This project uses the trace element content of sedimentary pyrite through time to interpret changes in the trace element content of past oceans and relationships to atmospheric oxygen. This study combines the results from two totally different methods to estimate atmosphere oxygen concentrations during the Precambrian and Phanerozoic. Firstly, measurements of oxygen concentrations in sedimentary halite fluid inclusions by Nigel Blamey and colleagues at the University of Western Ontario. Secondly, measurement of the Se/Co ratio in 2037 sedimentary pyrite grains from 310 black shale samples spread throughout the Precambrian determined by the CODES research team. By combining these two approaches we have derived the following relationship:

Atmosphere O₂% = 8.5 x log (Se/Co_pyrite) + 16.2; R² = 0.94

The oxygen curve revealed by this relationship suggests that atmospheric oxygen in the Archean varied from <1–8%, with a median value of 4%, and in the Proterozoic from <1–21% with a median value of 8%. The broad first order trend is one of increasing O₂ of <1–~20% from 3500 to 1850 Ma followed by a general decrease to around 1–5% at 1000 Ma, but with a spike around 1400 Ma. Oxygen rises gradually through the Ediacaran to reach a maximum of 20–25% by the early Cambrian. These estimates of atmospheric oxygen are very different to the current published view that oxygen was very low (less than 1%) throughout most of the Archean and Proterozoic. The results are to be published in Precambrian Research (Steadman et al., in press).

An additional study of S-isotopes in sedimentary pyrite and their relationship to trace elements in the global ocean has shown the influence of major Large Igneous Province (LIP) events on past ocean chemistry and S-isotope cycles through time. This study has been submitted as a chapter in a book to be published by the American Geophysical Union, titled The Deadly Kiss of LIPS.

In 2018 Indrani Mukherjee gained her PhD and published a paper in Nature Science Reports on ‘The Boring Billion, a slingshot for complex life on Earth’. Indrani presented a new hypothesis that it is ocean trace element nutrients rather than atmosphere oxygen that has controlled evolutionary processes through the middle Proterozoic. The paper has been particularly well received.

Ross Large prepared an invited paper for a special issue of the American Geophysical Union, based on a large data set of sulfur isotopes in sedimentary pyrite through time, collected in a collaboration with his PhD students and Trevor Ireland from ANU. The research demonstrates a correlation between the temporal variation in sedimentary pyrite sulfur isotopes and trace element chemistry with Large Igneous Province events (LIPS). If correct, this finding will rewrite the book on sulfur isotope trends in the ocean through time.

Ross and the team have been investigating further the development of a proxy for atmosphere oxygen concentration based on trace element ratios in sedimentary pyrite. This represents the ‘Holy Grail’ in deep time geochemistry and has become a highly controversial subject. In particular, ‘was the Proterozoic atmosphere very depleted in oxygen (< 0.2 % O2), or was the oxygen concentration much higher (2–20 % O2)?’.

The global impact of the CODES research on understanding ocean and atmosphere chemistry through time has been recognised by an invitation from Elsevier to Ross Large to write a chapter on the ‘Evolution of the Earth’s atmosphere’ for the next edition of the Elsevier Encyclopedia of Geology. Ross was also awarded the Royal Society of Tasmania Medal in 2018, for his career research achievements.

In 2017 this project focused on determining temporal REE patterns measured by LA-ICP-MS on the matrix of black shales. The results demonstrate that the oceans during the Boring Billion or Middle Proterozoic were enriched in REE, U, Th and Tl, but depleted in most bioessential elements. This anomalous situation resulted from a change in the uppermost continental crust around 1800 Ma from a mixed mafic-felsic composition to one dominated by felsic intrusives, particularly K-Th-U-anorogenic granites, and had a major influence on biological evolution during the period. The results were published in Earth and Planetary Science Letters.

This research uses analytical LA-ICP-MS technology developed at CODES to track changes in the trace element history of sedimentary pyrite through the Precambrian and Phanerozoic. Currently, this project is funded by an ARC Discovery grant, and is being conducted in collaboration with members of the CODES Ore Deposits: Characterisation and Context Module.

A highlight of the year was undoubtedly the team receiving an Australian Museum Eureka Prize in the category of Excellence in Interdisciplinary Scientific Research. The coveted prize was awarded for studies into the relationships between plate tectonics, past ocean chemistry (particularly the role of the trace element selenium) and evolution and extinction cycles.

A key factor in this research was an increase in the marine pyrite database to include over 5,000 analyses. This enabled the team to relate changes in ocean chemistry to atmosphere oxygenation and supercontinent cycles. It was found that many trace elements in the ocean dropped to very low concentrations during particular short time intervals over the past 500 million years.

Flinders University paleontologist John Long played an important role in helping the team understand this phenomena, and how it related to biological evolution. He brought together a team of international biologists and palaeontologists from the UK, USA and Australia to critically dissect the new data. Subsequently, it was found that the concentration of selenium, which is vital for life, dropped so low that life would be impossible to sustain through these short periods. Remarkably, this has happened three times, at 450, 375 and 200 million years ago, corresponding precisely to the timing of three of the five biggest mass extinction events on the Earth.

The pyrite data also enabled researchers to track the concentration of gold in the oceans, which revealed two significant features, i.e., gold in the Archean oceans was concentrated several times more than in the Proterozoic oceans; and gold showed a cyclic pattern of concentration through the Phanerozoic corresponding to the cycles of gold ore deposits.

This project uses analytical LA-ICP-MS technology developed at CODES to track changes in the trace element history of sedimentary pyrite through the Precambrian and Phanerozoic. Currently, this project is funded by an ARC Discovery grant, and is being conducted in collaboration with members of the CODES Ore Deposits: Characterisation and Context Module.

During 2015, the marine pyrite analytical database was increased from 3,000 to over 5,000 LA-ICP-MS analyses. This database has a unique potential to supply information on ocean nutrient supply through time, pO2 variations in the atmosphere, and trends in bio-essential trace elements.

Four significant papers were published during the year:

• An overview of the chemical composition of sedimentary pyrite and chemical criteria for distinguishing sedimentary pyrite from other types of pyrite.
• A study of the gold content of marine pyrite has, for the first time, assessed the gold content of paleo-oceans through time and revealed a low gold content of the oceans in the mid-Proterozoic compared to the Archean and Phanerozoic.
• Trace elements contained in marine pyrite were used to reveal cycles of bio-essential nutrient trace element supply and define periods when the oceans were nutrient-rich, which were followed by periods when they were nutrient-poor. A relationship between tectonics, nutrient supply, pO2 and evolution of marine life was proposed.
• A follow-up paper showed that during the nutrient-poor periods the global ocean was severely depleted in the bio-essential element selenium, ultimately causing three of the five mass extinction events in the Phanerozoic.