HOW MUCH TIME IS MISSING AT PARASEQUENCE BOUNDARIES? SUBAERIAL EXPOSURE AND METEORIC DIAGENESIS OF CYCLIC CARBONATES

Subaerial exposure and subsequent early meteoric diagenesis of carbonates imparts a predictable and distinct geochemical and stratigraphic expression. While the primary isotopic record of the ancient ocean can be erased, meteoric diagenesis provides a unique window into the duration and local hydro-dynamics of subaerial exposure. Carbonate sequences from the late Paleozoic Ice Age are rife with evidence of meteoric diagenesis that may be recording glacio-eustatic falls resulting from farfield ice-sheet expansion or orbitally forced glacial-interglacial cycles. In the middle Carboniferous, this diagenesis manifests as 10-100 meters of recrystallized and isotopically altered carbonate beneath regionally extensive and karsted exposure surfaces. However, throughout the Pennsylvanian, diagenesis is limited to parasequence bounded meter-scale isotopic excursions. These differences in expression of meteoric diagenesis may arise from distinct ice sheet dynamics during the middle and late Carboniferous. Here we quantitatively explore the duration of diagenesis and range of advection and reaction rates necessary to create these disparate isotopic and stratigraphic records using a newly developed 2D reactive transport model. This model is coupled with Bayesian inverse methods and chemical and physical stratigraphic data from meteorically altered sections throughout the Carboniferous to generate model parameter estimation while maintaining the full range of uncertainty in the parameter space. These results provide new insight into the origin of Pennsylvanian parasequences, and represent a novel approach for inferring exposure duration from the record of meteoric diagenesis in carbonates.