METEORIC ALTERATION OF MARINE δ13CCARB BELOW SUBAERIAL UNCONFORMITIES: PALEOZOIC EXAMPLES ACROSS A RANGE OF SPATIAL AND TEMPORAL SCALES

Despite decades of study, the interpretation of the carbon isotope record preserved in carbonate successions continues to be highly debated, particularly in regard to the effects of meteoric diagenesis on primary marine δ¹³C_carb. Studies of Recent carbonates that show substantial modification of fabrics and chemical composition through processes related to subaerial exposure and shallow burial have introduced varying levels of uncertainty as to how δ¹³C_carb values measured from ancient stratigraphic sequences should be interpreted. To better understand the signal origin of δ¹³C_carb in ancient carbonates, we evaluate successions containing unconformities of varying scales, focusing on evidence for the modification of primary δ¹³C_carb values by meteoric processes.

Subhorizontal, shallowly buried Paleozoic carbonates along structural arches in the U.S. Midcontinent contain infilled paleokarst cavities and locally thick paleosols preserved at unconformity surfaces, that range between a few hundred thousand to millions of years in duration. Here we present multiple high-resolution δ¹³C_carb and δ¹⁸O_carb profiles across several unconformities to evaluate the resetting of primary values. While we document distinct negative shifts in the range of -2 to -10‰, cements bearing these values are generally minor and highly localized. In an extreme example, δ¹³C_carb negative shifts below the high-relief, 40 million-year-duration Walbridge Unconformity are relatively minor and generally restricted to rock clearly affected by paleo-weathering. Surprisingly, shorter duration subaerial surfaces associated with low-relief 3^rd and 4^th order depositional sequences show little evidence of δ¹³C_carb modification. In fact, the largest δ¹³C_carb shifts appear to be produced by marine diagenesis below unconformity surfaces through infilling of secondary porosity with younger marine cements and plugging of paleokarst features with transgressive sediments. Also noteworthy is the preservation of primary values and trends despite post-deposition dolomitization of some of these stratigraphic sections. Together, our extensive evaluation of subaerial unconformity surfaces testifies to the robust preservation of primary δ¹³C_carb signatures in Paleozoic whole-rock carbonates.