Does the Largest Recorded Negative Carbon Isotope Excursion from the Neoproterozoic Krol Formation (India) Represent a Globally Synchronous Diagenetic Event?

The "Shuram Excursion (SE)" stands as the largest known negative carbon isotope excursion (CIE) in geological history and is thought to represent the largest carbon cycle perturbation, possibly pivotal in the evolution of complex life forms and, thereafter, the Cambrian biotic explosion. Apart from the Shuram Formation at its type locality in Oman (the maximum depleted stable carbon isotopic ratio [δ¹³C value] in carbonate [peak δ¹³C_Carbonate] of ~ –12 ‰), comparable negative CIE of similar ages has also been documented from different parts of the globe, among which the well constrained globally recognized SE sections are the Wonoka Formation of Australia (peak δ¹³C_Carbonate of ~ −10 ‰), the Doushantuo Formation of South China (peak δ¹³C_Carbonate of ~ −14 ‰), and the Rainstorm Member in the Johnnie Formation of Death Valley, California (peak δ¹³C_Carbonate of ~ −11‰). Considering the inherent problem with the exact depositional age estimation for sedimentary rocks, although all the global locations show similar CIE patterns, the peak δ¹³C_Carbonate values, and stratigraphic thicknesses vary from section to section. Because of these disparities, two distinct perspectives exist regarding the origin of the SE. While one group argues that a globally synchronous diagenetic event is responsible for the SE CIE, the other suggests it is a record of the temporal variation in the primary δ¹³C composition of the seawater dissolved inorganic carbon (DIC) and, hence, represents the actual carbon cycle perturbation. Based on the preliminary data obtained by other studies from the late Neoproterozoic Lesser Himalayan Krol Formation, previous workers tentatively suggested that the succession may contain the SE. However, because of the composite nature of the studied sections and the large spread in carbonate δ¹³C values in lithologically correlative stratigraphic (temporal) intervals, the proposition was not confirmed. So, the current study tries to recognize the possible SE CIE in the Lesser Himalayan Krol Formation, India, by studying continuously measured structurally undisturbed sections by employing vigorous fieldwork, detailed high-resolution carbonate δ¹³C data, and Pb-Pb dating of the carbonates after careful thin section and cathodoluminescence (CL) based screening for any possible diagenetic alterations. The transmitted light and CL-based petrographic observation, along with bulk and fabric-specific (micro-sampling) carbonate δ¹³C data, suggests the presence of distinct negative CIE with a peak δ¹³C_Carbonate value of ~ –10.7 ‰ (CIE magnitude of ~10.7 ‰) in the Lesser Himalayan Krol Formation. The depositional age of ~560 ± 12 Ma, estimated by Pb-Pb dating of the carbonate across this CIE, further suggests the possible presence of SE CIE in the investigated section.