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

Abstract

Calculation of [CO 2 (aq)] and p CO 2-atm based on proxy temperature and salinity data (δ 18 O and δ 13 C from belemnite rostra), and δ 13 C TOC (TOC—total organic carbon) from Toarcian marine organic-rich shales, yields unreasonably large intraformational p CO 2 (atm) variations. We postulate that the low δ 13 C TOC values in the shales can instead be explained by episodic release of isotopically light CO 2 that had built up below a relatively deep pycnocline (halocline?). The lowest δ 13 C 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 CO 2 . In addition, the higher productivity associated with the nutrient release would have resulted in depletion of the mixed-layer CO 2 , leading to lower fractionation and higher δ 13 C TOC values.