Volcanic ash is known to influence a range of biogeochemical processes once deposited in the oceans. These processes include the fertilisation of phytoplankton, and the enhancement of organic carbon burial, and the impact typically scales with the volume of ash. It has been shown that during periods of intense volcanic ash deposition, the impact on the ocean carbon cycle can be significant enough to cause global cooling. As a result, knowing the volume of ash entering the world’s oceans through time is vital to understanding the role explosive volcanism plays in setting global climate states. However, records of ash deposition consist of a small number of individual archives of ash input estimated via either layer counting or from multivariate statistical partitioning. Here, we compile the discontinuous and patchy records of volcanic ash deposition in the Pacific Ocean over the past 70 million years and synthesise all available data to produce a coherent record of ash accumulation rates. We show how the development of certain provinces, such as the Izu-Bonin Arc led to considerable upticks in ash input, and discuss how changing levels of ash deposition may have impacted Cenozoic climate change. Using a global biogeochemical model, we demonstrate mechanistically the role changing ash supply has in controlling global climate.

How to cite: Longman, J., Dunlea, A., Anderson, C., and Scudder, R.: Reconstructing the input of volcanic ash to the Pacific Ocean over the Cenozoic era, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9535, https://doi.org/10.5194/egusphere-egu23-9535, 2023.

The pattern of taxonomic richness peaking at the equator and dwindling off towards the Earth’s poles is known as the latitudinal diversity gradient (LDG). It was among the first global biodiversity pattern to be discovered, but is also one that, to date, has resisted comprehensive explanation. The Paleozoic provides a unique perspective to study how the LDG has variated and its form regulated is in the deep time. Here a summary of genus-level global marine invertebrates fossil data was obtained from the Paleobiology Database and used to evaluate LDG patterns for each Paleozoic stage. After data cleaning, this dataset included 485,129 occurrences of 18,234 genera distributed across 85,259 fossil localities. Bias in the observed fossil record, as recorded by these data, was addressed via rarefaction, shareholder quorum subsampling and bootstrap approaches. Pattern analyses of LDG form across Paleozoic stages reveal seven main patterns. Location of latitudinal peak diversity relative to the paleoequator and the steepness of the LDG slopes across latitudes, represent the features that distinguish these seven LDG patterns across Paleozoic time. Although all observed LDG patterns are characterized by a low-latitude richness peak from 30°N to 30°S, they are not symmetrical about the paleoequator in most stages. Comparison of variation in LDG geometries with time-series variation in a suite of environmental proxies suggests that variation in tectonic plate configurations and aspects of global climate change are associated closely with the time-series of LDG variation. These associations suggest that the form of the LDG at different points in earth history was structured by secular changes in global climate states since tectonic configurations would be expected to affect organismal populations primarily through variation in environmental state. Moreover, our results suggest that the current LDG represents a form that is exceptionally balanced – and so somewhat atypical – when compared with LDG forms that existed over much of the Paleozoic.

How to cite: Wen, D., Fan, J., MacLeod, N., and Shi, Y.: Forms and Regulation of the Paleozoic Marine Latitudinal Diversity Gradient, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8230, https://doi.org/10.5194/egusphere-egu23-8230, 2023.

Life originated in the sea about 4 billion years ago and, for the first 3.5 billion years, it was exclusively microscopic. Something happened early in the Cambrian period that made multicellular life to thrive, giving rise to the spectacular biodiversity that has been this planet's hallmark for the past 500 million years. This staggering increase in diversity has raised a fundamental question among evolutionary ecologists: are there limits to the diversity of life? Some scientists say that global diversity increases to an equilibrium point or saturation level that is determined by the system’s carrying capacity. Alternatively, others claim that biodiversity is well below the saturation level and thus we can ignore the existence of any limit. These contrasting views have been fueled by interpretations of the fossil record which is severely biased in space and time. To theoretically test these contrasting views, we have developed a regional diversification model. In the model, we let 1 genus to diversify everywhere in the global ocean from 500 million years ago until present according to a model of paleogeography that constrains the diversification time and a paleo Earth System model that constrains the diversification rate as a function of seawater temperature and food supply. By externally imposing mass extinctions, we explore how the oceans filled with life. The regional model fits surprisingly well global fossil diversity curves and modern biodiversity distributions. According to the model, the ocean is far from saturation except for in the biodiversity hotspots, regions of extraordinarily high levels of diversity, which evolved under prolonged conditions of Earth system stability and maximum continental fragmentation. The model allows us to recreate many things, such as the history of biodiversity hotspots and the dynamics of the latitudinal biodiversity gradient. 

How to cite: García-Comas, C. and Cermeño, P.: INDITEK: A model to understand the emergence of marine biodiversity hotspots in the last 500 million years, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9759, https://doi.org/10.5194/egusphere-egu23-9759, 2023.

Plants respond to environmental change but also impact the Earth system by altering biogeochemical fluxes. Plants and their distribution have also changed profoundly across deep time as major evolutionary groups have evolved, prospered or declined. However, the current representation of this change in models is simplistic. For example, a simple succession of biogeochemical impacts along with evolution of major plant groups (i.e.,sporophytes, gymnosperm, and angiosperms) is assumed through eras of deep time, whereas major plant innovations are known to have evolved through appearance of novel combinations of traits that show variation within these lineages.

In the TERRAFORM project we are integrating trait ecology of extinct plants, state-of-the-art weathering experiments, and multi-scale modeling to study the terrestrial biosphere’s impact on the carbon, nutrient and hydrological cycles in deep-time. We focus on the effects of evolution of plant traits on chemical weathering and the global silicate cycle and how it varies over time, we hope to track how plants have impacted major episodes of biosphere upheaval. Pursuant to that focus, we perform palaeo-Earth weathering and decomposition experiments to understand the difference in effect of plant evolutionary groups and their traits on  biogeochemical fluxes. We apply these empirical insights to plant fossils spanning episodes of major environmental change (Pennsylvanian-Permian glacial interglacial cycles; the Triassic-Jurassic mass extinction; Cretaceous OAEs) to evaluate the impact of environmental change on extinct plants and their traits. This knowledge will improve the chemical weathering and terrestrial ecosystem parameterization and performance of biogeochemical models, to evaluate the plant feedback on other components of the earth system. Ultimately we aim to understand how plants TERRAFORMed the Earth, and how plant functional traits and function evolved over the past 300 million years.

How to cite: Chondrogiannis, C., Nair, R., Matthaeus, W., Kwasniewska, K., O’Dea, K., Barbosa, C., Knettge, A., Jackson, B., and McElwain, J.: TERRAFORM: Trait Ecology and Biogeochemical Cycles in Deep Time, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17419, https://doi.org/10.5194/egusphere-egu23-17419, 2023.

Traditionally, the evolution of trees and the establishment of the first forests during the Devonian (419–359 Ma) have been linked to an enhancement of terrestrial weathering processes and a subsequent reduction of atmospheric carbon dioxide (CO₂) levels by one order of magnitude. However, empirical estimates of early-Devonian CO₂ concentrations are sparse and carry large error bars. Here we use leaf carbon isotopes, stomata density, and stomata pore length from fossilized lycophytes to estimate atmospheric CO₂ levels 410–380Ma based on a mechanistic model for gas exchange calibrated using their closest modern lycophyte relatives. We find that Earth's atmosphere contained about 525–715 ppm of CO₂ before the emergence of forested ecosystems, much less than previously thought. Using a coupled climate model, we show that Earth was partially glaciated at these moderate CO₂ levels and that this cool climate state is in principle agreement with available climate proxies and fossil evidence for the distribution terrestrial vegetation. Finally, we use a process-based biogeochemical model to demonstrate that our results are consistent with a scenario in which enhanced weathering, climate cooling, and atmospheric oxygenation are associated with the earlier emergence of shallow-rooted vascular ecosystems rather than the appearance of the first forests.

How to cite: Feulner, G., Dahl, T. W., Harding, M. A. R., Brugger, J., Norrman, K., Lomax, B. H., and Junium, C. K.: New proxy estimates reveal low atmospheric CO2 levels before the emergence of forested ecosystems, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13901, https://doi.org/10.5194/egusphere-egu23-13901, 2023.

Devonian plants in Siberia present protracted pioneer succession. During the Devonian Angarida was a large and semi-isolated continent within warm, arid zones of the northern hemisphere. Early plants in the Devonian did not originate upon Angarida, instead they are known to migrate to Angarida allowing a unique opportunity to study their changing biogeography and their influence upon the virgin landmass. In the Late Devonian, the Viluy-Yakutsk Large Igneous Province had two active phases and dramatically altered the physical and chemical environment of both the early plants and the marine systems. Our research into the survivourship dynamics of early plant communities upon the palaeocontinent Angarida have demonstrated that transgression and volcanogenic nutrient influx were key to the survival of colonising plants. Taxic proportions show that migrating taxa entered Angarida from the southwest, Kuznetsk and Minusinsk basins, dispersing across the continent in waves through central areas northwards. The patterns of dispersal are consistent throughout the Devonian and into the Early Carboniferous. Increased nutrient load from the active pulses of the Viluy-Yakutsk Large Igneous Province, biogeomorphic ecosystem engineering, and the increased biomass of Angaridan plants are assisted by Late Devonian transgression. These cumulative factors can be linked to the Late Devonian marine extinctions observed in Siberia.

How to cite: Dowding, E., Akulov, N., and Mashchuk, I.: Early Plant responses to Large Igneous Province Activity: the Devonian of Angarida, Siberia., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14439, https://doi.org/10.5194/egusphere-egu23-14439, 2023.

The Upper Devonian deposits of Latvia are rich in fossil vertebrate remains that have been studied for several decades. Attention to pathologies in these remains has been given only recently.

The paleontological record of parasitic behaviour is quite incomplete, particularly regarding the parasites of vertebrates. Although fossil remains of parasitic organisms are very rare, traces of their action and parasite-induced pathologies can be found more often even though these are hard to identify and it is even more complicated to determine what was the organism that left the traces. Various skeletal pathologies from Middle (Givetian) to Upper Devonian (Frasnian, Famennian) fishes from Latvia, Estonia and Western Russia were attributed to attacks of parasites (Lukševičs et al. 2009) and predators (Lebedev et al. 2009). The pathologies caused by parasite action described by Lukševičs et al. 2009 include round and oval fossulae, swellings, variously shaped attachment buttresses, openings on various skeletal elements of fishes, and porous spongy formations found on the non-overlapped surface of the scales of porolepiform sarcopterygians. These pathologies were explained as caused by cestode and trematode infestations, attacks of parasites similar to copepod crustaceans, by activity of pathogenic fungi, bacteria, protozoans or algae. Bite marks on skeletal parts of placoderm antiarchs and arthrodires, sarcopterygian porolepiforms and osteolepiforms, and agnathan pteraspidiforms and psammosteiforms were also reported (Lebedev et al. 2009).

Subsequent excavations from 2018-2020 have provided new paleontological material. The paleontological material from the field work and collection of the Museum of University of Latvia has been studied. Pathologies have been found in porolepiform holoptichiid Holoptychius sp. and Ventalepis ketleriensis scales, osteolepiform Cryptolepis grossi scales from the Upper Devonian Tērvete and Ketleri Formations, antiarch bothriolepid Bothriolepis ciecere from the Ketleri Formation and B. ornata armour plates from the Tērvete Formation. Most common pathologies are regular pits on the overlapped and non-overlapped sarcopterygian scale surface similar to those described earlier. Thin sections and CT scans  that have been carried out for the first time confirm that they were formed during animal life. Shallow pits have also been found on the surface of the placoderm armour plates usually close to plate boundaries. Small drill holes on the placoderm armour plates and sarcopterygian scales, in one case connected with a swelling within the scale, might indicate endoparasite activity. Predator bite marks have also been found on the bones both as lifetime damage traces, in some cases accompanied by pathological regrowth of the bone element, as well as post-mortem scratches and marks. Some of the scales and the armour plates also exhibit lesions of unspecific shape on the bone surface, exact cause of these is currently unclear. Difficulties in interpreting the exact cause of the pathologies still should not disencourage further research.

How to cite: Alksnitis, V.: Pathologies in the fish fossils from the Upper Devonian, Famennian deposits of Latvia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7156, https://doi.org/10.5194/egusphere-egu23-7156, 2023.

How to cite: Husson, L., Salles, T., Lorcery, M., and Hadler Boggiani, B.: Global physiography dynamics controled the Phanerozoic diversification of the biosphere, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5323, https://doi.org/10.5194/egusphere-egu23-5323, 2023.

Aim: Using geochemical proxies to record changes in environmental conditions during the Ediacaran-Cambrian time

Methods: Thin section petrography combined with high-resolution analyses of trace elements and U-Pb, Sr, B and Li isotopes by LA-ICPMS (single and multi-collector)

Problem: Identification of pristine early diagenetic carbonate phases and their distinction from late diagenetic and altered carbonates. 

In this study, we use high spatial resolution multi-element and isotope analyses to identify pristine carbonate phases in samples from recent drill cores from Namibia, drilled by International Continental Scientific Drilling Program project GRIND-ECT. GRIND-ECT aims to capture, in drill core, the Ediacaran-Cambrian transition in key successions worldwide.

The method applied allows for relatively high sample throughput (>200) and precise detection of heterogeneous altered domains. Exclusion criteria are the presence or increased concentration of various trace elements (Al, Rb, LREE ...), heterogeneity of the ⁸⁷Sr/⁸⁶Sr isotope ratio, a lack of discernible populations in U/Pb isotope space, resulting in non-correlated scattering, and too young U-Pb dates.

To date, about 10-15% of the samples analyzed yielded a U-Pb date consistent with depositional age or an early post-depositional age. Geochronologic control of ancient sedimentary sequences is critical for evaluating cause and effect and rates of change in ancient environments. However, precision of LA-ICPMS ages (ca. 1-2%, e.g., 6-11 Ma) is insufficient to resolve changes in the Ediacaran-Cambrian oceans on timescales relevant for biological and environmental change. Therefore, the ages of these pre-characterized samples need to be refined using high-precision methods (Cantine et al., EGU 2023).

Analyses of Li and B isotopes are in progress on the pre-characterized, potentially pristine carbonate areas. We expect to see a change in the Li, B and Sr isotope records through Ediacaran-Cambrian time hopefully with significant anomalies. The correlation of the three isotope systems would be a confirmation for us that we are targeting pristine, early diagenetic carbonates.

How to cite: Gerdes, A., Cantine, M., and Eitel, S.: Ancient carbonates as archives for global environmental changes during Ediacaran-Cambrian time: a geochemical perspective, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14217, https://doi.org/10.5194/egusphere-egu23-14217, 2023.

The Marinoan Snowball Earth event marks a critical time in Earth’s history, with significant climate change characterized by low-latitude glaciation. The ocean is hypothesized to be anoxic during the Marinoan glaciation (ca. 649 to 635 Ma). However, this hypothesis is contradicted with global marine red beds (MRB) deposited in the Marinoan glaciation and diversification of complex eukaryotes after the termination of the Marinoan. To better comprehend the redox conditions of the late Cryogenian seawater, we present systematic sedimentological, mineralogical (SEM), and Fe geochemical analyses (Fe isotope and Fe speciation) of the MRB from the Nantuo Formation in South China.

Two continuous ice advance-retreat cycles in the Nantuo Formation are separated by a MRB sequence, indicating an interglacial period with limited influence from glaciation. The results of Fe speciation show that the predominant phase of Fe_HR (highly reactive Fe) is Fe-oxide with extremely low content of Fe-pyrite and Fe-carbonate of the Nantuo Formation. Furthermore, well-preserved and nanometer-sized hematite (Fe-oxide) particles randomly dispersed in the matrix, suggesting that Fe(II) oxidation occurred in the water column rather than in the sediments. High Fe_HR/Fe_T ratios and near-zero δ⁵⁶Fe_HR values of the MRB suggest scavenging of dissolved Fe(II) by quantitative oxidation at the basal of the Nantuo Formation. A positive δ⁵⁶Fe_HR shift above the MRB further reflects the partial oxidation of Fe(II) to Fe(III) in the water column. These suggest sufficient oxygen derived from meltwater and/or atmosphere during the inter-glacial episode, facilitating the deposition of MRB. The oceanic redox conditions switch to anoxic in response to the re-appearance of the second glacial episode. Our findings suggest that the dynamic redox state of seawater was likely controlled by ice-sheet advancing-retreating cycles and provide new insight into redox conditions during the end-Cryogenian Marinoan glaciation.

How to cite: Meng, Z., Zhou, X., Wang, Z., Ju, P., and Huang, K.: Marine redox fluctuation during the Marinoan Snowball Earth: evidence from red beds of the Nantuo Formation in South China, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4776, https://doi.org/10.5194/egusphere-egu23-4776, 2023.

Glacier-archived records are powerful windows into past climates and ecosystems, but variation in co-preserved microbiota is rarely characterized over long time periods or connected to changing climates. In a 310-meter ice core from the Guliya ice cap, Tibetan Plateau, we linked microbial communities to concomitant climatic conditions across 33 depths, spanning at least the last 150,000 years. Communities differed significantly between cold and warm periods, and among the three major climate epochs Holocene, Last Glacial Stage (LGS), and Pre-LGS. Although source inputs varied during these periods, the microbial changes appeared independently impacted by climate as well. Co-occurrence network analyses suggested the importance of glacial surface-growing communities, and their phototrophs, to the preserved microbial record. The inferred microbial growth (by cell densities, diversity, and potential doubling times pre-compaction into solid ice) on Guliya’s surfaces was higher during cold periods than warm periods, likely associated with deeper snow and firn layers under colder conditions. Three Cyanobacteria “blooms” were captured in the record, and were significantly correlated to overall microbial concentration and diversity, as well as the abundance of two heterotrophic photosynthetic clades (the genus Geodermatophilus and the class Chloroflexia). Taxonomically, 6 genera were historically persistent and dominant throughout the record (Polaromonas, Flavobacterium, Massilia, Aquaspirillum, Pedobacter, and Cryobacterium), one of which (Cryobacterium) exhibited a shift in dominant strains from the Pre-LGS to the LGS, indicating a possible speciation event. Collectively, these findings advance our understanding of ancient glacier-archived microbial communities and the ecological forces they experienced, provide evidence for microbiological responses to the prevailing climate regime, and may shed new light on the microbial evolution across a long-term history over at least the last >150,000 years.

How to cite: Zhong, Z.-P., Li, Y.-F., Davis, M., Van Etten, J., Mosley-Thompson, E., Sullivan, M., Rich, V., and Thompson, L.: Microbial communities differ in warm and cold periods over >150,000 years, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3852, https://doi.org/10.5194/egusphere-egu23-3852, 2023.

Establishing an absolute timeline for the evolution of Ediacaran (latest Neoproterozoic) biota is an outstanding challenge for the geochronology community. Ediacaran biota (“Vendian” in Russian literature; 580-539 Ma) is traditionally divided into three assemblages: Avalon, White Sea and Nama, considered as a partially overlapping evolutionary and temporal successions. The geochronological record of the White Sea assemblage is hampered by the lack of datable volcanic beds, with very few exceptions like in Onega peninsula and along the Winter coast in northwestern Russia. There, we find examples of the most complete and diversified White Sea assemblage of the entire East European Patform (EEP).

We report here results from a field study and U-Pb geochronology on the Lyamtsa and Verkhovka formations from Onega peninsula (Lyamtsa and Agma rivers) and Zimnie Gori formation along the Winter coast. The studied sedimentary successions consist mainly of fine-grained sandstone, mudstone and claystone, reflecting shallow marine conditions, influenced by a large river delta. The base of the White Sea Ediacaran deposits starts with sandstones and conglomerates, overlain by the brown clay with volcanic tuff (only in drillcore) in the base of the Lyamtsa formation, which contain  first simple horizontal trace fossils, worm tubes Calyprtina and cyanobacterial colonies Beltanelloides, predating the onset of the rich Ediacaran fossil assemblage. In the middle part of the Lyamtsa formation (accessible in outcrops) the first Dickinsonia, Parvancorina and Aspidella fossils are found, as well as the oldest trace fossils of arthropods Cruzianа, vertical burrows Altichnus and traces of polychaete-like worms, previously known only from the uppermost Edicaran and Cambrian. The full range of diverse Ediacaran fossils (e.g., various dickinsoniids, trilobozoans, Kimberella, Charnia, Rangea, Pteridinium, etc.) is reached further up in the section (Verkhovka, Zimnie Gori and Erga formations).

We have sampled two ash layers from the base of Verkhovka formation (Agma river) and one from the base of Zimnie Gori formation (along the Winter coast). Our high precision U-Pb zircon CA-ID-TIMS age determinations yielded weighted mean ²⁰⁶Pb/²³⁸U dates of 555.7 ± 0.6 Ma and 555.1 ± 0.7 Ma for the two ash layers of the Verkhovka formation and 552.6 ± 0.6 Ma for the base of Zimnie Gori formation. Applying age depth model calculations we can estimate that the onset of the Ediacaran White Sea assemblage in Lyamtsa formation must be older than 561 Ma. These results, combined with the age of the top of Zimnie Gori formation (550 ± 5 Ma, Llianos et al., 2005) have important implications for: i) estimate of the evolutionary trend of Ediacaran White Sea assemblage or at least for some Ediacaran organisms, which are showing philogenetic evolution (for examle Dickinsonia); ii) better correlations between distribution of Ediacaran macrofossils in the sedimentary sequences of the Eastern and Northeastern part of the EEP.

REFERENCES

Llanos, M.P.I., Tait J.A., Popov, V. & A. Abalmassova (2005) Palaeomagnetic data from Ediacaran (Vendian) sediments of the Arkhangelsk region, NW Russia: An alternative apparent polar wander path of Baltica for the Late Proterozoic–Early Palaeozoic. Earth and Planetary Science Letters 240, 732–747.

How to cite: Ovtcharova, M., Ivantsov, A., Nagovitsyn, A., Zakrevskaya, M., Linnemann, U., Ivleva, A., and Ershova, V.: U-Pb zircon ID-TIMS geochronology of the Ediacaran White Sea assemblage from Onega peninsula, NW Russia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9234, https://doi.org/10.5194/egusphere-egu23-9234, 2023.

The Kuibis Subgroup (~551- <547.36 Ma) in the Nama Basin records an important fauna assemblage change during the Ediacaran. Current models support a transition from a more diverse ‘White Sea’ biotic assemblage to a less diverse ‘Nama’ biotic assemblage, for which the drivers remain unresolved. In spite of this decrease in diversity, the Nama assemblage hosts the first appearance of metazoan biomineralization in the geologic record. High resolution chemostratigraphic data are required to globally anchor the timeline of Earth System changes leading to this innovation of life. In order to track these changes at high-resolution, we performed a sampling campaign of Namibian cores from the ICDP project ‘Geological Research through Integrated Neoproterozoic Drilling: The Ediacaran-Cambrian Transition’ (GRIND-ECT), which focuses on terminal Ediacaran stratigraphy of the Witputs and Vioolsdrif sub-basins of southern Namibia. Here we present new Carbon and Oxygen isotope data for carbonate rocks of the Kuibis Subgroup retrieved from drill core 1G. This core records an approximately 200 m-thick succession of mixed carbonate-siliciclastic sedimentary rocks of that unit. Both dolostones and limestones within the lowermost 100 m display negative δ¹³C values between -5 and 0 ‰, consistent with laterally-correlative outcrop data that record the Basal Nama Excursion (BANE). The oxygen isotope profile recorded in core 1G is also very distinctive throughout the BANE. It starts with a negative shift of the δ¹⁸O values from -5 to -15 ‰, which then progressively increase to values around -12 ‰. The uppermost 100 m of core 1G display a gradual increase in δ¹³C values that reach +5 ‰ and are accompanied by increasing δ¹⁸O values that approach -10 ‰. The 0 ‰ crossing point (carbon isotope value) occurs at ~90 m depth, below strata that confidently record the first appearance of fossils of biomineralizing animals. Magnesium isotopic and paleontological data added to our high-resolution carbon isotope profile have the potential to refine the knowledge about the biological and environmental changes of this time period. 

How to cite: Paula-Santos, G., Kasemann, S., Bowyer, F., Trindade, R., Babinski, M., Wood, R., and Leme, J.: High-resolution carbon, oxygen and magnesium isotope chemostratigraphy of the Kuibis Subgroup, Nama Basin, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11907, https://doi.org/10.5194/egusphere-egu23-11907, 2023.

The marine carbon isotope record (δ¹³C) used for chemostratigraphy and reconstruction of carbon cycle dynamics is constructed using carbonate rocks, but there is evidence that carbonate cements hosted within fine-grained clastics (shales and mudstones) in some settings may also express δ¹³C trends that covary with the record from carbonates. We present new carbon and oxygen isotopic data from shale-hosted carbonate cements (δ¹³C_carb-sh and δ¹⁸O_carb-sh, n = 107, <16 wt% CaCO₃) of the terminal Ediacaran Nama Group, Namibia (≥550.5 to <539.6 Million years ago; Ma). These data are compared with the published carbon and oxygen isotopic record from coeval carbonates (δ¹³C_carb and δ¹⁸O_carb, n = 1611) and total organic carbon (TOC) concentrations. We show that δ¹³C_carb-sh compositions in samples of intermediate to high CaCO₃/TOC can approximate contemporaneous δ¹³C_carb in open marine mixed carbonate-clastic settings. By contrast, δ¹³C_carb-sh values in samples with low CaCO₃/TOC that were deposited in clastic settings distant from the locus of carbonate deposition are more negative than contemporaneous δ¹³C_carb. These data suggest that δ¹³C_carb-sh may approach seawater composition in samples of low TOC when deposited in high dissolved inorganic carbon (DIC) settings, where carbonate can rapidly precipitate from seawater during early diagenesis. However, the use of δ¹³C_carb-sh to infill gaps in the existing δ¹³C_carb record remains uncertain, even when these criteria are fulfilled. Intervals of δ¹³C-δ¹⁸O co-variability in the Nama Group succession appear to correlate with units where seawater mixing with meteoric fluids was more likely during early diagenesis, such as clastic-dominated settings, which also show significant decreasing δ¹⁸O through time with gradual sub-basin infill.We further consider uncertainties in lithostratigraphic correlation of the upper Urusis Formation of the Nama Group that enable three new possible correlations to be proposed for δ¹³C_carb-sh data within the terminal Ediacaran to lower Cambrian (<542.65 Ma to >535 Ma) regional and global δ¹³C_carb records.

How to cite: Yilales, M., Bowyer, F., Wood, R., and Poulton, S.: Insights into the terminal Ediacaran marine carbonate record from shale-hosted carbonate carbon isotopes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15489, https://doi.org/10.5194/egusphere-egu23-15489, 2023.

The early evolution of macroscopic life, including the Cambrian ‘explosion’ of animals, took place over more than 100 million years against the backdrop of dynamic environmental changes like unstable redox conditions and nutrient supply. The terminal Ediacaran to early Cambrian transition is marked by the appearance of macroscopic biomineralizers, but also a seeming decline in diversity, in contrast to rich communities of macroscopic Ediacara-type biota preserved in older units worldwide. Our understanding of this biotic change as well as its environmental divers suffers from stratigraphic incompleteness, as continuous mid and upper Ediacaran, and Cambrian strata are rarely exposed in a continuous sequence in the same area, or represent different depositional settings.

The Mackenzie Mountains in the Northwest Territories, Canada are a home to a nearly continuous sedimentary succession from the Cryogenian to the Cambrian and as such, provide a great target to analyse biotic and environmental changes through time. We have investigated the interval of carbonates and siliciclastics through the Blueflower, Risky, Ingta, Backbone, and Vampire formations and present integrated palaeontological and geochemical data to produce a high resolution biostratigraphic and chemostratigraphic profile for the Ediacaran-Cambrian boundary interval.

The Ediacaran-Cambrian boundary occurs in the Ingta Formation, marked by sporadic trace fossils Treptichnus and Harlaniella. Simple surficial traces Planolites occur throughout the formation. Also common are well-preserved bacterial filamentous organic-walled microfossils in mudrocks, but the boundary interval lacks complex acritarchs or cuticular animal remains. Macroscopic carbonaceous fossil problematica were also recovered. Complex trace fossils occur in the overlying Backbone Ranges Formation and become more common in the Vampire Formation, but are absent from the lowermost Cambrian strata in the Mackenzie Mountains. The primary producer dominated environment and the absence of organically preserved animal remains in a setting conducive to organic preservation imply a true decline in diversity through the boundary interval – possibly a result of either phylogeny or ecology.

How to cite: Agic, H., Smith, M., and Kovalick, A.: Primary producer dominated environments of the Ediacaran-Cambrian transition: palaeontological insights from the Mackenzie Mountains, Canada, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16040, https://doi.org/10.5194/egusphere-egu23-16040, 2023.

Global Boundary Stratotype Section and Points (GSSP’s) are often regarded as immutable points, that once ratified should “remain fixed in spite of discoveries stratigraphically above and/or below” (Cowie, 1986, p. 79). The Precambrian–Cambrian boundary, defined at Fortune Head by the FAD of the trace fossil Treptichnus pedum, has been a source of controversy and debate since its ratification. Treptichnus pedum has proven to be a difficult marker to apply to global correlation, and lack of other markers at Fortune Head (skeletal fossils, chemostratigraphic, magnetostratigraphic or radiometric data) has prompted the use of other “unofficial” indicators of the Precambrian–Cambrian boundary in many sections around the world. δ¹³C chemostratigraphy has become a standard global correlation tool, and the integration of δ¹³C isotopes with other “multi-proxy” data is an approach that is becoming increasingly adopted. Discoveries and advances in such methods and techniques demonstrate that new data can (and should) enable the fine-tuning of stratigraphic boundaries. Recent work in southwestern Mongolia through the Precambrian–Cambrian boundary section at Bayan Gol demonstrates the utility of multi-proxy stratigraphic data to defining and correlating the base of the Cambrian. Here, two proxies are used in concert to identify the Precambrian–Cambrian boundary; the first occurrence of the small shelly fossil Protohertzina anabarica and the nadir of the BACE (δ¹³C excursion). This section demonstrates 1) the value of carbonates in their capacity to preserve a wider variety of stratigraphic proxies than siliciclastics, 2) the importance of systematically measured and sampled stratigraphic sections (rather than composite sections) for regional and global correlation and 3) the need for a redefined Cambrian GSSP, with a supporting ASSP as a pathway toward global correlation of the base of the Cambrian.

How to cite: Topper, T., Betts, M., Dorjnamjaaa, D., Li, G., Li, L., Altanshagai, G., Enhkbaatar, B., and Skovsted, C.: Two out of three ain’t bad: Dealing with proxy paucity when correlating the base of the Cambrian, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5782, https://doi.org/10.5194/egusphere-egu23-5782, 2023.

Trilobites are the most diverse animal phylum in the Cambrian, and are critical stratigraphic markers. Despite their significance, the exact sequence of trilobite appearances across the globe, and their respective ages, are difficult to reconstruct due to a scarcity of radiometric dates. Using visual correlation of carbon isotope (δ¹³C) excursions and integration of available U-Pb age constraints, Landing et al. (2021) have estimated the age of the first Siberian trilobites at roughly 521 million years ago (Ma), which would make them the world’s oldest trilobites. Here, we aim to provide a more precise date, with uncertainty, using a Bayesian stratigraphic age model that takes the global signature of δ¹³C excursions to correlate proxy records from multiple locations. Our model integrates radiometric dates across sections, using ages from well-dated sections to inform age estimates in sections with little or no age information. The model works by evaluating the fit of Bayesian splines to different alignments of the sections, using Markov chain Monte Carlo methods to obtain the posterior distributions. This approach provides an objective evaluation of different possible alignments, generating a probabilistic age-depth model. We apply this model to the lower Cambrian sections of the Anti-Atlas mountains in Morocco, and to the Sukharikha River section of the northwestern Siberian platform. The Moroccan sections provide a uniquely detailed δ¹³C record and multiple radiometric dates, and this age information is transferred to the Siberian section and the emergence of trilobites in the fossil record. Preliminary results indicate that the first appearance of Siberian trilobites is younger than previously estimated, and consequently closer in time to the first trilobites in Morocco and elsewhere.

Landing, E., Schmitz, M., Geyer, G., Trayler, R., & Bowring, S. (2021). Precise early Cambrian U–Pb zircon dates bracket the oldest trilobites and archaeocyaths in Moroccan West Gondwana. Geological Magazine, 158(2), 219-238.

How to cite: Eichenseer, K., Sinnesael, M., Smith, M. R., and Millard, A. R.: Dating the first Siberian trilobites with a Bayesian, stratigraphic age model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16572, https://doi.org/10.5194/egusphere-egu23-16572, 2023.

Oxygenation during the Cambrian Radiation progressed via a series of short-lived pulses. However, the metazoan biotic response to this episodic oxygenation (and potentially productivity changes) has not been quantified, nor have the causal evolutionary processes been constrained. Here we present analyses of the dynamics of early Cambrian metazoan body size changes, habitat distribution, and ecological complexity on the Siberian Platform (525–510 Ma).

First, we quantify high-resolution changes in species body size in archaeocyath sponges, hyolith lophophorates, and helcionelloid molluscs, and brachiopods. Archaeocyath, hyoliths, and helcionelloids, show dynamic and synchronous trends over million-year timescales, with peaks in body size during the latest Tommotian/early Atdabanian (~521–519 Ma) and late Atdabanian/early Botoman (~519–516.5 Ma), and notably small body sizes in the middle Atdabanian and after the Sinsk anoxic extinction event, starting ca. 513 Ma. These intervals of body size changes are also mirrored in individual species and correlate positively with increased rates of origination and broadly with total species diversity. Calcitic brachiopods (rhynchonelliformeans), however, show a general increase in body size following the increase in species diversity through this interval; phosphatic brachiopods (linguliformeans) show a body size decrease that negatively correlates with diversity. Both brachiopod groups show a rapid recovery at the Sinsk Event. The synchronous changes in these metrics in archaeocyaths, hyoliths and helcionelloids suggest the operation of external drivers through the early Cambrian, coincident with two oxic or productivity pulses. But the trends shown by brachiopods suggests a differing physiological response. Together, these dynamics created both the distinct evolutionary record of metazoan groups during the Cambrian Explosion and determined the nature of its termination.

Second, during the oxic pulse at ~521–519 Ma, we quantify the expansion of archaeocyath sponge reef habitat coupled to an increase in reef size and metacommunity complexity, from individual within-community reactions to their local environment, to ecologically complex synchronous community-wide response, accompanied by an increase in rates of origination. Subsequently, reef and archaeocyath body size are reduced in association with increased rates of extinction due to inferred expanded marine anoxia (~519–516.5 Ma). The later oxic pulse at ~515 Ma shows further reef habitat expansion, increased archaeocyath body size and diversity, but weaker community-wide environmental responses.

These metrics confirm that oxygenation events created temporary pulses of evolutionary diversification and enhanced ecosystem complexity, potentially via the expansion of habitable space, and increased archaeocyath individual and reef longevity in turn leading to niche differentiation. Most notably, we show that progression towards increasing biodiversity and ecosystem complexity was episodic and discontinuous, rather than linear, during the Cambrian Radiation.

How to cite: Wood, R., Zhuravlev, A., Mitchell, E., Bowyer, F., and Penny, A.: Biotic responses to oxygenation pulses during the Cambrian Radiation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6317, https://doi.org/10.5194/egusphere-egu23-6317, 2023.