The Dover 33 Reef, part of the Niagaran Reef Complex in Northern Michigan (USA), has been the focus of an enhanced oil recovery/carbon capture utilization and storage (EOR/CCUS) project as part of ...the Phase III-Midwest Regional Carbon Sequestration Partnership (Gupta et al., 2013a,b). The Dover 33 structure has experienced significant CO2 flooding in the past two decades, and over the course of the current injection study (between February 2013 and July 2016) has received approximately 100 to 1000 tonnes/day into the central injection well (L-M) 1–33). As part of the geochemical monitoring effort of the study, gas and fluid samples were collected from Dover 33 reef, and several other nearby reef structures, to assess the impact of CO2 injection on the geochemical processes occurring within in the reef.
The injected gas is composed of approximately 95% CO2, with a δ13CCO2 of ∼20.5‰, which is consistent with previously published compositions of Antrim shale gas, the source of the CO2. The concentrations and isotopic compositions of higher pressure gas collected from the L-M 5–33 monitoring well were similar to those measured in the injection well, but did exhibit a small but systematic shift in isotopic composition towards lower values over the course of the study, suggesting mixing and dilution between the gas in the reservoir and the injected gas. In contrast the δ13CCO2 of gas samples from the monitoring well with the lower surface pressure, L-M 2–33, are consistently lower throughout the study, ∼18.5‰, indicating that reactions with the injected CO2 are occurring within the reef or with the well casing.
Fluid samples were collected to assess the extent of interaction among the injected gas, the reservoir rock, and the brine. The brine samples are acidic (pH ∼ 4.1 to 4.9) with a total salt content of nearly 400 g/L. Analysis of the isotopic composition of dissolved inorganic carbon (DIC) in the Dover 33 brine shows that δ13C is higher than the injected gas (27–33‰) suggesting that the gas is not in equilibrium with DIC in the brine, and that there has been little isotopic exchange with carbonate minerals in the reef. The water isotope composition of the brine, δ18O and δD, plot below the meteoric water line, indicating that the water is not of recent meteoric origin and has undergone isotopic exchange with both gas and minerals within the reef structure. The 87Sr/86Sr ratios of the brine samples range from 0.70865 to 0.70869, consistent with Silurian seawater composition.
Geochemical modelling of the brine composition shows that the predicted CO2 solubility as DIC is much greater than the measured DIC, and that the brines are supersaturated with respect to carbonate minerals, suggesting the potential for significant trapping of CO2 in both dissolved and mineral form.
•The Dover 33 Reef has been the focus of an EOR/CCUS project as part of the MRCSP.•The composition of gas within the reef structure closely reflects the injected gas.•δ18O reflects isotopic exchange between the injected gas and the brine.•Geochemical modelling suggest the potential for CO2 trapping in dissolved and mineral form.
While it is widely accepted that early animals originated and primarily evolved during the Neoproterozoic to Cambrian period, there remains ongoing debate over how fluctuations in marine-atmospheric ...oxygen levels influenced their evolution and diversification. To investigate this, we analyzed pristane/phytane ratios—a redox-proxy based on organic geochemistry—in surface sediments from the Cryogenian to Cambrian successions in South China, Oman, and Australia. The temporal changes in this proxy exhibited consistent patterns across all sites, revealing five cycles of anoxic to oxic conditions in the ocean between 660 and 510 Ma. By examining the average pristane/phytane ratios, we identified three events when oxygen levels increased. This represents a transition from anoxic conditions to anoxic-dysoxic boundary at 630–600 Ma, followed by a shift to dysoxic conditions at 570 Ma (Shuram event), and finally reaching oxic conditions at 520 Ma (Cambrian explosion). Significantly, these oxygenation events align with eumetazoan evolution. An inverse relationship was observed between oceanic redox events and positive/negative shifts of δ13Ccarb, occurring between the first and second oxygenation events, suggesting an increase in atmospheric oxygen levels. Based on these findings, we propose that there were global increases in oceanic and atmospheric oxygen levels, at least during the early Ediacaran period. These shifts in oxygen levels likely played a role in influencing the rate of evolution among early animals.
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•Organic proxy shows five anoxic-oxic cycles in the Cryogenian to Cambrian oceans.•Temporal patterns of redox change from three different areas are in agreement.•Ocean oxygenation events coincided with climate changes and evolution of eumetazoans.•The oxygen-carbon cycle turnover ∼600 Ma shows an increase in atmospheric oxygen.•The early Ediacaran oxygenation was followed by the evolution of bilateria.
Carbon isotope (δ
13C) analyses of marine carbonates spanning the Silurian–Devonian transition are compared from three richly fossiliferous, well-dated sequences in North America. The three sections, ...in the central Appalachian Mountains (West Virginia), Great Basin (Nevada), and the southern Mid-continent (Oklahoma), reveal positive δ
13C shifts beginning in the late Pridoli and reaching peak values as heavy as +5.8‰ in the earliest Lochkovian following the first occurrence of the conodont species
Icriodus woschmidti and the graptolite
Monograptus uniformis. A positive shift in δ
13C is also recorded at this time in Gondwanan regions, including the global stratotype section and point for the Silurian–Devonian boundary at Klonk in the Czech Republic, as well as in sections in the Carnic Alps of Austria (Cellon), and Queensland, Australia. The available data from Euramerica and Gondwana are consistent with a scenario linking seawater δ
13C enrichment to a eustatic drop during the Silurian–Devonian transition. Seawater δ
13C likely increased as a result of enhanced carbonate weathering during exposure and erosion of older Silurian platform deposits. In addition, the fall in sea level appears to have enhanced nutrient delivery to the oceans and triggered an increase in organic carbon burial rates at or near the Silurian–Devonian boundary, as indicated by organic-rich deposition in Gondwanan basins. In terms of its magnitude, the Silurian–Devonian δ
13C excursion of ≥+5‰ appears to be among the largest well-documented events in the Paleozoic, comparable to the shifts in the Late Ordovician and Early Mississippian.
A stepwise change in atmospheric oxygen (O2) levels during the Ordovician has been attributed to the emergence of land plants. This phenomenon is tied to a major baseline shift in the stable carbon ...isotope (δ13C) curve and inferred increase in nutrient delivery and enhanced primary productivity in nearshore settings, which led to high organic carbon burial. The timing and magnitude of this baseline shift, however, is still elusive in part because of the lack of high-resolution δ13C data that span this period. Much of the existing Ordovician δ13C literature is focused on isotopic excursions with less emphasis on identifying long-term shifts in baseline (pre- and post-excursion) values.
This study presents new high resolution δ13C data from stratigraphic sections at Germany Valley (West Virginia) and Union Furnace (Pennsylvania) in the Central Appalachian Basin. These sections span the entire Sandbian Stage and continue into the lower Katian Stage. The δ13C data from both sections are characterized by relative stability carbon isotope values (mean = −0.61‰) in the lower Sandbian, followed by a + 1.2‰ shift in the upper Sandbian (mean = +0.62‰). Herein, we propose that the positive shift represents a long-term global shift in baseline δ13C values of dissolved inorganic carbon. The timing of this positive shift coinciding with the diversification of early land plants (i.e., bryophytes) supports earlier models that suggested enhanced organic carbon burial rates served as a mechanism for the stepwise oxygenation of the atmosphere during the Late Ordovician.
•New high-resolution δ13C dataset of marine carbonate rocks from the Appalachian Basin•The δ13C dataset shows a 1.2‰ positive shift of baseline values•The shift reflects a fundamental change of global carbon cycles in the Sandbian.•The change is related to an O2 rise, driven by diversifications of land plants.
Marine carbonates from two well-studied areas of the Silurian of North America were analyzed for stable carbon isotope (δ
13
C
carb
) stratigraphy. A graptolite-bearing sequence from the eastern ...margin of the Panthallasic Ocean (Nevada) and a conodont-bearing sequence from the mid-continent epeiric seaway (Tennessee) were sampled for δ
13
C
carb
stratigraphy in order to improve the correlation between these areas and the Swedish island of Gotland, which has become the global standard for Wenlock conodont and carbon isotope stratigraphy. The Homerian (Late Wenlock) Mulde positive carbon isotope excursion serves as a useful chronostratigraphic marker for Homerian sequences, especially in regions such as the two included in this investigation, where zonal fossils are absent or poorly represented. In Nevada, using presently available biostratigraphic data, a detailed modern graptolite zonation cannot be applied due to a lack of several key species. Likewise, the zonally important conodont species are poorly represented in Tennessee. Our recognition of the dual-peaked Mulde δ
13
C
carb
excursion in North America allows improved correlation between these sequences and any other locality where the Mulde Excursion has been recorded in sufficient detail.
Recent revisions to the biostratigraphic and chronostratigraphic assignment of strata from the type area of the Niagaran Provincial Series (a regional chronostratigraphic unit) have demonstrated the ...need to revise the chronostratigraphic correlation of the Silurian System of North America. Recently, the working group to restudy the base of the Wenlock Series has developed an extremely high-resolution global chronostratigraphy for the Telychian and Sheinwoodian stages by integrating graptolite and conodont biostratigraphy with carbonate carbon isotope (δ13Ccarb) chemostratigraphy. This improved global chronostratigraphy has required such significant chronostratigraphic revisions to the North American succession that much of the Silurian System in North America is currently in a state of flux and needs further refinement. This report serves as an update of the progress on recalibrating the global chronostratigraphic correlation of North American Provincial Series and Stage boundaries in their type area. The revised North American classification is correlated with global series and stages as well as regional classifications used in the United Kingdom, the East Baltic, Australia, China, the Barrandian, and Altaj. Twenty-four potential stage slices, based primarily on graptolite and conodont zones and correlated to the global series and stages, are illustrated alongside a new composite δ13Ccarb curve for the Silurian.
Positive excursions in the carbon isotopic ratio of marine carbonates (
δ
13C
carb) are commonly attributed to an increase in the burial ratio of organic versus carbonate carbon in the global oceans. ...Regardless of whether this increased deposition of organic carbon was oceanographically or biologically induced, the fact that high carbon isotope values are most often attributed to the sequestration and burial of organic carbon remains. The notion that high
δ
13C
carb values should be the result of organic rich deposition in epeiric sea settings is not surprising considering that the overwhelming majority of organic carbon burial in the modern oceans occurs in continental shelf and margin settings. What is surprising is that major positive carbon isotope excursions during the Silurian are recorded in clean carbonate successions with little to no appreciable black shale deposition in epeiric seas. In fact, Silurian
δ
13C
carb excursions occurred during intervals of prolific reef development and an overall expansion of carbonate platform environments globally.
This discrepancy has sparked considerable debate among Silurian carbon isotope specialists but has offered a valuable opportunity to re-examine the intricate connections among nutrients, primary productivity, carbonate sedimentology, and the global carbon cycle in the lower Paleozoic. Here, we consider several sedimentological and geochemical features of one of these Silurian positive carbon isotope (
δ
13C
carb) excursions and discuss the broader implications of current Silurian
δ
13C
carb models to the interpretation of lower Paleozoic carbonate sequences.
The Global Standard Stratotype‐section and Point (GSSP) of the Furongian Series (uppermost series of the Cambrian System) and the Paibian Stage (lowermost stage of the Furongian Series), has been ...recently defined and ratified by the International Union of Geological Sciences (IUGS). The boundary stratotype is 369 metres above the base of the Huaqiao Formation in the Paibi section, northwestern Hunan Province, China. This point coincides with the first appearance of the cosmopolitan agnostoid trilobite Glyptagnostus reticulatus, and occurs near the base of a large positive carbon isotopic excursion (SPICE excursion).
The Guttenberg carbon isotope excursion (GICE) documented from eastern North America demonstrates the effects that regional, geochemically distinct water masses, upwelling, and ocean circulation have ...on the carbon isotope record from carbonate platforms. Late Turinian–Chatfieldian carbonates from Oklahoma, Kentucky, Virginia, and West Virginia record a positive carbon isotope excursion (≥
+
3.0‰), the GICE excursion. The GICE excursion has relations to established biostratigraphy (beginning in the North American Midcontinent
Phragmodus undatus Conodont Zone and continuing through the
Plectodina tenuis Zone), sequence, and event stratigraphy. Previously established models for positive carbon isotope shifts on carbonate platforms have been tested during the GICE excursion, where geochemically distinct water masses are defined for the Upper Ordovician. A major eustatic sea-level rise before the GICE promoted a greater exchange of open ocean waters onto the carbonate platform of Laurentia; however, restricted or sluggish circulation and exchange between water masses within the epeiric seas and the adjacent Iapetus Ocean were still apparent. Local variations documented in the GICE excursion are directly related to upwelling of nutrient rich isotopically light waters, increased primary productivity, and the subsequent organic carbon production and burial.
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