Abstract
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
δ13C 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 δ13C 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
(δ44/40Ca) isotopic compositions. Elemental
compositions were measured using an Inductively Coupled Plasma Mass
Spectrometer (ICP-MS, X series II) while
87Sr/86Sr and
δ44/40Ca (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 δ13C values is evident in samples
with low Y/Ho (<40), samples with high Y/Ho (>
40) preserve low δ13C values. No significant
correlation has been observed between δ13C 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
δ44/40CaSRM915a compositions
(1.44‰-2.21‰) typical of Neoproterozoic post-glacial
successions2. Our study confirms primary origin and
open ocean nature of the late Neoproterozoic δ13C
excursions in the Marwar Basin. [1] Ansari et al., (2018)
Precambrian Research, [2] Silva-Tamayo et al., (2010) Precambrian
Research