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1. The Brachiopod δ 11 B Record Across the Carboniferous‐Permian Climate Transition

   https://doi.org/10.1029/2019PA003838  

   Legett, S. A. ; Rasbury, E. T. ; Grossman, E. L. ; Hemming, N. G. ; Penman, D. E. ( September 2020 , Paleoceanography and Paleoclimatology)

   We present the δ¹¹B of well‐preserved brachiopod fossils coupled with geochemical modeling to examine how seawater boron responded to abrupt and dynamic climate changes in the Late Paleozoic. The Late Carboniferous, a time of major coal formation and glacioeustatic sea level changes, is characterized by relatively stable brachiopod δ¹¹B of 15–17‰, similar to values seen in modern brachiopods. Brachiopod δ¹¹B dropped by ~5‰ in the early Permian and then restabilized at a new value of 10‰ within a few million years. Mass balance models of seawater δ¹¹B reproduced the overall trends in our brachiopod data but failed to capture the large drop in δ¹¹B in the early Permian. Published seawater⁸⁷Sr/⁸⁶Sr and δ^44/40Ca data based on brachiopod shells also shift to lower values in the early Permian, suggesting a common control on all three seawater isotope systems. The Permian terrestrial record of evaporites and eolian deposits suggests a prolonged reduced delivery of dissolved weathering products to the ocean, accounting for the change in seawater⁸⁷Sr/⁸⁶Sr. This reduced weathering, in turn, led to increased atmospheric CO₂and lowered seawater pH, which may have significantly decreased major removal mechanisms for seawater calcium and boron leading to declines in both isotope systems. We propose that boron removal via coprecipitation in carbonates and adsorption onto clay minerals was significantly diminished due to a reduction in the availability of the borate aqueous species caused by lowered seawater pH.

    

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2. Geochemical Evolution of Eocene Lakes in the Nevada Hinterland of the North American Cordillera

   https://doi.org/10.1029/2021GC009863  

   Canada, Andrew S. ; Cassel, Elizabeth J. ; Smith, M. Elliot ( October 2021 , Geochemistry, Geophysics, Geosystems)

   Eocene strata of the Elko Formation record lacustrine deposition within the Nevada hinterland of the North American Cordillera. We present a detailed geochemical stratigraphy enabled by high‐sampling‐resolution geochronology from lacus trine limestone and interbedded volcanic rocks of the Elko Formation. Two intervals of lacustrine deposition, an early Eocene “Lake Adobe” of limited aerial extent and a laterally extensive middle Eocene “Lake Elko,” are separated by ∼5 m.y. of apparent unconformity. Sediments deposited in the apparently short‐lived (49.5–48.5 Ma) early Eocene Lake Adobe exhibit high‐amplitude covariation of δ¹⁸O, δ¹³C and⁸⁷Sr/⁸⁶Sr, which suggests a dynamically changing catchment and precipitation regime. Lake Elko formed during the middle Eocene, and its strata record three geochemically distinct phases, indicating it was a single interconnected water body that became increasingly evaporative over time. The lower Elko Formation (44.0–42.5 Ma) was deposited in a freshwater lake. Middle Elko Formation (42.5–41.2 Ma) lithofacies and geochemistry suggest that an increasingly saline and alkaline Lake Elko experienced salinity stratification‐induced hypolimnion disoxia and burial of¹²C‐rich organic matter. The upper Elko Formation (41.2–40.5 Ma) records a shallow final phase of Lake Elko that experienced short residence times and a breakdown in stratification. A sharp decline of⁸⁷Sr/⁸⁶Sr in the upper Elko Formation reflects an increasing aerial extent of low‐⁸⁷Sr/⁸⁶Sr volcanic deposits from nearby calderas. Middle Eocene strata record ponding of paleodrainage, increasing hydrologic isolation and volcanism, consistent with progressive north to south removal of the Farallon flat slab and/or delamination of the lower lithospheric mantle of the North American plate.

    

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3. Fluid Mixing and Spatial Geochemical Variability in the Lost City Hydrothermal Field Chimneys

   https://doi.org/10.1029/2023GC011011  

   Aquino, Karmina A. ; Früh‐Green, Gretchen L. ; Bernasconi, Stefano M. ; Rickli, Jörg ; Lang, Susan Q. ; Lilley, Marvin D. ( February 2024 , Geochemistry, Geophysics, Geosystems)

   Carbonate‐brucite chimneys are a characteristic of low‐ to moderate‐temperature, ultramafic‐hosted alkaline hydrothermal systems, such as the Lost City hydrothermal field located on the Atlantis Massif at 30°N near the Mid‐Atlantic Ridge. These chimneys form as a result of mixing between warm, serpentinization‐derived vent fluids and cold seawater. Previous work has documented the evolution in mineralogy and geochemistry associated with the aging of the chimneys as hydrothermal activity wanes. However, little is known about spatial heterogeneities within and among actively venting chimneys. New mineralogical and geochemical data (⁸⁷Sr/⁸⁶Sr and stable C, O, and clumped isotopes) indicate that the brucite and calcite precipitate at elevated temperatures in vent fluid‐dominated domains in the interior of chimneys. Exterior zones dominated by seawater are brucite‐poor and aragonite is the main carbonate mineral. Carbonates record mostly out of equilibrium oxygen and clumped isotope signatures due to rapid precipitation upon vent fluid‐seawater mixing. On the other hand, the carbonates precipitate closer to carbon isotope equilibrium, with dissolved inorganic carbon in seawater as the dominant carbon source and have δ¹³C values within the range of marine carbonates. Our data suggest that calcite is a primary mineral in the active hydrothermal chimneys and does not exclusively form as a replacement of aragonite during later alteration with seawater. Elevated formation temperatures and lower⁸⁷Sr/⁸⁶Sr relative to aragonite in the same sample suggest that calcite may be the first carbonate mineral to precipitate.

    

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4. Changing Mantle Sources and the Effects of Crustal Passage on the Steens Basalt, SE Oregon: Chemical and Isotopic Constraints

   https://doi.org/10.1029/2020GC008910  

   Moore, N. E. ; Grunder, A. L. ; Bohrson, W. A. ; Carlson, R. W. ; Bindeman, I. N. ( August 2020 , Geochemistry, Geophysics, Geosystems)

   Continental flood basalts are more prone to compositional modification from passage through thicker and (or) more felsic crust in comparison to their oceanic counterparts. The Steens Basalt in southeast Oregon (~17 Ma) is among the oldest and most mafic members of the Columbia River Basalt Group and provides a record of the early stages of flood basalt volcanism. We evaluate the balance of mantle sources in time during the onset of Columbia River Basalt Group magmatism and assess the effect of crustal passage using stratigraphically controlled Sr, Nd, Pb, Hf, Os, and O isotopic compositions, as well as whole rock major and trace element data.

   Mixing models indicate that depleted and enriched mantle sources identified by previous workers contribute in varying proportions during the life of the magmatic system, with the greatest contribution by depleted mantle when eruption rate and presumed intrusion rate increase. During waxing, enrichment of δ¹⁸O in some flows signals cryptic deep fractionation of abundant clinopyroxene followed by shallow fractionation of olivine ± clinopyroxene ± plagioclase. Os concentrations are among the highest worldwide at a given MgO (0.29–0.86 ppb at 6.0 to 10.9 wt.%). We argue that high Os results from scavenging of sulfides by recharging magmas passing through earlier crystallized magmas. Elevated⁸⁷Sr/⁸⁶Sr in the latest stage supports modest assimilation of partial melts from mafic accreted terranes, facilitated by thermal priming of crust by persistent magmatism. This work provides a more detailed schematic view of the Steens Basalt magmatic system, from mantle origin through crustal staging.

    

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5. Uncovering the spatial heterogeneity of Ediacaran carbon cycling

   https://doi.org/10.1111/gbi.12222  

   Li, C. ; Hardisty, D. S. ; Luo, G. ; Huang, J. ; Algeo, T. J. ; Cheng, M. ; Shi, W. ; An, Z. ; Tong, J. ; Xie, S. ; et al ( December 2016 , Geobiology)

   Records of the Ediacaran carbon cycle (635–541 million years ago) include the Shuram excursion (SE), the largest negative carbonate carbon isotope excursion in Earth history (down to −12‰). The nature of this excursion remains enigmatic given the difficulties of interpreting a perceived extreme global decrease in the δ¹³C of seawater dissolved inorganic carbon. Here, we present carbonate and organic carbon isotope (δ¹³C_carband δ¹³C_org) records from the Ediacaran Doushantuo Formation along a proximal‐to‐distal transect across the Yangtze Platform of South China as a test of the spatial variation of theSE. Contrary to expectations, our results show that the magnitude and morphology of this excursion and its relationship with coexisting δ¹³C_orgare highly heterogeneous across the platform. Integrated geochemical, mineralogical, petrographic, and stratigraphic evidence indicates that theSEis a primary marine signature. Data compilations demonstrate that theSEwas also accompanied globally by parallel negative shifts of δ³⁴S of carbonate‐associated sulfate (CAS) and increased⁸⁷Sr/⁸⁶Sr ratio and coastalCASconcentration, suggesting elevated continental weathering and coastal marine sulfate concentration during theSE. In light of these observations, we propose a heterogeneous oxidation model to explain the high spatial heterogeneity of theSEand coexisting δ¹³C_orgrecords of the Doushantuo, with likely relevance to theSEin other regions. In this model, we infer continued marine redox stratification through theSEbut with increased availability of oxidants (e.g., O₂and sulfate) limited to marginal near‐surface marine environments. Oxidation of limited spatiotemporal extent provides a mechanism to drive heterogeneous oxidation of subsurface reduced carbon mostly in shelf areas. Regardless of the mechanism driving theSE, future models must consider the evidence for spatial heterogeneity in δ¹³C presented in this study.

    

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