Orbitally synchronized late Pliensbachian–early Toarcian glacio-eustatic and carbon-isotope cycles

Abstract

The late Pliensbachian–early Toarcian δ13Corg in the Mochras Core (UK) consists of segments (rising and falling limbs, valleys, plateaus) that correlate to coeval segments in Europe and America. The Mochras global δ13Corg segments were dated with cyclostratigraphic analysis and U-Pb dating and found to be anti-correlated to eustatic cycles and model cycles predicted by the orbital scale of glacio-eustasy. In the interval 188–181 Ma five lowstands correlate to minor (<25 m), medium (25–75 m) or major (>75 m) global sequence boundaries (SB): minor early margaritatus SB JPl6 (187.89 Ma); major late margaritatus SB JPl7 (186.27 Ma); medium spinatum SB JPl8 (184.24 Ma); medium tenuicostatum SB JTo1 (183.03 Ma); and medium serpentinum SB JTo2 (182.22 Ma). Minor SBs correlate to small δ13Corg positive spikes without apparent hiatus. Medium and major SBs correlate to positive δ13Corg jumps (unconformity), or occur in rising limbs and plateaus. Maximum flooding surfaces follow SBs by >0.4 Myr at the intersection of δ13Corg falling limbs and valleys. The δ13Corg segments and correlative cycles are tuned to the 0.405 Myr eccentricity cycle and its multiples, implying orbital-forcing of insolation synchronized glacio-eustatic, climate and carbon cycles and modulated the atmospheric CO2 levels during glacial-interglacial intervals. Orbitally paced changes in the Earth's cryosphere, climate and carbon cycle are therefore considered a major driver of late Pliensbachian-early Toarcian environmental instabilities and perturbations.