Calculation of [CO2 (aq)] and pCO2-atm based on proxy temperature and salinity data (δ18O and δ13C from belemnite rostra), and δ13CTOC (TOC—total organic carbon) from Toarcian marine organic-rich shales, yields unreasonably large intraformational pCO2 (atm) variations. We postulate that the low δ13CTOC 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 δ13CTOC 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 δ13CTOC values.