Strontium isotopes trace the dissolution and precipitation of mineral organic carbon interactions in thawing permafrost

Interactions between minerals and organic carbon (OC) in soils are key to stabilize OC and mitigate greenhouse gas emissions upon permafrost thaw. However, changes in soil water pathways upon permafrost thaw are likely to affect the stability of mineral OC interactions by inducing their dissolution and precipitation. This study aims to assess and quantify how mineral OC interactions are affected by dissolution and precipitation in thawed relative to unthawed layers. We hypothesize that a change in the radiogenic strontium (Sr) isotopic ratio (87Sr/86Sr) involved in mineral OC interactions upon changing water saturation conditions implies a destabilization of the mineral OC interaction. We quantified mineral OC interactions using selective extractions in soils facing gradual thaw (Eight Mile Lake, AK, USA) and in sediments with a thawing history of abrupt thaw (Duvanny Yar, Russia), and we measured the 87Sr/86Sr ratio of the selective extracts targeting the Sr associated to mineral OC interactions. Firstly, for water saturated layers with a higher proportion of mineral OC interactions, we found a difference in the 87Sr/86Sr ratio relative to the surrounding layers, and this supports the preservation of a Sr “stable” pool in these mineral OC interactions. We estimated that a portion of these mineral OC interactions have remained undissociated since their formation (between 4% and 64% by Sr isotope mass balance). Secondly, we found no difference in 87Sr/86Sr ratio between layers accumulating Fe oxides at redox interfaces regularly affected by water table changes (or upon thermokarst processes) relative to surrounding layers. This supports the dominance of a Sr “labile” pool inherited from processes of dissolution and precipitation of the mineral OC interactions. Thirdly, our estimations based on a Sr isotope mass balance support that, as a consequence of permafrost thaw, a larger proportion of Sr from primary mineral weathering (>80%) controls the Sr in mineral OC interactions in the saturated zone of deeply thawed soils relative to poorly thawed soils (∼50%). In conclusion, we found that the radiogenic Sr isotope method, applied for the first time in this context, is promising to trace dissolution-precipitation processes of mineral OC interaction in thawing permafrost

Long-term CO₂ production following permafrost thaw

Thawing permafrost represents a poorly understood feedback mechanism of climate change in the Arctic, but with a potential impact owing to stored carbon being mobilized1–5. We have quantified the long-term loss of carbon (C) from thawing permafrost in Northeast Greenland from 1996 to 2008 by combining repeated sediment sampling to assess changes in C stock and>12 years of CO2 production in incubated permafrost samples. Field observations show that the active-layer thickness has increased by>1 cm yr−1 but thawing has not resulted in a detectable decline in C stocks. Laboratory mineralization rates at 5 ◦C resulted in a C loss between 9 and 75%, depending on drainage, highlighting the potential of fast mobilization of permafrost C under aerobic conditions, but also that C at near-saturated conditions may remain largely immobilized over decades. This is confirmed by a three-pool

Elucidating the nutritional dynamics of fungi using stable isotopes

Abstract Mycorrhizal and saprotrophic (SAP) fungi are essential to terrestrial element cycling due to their uptake of mineral nutrients and decomposition of detritus. Linking these ecological roles to specific fungi is necessary to improve our understanding of global nutrient cycling, fungal ecophysiology, and forest ecology

The impact of the permafrost carbon feedback on global climate

Degrading permafrost can alter ecosystems, damage infrastructure, and release enough carbon dioxide (CO2) and methane (CH4) to influence global climate. The permafrost carbon feedback (PCF) is the amplification of surface warming due to CO2 and CH4 emissions from thawing permafrost. An analysis of available estimates PCF strength and timing indicate 120 ± 85 Gt of carbon emissions from thawing permafrost by 2100. This is equivalent to 5.7 ± 4.0% of total anthropogenic emissions for the Intergovernmental Panel on Climate Change (IPCC) representative concentration pathway (RCP) 8.5 scenario and would increase global temperatures by 0.29 ± 0.21 °C or 7.8 ± 5.7%. For RCP4.5, the scenario closest to the 2 °C warming target for the climate change treaty, the range of cumulative emissions in 2100 from thawing permafrost decreases to between 27 and 100 Gt C with temperature increases between 0.05 and 0.15 °C, but the relative fraction of permafrost to total emissions increases to between 3% and 11%. Any substantial warming results in a committed, long-term carbon release from thawing permafrost with 60% of emissions occurring after 2100, indicating that not accounting for permafrost emissions risks overshooting the 2 °C warming target. Climate projections in the IPCC Fifth Assessment Report (AR5), and any emissions targets based on those projections, do not adequately account for emissions from thawing permafrost and the effects of the PCF on global climate. We recommend the IPCC commission a special assessment focusing on the PCF and its impact on global climate to supplement the AR5 in support of treaty negotiation

Biomass offsets little or none of permafrost carbon release from soils, streams, and wildfire: an expert assessment

As the permafrost region warms, its large organic carbon pool will be increasingly vulnerable to decomposition, combustion, and hydrologic export. Models predict that some portion of this release will be offset by increased production of Arctic and boreal biomass; however, the lack of robust estimates of net carbon balance increases the risk of further overshooting international emissions targets. Precise empirical or model-based assessments of the critical factors driving carbon balance are unlikely in the near future, so to address this gap, we present estimates from 98 permafrost-region experts of the response of biomass, wildfire, and hydrologic carbon flux to climate change. Results suggest that contrary to model projections, total permafrost-region biomass could decrease due to water stress and disturbance, factors that are not adequately incorporated in current models. Assessments indicate that end-of-the-century organic carbon release from Arctic rivers and collapsing coastlines could increase by 75% while carbon loss via burning could increase four-fold. Experts identified water balance, shifts in vegetation community, and permafrost degradation as the key sources of uncertainty in predicting future system response. In combination with previous findings, results suggest the permafrost region will become a carbon source to the atmosphere by 2100 regardless of warming scenario but that 65%–85% of permafrost carbon release can still be avoided if human emissions are actively reduced

Permafrost

Permafrost is perennially frozen ground, such as soil, rock, and ice. In permafrost regions, plant and microbial life persists primarily in the near-surface soil that thaws every summer, called the ‘active layer’ (Figure 20). The cold and wet conditions in many permafrost regions limit decomposition of organic matter. In combination with soil mixing processes caused by repeated freezing and thawing, this has led to the accumulation of large stocks of soil organic carbon in the permafrost zone over multi-millennial timescales. As the climate warms, permafrost carbon could be highly vulnerable to climatic warming. Permafrost occurs primarily in high latitudes (e.g. Arctic and Antarctic) and at high elevation (e.g. Tibetan Plateau, Figure 21). The thickness of permafrost varies from less than 1 m (in boreal peatlands) to more than 1 500 m (in Yakutia). The coldest permafrost is found in the Transantarctic Mountains in Antarctica (−36°C) and in northern Canada for the Northern Hemisphere (-15°C; Obu et al., 2019, 2020). In contrast, some of the warmest permafrost occurs in peatlands in areas with mean air temperatures above 0°C. Here permafrost exists because thick peat layers insulate the ground during the summer. Most of the permafrost existing today formed during cold glacials (e.g. before 12 000 years ago) and has persisted through warmer interglacials. Some shallow permafrost (max 30–70m depth) formed during the Holocene (past 5000 years) and some even during the Little Ice Age from 400–150 years ago. There are few extensive regions suitable for row crop agriculture in the permafrost zone. Additionally, in areas where large-scale agriculture has been conducted, ground destabilization has been common. Surface disturbance such as plowing or trampling of vegetation can alter the thermal regime of the soil, potentially triggering surface subsidence or abrupt collapse. This may influence soil hydrology, nutrient cycling, and organic matter storage. These changes often have acute and negative consequences for continued agricultural use of such landscapes. Thus, row-crop agriculture could have a negative impact on permafrost (e.g. Grünzweig et al., 2014). Conversely, animal husbandry is widespread in the permafrost zone, including horses, cattle, and reindeer

The Permafrost Regionalization Map (PeRM): How well do observations, models and experiments represent the circumarctic-scale spatial variability in permafrost carbon vulnerability?

A large amount of organic carbon stored in permafrost soils across the high latitudes is vulnerable to thaw, decomposition and release to the atmosphere as a result of climate warming. Findings from observational, experimental and modeling studies all suggest that this process could lead to a significant positive feedback on future radiative forcing from terrestrial ecosystems to the Earth’s climate system. With respect to the magnitude and timing of this feedback, however, observational data show large variability across sites, experimental studies are few, and different models result in a wide range of responses. These issues represent fundamental limitations on improving our confidence in projecting future permafrost carbon release and associated climate feedbacks. Recent studies have brought new insight into – and even quantitative estimates for – these issues through broader data synthesis and model-data integration approaches. But, how representative of the circumarcticscale variability in permafrost carbon vulnerability are the data and models from these studies? To address this question, we developed a geospatial data synthesis and analysis framework designed to represent and characterize the variability in permafrost carbon vulnerability across the northern high latitudes. Here, we describe the rationale and methods used to develop the regionalization scheme, and then use the framework to assess the spatial representativeness of, and the variability described by, existing data sets defining the fundamental components and environmental drivers of permafrost carbon vulnerability. The Permafrost Regionalization Map (PeRM) considers the regional-scale environmental factors that generally determine the spatial variability in permafrost carbon vulnerability across the Arctic. The broadly-defined regional classification is based on a circumarctic spatial representation of the major environmental controls on a) the rate and extent of permafrost degradation and thaw, b) the quantity and quality of soil organic matter stocks, and c) the form of permafrost carbon emissions as CO2 and CH4. We chose a generalized, pragmatic approach that resulted in a feasible number of regional subdivisions (i.e.,‘reporting units’) based on an intersection of spatial data layers according to permafrost extent, permafrost distribution, climate regime, biome and terrain. The utility of the PeRM framework is demonstrated here through areal density analysis and spatial summaries of existing data collections describing the fundamental components of permafrost carbon vulnerability. We use this framework to describe the spatial representativeness and variability in measurements within and across PeRM regions using observational data sets describing active layer thickness, soil pedons and carbon storage, long-term incubations for carbon turnover rates, and site-level monitoring of CO2 and CH4 fluxes from arctic tundra and boreal forest ecosystems. We then use these regional summaries of the observational data to benchmark the results of a process-based biogeochemical model for its skill in representing the magnitudes and spatial variability in these key indicators. Finally, we discuss the on-going use of this framework as a basis for higher-resolution mapping of key regions of particular vulnerability to both press (active layer thickening) and pulse (thermokarst development) disturbances. This work is guiding on-going research toward characterizing permafrost degradation and associated vegetation changes through multi-scale remote sensing. Overall, this spatial data synthesis framework work provides a critical bridge between the abundant but disordered observational and experimental data collections and the development of higher-complexity process representation of the permafrost carbon feedback in geospatial modeling frameworks

Deep Yedoma permafrost: A synthesis of depositional characteristics and carbon vulnerability

Permafrost is a distinct feature of the terrestrial Arctic and is vulnerable to climate warming. Permafrost degrades in different ways, including deepening of a seasonally unfrozen surface and localized but rapid development of deep thaw features. Pleistocene ice-rich permafrost with syngenetic ice-wedges, termed Yedoma deposits, are widespread in Siberia, Alaska, and Yukon, Canada and may be especially prone to rapid-thaw processes. Freeze-locked organic matter in such deposits can be re-mobilized on short time-scales and contribute to a carbon cycle climate feedback. Here we synthesize the characteristics and vulnerability of Yedoma deposits by synthesizing studies on the Yedoma origin and the associated organic carbon pool. We suggest that Yedoma deposits accumulated under periglacial weathering, transport, and deposition dynamics in non-glaciated regions during the late Pleistocene until the beginning of late glacial warming. The deposits formed due to a combination of aeolian, colluvial, nival, and alluvial deposition and simultaneous ground ice accumulation. We found up to 130 gigatons organic carbon in Yedoma, parts of which are well-preserved and available for fast decomposition after thaw. Based on incubation experiments, up to 10% of the Yedoma carbon is considered especially decomposable and may be released upon thaw. The substantial amount of ground ice in Yedoma makes it highly vulnerable to disturbances such as thermokarst and thermo-erosion processes. Mobilization of permafrost carbon is expected to increase under future climate warming. Our synthesis results underline the need of accounting for Yedoma carbon stocks in next generation Earth-System-Models for a more complete representation of the permafrost-carbon feedback