The Hero of Black Shale Creation
The core topic of my ELP is studying black shales and how there are endowed with high concentrations of metals. A key descriptor of black shales is their colour – black –due to high concentrations of organic material. The organic material is often sourced from dead marine plankton, photosynthetic organisms living in the ocean’s photic zone (i.e., where sunlight reaches). These tiny organisms are only 1% of the Earth’s biomass, yet they account for nearly 50% of the world’s oxygen production, more than double the contribution of the entire Amazon forest.
When plankton dies, their organic matter typically gets consumed in the ocean water through oxidation (think advanced composting). However, in certain situations, the organic debris can accumulate on the seafloor and create “organic-rich” sediment. If the organic debris exceeds 2% of the sediment’s total weight, it is known as a black shale after it turns into a rock (i.e., lithification).
In a subset of these cases, a particular type of bacteria, creatively called sulphate-reducing bacteria, eat organic debris for their energy and, in that process, convert seawater sulfate (SO42-) to sulphide (H2S). This latter type of sulphur has significant implications for concentrating metals in the seafloor sediment. Sulphide minerals, such as pyrite (iron sulfide), sphalerite (zinc sulfide) and millerite (nickel sulfide), can enhance the drawdown of elements from seawater into the sediment. These three sulfides occur in the metal- and sulphur-rich shales I study in Yukon, Canada, and southeast China. Although the source of sulphur is undoubtedly seawater, I have gathered evidence early in my ELP that other processes have impacted that sulphur. These additional processes can be investigated by studying sulphur isotopes.
Isotopes are different versions of the same element that have the same chemical properties but different masses. A simple example is carbon, which has two essential isotopes with atomic masses of 12 and 13. Understanding the ratio of these isotopes can reveal a plethora of information about biomass and it utilized in pollution or remediation studies. The two sulphur isotopes I study are sulphur-32 (S-32) and sulphur-34 (S-34). Some natural processes affect one isotope more (or less) than the other – for example, in their metabolism of sulphate reducing bacteria, their prefer to consume S-32 because the bonding energy is fractionally weaker than S-34, and therefore more easily accessed. Therefore, when sulphate is converted to sulfide there is a change in the S-32/S-34 ratio, which we call a fractionation.
The fractionation of sulphur isotope can tell us a lot about the origin of sulfide minerals and is particularly helpful when analyses are performed on individual minerals using a machine called secondary ion mass spectrometry or just SIMS. There are only a few SIMS in Canada, which I was fortunate enough to access in collaboration with Professor Fayek at the University of Manitoba.
Figure 1. SIMS machine at the University of Manitoba
An exciting aspect to this study is it will first time where SIMS analysis is performed on millerite (nickel sulfide). A key component of such analysis is developing a reference material, which is key when performing these analyses. Although the details are more complicated, this collaboration is the core thread of my ELP, whereby I learn how to develop new analytical protocols, so that in my future career I can undertake tackle more complicated research questions. This study will perform SIMS analysis on pyrite, sphalerite and millerite and will also combine additional sulphur analysis performed at the University of Calgary.
Early results of this study suggest that my previous comment about the source of sulphur being seawater my not be entirely true. The processes that can impact sulphur isotope fractionation are myriad, and unravelling the possible mechanism will yield some exciting results.