High-latitude δ
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O archives deriving from meteoric water (e.g., tree-rings and ice-cores) can provide valuable information on past temperature variability, but stationarity of temperature signals ...in these archives depends on the stability of moisture source/trajectory and precipitation seasonality, both of which can be affected by atmospheric circulation changes. A tree-ring δ
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O record (AD 1780–2003) from the Mackenzie Delta is evaluated as a temperature proxy based on linear regression diagnostics. The primary source of moisture for this region is the North Pacific and, thus, North Pacific atmospheric circulation variability could potentially affect the tree-ring δ
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O-temperature signal. Over the instrumental period (AD 1892–2003), tree-ring δ
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O explained 29 % of interannual variability in April–July minimum temperatures, and the explained variability increases substantially at lower-frequencies. A split-period calibration/verification analysis found the δ
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O-temperature relation was time-stable, which supported a temperature reconstruction back to AD 1780. The stability of the δ
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O-temperature signal indirectly implies the study region is insensitive to North Pacific circulation effects, since North Pacific circulation was not constant over the calibration period. Simulations from the NASA-GISS ModelE isotope-enabled general circulation model confirm that meteoric δ
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O and precipitation seasonality in the study region are likely insensitive to North Pacific circulation effects, highlighting the paleoclimatic value of tree-ring and possibly other δ
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O records from this region. Our δ
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O-based temperature reconstruction is the first of its kind in northwestern North America, and one of few worldwide, and provides a long-term context for evaluating recent climate warming in the Mackenzie Delta region.
The combined effects of climate warming (i.e., increased storminess, reduced sea ice extent, and rising sea levels) make low‐lying Arctic coastal regions particularly susceptible to storm surges. The ...Mackenzie Delta, a biologically significant and resource‐rich region in northwestern Canada, is particularly vulnerable to flooding by storm surges. To properly manage the consequences of climate warming for Arctic residents, infrastructure, and ecosystems, a better understanding of the influence of climate change on storm surge activity is required. Here we use particle size analysis of lake sediment records to show that the occurrence and magnitude of storm surges in the outer Mackenzie Delta are significantly related to temperature and that the frequency and intensity of storm surges is increasing. Our results demonstrate the effects of changing climate on storm surge activity and provide a cautionary example of the threat of inundation to low‐lying Arctic coastal environments under future climate warming scenarios.
Key PointsArctic storm surge activity is related to climate warming and reduced sea iceA 1999 surge in the Mackenzie Delta was the most severe in at least 400 yearsAs the Arctic warms, planning for increased storm surge activity is required
Summary
1. Rapid environmental change occurring in high‐latitude regions has the potential to cause extensive thawing of permafrost. Retrogressive thaw slumps are a particularly spectacular form of ...permafrost degradation that can significantly impact lake–water chemistry; however, to date, the effects on aquatic biota have received little attention.
2. We used a diatom‐based palaeolimnological approach featuring a paired lake study design to examine the impact of thaw slumping on freshwater ecosystems in the low Arctic of western Canada. We compared biological responses in six lakes affected by permafrost degradation with six undisturbed, reference lakes.
3. Slump‐affected lakes exhibited greater biological change than the paired reference systems, although all systems have undergone ecologically significant changes over the last 200 years. Four of the six reference systems showed an increase in the relative abundance of planktonic algal taxa (diatoms and scaled chrysophytes), the earliest beginning about 1900, consistent with increased temperature trends in this region.
4. The response of sedimentary diatoms to thaw slumping was understandably variable, but primarily related to the intensity of disturbance and associated changes in aquatic habitat. Five of the slump‐affected lakes recorded increases in the abundance and diversity of periphytic diatoms at the presumed time of slump initiation, consistent with increased water clarity and subsequent development of aquatic macrophyte communities. Slump‐affected lakes generally displayed lower nutrient levels; however, in one system, thaw slumping, induced by an intense fire at the site in 1968, ostensibly led to pronounced nutrient enrichment that persists today.
5. Our results demonstrate that retrogressive thaw slumping represents an important stressor to the biological communities of lakes in the western Canadian Arctic and can result in a number of limnological changes. We also show that palaeolimnological methods are effective for inferring the timing and response of aquatic ecosystems to permafrost degradation. These findings provide the first long‐term perspective on the biological response to permafrost thaw, a stressor that will become increasingly important as northern landscapes respond to climate change.
One of the most ominous predictions related to recent climatic warming is that low-lying coastal environments will be inundated by higher sea levels. The threat is especially acute in polar regions ...because reductions in extent and duration of sea ice cover increase the risk of storm surge occurrence. The Mackenzie Delta of northwest Canada is an ecologically significant ecosystem adapted to freshwater flooding during spring breakup. Marine storm surges during the open-water season, which move saltwater into the delta, can have major impacts on terrestrial and aquatic systems. We examined growth rings of alder shrubs (Alnus viridis subsp. fruticosa) and diatoms preserved in dated lake sediment cores to show that a recent marine storm surge in 1999 caused widespread ecological changes across a broad extent of the outer Mackenzie Delta. For example, diatom assemblages record a striking shift from freshwater to brackish species following the inundation event. What is of particular significance is that the magnitude of this recent ecological impact is unmatched over the > 1,000-year history of this lake ecosystem. We infer that no biological recovery has occurred in this lake, while large areas of terrestrial vegetation remain dramatically altered over a decade later, suggesting that these systems may be on a new ecological trajectory. As climate continues to warm and sea ice declines, similar changes will likely be repeated in other coastal areas of the circumpolar Arctic. Given the magnitude of ecological changes recorded in this study, such impacts may prove to be long lasting or possibly irreversible.
We present a new 23‐site network of white spruce ring‐width chronologies near boreal treeline in Old Crow Flats, Yukon Territory, Canada. Most chronologies span the last 300 years and some reach the ...mid‐16th century. The chronologies exhibit coherent growth patterns before the 1930s. However, since the 1930s, they diverge in trend and exhibit one of two contrasting, but well‐replicated patterns we call Group 1 and Group 2. Over the instrumental period (1930–2007) Group 1 sites were inversely correlated with previous‐year July temperatures while Group 2 sites were positively correlated with growth‐year June temperatures. At the broader northwestern North America (NWNA) scale, we find that the Group 1 and Group 2 patterns are common to a number of white spruce chronologies, which we call NWNA 1 and NWNA 2 chronologies. The NWNA 1 and NWNA 2 chronologies also share a single coherent growth pattern prior to their divergence (ca. 1950s). Comparison of the NWNA 1/NWNA 2 chronologies against gridded 20th‐century temperatures for NWNA and reconstructed northern hemisphere summer temperatures (ad 1300–2000) indicates that all sites responded positively to temperature prior to the mid‐20th century (at least back to ad 1300), but that some changed to a negative response (NWNA 1) while others maintained a positive response (NWNA 2). The spatial extent of divergence implies a large‐scale forcing. As the divergence appears to be restricted to the 20th century, we suggest that the temperature response shift represents a moisture stress caused by an anomalously warm, dry 20th‐century climate in NWNA, as indicated by paleoclimatic records. However, because some sites do not diverge and are located within a few kilometres of divergent sites, we speculate that site‐level factors have been important in determining the susceptibility of sites to the large‐scale drivers of divergence.
Tree growth at northern treelines is generally temperature‐limited due to cold and short growing seasons. However, temperature‐induced drought stress was repeatedly reported for certain regions of ...the boreal forest in northwestern North America, provoked by a significant increase in temperature and possibly reinforced by a regime shift of the pacific decadal oscillation (PDO). The aim of this study is to better understand physiological growth reactions of white spruce, a dominant species of the North American boreal forest, to PDO regime shifts using quantitative wood anatomy and traditional tree‐ring width (TRW) analysis. We investigated white spruce growth at latitudinal treeline across a >1,000 km gradient in northwestern North America. Functionally important xylem anatomical traits (lumen area, cell‐wall thickness, cell number) and TRW were correlated with the drought‐sensitive standardized precipitation–evapotranspiration index of the growing season. Correlations were computed separately for complete phases of the PDO in the 20th century, representing alternating warm/dry (1925–1946), cool/wet (1947–1976) and again warm/dry (1977–1998) climate regimes. Xylem anatomical traits revealed water‐limiting conditions in both warm/dry PDO regimes, while no or spatially contrasting associations were found for the cool/wet regime, indicating a moisture‐driven shift in growth‐limiting factors between PDO periods. TRW reflected only the last shift of 1976/1977, suggesting different climate thresholds and a higher sensitivity to moisture availability of xylem anatomical traits compared to TRW. This high sensitivity of xylem anatomical traits permits to identify first signs of moisture‐driven growth in treeline white spruce at an early stage, suggesting quantitative wood anatomy being a powerful tool to study climate change effects in the northwestern North American treeline ecotone. Projected temperature increase might challenge growth performance of white spruce as a key component of the North American boreal forest biome in the future, when drier conditions are likely to occur with higher frequency and intensity.
We examined how the observed phases of 20th century PDO (pacific decadal oscillation) variability influenced the temporal relationships between tree growth (assessed using wood anatomical traits and tree‐ring width) and water availability in a dominant conifer species (white spruce) of the North American treeline ecotone along a longitudinal gradient. We found xylem anatomical traits being significantly associated with water availability during positive (dry/warm) phases of the PDO, while there were no or spatially contrasting associations during the cool/wet phase. Our findings indicate temporally and spatially adapted growth of white spruce and provide insights on its physiological functioning under climate change.
Macroscopic charcoal analysis of lake sediment stratigraphies is a widely used approach to reconstruct past biomass burning patterns of ecosystems. The development of fire records often relies on a ...single quantification method of charcoal in a sediment subsample; however, recent studies have shown that additional paleoecological information can be obtained by classifying charcoal morphologies. The morphologies and diagnostic features of charcoal yields information about fuel sources, fire type, and charcoal taphonomy, and can aid in calibrating sediment records to known historical fires. This additional information enhances paleoecological inferences by providing more paleoenvironmental information than studies of total charcoal as the only metric. Here we present a classification of 27 macroscopic charcoal morphologies observed in Holocene sediments of lakes located in the mixed-conifer forests of southeastern British Columbia, Canada. This classification system builds on other morphological classifications that have been previously utilized, but is more inclusive of the morphological variability observed and is flexible to modification for use when applied to other study settings. The morphological classification presented here was developed following the observation of >100,000 macroscopic charcoal fragments >150 µm. This paper focuses on the observed morphological classes, their identification, potential fuel sources, and the morphotype assemblage stratigraphy from one site as an example. The charcoal assemblages varied throughout the mid-to-late Holocene contemporaneously with known regional scale hydroclimatic changes in British Columbia. Major changes in fire frequency were also concomitant with morphotype assemblage changes. Future work focusing on linking fuel types with charcoal morphotypes, post-fire observations of charcoal taphonomy, and the analysis of multiple attribute charcoal data sets from a variety of ecosystems will improve our understanding of biomass burning and long-term fire ecology.
Aim Top-down (climatic) controls of fire occurrence are expected to homogenize fire regimes in a given area over long (millennial) temporal scales. Previous investigations in south-eastern British ...Columbia have shown that bottom-up (local site) factors can override long-term climate as a dominant control. Here, we examine the interactions between fire regime controls using five 5000-year-long lake sediment records. Location All lakes are located in separate watersheds within a 550-km2 region of Engelmann spruce–subalpine fir forest in the Columbia Mountains, British Columbia, Canada. Methods Sedimentary macroscopic (> 150 μm) charcoal analysis was used to produce local fire history records. Fire events were identified by decomposing the charcoal concentration series using the program CharAnalysis. We analysed the temporal coherence of the reconstructed fire events and the distributions of the fire return intervals at all sites and pooled sites by their northfacing or south-facing aspect. Results Since 5000 cal. yr bp, fire frequency was highest between 4250 and 2750 cal. yr bp and lowest from 750 cal. yr bp to the present. Median fire return intervals were shorter on the warmer and drier south-facing slopes (135–190 years) and longer on the cooler and moister north-facing slopes (226–241 years). Significant synchrony existed between sites with similar aspect, but no synchrony was found at sites with opposing aspect, providing evidence for the importance of bottom-up controls. Smoothed fire frequencies suggested that the influence of aspect varied throughout the period and that the importance of aspect could be overridden by other controls. The asynchrony between sites with opposite aspects suggests that local conditions for fire are important spatial controls on the fire regime. Main conclusions Aspect is an important bottom-up control of fire regimes in mid-elevation forests and its influence varies through time. The variability of climate–fire–vegetation interactions in the region needs to be investigated to understand the importance of top-down and bottom-up controls.
We examined dated sediment cores from 14 thermokarst affected lakes in the Mackenzie Delta uplands, NT, Arctic Canada, using a case-control analysis to determine how retrogressive thaw slump ...development from degrading permafrost affected the delivery of mercury (Hg) and organic carbon (OC) to lakes. We show that sediments from the lakes with retrogressive thaw slump development on their shorelines (slump-affected lakes) had higher sedimentation rates and lower total Hg (THg), methyl mercury (MeHg), and lower organic carbon concentrations compared to lakes where thaw slumps were absent (reference lakes). There was no difference in focus-corrected Hg flux to sediments between reference lakes and slump-affected lakes, indicating that the lower sediment Hg concentration in slump-affected lakes was due to dilution by rapid inorganic sedimentation in the slump-affected lakes. Sedimentation rates were inversely correlated with THg concentrations in sediments among the 14 lakes considered, and explained 68% of the variance in THg concentration in surface sediment, further supporting the dilution hypothesis. We observed higher S2 (algal-derived carbon) and particulate organic carbon (POC) concentrations in sediment profiles from reference lakes than in slump lakes, likely because of dilution by inorganic siliciclastic matter in cores from slump-affected lakes. We conclude that retrogressive thaw slump development increases inorganic sedimentation in lakes, and decreases concentrations of organic carbon and associated Hg and MeHg in sediments.
Mining in northern Canada has been known to cause major environmental problems; however, historical monitoring data are scarce or non-existent. Here, we use a multi-proxy (metals, bioindicators, ...pigments) paleolimnological approach to track the impacts of mining activity near Keno City, on the traditional land of the First Nation of Na-Cho Nyäk Dun, in central Yukon (Canada). Silver was discovered in the early 1900s, primarily on or between two hills (Galena Hill and Keno Hill). Intensive mining has taken place ever since, with brief hiatuses dependent on ore prices and ownership of the claims. Christal Lake, a shallow site located in the valley between both hills, lies near many historical and current mines, and was once the site of a processing mill. Geochemical data show elevated background concentrations of many metals and faithfully track known mining activity. Interestingly, background (pre-mining) sediment concentrations of arsenic, cadmium, and zinc were all elevated above the Canadian Sediment Quality Guidelines for the Protection of Aquatic Life, reflecting the natural weathering of elements in high concentrations from the local catchment. These, and other metal(loid)s, increased and peaked in sedimentary concentration after ca. 1920s, when intensive mining began. Sedimentary chlorophyll-
a
concentrations declined with the rise of metal concentrations, although values increased again slightly in more recent sediments, perhaps reflecting the decline in recent metal inputs and reclamation of historic mine sites. Meanwhile, subfossil diatom assemblages were dominated by small benthic
Fragilaria sensu lato
taxa, whose assemblage composition only changed subtly with mining (similar to other shallow, non-acidified sites in the highly metal-impacted area of Norilsk, Siberia). There was no biological evidence of acidification, likely due to the neutralizing effect of the carbonate-rich catchment. Cladoceran subfossils were only present in very low numbers throughout the core, reflecting both the shallow nature and high background metal concentrations in the lake. Collectively, these data show the long-term impacts of silver mining in this subarctic environment.
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