Clay-rich marine sediments form Earth's main sink for organic carbon. This determines atmospheric oxygen levels, as well as fluctuations of global climate. For example, an increase in carbon burial effectiveness lowers atmospheric CO2 and induces global cooling, with an increase creating the opposite effect. Understanding oceanic carbon burial therefore has a direct bearing on how we model past changes of the earth system and predict future developments, such as global warming due to fossil fuel consumption. This research carries out an ambitious and holistic controlled laboratory examination of the transport, deposition, and degradation of organic matter in clay-rich sediments. It involves construction of a novel, new flume that allows visualization of transported organo-clay flocs and bed forms and allows for the control of temperature and system redox conditions. The work also involves detailed transmission and electron microscopic studies of sediment and organic matter textural relations, a novel integrated organic matter degradation study, and development and parameterization of theoretical and mathematical models of carbon sequestration and degradation. Broader impacts of the work include support of two researchers in an EPSCoR state (Mississippi), student training, and public outreach through YouTube and the Internet. It is also applicable to increasing our understanding of the dispersal of pollutants by sedimentary processes and may also help the search for unconventional fossil fuel resources that can ease our transition to the next generation of fuels and power sources.
The processes that control deposition and burial of organic carbon are at the heart of the Earth's carbon cycle, atmospheric oxygen levels, and fluctuations of global climate. Roughly half of global organic carbon remineralization occurs in the marine realm, mostly in muddy sediments, and is controlled by a wide range of biological, chemical, and physical processes that operate at multiple time scales and a wide range of physical scales. Biological and chemical aspects of carbon remineralization have been studied in considerable detail, however, the role of physical mechanisms in the deposition, degradation, and preservation of marine sedimentary organic matter are poorly understood. This is because sediment clay fabric and physico-chemistry, the key sources of information about the physical aspects, are not readily quantified as, for example, general bulk chemical and biological parameters, and also because historically it was difficult to delineate and observe fabric features that sequester organic matter in clays. Spatial relationships between organic matter and minerals in marine muds play a critical role with regard to the preservation potential of freshly deposited organic matter. They impact the effectiveness of bacterial enzymes emitted by sediment pore dwelling microbes, the degree to which organic matter is shielded from enzymes, and the rate at which oxygen can diffuse into the sediment. The physical form organic matter arrives at the seabed does matter, and the centerpiece of our study is an exploration of the consequences of recent insights into the nature of organo-clay interaction during hydro-dynamic stresses and mud deposition. Our multi-disciplinary team analyzed sediment samples from a laboratory-based flume that simulated natural conditions under which marine sediment and organic matter might be deposited under controlled conditions. Flume samples of fine-grained sediment were deposited under experimental conditions of dynamic and static modes of deposition and under four combinations of temperature and oxygen tension (warm oxic, cold oxic, warm sub-oxic and cold sub-oxic). Analysis of samples addressed clay fabric morphology and sequestration of organic matter. We tested the broad hypothesis that marine sediment deposited under static conditions would accumulate less organic matter than sediment deposited under dynamic flow conditions. Also tested was the effect of temperature and oxygen levels on the rate of degradation of organic matter. We found that static deposition of marine sediment results in denser fabric (lower porosity) with less organic matter than its dynamic counterpart. Differences in fabric led to speculations about the hierarchical structure of small aggregates within larger aggregates that might characterize depositional dynamics. Many large aggregates were built by attachment of smaller aggregates that formed a complex compound aggregate and the deposits contained considerably more organic matter than their statically-settled counterparts. Understanding the rate of degradation for organic matter is important to support the analysis of flume sample fabric and organic matter sequestration. Our study analyzed degradation of kelp, as representative organic matter, to provide supporting data for interpreting rates of degradation that were explored in the flume experiments. Degradation rate was monitored over a period of nearly two years generating data about changes in total organic carbon (TOC) and bulk density of the degraded product that could be used to interpret fabric morphology and dynamics from the flume experiments. Three-dimensional reconstruction of marine sediment was performed in order to develop a technique for achieving such reconstruction at the nanometer level of sediment clay particle organization. The utility of such reconstructions was demonstrated by the accompanying analysis of tortuosity, a key factor in the distribution of microbial enzymes that degrade organic matter trapped in fine-grained sediment. The technique we developed is the first of which we are aware for creating 3-D reconstructions of clay fabric at the micro-and nanometer level. The three-dimensional reconstruction of marine sediment gave us fresh insight into measures of tortuosity; it demonstrated that measurements taken from 2-D pictures or 3-D pictures yield similar measurements for high porosity, randomly arranged, particles, but that 3-D reconstructions yield far more paths for dynamic flow. Results from this NSF-funded research will provide new perspectives for understanding the underlying mechanisms of oceanic anoxic events, black shale formation, mass extinctions, the late Proterozoic rise in atmospheric oxygen, and the global carbon cycle in general. They will also enhance our ability to model sea-to-air CO2 flux, and help to refine methods of evaluating offshore petroleum source rock potential and the early development of organic-rich stratigraphic sequences. Our research results will benefit a wide range of disciplines, including oceanography, sedimentary geology, petroleum geology, marine benthic microbiology and ecology, organic geochemistry, biogeochemical cycling, hydrocarbon exploration, and coastal & continental shelf geotechnical engineering. The project also served to train two graduate and two undergraduate students that will enhance the infrastructure of research and education. Broad dissemination of results via presentations at scientific meetings, refereed publications, and university web sites is making results accessible to the general public as well as to specialists.
