Mineral ‘roses’ that bloomed on the seafloor ca. 600 Ma reveal the tortuous road of early animal evolution

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    Multiple, short-lived oxygenation events occurred on Early Ediacaran continental shelves in the aftermath of the Marinoan glaciation, which may have served to promote the contemporaneous emergence and diversification of complex metazoans (animals). However, geochemical proxies for redox conditions have yielded conflicting results, and spatiotemporal patterns of oceanic oxygenation during the Early Ediacaran thus remain controversial, limiting our understanding of the co-evolution of early animal life and environments. To address this problem, we looked for direct mineral evidence of the introduction of oxygen to Early Ediacaran oceans—and we found it!

 

Pyrite-marcasite rosettes in Ediacaran Doushantuo phosphorites from Guizhou Province, South China. The rosettes have a layered distribution pattern and feature flower-like structures, ranging in size from a few to hundreds of microns.

 

Abundant iron-sulfide mineral associations with distinct structures are preserved in these rocks. These mineral associations resemble a flower, with yellowish, rough ‘stamens’ and white, smooth ‘petals’—a sort-of iron-sulfide ‘rose’. More accurately, they should be called pyrite-marcasite rosettes (PMRs), but for romantic reasons I prefer to call them ‘roses’. It was a typical summer afternoon in 2017 when I was doing microscope observations and noticed these distinctive dark minerals. At that moment, I did not suspect that this quotidian observation would lead me into five years of study of these minute petrographic features.

In July 2017, I went on a fieldtrip to phosphorite mining areas in Guizhou Province, South China, accompanied by my supervisors Zhenbing She (China University of Geosciences-Wuhan) and Dominic Papineau (University College London, or UCL), and my dear friends Guang Ouyang, Kenan Cao, Majed Algabri, Yalin Yu, and Yiyun Dai. Looking around the field site, tons of dark phosphate rock dyed the mountains black. In these Ediacaran rocks, numerous fossils are preserved, as well as what we thought were ‘roses’ made of pyrite. I squatted on the roadside and examined the rock, imagining that hundreds of millions of years ago at this very site life was struggling to adapt to a changing environment, and that countless metal roses were blooming on the seafloor on which my feet now rested. What a magnificent and romantic scene this must have been. Despite all these thoughts, the metal roses did not actually get all that much attention at that time. This is because, based on my past experience, I assumed that pyrite rosettes were hydrothermal products and did not provide much useful information about the contemporaneous marine environment.

 

The pH-redox model of PMR formation. O2 in the bottom water oxidizes and dissolves sedimentary pyrite, causing acidification of the pore-water environment, which favors marcasite formation.

 

The first turning point in my research came in 2018. At that time, I took these precious geological samples to London, UK, where I first used a laser Raman microscope in Dominic's laboratory at the London Center for Nanotechnology (LCN), UCL. The spatial resolution of the confocal laser Raman instrument is as fine as 300 nm, and its powerful ability to image nm-scale minerals allowed us to easily distinguish pyrite from marcasite (FeS2 polymorphs). What we initially thought were hydrothermal pyrite turned out to have more complicated compositions and structural characteristics. What we discovered were well-developed ‘roses’ usually consisting of a framboidal or irregular pyrite nucleus and a cortex composed of radiating marcasite blades intergrown with quartz. Thus, we identified the first pyrite-marcasite rosettes (PMRs). Marcasite formation requires acidic conditions (pH = ~4-5), whereas pyrite formation is favored by neutral-alkaline conditions (pH = ~7-8). Therefore, intervals of acidification must have occurred during the formation of PMRs.

What followed were four years of intensive research to better understand the genesis and significance of these unusual metal rosettes. At CUG-Wuhan, we made further use of high-precision laser Raman spectroscopy (LRS) as well as scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) in the State Key Laboratory of Biogeology and Environmental Geology (BGEG), and we undertook measurements of in-situ S isotopes of pyrite and marcasite and C-O isotopes of wall rocks at the State Key Laboratory of Geological Process and Mineral Resources (GPMR). At the University of Hanover in Germany, in collaboration with Prof. Chao Zhang, we measured the in-situ Fe-isotopic compositions of pyrite and marcasite. During these 4 years, I frequently discussed the analytical results with my supervisors Zhenbing She and Dominic Papineau, and I repeatedly revised the manuscript reporting these findings, which ultimately went through 17 versions! With these new data, we were finally able to develop a pH-redox model to explain PMR formation: a transient influx of O2 into bottom waters of the Early Ediacaran ocean oxidized and partly dissolved pyrite in the sediment, resulting in pore-water acidification and, thus, conditions favorable for marcasite formation. On this basis, we determined that the oxidative dissolution of pyrite and concurrent precipitation of marcasite occurred in the early diagenetic environment, close to the seafloor.

The second turning point in my research came in March 2022,  I met Professor Chao Li, who become the mentor of my postdoc later in Chengdu University of Technology. Looking at the PMR images, Chao turned to me and said excitedly,“Your work provides direct evidence for our model!”After reading more papers suggested by him, I understood what Professor Li meant—the occurrence of PMRs in my study samples provides direct mineral evidence for high-frequency oxygenation of Early Ediacaran continental shelves (i.e., for first time we have direct evidence for that dissolved O2 periodically diffused into the early Ediacaran seafloor!). Such multiple oxygenation episodes in an otherwise anoxic environment are likely to have promoted the contemporaneous emergence and diversification of early metazoans such as the Weng’an and Lantian biotas. At that point, the final piece of this study was complete.

Five years on, I still sometimes think back on the summer of 2017. I squatted on the side of the road, watching the dust behind the trucks. Every passing truck was filled with phosphate ore rocks, which may then be used to make fertilizer, growing more crops, feeding more people. Human beings are constantly striving for survival and for a bright future, like the ancient marine creatures that lived here hundreds of millions of years ago.

Beidoushan phosphorite mining area in Guizhou Province, South China

I dedicate this immortal rose to my wife, my friend, and my best work partner, Dr. Liang Qi
by Liangxuan Jiao

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