Quantifying the deep: The importance of diagenetic reactions to marine geochemical cycles

Abstract

Marine sediments play a fundamental role in long-term element cycles on Earth and host an expansive microbial ecosystem known as the “oceanic deep biosphere”. The biogeochemical and inorganic reactions that occur in the sediments as they are buried alter both their physical and chemical properties, as well as control the chemistry of the surrounding pore waters. While the alteration of sediments can affect subduction zone dynamics and mantle geochemistry, chemical alteration of pore waters affects ocean chemistry through the diffusional and advective communication between pore waters and the overlying ocean. The alteration of pore waters also provides a sensitive indicator of the chemical reactions taking place in marine sediments. However, the same mobility of solutes in pore water that results in marine sediments’ influence on ocean chemistry also results in challenges in quantifying and characterizing these reactions. Reactive-transport modeling is an effective approach employed to characterize the reactions taking place in marine sediments that accounts for the mobility of solutes and pore waters. Data collected and archived during the past ~50 years of scientific ocean drilling provides the necessary information to parameterize and apply reactive-transport modeling to study marine sedimentary reactions on local, regional, and global scales. The research detailed in this dissertation utilizes reactive-transport modeling of ocean drilling data, supplemented with new measurements, to evaluate the influence of dehalogenation reactions on the deep biosphere, and the role of authigenic mineral formation reactions in global geochemical cycles. Reactive-transport modeling is complementary to measurements that characterize microbial communities in the deep biosphere. Genomics, metagenomics, metabolomics, and other methods provide information about the composition and function of microbes in marine sediments, but geochemically-derived reaction rates are needed to understand the magnitudes of the in situ activity in these communities. The research described in Chapter 2 supplements reactive-transport modeling of pore water bromide with new measurements of solid-phase organobromine content to constrain the maximum rates of microbial organobromine respiration and to investigate the depth distribution of debromination activity in continental margin sediments. The reactive-transport modeling results and organobromine profiles indicate that debromination is most active in the upper sediment column, and is largely limited by substrate availability. Maximum depth-integrated rates of debromination on the order of 101 to 103 μmol m-2 y-1 indicate that the amount of energy that is provided through organobromine respiration is low relative to other metabolic pathways such as sulfate reduction and methanogenesis, but may still serve an important niche in the microbial community. In addition, a close connection between debromination and ammonium production is apparent in the pore water profiles, suggesting a relationship between debromination and degradation of the amino acid fraction of organic matter, which may allow debromination to be a useful tracer for degradation of this labile pool of carbon and nitrogen. Ocean drilling data supplemented with new measurements are also effective for investigating oceanic geochemical cycles on a global scale, such as the oceanic magnesium cycle. The oceanic magnesium cycle is intimately connected to long-term climate on Earth through its relationship to continental weathering and formation of aluminosilicate and carbonate minerals. Uncertainties in the oceanic magnesium cycle propagate into other chemical budgets such as carbon and calcium, and into interpretations of paleo-oceanographic reconstructions of seawater δ26Mg and Mg/Ca ratios. In Chapter 3, dissolved magnesium fluxes at 269 ocean drilling sites are calculated to create a detailed global map of the diffusive and burial flux of dissolved magnesium across the sediment-water interface using a machine learning regression with several globally-gridded environmental parameters. In addition, the isotopic fractionations associated with those fluxes are calculated using data from a variety of ocean drilling locations and extrapolated globally using a lithologically-binned regression. These analyses show that the magnesium flux into marine sediments accounts for about 15 – 20% of the magnesium sink from the ocean, with a flux-weighted fractionation factor of approximately 0.9997 acting to increase the magnesium isotopic ratio in the ocean. This analysis of global magnesium fluxes and isotopic fractionation provides the best constraints to date on the sources and sinks that define the oceanic magnesium cycle, including new constraints on the loss of magnesium during low-temperature ridge flank hydrothermal circulation. The in situ reactions influencing the magnesium flux into marine sediments are also important for other oceanic geochemical cycles, such as the oceanic alkalinity, 13C, and H218O cycles, as well as mineral-bound water delivery to subduction zones. A new multicomponent reactive transport model is applied to nine ocean drilling cores, including sites characterized by pelagic and hemipelagic sedimentation. The model results indicate that authigenic clay formation in the deep subsurface (>1 meter below seafloor) is a widespread process that accounts for up to 5 wt% of the bulk sediment. The rate of authigenic clay formation in the deep subsurface could amount to the equivalent of a few percent of the total sediment input into the ocean from rivers, and up to 5% of the structural water content of subducting sediment. This geochemical sink of 18O-enriched structural water may account for the imbalance calculated in the 108-year oxygen isotope budget of the ocean. The model indicates that the rates of authigenic carbonate formation in the upper sediment column are underestimated by a factor of at least 1.5 to 2 by models that solely utilize the calcium concentration profiles, suggesting authigenic carbonate formation has a greater role in the 13C cycle of the ocean than previously estimated. The results from this study add further support to evidence for the importance of authigenic mineral formation reactions in global geochemical cycles.