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Evidence of recycling of isotopically light CO₂ (aq) in stratified black shale basins: Contrasts between the Whitby Mudstone and Kimmeridge Clay formations, United Kingdom

¹ RF-Rogaland Research, Thormøhlensgate 55, N-5008 Bergen, Norway
² Fossil Fuels and Environmental Geochemistry (Postgraduate Institute), Drummond Building, University of Newcastle, Newcastle upon Tyne, NE1 7RU, England
³ Geological Institute, University of Bergen, Allégaten 41, N-5007 Bergen, Norway
⁴ Norsk Hydro Research Centre, Sandsliveien 90, N-5020 Bergen, Norway

Calculation of [CO₂ (aq)] and pCO_2-atm based on proxy temperature and salinity data (¹⁸O and ¹³C from belemnite rostra), and ¹³C_TOC (TOC—total organic carbon) from Toarcian marine organic-rich shales, yields unreasonably large intraformational pCO₂ (atm) variations. We postulate that the low ¹³C_TOC values in the shales can instead be explained by episodic release of isotopically light CO₂ that had built up below a relatively deep pycnocline (halocline?). The lowest ¹³C_TOC values would thus be associated with mixing and erosion of the pycnocline following extended periods of stable stratification. In contrast, the studied Tithonian oil shales were probably deposited during periods of only slight perturbation of a photic zone chemocline, which probably led to only modest release of isotopically light CO₂. In addition, the higher productivity associated with the nutrient release would have resulted in depletion of the mixed-layer CO₂, leading to lower fractionation and higher ¹³C_TOC values.

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