Photoinduced Evolutions of Permafrost-Derived Carbon in Subarctic Thermokarst Pond Surface Waters
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In subarctic regions, rising temperature and permafrost thaw lead to the formation of thermokarst ponds, where organics from eroding permafrost accumulate. Despite its environmental significance, limited knowledge exists regarding the photosensitivity of permafrost-derived carbon in these ponds. In this study, laboratory experiments were conducted to explore the photochemical transformations of organic matter in surface water samples from thermokarst ponds from different environments in northern Quebec, Canada. One pond near Kuujjuarapik is characterized by the presence of a collapsing palsa and is therefore organically rich, while the other pond near Umiujaq is adjacent to a collapsing lithalsa and thus contains fewer organic matters. Photobleaching occurred in the Umiujaq sample upon irradiation, whereas the Kuujjuarapik sample exhibited an increase in light absorbance at wavelength related to aromatic functionalities, indicating different photochemical aging processes. Ultrahigh-resolution mass spectrometry analysis reveals that the Kuujjuarapik sample preferentially photoproduced highly unsaturated CHO compounds with great aromaticity, while the irradiated Umiujaq sample produced a higher proportion of CHON aromatics with reduced nitrogen functionalities. Overall, this study illustrates that the photochemical reactivity of thermokarst pond water varies with the source of organic matter. The observed differences in reactivity contribute to an improved understanding of the photochemical emission of volatile organic compounds discovered earlier. Further insights into the photoinduced evolutions in thermokarst ponds may require the classification of permafrost-derived carbon therein.Keywords:
Thermokarst
Subarctic climate
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Rapid temperature rise during recent decades (IPCC 2013) is causing permafrost in the Arctic to warm and thaw. This thaw exposes previously frozen soil organic carbon (SOC) to microbial decomposition, generating greenhouse gases methane (CH4) and carbon dioxide (CO2) in a feedback process that leads to further warming and thaw. A growing number of studies model the future permafrost carbon feedback (PCF) to climate warming [Koven et al., 2015, Schneider von Deimling et al., 2015]. However, despite observations of widespread permafrost thaw during recent decades and forecasts of thaw during the next 25-100 years [Koven et al., 2015], no research has quantified the PCF for recent decades. This is in part due to the difficulty of detecting the net movement of old carbon from permafrost to the atmosphere over years and decades amidst large input and output fluxes from ecosystem carbon exchange. In contrast to terrestrial environments, thermokarst lakes provide a direct conduit for processing and emission of old permafrost carbon to the atmosphere, and these emissions are more readily detectable. Results here are based on Walter Anthony et al. [submitted], whereby we quantified the permafrost SOC input to a variety of thermokarst and glacial lakes in Alaska and Siberia in thermokarst zones, defined as areas where land surfaces have transitioned to open lakes due to permafrost thaw during the past 60 years, the historical period most commonly covered by remote-sensing data sets. We also quantified the resulting methane emitted from these active thermokarst lake zones. Using field work, numerical modeling of thaw bulbs, remote sensing and spatial data analysis we will report on the relationship between methane emissions from thermokarst zones and SOC inputs to lakes across gradients of permafrost and climate in Alaska. We will also define the relationship between radiocarbon ages of methane and permafrost soil carbon entering into lakes upon thaw. We will report on the presentday PCF relationship between thaw of permafrost SOC and resulting greenhouse gas release. An extrapolation of our results to the panarctic permafrost region will be presented and compared to permafrost carbon mass balance approaches. The fraction of the terrestrial permafrost carbon pool that has been released as methane from thermokarst along lake margins during the past 60 years will be evaluated relative to early Holocene thermokarst lake emissions and projected permafrost carbon emissions by year 2100. The data will be placed in the context of large regional temperature increases in the Arctic, up to 7.5 °C by 2100, and thicker, organic-rich Holocene-aged deposits subject to thaw and aerobic decomposition as active layer deepens. We will report on the inflection of large permafrost carbon emissions that is imminently expected to occur and whether or not it has commenced. References: Koven, C.D.; Schuur, E.A.G.; Schadel, C.; Bohn, T.J.; Burke, E.J.; Chen, G.; Chen, X.; Ciais, P.; Grosse, G.; Harden, J.W.; Hayes, D.J.; Hugelius, G.; Jafarov, E.E.; Krinner, G.; Kuhry, P.; Lawrence, D.M.; MacDougall, A.H.; Marchenko, S.S.; McGuire, A.D.; Natali, S.M.; Nicolsky, D.J.; Olefeldt, D.; Peng, S.; Romanovsky, V.E.; Schaefer, K.M.; Strauss, J.; Treat, C.C. and Turetsky, M. [2015]: A simplified, data-constrained approach to estimate the permafrost carbon–climate feedback. Trans. R. Soc. A, 373, doi:10.1098/rsta.2014.0423. Schneider von Deimling, T.; Grosse, G.; Strauss, J.; Schirrmeister, L.; Morgenstern, A.; Schaphoff, S.; Meinshausen, M. and Boike, J. [2015]: Observationbased modelling of permafrost carbon fluxes with accounting for deep carbon deposits and thermokarst activity. Biogeosciences, 12(11):3469–3488, doi:10.5194/bg-12-3469-2015. Walter Anthony, K.; Daanen, R.; Anthony, P.; Schneider von Deimling, T.; Ping, C.-L.; Chanton, J. and Grosse, G. [submitted]: Ancient methane emissions from ˜60 years of permafrost thaw in arctic lakes.
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Taking the thermokarst lake by the Hongliang River,a typical thermokarst lake in the permafrost regions of the Tibetan Plateau,as a case,the effect of thermokarst lake on the thermal regimes of soil and permafrost table under and around the lake is studied.The results show that the predominant lateral heat effect of the thermakarst lake changes the thermal regimes of the soils in the lakebed and the lakeshore.As a result,the surrounding soil thermal regime,affected by the thermakarst lake and the surrounding permafrost,is in a dynamic balance process.The thermokarst lake itself has less impact on the subsurface temperature and the permafrost table in the surrounding soils,but the thermal effect of thermokarst lake results in permafrost table rising and ground temperature rising in the margins of lakeshore.Therefore,the thermokarst lake has a great influence on the permafrost temperature and the permafrost depth for the surrounding soils,especially near the margins of lakeshore.
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The thermal influence of a thermokarst lake on permafrost in the Beiluhe Basin of the Qinghai-Tibet Plateau was examined over nearly 10 yr (2006–2014), and lake development involved both downward and lateral heat transfers. Downward heat transfer rapidly thawed 8 m of permafrost beneath the lake bottom center, forming a through talik (i.e., year-round unfrozen ground in permafrost that is open to top and unfrozen layers beneath permafrost) by October 2008. Lateral heat transfer resulted in permafrost temperatures and permafrost table depths at the lakeshore that decreased with distance from the lake. In 2014, the maximum differences in the mean annual ground temperature and permafrost table depth within 75 m of the lake were 0.4 °C and 0.8 m, respectively. The horizontal extent of the talik has expanded gradually from the lake center to the lakeshore. The development of the thermokarst lake on the Qinghai-Tibet Plateau is discussed in terms of four stages, initiation, development, stabilization, and termination, resulting from changes in the surface energy balance.
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Permafrost soils are widespread in the Northern Hemisphere and contain high amounts of carbon (C) and nitrogen (N). Due to the changing climate, permafrost thaws and makes previously frozen C and N available for the active C and N cycles. While additional N might increase plant growth in the nutrient limited tundra, additional C will be decomposed by microbial activity and released through respiration as CO2 or CH4 into the atmosphere. This, in turn, increases the amount of C in the atmosphere, which triggers climate warming and is thus considered a positive feedback loop to climate warming termed the “permafrost-carbon-climate feedback”.
The focus of this dissertation is on deep, ice-rich permafrost deposits, which are prone to rapid degradation through thermokarst processes. Thermokarst involves the thawing of ice-rich permafrost and leads to surface subsidence, lake formation but also lake drainage. Therefore, once thaw is initiated, irreversible changes occur on the landscape and affect C and N that have been freeze-locked for millennia. However, thermokarst processes in permafrost environments are not yet implemented in global permafrost-carbon and climate models, even though they have the potential to degrade and change the surface of a landscape and to mobilize C and N within short time scales.
In this dissertation, I investigated different, rapidly changing permafrost environments in terms of their C and N contents in the top two meters of soil. This includes the yedoma-dominated study sites on Sobo-Sise Island and Bykovsky Peninsula in the north of east Siberia, the thermokarst lake landscape north of Teshekpuk Lake on the Arctic Coastal Plain of northern Alaska, and five different Arctic river deltas in the north and west of Alaska. For this analysis, a total of 77 permafrost soil cores were collected and more than 1,000 samples analyzed for their C and N contents. With a combination of field, laboratory, and remote sensing methods, landscape soil organic C and soil N was characterized. A coherent, continuous sampling collection and analyzing scheme was applied to compare the different study regions and to investigate the overarching research question “How much organic carbon and nitrogen is stored in the top two meters of dynamic thermokarst-affected permafrost environments?”
In particular, the analysis from the yedoma dominated sites in Siberia reveal that soils in drained thermokarst lake basins cannot be generalized. Especially on Sobo-Sise, soils in drained thermokarst lake basins are more depleted in organic C than the Holocene cover layer on top of yedoma deposits. The results of this study indicate a high variability in soil C and N contents within study regions, which is also reflected in a high variability in C accumulation rates. Nevertheless, there are still large amounts of frozen C (13 kg C m-2) potentially available for mobilization with future active layer deepening.
A thermokarst lake landscape north of Teshekpuk Lake on the Arctic Coastal Plain was analyzed by comparing C, N and geochronological characteristics in sediment cores from a primary surface (upland), from a thermokarst lake and from several drained thermokarst lake basins of different lake level stages. This analysis along the thermokarst lake sequence (upland – lake – drained lake) revealed the degradation of organic C through the lake phase but also the accumulation of C and N rich organic matter after lake drainage. Nevertheless, both lacustrine and terrestrial sediments are C and N rich, leading to overall high C landscape contents (~60 kg C m-2 for 0-200 cm) for this thermokarst lake landscape, which is shaped by up to five different stages of lake levels indicating the dynamic nature of this thermokarst landscape over the last 7,000 years.
Also, soils in north Alaskan Arctic river deltas contain a large amount of frozen C and N. In relation to the entire two meter soil profile, there is a considerable amount of C (46%) and N (51%) stored in the 100-200 cm depth interval. This is particularly the case for sandy surface areas, where the first meter of soil is C and N poor. This finding shows the importance to include deeper Arctic river delta deposits in permafrost soil C and N estimations. In addition, radiocarbon dates from the delta sediments reveal coherent carbon and sediment accumulation rates among different sites in the investigated river deltas indicating stable depositional environments for the past 2,000 years.
This dissertation contributes to the circum-arctic C and N estimations by investigating rapidly changing, thermokarst-affected permafrost landscapes. The range of mean landscape C and N storages for these landscapes is between 31.3 and 60.2 kg C m-2 and 2.0 – 4.2 kg N m-2 for 0 200 cm. The consequent sampling down to two meter depth and the high sample resolution within cores allows estimates on the potential availability of C and N as a consequence of future permafrost thawing and active layer deepening which is expected to reach well beyond the standard soil survey depth (100 cm) in many Arctic permafrost regions by the end of the 21st Century. This assessment is complemented by extensive radiocarbon dating, which allows new insights into carbon and sediment characteristics for rapidly changing, thermokarst-affected permafrost landscapes. The high amounts of C and N stored in these deposits reveal the climate warming potential and urgent need to include thermokarst-affected permafrost terrain in future permafrost-climate models.
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In permafrost regions of the Qinghai-Tibet Plateau, road disaster caused by permafrost degradation cannot be ignored. As a common thermal disaster in permafrost regions, thermokarst lake has serious thermal erosion on permafrost and results in permafrost degradation aggravating. This study focused on two subgrade cross-sections of Gonghe-Yushu Highway in the Qinghai-Tibet Plateau to analyze thermal effect of thermokarst lake on the permafrost under embankment. The analysis infers that thermokarst lake can transfer heat to permafrost under the embankment, as a heat resource, and the heat flux decreases with the distance away from thermokarst lake in horizontal and vertical direction. Thermokarst lake can cause average ground temperature of permafrost under the embankment increasing, and with less distance from the thermokarst lake the temperature increases more severely. Thermokarst lake results in 14 m thickness melting interlayer in soil under lake and change shape of melting area.
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Abstract Both the inflow and outflow of supra‐permafrost water to lakes play important roles in the hydrologic process of thermokarst lakes. The accompanying thermal effects on the adjacent permafrost are required for assessing their influences on the development of thermokarst lakes. For these purposes, the lake water level, temperature dynamics, and supra‐permafrost water flow of a lake were monitored on the Qinghai‐Tibet Plateau. In addition, the spatial and temporal variation of the active layer thickness and permafrost distribution around the lake were investigated by combining ground penetrating radar, electrical resistivity tomography, and borehole temperature monitoring. The results revealed that the yearly unfrozen supra‐permafrost water flow around the lake lasted approximately 5 months. The temperature and water level measurements during this period indicate that the lake water was recharged by relatively colder supra‐permafrost water from the north‐western lakeshore and was discharged through the eastern lakeshore. This process, accompanied by heat exchange with the underlying permafrost, might cause a directional difference of the active layer thickness and permafrost characteristics around the lake. Specifically, the active layer thickness variation was minimal, and the ice‐rich permafrost was found adjacent to the lakeshore along the recharge groundwater pathways, whereas a deeper active layer and ice‐poor permafrost were observed close to the lakeshore from which the warm lake water was discharged. This study suggests that the lateral flow of warm lake water can be a major driver for the rapid expansion of thermokarst lakes and provides clues for evaluating the relationships between the thermokarst expansion process and climate warming.
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