by relationships between MT and the host rocks with facies interpretations are classified and discussed in this paper. The sedimentary environments of MT formation will be diverse if their morphologies are different.

A study on the environmental conditions of the microsparite (Molar-tooth) carbonates

Along with the progress in research on the Precambrian, Molar-tooth carbonates (simplified as MT, or microsparite carbonates or MT structure) which were formed in the Middle-Late Proterozoic have become a hot subject recently. The Proterozoic Molar-tooth (MT) carbonate rocks refer to those Meso-to Neoproterozoic (1600–650 Ma) carbonates with MT structure, i.e., a series of peculiar, ptygmatically folded and spar-filled cracks in fine-grained carbonates of Precambrian age, located in the environment of mid-to inner ramp and shallow platform. MTS, like a bridge connecting the inorganic world with the organic one, are closely related to the evolution of paleo-oceans, atmosphere and biosphere. Their development and/or recession are/is related to the origin of life and the abruption of sedimentary geochemistry events of marine carbonates. By using modern instruments and testing methods adequately, the contents of oxides in sandstones were measured and the REE distribution pattern curves were established; an accurate value of isotopic ratio of 87Sr/86Sr was obtained, that is, the age of MT formation is about 750–900 Ma; C and O isotopes of some fresh micrite limestone samples were analyzed; the energy spectrum analysis revealed that the MT consists mainly of microspar calcite, while as for its chemical composition, the matrix shows outstanding peaks of Ca, Mg, Al, Si, and K. The geochemical indicators proved that Neoproterozoic MT carbonates in the Jilin-Liaoning region were developed at the margin of a stable continent, in the torrid zone where the paleo-temperature was about 50°C, the seawater had normal salinity when MT was formed during the Wanlong period in southern Jilin and during the Yingchengzi and Xingmincun periods in eastern Liaoning. The sedimentary environment was located in the inner ramp. In summary, it is of great importance to understand the origin of MT, ascertain the paleo-climate and paleo-environment characteristics, constrain the age and the stratigraphic division and comparison of the Proterozoic so as to study the geochemical characteristics of MT carbonates and their formation environment.

Tomographic study of Paleoproterozoic carbonates as key to understanding the formation of molar-tooth structure

X-ray computed tomographic studies of relatively pure Paleoproterozoic limestones from the George Formation, Muskwa Assemblage, northern British Columbia, Canada indicate that molar-tooth structures developed along linked fractures in gel-like semi-plastic carbonate mud, with a high organic content. Where pore fluid and/or gas pressures matched confining loads, MT blobs developed. Where pressure exceeded loads, cracks propagated into adjacent semi-elastic sediment and were rapidly filled by clusters of uniform, equant, microcrystalline carbonate. Where abundant carbonate was not precipitated, incipient cracks and sheets collapsed leaving residual trains of microcrystalline carbonate with similar density to the molar-tooth carbonate. Tomographic studies show that the density of calcite domains within petrographically uniform sheets of MT void-filling calcite is uneven, suggesting that precipitation was not instantaneous, but was propagated from discrete centres. It is here suggested that carbonate production and sediment rheology were both strongly influenced by organic matter. During early sea-floor diagenesis microcrystalline carbonate precipitated within organic-rich sediment with high water content, possibly within decomposing mats of microbial extracellular polymeric substances (EPS). When pore pressures in the host sediment increased in response to cyclic loading by long-period waves, pore fluids containing EPS were injected into newly created fractures, allowing rapid precipitation of molar-tooth carbonate. Because tomographic studies allow detailed resolution of minor density differences, they provide a useful method of evaluating structures in relatively uniform carbonate rocks of any age.

Biomarkers in the Molar Tooth (MT)-bearing limestones in the Jilin-Liaoning area of China

The origin of Molar Tooth (MT) carbonates has been argued for more than 100 years, which are a kind of Proterozoic carbonates especially composed of microsparite with ptygmatically folded and sheet-like structures. Biomarkers detected in the microcalcsparite from the Wanlong and Xingmincun formations in the Jilin-Liaoning area showed there are abundant normal alkanes, isoprenoids, hopanes, steranes, alkylmethylcyclohexanes, and alkylcyclohexanes, indicating a diversity of biological source: long-chain isoprenoids, the major components of chlorophyll, such as C19, C20, a kind of major biomarkers synthesized early by isoprenoid monomers; hopanes a type of characteristic biomarkers from prokaryote, such as archaebacteria and cyanobacteria; sterane a biomarker for eukaryote; and two kinds of alkanes with C17, C18 as the main peaks representing aquatic bacteria and with C23, C24 as the main peaks representing fungi, respectively. Biomarker analysis showed that MT is the result of bacterial and algal activities, which is a kind of organisms between aerobic and anaerobic bacteria, reproducing well in normal or slightly saline sea water under weak oxidation-reduction conditions, resulting in rapid deposition of calcite as microsparite due to some mechanisms.

Microorganisms Linked to Neoproterozoic Microspar Carbonate Sedimentation in the Jilin‐Liaoning Area

Abstract Molar‐tooth carbonate refers to a sort of rock that has ptygmatical folded structure comparable to the ivory. This kind of carbonate exists in a special time range (from Middle to Neoproterozoic). Its origin and the possibility to use it in stratigraphic correlation of the paleocontinent is the key task of the IGCP447, a project on Proterozoic molar tooth carbonates and the evolution of the earth (2001–2005). The importance lies in that the molar‐tooth structure is the key to solving problems related to Precambrian biological and global geochemical events. The molar‐tooth structure is associated with microorganisms. Development and recession of such carbonates have relations with the evolution process of early lives and abrupt changes in sea carbonate geochemistry. In recent years, based on researches on petrology, geochemistry and Sr isotope of molar‐tooth carbonate in the Jilin‐Liaoning and Xuzhou‐Huaiyang area, the authors hold that it can be used as a marker for stratigraphic sequence and sedimentary facies analyses.

Molar‐tooth Carbonate Sequences and Sr Isotopes in the Neoproterozoic for Stratigraphic Correlation: Research in the Jilin‐Liaoning‐Xuzhou‐Huaiyang Area of the Sino‐Korean Plate and Its Correlation with the Yangtze Plate

Abstract Based on a study of Neoproterozoic carbonates in the Jilin‐Liaoning‐Xuzhou‐Huaiyang area, especially its cyclic sequence stratigraphy and Sr isotopes, two maximum sea flooding events (at 820 Ma and 835 Ma) have been identified. The resulting isochronous stratigraphic correlation proves that these Precambrian strata were connected between the Qingbaikou and the Nanhuan systems with a time range from 750 Ma to 850 Ma. The disappearance of microsparite carbonate and coming of a glacial stage offer important evidence for worldwide stratigraphic correlation and open a window for further correlation of the stratigraphic successions across the Sino‐Korean and Yangtze Plates. A new correlation scheme is therefore provided based on our work.

Cyclic Sequences, Events and Evolution of the Sino‐Korean Plate, with a Discussion on the Evolution of Molar‐tooth Carbonates, Phosphorites and Source Rocks

Abstract This paper gives an account of the research that the authors conducted on the cyclic sequences, events and evolutionary history from Proterozoic to Meso‐Cenozoic in the Sino‐Korean plate based on the principle of the Cosmos‐Earth System. The authors divided this plate into 20 super‐cyclic or super‐mega‐cyclic periods and more than 100 Oort periods. The research focused on important sea flooding events, uplift interruption events, tilting movement events, molar‐tooth carbonate events, thermal events, polarity reversal events, karst events, volcanic explosion events and storm events, as well as types of resource areas and paleotectonic evolution. By means of the isochronous theory of the Cosmos‐Earth System periodicity and based on long‐excentricity and periodicity, the authors elaborately studied the paleogeographic evolution of the aulacogen of the Sino‐Korean plate, the oolitic beach platform formation, the development of foreland basin and continental rift valley basin, and reconstructed the evolution of tectonic paleogeography and stratigraphic framework in the Sino‐Korean plate in terms of evolutionary maps. Finally, the authors gave a profound discussion on the formation and development of molar‐tooth carbonates, phosphorites and source rocks.

Molar-Tooth Carbonates: Shallow Subtidal Facies of the Mid- to Late Proterozoic: Discussion

From field work in northern Canada and many other examples documented in the literature, James et al. (1998) highlighted the potential use of molar-tooth structures (i.e., a series of spar-filled cracks) as a stratigraphic indicator for the Mesoproterozoic and Neoproterozoic. These authors also stated that all known occurrences where molar-tooth structures are volumetrically significant are from shallow-water carbonates. Although these are comprehensive statements, considering that most molar-tooth structures reported so far are from shallow-water Mesoproterozoic and Neoproterozoic deposits, I am concerned about such proposed temporal and environmental constrictions, which are not supported by the field data, summarized in the following. Detailed field work in the Grajau Basin, northern Brazil, revealed that structures similar to the ones reported by James et al. (1998) are volumetrically widespread in the Codo Formation. This is a lacustrine lithostratigraphic succession formed during the early stages of the South Atlantic rifting in the Aptian (e.g., Petri and Fulfaro 1983). The sedimentary features described here occur in lime mudstone to grainstone, ostracodal grainstone, and crystalline limestone interbedded with shales strongly affected by penecontemporaneous deformation, as indicated by a variety of complex convolute folds, small faults, fissures, and bed collapses. The molar-tooth structures occur throughout these beds, although they substantially increase in volume downward, where evidence for soft-sediment deformation diminishes. These …

Molar-Tooth Carbonates: Shallow Subtidal Facies of the Mid- to Late Proterozoic: Reply

ABSTRACT Molar tooth is a series of peculiar, ptygmatically folded and spar-filled cracks in fine-grained carbonates of Precambrian age. This sedimentary structure appears to be limited in time and space. Our field studies in northern Canada and a survey of literature on Precambrian carbonates worldwide indicates that such features are localized to mid/inner ramp and shallow platform paleoenvironments of Mesoproterozoic and Neoproterozoic age (ca. 1500-650 Ma) with only sporadic occurrences in older and younger strata. As such they are equivalent in terms of texture, location, and abundance to the ubiquitous shallow subtidal, fossiliferous limestones of the Phanerozoic. The amount of synsedimentary calcite spar in molar tooth, in excess of 50% of individual beds and 5-25% of entire formations, is further roughly equivalent to the amount of calcite in calcareous benthic invertebrates in these younger rocks. Thus, there is an evolving trend in such facies through geologic time; Archean-Paleoproterozoic = massive seafloor precipitates; Mesoproterozoic-Neoproterozoic = molar-tooth mudstones and grainstones; Phanerozoic = burrowed and fossiliferous limestones.

Molar-tooth carbonates: shallow subtidal facies of the mid- to late Proterozoic

ABSTRACT Molar tooth is a series of peculiar, ptygmatically folded and spar-filled cracks in fine-grained carbonates of Precambrian age. This sedimentary structure appears to be limited in time and space. Our field studies in northern Canada and a survey of literature on Precambrian carbonates worldwide indicates that such features are localized to mid/inner ramp and shallow platform paleoenvironments of Mesoproterozoic and Neoproterozoic age (ca. 1500-650 Ma) with only sporadic occurrences in older and younger strata. As such they are equivalent in terms of texture, location, and abundance to the ubiquitous shallow subtidal, fossiliferous limestones of the Phanerozoic. The amount of synsedimentary calcite spar in molar tooth, in excess of 50% of individual beds and 5-25% of entire formations, is further roughly equivalent to the amount of calcite in calcareous benthic invertebrates in these younger rocks. Thus, there is an evolving trend in such facies through geologic time; Archean-Paleoproterozoic = massive seafloor precipitates; Mesoproterozoic-Neoproterozoic = molar-tooth mudstones and grainstones; Phanerozoic = burrowed and fossiliferous limestones.

Molar-Tooth Carbonates: Shallow Subtidal Facies of the Mid- to Late Proterozoic

Molar-Tooth Carbonates: Shallow Subtidal Facies of the Mid- to Late Proterozoic