Advanced Inverse Modeling of Sediment Thermal Diffusion Processes : Reconstructing Temporal Variant Boundary Conditions for the One-Dimensional Heat Equation

Temperatures in marine sediments are driven by the geothermal heat flow from the Earth's crust and the evolution of the bottom water temperature. Mathematically, the temperature field can be modeled with the heat equation, a Robin boundary condition at the sediment-water interface, and a Neumann condition at the lower boundary. Given the thermal properties of the sediment and a model for the bottom water temperature function the forward problem is well-posed. The inverse problem, i.e. reconstructing the bottom water temperature function from measurements of the sediment temperature, on the other hand is ill-posed; the parameterized model is non-linear but low-dimensional. Different Newton-linke methods, as well as a linear fitting approach with Tikhonov minimization, and a Markov Chain Monte Carlo method are shown and their performances for this problem are compared. The algorithms work differently well on this problem, and regularising methods are not necessarily better. The heuristic linear fitting has the best accuracy in reasonable computing time, while the Markov Chain Monte Carlo method has proven convergence for enlarging ensembles

In-situ measurements of sediment temperature under shallow water bodies in Arctic environments

The thermal regime under lakes, ponds, and shallow near shore zones in permafrost zones in the Arctic is predominantly determined by the temperature of the overlying water body throughout the year. Where the temperatures of the water are warmer than the air, unfrozen zones within the permafrost, called taliks, can form below the water bodies. However, the presence of bottom-fast ice can decrease the mean annual bed temperature in shallow water bodies and significantly slow down the thawing or even refreeze the lake or sea bed in winter. Small changes in water level have the potential to drastically alter the sub-bed thermal regime between permafrost-thawing and permafrost-forming. The temperature regime of lake sediments is a determining factor in the microbial activity that makes their taliks hot spots of methane gas emission. Measurements of the sediment temperature below shallow water bodies are scarce, and single temperature-chains in boreholes are not sufficient to map spatial variability. We present a new device to measure in-situ temperature-depth profiles in saturated soils or sediments, adapting the functionality of classic Lister-type heat flow probes to the special requirements of the Arctic. The measurement setup consists of 30 equally spaced (5cm) digital temperature sensors housed in a 1.5 m stainless steel lance. The lance is portable and can be pushed into the sediment by hand either from a wading position, a small boat or through a hole in the ice during the winter. Measurements are taken continuously and 15 minutes in the sediment are sufficient to acquire in-situ temperatures within the accuracy of the sensors (0.01K after calibration at 0°C). The spacing of the sensors yield a detailed temperature-depth-profile of the near-surface sediments, where small-scale changes in the bottom water changes dominate the temperature field of the sediment. The short time needed for a single measurement allows for fine-meshed surveys of the sediment in areas of interest, such as the transition zone from bottom-fast to free water. Test campaigns in the Canadian Arctic and on Svalbard have proven the device to be robust in a range of environments. We present data acquired during winter and summer, covering non-permafrost, thermokarst lake and offshore measurements

In-situ measurements of sediment temperature under shallow water bodies in Arctic environments

The thermal regime under lakes, ponds, and shallow near shore zones in permafrost zones in the Arctic is predominantly determined by the temperature of the overlying water body throughout the year. Where the temperatures of the water are warmer than the air, unfrozen zones within the permafrost, called taliks, can form below the water bodies. However, the presence of bottom-fast ice can decrease the mean annual bed temperature in shallow water bodies and significantly slow down the thawing or even refreeze the lake or sea bed in winter. Small changes in water level have the potential to drastically alter the sub-bed thermal regime between permafrost-thawing and permafrost-forming. The temperature regime of lake sediments is a determining factor in the microbial activity that makes their taliks hot spots of methane gas emission. Measurements of the sediment temperature below shallow water bodies are scarce, and single temperature-chains in boreholes are not sufficient to map spatial variability. We present a new device to measure in-situ temperature-depth profiles in saturated soils or sediments, adapting the functionality of classic Lister-type heat flow probes to the special requirements of the Arctic. The measurement setup consists of 30 equally spaced (5cm) digital temperature sensors housed in a 1.5 m stainless steel lance. The lance is portable and can be pushed into the sediment by hand either from a wading position, a small boat or through a hole in the ice during the winter. Measurements are taken continuously and 15 minutes in the sediment are sufficient to acquire in-situ temperatures within the accuracy of the sensors (0.01K after calibration at 0°C). The spacing of the sensors yield a detailed temperature-depth-profile of the near-surface sediments, where small-scale changes in the bottom water changes dominate the temperature field of the sediment. The short time needed for a single measurement allows for fine-meshed surveys of the sediment in areas of interest, such as the transition zone from bottom-fast to free water. Test campaigns in the Canadian Arctic and on Svalbard have proven the device to be robust in a range of environments. We present data acquired during winter and summer, covering non-permafrost, thermokarst lake and offshore measurements

Glacial isostatic adjustment reduces past and future Arctic subsea permafrost

Sea-level rise submerges terrestrial permafrost in the Arctic, turning it into subsea permafrost. Subsea permafrost underlies ~ 1.8 million km2 of Arctic continental shelf, with thicknesses in places exceeding 700 m. Sea-level variations over glacial-interglacial cycles control subsea permafrost distribution and thickness, yet no permafrost model has accounted for glacial isostatic adjustment (GIA), which deviates local sea level from the global mean due to changes in ice and ocean loading. Here we incorporate GIA into a pan-Arctic model of subsea permafrost over the last 400,000 years. Including GIA significantly reduces present-day subsea permafrost thickness, chiefly because of hydro-isostatic effects as well as deformation related to Northern Hemisphere ice sheets. Additionally, we extend the simulation 1000 years into the future for emissions scenarios outlined in the Intergovernmental Panel on Climate Change’s sixth assessment report. We find that subsea permafrost is preserved under a low emissions scenario but mostly disappears under a high emissions scenario

The Expedition to the Peel River in 2019: Fluvial Transport Across a Permafrost Landscape

The Expedition to the Peel River in 2019: Fluvial Transport Across a Permafrost Landscape EU Horizon2020 project, Nunataryuk. Consisting of 7 legs from 16 June 2019 - 11 August 2019

Serpentine (Floating) Ice Channels and their Interaction with Riverbed Permafrost in the Lena River Delta, Russia

Arctic deltas and their river channels are characterized by three components of the cryosphere: snow, river ice, and permafrost, making them especially sensitive to ongoing climate change. Thinning river ice and rising river water temperatures may affect the thermal state of permafrost beneath the riverbed, with consequences for delta hydrology, erosion, and sediment transport. In this study, we use optical and radar remote sensing to map ice frozen to the riverbed (bedfast ice) vs. ice, resting on top of the unfrozen water layer (floating or so-called serpentine ice) within the Arctic’s largest delta, the Lena River Delta. The optical data is used to differentiate elevated floating ice from bedfast ice, which is flooded ice during the spring melt, while radar data is used to differentiate floating from bedfast ice during the winter months. We use numerical modeling and geophysical field surveys to investigate the temperature field and sediment properties beneath the riverbed. Our results show that the serpentine ice identified with both types of remote sensing spatially coincides with the location of thawed riverbed sediment observed with in situ geoelectrical measurements and as simulated with the thermal model. Besides insight into sub-river thermal properties, our study shows the potential of remote sensing for identifying river channels with active sub-ice flow during winter vs. channels, presumably disconnected for winter water flow. Furthermore, our results provide viable information for the summer navigation for shallow-draught vessels

Organic carbon in subsea permafrost: a globally significant but inert carbon pool Frederieke Miesner

Subsea permafrost underlays 2.4 million km2 of the Arctic Shelf, an area equaling ~18% of the terrestrial permafrost region. Most of it was inundated at some point after the last glacial maximum and is in an advanced state of warming. How much organic carbon (OC) accumulated, how this carbon pool was affected by permafrost presence and degradation over time, how much carbon still remains today and how much of it may be mobilized are major unknowns in the global carbon cycle. Recent estimates of OC decomposition from thawing submarine permafrost were as high as 8 Tg OC per year in methane alone. Here, we combine a numerical model of sedimentation and permafrost evolution with simplified carbon turnover to estimate accumulation and microbial decomposition of organic matter on the pan-Arctic shelf over the past four glacial cycles (450 kyr). Organic carbon decomposition is modeled with a reactivity continuum model using inversely determined parameters from incubation experiments and liquid water content within the permafrost as the limiting factor rather than temperature alone. We find that Arctic shelf permafrost is a long-term carbon sink storing 2822 (1518 - 4982) Pg OC, two to four times the amount stored in lowland permafrost. Although subsea permafrost is currently thawing, prior microbial decomposition and organic matter aging would limit decomposition rates to less than 48 Tg OC per year even if all frozen sediment deposited in the past 450 kyr thawed immediately. Since actual thaw rates are orders of magnitude lower, true emissions due to subsea permafrost thaw are also orders of magnitude lower than this. The OC pool in shelf permafrost is therefore largely immobilized. Compared to the organic matter in thawing permafrost large emissions are more likely derived from older and deeper sources as shelf’s frozen lid, the permafrost, becomes more permeable

Submarine Permafrost as a Long-term Late Quaternary Carbon Sink

Organic carbon (OC) stored in Arctic continental shelf sediment is a climate-sensitive but poorly quantified component of the global carbon cycle. The current interglacial period means that most shelf permafrost, along with its OC, is currently warmer than -2 °C, and therefore susceptible to small additional warming in the near future. Estimating how much OC is potentially stored in subsea permafrost is thus key to a quantitative understanding of potential impacts of permafrost thaw on carbon mobilization in a warming Arctic. We developed a process-based model of permafrost distribution and organic matter (OM) sedimentation and decomposition to estimate the contribution of submarine permafrost to Arctic shelf organic carbon stocks. Driven by Earth System Model forcing, our model calculates 1D heat flow below the earth surface, ice caps and sea bed, and uses a reactivity continuum model of OM decomposition. We restrict our modeling to sediment that was buried within the last four glacial cycles (450 kyr), and therefore neglect OC stocks deeper than about 100 m, including any gas hydrates. Restricting OM decomposition to the liquid habitat for microbial activity in the sediment, we estimated that permafrost below the Arctic Shelf stores at least as much OC as the terrestrial counterpart at pre-industrial time, and probably in the range of twice to three times as much OC. We compared the effect of varying the OC sedimentation rates and OC reactivity. Higher reactivity in marine sediments combined with lower ice contents to increase the rate of OM decomposition, relative to sediment deposited in terrestrial settings. As a result, permafrost in our model preserved a greater proportion of marine OM from decomposition while having little effect (< 5%) on the amount of recalcitrant terrestrial OC. These differences in sedimentation rate and reactivity influence the distribution of OC preservation on the Arctic shelf. Our modeling shows that subsea permafrost is a relevant OC stock and that more research is needed to understand microbial OM decomposition in cold but not necessarily frozen sediments. Given that deeper deposits and gas hydrates are not included, we provide conservative estimates of Arctic shelf OC stocks and suggest that the shelves have acted as long-term carbon sinks over multiple glacial--interglacial cycles

Submarine permafrost map in the arctic modeled using 1-D transient heat flux (SuPerMAP)

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Oceans 124(6), (2019): 3490-3507, doi:10.1029/2018JC014675.Offshore permafrost plays a role in the global climate system, but observations of permafrost thickness, state, and composition are limited to specific regions. The current global permafrost map shows potential offshore permafrost distribution based on bathymetry and global sea level rise. As a first‐order estimate, we employ a heat transfer model to calculate the subsurface temperature field. Our model uses dynamic upper boundary conditions that synthesize Earth System Model air temperature, ice mass distribution and thickness, and global sea level reconstruction and applies globally distributed geothermal heat flux as a lower boundary condition. Sea level reconstruction accounts for differences between marine and terrestrial sedimentation history. Sediment composition and pore water salinity are integrated in the model. Model runs for 450 ka for cross‐shelf transects were used to initialize the model for circumarctic modeling for the past 50 ka. Preindustrial submarine permafrost (i.e., cryotic sediment), modeled at 12.5‐km spatial resolution, lies beneath almost 2.5 ×106km2 of the Arctic shelf. Our simple modeling approach results in estimates of distribution of cryotic sediment that are similar to the current global map and recent seismically delineated permafrost distributions for the Beaufort and Kara seas, suggesting that sea level is a first‐order determinant for submarine permafrost distribution. Ice content and sediment thermal conductivity are also important for determining rates of permafrost thickness change. The model provides a consistent circumarctic approach to map submarine permafrost and to estimate the dynamics of permafrost in the past.Boundary condition data are available online via the sources referenced in the manuscript. This work was partially funded by a Helmholtz Association of Research Centres (HGF) Joint Russian‐German Research Group (HGF JRG 100). This study is part of a project that has received funding from the European Unions Horizon 2020 research and innovation program under grant agreement 773421. Submarine permafrost studies in the Kara and Laptev Seas were supported by Russian Foundation for Basic Research (RFBR/RFFI) grants 18‐05‐60004 and 18‐05‐70091, respectively. The International Permafrost Association (IPA) and the Association for Polar Early Career Scientists (APECS) supported research coordination that led to this study. We acknowledge coordination support of the World Climate Research Programme (WCRP) through their core project on Climate and Cryosphere (CliC). Thanks to Martin Jakobsson for providing a digitized version of the preliminary IHO delineation of the Arctic seas and to Guy Masters for access to the observational geothermal database. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.2019-10-1

Inverse Modelierung von Thermischer Diffusion in Marinen Sedimenten : Rekonstruktion von Zeitabhängigen Randbedingungen der Eindimensionalen Wärmeleitungsgleichung

Temperatures in marine sediments are driven by the geothermal heat flow from the Earth's crust and the evolution of the bottom water temperature. Mathematically, the temperature field can be modeled with the heat equation, a Robin boundary condition at the sediment-water interface, and a Neumann condition at the lower boundary. Given the thermal properties of the sediment and a model for the bottom water temperature function the forward problem is well-posed. The inverse problem, i.e. reconstructing the bottom water temperature function from measurements of the sediment temperature, on the other hand is ill-posed; the parameterized model is non-linear but low-dimensional. Different Newton-linke methods, as well as a linear fitting approach with Tikhonov minimization, and a Markov Chain Monte Carlo method are shown and their performances for this problem are compared. The algorithms work differently well on this problem, and regularising methods are not necessarily better. The heuristic linear fitting has the best accuracy in reasonable computing time, while the Markov Chain Monte Carlo method has proven convergence for enlarging ensembles