Paleogeographic Distribution Research Articles

The first record of gars (Ginglymodi: Lepisosteidae) from the Upper Cretaceous of Bulgaria reveals a wider paleogeographic distribution of lepisosteids within the European Archipelago

The bony-fishes of the clade Lepisosteidae, commonly called ‘gars’ or ‘gar-fish’, are a lineage of proficient piscivores with evolutionary history spanning about 150 million years, which are today represented by two genera inhabiting the freshwater and brackish areas of southeastern North America, Central America, and Cuba. This was not the case during the Late Cretaceous when lepisosteids were more diverse and had much wider geographical distribution. Gar fossils, albeit very fragmentary, are a common component of the Upper Cretaceous freshwater and brackish vertebrate assemblages in Europe, yet all of the pre-upper Campanian records come from the western and central parts of the Late Cretaceous European Archipelago. Here we describe new lepisosteid material from Bulgaria, comprising nine teeth and three scales found at the uppermost Santonian–lowermost Campanian Vrabchov Dol vertebrate locality. These fossils represent the easternmost record of gars within the European Archipelago to date. Despite being found in a lagoonal to foreshore deposits, the paleontological content of the locality, the incompleteness and preservational state of the material, as well as the predominantly non-marine ecology of modern and fossils gars suggest that these fossils belong to fishes which inhabited more inland, freshwater environments. The Vrabchov Dol lepisosteids remains are the first record of gars in Bulgaria and one of the rare documented occurrences of Mesozoic osteichthyans in the country. This material expands the paleobiogeographic distribution of the Lepisosteidae within the European Archipelago.

Middle Devonian actinopterygians from Lithuania and Belarus

In the Baltic States and Belarus, the Middle Devonian period is characterised by an abundant fossil record of invertebrates such as scolecodonts, brachiopods, ostracods, trilobites, bivalves, crinoids, gastropods, and tentaculites. On the other hand, there was limited diversity in the conodont and other vertebrate fauna. In this study, we introduce a newly refined ichthyofaunal assemblage from the Eifelian and Givetian epochs from the present-day regions of Belarus and Lithuania. The isolated scales of Cheirolepis are identified as C. gaugeri, C. cf. gaugeri, C. aleshkai, C. cf. aleshkai, and Cheirolepis sp., while Orvikuina is represented by the isolated scales of O. vardiaensis and Orvikuina sp. The histological analyses for the scales of Orvikuina are provided here. Moreover, Orvikuina and Cheirolepis taxa are now recognised to be widely distributed throughout the Baltic States, especially Lithuania and Belarus, as a result of these findings. Thus, this ichthyofaunal data markedly improved the biostratigraphic correlation within the study region and expanded the paleogeographic distribution of these taxa during the Eifelian and Givetian time in Laurasia.

New Discovery of Calamitaceae from the Cisuralian in Northwest China: Morphological Evolution of Strobilus.

Calamitaceae was the predominant arborescent element of the Late Paleozoic plant assemblages. However, there is currently controversy surrounding the evolutionary relationships of their four reproductive organs, and there is no unified understanding of the geographical distribution and species richness characteristics. This paper is based on the detailed description of the macro- and microstructures of Calamites and Macrostachya discovered in the Shanxi Formation of the Cisuralian in the eastern Hexi Corridor of northwestern China, and it discusses the evolutionary patterns of calamitean reproductive organs. The results indicate that the current specimens can be identified as the Calamites cistii and Macrostachya sp., and the in situ spores should exhibit distinct trilete marks, indicating that they belong to the Punctatisporites punctatus. The abundant fossil records suggest that the "Calamostachys" type should be considered as the ancestral type of strobili. Additionally, Calamitaceae was widely distributed in mid to low latitudes globally from the Pennsylvanian to the Cisuralian and coupled with paleogeographic distribution characteristics.

Lead-up and manifestation of the Oceanic Anoxic Event 2 at the DSDP Site 398 (Vigo Seamount, NW Iberian offshore): Palynological and geochemical insights

Extreme climatic conditions, volcanism, and paleogeographical distribution presumably led to the Oceanic Anoxic Event 2 (OAE 2), one of the most disruptive events for Cretaceous ecosystems. Although the terrestrial response to this issue is barely studied, a turnover within the plant communities seems to be linked to the OAE 2, besides the well-known extinction of several marine groups. In this study, palynological and δ13Corg analyses were combined to research the sediments from DSDP Site 398 (Vigo Seamount) cores 398D-58 to 398D-56 (upper Albian to lower Turonian). The δ13Corg curve exhibits a Carbon Isotopic Excursion (CIE) observed between the interval 948,60 and 947,77 m below seafloor (mbsf), corresponding to the OAE 2. Four palynological assemblages were differentiated, ranging from the uppermost Albian to the lower Turonian. These assemblages comprised diverse marine and terrestrial communities with generally excellent preservation. In addition to previous biostratigraphic works, we present a more detailed chronostratigraphy of DSDP Hole 398D based on palynomorphs. The paleoecological reconstruction reveals the significant changes during the OAE 2 period: a decrease in the diversity and abundance of dinoflagellate cysts and a shift to angiosperm-dominated terrestrial communities (i.e., Normapolles) which, unlike other sites, occurs prior to the maximum values of δ13C.

Comments on “Morphogenesis of Balkhania balkhanica Mamontova, 1966 (benthic foraminifera) from Lower Cretaceous successions along the northern Tethyan margin (NE and Central Iran)”

The study by Gheiasvand and Bartolini (2023) treated Balkhania balkhanica and Torinosuella peneropliformis as morphotypes of a single species, Balkhania balkhanica, leading to incorrect conclusions about its characteristics. Their analysis overlooked key aspects of the life cycle, morphotypes, ecological roles, and paleogeographical distribution of B. balkhanica due to a misunderstanding of foraminiferal systematics. Notably, its megalospheric forms can reach a maximal diameter of 5.2 mm, whereas its microspheric forms may reach up to 15 mm. Its stratigraphic range is limited to the Barremian-early Aptian interval. Although B. balkhanica and Choffatella decipiens share equivalent ecological niches, they are not found together in the same strata. Ch. decipiens has a much wider distribution, over most of the Tethys realm, whereas Balkhania balkhanica is primarily found in the northern Tethys platforms, with the notable exception of Lebanon in the southern Tethys platforms.

New findings of Changxingaspis (Xiushuiaspidae, Galeaspida) from the Silurian of Tarim Basin and Zhejiang Province, China

AbstractNew discovery of the early Silurian fossil fish Changxingaspis (Xiushuiaspidae, Galeaspida), Changxingaspis nianzhongi sp. nov. and C. gui, are described from the Tataertag Formation in Tarim Basin and the Kangshan Formation in Zhejiang Province, respectively. C. nianzhongi mainly differs from C. gui in the shape of the median dorsal opening that is transverse elliptic with a width/length ratio of about 3.0, the long lateral transverse canals extending to the lateral margin of the headshield, and the second lateral transverse canal with dichotomous branchings. Discovery of C. nianzhongi from the Tataertag Formation and C. gui from the Kangshan Formation provide direct evidence on the specific level for the correlation between these two formations, which further supports the Silurian fish‐bearing red beds in northwest Zhejiang belonging to the Silurian Lower Red Beds (LRBs) rather than the Upper Red Beds (URBs). Additionally, as the first record of the Changxingaspis in Tarim Basin, it extends the paleogeographical distribution of this genus from the South China Block to the Tarim Block, providing new evidence to support faunal exchanges between these two blocks and the hypothesis of a united Tarim–South China Block during the early Silurian.

New material of the zosterophyllopsid Gosslingia from the Lower Devonian of Guizhou, southwestern China

Abstract The zosterophyllopsids had a widespread distribution and constituted a dominant component in many plant assemblages during the Early Devonian. Although a large number of zosterophyllopsids have been documented, knowledge about the paleogeographic distribution of different genera/species remains to be expanded by further fossil evidence. In this article, new material assigned to the genus Gosslingia Heard, 1927 and designated as Gosslingia cf. G. breconensis Heard, 1927 is described from a new locality of the Lower Devonian of Guizhou Province, China. The Guizhou material shows main axes that are pseudomonopodially branched, pseudomonopodial lateral branching systems, subaxillary tubercle branches, circinate apices, elliptical xylem strand, exarch maturation of xylem, and G-type tracheids, and exhibits considerable similarities with the type and only species of Gosslingia, Gosslingia breconensis. Our finding represents the first report of Gosslingia in the South China Block and the most convincing occurrence of this genus outside of Wales, UK. Gosslingia adds to the diversity of genera shared among the Early Devonian floras of South China, western Europe, and North America, along with Distichophytum Mägdefrau, Estinnophyton Fairon-Demaret, Zosterophyllum Penhallow, and others, and indicates that the dispersal of early vascular plants among different paleocontinents was more common than previously appreciated.

The middle miocene (Badenian) holoplanktonic mollusks (Euthecosomata - Pteropoda) from Serbia, central paratethys

The present paper deals with holoplanktonic gastropods (Pteropoda) from the Middle Miocene (Badenian) deposits of Serbia. The material for the present study comes from three localities at the southern margin of the Pannonian Basin (Central Paratethys): Konjarnik (greater center of Belgra de), Sumijevac stream near Koceljeva (western Serbia), and Radljevo (Kolubara Basin, central Serbia). A total of four taxa were recorded and their stratigraphic value is discussed. Two taxa were identified to species level: Vaginella austriaca KITTL, 1886, belonging to the family Cavoliniidae GRAY, 1850 (1815), and Limacina valvatina (REUSS, 1867)?, from the family Limacinidae GRAY, 1847. Four specimens could only be identified to genus level (Vaginella sp. indet. and Clio? sp. indet.). The specimen of Clio? sp. indet. is possibly the first find of the genus Clio in Serbia. Here presented records of the genera Vaginella and Clio? from the late Badenian (NN6 nannozone) modify the insight into their paleogeographical distribution in the Middle Miocene of the Central Paratethys.

Discovery of the earliest Devonian conodonts from Xinjiang, Northwest China

The base of the Devonian is defined at the first appearance of graptolite Uncinatograptus uniformis uniformis, with the occurrence of trilobite Warburgella rugulosa rugosa and conodont Caudicriodus woschmidti as the auxiliary indicators. Recently some researchers questioned whether Caudicriodus woschmidti can be an indicator, and proposed Caudicriodus hesperius to mark the base of the Devonian in carbonate successions. However, the paleogeographic distribution of this species is mainly in the southern hemisphere or near the equator, and there was no definite record of it in the northern hemisphere during the Early Devonian. Due to lithological restrictions, conodont data of the Silruain–Devonian Boundary (SDB) are relatively rare in China, and Caudicriodus hesperius has not been identified. In this study, specimens of Caudicriodus hesperius were obtained from the base of the Kaokesaiergai Formation in the eastern Junggar, northern Xinjiang, which represents the first record of this taxon in China, suggesting that the age of the formation is Early Devonian, rather than late Silurian as previously believed based on the benthic faunas. The encrinurid trilobites, which commonly occur in the Silurian, were also recorded in the same horizon with C. hesperius, indicating that encrinurids may cross the SDB and range into the earliest Devonian. The discovery of Caudicriodus hesperius in northern Xinjiang significantly expands the paleogeographic range of this index conodont to the Central Asian Orogenic Belt in the northern hemisphere, and suggests that Caudicriodus hesperius may be used as a global index for the base of the Devonian and for possible stratigraphic correlation of the SDB in different paleogeographic and paleotectonic settings.

Late Cenomanian–early Turonian ammonite successions of north Eastern Desert, Egypt: biostratigraphy and paleobiogeographic implications

The upper Cenomanian-lower Turonian sequence of Egypt yield a remarkably diverse and plentiful ammonite fauna. Three stratigraphic sections located on Wadi Araba, north of the Southern Galal Plateau were investigated. Lithostratigraphically, the measured sections are subdivided from base to top into the Malha (pre-Cenomanian), Galala (Cenomanian) and Maghra El Hadida (Turonian) formations. Over a hundred well-preserved and nearly complete ammonite internal molds were collected. The studied ammonites are systematically classified as Vascoceratidae, Pseudotissotiidae, Acanthoceratidae and Engonoceratidae. Vascoceratidae and Pseudotissotiidae comprise 75% of the identified ammonites. Twelve ammonite species have been identified. Six late Cenomanian-early Turonian ammonite biozones are proposed herein: Neolobites vibrayeanus, Vascoceras gamai, Vascoceras cauvini, Vascoceras proprium, Choffaticeras segne, and Wrightoceras munieri. The identified ammonite biozones are correlated with NW European standard zonation and the Tethyan biostratigraphical schemes was performed. The first occurrences of Vascoceras proprium, as well as the first occurrences of the inoceramids Mytiloides kosmati and Mytiloides puebloensis define the Cenomanian-Turonian boundary. The diversity and abundance of studied fossil groups varied dramatically over the C/T transition. In the latest Cenomanian, all identified Cenomanian species had become extinct. Only three species have reached the Turonian time. The paleobiogeographic implications of the descried ammonites are thoroughly discussed. The paleogeographic distribution of the studied ammonite fauna shows that the majority of the described species are restricted to the shallow shelf of the southern Tethys.

Tracing the origin of Lago Mare biota: Ostracods and mollusks from the late Neogene of the Slavonian mountains in the southern Pannonian Basin (NE Croatia)

Lake Pannon was a huge central European, long-lived endorheic lake occupying the Pannonian Basin System during the late Neogene. The benthic fauna of this originally sparsely populated brackish lake went through a spectacular adaptive radiation leading to a great number of autochthonous species. Although largely endemic, some of these species are excellent stratigraphic markers of the Lago Mare interval in the Mediterranean Basin. This situation apparently reflects a short-termed unidirectional migration-wave in the latest Miocene. The latter interval marks the final phase of a major environmental perturbation followed by a massive evaporite deposition termed the Messinian Salinity Crisis. The Section Bozara, located in the southern Pannonian Basin at the southern slopes of Mt. Papuk, bears a well-preserved benthic fauna representative of Lake Pannon. It is embedded into a 25-m-thick succession of alternating mud and sand packages. We detected therein 25 ostracod and 18 mollusk taxa enabling, along with a sedimentary facies analysis, an integrated evaluation of the depositional setting, biostratigraphic position, and paleogeographic distribution pattern. Accordingly, the depositional environment shows a shallowing upward trend from deep-water sublittoral and distal prodelta settings to deltaic high-energy littoral conditions. The biostratigraphic markers, such as the bivalve Rhombocongeria rhomboidea (Hörnes, 1867) and the ostracod Caspiocypris pontica (Sokač, 1972), constrained its stratigraphic position to the Portaferrian interval (8.0–4.5 Ma). Of 16 ostracod taxa determined at the species level, 10 are shared with the Eastern Paratethys, whereas only 3 are present in the Mediterranean Lago Mare deposits as well. In contrast, among 12 corresponding mollusk taxa, only 4 are known from the Eastern Paratethys, all being absent in the Lago Mare interval. Such a paleobiogeographic pattern suggests that the time window of the outflow event from Lake Pannon into the Eastern Paratethys was not synchronous with, but must have preceded, the hypothetic Eastern Paratethys drainage event generating the Lago Mare depositional conditions in the Mediterranean Basin.

Macroalgal fossil Longfengshania from the Tonian Shiwangzhuang and Jiuliqiao formations in western Shandong and northern Anhui, North China

Longfengshania is among the few examples of Pre-Cryogenian macroscopic fossils with an unambiguous eukaryotic affinity. It has received great attention since this genus is the earliest known Pre-Cryogenian macroalga with blade, stipe, and holdfast differentiation. However, compared to other macrofossils, such as Chuaria and Tawuia, reported occurrences (lithostratigraphic units) of Longfengshania are limited in Tonian strata, restricting the assessment of paleogeographic distribution of Longfengshania and therefore hindering our understanding of its paleoecological role in the Tonian oceans. Here, we describe two new localities of Longfengshania, including the Tonian Shiwangzhuang Formation (∼850–720 Ma) in western Shandong and Jiuliqiao Formation (∼950–720 Ma) in northern Anhui, North China. Specimens of Longfengshania from the Shiwangzhuang and Jiuliqiao formations are smaller than specimens from other localities, which has been interpreted as adaptations to relatively stronger hydrodynamic conditions. Biometric and principal component analyses suggest that most other species of Longfengshania actually fall within the morphospace range of the type species L. stipitata and therefore a re-examination on previously reported specimens and a taxonomic revision for this genus are needed in future study.

Imprecision and instability of the Phanerozoic chronostratigraphic scale: A solution

The use of the GSSP (Global Stratotype Section and Point) method to define a Phanerozoic chronostratigraphic timescale has resulted in imprecision and instability because most of the primary signals for correlation of the GSSPs are single taxon biotic events that are inherently diachronous and restricted in their paleogeographic distributions by the limitations of sampling, facies and provincialism. Greater precision and stability can be achieved by using non-biotic criteria as the primary signals of GSSPs-numerical ages, magnetostratigraphy, isotope excursions and others. If correlation is aided by close association of the non-biotic primary signal with secondary signals of geographically widespread, ergo global, extent, such non-biotic primary signals of GSSPs will produce a more precise and more stable chronostratigraphic timescale.

Foraminiferal distribution and biostratigraphy of the Lower Miocene Qom formation, north of Bijar, NW Iran

This study is based on larger benthic foraminifera that dominate the Lower Miocene shelf carbonates of the Qamcheqai section, NW Iran. During the time of deposition, the investigated area was positioned on the Eurasian side of the Tethyan Seaway between the Eastern and Western Tethys. Seven porcelaneous and hyaline perforated species were determined, including Praebullalveolina curdica, Peneroplis evolutus, Androsina cf. diyarbakirensis, Sivasina egribucakensis, Miogypsina cf. intermedia, Operculina complanata, and Amphistegina bohdanowiczi. The presence of Miogypsina cf. intermedia, and Praebullalveolina curdica allowed to correlate the assemblage with SBZ 25 Zone, indicating a Burdigalian age. The results of this study extend the paleogeographic distribution of the endemic taxa Androsina and Sivasina eastwards as far as Central Iran.

The first Eugaleaspiforme fish from the Silurian of the Tarim Basin reveals a close relationship between the Tarim and South China blocks at 438 mya

The first Eugaleaspiforme fish, Jiangxialepis rongi sp. nov., from the lower Telychian (Llandovery, Silurian) Tataertag Formation in the Tarim Basin is described and its palaeogeographic significance is evaluated. The new species can be assigned to the family Shuyuidae and distinguished from J. retrospina and J. jiujiangensis by its comparatively broader headshield, ornamentation, and serrated margin of the headshield and median dorsal opening. The discovery of J. rongi provides direct fossil evidence for the correlation of lower Telychian marine red beds between the Tarim Basin and South China and expands our knowledge of the distribution of Eugaleaspiforme fish into the Tarim Basin. An analysis of galeaspid paleogeographic distribution within a phylogenetic framework reveals at least five galeaspid dispersal events between the blocks of South China and the Tarim Basin during the early Telychian age. This indicates that the South China and Tarim blocks were close enough to share the same shallow marine environment, and perhaps comprise a united Tarim-South China Block during the early Silurian.

“LARGER” BENTHIC FORAMINIFERA OF THE CENOMANIAN. A REVIEW OF THE IDENTITY AND THE STRATIGRAPHIC AND PALAEOGEOGRAPHIC DISTRIBUTION OF NON-FUSIFORM PLANISPIRAL (OR NEAR-PLANISPIRAL) FORMS

A key but challenging task for biostratigraphers is to provide a biozonal/bioevent framework for geological correlation. Species of Larger Benthic Foraminifera (LBF) are important biostratigraphic markers in depositional environments where classical biostratigraphic fossils such as planktonic micro- and macrofossils are rare or absent – e.g., tropical-subtropical shallow water platforms. However, a lack of taxonomic rigour in identifying some LBF species, together with a lack of good age-calibration of their occurrences, has given rise to artificially extended biostratigraphic and paleogeographic ranges for many taxa, diluting their usefulness. In this study, the occurrences of Cenomanian LBF belonging to a “planispiral morphogroup”, both agglutinated and calcareous, have been critically evaluated to determine (i) identity; (ii) stratigraphic range; and (iii) palaeogeographic distribution. Since the last major review of the group in 1985, a voluminous literature has appeared reporting occurrences and adding new taxa. An extensive review of some 600+ published items on Cenomanian planispiral LBF – mostly published after 1985 – and a critical review of the confidence in species identification and age-calibrations therein, has led us to identify 39 taxa (three in “open” status) which appear to have distinct identity. The vast majority of these records are from Neotethys although some also occur in (or are endemic to) the Caribbean/West Atlantic and the Eastern Pacific. The quality of the published taxonomic data is variable and many published records based on identity can be discounted or termed “unconfirmed”. Likewise, many records (confirmed or otherwise) are poorly age-calibrated due to lack of corroborating biostratigraphy or chemostratigraphy, or by using circular reasoning. We summarise and illustrate the main defining characteristics of each taxon and their possible confusion species, including new taxa described since the mid-1980s. We publish new, more confident, age-ranges for these taxa – confirmed by identity and/or age-calibration – and identify where published range data may be unreliable. Paleogeographic distribution maps for each taxon are also provided. Particular stratigraphic issues around the Cenomanian- Turonian boundary are observed due to the difficulty of identifying that boundary, or its preservation, in shallow marine carbonate settings. Although most Cenomanian planispiral LBF are somewhat long-ranging, an increase in diversity throughout the middle – late Cenomanian has shown potential for biostratigraphic resolution to at least substage level using this group. Integration of the planispiral taxa with other LBF morphogroups, after similar treatment, will yield even higher biostratigraphic resolution of Cenomanian LBF and provide a sound basis for biozonation (both local and global), correlation, and age calibration.

TAXONOMIC CONSIDERATION OF THE YPRESIAN BENTHIC FORAMINIFERAL SPECIES OF IRAN AND ITS PALEOGEOGRAPHIC DISTRIBUTION IN THE TETHYS

An Ypresian Iranian twenty-five smaller benthic foraminiferal species belonging to twenty genera, were identified by which represent good example of the Southern Tethyan benthic foraminiferal assemblage. The present study deals with the modern taxonomical consideration of this assemblage. These taxa are: Ammodiscus glabrata Cushman & Jarvis, Spiroplectinella knebeli, Pseudogaudryinella iranica Anan, Verneuilina aegyptiaca Said & Kenawy, Verneuilina luxorensis Nakkady, Dorothia bulletta (Carsey), Textularia nilotica (Schwager), Textularia punjabensis Haque, Textularia salahii Anan, Vulvulina advena Cushman, Lenticulina turbinata (Plummer), Percultazonaria fragaria (Gümbel), Orthokarstenia applinae (Plummer), Bulimina alazanensis Cushman, Bulimina midwayensis Cushman & Parker, Globobulimina pyrula (d’Orbigny), Uvigerina mediterranea (Hofker), Aragonia aragonensis (Nuttall), Nonionella auris (D’Orbigny), Anomalinoides acutus (Plummer), Cibicidoides proprius Brotzen, Cibicidoides vulgaris Plummer, Oridorsalis umbonatus (Reuss), Gyroidinoides girardanus (Reuss), Hanzawaia cubensis. The Southern Tethyan assemblage of Iran indicates an open marine environment, which represents outer neritic environment, and shows an affinity with Midway-Type Fauna (MTF). These wide paleogeographic distribution indicate that the ancestral Tethys is connected with the ancestral Atlantic and Indian Oceans via Mediterranean Sea, as well as Arctic sea in that time.

New record of aquatic snakes (Squamata, Palaeophiidae) from the Paleocene of South America

ABSTRACT The Paleocene deposits from the Cerrejón Formation (Colombia, South America) have provided abundant snake remains. All this material has been assigned to the giant snake Titanoboa cerrejonensis. Here, we describe six vertebrae from among the referred materials that do not correspond to Titanoboa. All six vertebrae are morphologically identical, of a similar size, and display a distinct morphology that differs from Titanoboa. This new and unnamed taxon is a large snake estimated to be around eight meters in total length. Anatomical comparisons show that these vertebrae possess certain features typical of the Palaeophiidae, a group of extinct snakes of uncertain relationships known almost exclusively from vertebrae. Among the Palaeophiidae, the new taxon resembles the more generalized forms assigned to the genus Palaeophis, but lacks the extreme aquatic adaptations that define forms assigned to the genus Pterosphenus. We regard the new taxon as an undetermined palaeophiine but do not assign it to Palaeophis as a number of features differ from that described genus. This new record expands the known diversity of aquatic snakes from the Paleogene of South America and provides a substantial new record for the paleogeographic distribution of the Palaeophiidae. The oldest records are from the Cretaceous of Africa, while Paleocene records are poorly known and restricted to Africa, North America, and Europe. The majority of vertebral forms assigned to Palaeophiidae are from the Eocene, principally from the Tethys region, with only one previous South American record coming from the Eocene of Ecuador.

The origin of cap carbonate after the Ediacaran glaciations

Cap carbonate is the most unique component of a Snowball Earth glaciation, i.e., two Cryogenian global glaciations. In the canonical model, cap carbonate precipitation requires a deglacial extremely high atmospheric pCO2 level, which is unique for a Snowball Earth glaciation but did not occur in other non-Snowball Earth glaciations. However, some Ediacaran glacial successions also have a carbonate unit directly overlying the glacial deposits, although the Ediacaran glaciation is widely accepted to be a non-Snowball Earth ice age. Therefore, occurrence of Ediacaran glacial cap carbonate is counterintuitive, challenging the traditional model of cap carbonate precipitation. Here, we investigate two Ediacaran glacial cap carbonates, i.e. the Zhoujieshan cap carbonate above the Hongtiegou glacial deposits in the Chaidam Block, and the Hankalchough cap carbonate overlying the Hankalchough glacial deposits in the Tarim Block. The Zhoujieshan cap carbonate shows a transitional contact with the underlying glacial deposits, and has a nearly invariant carbonate carbon isotope value (δ13Ccarb) of ∼ + 1‰. The Hankalchough cap carbonate from the Luobupo section has a persistent δ13Ccarb value of ∼ − 5‰, but display a ∼ 15‰ spatial isotopic gradient in the Quruqtagh area. Combining with other occurrences, the Ediacaran glacial cap carbonate differs from the Snowball Earth cap carbonate in paleogeographic distributions, sedimentary structures, and mineralogical and geochemical compositions. We noticed that the Ediacaran glacial cap carbonates were restricted at ∼30–50° N/S, which may reflect a rapid transition from glaciation to carbonate deposition in subtropics. Thus, we speculate that the Ediacaran glaciation might have extended to latitude low enough, while the subsequent deglaciation and global warming may associate with an expansion of carbonate precipitation toward higher latitudes. Thus, the Ediacaran glacial cap carbonate precipitation occurred when carbonate precipitation and glaciation spatially overlapped in subtropics. Alternatively, considering paleomagnetic evidence for a ∼ 90° reorientation of all continents due to the Ediacaran true polar wander (TPW) event, some Ediacaran glacial cap carbonates could also be driven by glaciated continents moving into lower latitudes, where glacier melt and carbonate precipitated. Either way, a typical Snowball Earth glaciation may lead to global cap carbonate depositions, while most of Phanerozoic glaciations are characterized by glacial deposits in mid- to high-latitude and carbonate precipitation in tropics, i.e., showing no spatial overlapping of glaciation and carbonate precipitation. Thus, the enigmatic cap carbonate precipitation after the Ediacaran glaciations imply a unique climatic condition that differs from either a Snowball Earth glaciation or a Phanerozoic ice age.

Limited change in silicate chemical weathering intensity during the Permian–Triassic transition indicates ineffective climate regulation by weathering feedbacks

Vast emissions of volcanogenic greenhouse gases resulted in rapid global warming during the latest Permian mass extinction (LPME, ∼252 Ma), the impact of which on continental chemical weathering conditions was complex. Here, we compile Chemical Index of Alteration (CIA) records from 25 terrestrial and 12 marine sections with a near-global paleogeographic distribution to reconstruct the evolution of chemical weathering intensity of silicates during the LPME. This compilation shows that the CIA profiles, except those from the Australian Sydney and Bowen basins of the high-latitude Southern Hemisphere, exhibit either indiscernible changes or decreases through the LPME interval, contrary to previous inferences of increased silicate chemical weathering intensity. The lack of major changes in silicate chemical weathering intensity is further supported by the near uniformity of clay mineral assemblages through the LPME interval, with only minor increases of illite and chlorite and, if present, decreases in kaolinite content in some terrestrial sections. We infer a shift from dominantly transport-limited to kinetic-limited weathering regimes, likely caused by environmental climate factors such as widespread devegetation, river-flow changes (i.e., toward flashier and more intense discharge), and aridification during the earliest Triassic that effectively reduced water availability on land for water/rock reactions. Combined with limited changes in chemical weathering fluxes from previous estimates, our data imply possibly ineffective climate regulation by silicate chemical weathering at a (near-)global scale, despite high erosion rates, amid the intense global warming that followed the LPME.