Seafloor hydrothermal vents are hotspots of biological activity on the deep seafloor, yet we know little about how the physical, chemical, and biological systems are tied together in order to yield such high productivity. This research, originates as a Ridge 2000 Postdoctoral Fellowship and is one of the first studies to apply reactive transport modeling to evaluate the interplay between hydrothermal vent fluid chemistry, chemosynthetic microbial metabolic processes, fluid flow, and mineral precipitation. This modeling project provides a unique and quantitative foundation that can be used to test hypotheses for mechanisms of interaction, gauging the sensitivity of the system to changes in governing parameters, and identifying linkages between large and small scale processes. Model predictions will be compared with data and observations from the well-characterized hydrothermal vent fields of the Endeavour Segment of the Juan de Fuca Ridge offshore of the northwestern US. The well established TOUGH REACT modeling code and independently developed larger scale modeling codes will be used for the calculations. Input parameters will come from available thermodynamic data bases and new kinetic data from experiments that measure the biological uptake rates of chemical species by various hydrothermal vent micro-organisms. Broader impacts of the work include interdisciplinary training of a postdoctoral researcher who will cross train in laboratories at the University of Georgia and Harvard University. The postdoc will also engage in training and mentoring undergraduate students, among other activities.
The drift of the Earth’s massive tectonic plates on top of a layer of molten lava is driven, in part, by the creation of new crust at divergent plate boundaries. As magma upwells from the mantle in this geologic foundry, seawater circulates within the crust, convectively cooling the solidifying magma like a tub of water cools a blacksmith’s iron, but at pressures and temperatures several hundred times higher than at the Earth’s surface. In this type of setting, known as a mid-ocean ridge (MOR) hydrothermal system, seawater undergoes profound chemical and physical transformations to become a scorching hot (sometimes boiling) hydrothermal brew brimming with electron-rich chemicals. This so-called hydrothermal fluid then mixes with seawater within the ocean crust to create a broad region of moderate temperature mixtures (or diffuse fluid). The diffuse fluid environment is less blistering than the original hydrothermal fluid and not as frigid as bottom seawater; just right for chemosynthetic microorganisms that can capitalize on the oxidation of reduced molecules in the fluid to drive their metabolic machinery, all in the complete absence of sunlight. Although the scientific community has a reasonable conceptual understanding of the subsurface environment, our quantitative knowledge is relatively sparse owing, in part, to the inaccessibility of the environment but also to the relatively young age of this field of study; the first discovery dates back only to the late 1970’s. The research funded by this grant used computer models of the subsurface in conjunction with chemistry data from outflow sites at the seafloor to paint a more detailed picture of the movement of hydrothermal fluids and the implications thereof for the subsurface biosphere. We pursued a two-pronged research approach. First, we created a geochemical model of the mixing process with commercially available software, and meshed it with a transport model of the seawater circulation path to get an idea of where conditions were most favorable for chemosynthetic microorganisms. From these energy maps (tailored so that modeled chemistry at the surface matched observations of fluid chemistry as closely as possible) we defined the best places for microbes to make a living by, for instance, oxidizing H2S or by converting CO2 to CH4. We were also able to calculate reaction rates for abiotic processes, and the peak rate of one process in particular (CaSO4 precipitation) helps to further define ecosystem dimensions because precipitation occurs at a temperature just above the upper limit to life (~ 120°C). A major limitation of this first approach is that it cannot handle the higher temperature and multiphase character of the hottest hydrothermal fluids. To address this shortcoming, we modified an open-source code from the geothermal energy community to make it suitable for seafloor hydrothermal systems, which have higher temperatures, pressures and salt concentrations than the analogous continental geothermal systems (e.g., The Geysers in California) for which the code was originally designed. Because salt water has much more complex phase relations than pure water, this is a very challenging problem, which impacts both the flow dynamics and the chemical composition of the hydrothermal fluid. Applying this novel modeling tool, we simulated the multiphase flow in a hydrothermal discharge zone, which is the conduit through which the hottest hydrothermal fluid flows from just above a magmatic heat source directly to a vent at the seafloor. The results of this model give us an idea of where in the ocean crust we should expect to find boiling hydrothermal fluid: the upper 700 meters in our model scenario. This information, in turn, points to the location of a phase contrast where we might expect enhanced precipitation of valuable minerals such as copper sulfide. The onset of boiling also impacts the distribution of the volatile substance, H2S, upon which some chemosynthetic microorganisms rely for food. Since H2S prefers to be in a vapor phase if one is present, the liquid left behind from boiling offers a reduced supply of sustenance for the rich ecosystem at vents. These computer-modeling advances represent important first steps towards a more integrated view of hydrothermal systems. Such a framework will be increasingly important to the study of MOR hydrothermal settings as underwater observatories currently under construction come online. These data sets can then be compared to the quantitative integrated analysis provided by process-based numerical models, with discrepancies highlighting new avenues of investigation.
