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[BPT27-12] Osmium isotope composition in the deep pelagic Panthalassa ocean during the Toarcian Oceanic Anoxic Event
The Toarcian Oceanic Anoxic Event (T-OAE) represents one of the most significant paleo-environmental perturbations in the Phanerozoic, which resulted in marked disruption to both the climate system and marine ecosystem. The causes for this widespread deposition of organic carbon-rich sediments under the reducing condition are still controversial. Several mechanisms have been proposed to explain the occurrence of T-OAE including; abrupt seawater warming, an increase in continental weathering rates, enhanced rates of biotic turnover, atmospheric pCO2 changes, and ocean acidification. In particular, volcanic activities in the large igneous Karoo-Ferrar province have been considered as the most fundamental trigger for the Toarcian perturbations. Radiogenic Os isotope compositions of sedimentary rocks are useful for estimating the influx from both ancient basaltic magmatism and continental weathering. However, the relationship between biotic crisis, anoxic event, and the volcanic activity in the deep pelagic environment is still ambiguous because of the lack of detailed Os isotope records during T-OAE. In this study, we reconstructed a detailed lithological column in the Inuyama area where the Pliensbachian to Toarcian deep-sea cherts well crop out along the Kiso river. For Os isotope analysis, we collected 12 chert samples from a section where two black bedded chert intervals T-OAE1 and T-OAE2 were observed.
The rock powder was spiked with 190Os and 185Re prior to digestion. The sample was digested by CrO3-H2SO4 in a sealed Carius tube at 240 oC for more than 48 hours. Subsequently, Os was separated by solvent extraction using CCl4 and back-extracted from CCl4 into HBr, followed by the final purification using the micro-distillation method. The Os isotope ratios were determined by N-TIMS at Tokyo Tech. Re-bearing solution after the CCl4 extraction was stored in a Teflon vessel, and Cr6+ in the solution was completely reduced to Cr3+ by the addition of ethanol in order to avoid the disturbance of the elution profile of the following chromatography due to the coexistence of Cr6+. After this process, Re was purified by chemical separation using an anion exchange resin, and the isotopic composition was determined using a quadrupole-type ICP-MS at Tokyo Tech. The Os and Re concentrations were determined by the isotope dilution method.
The Re and Os abundances in 12 chert samples across T-OAE varied from 2.1 to 128.6 ng/g, and from 32.8 to 2284.3 pg/g, respectively. The 187Os/188Os ratio in the samples before T-OAE1 decreased from 0.44 to 0.27, resulting in the minimum value of 0.24 at the onset of T-OAE1. However, the 187Os/188Os ratio increased abruptly to be 0.77 towards the end of T-OAE1, and decrease down to 0.30 at the interval between T-OAE1 and T-OAE2. During T-OAE2, the 187Os/188Os ratio gradually increased up to 0.56. Take into account organic carbon isotope record in the same section by Grocke et al. (2011), we envisage the following scenario for the cause of Os and C isotopic variation during T-OAE observed in the chert samples: The Karoo-Ferrar volcanic activity induced an anoxic condition, which resulted in negative 187Os/188Os before T-OAEs. The Karoo-Ferrar volcanism released significant amount of CO2, which triggered the global warming and continental erosion during T-OAE1. Subsequent gradual increase of Os isotope ratios most likely reflects its aftermath. These indicate that the Karro-Ferrar volcanism played an important role for induing anoxic condition and global worming during Toarcian in the deep pelagic Panthalassa ocean.
The rock powder was spiked with 190Os and 185Re prior to digestion. The sample was digested by CrO3-H2SO4 in a sealed Carius tube at 240 oC for more than 48 hours. Subsequently, Os was separated by solvent extraction using CCl4 and back-extracted from CCl4 into HBr, followed by the final purification using the micro-distillation method. The Os isotope ratios were determined by N-TIMS at Tokyo Tech. Re-bearing solution after the CCl4 extraction was stored in a Teflon vessel, and Cr6+ in the solution was completely reduced to Cr3+ by the addition of ethanol in order to avoid the disturbance of the elution profile of the following chromatography due to the coexistence of Cr6+. After this process, Re was purified by chemical separation using an anion exchange resin, and the isotopic composition was determined using a quadrupole-type ICP-MS at Tokyo Tech. The Os and Re concentrations were determined by the isotope dilution method.
The Re and Os abundances in 12 chert samples across T-OAE varied from 2.1 to 128.6 ng/g, and from 32.8 to 2284.3 pg/g, respectively. The 187Os/188Os ratio in the samples before T-OAE1 decreased from 0.44 to 0.27, resulting in the minimum value of 0.24 at the onset of T-OAE1. However, the 187Os/188Os ratio increased abruptly to be 0.77 towards the end of T-OAE1, and decrease down to 0.30 at the interval between T-OAE1 and T-OAE2. During T-OAE2, the 187Os/188Os ratio gradually increased up to 0.56. Take into account organic carbon isotope record in the same section by Grocke et al. (2011), we envisage the following scenario for the cause of Os and C isotopic variation during T-OAE observed in the chert samples: The Karoo-Ferrar volcanic activity induced an anoxic condition, which resulted in negative 187Os/188Os before T-OAEs. The Karoo-Ferrar volcanism released significant amount of CO2, which triggered the global warming and continental erosion during T-OAE1. Subsequent gradual increase of Os isotope ratios most likely reflects its aftermath. These indicate that the Karro-Ferrar volcanism played an important role for induing anoxic condition and global worming during Toarcian in the deep pelagic Panthalassa ocean.