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• 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. [ABSTRACT FROM AUTHOR]

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 (δ34S seawater) are essential for estimating changes in redox conditions in ancient environments, particularly for geochemical models that utilize isotope mass balance. Thus, accurate estimates of δ34S seawater 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 δ34S CAS 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 δ34S CAS 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 δ34S CAS 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 δ34S CAS values, as well as no significant difference in pyrite concentrations and pyrite δ34S (δ34S PY) 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 δ34S CAS 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 δ34S seawater. This has clear implications for modeling studies aimed at reconstructing paleo-redox conditions using δ34S data assumed to record global δ34S seawater trends if these δ34S data are generated using a non-optimized CAS protocol. [ABSTRACT FROM AUTHOR]

Abstract: The impact of climate and ocean circulation changes on marine carbon and nitrogen cycling was investigated for a period covering the Late Ordovician Hirnantian glaciation. Three sections were studied: two from Nevada, one proximal and one distal; and one from the Yukon. The opportunity to compare δ 13 C carb, δ 13 C org and δ 13 C grap profiles between ocean and epeiric platform settings enabled a more thorough assessment of environmental and diagenetic factors influencing the preservation of seawater δ 13 C values at each stratigraphic section. Using the best preserved sedimentary components from the two Nevada sections, a lowstand sedimentary gradient of ∼4‰ was found between platform and ocean settings during the Hirnantian glaciation. Gradients of similar magnitude have been reported from Hirnantian platforms in the Canadian Arctic and Baltic basins. Taking the outboard sections as the most representative of the ocean carbon pool, a mean, positive δ 13 C shift of just 2.7±0.4‰ is calculated, which is significantly smaller than the 5–7‰ positive shifts recorded in epeiric platform settings. The discrepancy is attributed to the effects of local carbon cycling in tropical and subtropical epeiric seas. It is hypothesized that increased carbonate weathering caused δ 13 C values of local carbon weathering fluxes to increase, which in turn caused seawater δ 13 C values to increase in basin proximal settings. In addition, nutrient dust fluxes may have contributed to increased levels of photosynthetic activity in proximal settings, driving sedimentary δ 13 C values even higher—much higher than sediment deposited in outboard settings where carbon exchanges with the surface ocean were less restrictive. A δ 15 N profile reconstructed from deposits of the distal Vinini Creek section records a 1.5‰ positive shift through the Hirnantian glacial interval. Interpreted as a major Late Ordovician upwelling zone, high organic carbon (>1 wt.%) and high hydrogen indices (400–600) make the Vinini Creek section a suitable candidate for the preservation of original sedimentary δ 15 N records. To explain the results, a conceptual model of ocean nitrogen cycling under greenhouse climate conditions is presented that highlights the role of cyanobacteria as both primary producers and nitrogen fixers. It is hypothesized that in greenhouse climates, the fixed nitrogen inventory of the abyssal ocean was low due to efficient denitrification in the ocean''s expansive oxygen minimum zones. This caused algal productivity to become strongly dependent on continuous supplies of fixed nitrogen from cyanobacteria in the photic zone, yielding sedimentary δ 15 N values between 0 and −2‰. By contrast, the cooler icehouse climate of the Hirnantian led to increased ocean ventilation, greater partitioning of atmospheric oxygen into downwelling surface waters, and oxygen minimum zone shrinkage. As a result, fixed nitrogen levels climbed in the ocean''s abyssal depths due to the lower denitrification rates. This in turn allowed nitrogen with high δ 15 N values to recycle back into the photic zone, driving sedimentary δ 15 N values higher during the Hirnantian glaciation. [Copyright &y& Elsevier]

Abstract Cambrian–Ordovician sponge-microbial mounds in the Great Basin of the western USA reveal reef structure and composition immediately prior to the Great Ordovician Biodiversification Event (GOBE). Here we describe lithistid sponge-microbial reefs from the upper Cambrian (Furongian, Stage 10) strata of the Arrow Canyon Range, Nevada. The reefs are mound-like structures up to 1 to 2 m high and a few meters wide that consist of an unidentified thin-walled, bowl-shaped anthaspidellid sponge, columnar microstromatolite fabric, and the calcified microbe Angusticellularia. The reefs formed in low-energy, subtidal environments in which lime mud filled spongocoels and inter-reef spaces around undisturbed, in place, thin-walled sponges. The reefs colonized stable substrates provided by oolitic and bioclastic grainstone shoals. The mutually attached lithistid sponges form the main framework of the reefs. These thin-walled and bowl-shaped lithistids most likely were adapted to low-energy environments. Spaces beneath the overhanging sponge walls were filled by microbial carbonates. These include pendent micro-dendritic Angusticellularia attached to dermal sponge surfaces and upward-growing masses of microstromatolites. After death the lithistid spongocoels were mainly filled by micritic sediment that hosted soft-bodied burrowing organisms and keratose-like sponges. These lithistid sponge-microbial reefs, together with an earlier example of late Cambrian (Paibian) dendrolite-lithistid reefs in the same area, characterize skeletal-microbial reefs immediately prior to the GOBE. Highlights • Lithistid sponges formed reef frameworks in the upper Cambrian of Nevada, USA. • Reefs dominated by thin-walled sponges developed under low-energy conditions. • Spaces beneath the overhanging sponge walls were filled by microbial carbonates. • In the Great Basin, gradual reef transition occurred across Cambro-Ordovician boundary. • Regional disparities in reef evolution pattern during early stage of the GOBE are recognized. [ABSTRACT FROM AUTHOR]

The Ordovician Meiklejohn Peak mud-mound in western Nevada contains sedimentary structures (e.g., stromatactis and “zebra-rock”) that resemble features found in modern methane hydrate deposits. However, the δ13C of the mud-mound carbonates (−1 to 1‰ PDB) does not suggest interaction with δ13C-depleted methane. Consequently, Krause (Sedimentary Geology 145 (2001) 189) proposed that the Meiklejohn structures might have been produced by the formation and dissociation of CO2 gas hydrate rather than methane gas hydrate. Structures in Neoproterozoic cap carbonates, sheet cracks and tepee-like structures have also been suggested to represent gas hydrate interaction with carbonate sediments (Geology 29 (2001) 443), but, similarly, δ13C values do not suggest direct interaction with methane. The objective of this contribution is to see if the presence of CO2 gas hydrate is plausible for either setting.To test the feasibility of CO2 gas hydrate involvement in the formation of these structures, necessary concentrations of TCO2 were calculated based on required phase equilibria using the quadruple points Q1 (−1.73 °C, 10.2 atm) and Q2 (10.2 °C, 44.5 atm) from the phase diagram for CO2 gas hydrate (Miller, S.L., 1974. The Nature and Occurrence of Clathrate Hydrates. In: Kaplan, I.R. (Ed.), Natural Gases in Marine Sediments. Plenum, New York, NY, pp. 151–178). A limit for Ca+2 was established based on the lack of evidence for gypsum formation in this environment and the assumption that sulfate was up to 100% of its present oceanic value; the calculated TCO2 concentration, however, is relatively insensitive to the concentration of SO4−2 used. The lowest permissible value of pCO2 and TCO2 were found at Q1, where TCO2 must be at least 300 times the present value. These conditions are implausible for the formation of the Meiklejohn Peak mud-mound. Although some models for Neoproterozoic “Snowball Earth” require highly elevated atmospheric CO2, these models predict pCO2 that is still too low to permit the formation of CO2 gas hydrates in carbonate sediments. In addition, if the required conditions could be achieved during “Snowball Earth”, one would expect to find the associated structures only at the base of the section, while they are commonly observed meters to tens of meters above the base. Therefore, we conclude that it is unlikely these structures were produced by the formation and dissociation of CO2 gas hydrate. [Copyright &y& Elsevier]