Isotope insight into microbial processes on the North Pond Leg, IODP expedition 336

Microbes are single celled organisms that live all over the surface of the Earth, including in the oceans and in the sediments within the oceans. Many microbes respire oxygen, like humans do, consuming organic carbon in the process. However, in marine sediments, there often isn't enough oxygen - in some places there is no oxygen at all. In these "anoxic" environments there are microbes that "respire" other molecules such as sulphate. Understanding the functioning of these organisms in what we call the "deep biosphere" is important because they are similar to some of the earliest life on Earth. We are particularly interested in how these organisms respire sulphate, produce sulphide, and how other organisms use this sulphide - this is called the microbial sulphur cycle. This is especially important because this is the dominant microbial process in marine sediments, therefore the amount of sulphate that is used in the deep biosphere influences how much organic carbon is ultimately buried in sediments, which links directly to the amount of oxygen in our atmosphere. A proper understanding of the sulphur cycle could help us explore and understand when and how oxygen evolved in the atmosphere and how this may have influenced the evolution of life on Earth. Also, methane, a very powerful greenhouse gas responsible for 20% of the global warming to date, is produced abundantly in marine sediments, but never makes it to the atmosphere because sulphate reduction consumes it. Understanding how sulphate reduction is coupled to methane consumption is important to determining if marine sediments could become a source of this potent greenhouse gas.

Furthermore, it is possible that the organisms which respire sulphide may also live in cracks and pores in the rock beneath the sediment on the seafloor. As these organisms use up sulphides, they produce acids which would corrode ("weather") the surrounding rock. This results in the release of calcium, silicon, and other important chemicals, which ultimately end up in seawater. It is important to understand the amount and rates of delivery of chemicals to the ocean because this is directly linked to the amount of inorganic carbon that is brought to, and stored in the ocean. Changes in the delivery of carbon to the ocean have important implications for climate over millions of years because the amount of carbon in the ocean has a large impact on atmospheric carbon dioxide levels and therefore global temperature.

P-I Turchyn's previous work has used ratios of isotopes in sulphate to understand the processes of sulphate reduction and sulphur cycling. An isotope of an element is a form of that element (sulphur or oxygen in this case) that has extra neutrons in its nucleus. These extra neutrons make molecules containing the heavier isotopes behave differently in chemical reactions. Sulphate (SO4) has two isotope ratios of interest to geochemists - 34S to 32S and 18O to 16O. We can use these differences to pinpoint where organisms are living in the subsurface, and how they are processing sulphur.

In this proposal we would like to explore this further. We will look at fluids from under the ocean floor at the North Pond drilling sites on the Mid-Atlantic Ridge. We will measure the isotope ratios for sulphur and oxygen in sulphate in the fluids, and combine this with measurements on various sediments from these same sites to better understand how sulphur is modified in the subsurface. This will allow us to reconstruct where and how the carbon is oxidized during microbial respiration, and whether there is sufficient sulphur-based microbiology in the underlying rock to significantly affect weathering processes. This research has the possibility to greatly expand our knowledge of processes in the subsurface deep biosphere.

The results from this project will provide key geochemical details of microbial processes. This will complement the microbial characterizations planned for expedition 336, and thus provide important data for the main aims for this IODP cruise.
Research published from this project will highlight the wealth of information which is available from IODP samples, and further reinforce the importance of IODP cruises and samples.

Furthermore, this project will impact the international community of researchers who study various aspects of the biogeochemical sulphur cycle on a range of timescales, providing new and relevant data to feed into our understanding of this essential component of the deep marine biosphere.