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

Coastal aquifers act as a major host of seawater-groundwater interaction and play an important role in modulating the marine elemental budget. Calcium stable isotopes (δ44/40Ca) have been used to identify mass dependent isotope fractionation processes such as carbonate dissolution and precipitation in a range of geological reservoirs that has major implications in constraining global geochemical cycles. However, there is limited Ca isotope data from coastal aquifers globally. Here we report δ44/40Ca values of groundwater collected in 2017-18 from multiple locations and depths from the Bakkhali delta front, Sundarbans, India. The sampling depth varied between 14 m below ground level (m bgl) and 333 m bgl and the salinity of the groundwater samples range from 1-25 ppt. The salinity of the water samples decreases with increasing depth indicating greater seawater incursion from the Bay of Bengal at shallower depths. Variable amounts of mixing of freshwater and seawater is also supported by Sr and Ca concentrations which vary between 1.6-62.8 μmol/l and 0.29-8.92 mmol/l, respectively, and show progressively lower concentrations with depth. The δ44/40Ca values of dissolved phase in groundwater samples (relative to NIST SRM 915a) were measured using a 43Ca-48Ca Double Spike TIMS technique at Centre for Earth Sciences, Indian Institute Science, Bangalore. The δ44/40Ca values of the groundwater samples show significant variability between 1.52-2.28‰ (2SD ~0.1‰) with several samples showing δ44/40Ca values higher than modern seawater (~1.88‰). Low δ44/40Ca values, mostly in deeper groundwater samples, is consistent with higher proportions of freshwater input. Samples showing high δ44/40Ca values are mostly from shallower depths and likely reflect carbonate precipitation which is consistent with high Sr/Ca (~8.13-12.26) in samples from 30-42 m depth. The high δ44/40Ca in groundwater samples from the Ganges-Brahmaputra delta could explain the high δ44/40Ca values reported in water samples from the Bay of Bengal1. [1] Chakrabarti et al., Goldschmidt Boston, 2018 Abstract.

Stable Ca isotopic composition (δ44/40Ca) of crustal carbonates are typically lighter than that of the bulk silicate Earth value (~1.05 ‰). Hence, δ44/40Ca of mantle-derived rocks can potentially trace recycled crustal carbonates into the mantle. We report the Ca isotopic compositions of globally distributed carbonatites (n = 46), which are unique igneous rocks with more than 50% modal carbonate minerals, with eruption ages ranging from Precambrian until recent. The δ44/40Ca (w.r.t. SRM915a) of these carbonatites show a large range (0.35 ‰ to 1.26 ‰), which is significantly higher than the analytical uncertainty (0.08‰) of the measurements performed using TIMS at CEaS, IISc. These samples are well-characterized in terms of their major and trace element geochemistry as well as Nd, Sr, B, C, and O isotopic compositions for selected samples. No systematic trend is observed between δ44/40Ca of the carbonatites and their eruption ages. Significant variability is observed in δ44/40Ca values in samples from individual provinces including those from the Oka complex in Canada (0.44 ‰ – 1.26 ‰, n= 8), Ambadongar (0.53 ‰ – 1.1 ‰, n= 8) and the Newania complexes (0.44 ‰ – 0.83 ‰, n= 4) in north-west India, the South Indian carbonatites (0.65 ‰ – 0.91 ‰, n= 3) and carbonatites from the Palabora complex in South Africa (0.35 ‰ – 0.84‰, n= 3). The δ44/40Ca of carbonatites from Oka, Newania and the Ambadongar show strong correlations with Ca/Mg, Ca/Fe as well as CaO and MgO contents. The δ44/40Ca of the Oka and Ambadongar carbonatites show correlated variations with their Mg# and K/Rb ratios, respectively. The large variability in δ44/40Ca of global carbonatites is explained in terms of: (1) presence of isotopically lighter ancient subducted carbonates in the mantle-source regions and carbonate metasomatism of the mantle, (2) partial melting and differentiation of the carbonatite magma and (3) heterogeneity in the source-mantle mineralogy of carbonatites.