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Roles of Thermokarst Lakes in a Warming World

2020; Elsevier BV; Volume: 28; Issue: 9 Linguagem: Inglês

10.1016/j.tim.2020.04.002

1878-4380

Michiel H. in ’t Zandt, Susanne Liebner, Cornelia U. Welte,

Geology and Paleoclimatology Research

Thermokarst lakes form as a result of permafrost thaw in predominantly ice-rich yedoma deposits and are therefore an ecosystem that is rapidly expanding with the onset of climate change.Thermokarst lakes are net greenhouse gas sources as century-old carbon deposits become bioavailable and are mineralized to CO2 and CH4.CH4 is a more potent greenhouse gas than CO2. Changed dynamics will therefore disproportionately affect global warming.Methane emissions are the net result of methane production by methanogenesis and methane oxidation by aerobic bacteria or anaerobic archaea, with high levels of heterogeneity and intricate interactions.Future climate change will have disproportionate effects on the Arctic, which implies potentially strong consequences for future greenhouse gas fluxes and thaw progression. Permafrost covers a quarter of the northern hemisphere land surface and contains twice the amount of carbon that is currently present in the atmosphere. Future climate change is expected to reduce its near-surface cover by over 90% by the end of the 21st century, leading to thermokarst lake formation. Thermokarst lakes are point sources of carbon dioxide and methane which release long-term carbon stocks into the atmosphere, thereby initiating a positive climate feedback potentially contributing up to a 0.39°C rise of surface air temperatures by 2300. This review describes the potential role of thermokarst lakes in a warming world and the microbial mechanisms that underlie their contributions to the global greenhouse gas budget. Permafrost covers a quarter of the northern hemisphere land surface and contains twice the amount of carbon that is currently present in the atmosphere. Future climate change is expected to reduce its near-surface cover by over 90% by the end of the 21st century, leading to thermokarst lake formation. Thermokarst lakes are point sources of carbon dioxide and methane which release long-term carbon stocks into the atmosphere, thereby initiating a positive climate feedback potentially contributing up to a 0.39°C rise of surface air temperatures by 2300. This review describes the potential role of thermokarst lakes in a warming world and the microbial mechanisms that underlie their contributions to the global greenhouse gas budget. Permafrost covers a quarter of the northern hemisphere land surface [1.Gruber S. Derivation and analysis of a high-resolution estimate of global permafrost zonation.Cryosphere. 2012; 6: 221-233Crossref Scopus (171) Google Scholar,2.Camill P. permafrost thaw accelerates in boreal peatlands during late-20th century climate warming.Clim. Change. 2005; 68: 135-152Crossref Scopus (131) Google Scholar]. Its carbon (C) pools are estimated at 1300 Pg C (range 1100–1500 Pg C) which equals twice the amount of carbon that is present in the atmosphere [3.Hugelius G. et al.Estimated stocks of circumpolar permafrost carbon with quantified uncertainty ranges and identified data gaps.Biogeosciences. 2014; 11: 6573-6593Crossref Scopus (489) Google Scholar,4.Schuur E.A.G. et al.The effect of permafrost thaw on old carbon release and net carbon exchange from tundra.Nature. 2009; 459: 556-559Crossref PubMed Scopus (0) Google Scholar]. The major fraction, about 1000 Pg, is present in the near surface (the upper 3 m) that is vulnerable to warming [3.Hugelius G. et al.Estimated stocks of circumpolar permafrost carbon with quantified uncertainty ranges and identified data gaps.Biogeosciences. 2014; 11: 6573-6593Crossref Scopus (489) Google Scholar]. 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One consequence of a warming Arctic and permafrost thaw is the formation of thermokarst (see Glossary) landscapes. These landscapes are associated with pingos, thermokarst troughs and pits that can collapse to form thermokarst lakes [18.Kokelj S.V. Jorgenson M.T. Advances in thermokarst research.Permafr. Periglac. Process. 2013; 24: 108-119Crossref Scopus (136) Google Scholar]. GHG emissions from these lakes can have disproportionate climate effects due to the rapid release of long-term stored carbon into the atmosphere, which initiates a strong positive climate feedback [17.Schaefer K. et al.Amount and timing of permafrost carbon release in response to climate warming.Tellus B. 2011; 63: 165-180Crossref Scopus (266) Google Scholar,19.Walter Anthony K.M. et al.Methane bubbling from northern lakes: present and future contributions to the global methane budget.Philos. Trans. R. Soc. A Math. Phys. Eng. 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About 40% (500 Pg) of the permafrost carbon stocks are found in ice-rich Yedoma deposits which have an organic carbon content ranging from 2% to 5% [23.Strauss J. et al.Deep Yedoma permafrost: a synthesis of depositional characteristics and carbon vulnerability.Earth-Science Rev. 2017; 172: 75-86Crossref Scopus (41) Google Scholar,24.Zimov S.A. et al.Permafrost and the global carbon budget.Science. 2006; 312: 1612-1613Crossref PubMed Scopus (612) Google Scholar]. These deposits are found in Alaska and Siberia, and they originate from the late Pleistocene [25.Schirrmeister L. et al.Yedoma: late Pleistocene ice-rich syngenetic permafrost of Beringia.in: Elias S. Encyclopedia of Quaternary Science. 2nd edn. Elsevier, 2013: 542-552Crossref Scopus (85) Google Scholar, 26.Grosse G. et al.Distribution of late Pleistocene ice-rich syngenetic permafrost of the Yedoma Suite in east and central Siberia, Russia.U.S. Geol. Surv. 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Yedoma landscapes are highly sensitive to climate change as observed in Western Siberia, Alaska, and Québec [29.Shirokova L.S. et al.Biogeochemistry of organic carbon, CO2, CH4, and trace elements in thermokarst water bodies in discontinuous permafrost zones of Western Siberia.Biogeochemistry. 2013; 113: 573-593Crossref Scopus (80) Google Scholar, 30.Osterkamp T.E. Characteristics of the recent warming permafrost in Alaska.J. Geophys. Res. Earth Surf. 2007; 112: 1-10Crossref Scopus (189) Google Scholar, 31.Payette S. et al.Accelerated thawing of subarctic peatland permafrost over the last 50 years.Geophys. Res. Lett. 2004; 31L18208Crossref Scopus (212) Google Scholar]. In addition, global warming projections estimate a loss of all Yedoma by the end of the 21st century [10.Lawrence D.M. Slater A.G. A projection of severe near-surface permafrost degradation during the 21st century.Geophys. Res. Lett. 2005; 32L24401Crossref Scopus (299) Google Scholar,32.Zimov S.A. et al.North Siberian lakes: a methane source fueled by Pleistocene carbon.Science. 2006; 277: 800-802Crossref Scopus (226) Google Scholar]. Gradual thawing of the ice-rich permafrost leads to the formation of thermokarst, of which thermokarst lakes are a characteristic feature [32.Zimov S.A. et al.North Siberian lakes: a methane source fueled by Pleistocene carbon.Science. 2006; 277: 800-802Crossref Scopus (226) Google Scholar,33.Osterkamp T.E. et al.Observations of thermokarst and its impact on boreal forests in Alaska, U.S.A.Arctic Antarct. Alp. Res. 2000; 32: 303-315Crossref Google Scholar]. Nowadays, thermokarst landscapes are estimated to cover 20–40% of the northern permafrost regions, including the Yukon delta, the Alaska north slope, and the coastal regions of the Kara Sea, the Laptev Sea, and the East Siberian Sea [13.Schuur E.A.G. et al.Climate change and the permafrost carbon feedback.Nature. 2015; 520: 171-179Crossref PubMed Scopus (0) Google Scholar,34.Osterkamp T.E. et al.Physical and ecological changes associated with warming permafrost and thermokarst in Interior Alaska.Permafr. Periglac. Process. 2009; 20: 235-256Crossref Scopus (123) Google Scholar,35.Olefeldt D. et al.Circumpolar distribution and carbon storage of thermokarst landscapes.Nat. Commun. 2016; 7: 1-11Crossref Scopus (101) Google Scholar]. Thermokarst lakes also occur with lower densities on the Qinghai-Tibetan Plateau, which covers 8% of the global permafrost surface [36.Mu C. et al.Dissolved organic carbon, CO2, and CH4 concentrations and their stable isotope ratios in thermokarst lakes on the Qinghai-Tibetan Plateau.J. Limnol. 2016; 75: 313-319Google Scholar]. Thermokarst lakes play a significant role in the current climate by functioning as point sources of GHG emissions in comparison with surrounding soils and sediments [32.Zimov S.A. et al.North Siberian lakes: a methane source fueled by Pleistocene carbon.Science. 2006; 277: 800-802Crossref Scopus (226) Google Scholar,37.Matveev A. et al.High methane emissions from thermokarst lakes in subarctic peatlands.Limnol. Oceanogr. 2016; 61: S150-S164Crossref Scopus (18) Google Scholar]. Thermokarst lake expansion therefore results in an increase in GHG emissions. Observation-based modeling predicts the largest methane (CH4) emission rates of 50 Tg year–1 from newly thawed permafrost around 2050 compared with current rates of around 1 Tg year–1 [5.Schneider von Deimling T. et al.Observation-based modelling of permafrost carbon fluxes with accounting for deep carbon deposits and thermokarst activity.Biogeosciences. 2015; 12: 3469-3488Crossref Scopus (46) Google Scholar,11.Dean J.F. et al.Methane feedbacks to the global climate system in a warmer world.Rev. Geophys. 2018; 56: 207-250Crossref Scopus (62) Google Scholar]. This peak coincides with the expected highest expansion of thermokarst landscapes [5.Schneider von Deimling T. et al.Observation-based modelling of permafrost carbon fluxes with accounting for deep carbon deposits and thermokarst activity.Biogeosciences. 2015; 12: 3469-3488Crossref Scopus (46) Google Scholar]. The expansion of thermokarst landscapes is accelerated by the physical phenomenon of rapid heat conduction through thawed water bodies leading to abrupt thaw beneath and around the thermokarst lakes. Modeled lake sediment temperatures are about 10°C higher than mean annual air temperatures [38.Jorgenson M.T. et al.Resilience and vulnerability of permafrost to climate change.Can. J. For. Res. 2010; 40: 1219-1236Crossref Scopus (247) Google Scholar], leading to vertical and radial expansion of thermokarst lakes. Therefore, heat conduction in thermokarst lakes significantly contributes to early (<100 years) thaw progression [39.Plug L.J. West J.J. Thaw lake expansion in a two-dimensional coupled model of heat transfer, thaw subsidence, and mass movement.J. Geophys. Res. 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Freshly thawed permafrost soils release more nutrients than soils that have already been exposed to thawing, which boosts initial microbial activity leading to GHG production [53.Mackelprang R. et al.Metagenomic analysis of a permafrost microbial community reveals a rapid response to thaw.Nature. 2011; 480: 368-371Crossref PubMed Scopus (378) Google Scholar,54.Reyes F.R. Lougheed V.L. Rapid nutrient release from permafrost thaw in Arctic aquatic ecosystems.Arctic Antarct. Alp. Res. 2015; 47: 35-48Crossref Scopus (23) Google Scholar]. Radiocarbon age profiles, which were obtained by analyzing the 14C composition of CH4 ebullition seeps from Yedoma thermokarst lakes showed oldest CH4-carbon origins at lake margins and younger profiles towards the center, indicating rapid degradation of freshly thawed old permafrost C associated with lake expansion [55.Wik M. et al.Climate-sensitive northern lakes and ponds are critical components of methane release.Nat. 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There is, however, conflicting evidence as to which methanogenic pathway is dominant in thermokarst lake sediments [56.Brosius L.S. et al.Using the deuterium isotope composition of permafrost meltwater to constrain thermokarst lake contributions to atmospheric CH4 during the last deglaciation.J. Geophys. Res. Biogeosci. 2012; 117: 1-16Crossref Scopus (52) Google Scholar,57.Heslop J.K. et al.Thermokarst lake methanogenesis along a complete talik profile.Biogeosciences. 2015; 12: 4317-4331Crossref Scopus (16) Google Scholar,71.Heslop J. et al.Utilizing pyrolysis GC-MS to characterize organic matter quality in relation to methane production in a thermokarst lake sediment core.Org. Geochem. 2017; 103: 43-50Crossref Scopus (1) Google Scholar]. The availability of methanogenic substrates may differ between thermokarst lake regions, or even single lakes, due to differences in local physicochemical parameters such as pH and organic matter types and inputs [72.Matheus Carnevali P.B. et al.Methane sources in Arctic thermokarst lake sediments on the North Slope of Alaska.Geobiology. 2015; 13: 181-197Crossref PubMed Scopus (11) Google Scholar]. A study on thaw lakes in the Arctic Foothills province of Alaska found that the acetoclastic pathway dominated over the hydrogenotrophic pathway with a 2:1 ratio that further increased with sediment depth [73.Lofton D.D. et al.Vertical sediment distribution of methanogenic pathways in two shallow Arctic Alaskan lakes.Polar Biol. 2015; 38: 815-827Crossref Scopus (10) Google Scholar]. A study on polygonal ponds, trough ponds, and lakes on Bylot Island, a continuous permafrost zone of the Eastern Canadian Arctic, also designated acetoclastic methanogenesis as the dominant process [74.Bouchard F. et al.Modern to millennium-old greenhouse gases emitted from ponds and lakes of the Eastern Canadian Arctic (Bylot Island, Nunavut).Biogeosciences. 2015; 12: 7279-7298Crossref Scopus (19) Google Scholar]. In contrast with the aforementioned studies, Blodau and coworkers showed that, for a thermokarst pond and adjacent thermokarst depressions in Igarka, Northern Siberia, CH4 was produced mainly through hydrogenotrophic methanogenesis [75.Blodau C. et al.A snapshot of CO2 and CH4 evolution in a thermokarst pond near Igarka, northern Siberia.J. Geophys. Res. 2008; 113: 1-8Crossref Scopus (10) Google Scholar]. When thermokarst lakes develop into fully thawed thermokarst bogs, methanogenesis may shift from acetoclastic to hydrogenotrophic, as found in a microcosm study in peat bog samples from west Siberia by Kotsyurbenko and coworkers due to the low pH (3.8) of the bog [67.Kotsyurbenko O.R. Trophic interactions in the methanogenic microbial community of low-temperature terrestrial ecosystems.FEMS Microbiol. Ecol. 2005; 53: 3-13Crossref PubMed Scopus (87) Google Scholar]. A molecular survey, based on methanogenic marker genes, of thaw lakes that formed upon the collapse of palsas (frozen peat cores) and lithalsas (mineral core mounds) found both hydrogenotrophic Methanomicrobiales and versatile Methanosarcinales in near-bottom waters [76.Crevecoeur S. et al.Environmental selection of planktonic methanogens in permafrost thaw ponds.Sci. Rep. 2016; 6: 1-10Crossref PubMed Scopus (10) Google Scholar]. The dominance of Methanomicrobiales in functional gene (methyl-coenzyme M reductase, mcrA) transcript data on these thaw lakes indicates that hydrogenotrophic methanogenesis is the major pathway [76.Crevecoeur S. et al.Environmental selection of planktonic methanogens in permafrost thaw ponds.Sci. Rep. 2016; 6: 1-10Crossref PubMed Scopus (10) Google Scholar]. However, a study of transcriptional activity in methanogenic archaea showed co-occurrence of Methanomicrobiales and Methanosarcinales in thawed permafrost, but only an increase in transcriptional activity in thawed permafrost for acetoclastic Methanosarcinales [77.Wei S. et al.Shifts of methanogenic communities in response to permafrost thaw results in rising methane emissions and soil property changes.Extremophiles. 2018; 22: 447-459Crossref PubMed Scopus (8) Google Scholar]. A study based on 16S rRNA gene amplification by Matheus Carnevali and coworkers showed that the archaeal population of thermokarst lakes on the north slope of Alaska is highly diverse and not yet characterized properly with possibly novel taxa with poorly characterized metabolism involved in methanogenesis [78.Matheus Carnevali P.B. et al.Distinct microbial assemblage structure and archaeal diversity in sediments of Arctic thermokarst lakes differing in methane sources.Front. Microbiol. 2018; 9: 1-15Crossref PubMed Scopus (4) Google Scholar]. These findings stress the need for activity-based studies to gain insight into methanogenesis dynamics in the field. One interesting aspect of thermokarst GHG emissions is that CO2 and CH4 production respond differently to warming. A study of 40 Alaskan lakes across a longitudinal transect that includes lakes formed in continuous, discontinuous, and isolated permafrost showed that all were net sources of CO2 and CH4, but that the warming impact on CH4 production was twice as high compared with CO2 [79.Sepulveda-Jauregui A. et al.Methane and carbon dioxide emissions from 40 lakes along a north–south latitudinal transe