Geochemical and 87Sr/86Sr and 44/40Ca Compositions of Ediacaran Carbonates from the Indian Shield: Understanding Preservation of Local versus Global Biogeochemical Signatures during Periods of Extreme Environmental Changes

The Neoproterozoic Era represents extreme environmental conditions with three major glaciation events. Associated with these glaciation and deglaciation periods are high amplitude positive and negative 13C excursions which have been observed in the rock records, suggesting perturbations in oceanic biogeochemical cycles. However, whether these isotopic records reflect primary depositional signature and open ocean condition are debated. The Marwar Basin of the Indian Shield preserves one such record of the Ediacaran time period. The magnitude of negative 13C excursion from the Marwar Basin is comparable with the Shuram excursion.1 We report elemental compositions of sixty-eight carbonate samples collected from five spatially distributed sections from the Bilara carbonates of the Marwar Basin. Selected samples from two of these sections were analyzed for their radiogenic Sr (87Sr/86Sr) and stable Ca (44/40Ca) isotopic compositions. Elemental compositions were measured using an Inductively Coupled Plasma Mass Spectrometer (ICP-MS, X series II) while 87Sr/86Sr and 44/40Ca (reported relative to NIST SRM 915a) values were measured using a Thermal Ionization Mass Spectrometer (TIMS, Triton Plus), both at the CEaS, IISc Bangalore, India. The Bilara Group carbonates are sub-divided into two populations based on non-redox REY anomalies and the Y/Ho ratio. Super-Chondritic Y/Ho (40-52) and positive La anomaly (1.01-2.65) of some samples suggest deposition under open ocean conditions and connectivity to the global ocean. While heterogeneity in 13C values is evident in samples with low Y/Ho (<40), samples with high Y/Ho (> 40) preserve low 13C values. No significant correlation has been observed between 13C and Ce anomaly (Ce/Ce*) suggesting absence of any paleo-redox gradient. The lowest 87Sr/86Sr (~0.7079) observed in the carbonates is comparable with Ediacaran seawater confirming retention of primary depositional signatures in these samples. These carbonates show heavy 44/40CaSRM915a compositions (1.44-2.21) typical of Neoproterozoic post-glacial successions2. Our study confirms primary origin and open ocean nature of the late Neoproterozoic 13C excursions in the Marwar Basin. [1] Ansari et al., (2018) Precambrian Research, [2] Silva-Tamayo et al., (2010) Precambrian Research