Pyrite (FeS2) is the most common sulfur-bearing mineral in the Earth's crust and can be found in all major types of rock: igneous, metamorphic, and sedimentary. It is the shiny, brass-colored mineral that you may know as fool's gold because it looks like gold and has fooled people throughout recorded history (Fig. 1). In some areas, it can be observed at the sides of roads or trails, especially where the ground has been recently disturbed. If you have ever caught the scent of rotten eggs while digging at the beach, in mud flats, or swamps, you were probably in an area where pyrite was forming. That smell is caused by hydrogen sulfide (H2S), and the black or blue muds that you were digging in would have contained microscopic pyrite that formed when the hydrogen sulfide that you smelled bonded with iron in the pore waters (Rickard 2012).
Despite pyrite's bad rap as fool's gold, it has some interesting geochemical properties that may help us uncover how the ancient oceans, atmosphere, and life evolved on Earth, including how oxygen changed during these processes. Here we specifically discuss how pyrite can record bacterial processes and how we can use pyrite's isotopes (see Nitty Gritty Details) to understand how the pyrite formed.
To gain energy for survival, bacteria must combine organic matter with an oxygen-bearing molecule. Oxygen (O2) itself is the best molecule for this and the one that we and most multicellular life forms use; however, at low oxygen levels other molecules are used. In descending order of energy gained after oxygen these are: nitrate (NO3), manganese oxides (e.g., MnO2), iron oxides (e.g., FeO2), and sulfate (SO4) (Loyd et al. 2012). When bacteria combine organic matter with the oxygen in sulfate (SO4), smelly H2S forms and reacts with iron (Fe) to make pyrite (FeS2).
In the ocean, many bacteria use sulfate to help produce energy because it is can be relatively abundant—it is a feast! However, bacteria can run out of sulfate in deep sediments or even in the oceans, such as at times in Earth's history when oxygen was low—it is a famine.
The sulfur isotopes contained within the pyrite we find in the rock record can provide us with information about how much sulfate was consumed by this process. The isotopes of sulfur provide us with additional information about how much sulfate is used up. Sulfur has an atomic mass of 32.07. The reason for the decimal is that sulfur has four different stable isotopes (32S, 33S, 34S, and 36S), that is atoms with the same number of protons but different numbers of neutrons and 32.07 is the average of the atomic masses of these isotopes. The relative abundances of these isotopes can be used to understand the source of the sulfur and what reactions it underwent. Here we focus on the most abundant isotopes of sulfur, 32S and 34S. Sulfur isotope ratios are reported as a comparison to a standard with known ratio of isotopes and how our sample differs from that standard is reported as a δ34S value, with units of ‰ or (“per mil”) (see Nitty Gritty Details).
Isotopes: Each atom is made up of protons, neutrons and electrons. The number of protons for a given element is always the same, but the number of neutrons can vary. Atoms with the same number of protons, but a different number of neutrons are isotopes. In the case of sulfur, there are 16 protons and 16, 17, 18, or 20 neutrons, corresponding to atomic masses of 32, 33, 34, and 36.
34S: We designate each particular isotope by using a superscript before the elemental symbol. For example, 34S refers to the sulfur isotope that has an atomic mass of 34 (16 protons plus 18 neutrons).
δ 34S: Isotopic ratios are normalized to a standard compound, which is a material that contains the same element with known isotopic abundances. For sulfur, the standard is a specific mineral in the Canyon Diablo meteorite, whose 32S, 33S, 34S, and 36S contents are known. To determine δ34S, the 34S/32S ratio in a sample is measured and compared to the ratio in the meteorite. If the ratio is higher in the sample, δ 34S is positive (higher values for higher ratios). If the ratio is lower in the sample, δ34S is negative (lower values for lower ratios).
 Pyrite: A Natural History of Fool's Gold, David Rickard, Oxford University Press.
Bacteria use sulfate to get energy by oxidizing organic matter in a manner analogous to burning. During this process sulfate is transformed to hydrogen sulfide (H2S), which can then form pyrite. When bacteria do this, they preferentially use the lighter 32S isotope, meaning that the H2S that forms would incorporate less of the heavier isotopes and therefore the ratio of heavy (34S) sulfur to lighter (32S) sulfur is much lower than normal. This means that the pyrite that formed from this process would have a negative δ34S value (typically –40‰; see Nitty Gritty Details). In the open oceans, because there is always a lot of sulfate around, the δ34S value remains nearly constant. When pyrite forms deep in the sediments at the ocean's floor, the δ34S of that pyrite will start out low (approximately –40‰ compared to starting sulfate δ34S). As long as there is ample sulfate to consume, the bacteria will choose to use the 32S first (Fig. 2), and the δ34S will remain low. But as the initial sulfate is used up the bacteria will consume more of the heavier 34S. More and more consumption leads to less and less sulfur overall, producing higher and higher δ34S values. Therefore, high δ34S in the rock record tells us that the sulfate reducing bacteria at that location and time, were running out of sulfate and, in a manner of speaking, starving.
Approximately 2.33 billion years ago there was a sudden increase in atmospheric oxygen. This event it known as the Great Oxygenation Event (GOE; Luo et al. 2016). Prior to the GOE, oxygen levels in the atmosphere were extremely low, possibly 100 000 times lower than present day. Today, with our oxygenated atmosphere, much of the sulfate in the oceans comes from the oxidation of sulfide minerals on land. Because very few sulfide minerals were oxidized on land, very little sulfate was formed, and sulfate levels in the ocean were quite low before the GOE. This meant that bacteria were in famine mode, as reflected in high δ34S values. However, after the GOE, atmospheric oxygen may have approached modern day levels (~21%; Lyons et al. 2014). This rise in oxygen increased sulfate levels, and δ34S values decreased dramatically as sulfate conditions transitioned from limited (“famine”) to unlimited (“feast”).
Due to its abundance and the information held within in its building blocks, pyrite holds important information on the history of oxygenation of the oceans and atmosphere throughout Earth history. Because these processes are inextricably linked to the process of evolution of life, it is an important part of the puzzle that allowed us to become who we are today. Pyrite may be called fool's gold, but for geoscientists it is a goldmine for understanding Earth's processes and history.
Acknowledgments and Funding
Funded by NSERC Discovery grant to D.D.G. and NSF grant EAR 1251443 to M.J.K. Thank you to Selina Wu for insightful comments to early and late versions of the manuscript. Thank you also to Michael Borda, David Rickard, and four anonymous reviewers for constructive criticism that greatly improved the manuscript. We also thank Heather Burgi, Nathan Moens, Morgan Peer, and Deryk Jackson for helpful comments on the final version of the manuscript. For the original version of Figure 2, we thank Uli Wortmann, and for the photos in Figure 1 we thank the Royal Ontario Museum.