Carbonates and Evaporites (2020) 35:38 https://doi.org/10.1007/s13146-020-00574-9 ORIGINAL ARTICLE Palaeoredox link with the late Neoproterozoic–early Cambrian Bilara carbonate deposition, Marwar Supergroup, India A. H. Ansari1 · S. K. Pandey1 · Kamlesh Kumar1 · Shailesh Agrawal1 · Shamim Ahmad1 · Mayank Shekhar1 Accepted: 27 February 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract This study performed a stable isotope and redox-sensitive trace-element investigation on the Gotan Limestone and Pondlo Dolomite of the Bilara Group, Marwar Supergroup to decipher the palaeodepositional redox condition. A multivariate- statistical analysis of geochemical data set (δ13C-carb, δ18O-carb, δ13C-org, TOC, V, Ni, Cu, Zn, Mo, Pb, and U) reveals that limestone and dolomite units of the Bilara Group were deposited under two different redox regimes. Dolomite has an average δ13C-org values ~ − 30.0 ‰ and shows a statistically significant negative correlation with 13δ C-carb which indicates an oxygen-depleted closed palaeodepositional environment. Dolomite δ13C-carb also shows a statistically significant nega- tive correlation with redox-sensitive trace elements V, Ce, Pb, and U which infer higher secondary productivity during the dolomite deposition. The limestone has an average δ13C-org values ~ − 28.9 ‰ and δ13C-carb shows a significant positive correlation with δ18O-carb and Sr, whereas δ13C-carb shows a significant negative correlation with V and U. These statistical relationships among the geochemical parameter are indicative of suboxic-to-oxic water column with a relatively higher depth that received a significant input of water discharge from the terrestrial region. This discharge may have brought nutrients and other weathering-derived ions into the basin. Keywords Palaeoredox · Bilara group · Marwar supergroup · Limestone · Dolomite Introduction subsequently in shallow marine water setting (Martin et al., 2000; Narbonne and Gehling 2003). First unambiguous Redox nature of the marine environments in the long Edi- calcified metazoan appeared around 549 Ma (Germs 1972; acaran Period (635–541 Ma) holds the answer to the emer- Grotzinger et al. 2000; Amthor et al. 2003; Chen et al. 2008; gence of complex megascopic multicellular animals and Wood 2011; Darroch et al. 2018, and reference therein). surface environmental disturbances (Xiao and Laflamme Redox-sensitive trace-element study by Schröder and 2009; Sawaki et al. 2018) in parallel with the evolution of Grotzinger (2007) on the Ediacaran–Cambrian boundary deep ocean oxygenation. Evolution of multicellular animals section of Oman suggested the presence of stagnant basinal required oxygen support for their metabolism and hard skel- water mass. Upwelling of this anoxic water mass supplied etal parts. Therefore, it is normally assumed that progres- redox-sensitive trace elements (RSTE) and light carbon to sive increase in oxygen concentration was affected by the the surface water which was related to the development of static anaerobic condition in early–middle Proterozoic which isotopic excursion (Fike et al. 2006; McFadden et al. 2008). must have provided the platform for the development of this Whereas, from the lower Cambrian succession of China, complexity (Li et al. 2015; Planavsky et al. 2014; Droser pyrite framboids and redox-sensitive element ratios revealed et al. 2017; Evans et al. 2018). Complex multicellular forms anoxic-to-euxinic water regime (Fan et al. 2018; Och and of life appeared about 571 Ma, represented by soft body Shields-Zhou 2012; Pi et al. 2013; Jin et al. 2016; Zhang enigmatic Ediacaran biota initially in deep marine water and et al. 2018). In this study, we reconstruct the palaeoredox environment of the Bilara carbonate deposition (Barna-II) 13 * A. H. Ansari using RSTE, δ C-carb, total organic carbon (TOC), and 13 [email protected] δ C-org. Barna-II has been suggested equivalent to the Yangtze Gorges section, South China based on high-ampli- 1 Birbal Sahni Institute of Palaeosciences, 53 University Road, tude negative δ13C excursion (Ansari et al., 2018). RSTE Lucknow 226007, India Vol.:(0123456789)1 3 38 Page 2 of 13 Carbonates and Evaporites (2020) 35:38 studies for palaeoredox reconstruction are commonly con- were digested in a mixture of HF + HNO3 + HClO4 acids by ducted on black shales; however, RSTE analysis has also ‘B-solution’ method (Shapiro and Brannock 1962) in Teflon been performed on carbonate units for the same purpose crucibles. All samples were analyzed by ICP-MS (Agilent (Bolhar and Van Kranendonk 2007; Schröder and Grotzinger Technologies 7700 Series) at Birbal Sahni Institute of Pal- 2007; Chang et al. 2016). aeosciences, Lucknow. Several USGS standards (SGR-1, In general, many RSTE in the marine water are present MAG-1, SCo-1) and a few in-house standards were used for either in adsorbed or soluble forms. Removal of RSTE from machine calibration. Precision for ICP-MS analyses obtained marine water columns takes place either by biotic or abiotic was ≤ 5%. processes (Tribovillard et al. 2006). Biotic removal of RSTE For the measurement of TOC content and δ13C-org, ~ 1 g usually occurs in an oxic environment. RSTE removal by of the sediment sample was taken. We have followed the the abiotic process is relatively limited in oxic water, but sample preparation procedure discussed in Agrawal et al. occurs more often in anoxic waters through diffusion across (2015, 2017). In short, ~ 1 g homogenized sediment sample water–sediment interfaces or through repartitioning/remo- was dried and powdered to a fine clay and transferred into bilization along redox gradient (Tribovillard et al. 2006). 50 ml centrifuge tubes. The samples were then treated with a Abiotic processes are more effective in reducing environ- 5% HCl solution thrice to remove the carbonates. To remove ment; for example, adsorption of metallic or ionic species the acid and soluble salts, samples were washed with milli-Q is dependent upon organic matter, forms of organo-metallic water using a centrifuge machine (~ 3000 rpm). Thereafter, complex, precipitation of sulfide, and insoluble oxyhydrox- the de-carbonated samples were kept in a hot air oven at ide. RSTE such as U, V, and Mo in the water column are 45 °C temperature until they were completely dried. The sensitive to redox changes and are enriched in sediments samples were weighed and carefully packed into tin capsules under anoxic water condition that makes them a robust proxy and introduced into the pre-filled and conditioned reactor of to determine the palaeoredox condition (Tribovillard et al. Elemental Analyzer (Flash EA 2000 HT). The combusted 2006). In this paper, we have applied a multivariate-statisti- CO2 gas was introduced to the Continuous Flow Isotope cal analysis on the RSTE, TOC, δ13C-carb, δ18O-carb, and Ratio Mass Spectrometer (CFIRMS, MAT 253) coupled δ13C-org composition of the carbonate rocks to reconstruct with Con-Flow IV interface for isotopic analysis. Interna- the palaeoredox environment of Barna-II (best open-cast tional standards (CH3 and CH6), as well as internal stand- mine exposed in the Bilara Group). ards (Sulfanilamide), were run to check the accuracy for the CO2 measurements with an external precision of ± 0.1‰ (1σ). The carbon isotopic data have been reported against Materials and methods VPDB. TOC content was calculated from the peak area of the integrated m/z 44, 45, and 46 signals (Jensen 1991). All The samples were collected from horizontally bedded car- the samples were analyzed in the Stable Isotope Laboratory, bonates in an open-cast mine; Barna-II (Figs. 1, 2, 3). The Birbal Sahni Institute of Palaeosciences (BSIP), Lucknow. generalized litho-section is divided into lower Gotan Lime- To visualize the relationships between the isotopic and stone and upper Pondlo Dolomite. Overall, limestone and RSTE, we used a pair-plot correlation chart using the graph- dolomite are deposited with intercalations of mudstone and ics package “Performance Analytics” by Peterson et al. claystone (Fig. 2). Parallel bedding planes, rare undulated (2014) for R. laminites, irregular small-to-big-size vugs, thin-to-thickly bedded chert lenses, and mud cracks are the main features Geological setting observed in the mine section. Rarely, cross-stratified cherty horizons are also present. The middle part of the mine sec- The Marwar Supergroup (MSG) is a marine sedimentary tion is characterized by brecciated bands or intra-formational succession exposed in Jodhpur, Pali, Bikaner, Nagaur, conglomerates. A pink-color, thin limestone band is also and Jaisalmer districts in western Rajasthan, India, hav- present in the uppermost part of the mine section. Similar ing a thickness of about 1000 m and covering an area bands were seen in most of the other carbonate mines of of more than 51,000 km2 (Fig. 1). The sediments com- the Bilara carbonate sections during the field investigation. prise of sandstone, siltstone, carbonates, conglomerate, Cavernous nature of carbonate rocks is a dominating charac- and evaporites (Paliwal 2007). MSG sits unconformably teristic of the Barna-II. Chunks of carbonates samples were over the Neoproterozoic Malani Igneous Suite (MIS) and collected from a 23.1 m vertical section at about 15 cm or overlain by the Permo-Carboniferous Bap Boulder Bed more intervals (depending upon accessibility). (Pareek 1984). MIS forms the southernmost boundary, Total of 55 samples was analyzed for redox-sensitive ele- whereas Aravali and Delhi Supergroup rocks form the ment. The samples were homogenized and crushed to 200 easternmost boundary of MSG (Pareek 1981, 1984). MSG mesh size in an agate mortar. For RSTE analysis, samples has been divided into three groups: the Jodhpur Group 1 3 Carbonates and Evaporites (2020) 35:38 Page 3 of 13 38 Fig. 1 Geological map of Marwar Supergroup along with the location of Barna-II mine section (modified after Pareek 1981, 1984) (125–240 m thick), the Bilara Group (100–300 m thick), and sandwiched between the Jodhpur and the Nagaur and the Nagaur Group (100–300 m thick) in stratigraphic Groups, which are composed of argillo-arenaceous sand- order (Table 1).