The primary goal within the group is to better understand the relationship between microorganisms and Earth surface evolution. Our work ranges from geologically rooted questions, where we aim to track the onset or environmental expression of different metabolic processes and follow the inventory of atmospheric/oceanic oxidant budgets, through to modern processes and environments (such as experimental work with extant organisms, purified protein, work in the modern ocean water column, and early diagenesis in marine sediments). A common theme through these various scientific paths is the use of stable isotopes, and often more novel isotope systems, to unlock Earth’s storyline.

A flavor for some of this work is demonstrated below and certainly reflects the general lab philosophy. This is also reflected in video shorts on the lab homepage and in our most recent publications. That noted, suggestions and new thinking is always welcome and is what keeps our motor running.



Linking geology and environmental change

Linking geology and environmental change: Changes in the geological sulfur cycle are inferred from the sulfur isotopic composition of marine barite. The structure of the isotopic record from the Mesozoic to present has been interpreted as the result of microbial isotope effects or abrupt changes to tectonics and associated pyrite burial. Untangling the physical processes that govern the marine sulfur cycle and associated isotopic change is critical to understanding how climate, atmospheric oxygenation, and marine ecology have coevolved over geologic time. Here we demonstrate that the sulfur outgassing associated with emplacement of large igneous provinces can produce the apparent stepwise jumps in the isotopic record when coupled to long-term changes in burial efficiency. The record of large igneous provinces map onto the required outgassing events in our model. This solution provides a quantitative picture of the last 120 My of change in the ocean's largest oxidant reservoir. Read more in Laaksoet al., 2020 PNAS.


Calibrating modern processes to interpret the past
Calibrating modern processes to interpret the past: The mass-independent minor oxygen isotope compositions (Δ′17O) of atmospheric O2 and CO2 are primarily regulated by their relative partial pressures, pO2/pCO2. Pyrite oxidation during chemical weathering on land consumes O2 and generates sulfate that is carried to the ocean by rivers. The Δ′17O values of marine sulfate deposits have thus been proposed to quantitatively track ancient atmospheric conditions. This proxy assumes direct O2incorporation into terrestrial pyrite oxidation-derived sulfate, but a mechanistic understanding of pyrite oxidation—including oxygen sources—in weathering environments remains elusive. To address this issue, we present sulfate source estimates and
Δ′17O measurements from modern rivers transecting the Annapurna Himalaya, Nepal. Sulfate in high-elevation headwaters is quantitatively sourced by pyrite oxidation, but resulting Δ′17O values imply no direct tropospheric O2 incorporation. Rather, our results necessitate incorporation of oxygen atoms from alternative, 17O-enriched sources such as reactive oxygen species. Sulfate Δ′17O decreases significantly when moving into warm, low-elevation tributaries draining the same bedrock lithology. We interpret this to reflect overprinting of the pyrite oxidation-derived Δ′17O anomaly by microbial sulfate reduction and reoxidation, consistent with previously described major sulfur and oxygen isotope relationships. The geologic application of sulfate Δ′17O as a proxy for past pO2/pCO2 should consider both 1) alternative oxygen sources during pyrite oxidation and 2) secondary overprinting by microbial recycling. Read more in Hemingway et al., 2020 PNAS.


Addressing long-term changes in ocean chemistry

Addressing long-term changes in ocean chemistry: The temperature and chemistry of early seawater have both been inferred from the isotopic composition of Precambrian chert(SiO2), a precipitated mineral formed on or within marine sediments. The temporal change δ18O has been hypothesized to reflect either the product of cooling surface ocean temperatures, a signature of evolving seawater δ18O composition, or the product of later stage diagenesis (where measured δ18O reflects the composition of diagenetic fluids). We address this uncertainty through the inclusion of the minor oxygen isotope δ17O(noted as Δ′17O ) in conjunction with δ18O. What is definitively the case is an equitable, modern-like Archean surface ocean temperature. Read more in Liljestrandet al., 2020 EPSL.


Enzyme-level isotope effects

Enzyme-level isotope effects: The precise interpretation of environmental sulfur isotope records requires a quantitative understanding of the biochemical controls on sulfur isotope fractionation by the principle isotope-fractionating process within the S cycle, microbial sulfate reduction (MSR). Here we provide a direct observation of the sulfur isotope fractionations imparted by a central enzyme in the energy metabolism of sulfate reducers, dissimilatory sulfite reductase (DsrAB). Results from in vitrosulfite reduction experiments allow us to calculate the in vitroDsrAB isotope effect. These observations facilitate a rigorous evaluation of the isotopic fractionation associated with the dissimilatory MSR pathway, as well as of the environmental variables that govern the overall magnitude of fractionation by natural communities of sulfate reducers. Read more in Leavitt et al., 2015, Frontiers in Microbiology.