Studies of the Silurian sequence on Gotland have significantly advanced our understanding of Silurian climate dynamics, with much of the research focusing on small outcrops and short cores. Gotland has an extensive network of abandoned oil and gas wells. Most of these include gamma-ray well logs, which have not yet been fully utilised for their stratigraphic value. While the Ordovician succession has been successfully correlated using these well logs, the correlation for the Silurian succession has yet to be fully realised. The present study addresses some of the limitations of correlations performed using Dynamical Time Warping (DTW) by combining DTW with Barycenter Averaging (DBA). This enables a semi-automated correlation of the Silurian well-logs over a ~60 km-long transect and the subdivision of gamma-ray log logs into parts that can be linked with their lithostratigraphic surface counterparts. Additionally, we tracked changes in gamma-ray logs associated with these subsurface units, providing new insights into biogeochemical events (e.g., δ¹³C excursions) previously mapped only at the surface of Gotland. Our results demonstrate the application of a DTW/DBA-based workflow for stratigraphic correlation and highlight the potential to integrate subsurface well-log data to refine Silurian stratigraphy on Gotland.

The Silurian record is punctuated by several large, positive stable carbon isotope excursions that coincide with extinction events. Increasingly, these events are interpreted as Silurian Ocean Anoxic Events (OAEs). Malformed chitinozoans (extinct organic-walled zooplankton) recently emerged as potential early indicators of these biogeochemical disturbances. This study is part of an integrated research effort testing the hypothesis that increased chitinozoan teratology is a hallmark signature of most of these Silurian OAEs. Here we focus on the Ireviken Event (lower Wenlock; 433 Ma). Examination of more than 500,000 chitinozoan specimens in 74 samples from the well-studied Lusklint 1 outcrop and the complementary Lusklint-1 core (Gotland, Sweden), allowed their classification into normal, abnormal, and potentially abnormal categories. Ratios of normal versus abnormal chitinozoans across the Ireviken Event reveal malformation concentrations 3 to 33 times higher than background levels. Notably, the rise in teratological forms begins below the lowest conodont extinction datums that traditionally define the onset of the extinction event. This extended anomaly suggests that environmental stress, likely related to marine redox instability and metal contamination, preceded the earliest biotic turnover. In vivo teratologic growth is an emerging powerful proxy for identifying original paleo-oceanographic stress. Our findings establish malformed chitinozoans as early harbingers of this pivotal Silurian OAE. By integrating detailed records of teratology with the stratigraphic context of the Ireviken OAE, this study demonstrates that malformations offer a sensitive and underutilized tool for reconstructing the sequence and drivers of Silurian OAEs.

The rock record preserves the physical, chemical, and biological history of the Earth including dozens of biogeochemical events that fundamentally altered the Earth system. In order to evaluate the structure and duration of these events and accompanying Earth system changes, both rock sequences and numerical time must be integrated. Historically, these two exercises, chronostratigraphy for the rock record, and geochronology for the numerical record, have been decoupled due to uneven or sparse data developed in globally dispersed rock sequences via traditionally disparate disciplines (stratigraphy and radioisotope geochemistry). Here, we present an interdisciplinary study of the Silurian Altajme drill core (Gotland, Sweden) to illustrate how high-resolution carbon isotope stratigraphy conditioned by new high-precision CA-ID-TIMS U/Pb zircon bentonite dates within a Bayesian age-depth model dramatically changes perspectives on the rates of change and detailed substructure of two major Silurian positive carbon isotope excursions (Ireviken and Mulde) with implications for the potential drivers of these biogeochemical events.

The Black Hills, South Dakota, USA, expose thick packages of Devonian-Carboniferous carbonates that represent the shallow-water succession on the NW margin of the Transcontinental Arch. These carbonates are the up-ramp facies of the well-studied Devonian-Carboniferous Boundary succession in the Williston Basin farther to the NW. This basin margin succession is critical to developing a detailed sequence stratigraphic history of the Williston Basin and the U.S. Midcontinent but little chronostratigraphic information is available from these strata beyond their designation to the Mississippian Subsystem. Here, we sampled the uppermost beds of the Englewood Formation and similar to 60 m of the Pahasapa Formation for integrated conodont and carbon isotope biochemostratigraphy at the Spearfish Canyon section in the Black Hills including a total of 34 conodont and 203 carbonate carbon isotope samples. Conodont yields were generally poor, but the Mickelson Member of the Englewood Formation contained conodonts of the Siphonodella sandbergi Zone and the lower portion of the Pahasapa Formation contained Siphonodella crenulata Zone fauna. No discernable carbon isotope excursion was identified, and delta C-13 values persist above +3 parts per thousand throughout the top of the sampled interval. The Kinderhookian-Osagean Regional Stages boundary could not be placed precisely at the outcrop. However, there is no evidence that the portion of the Pahasapa Formation sampled extends any higher than within the global Tournaisian Stage.

The tristate area of Iowa, Illinois, and Missouri contains some of the best-exposed Mississippian strata in the world, including the type area for the Mississippian subsystem, across a broad carbonate platform known as the Burlington shelf. Strata have been mapped as thinnest along the central middle shelf and thickening both up-ramp and down-ramp, forming a complex dumbbell-like stratigraphic pattern rather than a simple clinoform geometry thinning into the basin. Additionally, two significant hiatuses at the Devonian-Carboniferous boundary and Kinderhookian-Osagean boundary greatly complicate stratigraphic correlations across the region. As a result, the precise temporal relationships between strata deposited across the region remain uncertain. Two large biogeochemical events occurred during this interval that provide facies-independent chronostratigraphic tools: the Hangenberg event, which marks the Devonian-Carboniferous boundary, and the Kinderhookian-Osagean boundary event. To target these events, we collected 66 conodont samples and 1005 carbonate carbon isotope samples from three cores and three outcrops and integrated the results with existing data from key facies/depth transitions across the Burlington shelf. Our new data demonstrate a complex relationship among complementary stratigraphic thicknesses, where the Devonian-Carboniferous boundary interval is thin or absent in the up-ramp inner-shelf setting and preserved in a significantly expanded interval in the central to distal middle-shelf deposits of southeast Iowa and northeast Missouri. However, the overlying Kinderhookian-Osagean boundary interval is not preserved in this down-ramp setting but is preserved in significantly expanded strata in the up-ramp inner-shelf setting of central Iowa.

A pre-Late Wisconsinan, post-Illinoian, till sheet has long been recognized in north-central Iowa, but has not been formally recognized or defined until now. Early researchers referred to these deposits as the 'Tazewell', and the term 'Sheldon Creek' was more recently used informally by the Iowa Geological Survey in guidebooks and reports. Recent mapping has extended the eastern margin significantly past previous interpretations. The Sheldon Creek Formation has similar lithologic characteristics to the overlying Alden Member of the Dows Formation, and the two units are distinguished mainly by stratigraphic position. Differentiation from underlying Pre-Illinoian till units is accomplished using lithology, primarily matrix grain-size and sand fraction lithology. A suite of 22 radiocarbon ages indicate two distinct, separate groupings within the Sheldon Creek data. These data strongly suggest ice advanced south to 42 degrees N twice, once during Marine Isotope Stage (MIS) 3 and again during late MIS 3 or possibly early MIS 2. The presence of the Laurentide Ice Sheet in northern Iowa during MIS 3 has significant implications for ice sheet reconstructions during this interval.

New high-resolution paired carbonate carbon (δ13Ccarb) and nitrogen (δ15Nbulk) isotope records of the Steptoean Positive Carbon Isotope Excursion (SPICE) from the Laurentian epicontinental platform reveal a transient negative excursion in δ15Nbulk that closely aligns with the onset of the SPICE event and the initiation of the positive δ13Ccarb excursion. The negative excursion in δ15Nbulk identified here likely demonstrates an increase in microbially fixed nitrogen by diazotrophs, or cyanobacteria capable of N2 fixation, in the marine system due to a reduction in bioavailable recycled N through several processes at the onset of the biogeochemical event. Expansion of reducing environments during the initiation of the SPICE event would have liberated phosphorous previously bound to organic matter and/or marine iron oxy-hydroxides and simultaneously promoted an expansion of denitrification in oxygen-poor settings that would have reduced bioavailable N in the marine system. The enhanced delivery of phosphorous to the photic zone would have stimulated primary productivity, which would have also put additional pressure on the marine bioavailable N budget. These feedback systems would have combined to lower the marine N:P ratio such that diazotrophs would become a significant contributor to the total bioavailable N in the marine realm. The data presented here are the first demonstration of this significant perturbation to local marine nutrient cycling during the onset of the SPICE event and provide new insight into what may ultimately be a consistent set of biogeochemical triggers for major Paleozoic biogeochemical events.

New δ 34 S py (pyrite) and δ 34 S CAS (carbonate‐associated sulfate) across the Llandovery‐Wenlock boundary (∼432 Ma) provide evidence for the expansion of reduced marine environments during the Ireviken Biogeochemical Event. This event consists of a major positive carbon isotope excursion, increased biotic turnover, and other major perturbations and changes within biogeochemical cycles. This interval of time has been hypothesized to coincide with an expansion of reducing marine environments that caused increased organic carbon burial and led to the Ireviken positive carbon isotope excursion (ICIE). Previous high‐resolution carbon isotope work in the Altajme core from Gotland, Sweden provides the highest resolution record of the ICIE yet documented and provides an ideal expanded stratigraphic section to study this event. Local expansion of reduced marine environments within the deeper shelf setting of the Altajme core is indicated by a positive shift in δ 34 S py values and increase in pyrite sulfur concentrations at the onset of the ICIE. These data are indicative of increased microbial sulfate reduction within this portion of the Baltic Basin. Combined with new δ 34 S CAS data from this core, as well as additional data from distant basins, the new data presented here suggest a global expansion of reduced environments led to an increase in organic carbon burial and the ICIE.

An oxygen-rich atmosphere is essential for complex animals. The early Earth had an anoxic atmosphere, and understanding the rise and maintenance of high O 2 levels is critical for investigating what drove our own evolution and for assessing the likely habitability of exoplanets. A growing number of techniques aim to reproduce changes in O 2 levels over the Phanerozoic Eon (the past 539 million years). We assess these methods and attempt to draw the reliable techniques together to form a consensus Phanerozoic O 2 curve. We conclude that O 2 probably made up around 5–10% of the atmosphere during the Cambrian and rose in pulses to ∼15–20% in the Devonian, reaching a further peak of greater than 25% in the Permo-Carboniferous before declining toward the present day. Evolutionary radiations in the Cambrian and Ordovician appear consistent with an oxygen driver, and the Devonian “Age of the Fishes” coincides with oxygen rising above 15% atm. ▪ An oxygen-rich atmosphere is essential for complex animals such as humans. ▪ We review the methods for reconstructing past variation in oxygen levels over the past 539 million years (the Phanerozoic Eon). ▪ We produce a consensus plot of the most likely evolution of atmospheric oxygen levels. ▪ Evolutionary radiations in the Cambrian, Ordovician, and Devonian periods may be linked to rises in oxygen concentration.

The Upper Silurian Salina Group of eastern North America is well known for its thick evaporite successions and hydrocarbon resources. These strata have been assigned to numerous chronostratigraphic schemes within Ohio and Michigan and are currently identified by varying subsurface and outcrop nomenclatural schemes. These chronostratigraphic challenges have persisted for over 50 yr and dramatically inhibit the correlation of events recorded in the Silurian section of eastern North America with the global record of Silurian biogeochemical events. To help resolve the chronostratigraphic correlation of these units, we provide new high-resolution δ13Ccarb chemostratigraphic analyses of a core located in central Ohio for strata assigned to the Greenfield and Tymochtee Formations and integrate existing biostratigraphic, chemostratigraphic, and subsurface geophysical data in western, southern, and eastern Ohio. The new data presented here, integrated for the first time with basinwide subsurface geophysical data, demonstrate a mid-late Homerian Stage global sea-level lowstand, suggest a short interval of tectonic stability within the study area at the beginning of “Salina B–G” deposition, during which accommodation was occupied by the Greenfield Formation and laterally equivalent strata, and provide chronostratigraphic constraints for basin flexure and potential forebulge migration associated with renewed tectonic activity. The new chronostratigraphic correlation of these strata provides a broader picture of Silurian environmental change across the eastern half of the Laurentian paleocontinent.

Conodonts are an extinct group of primitive jawless vertebrates whose elements represent the earliest examples of a mineralized feeding apparatus in vertebrates. Their relative relationship within vertebrates remains unresolved. As teeth, conodont elements are not homologous with the dentition of vertebrates, but they exhibit similarities in mineralization, growth patterns, and function. They clearly represent an early evolutionary experiment in mineralized dentition and offer insight into analogous dentition in other groups. Unfortunately, analysis of functional performance has been limited to a handful of derived morphologies and material properties that may inform ecology and functional analysis are virtually unknown. Here we applied a nanoscale approach to evaluate material properties of conodont bioapatite by utilizing Atomic Force Microscopy (AFM) nanoindentation to determine Young's modulus (E) along multiple elements representing different ontogenetic stages of development in the coniform-bearing apparatus of Dapsilodus obliquicostatus. We observed extreme and systematic variation in E along the length (oral to aboral) of each element that largely mirrors the spatial and ontogenetic variability in the crystalline structure of these specimens. Extreme spatial variability of E likely contributed to breakage of elements that were regularly repaired/regrown in conodonts but later vertebrate dentition strategies that lacked the ability to repair/regrow likely required the development of different material properties to avoid structural failure.

Evidence for a glacial advance into the Upper Mississippi River Basin between Marine Isotope Stage (MIS) 2 and MIS 6 has been debated for over a century. New sedimentological and chronological data provide evidence for two advances of the Laurentide Ice Sheet (LIS) into Iowa during MIS 3 to the east and west of the local glacial maximum margin. The lithology of the MIS 3 till is similar to the MIS 2 Des Moines Lobe till, which suggests both tills were sourced from the Keewatin Dome. A synthesis of new and previously collected radiocarbon ages indicate the two advances reached their terminus near 42⁰ N around 42 ka and 30 ka. A comparison of this record with published loess chronology and provenance studies suggests that these advances were likely a glacial source for the Mississippi Valley Roxana Silt and the Pisgah Formation loess in the Missouri River Valley. The apparent discrepancy between the basal ages of the Roxana and Pisgah loesses may be due to the timing of Des Moines Lobe entering the Mississippi River watershed before the Missouri River watershed. The results of this study demonstrate that the LIS advanced much farther south during MIS 3 than previously recognized, which has significant implications for ice sheet and paleoclimate modeling.

New high-resolution organic (delta C-13(org)) and carbonate (delta C-13(carb)) carbon isotope data from middle shelf deposits in southeastern Iowa demonstrate decoupled signals during the onset of the Hangenberg Event and across the Devonian-Carboniferous Boundary. High-resolution sampling captures a transient negative excursion in delta C-13(org) during the initiation of rising delta C-13(carb) values at the onset of the Hangenberg Event that ends prior to the onset of the final rise in delta C-13(carb) to values greater than +6.0% in the Louisiana Limestone. This negative excursion in delta C-13(org) is coincident with a significant increase in Total Organic Carbon (TOC) content in the underlying English River Formation, which likely corresponds to the well-known Hangenberg Black Shale of the classical European sections. The complex behavior of the carbon isotope record recovered here, combined with recently published geochemical data from classical European sections, demonstrate that a succession of geochemical events took place during the initiation of this global biogeochemical event that include a negative excursion in both delta C-13(carb) and delta C-13(org) prior to the major positive carbon isotope excursion, and that the role of organic carbon burial in this Devonian Oceanic Anoxic Event (OAE) extends well beyond the depositional interval of the Hangenberg Black Shale.

High-resolution paired analyses of delta C-13(carb) and delta C-13(org) from a new drill core from Gotland, Sweden, demonstrate asynchronous positive change in the carbon isotope records during the onset of one of the major Silurian biogeochemical events known as the Mulde Event or "Big Crisis". The detailed carbon isotope record presented here provides Delta C-13 (the difference between delta C-13(carb) and delta C-13(org)) and allows the calculation of changes in organic carbon burial (f(org)) throughout the late Wenlock. The paired data suggest a similar to 38% increase in f(org) during the peak of the positive delta C-13(carb) excursion and the high-resolution record reveals several short-lived inflections in Delta C-13 that have not been previously identified. When combined with sedimentological and sequence stratigraphic data from multiple paleocontinents, the new data presented here provide strong evidence for a transient global decrease in CO2, in support of previous interpretations of regression and global cooling coinciding with the Mulde Extinction Event.

Paired records of δ13Ccarb and δ13Corg across the Llandovery-Wenlock boundary demonstrate asynchronous behavior during the onset of the Ireviken Biogeochemical Event (IBE). The extremely high-resolution data produced from the Altajme Core, drilled from Gotland, Sweden, capture a negative excursion in δ13Corg during the initiation of the Ireviken Extinction Event (IEE) and prior to the onset of the Ireviken positive δ13Ccarb Excursion (ICIE). The record of carbon isotopic changes through this interval illustrate that both Δ13C (the difference between δ13Ccarb and δ13Corg) as well as the relative flux of organic carbon burial (forg) vary in unique ways and at different times during the progression of the IBE. Both process-oriented variables within the global carbon cycle (Δ13C and forg) track a series of events that help to demonstrate potential causative mechanisms of both the extinction and carbon cycle perturbation. The sequence of events demonstrated here largely mirror the cascade of events that took place during the Cretaceous Oceanic Anoxic Event 2 (OAE2) and a detailed comparison between the two events is provided here for the first time. The unique insight into the IBE presented in this work results primarily from the novel, nearly Neogene-scale resolution of the paired isotope data, which demonstrates the critical importance of high-resolution chemostratigraphic research to evaluating ancient perturbations to the Earth-life system. Additional data sets of equal or greater resolution through this interval will be critical to evaluate the global synchroneity of these short-lived events during the IBE, and similar high-resolution studies of other Paleozoic biogeochemical events may shed light on potentially similar causative mechanisms.