Clay mineral evidence for restricted, evaporitic environments and reverse weathering

Chemical sediments are widely recognized as environment indicators. Authigenic clay in the Ediacaran Doushantuo Formation provide evidence for a restricted evaporitic environment during the occurrence of potential earliest metazoan life as well as enhanced reverse weathering in Ediacaran seawater.
Published in Earth & Environment
Clay mineral evidence for restricted, evaporitic environments and reverse weathering

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The Ediacaran Period was a time of drastic environmental changes on Earth during which there was a major expansion of eukaryotic life. The first animal life, tiny organisms called metazoan, appear in the fossil record during the Ediacaran. The environments in which these organisms emerged are of interest to understand the co-evolution of early Earth System and biosphere.

Our paper published in Communications Earth & Environment examines one of most well-studied Ediacaran sedimentary records, the Doushantuo Formation in South China. The Doushantuo gained attention as garnering fossils of multicellular acritarchs (potentially metazoans), but the environment in which it was deposited remains disputed. The debate centers around a clay mineral, saponite. In this paper we seek to determine the origin of saponite clay minerals in the lower Doushantuo Formation and discuss what this tells us about the depositional environment and water chemistry as well as its implications for the Earth System.

Saponite is a Mg-rich trioctahedral clay mineral in smectite group, while the debate largely centers around how saponite is formed. Saponite can be formed via 1) authigenic origin: precipitation from gel precursors under alkaline conditions or direct precipitation from supersaturated solutions (including pore fluids) with high Si and Mg concentration, 2) detrital origin: pedogenic weathering of (ultra)mafic silicates under humid conditions or 3) hydrothermal origin: alteration of (ultra)mafic igneous rocks, volcanic ash or siliceous dolostone and transformation from other clay minerals. The abundant saponite in the lower Doushantuo Formation in the Yangtze Gorges Area is not seen in time equivalent sequences in open marine slope to basin settings. This was initially reported and interpreted as authigenic saponite, indicating that the lower Doushantuo Formation in this region was deposited in an alkaline lake, disconnected from the ocean. But subsequent work, inferred an open marine setting for the YGA, arguing that the saponite was likely of detrital terrestrial weathering origin and thus not important in constraining the depositional environment. A fundamental limitation of previous studies is their reliance on bulk mineralogical and indirect geochemical techniques, which are unable to differentiate how and where saponite formed.

To fill this knowledge gap, we used a novel SEM-EDS mineral mapping system for detailed mineralogical - petrographic investigations, which provided automatic mineral recognition results together with high resolution backscatter images. We demonstrate that these mineral maps can be used to determine the origin of the saponite. Specifically, the saponite that is dominant in the studied interval consists of small crystals, with curvy, fragile texture and evidence of deformation due to compaction (Fig. 1). These saponite grains are closely associated with carbonate and other authigenic sediments in domains largely free of detrital clasts (Fig. 1). Together, these observations allow us to confirm a pre-compaction, authigenic origin for these saponite.

 Credit: Han et al. [2022]

Fig. 1 Saponite (partially chloritized) in laminae domain. Scale bar represents 10 μm. Credit: Han et al. [2022]

Now that we knew how the saponite formed, what does it tell us about the environment? Authigenic saponite formation requires specific water chemistry with high Si, Mg concentration and elevated pH. From this, we infer that the lower Doushantuo sediments were deposited in a restricted, mildly evaporitic lagoonal basin at midlatitude. In this type setting, the circulation between the basin and open ocean was restricted and the seawater constituents such as Si and Mg can be concentrated by mild evaporation. We use hydrochemical modelling to illustrate how saponite precipitation is related to Si, Mg concentration and pH value and find that the Si, Mg-rich Ediacaran seawater may have been crucial. This offers a potential explanation for why saponite is not found in modern evaporitic marine settings. It also provides indirect evidence for the reverse weathering hypothesis because marine clay authigenesis would have been favored by Si-rich Precambrian seawater and played an important role in stabilizing both ocean pH and Earth’s climate. We infer that authigenesis of low-Mg but Fe- and K-rich marine clays likely occurred more widely outside of evaporitic settings, representing an important sink for various elements during the Precambrian.

As for the ties to biosphere evolution, the lower Doushantuo Formation hosts various phosphatically or siliceously preserved multicellular acritarchs, including embryo-like fossils which are potential candidates for the earliest animal (Fig. 2). The fossil localities are largely restricted to shallow water shelf lagoon and shelf margin settings, and rare in the deeper slope to basin. Therefore, we suggest that the lagoon setting is likely to have provided an unusually favorable environment for prokaryotic and eukaryotic organisms (including, potentially, metazoans) as it would have facilitated accumulation of nutrients, such as phosphorous, that were in scarce supply in the early Ediacaran Ocean. The subsequent radiation of life and expansion into open marine environments would have been facilitated by the gradual rise in marine nutrients as well as dissolved oxygen over the Ediacaran, which ultimately set the scene for the major environmental and biological changes that ushered in the Phanerozoic.

Credit: Han et al. [2022]

Fig. 2 Phosphatic embryo-like fossils resembling Archaeophycus yunnanensis, preserved in a dolomite nodule. Scale bar represents 20 μm. Credit: Han et al. [2022]



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