Over the last decades, it has emerged that increased CO2 in the atmosphere since the industrial revolution is one of the main drivers of accelerated global climate change. As marine geoscientists, we are interested in the capability of the world’s oceans to sequester atmospheric CO2 into photosynthetic algal biomass, and to ultimately lock it away in seafloor sediments when this biomass dies and sinks to the ocean depths. This sequestration of CO2 though organic matter burial has regulated global climate since the dawn of photosynthesis some 3 billion years ago, and is of paramount importance today as we seek to understand and mitigate the CO2-driven greenhouse effect.
A key question around organic matter burial in seafloor sediments is: What controls the capability of sediments to accumulate and stabilise organic matter? It has been known for decades that in Earth’s history, there have been repeated, million year-long intervals when large parts of the oceans accumulated huge amounts of organic matter in so-called “black shales”, fine-grained sediments that contain at least 2 and up to ~40 % of organic carbon. These intervals were often characterised by the lack of oxygen in vast swaths of the oceans (“Oceanic Anoxic Events”), preventing the breakdown of organic matter and enhancing its burial at the seafloor.
However, oxygen is only part of the story. It has also been shown that the association of organic matter with sulphur on a molecular level (a process called sulphurisation or natural vulcanisation) enhances the preservation of organic matter. Put simply, organic matter in the ocean or at the seafloor is partly degraded by microbial activity. This process consumes oxygen, and once this oxygen pool is exhausted, degradation continues via bacterial sulphate reduction takes. Sulphate reduction produces hydrogen sulphide, and it is this highly toxic sulphur species that reacts with organic molecules in the sediment. This process is of interest to the petroleum industry, since organic matter sulphurisation leads to richer hydrocarbon source rocks, but also generates “sour” oil and gas. From an environmental point of view, organic matter sulphurisation as a potential pathway to increasing the removal of atmospheric CO2 into marine sediments is poorly studied, but potentially relevant in the global carbon cycle.
In the relationship between carbon and sulphur, iron plays a very important role as well: The hydrogen sulphide produced during bacterial sulphate reduction also likes to react with various iron minerals (usually delivered from land by rivers, wind or ice), in particular iron (oxyhydr)oxides, to form iron sulphides like pyrite. This “iron sulphidisation” process competes with the “organic matter sulphurisation” process for the available hydrogen sulphide – and the rules of this chemical competition are not well-understood.
Traditionally, the view was that “sulphidisation” comes before “sulphurisation” – organic matter only becomes sulphurised once the available reactive iron pool is used up by pyrite formation. However, it has recently been shown that organic matter sulphurisation can occur simultaneously with, or even prior to, iron sulphidisation. Given its fundamental importance in coupled Fe-C-S cycling in past and present oceans, a better understanding of this chemical competition is required.
This PhD project will provide vital new insights into the “sulphurisation versus sulphidisation” problem by investigating the geochemical interactions between iron, sulphur and carbon from a fundamental point of view, and applying the newly gained insights to deciphering Fe-C-S cycling in various organic-rich sediments. This will be achieved by tightly controlled lab experiments of increasing complexity (adding varying amounts of different types of organic matter, iron minerals and potentially trace metals to sulphidic solutions) followed by analysis of the products using a range of techniques (including SEM-EDX, sequential extractions, isotope ratio mass spectrometry, synchrotron-based speciation). This will followed by analyses of natural sediments known to contain various amounts of iron sulphides and sulphurised organic matter.
1) Setting up and running laboratory experiments introducing different types of iron minerals, organic matter and trace metals into a sulphidic solution.
2) Determining the amounts, speciation and partly isotopic composition of different iron, carbon, sulphur and trace metals in particulates produced during the experiments using a range of in-house and external analytical methods.
3) Establishing a conceptual framework of the geochemical controls on sulphur partitioning between iron and organic matter based on experimental results.
4) Applying this conceptual framework to natural sediments with different relative proportions of iron sulphides and sulphurised organic matter, and testing to what extent the framework established by a simplistic and abiotic experimental setup is valid for a more complex natural system.