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1. Changing inputs of continental and submarine weathering sources of Sr to the oceans during OAE2

   https://doi.org/doi.org/10.1016/gca.2021.03.013  

   Nana Yobo, L.; Brandon, A.D.; Holmden, C.; Lau, K.V.; Eldrett, J. (January 2022, Geochimica et cosmochimica acta)

   Jacobson, A. (Ed.)

   Ocean anoxic events (OAE) are characterized by increased organic content of marine sediment on a global scale with accompanying positive excursions in sedimentary organic and inorganic d 13C values. To sustain the increased C exports and burial required to explain the C isotope excursion, increased supplies of nutrients to the oceans are often invoked during ocean anoxic events. The potential source of nutrients in these events is investigated in this study for Oceanic Anoxic Event 2, which spans the Cenomanian-Turonian boundary. Massive eruptions of one or more Large Igneous Provinces (LIPs) are the proposed trigger for OAE 2. The global warming associated with volcanogenic loading of carbon dioxide to the atmosphere has been associated with increased continental weathering rates during OAE 2, and by extension, enhanced nutrient supplies to the oceans. Seawater interactions with hot basalts at LIP eruption sites can further deliver ferrous iron and other reduced metals to seawater that can stimulate increased productivity in surface waters and increased oxygen demand in deep waters. The relative importance of continental and submarine weathering drivers of expanding ocean anoxia during OAE 2 are difficult to disentangle. In this paper, a box model of the marine Sr cycle is used to constrain the timing and relative magnitudes of changes in the continental weathering and hydrothermal Sr fluxes to the oceans during OAE 2 using a new high-resolution record of seawater 87Sr/86Sr ratios preserved in a marl-limestone succession from the Iona-1 core collected from the Eagle Ford Formation in Texas. The results show that seawater 87Sr/86Sr ratios change synchronously with Os isotope evidence for the onset of massive LIP volcanism 60 kyr before the positive C isotope excursion that traditionally marks the onset of OAE 2. The higher temporal resolution of the seawater Sr isotope record presented in this study warrants a detailed quantitative analysis of the changes in continental weathering and hydrothermal Sr inputs to the oceans during OAE 2. Using an ocean Sr box model, it is found that increasing the continental weathering Sr flux by
   1.8-times captures the change in seawater 87Sr/86Sr recorded in the Iona-1 core. The increase in the continental weathering flux is smaller than the threefold increase estimated by studies of seawater Ca isotope changes during OAE 2, suggesting that hydrothermal forcing may have played a larger role in the development of ocean anoxic events than previously considered. 

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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. Carbonate uranium isotopes record global expansion of marine anoxia during the Toarcian Oceanic Anoxic Event

   https://doi.org/10.1073/pnas.2406032121  

   Remírez, Mariano N; Gilleaudeau, Geoffrey J; Gan, Tian; Kipp, Michael A; Tissot, François_L H; Kaufman, Alan J; Parente, Mariano (July 2024, Proceedings of the National Academy of Sciences)

   The Toarcian Oceanic Anoxic Event (T-OAE; ~183 Mya) was a globally significant carbon-cycle perturbation linked to widespread deposition of organic-rich sediments, massive volcanic CO₂release, marine faunal extinction, sea-level rise, a crisis in carbonate production related to ocean acidification, and elevated seawater temperatures. Despite recognition of the T-OAE as a potential analog for future ocean deoxygenation, current knowledge on the severity of global ocean anoxia is limited largely to studies of the trace element and isotopic composition of black shales, which are commonly affected by local processes. Here, we present the first carbonate-based uranium isotope (δ²³⁸U) record of the T-OAE from open marine platform limestones of the southeastern Tethys Ocean as a proxy for global seawater redox conditions. A significant negative δ²³⁸U excursion (~0.4‰) is recorded just prior to the onset of the negative carbon isotope excursion comprised within the T-OAE, followed by a long-lived recovery of δ²³⁸U values, thus confirming that the T-OAE represents a global expansion of marine anoxia. Using a Bayesian inverse isotopic mass balance model, we estimate that anoxic waters covered ~6 to 8% of the global seafloor during the peak of the T-OAE, which represents 28 to 38 times the extent of anoxia in the modern ocean. These data, combined with δ²³⁸U-based estimates of seafloor anoxic area for other CO₂-driven Phanerozoic OAEs, suggest a common response of ocean anoxia to carbon release, thus improving prediction of future anthropogenically induced ocean deoxygenation. 

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4. Biotic extinction at the Norian/Rhaetian boundary (Upper Triassic): geochemical and isotope evidence of a previously unrecognised global event

   https://doi.org/10.46427/gold2022.11474  

   Rigo, Manuel; Onoue, Tetsuji; Sato, Honami; Tomimatsu, Yuki; Soda, Katsuhito; Godfrey, Linda; Katz, Miriam; Campbell, Hamish; Tackett, Lydia; Golding, Martyn; et al (October 2022, Goldschmidt 2022 Abstract)

   turnovers culminating in the so-called End-Triassic Extinction. We attribute onset of this interval of declining diversity to unusually high volcanic activity at the Norian/Rhaetian boundary (NRB) that may have initiated the stepwise extinctions of the Late Triassic [1]. We correlate the initiation of a rapid decline in 87Sr/86Sr and 187Os/188Os seawater values [2, 3] to a negative organic carbon isotope shift, which we attribute to volcanogenic CO2 outgassing to the ocean-atmosphere system by the Angayucham large igneous province (LIP). By studying the geochemical and isotope composition of bulk rocks from different sections located at different latitudes, sides of the Pangea continent and Hemispheres, we documented an accelerated chemical weathering due to global warming by elevated CO2, which enhanced nutrient discharge to the oceans and thus greatly increased biological productivity; higher export production and oxidation of organic matter led to oceanic dysoxia to anoxia at the NRB. Biotic consequences of these climatic and environmental changes include severe extinctions of several fossil groups, such as ammonoids, bivalves and radiolarians, as has been documented worldwide [1]. 

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5. Reconstructing the depositional environment and diagenetic modification of global phosphate deposits through integration of uranium and strontium isotopes

   https://doi.org/10.1016/j.chemgeo.2024.122214  

   Hill, Robert C; Wang, Zhen; Williams, Gordon DZ; Polyak, Victor; Singh, Anjali; Kipp, Michael A; Asmerom, Yemane; Vengosh, Avner (September 2024, Chemical Geology)

   The geochemistry of phosphate rocks can provide valuable information on their depositional environment and the redox condition of global oceans through time. Here we examine trace metal concentrations and uranium (δ238U, δ234U) and strontium (87Sr/86Sr) isotope variations of marine sedimentary phosphate rocks and the phosphate-bearing carbonate fluorapatite (CFA) mineral phase, originating from Precambrian to mid-Miocene aged major global phosphate deposits. We find elevated concentrations of several trace elements (Al, V, Cr, Cd, U, Mn, Co, Cu, As, and Rb) in the CFA mineral phase of young phosphate rocks (Miocene to Late Cretaceous) relative to those of older (Devonian to Precambrian) rocks. The δ238U of phosphate rocks of Mid-Miocene to Permian age range from −0.311‰ to 0.070‰, exhibiting a positive fractionation relative to modern seawater (−0.38‰). This is similar to the isotope fractionation reported for carbonate and shale sediments, likely resulting from the reduction of uranium in porewaters during CFA precipitation. Cambrian to Precambrian phosphate rocks have lower δ238U of −0.583‰ to −0.363‰, indicating different depositional redox conditions likely resulting from seafloor anoxia and/or diagenetic modification. The 87Sr/86Sr ratios of phosphate rocks of Cretaceous to Mid-Miocene age generally follow the secular 87Sr/86Sr seawater curve. Phosphate rocks with 87Sr/86Sr that deviate from this curve have characteristic trace metal trends, such as lower Sr/Ca and Sr concentrations, suggesting later diagenetic modification. Older phosphate rocks of Precambrian age are systematically more radiogenic than the expected secular Sr seawater composition at the time of deposition, possibly due to the greater influence of terrestrial input in peritidal zones and/or more pervasive diagenetic modification. Overall, our study reveals connections between U and Sr isotope variations for reconstructing the depositional and diagenetic conditions of global phosphate rock formation through Earth history and the transition to an oxic ocean following the Paleozoic Oxygenation Event. 

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