Life in the aftermath of Marinoan glaciation: The giant stromatolite evolution in the Puga cap carbonate, southern Amazon Craton, Brazil

Highlights

• The occurrence of giant stromatolites within extreme palaeoceanographic and palaeoenvironmental changes provides an opportunity to understand the interaction between biological and sedimentological processes.

• The transition from stratiform to domal morphology reflects the progressive deepening of the platform, concomitant with increases in microbial activity.

• The progressive generation of accommodation space related to sea-level rises and glacial isostatic adjustment (GIA) caused subsidence in the coastal zone, as proposed by the Snowball Earth hypothesis.

• The decline in microbial communities in carbonate platform caused by the massive influx of siliciclastic sediments in a mixed water setting, influenced by intense turbidity and continuous burial of the communities.

Abstract

Giant domal stromatolites associated with vertical tubestone are reported for the first time in the Marinoan (635 Ma) Puga cap dolostone on the southern Amazon Craton, Brazil. The well-preserved giant domal mounds reach 12 m in diameter and 10 m in height, forming laterally continuous biostromes of at least 200 m. The giant mounds occur mainly on the top cap carbonate succession overlying stratiform stromatolites directly underlying glacial diamictites marking the Cryogenian-Ediacaran boundary. We discuss the sedimentological and biological conditions during the greenhouse period that led to the development of these exceptional stromatolitic features. The microbial communities primordially colonized the diamictite substrate, forming stratiform bioconstructions in a shallow platform under hypersaline calm water conditions. The transition from stratiform to domal morphology reflects the progressive deepening of the platform, concomitant with increases in microbial activity. The high production and pre-lithification of carbonate muds, that formed a rigid load-resistant framework, allowed for the accumulation by vertical accretion, increasing the potential for preservation of giant mounds. The progressive generation of accommodation space related to sea-level rises and glacial isostatic adjustment (GIA) caused subsidence in the coastal zone, as proposed by the Snowball Earth hypothesis. The maximum flooding was associated with an increase in siliciclastic inflow, causing burial of microbial communities and the demise of giant stromatolites. The succession was succeeded by the deposition of long-term transgressive lime mud in a CaCO₃-oversaturated sea. The giant stromatolites consist of a new element of the Marinoan Puga cap carbonate and provide essential insights to enlarge the discussion about the biological and sedimentological interaction that occurred on post-glacial carbonate platforms worldwide.

Introduction

Stromatolites are the oldest records of life on several Precambrian cratons and represent one of the main components of cap dolostones, which are anomalous post-glacial deposits related to the snowball Earth event (Schopf, 1994, Chacon, 2010, Pruss et al., 2010, Bosak et al., 2013a, Hoffman et al., 2017). This theory admits that the late Neoproterozoic was marked by extreme climatic events and low-latitude glaciations that affected the entire planet (Hoffman et al., 1998, Hoffman and Schrag, 2002). The main glacial events of Cryogenian occurred in the Sturtiana (717–660 Ma) and Marinoan (645–635 Ma), generating environments stressed by cyclic icehouse and greenhouse conditions that triggered the evolution of life (Corsetti and Lorentz, 2006, Fairchild and Kennedy, 2007, Rooney et al., 2015) The record of these extreme conditions is well recorded in Marinoan cap carbonates, that consist mainly of dolomites and limestone overlapping diamictites and presenting typical anomalous structures, such as stromatolites with associated tubestone, megaripple bedding, macropeloids, aragonite crystal fans and δ¹³C negative excursion (Hoffman et al., 1998, Hoffman et al., 2007, Hoffman et al., 2017, James et al., 2001, Hoffman and Schrag, 2002, Nogueira et al., 2003, Nogueira et al., 2007, Nogueira et al., 2019, Jiang et al., 2003, Xiao et al., 2004, Lorentz et al., 2004; Halverson et al., 2005, Gammon et al., 2005, Gammon, 2012, Shields, 2005, Jiang et al., 2006, Bosak et al., 2013a, Soares et al., 2020, Romero et al., 2020).

Recent advances in palaeobiology research have demonstrated that many resistant forms of life could have survived and likely evolved during a snowball Earth event and experienced fast post-glaciation blooming (Corsetti and Grotzinger, 2005, Olcott et al., 2005, Corsetti et al., 2006, Elie et al., 2007, Bosak et al., 2011a, Bosak et al., 2011b, Knoll, 2015, Moore et al., 2017, Hoffman et al., 2017). Microbialites (stromatolites) have existed on Earth for at least 3.5 Ga and are exceptionally sensitive to recording geobiological changes through geologic time (Hofmann, 2000, Riding, 2006, Schopf et al., 2007, Allwood et al., 2007, Spear and Corsetti, 2013a, Spear and Corsetti, 2013b, Knoll, 2015). Although stromatolites have a low biostratigraphic resolution, and most works have emphasized mainly their biogenicity, their application as a palaeoenvironmental proxy remains underused in cap carbonate deposits (Cloud et al., 1974, Hegenberg, 1987, Kennedy et al., 2001, Nogueira et al., 2003, Corsetti and Grotzinger, 2005, Macdonald et al., 2009, Pruss et al., 2010, Bosak et al., 2013a.

The occurrence of stromatolites within extreme palaeoceanographic and palaeoenvironmental changes provides an opportunity to understand the interaction between biological and sedimentological processes. Moreover, the occurrence of stromatolites in cap carbonate beds has been attributed to a warm climate with an increase in nutrient-rich ice meltwaters after snowball Earth conditions (Corsetti and Grotzinger, 2005, Fabre and Berger, 2012, Hoffman et al., 2017). In contrast, the sudden disappearance of these structures is related mainly to transgression and reworking by currents in an ice-free sea (Nogueira et al., 2019).

Stromatolites with tubestone are a common feature in the Puga cap carbonate (PCC) in the southern Amazon Craton (Fig. 1 A). (Nogueira et al., 2003, Font et al., 2006; Romero et al., 2020), but the occurrence of giant domed stromatolites is record here for the first time with stromatolites comparable to the recorded in the Death Valley Ediacaran succession by Cloud et al., (1974). Our study describes giant stromatolites from post-Marinoan cap dolostone beds in the Amazon Craton, exposed in an open pit quarry in the Tangará da Serra region, Central Brazil (Fig. 1A). The sedimentology and petrography studies, combined with biological inferences, have allowed for interpreting the paleoenvironment of these features, as well as to discuss the influence of glacial isostatic adjustment (GIA) on the growth and preservation of mounds. Due to the giant stromatolites exceptional preservation, the PCC is a prime candidate for evaluating the interaction of sedimentary and biological processes and contextualize this paleontological record within the post- Marinoan glaciation event in the Amazon Craton.