The late Cambrian Steptoean positive carbon isotope excursion (SPICE) is a distinct chemostratigraphic feature of the Paleozoic, marked by a 4–5‰ shift in carbonate δ13C that has been recognized ...across the globe during the Paibian Stage. The SPICE may be related to enhanced burial of organic matter and pyrite during the expansion of marine euxinia, which as a source of O2 also results in a pulse of atmospheric oxygen. However, geochemical proxies have not clearly illustrated how the ocean redox evolved with atmospheric oxygen changes during the SPICE. This study presents new carbonate I/Ca data, a redox proxy for the upper ocean, from three basins. I/Ca values are low at Great Basin and South China from early into the peak of SPICE, indicating generally anoxic conditions in shallow waters. The overall increasing trend in I/Ca through the peak and recovery phase of the SPICE roughly correlates with the previously modeled rise in atmospheric oxygen. Spatially, the Georgina Basin (Mt. Whelan) might have recorded a relatively more oxic upper ocean compared to the Great Basin and South China. Earth system model simulations also demonstrate the importance of paleogeographic and oceanographic settings on local redox conditions, highlighting the redox heterogeneity during the SPICE.
•I/Ca proxy was used for reconstructing redox condition during SPICE.•An oxygenation event in the upper ocean was found during the peak and recovery phase of SPICE.•The ocean redox changes varied in different paleogeographic settings.
Sulfur isotopes measured from sedimentary rocks are used to reconstruct the global sulfur cycle and chemostratigraphic correlation. Relative and absolute changes in the sulfur isotopic composition of ...the ocean (δ34Sseawater) are essential for estimating changes in redox conditions in ancient environments, particularly for geochemical models that utilize isotope mass balance. Thus, accurate estimates of δ34Sseawater must be measured using a laboratory protocol optimized for the extraction of primary sulfate (preserved as carbonate-associated sulfate; CAS) by removing any contaminant secondary sulfate (e.g., from diagenetic pyrite oxidation).
Here we use two CAS-extraction protocols on Lower–Middle Ordovician carbonate rocks to test the degree to which similar absolute values and stratigraphic trends in δ34SCAS are produced by different CAS extraction methods. One method processes carbonate powders once with a single rinse of a 10% NaCl solution to remove sulfate minerals or weakly bonded sulfate ions, followed by a rinse in 5–6% bleach to remove organically bound sulfur before dissolution in hydrochloric acid (HCl). The second method treats carbonate powders with three rinses of NaCl, without a bleach rinse, to more aggressively remove secondary sulfate before dissolution in HCl. Isotopic results show that samples treated in three NaCl rinses produce δ34SCAS values that are on average 7‰ more positive than samples treated with a single NaCl rinse, but a similar stratigraphic trend is preserved in samples processed with either method. We interpret the difference in δ34SCAS between methods to reflect the incomplete removal of secondary sulfate derived from the oxidation of 32S-enriched pyrite when employing only a single NaCl rinse. Surprisingly, rocks with low CAS concentrations (≤10 mg/kg) using both methods show no significant difference in δ34SCAS values, as well as no significant difference in pyrite concentrations and pyrite δ34S (δ34SPY) between methods. Samples with low CAS concentrations are likely altered and secondary sulfate sourced from pyrite oxidation has likely become incorporated into recrystallized carbonate minerals or incorporated into pore-filling calcite cement, all of which is captured as CAS. Results suggest that both CAS-extraction protocols can produce broadly similar δ34SCAS trends, but a single NaCl rinse may not completely remove contaminant sulfate sorbed onto carbonate grains and thus will not yield meaningful data for reconstructing δ34Sseawater. This has clear implications for modeling studies aimed at reconstructing paleo-redox conditions using δ34S data assumed to record global δ34Sseawater trends if these δ34S data are generated using a non-optimized CAS protocol.
Highlights ► California mice display pronounced changes in plasma corticosterone (CORT) over a 24-h period. ► Circulating CORT levels do not differ between males and females. ► A relatively high dose ...of dexamethasone (5 mg/kg) was needed to suppress CORT for 8 h. ► Plasma CORT responses to adrenocorticotropic hormone and stress change over the course of the day. ► Excretion pattern of fecal glucocorticoid metabolites differs with time of CORT injection.
Profound changes in environmental conditions, particularly atmospheric oxygen levels, are thought to be important drivers of several major biotic events (e.g. mass extinctions and diversifications). ...The early Paleozoic represents a key interval in the oxygenation of the ocean–atmosphere system and evolution of the biosphere. Global proxies (e.g. carbon (δ13C) and sulfur (δ34S) isotopes) are used to diagnose potential changes in oxygenation and infer causes of environmental change and biotic turnover. The Cambrian–Ordovician contains several trilobite extinctions (some are apparently local, but others are globally correlative) that are attributed to anoxia based on coeval positive δ13C and δ34S excursions. These extinction and excursion events have yet to be coupled with more recently developed proxies thought to be more reflective of local redox conditions in the water column (e.g. I/Ca) to confirm whether these extinctions were associated with oxygen crises over a regional or global scale.
Here we examine an Early Ordovician (Tremadocian Stage) extinction event previously interpreted to reflect a continuation of recurrent early Paleozoic anoxic events that expanded into nearshore environments. δ13C, δ34S, and I/Ca trends were measured from three sections in the Great Basin region to test whether I/Ca trends support the notion that anoxia was locally present in the water column along the Laurentian margin. Evidence for anoxia is based on coincident, but not always synchronous, positive δ13C and δ34S excursions (mainly from carbonate-associated sulfate and less so from pyrite data), a 30% extinction of standing generic diversity, and near-zero I/Ca values. Although evidence for local water column anoxia from the I/Ca proxy broadly agrees with intervals of global anoxia inferred from δ13C and δ34S trends, a more complex picture is evident where spatially and temporally variable local trends are superimposed on time-averaged global trends. Stratigraphic sections from the distal and deeper part of the basin (Shingle Pass and Meiklejohn Peak) preserve synchronous global (δ13C and δ34S) and water column (I/Ca) evidence for anoxia, but not at the more proximal section (Ibex, UT). Although geochemical and paleontological evidence point toward anoxia as the driver of this Early Ordovician extinction event, differences between I/Ca and δ13C–δ34S signals suggest regional variation in the timing, extent, and persistence of anoxia.
•An Early Ordovician conodont and trilobite extinction event is linked to anoxia.•Global proxies for anoxia are confirmed by paired C and S isotope excursions.•Low I/Ca, a local anoxia proxy, is synchronous with C and S excursions.•Geochemical proxies for anoxia are coeval with a 30% extinction of generic diversity.•This strongly coupled anoxic and biotic event is one of the last prior to the GOBE.
The Evans Ferry section from the Appalachian Basin of eastern North America has been analyzed for chemostratigraphic trends to elucidate possible causal mechanisms facilitating the Great Ordovician ...Biodiversification Event (GOBE). Paired stable isotope (δ13C and δ34S) analyses were used in this carbonate-dominated locality from the Appalachian Basin to reconstruct the marine redox states during this key period of rapid biodiversification. This succession is one of the most expanded Sandbian Stage (Upper Ordovician) deposits known from North America, and it allows new high-resolution reconstructions of long-term, global carbon and sulfur cycle fluctuations. The integrated geochemical and sequence-stratigraphic investigation presented here from the Laurentian epeiric seaway allows for possible identification of the contraction and expansion of reducing water masses during the Middle–Late Ordovician that may have been linked to global-marine paleoredox dynamics. Utilizing new conodont-based 87Sr/86Sr isotope stratigraphy along with previous conodont biostratigraphy, we can confidently correlate our stable isotope profiles to other carbonate successions globally. Carbonate facies indicate that this area experienced partial restriction from open-marine conditions during certain intervals in the Sandbian. These semi-restricted environments record carbon (δ13Ccarb) isotope trends that are secular in nature, as they can be correlated to other successions across Laurentia and Baltica. We identify several intervals when δ13Ccarb and δ34SCAS (carbonate-associated sulfate) trends are decoupled. These inverse stratigraphic trends are, at times, followed by parallel positive shifts in δ13Ccarb and δ34SCAS. Causal mechanisms for the observed decoupled δ13C and δ34S trends may include a wide variety of factors such as more closed-system, local biogeochemical processes associated in part with diagenesis, or they could instead reflect variations in the global fluxes of organic matter and pyrite burial linked to changing marine paleoredox conditions. The positive covariation in trends presented here likely represents transient increases in organic carbon and pyrite burial in response to expansion of reducing marine environments.
•Tracking oxygen dynamics through Middle to Upper Ordovician stratigraphy•Evidence of oxygenation pulses using carbon and sulfur stable isotope ratios•Linkages between increased biodiversity and marine ventilation events•Modeling indicates decreased pyrite burial as principle driver for isotope trends.
The Early Carboniferous was a climatic transition interval from a mid-Paleozoic greenhouse world into the Late Paleozoic Ice Age. It was marked by a long-term cooling trend that was punctuated by ...short glaciations in the Tournaisian and Visean (early to mid-Early Carboniferous). Here, we generated organic carbon and nitrogen isotope profiles for two widely separated Lower Carboniferous sections (Arrow Canyon Range, Nevada, and Namur-Dinant Basin, Belgium) in order to assess the global nature and timing of changes in the carbon‑nitrogen cycles linked to oceanic productivity and redox conditions during this climatic transition interval. The carbon and nitrogen isotope profiles of both study sections record major perturbations during the mid-Tournaisian and Visean. A mid-Tournaisian event (TICE) is marked by increases in δ13Ccarb, δ13Corg, and δ15Nbulk at Arrow Canyon (from +0.6 to +7.0‰, −25.2 to −23.7‰, and –0.6 to +8.9‰, respectively) and Namur-Dinant (from −0.3 to +4.7‰, −27.9‰ to −22.2‰, and –5.7 to +5.2‰, respectively). An early Visean event (VICE; named herein) is marked by increases in δ13Ccarb, δ13Corg, and δ15Nbulk at Arrow Canyon (from −0.6 to +3.1‰, −28.4 to −23.6‰, and +1.4 to +9.4‰, respectively) and Namur-Dinant (from +2.1 to +4.3‰, −26.7 to −25.0‰, and –1.4 to +3.3‰, respectively). The positive excursions in all three isotopic records during these events are consistent with increased fractional burial of organic carbon and enhanced denitrification, implying intensification of marine productivity and expansion of hypoxia in the global ocean. Given that each event coincided with lowering of sea-surface temperatures and increased glaciation (as documented from published conodont δ18O and sea-level records), we hypothesize that global cooling led to intensified oceanic circulation and upwelling on continental margins, triggering increased marine productivity and attendant redox changes within the affected upwelling zones. Our results provide new insights into changes in Early Carboniferous oceanic conditions in response to the initial stages of cooling leading into the Late Paleozoic Ice Age.
•Carbon & nitrogen isotope profiles for Lower Carboniferous sections in Arrow Canyon, Nevada and Namur-Dinant Basin, Belgium•Positive excursions in δ13Ccarb, δ13Corg, and δ15Nbulk during the mid-Tournaisian (TICE) and Visean (VICE) cooling events•These excursions due to increased organic C burial and water-column denitrification linked to enhanced marine productivity.•Global cooling drove intensified oceanic overturning circulation and nutrient upwelling, enhancing marine productivity.•These findings highlight the importance of Early Carboniferous oceanic changes in development of the Late Paleozoic Ice Age.
•Rise in Middle Ordovician global sea level suggests increased seafloor production.•Hydrothermal weathering drives inflection in marine strontium isotopes (87Sr/86Sr).•Oxygen isotope (δ18O) data ...demonstrate cooling concurrent with 87Sr/86Sr inflection.•Continental silicate weathering can drive cooling.•Cooling from weathering can counteract volcanic carbon dioxide (CO2) degassing.
The global climate of the Ordovician Period (486.9 to 443.1 Ma) is characterized by cooling that culminated in the Hirnantian glaciation. Chemical weathering of Ca- and Mg-bearing silicate minerals and the subsequent trapping of carbon in marine carbonates act as a sink for atmospheric CO2 on multi-million-year time scales, with basaltic rocks consuming CO2 at a greater rate than rocks of granitic composition. The oceanic Sr isotope ratio (87Sr/86Sr) can act as a geochemical proxy for the relative proportion of basaltic versus granitic weathering. Oxygen isotopes (δ18O) act as a proxy for paleotemperature and ice volume, providing a useful complement to 87Sr/86Sr in studies of ancient climate. Previous studies have reported stepwise cooling (increasing δ18O) during the Middle to Late Ordovician. Combined with Sr and C cycle models, this has led to the hypothesis that continental silicate weathering of mafic material drove Ordovician cooling (e.g., the Taconic Orogeny). However, Sr and C cycle models have not accounted for an apparent rise in sea level and seafloor production in the Middle Ordovician (Darriwilian), which would increase the hydrothermal Sr flux as well as degassing along continental volcanic arcs. Furthermore, some Ordovician studies contain temporal uncertainty between 87Sr/86Sr and δ18O curves if they are not based on paired analyses, which can obscure the relationship between silicate weathering and cooling. Here, we present new paired 87Sr/86Sr and δ18O data from conodont apatite and integrate this with both a deterministic (forward) and stochastic (reverse) modeling approach to argue that increased hydrothermal weathering played a role in driving marine 87Sr/86Sr, specifically an inflection occurring in the Pygoda serra conodont zone of the mid-Darriwilian Stage (∼ 460.9 Ma ± 1 My). This 87Sr/86Sr inflection is accompanied by an increase in δ18O, consistent with climate cooling. Clarifying the role of seafloor production for marine 87Sr/86Sr and the implications for Ordovician cooling allows for a more nuanced understanding of the factors that drive multi-million-year shifts in climate.
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