Rapid bioproductivity recovery following the Marinoan glaciation: Evidence from Sr-Cr-Cd isotopes and trace elements in the Morraria do Sul cap dolostone, Brazil

Abstract

Early Ediacaran cap dolostones are distinctive strata deposited directly above glaciogenic rocks from the Marinoan Glaciation (650–635 Ma), one of the most dramatic ice ages in Earth's history. These cap dolostones were rapidly deposited during global synchronous deglaciation, being a superb depositional window into the immediate glaciation aftermath and suitable for investigating the icehouse-hothouse transition in this period. However, some aspects of this transition are still unclear, such as the timing of primary productivity recovery. Aiming to explore this question, we present new sedimentological, trace element, and Sr-Cr-Cd isotope data for the Puga cap dolostone in the Southern Paraguay Belt (Brazil), here termed Morraria do Sul Formation. The cap dolostone overlies diamictites, siltstones with outsized clasts, and basement rocks. It is divided into a basal dolograinstone and an upper tubestone-microbialite succession, with abundant vertical tube-like structures. The diamictites and siltstones were formed in a glaciomarine setting, within the range of ice-rafted debris. The basal dolograinstone facies of the cap dolostone was formed under high-energy post-glacial conditions, and the overlying tubestone-microbialite facies was deposited in a low-energy, well-lit environment. Rare earth elements and yttrium (REY) preserved the original seawater-like pattern, displaying true negative Ce anomalies down to 0.84, indicating oxygenated shallow waters. The cap dolostone displays relatively high ⁸⁷Sr/⁸⁶Sr ratios, up to 0.7123, above the assumed value for the Early Ediacaran seawater, suggesting the influence of meltwater plumes in a salinity-stratified setting. Authigenic Cr isotope values (δ⁵³Cr_auth) are within the range of Bulk Silicate Earth (BSE) to slightly positive in the basal dolograinstone and negatively fractionated below BSE values in the tubestone-microbialite facies, implying that microbial-influenced Cr reduction progressively increased upwards, controlling the isotopic fractionation. A rise upwards in δ¹¹⁴Cd, from −0.11 to 0.17 ‰, reveals a substantial increase in Cd-uptake by biomass, and maximum estimates for seawater δ¹¹⁴Cd show values within the range of present-day surface seawater. The data presented herein suggest that significant primary productivity recovery occurred in the immediate aftermath of the Marinoan Glaciation. This study also shows the efficiency of combined trace elements and Sr-Cr-Cd isotope analyses on reconstructing past seawater chemistry and productivity levels.