Iron isotope signatures of recent sedimentary pyrite in the Baltic Sea - Contribution to the development of a Paleoceanograpic proxy

The marine environment is vulnerable to profound future modification as a result of environmental change. Along with ocean acidification, a widespread increase of oxygen deficient conditions is likely over the coming years. How these modifications will be manifest in the modern oceans is still uncertain. Parts of the bottom waters of the Baltic Sea and the Black Sea are at present deficient in oxygen since very limited exchange with surface waters and inflowing freshwater occurs. Such conditions are rare in the present ocean, but have been more widespread in the Earth's history and may become more widespread in the future as a result of a global temperature increase. Modern oxygen-deficient environments that are present in the Baltic Sea and Black Sea are analogues for ancient oceans and provide important information on how ocean chemistry has changed over time. This project will study two locations in the Baltic Sea, the Gotland Deep and the Landsort Deep, in order to test and calibrate a new tool, iron isotopes, so that it can be applied to past changes in the oxygen content of the ocean.
Iron is the fourth most abundant element in the Earth's crust. It is much more soluble during oxygen-deficient conditions than during oxic conditions and therefore, iron concentrations in modern oceans are very low. Iron is also an essential trace element for most living organisms. Marine phytoplankton use CO2 from the atmosphere for growing. The phytoplankton grows faster when more iron is available and therefore, iron is linked to the global climate. Consequently, iron is an important element to study, and recent developments in measuring iron isotopes provide a critical new tool to study the role of iron in the oceans. For iron isotope studies, the relative abundance of iron atoms that have slightly different masses (isotopes) needs to be measured to high precision using a mass spectrometer. Each isotope of iron has slightly different chemical and physical properties due to their different masses and as a result, behaves differently in many processes that involve iron. In particular, iron isotopes give important information about changes in oxygen contents and biological processes.
The Baltic Sea is a unique setting where changes from oxygen-rich to oxygen-deficient conditions can be studied, since these occurred repeatedly over the last few hundreds of years. These changes are recorded in the sediments of the Baltic Sea and result in particular in different formation mechanisms of the iron-sulphide mineral pyrite. Pyrite is the most stable iron sulfide mineral and has therefore been intensively studied. The processes that determine the Fe isotopic composition of pyrite are, however, not well understood. This project will investigate how the iron isotope composition of pyrite changes with different formation mechanisms and oxygen contents of the bottom water.
The Baltic Sea has problems with heavy-metal pollution and spring algal blooms that are related to redox conditions and iron. Some species of blue-green algae are toxic to humans and the blooms further reduce the oxygen content of the water. The proposed study of iron-redox cycling in the Baltic Sea using Fe isotopes will contribute to a better understanding of the processes that cause the environmental problems in the Baltic Sea.

This project investigates the processes that govern the iron isotopic composition of sedimentary pyrite in the marine environment by studying a present day environment. The proposed study complements experimental and theoretical approaches to determine Fe isotope fractionation factors for pyrite in order to establish it as a Paleoceanographic proxy. This project is therefore intended to provide fundamental knowledge rather than information that can be transferred into policy or industry directly.

The project has several scientific user groups: 1) Those concerned with the application and understanding of Fe isotopes and other metal isotope systems in general and in particular in sedimentary environments and/or to study pyrite formation processes. 2) Paleoceanographers 3) Earth system scientists, in particular those investigating oxygen-deficient environments or time-periods in the Earth's history when such conditions existed and/or the iron biogeochemical cycle.
The methodologies that we have developed will be applied for the first time to isotope analyses of very low levels of Fe. This is an analytical capability that is only available in a few laboratories at present. Therefore, these analytical developments are expected to be of interest to a broader scientific community including environmental studies and medical applications (1).

Participation at the Goldschmidt conferences 2013 and 2014 allows Dr. Fehr to interact and present results of the proposed project to a broad scientific audience that includes the main identified user groups with the likely exception of medical scientists. Conference abstracts will be published in Geochimica et Cosmochimica Acta.
Organization of a workshop on metal isotope signatures of sulfides in 2013 allows Dr. Fehr to engage in depth with scientists that study the mechanism of Fe and other metal isotope fractionation in sulfides.

Since Fe is linked to the ocean-atmosphere exchange of CO2 and global climate, this project will also be relevant for the understanding of climate changes.
The Baltic Sea currently has problems with heavy-metal pollution and associated spring algal blooms that are related to redox conditions and iron. The present study of iron-redox cycling in the Baltic Sea using Fe isotopes will contribute to a better understanding of the processes that cause these environmental problems in the Baltic Sea, and new insights will be made accessible to the public via our Swedish project partners.
Other users who may benefit from the proposed project include potentially landfill site managers, environmental consultancies and water companies, as they are interested in a better understanding of the influence of redox changes on heavy metal mobility.

A project website will be set up within the first 3 months of the project for access by anyone with interest. It will be hosted at the Open University whose main web site (http://www.open.ac.uk/) receives on average about 5 million visits per month (2).
A video will be made to document the sampling cruise and document the analytical work. These videos are intended for the general public and will be posted on a readily accessible site, such as Youtube as well as permanently archived at an academic site.

School visits and public lectures (through local connections or the Science, Technology, Engineering & Mathematics (STEM) network) will be used to engage the younger generation and the wider public in the topical science issues such environmental pollution and climate change.
The Science Faculty of the Open University has four London Technology Network 'Business Fellows', including a member of the Isotope Geochemistry Group at the Department of Earth and Environmental Sciences, Dr. Anthony Cohen. He promotes the Department's research outside academic circles and actively engages with potential business and commercial users.

References
(1) Bullen and Walczyk, 2009, Elements (2) Online Services, 2009, The Open University