The fate of a Neoproterozoic intracratonic marine basin: Trace elements, TOC and IRON speciation geochemistry of the Bambuí Basin, Brazil

The Nitrogen Cycle in an Epeiric Sea in the Core of Gondwana Supercontinent: A Study on the Ediacaran-Cambrian Bambuí Group, East-central Brazil

“…Under anoxic environments, studies have shown that diagenetic effects increase primary δ 15 N signal of bulk sediments by less than +1‰, while in oxic environments the increment can reach +4‰ (Altabet et al, 1999;Lehmann et al, 2002). Studies using Rare Earth Elements and iron speciation indicate that the bottom waters of the Bambuí sea were anoxic in the intervals reported on this paper (Paula- Hippertt et al, 2019), which likely diminished the effects of diagenesis on the N isotopic signatures of the samples. Well 1 and Arcos section samples show a moderate and strong correlation (R 2 0.57 and 0.89, respectively) between TN decarb and TOC decarb (Figures 7A,B), indicating that N came mainly from the primary organic matter (Calvert, 2004).…”

“…In the Bambuí Group, the reported low values on the TST interval (∼+1‰) cannot be explained by a fully oxygenated ocean without denitrification on the water column. This scenario is not only unrealistic for the Precambrian (Canfield et al, 2010;Ader et al, 2016;Stüeken et al, 2016), it does not fit the published Caxito et al, 2018;Hippertt et al, 2019). Hence, the reported low δ 15 N bulk signature of Well 1 and Janu aria sections was likely caused by N 2 fixation using Mo-based nitrogenase in a redox stratified water body.…”

“…Besides that, as anoxic conditions enhance P residence time in the water column, environmental conditions prevented it from binding with Fe oxyhydroxides and being sequestered into sediments (Van Cappellen and Ingall, 1994). Finally, previous studies on the Bambuí Group show that Mo availability was high enough to support N 2 fixation (Hippertt et al, 2019). In fact, despite microbiological culture studies showing that low Mo-conditions might harm nitrogenase enzyme operation (Zerkle, 2005), there is no evidence that Moscarcity played a role in diazotrophy activity in geological time (Stüeken et al, 2016).…”

“…The Bambuí Group encompasses carbonates and siliciclastic rocks that were deposited above glaciogenic units Martins-Neto, 2001, 2012;Martins-Neto and Hercos, 2002;Martins-Neto, 2009;Reis and Alkmim, 2015) and lately, it has been a matter of intense research regarding the conditions that could have prevented the Ediacaran fauna to thrive at the core of West Gondwana (e.g., Paula-Santos et al, 2017, 2020Uhlein et al, 2017Uhlein et al, , 2019Okubo et al, 2018Okubo et al, , 2020Caetano-Filho et al, 2019Crockford et al, 2019). Geochronological and chemostratigraphic data suggest that the Bambuí evolved from a sea connected to the global ocean to a fully restricted environment (Paula-Santos et al, 2015, which might have influenced its paleoenvironmental conditions, leading to very high δ 13 C carb values (Iyer et al, 1995;Martins and Lemos, 2007;Kuchenbecker et al, 2016b;Guacaneme et al, 2017;Caetano-Filho et al, 2019;Uhlein et al, 2019) and anoxia (Paula-Santos et al, 2017;Hippertt et al, 2019;Uhlein et al, 2019). These in turn may have prevented complex life forms to thrive (Hippertt et al, 2019).…”

“…Geochronological and chemostratigraphic data suggest that the Bambuí evolved from a sea connected to the global ocean to a fully restricted environment (Paula-Santos et al, 2015, which might have influenced its paleoenvironmental conditions, leading to very high δ 13 C carb values (Iyer et al, 1995;Martins and Lemos, 2007;Kuchenbecker et al, 2016b;Guacaneme et al, 2017;Caetano-Filho et al, 2019;Uhlein et al, 2019) and anoxia (Paula-Santos et al, 2017;Hippertt et al, 2019;Uhlein et al, 2019). These in turn may have prevented complex life forms to thrive (Hippertt et al, 2019). Further knowledge on the interplay between the redox state of Bambuí seawater and the biological metabolic paths could help understanding the limiting conditions for life during the Ediacaran-Cambrian boundary, and thus on the comprehension of how life may have progressed to more complex forms at that period.…”