UNRAVELING POST-MARINOAN SEAWATER EVOLUTION IN THE AMAZON CRATON

The aftermath of the Marinoan glaciation (~ 635 Ma) is recorded in cap carbonate sequences worldwide, documenting eustatic sea level rise during post-glacial transgression. Recent models suggest that interactions between meltwater and marine water during deglaciation created a stable stratification, with low-salinity surface waters overlaying higher-salinity deeper waters in marine platforms. The basal deposits of the Araras–Alto Paraguai Basin, located within the Southern Amazon Craton, mark the onset of sedimentation associated with the Marinoan glaciation (~ 650–635 Ma). These deposits include the glaciomarine Puga Formation, overlain by the Puga cap carbonate of the Araras Group. This sequence comprises the Mirassol d'Oeste Formation (cap dolostone), indicating shallow platform conditions, and the Guia Formation, composed of carbonates and shales deposited in a moderately deep platform setting. Samples in this study were collected from two drill cores and an outcrop at the Calcário Tangará quarry in the Tangará da Serra region of Mato Grosso, Central-Western Brazil. While previous studies have extensively analyzed sedimentology, stratigraphy, and isotopic systems such as δ¹³C, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr, the combined use of isotopic systems, particularly ⁸⁷Sr/⁸⁶Sr and εNd, to investigate continental weathering processes remains underexplored. Here, we explore the whole-rock isotope analyses conducted (18 samples) by εNd, ⁸⁷Sr/⁸⁶Sr, and δ¹³C (50 samples), isotope composition of the Puga cap carbonate on the Amazon Craton to trace weathering sources and oceanic circulation during syn-deglacial conditions. The bulk-rock samples from the diamictite exhibit TDM values ranging from 1.5 to 2.0 Ga (average: 1.8 ± 0.8 Ga), while the pebble samples exhibit values from 1.6 to 1.8 Ga (average = 1.7 ± 0.1 Ga). Similarly, the cap dolostone shows TDM values ranging from 1.8 to 2.9 Ga with an average of 2.2 ± 0.3 Ga. This similar Paleoproterozoic to Mesoproterozoic Nd model ages (TDM) indicates continental sources consistent with the Amazon Craton basement for glacial diamictite and cap dolostone. Less radiogenic (−14) _ƐNd (635 Ma) and variable and higher ⁸⁷Sr/⁸⁶Sr values (0.7264–0.7084) in the cap dolostone suggest meltwater influence during precipitation. The intense chemical weathering of regolith (e.g., diamicton) was a significant source of alkalinity, accelerating cap dolostone precipitation during this period and likely homogenizing the Nd isotopic signatures of the cap dolomuds. In addition, persistent negative δ¹³C isotopes (mean = -5.23 ± 0.84) suggest enhanced weathering of freshly exposed carbonate platforms, and paleoproductivity contributed to the rapid precipitation. Our findings suggest potential weathering sources and oceanic circulation along the margin of the Amazon Craton following the Marinoan glaciation. In conjunction with other isotopes, such as δ13C and ⁸⁷Sr/⁸⁶Sr, Nd isotopes provide valuable insights into the mixing of ancient water masses, meltwater, and hypersaline waters linked to upwelling processes during the syndeglacial phase. The geochemical trends reveal the influence of meltwater mixing during deglaciation, supporting the model of rapid cap carbonate precipitation while providing critical insights into the transition from glacial to typical Ediacaran Ocean conditions at the Cryogenian-Ediacaran boundary.