STEPWISE ONSET OF CARBONIFEROUS GLACIATION CAPTURED IN C AND SR ISOTOPIC RECORDS OF PLATFORM CARBONATES

Long-lived and extensive glaciation during the Carboniferous has been the focus of many studies over the past decade revealing systematic changes in sea level, carbon cycling, and oceanic and climatic conditions. The nature of the inception of Carboniferous glaciation, however, remains poorly resolved with uncertainty regarding the timing and dynamics of build up of continental ice sheets. Here we present coupled C and Sr isotopic records within a sedimentologic and chronostratigraphic framework from late Mississippian shallow-marine, carbonate platform and contemporaneous carbonate slope successions from the east Paleotethys Ocean region (South China) to reveal systematic variations that provide insight into the sedimentary response to the onset of Carboniferous glaciation. Overall, temporal variability in the facies stacking patterns in both shallow platform and slope successions suggest changes in the inferred duration and magnitude of sea-level fluctuations through the late Visean to late Serpukhovian. In contrast, C and Sr isotopic compositions of diagenetically screened micrite and brachiopods from the platform carbonates exhibit systematic fluctuations in step with inferred sea-level changes, whereas slope carbonate and conodont isotopic compositions show minimal variability. These differences are interpreted to record the influence of local depositional, but not diagenetic, processes operating on the carbonate platform in response to glacioeustasy. Repeated large magnitude shifts in C and Sr isotopic compositions of late Visean to early Serpukhovian carbonates correspond to 1 to 2 myr-scale cycles interpreted to record large-magnitude sea-level fluctuations driven by the buildup of continental glaciers. Decreased magnitude of isotopic shifts through the overlying middle to late Serpukhovian interval coincident with a shift to shorter duration cycles and smaller magnitude sea-level fluctuations, is interpreted to record dampening influence of local processes with long-term sea-level rise in the late Serpukhovian. The coupled stratigraphic and isotopic records indicate stepwise initiation of ice-buildup prior to widespread glaciation across the mid-Carboniferous boundary, likely involving retraction of ice sheets in response to late Serpukhovian warming.