Geochemistry

Earth’s surface is covered in rocks (such as those seen exposed on the shores of Lake Onega in Russia, above; photo by Kärt Paiste). All rocks are made of chemicals. The chemicals present in rocks are a unique fingerprint of the chemistry of the environments in which they formed. Similarly, the chemistry of rocks that form in the ocean, such as limestones, dolostones, and shales, usually reflect the chemistry of the ocean. “Usually” because sometimes rocks form on or buried within mud/sediment in the ocean floor, where chemical conditions can evolve to be quite different from the salty water above. The chemical evolution of sediments is often referred to as diagenesis, and is seen by many as confounding to our ability to interpret past ocean chemistry from old rocks. Rather than shying away from the complications of diagenesis, I see the uniquely altered chemical signals it produces as a way to discover information about local environmental conditions, in addition to carefully teasing out information about global ocean chemistry.

One chemical I have built my career studying, is sulfur (S). Sulfur is special because it is sensitive to redox (i.e., how reducing or oxidized an environment is). In more oxidizing conditions, like those in the modern day ocean, sulfur is almost exclusively present as fully oxidized S(VI) in the chemical compound sulfate (SO₄^2-). In more reducing conditions, like those in the mud at the bottom of the modern day ocean, some sulfur is often present as partially reduced S(-I) or fully reduced S(-II) in diagenetic iron sulfide minerals like mackinawite (FeS), greigite (Fe₃S₄), pyrite (FeS₂; see above), and marcasite (FeS₂). Generally, the amount of oxidized versus reduced sulfur present in marine sedimentary rocks can tell us about the ocean redox conditions at the time they formed. Because almost all sulfur in three billion year old sedimentary rocks is in reduced forms, it leads us to believe that there was very little oxygen in the atmosphere and ocean on early Earth.

Another interesting quality of sulfur is that it comes in many ‘flavors’, called isotopes – these are types of sulfur with different numbers of neutrons in their nuclei, and therefore slightly different masses. The most common isotopes of sulfur are ³²S, ³³S, ³⁴S, and ³⁶S. These isotopes are stable, meaning that they do not decay to daughter species (such as during the transformation of ²³⁸U to ²³⁴Th). The advantage of stable isotopes is that once they are locked into rocks, they tend to stick around for a long time. Just like chemicals, isotopes can provide information about past environmental conditions. In the case of sulfur, the conversion of sulfate to sulfide is often catalyzed by microbes, and is mass-dependent, meaning that the lighter isotopes (³²S, ³³S) are preferentially partitioned into sulfide relative to the heavier isotopes (³⁴S, ³⁶S). As a result, the ratio of ³⁴S:³²S in sulfate is often higher than that in sulfide. The more oxidized the ocean is, the greater the offset between the ³⁴S:³²S ratio in sulfate vs. sulfide is – or so the traditional view dictates. This concept has underpinned much of modern biogeochemistry over the last several decades. However, recent work suggests that sulfur isotopes might reflect more about local environmental conditions that global ocean redox – I sought to investigate this assertion during my Ph.D. research as Washington University in St. Louis with Prof. David Fike.

My Ph.D. research focused specifically on how local environmental conditions (e.g., the rate at which sediments accumulate on the seafloor) affect sulfur isotope ratios in iron sulfide minerals that form in marine sediments and eventually become part of sedimentary rocks. Using a secondary ion mass spectrometer (SIMS; see Wash U’s above) to measure sulfur isotope ratios in individual grains of iron sulfide minerals, I found that local conditions can have a great effect on sulfur isotope ratios. This is not because microbes become more or less selective when it comes to the average mass of sulfur they like to transform, as many had suspected. Rather, it is because physical conditions change the likelihood for sulfate to enter sediments via diffusion versus simple ‘burial’. These findings will fundamentally change the way sulfur isotopes in sedimentary rocks are interpreted.

For my postdoc work at the University of Chicago, I am investigating how local environmental (i.e., diagenetic) conditions affect sulfur isotope ratios in sulfate that has been substituted into the crystal lattice of carbonates (see above) such as calcite (CaCO₃), aragonite (CaCO₃), and dolomite (CaMg(CO₃)₂). The reason I chose U Chicago to host this work is because Prof. Clara Blättler has in the course of her career developed an exciting geochemical framework to constrain past diagenetic conditions using calcium and magnesium isotopes. Using a new ion chromatogaph (IC) and multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS), we will obtain sulfur, calcium and magnesium isotope measurements of carbonates of all ages. In doing so, we will pull back the confounding curtain of diagenesis and more accurately constrain the sulfur isotopic composition of the ocean through Earth’s history. These efforts will be crucial to determining the timing and magnitude of Earth’s surface redox evolution, which may in turn have affected the evolution and diversification of life.