Award: OCE-1558829

Deep-sea environments above sections of the Earth?s crust where magma chambers lie close by often experience hydrothermal circulation. These circulation cells result from heat of the magma chamber warming porewater in the overlying geology, making it less buoyant and therefore likely to rise through the seafloor and discharge through either vigorous chimneys or more diffuse seepage zones. This discharge carries unique chemicals that support entire ecosystems from microbes to crabs, fish, and mollusks. While vigorous venting can be relatively easily observed and measured, the slower seepage is not nearly as measurable despite being thought to deliver comparatively more heat and chemicals to the deep ocean than the major vents. However, our ability to understand the importance of this diffuse seepage and the related biological functions that utilize the seepage products are limited by an inability to measure this slow discharge process. This project, therefore, aimed to develop a new geochemical tool that uses a natural isotope of radium (radium-224) as a tracer of this diffuse seepage in order to quantify discharge rates. Through the course of two research cruises in which we had the human-occupied research submersible Alvin, we collected a total of 33 sediment cores around a hydrothermal seepage environment in Guaymas Basin within the Gulf of California. We measured radium-224 activities within porewaters of these sediment cores at 2-4 cm intervals. We then measured the associated sediments to determine the maximum amount of radium-224 that they could support within porewaters based upon radioactive principles. Using concepts of vertical transport and radioactive decay/ingrowth, we developed a vertical exchange model for radium-224 to estimate the porewater flow rate through those sediments. This model compares the dissolved activity observed within a sediment layer with the radium-224 activities in adjoining layers and also with the maximum amount that could be supported by the sediments to determine how long those porewaters remained in contact with the sediment layer from which they were recovered. Dividing the porewater volume in that layer by its residence time yields an estimate of flow rate. We utilized this vertical exchange model across the 33 sediment cores we recovered from Guaymas Basin. Cores were collected through different types of microbial mats (which are thought to respond uniquely to different flow rates) and also bare sediments. We found that significantly higher flow rates through sediments colonized by microbial mats (ranging from 0.2 to 10.26 mL/cm2/day) than bare sediments (ranging from 0.07 to 2.0 mL/cm2/day). These results are somewhat intuitive – we understand that the metabolic function of these microbial communities requires a supply of chemical solutes that must come from deep supply in the sediments. However, for the first time, we have provided a range of flow rates associated with these communities. We also found that orange strains of these bacteria (Beggiotoa spp.) are associated with the highest discharge rates, but also a wider range of magnitude and direction of the flows than white Beggiotoa communities. Finally, we constrained the relationship between temperature of the porewater and flow rate – a relationship which holds remarkably well for bare sediments and white Beggiotoa, but not for orange Beggiotoa mats. These findings offer unique new insights into the supply of metabolic solutes and the responding community dynamics of these microbial mats. We have provided valuable knowledge on a range of flow rates across diffuse seepage environments which could only be inferred based upon microbial mat presence prior to this study. In addition, the geochemical tool that we developed here will provide a valuable approach to constraining deep-sea connectivity between the geology and overlying water column across a wide variety of seepage habitats. This project involved students from elementary school through all phases of college education. A total of 8 undergraduate and graduate students were involved in this project, from entering freshmen to graduating Ph.D.s. These students gained valuable research experiences in designing a research project, analytical methods, and result interpretation approaches. Several of these students participated in the research cruises and gained valuable experience with sampling in deep ocean settings using submersibles. Last Modified: 10/03/2019 Submitted by: Richard N Peterson