Where did all the sulphur go? Understanding reactions of sulphur with iron and organic matter in anoxic oceans

Background:

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.

 

Objectives:

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. 

 

Approach & training:

As a first step, the PhD student will establish experimental protocols to perform sulphidic experiments in the lab, including the selection of iron and organic phases as well as a set of experimental conditions (salinity, pH, temperature, sulphide concentration etc) representative of natural sulphidic systems. Following the setup and completion of the experiments, solid phase products will be analysed using a range of analytical techniques (bulk C/S quantification using Leco combustion analysis; extraction & quantification of organic & inorganic S species using sequential extraction procedures; S isotope analysis of organic & inorganic S compounds; S speciation analysis by synchrotron-based spectroscopy).

Following this experimental approach to understanding iron sulphidisation versus organic matter sulphurisation in a controlled setup, the PhD student will develop a conceptual framework and apply it to modern and ancient sediments or water column particles (e.g., Black Sea, Baltic Sea, Cretaceous and Jurassic black shales) that have different relative proportions of iron sulphides and sulphurised organic matter due to their different depositional conditions. These natural samples, as well as the required geochemical data, are partly available, but new samples may be taken and analysed by the PhD student.

Most analyses will be conducted in the world-leading Cohen Geochemistry labs at the University of Leeds. The student will also have the opportunity to look at the Fe-C-S coupling in very fine detail using an especially powerful type of microscopy and spectroscopy available at the world-leading Diamond Light Source synchrotron facility near Oxford. In addition, the student will spend a 3 month paid internship in the labs of our CASE partner Iso-Analytical (Crewe), who are experts in analysing S isotopes in variety of sample matrixes. The PhD student will benefit from world-leading expertise in the Cohen group and the supervisory team. A unique set of sediment samples and relevant background data has been gathered by the supervisory team and collaborators in the frame of past and ongoing projects, and these samples are available for investigation.

The student will be trained in geochemical procedures and analyses by Drs März, Newton and Profs Poulton and Peacock who have substantial experience in extracting different metal and sulphur phases from sediment samples and total acid digestions, analysis of dissolved phases using AAS, ICP-OES and ICP-MS and of solid samples using XRD, CN combustion analysis and IRMS, and synchrotron-based spectroscopy. Hands-on training and support will further be provided by highly qualified technicians in the Cohen Geochemistry labs. At Iso-Analytical, the student will be trained on a specialised setup for S isotopes by leading experts in the field. The successful candidate will have access to a wide range of training workshops (scientific writing and presentation skills, statistics, science communication and outreach), and will be supported by the supervisors in preparing conference presentations and peer-reviewed publications.

 

Student qualification:

The successful candidate should have an excellent degree in an Earth Science or closely related subject, or Environmental Sciences discipline, and have a keen interest in, and some experience of lab work, analytical methods and interdisciplinary research. Ideally the successful candidate will have experience in conducting a research project and presenting research results to a scientific audience.