Middle–Late Ordovician sequences from the Appalachian Basin and Arbuckle Mountain regions of North America were analyzed for carbonate-associated sulfate (δ34SCAS) and pyrite (δ34Spyr) paired with ...carbonate (δ13Ccarb) and organic matter (δ13Corg) chemostratigraphy. Two major negative drops in δ34SCAS (12‰ excursions) are recognized: the older decline in δ34SCAS occurs within the Histiodella holodentata–Phragmodus polonicus Conodont Zones and the younger drop is within the Cahabagnathus sweeti–Amorphognathus tvaerensis (Baltoniodus gerdae subzone) Zones. These overall these negative shifts in δ34SCAS have an antithetical relationship with positive shifts in δ34Spyr (~+10‰) and δ13Ccarb (~+2‰) recorded in the same successions. The older negative δ34SCAS shift is coincident with the widely documented mid-Darriwilian δ13C excursion (MDICE), and the younger negative δ34SCAS shift is coincident with another positive δ13Ccarb shift in the early Sandbian. Geochemical modeling of these sulfur isotope shifts suggests that a decrease in the global rate of pyrite burial or isotope fractionation between seawater sulfate and sedimentary pyrite could account for these negative δ34SCAS trends. Additionally, a substantial increase in the weathering flux of pyrite to the global oceans could also explain these secular sulfur isotope trends. While increased crustal weathering is broadly consistent with a sea-level lowstand, and the seawater 87Sr/86Sr isotope record of change in continental weathering in the late Darriwilian Stage of the Ordovician, geologic and geochemical proxy evidence do not support distinct pulses of continental weathering required to generate two separate negative shifts in δ34SCAS. These antithetical isotope trends may be best explained by changes in the marine redox state that significantly reduced microbially mediated pyrite burial and organic matter remineralization rates. Pulses of oceanic ventilation would have expanded habitable environments for marine organisms, and thus is broadly consistent with major increases in biodiversification during this period of the Ordovician.
•Newly paired sulfur and carbon isotope data of Ordovician carbonates from Laurentia•Two large drops in δ34SCAS correspond to positive shifts in δ13Ccarb values.•First isotopic evidence for Ordovician decoupling of carbon and sulfur cycles•These data support intervals of oceanic ventilation in the Middle–Late Ordovician.•Oceanic oxygenation provides bolstering conditions for coincident GOBE pulses.
The Late Ordovician Hirnantian Stage (44million years ago) was one of three time periods during the past half billion years in which large continental glaciers formed over Earth's polar regions. The ...effects of this glaciation were far-reaching and coincided with one of the largest marine mass extinction events in Earth history. The cause of this ice age is uncertain, and a paradoxical association with evidence for high atmospheric CO2 levels has been debated. Precise linkages between sea level, ice volume, and carbon isotope ( delta 13Ccarb and delta 13Corg) proxy records of pCO2 have been poorly understood due in part to uncertainties in stratigraphic correlation and the interpretation of globally important sections. Although correlation difficulties remain, recent Hirnantian biostratigraphic studies now allow for improved correlations. Here we show that consistent trends in both delta 13Ccarb and delta 13Corg from two well-dated stratigraphic sequences in Estonia and Anticosti Island, Canada coincide with changes in Late Ordovician (Hirnantian) climate as inferred from sea level and the extent of ice sheets. The integrated datasets are consistent with increasing pCO2 levels in response to ice-sheet expansion that reduced silicate weathering. Ultimately, the time period of elevated pCO2 levels is followed by geologic evidence of deglaciation.
Application of the recently introduced Baltic δ^sup 13^C isotope zonation to a composite North American Darriwilian through Hirnantian succession shows that in most intervals there is good ...trans-Atlantic agreement not only between the isotope zones but also with the available biostratigraphic data. This indicates that this isotope zonation is a useful tool for improving previously uncertain long-distance correlations.
Concurrent gaps in the Late Devonian/Mississippian fossil records of insects and tetrapods (i.e. Romer's Gap) have been attributed to physiological suppression by low atmospheric
O
Here, updated ...stable isotope inputs inform a reconstruction of Phanerozoic oxygen levels that contradicts the low oxygen hypothesis (and contradicts the purported role of oxygen in the evolution of gigantic insects during the late Palaeozoic), but reconciles isotope-based calculations with other proxies, like charcoal. Furthermore, statistical analysis demonstrates that the gap between the first Devonian insect and earliest diverse insect assemblages of the Pennsylvanian (Bashkirian Stage) requires no special explanation if insects were neither diverse nor abundant prior to the evolution of wings. Rather than tracking physiological constraint, the fossil record may accurately record the transformative evolutionary impact of insect flight.
•Carbonate δ44/40Ca and Sr/Ca from Meiklejohn Peak, Nevada during positive δ13C shift.•Significant covariation between δ44/40Ca – Sr/Ca, but not δ44/40Ca – δ13C.•Numerical models used to characterize ...contributions from mineralogy and diagenesis.•Variations in mineralogy and diagenesis may influence δ44/40Ca and Sr/Ca.•A change in DIC (locally or globbaly) is needed to reproduce the δ13C trend.
The Middle Ordovician Darriwilian Stage (∼469 – 458 Ma) records a ∼2‰ positive carbon isotope shift known as the MDICE (Mid-Darriwilian Carbon Isotope Excursion). Although studies have shown that the MDICE is a globally synchronous event, the link between the MDICE and changes in the global carbon cycle remains unclear. This is largely due to possible local processes including diagenesis and variations in carbonate polymorphs that can obscure δ13C signals recorded in shallow marine carbonate rock from the global dissolved inorganic carbon reservoir. Here, we use paired measurements of δ13C, δ44/40Ca, and Sr/Ca from a stratigraphic section at Meiklejohn Peak in southwest Nevada (USA) to constrain the potential for local processes in decoupling the recorded δ13C signals from the global carbon cycle. We find that variations in δ44/40Ca and Sr/Ca in this section at Meiklejohn Peak can in part be explained by changes in primary mineralogy and diagenesis. However, these processes are unable to reproduce the entire shift observed in δ13C, with the remaining portion of the δ13C curve driven by either local changes in platform water mass dissolved inorganic carbon (DIC) or global carbon cycling. While we cannot completely deconvolve local signals from the recorded δ13C, we argue that the MDICE may, at least in part, reflect a primary signal of increase organic carbon burial related to changes in primary productivity and nutrient delivery during increased basaltic weathering associated with the Taconic uplift. The proposed link between tectonic uplift and increase in primary productivity strengthens the notion that tectonic processes played a significant role in modulating changes in the global carbon cycle during the Ordovician Period.
The carbon isotopic composition of the global dissolved inorganic carbon (DIC) reservoir is best estimated from open ocean pelagic carbonate sediments (δ13Ccarb). However, this is not practical for ...most of geologic time because seafloor subduction has removed the pre-Jurassic record and these time periods may have lacked planktonic calcifying organisms, and therefore shallow water carbonate platform or periplatform sediments are utilized. Shallow water deposits are susceptible to a wide range of post-depositional alteration processes and syn-depositional controls on δ13Ccarb that include carbonate mineralogy, water mass restriction, and a host of related variables (e.g., pH, temperature, organic decomposition, evaporation and CO2 solubility) that can produce local gradients in DIC. The degree to which shallow water δ13C curves diverge from open marine deposits may be critical to understanding how well global carbon cycle isotope mass balance models can predict organic carbon burial rates, but documentation of such divergence is often hindered by factors that limit chronostratigraphic correlation in restricted water masses (e.g., endemic faunas). Here we integrate strontium isotope (87Sr/86Sr) stratigraphy and biostratigraphy to compare δ13C curves in a case study along a depth transect in Middle–Late Ordovician carbonate platform settings. The restricted tidal flat and more open marine deposits are offset by a maximum of ∼2‰ during sea level drop and ∼0‰ during highstand flooding of the platform. Global carbon cycle models such as GEOCARBSULF use published δ13Ccarb curves to drive organic carbon burial rates under the assumption that δ13Ccarb reflects a global seawater signal. We show here the potential pitfalls of using a published δ13Ccarb curve that violates this global assumption. For the 460 million year Middle–Late Ordovician time bin in GEOCARBSULF, improper usage of our locally depleted δ13C curve to drive global organic carbon burial would result in erroneous atmospheric O2 (∼10.8% O2, equal to about a 30% reduction from what it should be at ∼15.2% O2 using global δ13C) and CO2 (∼400 ppm too high, equal to about a 13% increase from what it should be at 2570 ppm CO2). With detailed sedimentologic analysis, it may be possible to identify and exclude δ13C samples that record the influences of local carbon cycling from global carbon cycle models such as GEOCARBSULF.
•87Sr/86Sr and bio-stratigraphy reveals δ13C gradient along depth transect in Ordovician.•δ13C gradient as large as 2‰ during sea level drop, approaches 0 during flooding.•2‰ lighter δ13C input to global carbon cycle models is a pitfall that lowers O2 by 5%.•Use of 2‰ lighter δ13C introduces less error in CO2 but levels still rise by 400 ppm.
A high-resolution
87Sr/
86Sr curve and paired
δ
13C carbonate-organic data set is generated for the Llandovery Series from the Ikla drill core in Estonia. A
δ
13C carbonate curve is also presented ...from the Pancake Range in Nevada. Observed
87Sr/
86Sr values in the Ikla drill core are at a minimum in the early Llandovery Rhuddanian Stage (∼
0.7079 to 0.7080), and then trend to more radiogenic ratios in the basal part of the Telychian Stage. An
87Sr/
86Sr high near ∼
0.7084 is observed in the Telychian at the top of the studied section. The range of values is in general agreement with the data from previous sample sets of brachiopods and conodonts recovered from localities in North America and Europe that record a rising trend in the
87Sr/
86Sr ratio throughout the Llandovery from approximately 0.7080 to 0.7084. The major increase in the
87Sr/
86Sr ratio during the late Llandovery may be due to weathering of radiogenic source rocks that were uplifted during early Silurian continent–continent collisions. The Sr rise potentially coincides with the occurrence of an unusually thick sequence of K-bentonite beds representing large-magnitude ash falls in the early Telychian.
A previously documented negative
δ
13C excursion in marine carbonates in the lower Telychian interval of the Ikla core is quasi-synchronous with the increase in
87Sr/
86Sr. Our new organic matter
δ
13C data from the Ikla core confirm that this negative
δ
13C carbonate excursion is not a result of diagenesis. Furthermore, a negative
δ
13C excursion in carbonates from the early Telychian portion of the Pancake Range section in Nevada seems to confirm the global scope of this carbon cycle perturbation.
New stable carbon isotope data (δ13Ccarb) from Lower–Middle Ordovician (Tremadocian to Darriwilian) carbonate mudstone and wackestone rocks of the Pogonip Group are presented from two sections in the ...Great Basin region (USA) — Shingle Pass (east-central Nevada) and the Ibex area (western Utah). The Pogonip Group is a succession of mixed carbonate and siliciclastic rocks that accumulated on a carbonate ramp under normal marine conditions during the Late Cambrian (Furongian) to Middle Ordovician (Darriwilian). The Shingle Pass and Ibex area sections have been previously studied for their conodont biostratigraphy and contain a North American Midcontinent conodont fauna that range from the Cordylodus intermedius Zone (uppermost Cambrian) to the Phragmodus polonicus Zone (Darriwilian). The δ13C trend has four distinct characteristics recognized in both Great Basin sections: 1) a drop in δ13C from +1‰ at the base of the Ordovician (Tremadocian) to −0.7‰, 2) a 1 to 2‰ positive δ13C shift in the uppermost Rossodus manitouensis Zone during the late Tremadocian, 3) a gradual δ13C increase from −2‰ to ca. 0‰ during the end of the Early Ordovician (Floian), and 4) a steady δ13C decrease from 0‰ to −4 to −5‰ during Middle Ordovician (Dapingian–Darriwilian).
In the Lower Ordovician, δ13C trends reported here from the Great Basin are not consistent with a causal mechanism involving sea level change and the migration of isotopically distinct water bodies. Instead, these Lower Ordovician isotope data most likely reflect primary seawater chemistry and changes in δ13C on a global scale. This interpretation is supported by the excellent correlation of δ13C in the Lower Ordovician to other δ13C trends reported from the sections in the Argentine Precordillera (La Silla and San Juan formations) and in western Newfoundland (St. George and Table Head groups). These correlations using δ13C are consistent with published biostratigraphic data and provide an integrated and high-resolution chemo-biostratigraphic framework for the Lower Ordovician sedimentary record of the Laurentian margin. The Middle Ordovician portion of the δ13C curves in the Great Basin represented by the Kanosh and Lehman formations shows significant isotopic depletion relative to the section in Argentina. Thus, although there is some indication that minima and maxima in the Middle Ordovician curves can be correlated, the Great Basin sections show clear evidence of overprinting by local variables related to both diagenesis (dolomitization) and platform restriction.
•New high-resolution δ13C data from Nevada and Utah are globally correlative.•Thin section analysis and isotope cross plots indicate that isotope data are primary.•Sea level changes are not the dominant driver of δ13C variability.•Other isotope proxy data may elucidate drivers of Ordovician biodiversification.
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