Collaborative Research: An Interdisciplinary Investigation of Groundwater-Carbon Coupling in Large Peat Basins and its Relation to Climate Change

The growth of northern peatlands during the Holocene created a globally important source and sink for greenhouse gases. The response of these large carbon reservoirs to Global Warming, however, remains uncertain. Different mathematical models predict that future warming could alter the carbon balance of peatlands by either increasing the rate of carbon sequestration or accelerating the emissions of greenhouse gases. However, these steady-state analytical models make unrealistic assumptions about natural peatlands, which are spatially and temporally variable ecosystems that are still accumulating carbon. This investigation will therefore develop a transient 3D numerical model that couples multiphase groundwater flow to solute transport, organic matter reactivity, and peat accumulation. The need for such a model is supported by our investigations in large peat basins over the past 25 years that demonstrate the close linkages among climate, groundwater, landscape, and peatland carbon fluxes. This new transient groundwater-peat accumulation model will be calibrated by multiple sets of field, lab, and remote sensing data collected at a range of scales. A sensitivity analysis of this calibrated model should then provide reliable predictions for the response of large peat basins to climate change at the regional level.This approach can best be tested in the Glacial Lake Agassiz Peatlands (GLAP) of northern Minnesota where a regional peat basin developed despite the relatively dry climate and periodic droughts. Recent advances in remote sensing, field instrumentation, geophysical exploration, and computational modeling will be used to develop and calibrate a transient coupled groundwater-peat accumulation model for the GLAP region. This interdisciplinary investigation will focus on several important problems including carbon cycling in the deeper peat and the hydraulics of a deformable media. This study will also determine if methane fluxes from large peatlands are dominated by ebullition (i.e., bubbling) from deep overpressured gas pockets representing a globally important and previously unaccounted for source of atmospheric methane. In addition, this investigation will evaluate whether the configuration of the regional river system amplifies the effects of climate change on peatland ecosystems.Broader Impacts: The broader impacts of this study will be extended by: 1) training graduate and undergraduate students in a large interdisciplinary investigation, 2) providing outreach to K-12 schools and the general public, 3) developing 3D computer visualization exhibits for state parks and other interpretive centers, and 4) linking our research results to regional and national educational programs organized by Morin. The visualization exhibits will be specifically designed for a new $1.6 million interpretive center at the Big Bog Recreational Area, which was recently established by the State of Minnesota adjacent to our field area. This exhibit will link results from our field measurements to the role of peatlands in the global carbon cycle for the general public. It will continue the long tradition of close cooperation between the GLAP research group and the officials that manage public land in northern Minnesota and elsewhere

Collaborative research: Geophysical evaluation of biogenic gasses in peatlands

Biogenic gas emission from northern peatlands, by wicking from vascular plants and by episodic ebullition events, accounts for approximately 7% of the global annual emission of methane to the atmosphere. This proposal involves experiments to apply ground penetrating radar (GPR) for (1) estimating the amount of biogenic gas stored in peatlands, (2) determining the spatial distribution of biogenic gas within the peat, and (3) monitoring biogenic gas release to the atmosphere. Data from a large northern peatland in Maine (EAR-0242353) show that (1) higher CH4 and CO2 concentrations correlate with high velocity/high attenuation zones in cross-borehole GPR data as well as shadow zones (loss of reflections) in surface GPR data, (2) shadow zones (indicative of high gas content) are frequently observed in the +11 km of surface GPR data collected in this peatland. The experimental objectives are: (1) a laboratory evaluation of the relationship between dielectric permittivity and gas content for a profile of peat cores from the surface to the mineral soil; (2) a cross-hole GPR and surface GPR monitoring experiment, supported by measurements of water levels, hydraulic conductivity and time domain reflectometry, to observe rates of biogenic gas release to the atmosphere; (3) a surface GPR study, supported by in situ measurements of biogenic gas concentration, to estimate the volume of biogenic gas stored in two well studied northern peatlands (Caribou Bog, ME and Glacial Lake Agassiz, MN). Important milestones include (a) a predictive equation for gas content estimation in peat as a function of depth from dielectric permittivity measurements, (b) new insight into the temporal pattern of gas release and ebullition flux from peatlands, and (c) new estimates of the free gas content of peatland carbon reservoirs accounting for the spatial/depth distribution of the gas. Broader Impacts This proposal incorporates educational activities, curriculum development, community outreach and international collaboration within an applied research framework. Honors UnderGraduate (HUG) researchers from the Rutgers-Newark Honors College (HC) will partner with a postdoctoral scientist to obtain the research training required to complete four of the primary research tasks. Each HUG will complete a yearlong senior project on their research and contribute to a publication. The status of Rutgers-Newark as the topranked National US University with respect to campus UG diversity facilitates HUG opportunities to minority students. Students in Earth/Environmental Sciences at Rutgers- Newark frequently express interest in fieldwork experiences. Part of the fieldwork will be conducted by students participating in a new class, Summer Field Camp in Applied Geophysics, developed as part of this project. Community outreach will occur via guided tours, poster boards and presentations on the hydrology, ecology and carbon cycle in Caribou Bog, facilitated by the recently opened 2 km long Orono boardwalk that now provides public access to this bog. Finally, collaboration with a prominent peatland scientist in Europe will draw international attention to our work and provide an opportunity to conduct comparative work on a unique European peat bog

Collaborative Research: Integrated Geophysical and Hydrogeologic Study of a Large Maine Peatland

Solute transport controls vegetation and water chemistry gradients in peatlands. Dispersive mixing and advective transport in peat will be measured in laboratory column experiments and in a natural gradient tracer test in a peatland to determine the relative importance of these processes. We will assess the retardation of solutes by matrix diffusion and the applicability of a dual-domain model. Electrical geophysical methods, verified through direct measurements, will be used to track tracer migration.An extensive geophysical and hydrogeologic characterization of the peatland will map variability in peat depth across the basin and identify stratigraphy. Ground-penetrating radar and resistivity/induced polarization imaging will be employed. Piezometer tests will provide a measure of the spatial variability in hydraulic conductivity within Caribou Bog, a large peatland in Central Maine. Correlations between hydrogeologic and geophysical parameters will be assessed and used to provide constraints on parameters for ground-water flow and mass transport modeling.Peat cores will be collected for laboratory tests to measure hydraulic conductivity, dispersion, effective porosity, specific surface and complex resistivity. The variability of peat properties with depth will be determined through laboratory testing. Relationships between peat electrical properties and hydrogeologic parameters will be evaluated.An NaBr tracer will be injected into the peatland and monitored for 12 months. The tracer will be tracked using surface and borehole electrical imaging. The pixels on the geophysical images will be a substitute for extensive direct water sampling points, allowing rapid and nearly continuous tracking of solute distribution. Geophysical and ground-water chemistry data collected from the tracer test will be used to calculate the spatial moments of the solute plume through time. Three-dimensional hydrogeologic and mass transport models will be calibrated to the geochemical and geophysical data to further evaluate mass transport parameters. Matrix diffusion, the migration of solutes into hydraulically isolated pores, will be incorporated into the numerical models to evaluate this important process.The project will evaluate: (1) the nature of dispersive mixing, (2) the correlation of hydrogeologic and geophysical parameters, and (3) the role of mass transporft in peatlands. Peatlands are a large carbon reservoir and a significant source of methane gas. Ground-water flow and mass transport are important in regulating geochemical conditions favorable for methane production and peat accumulation. The work will add to the understanding of processes that impact carbon and nutrient dynamics for peatlands. Geophysical monitoring of a tracer will provide important information on the utility of this non-invasive method for tracking the movement of solute plumes. This project will demonstrate the potential benefits of including electrical geophysics in hydrogeologic assessments and wetland characterization

SpinLink: An interconnection system for the SpiNNaker biologically inspired multi-computer

SpiNNaker is a large-scale biologically-inspired multi-computer designed to model very heavily distributed problems, with the flagship application being the simulation of large neural networks. The project goal is to have one million processors included in a single machine, which consequently span many thousands of circuit boards. A computer of this scale imposes large communication requirements between these boards, and requires an extensible method of connecting to external equipment such as sensors, actuators and visualisation systems. This paper describes two systems that can address each of these problems.Firstly, SpinLink is a proposed method of connecting the SpiNNaker boards by using time-division multiplexing (TDM) to allow eight SpiNNaker links to run at maximum bandwidth between two boards. SpinLink will be deployed on Spartan-6 FPGAs and uses a locally generated clock that can be paused while the asynchronous links from SpiNNaker are sending data, thus ensuring a fast and glitch-free response. Secondly, SpiNNterceptor is a separate system, currently in the early stages of design, that will build upon SpinLink to address the important external I/O issues faced by SpiNNaker. Specifically, spare resources in the FPGAs will be used to implement the debugging and I/O interfacing features of SpiNNterceptor

Investigating Peatland Stratigraphy and Hydrogeology Using Integrated Electrical Geophysics

Hydrology has been suggested as the mechanism controlling vegetation and related surficial pore-water chemistry in large peatlands. Peatland hydrology influences the carbon dynamics within these large carbon reservoirs and will influence their response to global warming. A geophysical survey was completed in Caribou Bog, a large peatland in Maine, to evaluate peatland stratigraphy and hydrology. Geophysical measurements were integrated with direct measurements of peat stratigraphy from probing, fluid chemistry, and vegetation patterns in the peatland. Consistent with previous field studies, ground-penetrating radar (GPR) was an excellent method for delineating peatland stratigraphy. Prominent reflectors from the peat-lake sediment and lake sediment-mineral soil contacts were precisely recorded up to 8 m deep. Two-dimensional resistivity and induced polarization imaging were used to investigate stratigraphy beneath the mineral soil, beyond the range of GPR. We observe that the peat is chargeable, and that IP imaging is an alternative method for defining peat thickness. The chargeability of peat is attributed to the high surface-charge density on partially decomposed organic matter. The electrical conductivity imaging resolved glaciomarine sediment thickness (a confining layer) and its variability across the basin. Comparison of the bulk conductivity images with peatland vegetation revealed a correlation between confining layer thickness and dominant vegetation type, suggesting that stratigraphy exerts a control on hydrogeology and vegetation distribution within this peatland. Terrain conductivity measured with a Geonics EM31 meter correlated with confining glaciomarine sediment thickness and was an effective method for estimating variability in glaciomarine sediment thickness over approximately 18 km(2). Our understanding of the hydrogeology, stratigraphy, and controls on vegetation growth in this peatland was much enhanced from the geophysical study

Dynamics of methane ebullition from a peat monolith revealed from a dynamic flux chamber system

Methane (CH4) ebullition in northern peatlands is poorly quantified in part due to its high spatiotemporal variability. In this study, a dynamic flux chamber (DFC) system was used to continuously measure CH4 fluxes from a monolith of near‐surface Sphagnum peat at the laboratory scale to understand the complex behavior of CH4 ebullition. Coincident transmission ground penetrating radar measurements of gas content were also acquired at three depths within the monolith. A graphical method was developed to separate diffusion, steady ebullition, and episodic ebullition fluxes from the total CH4 flux recorded and to identify the timing and CH4 content of individual ebullition events. The results show that the application of the DFC had minimal disturbance on air‐peat CH4 exchange and estimated ebullition fluxes were not sensitive to the uncertainties associated with the graphical model. Steady and episodic ebullition fluxes were estimated to be averagely 36 ± 24% and 38 ± 24% of the total fluxes over the study period, respectively. The coupling between episodic CH4 ebullition and gas content within the three layers supports the existence of a threshold gas content regulating CH4 ebullition. However, the threshold at which active ebullition commenced varied between peat layers with a larger threshold (0.14 m3 m−3) observed in the deeper layers, suggesting that the peat physical structure controls gas bubble dynamics in peat. Temperature variation (23°C to 27°C) was likely only responsible for small episodic ebullition events from the upper peat layer, while large ebullition events from the deeper layers were most likely triggered by drops in atmospheric pressure

In Situ Monitoring of Free-Phase Gas Accumulation and Release in Peatlands Using Ground Penetrating Radar (GPR)

We tested a set of surface common mid-point (CMP) ground penetrating radar (GPR) surveys combined with elevation rods ( to monitor surface deformation) and gas flux measurements to investigate in-situ biogenic gas dynamics and ebullition events in a northern peatland ( raised bog). The main findings are: ( 1) changes in the two-way travel time from the surface to prominent reflectors allow estimation of average gas contents and evolution of free-phase gas (FPG); ( 2) peat surface deformation and gas flux measurements are strongly consistent with GPR estimated changes in FPG content over time; ( 3) rapid decreases in atmospheric pressure are associated with increased gas flux; and ( 4) single ebullition events can induce releases of methane much larger ( up to 192 g/m(2)) than fluxes reported by others. These results indicate that GPR is a useful tool for assessing the spatial distribution, temporal variation, and volume of biogenic gas deposits in peatlands

Geophysical and Hydrological Evaluation of Two Bog Complexes in a Northern Peatland: Implications for the Distribution of Biogenic Gases at the Basin Scale

Ground penetrating radar (GPR) was used to determine peat basin geometry and the spatial distribution of free-phase biogenic gasses in two separate units of a northern peatland (Central and Southern Unit of Caribou Bog, Maine). The Central Unit is characterized by a deep basin structure (15 m maximum depth) and a raised (eccentric) bog topographic profile (up to 2 m topographic variation). Here numerous regions of electromagnetic (EM) wave scattering are considered diagnostic of the presence of extensive free-phase biogenic gas. In contrast, the Southern Unit is shallower (8 m maximum depth) and has a slightly convex upwards bog profile (less than 1 m topographic variation), and areas of EM wave scattering are notably absent. The biogenic gas zones interpreted from GPR in the Central Unit are associated with: (1) wooded heath vegetation at the surface, (2) open pools at the surface, (3) high water table elevations near the center of the basin, and (4) a region of overpressure (at approximately 5 m depth) immediately below the zone of free-phase gas accumulation. The latter suggests (1) a transient pressure head associated with low hydraulic conductivity resulting from the biogenic gasses themselves or confining layers in the peat that restrict both gas release and groundwater flow and/or (2) overpressure in the peat column as a result of the gas buildup itself. In contrast, the Southern Unit, where zones of EM scattering are absent, is characterized by: (1) predominantly shrub vegetation, (2) a lack of open pools, (3) only minor variations (less than 1 m) in water table elevation throughout the entire unit; and (4) generally upward groundwater flow throughout the basin. The results illustrate the nonuniformity of free-phase biogenic gas distribution at the peat basin scale and provide insights into the processes and controls associated with CH4 and CO2 accumulation in peatlands

Spatial Variability in Biogenic Gas Accumulations in Peat Soils Is Revealed By Ground Penetrating Radar (GPR)

We performed surface and borehole ground penetrating radar (GPR) tests, together with moisture probe measurements and direct gas sampling to detect areas of biogenic gas accumulation in a northern peatland. The main findings are: (1) shadow zones (signal scattering) observed in surface GPR correlate with areas of elevated CH4 and CO2 concentration; (2) high velocities in zero offset profiles and lower water content inferred from moisture probes correlate with surface GPR shadow zones; (3) zero offset profiles depict depth variable gas accumulation from 0-10% by volume; (4) strong reflectors may represent confining layers restricting upward gas migration. Our results have implications for defining the spatial distribution, volume and movement of biogenic gas in peatlands at multiple scales