Chemostratigraphy of Neoproterozoic carbonates: implications for ‘blind dating’

Abstract

The δ13Ccarb and 87Sr/86Sr secular variations in Neoproteozoic seawater have been used for the purpose of ‘isotope stratigraphy’ but there are a number of problems that can preclude its routine use. In particular, it cannot be used with confidence for ‘blind dating’. The compilation of isotopic data on carbonate rocks reveals a high level of inconsistency between various carbon isotope age curves constructed for Neoproteozoic seawater, caused by a relatively high frequency of both global and local δ13Ccarb fluctuations combined with few reliable age determinations. Further complication is caused by the unresolved problem as to whether two or four glaciations, and associated negative δ13Ccarb excursions, can be reliably documented. Carbon isotope stratigraphy cannot be used alone for geological correlation and ‘blind dating’. Strontium isotope stratigraphy is a more reliable and precise tool for stratigraphic correlations and indirect age determinations. Combining strontium and carbon isotope stratigraphy, several discrete ages within the 590–544 Myr interval, and two age‐groups at 660–610 and 740–690 Myr can be resolved.

C, O, and Sr isotopic variations in Neoproterozoic-Cambrian carbonate rocks from Sete Lagoas Formation (Bambuí Group), in the Southern São Francisco Basin, Brazil

“…The 87 Sr/ 86 Sr ratios between 0.7074 and 0.7080 indicate ambiguity in the age of the Bambuí Group (Misi et al, 2007;Babinski et al, 2007;Kuchenbecker, 2011;Alvarenga et al, 2014;Paula-Santos et al, 2015) in comparison with the seawater Sr isotope evolution curves proposed in literature (Jacobsen & Kaufman, 1999;Melezhik et al, 2001;Halverson et al, 2010;Kuznetsov et al, 2013). Considering the maximum depositional age of 560 Ma, such Sr isotope ratios are lower than the ones expected for the terminal Ediacaran.…”

“…The latter authors suggest that the difference between the low radiogenic 87 Sr/ 86 Sr ratios commonly obtained for Bambuí Group and the ratios presented for the late Neoproterozoic (Melezhik et al, 2001;Halverson et al, 2010;Kuznetsov et al, 2013) may be due to the fact that carbonates of the Sete Lagoas Formation have been deposited in a restricted epi-continental sea, and its isotopic composition was not homogenized with contemporary global oceans (Paula- Santos et al, 2015). Silva-Tamayo et al (2010) propose that the Ca isotopic signature recorded in the Sete Lagoas Formation is post-Sturtian with δ 44 Ca values ranging from 0 to 1.0‰, which is very different of post-Marinoan cap carbonates that present δ 44 Ca values varying between 0 and 2.0‰.…”

“…It is likely that during the dolomitization process, the primary Sr was leachate and radiogenic Sr from interstitial fluids has been incorporated into the chemical structure of carbonates. However, disturbances of Sr isotope composition depend not only on the extent of post-depositional alteration but also on the initial composition of carbonates (Melezhik et al, 2001). Carbonates from lower Sete Lagoas Formation present significant amounts of Si, Al and K, which indicate a contribution of detrital sediments and possibly influence of freshwater on the carbonate platform.…”

“…However, some carbonates are usually susceptible to post-depositional alterations, especially those of Precambrian age, which change the original isotopic and geochemical seawater record. Recognition of post-depositional alteration processes is extremely important to ensure the reliable application of C isotopic chemostratigraphy (Melezhik et al, 2001). According to Brand and Veizer (1980) and Banner and Hanson (1990), the interaction of carbonates with interstitial fluids during diagenesis, dolomitization or metamorphism leads to an increase in the Rb/Sr, Mn/Sr, Fe/Sr and 87 Sr/ 86 Sr ratios, Note: n.a.= not analyzed.…”

“…During the Ediacaran, the Gaskiers glaciation (~582 Ma) may have had a different scenario, with geochemical signals for a significant oxygenation event in deep sea and through the appearance of complex eukaryotes (Narbonne, 2008). Nevertheless, many researchers find the overall character of δ 13 C values a challenge (Melezhik et al, 2001;Eyles & Januszczak, 2004;Derry, 2010;Frimmel, 2010), since the carbon isotope anomalies may represent post-depositional processes, rather than global or restricted depositional environments.…”