# Pyrite trace element geochemistry of black shales of the “Boring Billion” period (1800-800 Ma) : implications for evolution of the atmospheric oxygen and complex life

Mukherjee, I ORCID: 0000-0002-2808-1821 2018 , 'Pyrite trace element geochemistry of black shales of the “Boring Billion” period (1800-800 Ma) : implications for evolution of the atmospheric oxygen and complex life', PhD thesis, University of Tasmania.

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## Abstract

This study focused on a particular time span in Earth’s history known as the “Boring Billion” i.e., 1800-800 million years ago (Ma). This period has been referred to as a period of environmental stasis (geological and biological) that prohibited complex microscopic life forms to evolve into their macroscopic counterparts. Apart from absence of major biological evolutionary events, the period also lacks certain types of major ore deposits specifically, volcanic-hosted massive sulfides, orogenic gold deposits, Banded Iron Formations (BIFs), Mn oxide deposits and phosphorites. Major glaciation events, mineral species evolution (Hg, Be, B) and evidence for modern style plate tectonics are also lacking. Several studies in the past have applied different geochemical techniques such as, C, S, Cr, Sr, Fe, Tl, Zn and Mo isotopes and redox sensitive trace element (Mo, U, Se etc.) enrichments in marine sediments towards understanding the above-mentioned characteristics of this time span. Outcomes of previous research point toward this geo-biological stasis having been caused primarily by lack of adequate oxygen in the atmosphere and ocean in the Proterozoic, which resulted in a billion-year delay in biologic evolution.
Even though previous research suggests stasis during the “Boring Billion”, primarily based on geochemical proxies, there appears to be a discord between geochemical signatures and paleo-biological observations in the rock record. Such as, certain key evolutionary breakthroughs including the evolution of the first complex eukaryotic cell and its cell organelles occurred during the “Boring Billion”, including the first major diversification of eukaryotes and appearance of metaphytes. Also, the two major oxygenation events in Earth’s history either pre-date or post-date major evolution events, for instance the Great Oxidation Event 1 at ~2.3 Ga (GOE 1) pre-dates the appearance of the first eukaryotic cell (~1.5 Ga) and the second great xidation event GOE 2 (~560 Ma) post-dates the rise of metazoans (~750 Ma). The discord exists mainly due to sole emphasis on the availability of oxygen in the atmosphere and the ocean (and their effect on evolution) by past researchers despite the fact that primitive organisms have known to require very small amounts of oxygen. Also, apart from oxygen, the role of bio-essential elements (Se, Ni, Mo, Zn, Mo etc.) and other toxic elements (W, Pb, As) have not been taken in to consideration even though they play a vital role in an organism’s life both in terms of scarcity and abundance. These roles include forming enzymes or protein molecules that aid in basic cellular level functions and cater to nutritional requirements for growth and sustenance. Variations in trace element availability through the “Boring Billion” is therefore critical to our understanding and was addressed in this thesis.
This study investigated the importance of bio-essential trace element availability in the Proterozoic oceans using a novel technique, i.e., analysing bio-essential trace element concentrations and sulfur isotopic compositions in sedimentary pyrite in Proterozoic black shales using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) and Sensitive High Resolution Ion Microprobe-Stable Isotope (SHRIMP-SI) respectively, to further our understanding of the “Boring Billion”. Considering high resolution temporal bio-essential trace elements trends are currently scarce in the literature, this study resolved the issue by systematic sampling of Proterozoic black shales from the McArthur and Mt Isa basins in northern Australia and Vindhyan Basin in central India. That was followed by pyrite LA-ICP-MS (~2000 analyses) to measure trace element concentrations of Se, Mo, Ni, Co, Bi, Cu, Zn, Mo, Pb, As, Tl, Cd, Ag, Au not only to reflect on their availability but also to infer paleo-redox structure of the Proterozoic oceans and atmosphere and its effect on biologic evolution. Additionally, pyrites used for LA-ICP-MS analyses were also analysed for sulfur isotope compositions using SHRIMP-SI to provide a more robust indicator of ocean redox conditions in the Proterozoic. Also, some black shales are known to host some of the world’s largest Zn-Pb SEDEX style ore deposits, such as the Barney Creek Formation hosting the McArthur River deposit. This study also explored the possibility of using trace element concentrations in pyrite as mineralization vectors to McArthur River style Zn-Pb deposits in sedimentary basins.
There are four main conclusions from this PhD research discussed in separate chapters in the thesis. First, the pyrite LA-ICP-MS technique adopted for the study to measure trace element concentrations was proven to be a sensitive tool, that incorporated petrography and allowed better analytical detection limits than whole rock method, which was critical. Particularly in the Proterozoic where trace element signatures in the rock record are suppressed, the sensitivity of the technique, unlike the whole rock method, allowed robust trace element trends to be recognised. Second, this research demonstrates how redox sensitive trace element concentrations in sedimentary pyrite and their ratios can be effectively used as proxies for tracking nutrient-productivity cycles and atmospheric oxygenation through the “Boring Billion” period. Elements like Se, Zn, Ni, Co, Mo and Bi and their ratios (Se/Co, Ni/Co, Zn/Co, Mo/Co, Se/Bi, Ni/Bi, Mo/Bi, Zn/Bi) are particularly robust indicators. The research concludes that whilst certain trace elements show an increase in their concentrations in pyrite in response to rise in atmospheric oxygen such as Se, Zn, Ni and Mo, some elements show a decrease (Co, Bi) as the latter is relatively more readily adsorbed onto Fe-Mn hydroxides and oxides owing to their ionic forms. Therefore, an increase in elements like Se, Mo, Zn coupled with a decrease in Bi and Co is a more robust indicator of any proposed oxygenation event. Also, pyrite-sulfur isotope compositions obtained using SHRIMP-SI provide useful insights regarding ocean oxygenation. We suggest that an increase in mean pyrite sulfur isotope compositions (δ34SVCDT values) for black shales depositing under open ocean conditions is possibly indicative of progressive ocean oxygenation and a simultaneous decrease in areal extent of anoxia making it a more efficient reducing system. This causes the pyrites to exhibit heavy δ$$^{34}$$S$$_{VCDT}$$ values. Third, variation in trace element concentrations in sedimentary pyrites from varying proximity to an ore body (in this case McArthur River deposit) are excellent mineralization vectors. Results demonstrate that elements like Mo, Ni, Co showed a consistent decrease in concentrations and Zn, Tl, Pb showed an increase, moving closer to mineralization. This was further used to construct pyrite vector diagrams that would aid in exploration for more Zn-Pb deposits (McArthur River style) in the McArthur Basin. Another important finding was that black shales affected by mineralization events and hydrothermal activity should not be used to infer paleo-seawater chemistry as the trace element chemistry becomes altered significantly due to both mineralization and hydrothermal fluid activity. Therefore, for black shales known to host mineral deposits, caution during sampling is recommended; sampling black shales farthest from a mineralized zone is most preferable. Fourth, the thesis provides a novel and fresh perspective on the “Boring Billion” and highlights evidence against the current consensus on the significance of this time span. Findings from this study, for the first time, point towards a fluctuating trace element availability unlike the low and flat trends suggested by previous studies. This study observes both periods of low (1800-1400 Ma) and relatively high (1400-800 Ma) TE availability during the Proterozoic. Also, unlike previous research, I propose low trace element availability as a major driving force for evolution. The study concludes that periods of low nutrient TE caused an evolutionary pressure, and were essential triggers promoting biological innovations such as the emergence of the first complex cell (eukaryote) and periods of high TE promoted rapid diversification of eukaryotes.