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1. 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)

   Abstract 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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2. Strontium and carbon isotopic evidence for decoupling of pCO2 from continental weathering at the apex of the late Paleozoic glaciation

   https://doi.org/10.1130/G40093.1  

   Chen, Jitao; Montañez, Isabel P.; Qi, Yuping; Shen, Shuzhong; Wang, Xiangdong (March 2018, Geology)

   Earth’s penultimate icehouse (ca. 340–285 Ma) was a time of low atmospheric pCO2 and high pO2, formation of the supercontinent Pangaea, dynamic glaciation in the Southern Hemisphere, and radiation of the oldest tropical rainforests. Although it has been long appreciated that these major tectonic, climatic, and biotic events left their signature on seawater 87Sr/86Sr through their influence on Sr fluxes to the ocean, the temporal resolution and precision of the late Paleozoic seawater 87Sr/86Sr record remain relatively low. Here we present a high-temporal-resolution and high-fidelity record of Carboniferous– early Permian seawater 87Sr/86Sr based on conodont bioapatite from an open-water carbonate slope succession in south China. The new data define a rate of long-term rise in 87Sr/86Sr (0.000035/m.y.) from ca. 334–318 Ma comparable to that of the middle to late Cenozoic. The onset of the rapid decline in 87Sr/86Sr (0.000043/m.y.), following a prolonged plateau (318–303 Ma), is constrained to ca. 303 Ma. A major decoupling of 87Sr/86Sr and pCO2 during 303–297 Ma, coincident with the Paleozoic peak in pO2, widespread low-latitude aridification, and demise of the pan-tropical wetland forests, suggests a major shift in the dominant influence on pCO2 from continental weathering and organic carbon sequestration (as coals) on land to organic carbon burial in the ocean. 

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3. Single Spot Rb‐Sr Isochron Dating of Biotite by LA‐MC‐ICP‐MS / MS

   https://doi.org/10.1111/ggr.12518  

   Cruz‐Uribe, Alicia M.; Craig, Grant; Garber, Joshua M.; Paul, Bence; Arkula, Cemil; Bouman, Claudia (July 2023, Geostandards and Geoanalytical Research)

   Laser ablation tandem mass spectrometry is a burgeoning field forin situRb‐Sr geochronology. Here, we determined simultaneous isotope ratios of⁸⁷Sr/⁸⁶Sr and⁸⁷Rb/⁸⁶Sr in metamorphic biotite from western Maine, using an ESL™ imageGEO™193 excimer laser ablation system coupled to a Thermo Scientific™ Neoma™ MC‐ICP‐MS/MS. Measurements were made on Faraday cups with Rb⁺at mass 87; Sr isotopes were reacted with SF₆gas and measured as SrF⁺at masses 103–107. Twenty‐two laser spots in biotite from a single sample yield a traditional Rb‐Sr isochron date of 289 ± 6 Ma. Time‐resolved signals reveal significant zoning in⁸⁷Sr/⁸⁶Sr and⁸⁷Rb/⁸⁶Sr within single spot analyses, which were used to construct single spot isochrons. Individual laser spots contain multiple isochronous subpopulations; some spots contain up to three distinct Rb‐Sr isochrons that are decoupled from variations in Rb/Sr. Thirty‐five isochron dates were determined using this sub‐spot approach, with⁸⁷Sr/⁸⁶Sr intercepts that systematically vary with Rb‐Sr date; two‐point isochrons were calculated for individual integrations (n= 780) based on these variable intercepts. Both methods yield age peaks at 303, 270 and 240 Ma. These data suggest that the Rb‐Sr system has the potential to record multiple heating, cooling or fluid‐alteration events spanning ~ 100 My within small domains in single biotite crystals. 

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4. Carbon Isotope Trends of Precambrian-Cambrian Carbonates in Southwestern Laurentia Are Robust to Diagenetic Overprinting

   https://doi.org/10.2475/001c.155346  

   Lonsdale, Mary C; Ahm, Anne-Sofie C; Nelson, Lyle L; Thompson, Jacob W; Higgins, John A; Smith, Emily F (January 2026, American Journal of Science)

   An extreme negative (<-6‰) carbon isotope (δ¹³C) excursion is recognized globally in strata recording the Ediacaran-Cambrian boundary. This excursion has been termed the BAsal Cambrian carbon isotope Excursion (BACE). It, like other carbon isotope excursions throughout the geologic record, has been interpreted to record a perturbation to the global carbon cycle based on the assumption that shallow water carbonate sediments and rock preserve an accurate time-series of the δ¹³C composition of the global ocean. However, this assumption has been demonstrated to be inaccurate in some shallow water settings; pervasive early diagenetic alteration of shallow water carbonate δ¹³C records and local carbon cycling within the platform environment can result in the decoupling of global ocean and shallow water carbonate δ¹³C values. Here, we test the extent and isotopic effects of early diagenetic alteration of shallow water carbonate sediments that record the BACE in southwestern Laurentia using carbonate stable isotope (δ¹³C, δ¹⁸O, δ^44/40Ca, δ²⁶Mg) and major and minor element (Mg/Ca, Sr/Ca) geochemistry. The δ^44/40Ca values of the three studied sections are consistent with different modes of diagenetic alteration, ranging from more sediment-buffered to more fluid-buffered early marine diagenesis. Dolostone δ²⁶Mg and δ¹⁸O values suggest that diagenetic alteration and dolomitization occurred prior to significant burial. Because the δ¹³C chemostratigraphic records are reproducible despite the variable diagenetic regimes among the three sites, we argue that the prominent δ¹³C stratigraphic trends were insensitive to diagenetic overprinting and instead reflect changes in the primary δ¹³C composition of dissolved inorganic carbon of platform seawater. Finally, we explore a potential link between the BACE, an increase in early marine fluid-buffered marine diagenesis of platform carbonates, and magmatism associated with the rifting of the southwest margin of Rodinia. 

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5. Revisiting Elevated δ ¹³ C Values of Sediment on Modern Carbonate Platforms

   https://doi.org/10.1029/2023GL107703  

   Trower, Elizabeth J; Hibner, Brianna M; Lincoln, Tyler A; Dodd, Jacqueline E; Hagen, Cedric J; Cantine, Marjorie D; Gomes, Maya L (August 2024, Geophysical Research Letters)

   Abstract The measured carbon isotopic compositions of carbonate sediments (δ¹³C_carb) on modern platforms are commonly¹³C‐enriched compared to predicted values for minerals forming in isotopic equilibrium with the dissolved inorganic carbon (DIC) of modern seawater. This offset undermines the assumption that δ¹³C_carbvalues of analogous facies in the rock record are an accurate archive of information about Earth's global carbon cycle. We present a new data set of the diurnal variation in carbonate chemistry and seawater δ¹³C_DICvalues on a modern carbonate platform. These data demonstrate that δ¹³C_carbvalues on modern platforms are broadly representative of seawater, but only after accounting for the recent decrease in the δ¹³C value of atmospheric CO₂and shallow seawater DIC due to anthropogenic carbon release, a phenomenon commonly referred to as the¹³C Suess effect. These findings highlight an important, yet overlooked, aspect of some modern carbonate systems, which must inform their use as ancient analogs. 

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