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The demanding precision of triple oxygen isotope (Δ17O) analyses in water has restricted their measurement to dual-inlet mass spectrometry until the recent development of commercially available infrared laser analyzers. Laser-based measurements of triple oxygen isotope ratios are now increasingly performed by laboratories seeking to better constrain the source and history of meteoric waters. However, in practice, these measurements are subject to large analytical errors that remain poorly documented in scientific literature and by instrument manufacturers, which can effectively restrict the confident application of Δ17O to settings where variations are relatively large (∼ 25–60 per meg). We present our operating method of a Picarro L2140-i cavity ring-down spectrometer (CRDS) during the analysis of low-latitude rainwater where confidently resolving daily variations in Δ17O (differences of ∼ 10–20 per meg) was desired. Our approach was optimized over ∼ 3 years and uses a combination of published best practices plus additional steps to combat spectral contamination of trace amounts of dissolved organics, which, for Δ17O, emerges as a much more substantial problem than previously documented, even in pure rainwater. We resolve the extreme sensitivity of the Δ17O measurement to organics through their removal via Picarro's micro-combustion module, whose performance is evaluated in each sequence using alcohol-spiked standards. While correction for sample-to-sample memory and instrumental drift significantly improves traditional isotope metrics, these corrections have only a marginal impact (0–1 per meg error reduction) on Δ17O. Our post-processing scheme uses the analyzer's high-resolution data, which improves δ2H measurement (0.25 ‰ error reduction) and allows for much more rich troubleshooting and data processing compared to the default user-facing data output. In addition to competitive performance for traditional isotope metrics, we report a long-term, control standard root mean square error for Δ17O of 12 per meg. Overall performance (Δ17O error of 6 per meg, calculated by averaging three replicates spread across distinct, independently calibrated sequences) is comparable to mass spectrometry and requires only ∼ 6.3 h per sample. We demonstrate the impact of our approach using a rainfall dataset from Uganda and offer recommendations for other efforts that aim to measure meteoric Δ17O via CRDS.

The demanding precision of triple oxygen isotope (Δ17O) measurements in water has restricted their measurement to dual-inlet mass spectrometry until the recent development of commercially available infrared-laser analyzers. Laser-based measurements of triple oxygen isotope ratios are now increasingly performed by laboratories seeking to better constrain the source and history of meteoric waters. However, in practice, these measurements are subject to large analytical errors that remain poorly documented in scientific literature and by instrument manufacturers, which can effectively restrict the confident application of Δ17O to settings where variations are relatively large (~25–60 per meg). We present our operating method of a Picarro L2140-i cavity ringdown spectrometer during the analysis of low-latitude rainwaters where confidently resolving daily variations in Δ17O (differences of ~10–20 per meg) was desired. Our approach was optimized over ~3 years and uses a combination of published best-practices plus additional steps to combat spectral contamination of trace amounts of dissolved organics, which, for Δ17O, emerges as a much more substantial problem than previously documented, even in pure rainwater. We resolve the extreme sensitivity of the Δ17O measurement to organics through their removal via Picarro’s micro-combustion module, whose performance is evaluated each sequence using alcohol-spiked standards. While correction for sample-to-sample memory and instrumental drift significantly improves traditional isotope metrics, these corrections have only marginal impact (0–1 per meg error reduction) on Δ17O. Our post-processing scheme uses the analyzer’s high-resolution data, which improves δ2H measurement (0.25 ‰ error reduction) and allows for much more rich troubleshooting and data-processing compared to the default user-facing data output. In addition to competitive performance for traditional isotope metrics, we report a long-term, control standard root-mean-square-error for Δ17O of 11 per meg. Overall performance (Δ17O error of 7 per meg, calculated by averaging 3 replicates spread across distinct, independently calibrated sequences) is comparable to mass spectrometry and requires only ~6.3 h per sample. We demonstrate the impact of our approach using a rainfall dataset from Uganda and offer recommendations for other efforts that aim to measure meteoric Δ17O via CRDS.

Evidence suggests that 5–15% of the vast pool of soil carbon stored in northern permafrost ecosystems could be emitted as greenhouse gases by 2100 under the current path of global warming. However, direct measurements of changes in soil carbon remain scarce, largely because ground subsidence that occurs as the permafrost soils begin to thaw confounds the traditional quantification of carbon pools based on fixed depths or soil horizons. This issue is overcome when carbon is quantified in relation to a fixed ash content, which uses the relatively stable mineral component of soil as a metric for pool comparisons through time. We applied this approach to directly measure soil carbon pool changes over five years in experimentally warmed and ambient tundra ecosystems at a site in Alaska where permafrost is degrading due to climate change. We show a loss of soil carbon of 5.4% per year (95% confidence interval: 1.0, 9.5) across the site. Our results point to lateral hydrological export as a potential pathway for these surprisingly large losses. This research highlights the potential to make repeat soil carbon pool measurements at sentinel sites across the permafrost region, as this feedback to climate change may be occurring faster than previously thought. Permafrost loses carbon at a faster rate than previously thought as climate warms, according to direct soil carbon observations over five years in the field in Alaska’s tundra ecosystem.

Organic carbon stored in high-latitude permafrost represents a potential positive feedback to climate warming as well as a valuable store of paleoenvironmental information. The below-freezing conditions have effectively removed permafrost organic material from the modern carbon cycle and preserved its pre-freezing bulk and molecular states. The conditions that lead to efficient burial of organic carbon (OC) within permafrost were investigated by measuring OC stocks, past accumulation rates, and biogeochemical composition of a permafrost core taken from Interior Alaska dating back to 40 ka. The post-glacial Marine Isotope Stage 1 is represented by the top 1.2 m of the core and contains 64.7 kg OC/m2 with an accumulation rate of 4.3 g OC/m2/yr. The sediments that accumulated around the Last Glacial Maximum contain 9.9 kg OC/m2 with an accumulation rate of 0.5 g OC/m2/yr. Carbon storage (144.7 kg OC/m2) and accumulation (26.1 g OC/m2/yr) are both observed to be greatest between 35 and 40 ka, late during the Marine Isotope Stage 3 global interstadial. The extent of OC degradation was assessed using lignin and amino acid biomarkers with both approaches indicating well-preserved contemporary active layer and interstadial OC, whereas stadial OC was highly degraded. Lignin compositional indices throughout the core appear altered by sorptive processes that confounded some expected trends in the overall organic matter composition, while amino acids provided a more integrated pattern of change. Significant correlations between carbon-normalized hydroxyproline and total lignin concentrations further support the usefulness of hydroxyproline as an indicator for the abundance of plant organic matter. A novel amino acid plant-microbial index of the ratio of microbial-specific muramic acid and diaminopimelic acid biomarkers to the plant-specific hydroxyproline biomarker, indicate a transition from plant-dominated organic matter in fresh organic soils (index values of 0.01–0.20) to more microbial-dominated organic matter in degraded mineral soils (index values of 0.50–2.50). The branched glycerol dialkyl glycerol tetraether composition is complex and is not immediately compatible with existing temperature transfer functions. Residence time within the active layer is interpreted to integrate key factors such as primary productivity, inorganic sediment delivery, and other climate factors that control soil organic matter degradation. The Marine Isotope Stage 3, mid-Wisconsin interstadial period at this locality was forest-dominated and suggests the currently prevailing tundra ecotone is sensitive to environmental change. The majority of buried permafrost OC is high in degradability and if thawed, would be expected to be highly vulnerable to microbial decomposition.

Chocolate Pots hot spring (CP) is an Fe-rich, circumneutral-pH geothermal spring in Yellowstone National Park. Relic hydrothermal systems have been identified on Mars, and modern hydrothermal environments such as CP are useful for gaining insight into potential pathways for generation of biosignatures of ancient microbial life on Earth and Mars. Fe isotope fractionation is recognized as a signature of dissimilatory microbial iron oxide reduction (DIR) in both the rock record and modern sedimentary environments. Previous studies in CP have demonstrated the presence of DIR in vent pool deposits and show aqueous-/solid-phase Fe isotope variations along the hot spring flow path that may be linked to this process. In this study, we examined the geochemistry and stable Fe isotopic composition of spring water and sediment core samples collected from the vent pool and along the flow path, with the goal of evaluating whether Fe isotopes can serve as a signature of past or present DIR activity. Bulk sediment Fe redox speciation confirmed that DIR is active within the hot spring vent pool sediments (but not in more distal deposits), and the observed Fe isotope fractionation between Fe(II) and Fe(III) is consistent with previous studies of DIR-driven Fe isotope fractionation. However, modeling of sediment Fe isotope distributions indicates that DIR does not produce a unique Fe isotopic signature of DIR in the vent pool environment. Because of rapid chemical and isotopic communication between the vent pool fluid and sediment, sorption of Fe(II) to Fe(III) oxides would produce an isotopic signature similar to DIR despite DIR-driven generation of large quantities of isotopically light solid-associated Fe(II). The possibility exists, however, for preservation of specific DIR-derived Fe(II) minerals such as siderite (which is present in the vent pool deposits), whose isotopic composition could serve as a long-term signature of DIR in relic hot spring environments.

Author(s): Mauritz, M; Celis, G; Ebert, C; Hutchings, J; Ledman, J; Natali, SM; Pegoraro, E; Salmon, VG; Schadel, C; Taylor, M; Schuur, EAG

Amino acids are effective tracers of organic carbon and nitrogen cycling in natural substrates. However, total analytical time can be long due to extraction, derivatization, and chromatography. Here, we present a liquid chromatographic separation of 19 naturally occurring amino acids requiring no derivatization using triple-quadrupole mass spectrometry. This method builds on recent advances in volatile ion pairing and was applied to natural soil samples collected from two distinct systems with highly variable organic C contents. Separation and quantification of amino acids were achieved with an 18 min sample-to-sample run time employing a one-point standard addition to account for variable matrix effects during ionization. Detection limits ranged from 5.9 to 187.5 fmol (mean 30.0 fmol) while instrumental precision averaged 5.8% and 12.0% for intra- and inter-day error, respectively. The highest yields (mean > 100 µmol/g C) in our natural substrates were observed for glycine, aspartic acid, alanine, and glutamic acid while low yields (mean

Subsistence adaptations respond to environmental instability as well as sociopolitical demands. On the Andean altiplano, camelid husbandry is a key adaptation utilized for thousands of years to mitigate environmental risks to agricultural productivity, to facilitate transport and to generate wealth for political purposes. Here, we reconstruct the history of camelid utilization on the altiplano with fecal stanol biomarkers from Lake Orurillo in southern Peru. Biomarker samples span a 4000-year interval and document changes at centennial timescales. The abundances of individual fecal stanols appear to be less diagnostic of changes in human occupation in the Orurillo region where human population levels apparently remained far lower than those of domesticated camelids. However, ratios of human (coprostanol) to ruminant (24-ethylcoprosanol) fecal stanol concentrations demonstrate that these proxies are reliable indicators of variations in camelid herd sizes. Our data show that camelid utilization intensified after ca. 1050 CE during a severe and protracted drought that was coincident with widespread population movement away from fertile farmlands to higher elevations favorable for grazing. Camelid herds remained in high abundance around Orurillo during improved climate conditions and through the Inca era until after ca. 1500 CE when our data point to a significant disruption after European conquest. The intensification of camelid husbandry among Andean cultures is therefore linked to major environmental and sociopolitical shifts.

River deltas have contributed to atmospheric carbon regulation throughout Earth history, but functioning in the modern era has been impaired by reduced sediment loads, altered hydrologic regimes, increased global sea-level rise and accelerated subsidence. Delta restoration involves environmental engineering via river diversions, which utilize self-organizing processes to create prograding deltas. Here we analyse sediment cores from Wax Lake delta, a product of environmental engineering, to quantify the burial of organic carbon. We find that, despite relatively low concentrations of organic carbon measured in the cores (about 0.4%), the accumulation of about 3 T m−2 of sediment over the approximate 60 years of delta building resulted in the burial of a significant amount of organic carbon (16 kg m−2). This equates to an apparent organic carbon accumulation rate of 250 ± 23 g m−2 yr−1, which implicitly includes losses by carbon emissions and erosion. Our estimated accumulation rate for Wax Lake delta is substantially greater than previous estimates based on the top metre of delta sediments and comparable to those of coastal mangrove and marsh habitats. The sedimentation of carbon at the Wax Lake delta demonstrates the capacity of engineered river diversions to enhance both coastal accretion and carbon burial. Substantial amounts of organic carbon have been buried in the Wax Lake delta, USA, over the past 20 years, according to sediment analyses. This suggests that river diversions can lead to both coastal accretion and carbon sequestration.

The Late Cretaceous sedimentary record of the North American Western Interior Seaway is characterized by cyclic deposition of organic carbon-rich sediments. One notable interval during the late Coniacian-Santonian is recorded by the Niobrara Formation. The organic carbon-rich interval within the Niobrara Formation has been identified as Oceanic Anoxic Event (OAE) 3. Understanding the reason for this distribution of organic carbon within the Niobrara Formation requires a refined understanding of the source and maturity of the organic matter. In this study, we present lipid biomarker records from the USGS Portland #1 core (Canon City, CO) to constrain the thermal maturity of the organic matter and the differing contributions of organic matter sources. Sterane and hopane thermal maturity indices indicate that the samples are somewhat immature with respect to oil formation and that there is strong agreement between different proxies for thermal maturity. Based on the distribution of n-alkanes, steranes, and hopanes, there is a significant increase in the contribution of algal organic matter during and after OAE 3, coeval with increased organic carbon accumulation. Although a consistent terrestrial contribution is observed, it is only a minor source of organic matter at the Portland core location and does not drive increased organic matter accumulation during OAE 3. Of particular note is the consistent influence of even-over-odd predominantly mid-chain length (C21 to C25) organic matter. This observation within the brackish to marine, not methanogenic WIS represents an expansion of the depositional settings in which even-over-odd predominance has been observed in mid-chain length n-alkanes. Pristane (Pr) and phytane (Ph) abundances are inconsistent with a redox control on Pr/Ph ratios and suggest an increase in the delivery and/or preservation of phototrophic organic matter as the source for pristane and phytane in the Portland core.

Current and future warming of high-latitude ecosystems will play an important role in climate change through feedbacks to the global carbon cycle. This study compares 6 years of CO2 flux measurements in moist acidic tundra using autochambers and eddy covariance (Tower) approaches. We found that the tundra was an annual source of CO2 to the atmosphere as indicated by net ecosystem exchange using both methods with a combined mean of 105 ± 17 g CO2 C m−2 y−1 across methods and years (Tower 87 ± 17 and Autochamber 123 ± 14). The difference between methods was largest early in the observation period, with Autochambers indicated a greater CO2 source to the atmosphere. This discrepancy diminished through time, and in the final year the Autochambers measured a greater sink strength than tower. Active layer thickness was a significant driver of net ecosystem carbon exchange, gross ecosystem primary productivity, and Reco and could account for differences between Autochamber and Tower. The stronger source initially attributed lower summer season gross primary production (GPP) during the first 3 years, coupled with lower ecosystem respiration (Reco) during the first year. The combined suppression of GPP and Reco in the first year of Autochamber measurements could be the result of the experimental setup. Root damage associated with Autochamber soil collar installation may have lowered the plant community's capacity to fix C, but recovered within 3 years. While this ecosystem was a consistent CO2 sink during the summer, CO2 emissions during the nonsummer months offset summer CO2 uptake each year.

Rapid Arctic warming is expected to increase global greenhouse gas concentrations as permafrost thaw exposes immense stores of frozen carbon (C) to microbial decomposition. Permafrost thaw also stimulates plant growth, which could offset C loss. Using data from 7 years of experimental Air and Soil warming in moist acidic tundra, we show that Soil warming had a much stronger effect on CO2 flux than Air warming. Soil warming caused rapid permafrost thaw and increased ecosystem respiration (Reco ), gross primary productivity (GPP), and net summer CO2 storage (NEE). Over 7 years Reco , GPP, and NEE also increased in Control (i.e., ambient plots), but this change could be explained by slow thaw in Control areas. In the initial stages of thaw, Reco , GPP, and NEE increased linearly with thaw across all treatments, despite different rates of thaw. As thaw in Soil warming continued to increase linearly, ground surface subsidence created saturated microsites and suppressed Reco , GPP, and NEE. However Reco and GPP remained high in areas with large Eriophorum vaginatum biomass. In general NEE increased with thaw, but was more strongly correlated with plant biomass than thaw, indicating that higher Reco in deeply thawed areas during summer months was balanced by GPP. Summer CO2 flux across treatments fit a single quadratic relationship that captured the functional response of CO2 flux to thaw, water table depth, and plant biomass. These results demonstrate the importance of indirect thaw effects on CO2 flux: plant growth and water table dynamics. Nonsummer Reco models estimated that the area was an annual CO2 source during all years of observation. Nonsummer CO2 loss in warmer, more deeply thawed soils exceeded the increases in summer GPP, and thawed tundra was a net annual CO2 source.

Summary We investigated soil organic carbon (SOC) characteristics in three forests along a north–south transect in Sweden where these forest types cover about 69% of the landscape. There was variation in 14C ages, and the median ages ranged from about 25 to > 2500 cal BP in SOC. Although total SOC and nitrogen (N) contents decreased, stable carbon isotope and humification indices increased with depth. These progressive changes with depth and age were related to degradation. The δ13C values and specific biomarkers indicated that organic carbon was primarily from C3 plants. Biomarkers were effective in distinguishing OC input from specific sources (i.e. angiosperms, gymnosperms and grasses). A sharp decrease in biomarkers with depth indicated degradation of OC in the upper soil horizon, and limited contribution in the subsoil towards the stabilization of SOC. The sharp decrease in carbon stocks and 14C age in the soil OC pool with increasing soil depth, and quite large values for the percentage of modern carbon, suggested a decrease in SOC pools. Overall, these results showed that carbon sequestration in high latitude forests was small, and their role as potential carbon sinks needs to be reassessed. Highlights We studied soil organic carbon (SOC) characteristics in Swedish boreal and temperate forests. The decline of SOC with depth contributes little to carbon sequestration in deeper soil horizons. Soil degradation causes a decline in total carbon, nitrogen, alkanes and lignin with depth and age. The role of these forests as carbon sinks needs to be reassessed because of degradation of SOC.

Significance During the Holocene (11,600 y ago to present), northern peatlands accumulated significant C stocks over millennia. However, virtually nothing is known about peatlands that are no longer in the landscape, including ones formed prior to the Holocene: Where were they, when did they form, and why did they disappear? We used records of peatlands buried by mineral sediments for a reconstruction of peat-forming wetlands for the past 130,000 y. Northern peatlands expanded across high latitudes during warm periods and were buried during periods of glacial advance in northern latitudes. Thus, peat accumulation and burial represent a key long-term C storage mechanism in the Earth system., Glacial−interglacial variations in CO2 and methane in polar ice cores have been attributed, in part, to changes in global wetland extent, but the wetland distribution before the Last Glacial Maximum (LGM, 21 ka to 18 ka) remains virtually unknown. We present a study of global peatland extent and carbon (C) stocks through the last glacial cycle (130 ka to present) using a newly compiled database of 1,063 detailed stratigraphic records of peat deposits buried by mineral sediments, as well as a global peatland model. Quantitative agreement between modeling and observations shows extensive peat accumulation before the LGM in northern latitudes (>40°N), particularly during warmer periods including the last interglacial (130 ka to 116 ka, MIS 5e) and the interstadial (57 ka to 29 ka, MIS 3). During cooling periods of glacial advance and permafrost formation, the burial of northern peatlands by glaciers and mineral sediments decreased active peatland extent, thickness, and modeled C stocks by 70 to 90% from warmer times. Tropical peatland extent and C stocks show little temporal variation throughout the study period. While the increased burial of northern peats was correlated with cooling periods, the burial of tropical peat was predominately driven by changes in sea level and regional hydrology. Peat burial by mineral sediments represents a mechanism for long-term terrestrial C storage in the Earth system. These results show that northern peatlands accumulate significant C stocks during warmer times, indicating their potential for C sequestration during the warming Anthropocene.

Transport of particles plays a major role in redistributing organic carbon (OC) along coastal regions. In particular, the global importance of fjords as sites of carbon burial has recently been shown to be even more important than previously thought. In this study, we used six surface sediments from Fiordland, New Zealand, to investigate the transport of particles and OC based on density fractionation. Bulk, biomarker, and principle component analysis were applied to density fractions with ranges of 2.5 g cm−3. Our results found various patterns of OC partitioning at different locations along fjords, likely due to selective transport of higher density but smaller size particles along fjord head-to-mouth transects. We also found preferential leaching of certain biomarkers (e.g., lignin) over others (e.g., fatty acids) during the density fractionation procedure, which altered lignin-based degradation indices. Finally, our results indicated various patterns of OC partitioning on density fractions among different coastal systems. We further propose that a combination of particle size-density fractionation is needed to better understand transport and distribution of particles and OC.

Despite the large contribution of rangeland and pasture to global soil organic carbon (SOC) stocks, there is considerable uncertainty about the impact of large herbivore grazing on SOC, especially for understudied subtropical grazing lands. It is well known that root system inputs are the source of most grassland SOC, but the impact of grazing on partitioning of carbon allocation to root tissue production compared to fine root exudation is unclear. Given that different forms of root C have differing implications for SOC synthesis and decomposition, this represents a significant gap in knowledge. Root exudates should contribute to SOC primarily after microbial assimilation, and thus promote microbial contributions to SOC based on stabilization of microbial necromass, whereas root litter deposition contributes directly as plant-derived SOC following microbial decomposition. Here, we used in situ isotope pulse-chase methodology paired with plant and soil sampling to link plant carbon allocation patterns with SOC pools in replicated long-term grazing exclosures in subtropical pasture in Florida, USA. We quantified allocation of carbon to root tissue and measured root exudation across grazed and ungrazed plots and quantified lignin phenols to assess the relative contribution of microbial vs. plant products to total SOC. We found that grazing exclusion was associated with dramatically less overall belowground allocation, with lower root biomass, fine root exudates, and microbial biomass. Concurrently, grazed pasture contained greater total SOC, and a larger fraction of SOC that originated from plant tissue deposition, suggesting that higher root litter deposition under grazing promotes greater SOC. We conclude that grazing effects on SOC depend on root system biomass, a pattern that may generalize to other C4-dominated grasslands, especially in the subtropics. Improved understanding of ecological factors underlying root system biomass may be the key to forecasting SOC and optimizing grazing management to enhance SOC accumulation.

Incomplete drill holes in shelled invertebrate prey produced by predatory gastropods are often interpreted as failed attacks, and the frequency of incomplete drill holes in a sample is thought to provide insight into the effectiveness of prey defensive adaptations. Here, we conduct an experimental test of the hypothesis that differences in the frequencies of incomplete drill holes do not necessarily indicate anything about the prey and can instead be triggered by disturbance competition among conspecific drilling gastropods. Our test employed the western Atlantic naticid gastropod Neverita delessertiana and its venerid bivalve prey Chione elevata. The experiment consisted of an isolation treatment, where predators fed in the absence of conspecifics, and a competition treatment, where predators fed in groups of three. Competitive interactions, which included grappling, prey theft, and cannibalism, were found to be important in determining the presence of incomplete drill holes, as this trace was absent in the isolation treatment but present in the competition treatment, with the frequency of incomplete drill holes increasing in interactions among larger gastropods. These results suggest that interpretations of both field and fossil data must consider the role of competitive disruption as an additional mechanism underlying long-term patterns in the frequency of incomplete drill holes in the fossil record.

Author(s): Schadel, C; Koven, CD; Lawrence, DM; Celis, G; Garnello, AJ; Hutchings, J; Mauritz, M; Natali, SM; Pegoraro, E; Rodenhizer, H; Salmon, VG; Taylor, MA; Webb, EE; Wieder, WR; Schuur, EAG | Abstract: In the last few decades, temperatures in the Arctic have increased twice as much as the rest of the globe. As permafrost thaws in response to this warming, large amounts of soil organic matter may become vulnerable to decomposition. Microbial decomposition will release carbon (C) from permafrost soils, however, warmer conditions could also lead to enhanced plant growth and C uptake. Field and modeling studies show high uncertainty in soil and plant responses to climate change but there have been few studies that reconcile field and model data to understand differences and reduce uncertainty. Here, we evaluate gross primary productivity (GPP), ecosystem respiration (Reco), and net ecosystem C exchange (NEE) from eight years of experimental soil warming in moist acidic tundra against equivalent fluxes from the Community Land Model during simulations parameterized to reflect the field conditions associated with this manipulative field experiment. Over the eight-year experimental period, soil temperatures and thaw depths increased with warming in field observations and model simulations. However, the field and model results do not agree on warming effects on water table depth; warming created wetter soils in the field and drier soils in the models. In the field, initial increases in growing season GPP, Reco, and NEE to experimentally-induced permafrost thaw created a higher C sink capacity in the first years followed by a stronger C source in years six through eight. In contrast, both models predicted linear increases in GPP, Reco, and NEE with warming. The divergence of model results from field experiments reveals the role subsidence, hydrology, and nutrient cycling play in influencing the C flux responses to permafrost thaw, a complexity that the models are not structurally able to predict, and highlight challenges associated with projecting C cycle dynamics across the Arctic.