Measurements of aerosol and CCN properties in the Mackenzie River delta (Canadian Arctic) during spring–summer transition in May 2014

Abstract. Within the framework of the RACEPAC (Radiation–Aerosol–Cloud Experiment in the Arctic Circle) project, the Arctic aerosol, arriving at a ground-based station in Tuktoyaktuk (Mackenzie River delta area, Canada), was characterized during a period of 3 weeks in May 2014. Basic meteorological parameters and particle number size distributions (PNSDs) were observed and two distinct types of air masses were found. One type were typical Arctic haze air masses, termed accumulation-type air masses, characterized by a monomodal PNSD with a pronounced accumulation mode at sizes above 100 nm. These air masses were observed during a period when back trajectories indicate an air mass origin in the north-east of Canada. The other air mass type is characterized by a bimodal PNSD with a clear minimum around 90 nm and with an Aitken mode consisting of freshly formed aerosol particles. Back trajectories indicate that these air masses, termed Aitken-type air masses, originated from the North Pacific. In addition, the application of the PSCF receptor model shows that air masses with their origin in active fire areas in central Canada and Siberia, in areas of industrial anthropogenic pollution (Norilsk and Prudhoe Bay Oil Field) and the north-west Pacific have enhanced total particle number concentrations (NCN). Generally, NCN ranged from 20 to 500 cm−3, while cloud condensation nuclei (CCN) number concentrations were found to cover a range from less than 10 up to 250 cm−3 for a supersaturation (SS) between 0.1 and 0.7 %. The hygroscopicity parameter κ of the CCN was determined to be 0.23 on average and variations in κ were largely attributed to measurement uncertainties. Furthermore, simultaneous PNSD measurements at the ground station and on the Polar 6 research aircraft were performed. We found a good agreement of ground-based PNSDs with those measured between 200 and 1200 m. During two of the four overflights, particle number concentrations at 3000 m were found to be up to 20 times higher than those measured below 2000 m; for one of these two flights, PNSDs measured above 2000 m showed a different shape than those measured at lower altitudes. This is indicative of long-range transport from lower latitudes into the Arctic that can advect aerosol from different regions in different heights.

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Measurements of aerosol and CCN properties in the Mackenzie River delta (Canadian Arctic) during Spring-Summer transition in May 2014

10.5194/acp-2017-745 ◽

2017 ◽

Cited By ~ 1

Author(s):

Paul Herenz ◽

Heike Wex ◽

Silvia Henning ◽

Thomas Bjerring Kristensen ◽

Florian Rubach ◽

...

Keyword(s):

Particle Number ◽

Cloud Condensation Nuclei ◽

River Delta ◽

The Arctic ◽

Air Mass ◽

Back Trajectories ◽

Air Masses ◽

North East ◽

The North ◽

Mackenzie River

Abstract. Within the framework of the RACEPAC (Radiation-Aerosol-Cloud Experiment in the Arctic Circle) project, the Arctic aerosol, arriving at a ground based station in Tuktoyaktuk (Mackenzie River delta area, Canada), was characterized during a period of 3 weeks in May 2014. The observations of basic meteorological parameters and particle number size distributions (PNSDs) were indicative for the rapid transition from Arctic spring to summer that took place during the measurement period. Two distinct types of air masses were found. One type were typical Arctic haze air masses, termed as spring-type air masses, characterized by a mono-modal PNSD with a pronounced accumulation mode at sizes above 100 nm. These air masses were observed during a period when back trajectories indicate an air mass origin in the north east of Canada. The other air mass type is characterized by a bi-modal PNSD with a clear minimum around 90 nm, and with an Aitken mode consisting of freshly formed aerosol particles. Back trajectories indicate that these air masses, termed as summer-type air masses, originated from the northern Pacific. Generally total particle number concentrations (NCN) ranged from 20 to 500 cm−3, while cloud condensation nuclei (CCN) number concentrations were found to cover a range between less than 10 up to 250 cm−3 for a supersaturation (SS) between 0.1 and 0.7 %. The hygroscopicity parameter κ of the CCN was determined to be 0.23 on average and variations in kappa were largely attributed to measurement uncertainties. Furthermore, simultaneous PNSD measurements at the ground station and on the Polar 6 research aircraft were performed. We found a good agreement of ground based PNSDs with those measured between 200 and 1200 m. During two of the four overflights, particle number concentrations at 3000 m were found to be up to twenty times higher than those measured below 2000 m, and for one of these two flights, PNSDs measured above 2000 m showed a different shape than those measured at lower altitudes. This is indicative for long range transport from lower latitudes into the Arctic that can advect aerosol from different regions in different heights.

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Bacterial diversity in snow from mid-latitude mountain areas: Alps, Eastern Anatolia, Karakoram and Himalaya

Annals of Glaciology ◽

10.1017/aog.2018.18 ◽

2018 ◽

Vol 59 (77) ◽

pp. 10-20 ◽

Cited By ~ 4

Author(s):

Roberto Sergio Azzoni ◽

Ilario Tagliaferri ◽

Andrea Franzetti ◽

Christoph Mayer ◽

Astrid Lambrecht ◽

...

Keyword(s):

Microbial Communities ◽

Bacterial Communities ◽

The Arctic ◽

Eastern Anatolia ◽

Mountain Areas ◽

Air Mass ◽

Back Trajectories ◽

Air Masses ◽

Operational Taxonomic Units ◽

High Throughput Dna Sequencing

ABSTRACTSnow can be considered an independent ecosystem that hosts active microbial communities. Snow microbial communities have been extensively investigated in the Arctic and in the Antarctica, but rarely in mid-latitude mountain areas. In this study, we investigated the bacterial communities of snow collected in four glacierized areas (Alps, Eastern Anatolia, Karakoram and Himalaya) by high-throughput DNA sequencing. We also investigated the origin of the air masses that produced the sampled snowfalls by reconstructing back-trajectories. A standardized approach was applied to all the analyses in order to ease comparison among different communities and geographical areas. The bacterial communities hosted from 25 to 211 Operational Taxonomic Units (OTUs), and their structure differed significantly between geographical areas. This suggests that snow bacterial communities may largely derive from ‘local’ air bacteria, maybe by deposition of airborne particulate of local origin that occurs during snowfall. However, some evidences suggest that a contribution of bacteria collected during air mass uplift to snow communities cannot be excluded, particularly when the air mass that originated the snow event is particularly rich in dust.

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In situ measurements of aerosol optical properties and number size distributions in a coastal region of Norway during the summer of 2008

Atmospheric Chemistry and Physics ◽

10.5194/acp-12-5841-2012 ◽

2012 ◽

Vol 12 (13) ◽

pp. 5841-5857 ◽

Cited By ~ 11

Author(s):

S. Mogo ◽

V. E. Cachorro ◽

J. F. Lopez ◽

E. Montilla ◽

B. Torres ◽

...

Keyword(s):

The Arctic ◽

Size Distributions ◽

Aerosol Optical Properties ◽

Air Mass ◽

Mean Values ◽

Air Masses ◽

The North ◽

Microphysical Parameters ◽

The One

Abstract. In situ measurements of aerosol optical properties and particle size distributions were made in the summer of 2008 at the ALOMAR station facility (69°16' N, 16°00' E), located in a rural site in the north of the island of Andøya (Vesterålen archipelago), approximately 300 km north of the Arctic Circle. The extended three-month campaign was part of the POLARCAT Project (Polar Study using Aircraft, Remote Sensing, Surface Measurements and Models, of Climate, Chemistry, Aerosols, and Transport) of the International Polar Year (IPY-2007-2008). Our goal was to characterize the aerosols of this sub-Arctic area, which are frequently transported to the Arctic region. Data from 13 June to 26 August 2008 were available and the statistical data for all instruments were calculated based on the hourly averages. The overall data coverage was approximately 72%. The hourly mean values of the light-scattering coefficient, σs, and the light-absorption coefficient, σa, at 550 nm were 5.41 Mm−1 (StD = 3.55 Mm−1) and 0.40 Mm−1 (StD = 0.27 Mm−1), respectively. The scattering/absorption Ångström exponents, αs,a, were used in a detailed analysis of the variations of the spectral shape of σs,a. While αs indicates the presence of two particle sizes corresponding to two types of aerosols, αa indicates only one type of absorbing aerosol particle. αa values greater than 1 were not observed. The single-scattering albedo, ω0, ranged from 0.62 to 0.99 (mean = 0.91, StD = 0.05), and the relationships between this parameter and the absorption/scattering coefficients and the Ångström exponents are presented. Any absorption value may lead to the lowest values of ω0, whereas only the lowest scattering values were observed in the lowest range of ω0. For a given absorption value, lower ω0 were observed for smaller αs. The submicrometer, micrometer and total concentrations of the particles presented hourly mean values of 1277 cm−3 (StD = 1563 cm−3), 1 cm−3 (StD = 1 cm−3) and 2463 cm−3 (StD = 4251 cm−3), respectively, and the modal correlations were also investigated. The optical and microphysical parameters, as well as their relationship with each other, are reported. σs correlated strongly with the number concentration of accumulation mode particles and more strongly with the micrometer fraction of particles, but weak correlations were observed for the Aitken and nucleation modes. The origins and pathways of the air masses were examined, and based on sector classification, a relationship between the air mass origin, the optical parameters and the size distributions was established. The low values of the optical and microphysical parameters indicate that the predominant regional aerosol is mostly clean and the shape of the size distribution is characterized by bimodal median size distributions. However, the relationships between the air mass origins and the parameters studied allow us to describe two characteristic situations: the one of the northern and western air masses, which were predominantly composed of marine aerosols and presented the lowest optical and microphysical values observed, indicating predominantly non-absorbent and coarser particles; and the one of the eastern and southern air masses, in which continental aerosols were predominant and exhibited higher values for all parameters, indicating the presence of smaller absorbent particles. The north-northeastern air masses presented the strongest Aitken mode, indicating more recently formed particles, and the southeastern air masses presented the strongest accumulation mode (however, the southeastern air masses were the least common, accounting for only 3% of occurrences).

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Continuous atmospheric CO<sub>2</sub>, CH<sub>4</sub> and CO measurements at the OPE station in France from 2011 to 2018

10.5194/amt-2019-128 ◽

2019 ◽

Author(s):

Sébastien Conil ◽

Julie Helle ◽

Laurent Langrene ◽

Olivier Laurent ◽

Michel Ramonet

Keyword(s):

Greenhouse Gases ◽

Observation System ◽

Atmospheric Concentration ◽

Close Link ◽

Back Trajectories ◽

Air Masses ◽

Mixing Ratios ◽

North East ◽

The North ◽

Carbon Dioxide Co2

Abstract. Located in the North East of France, the Observatoire Pérenne de l'Environnement (OPE) station was built during the Integrated Carbon Observation System (ICOS) Demonstration Experiment to monitor the atmospheric concentration of greenhouse gases. Its continental rural background setting allows to fill the gaps between oceanic or mountain background stations and urban stations within the ICOS network. Continuous measurements of several greenhouse gases using high precision spectrometers started in 2011 on a tall tower with three sampling inlets at 10 m, 50 m and 120 m above the ground. The measurements quality are regularly assessed using several complementary approaches based on reference high pressure cylinders, traveling instruments audit and sets of travelling cylinders (so-called cucumber intercomparison). Thanks to the quality assurance strategy recommended by ICOS, the measurements precision are within the WMO compatibility goals for carbon dioxide (CO2), methane (CH4) and carbon monoxide (CO). The mixing ratios time series from 2011 to end of 2018 allow analyses of trends and diurnal and seasonal cycles. The CO2 and CH4 annual growth rates are respectively 2.4 ppm/year and 8.8 ppb/year for the 120 m above ground measurements over the investigated period. No significant trend has however been recorded for the CO mixing ratios. The afternoon mean residuals of these three compounds are significantly stronger during the cold period when inter-species correlations are high, compared to the warm period. The residuals variabilities show a close link with the air masses back-trajectories.

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UV/PAR radiations and DOM properties in surface coastal waters of the Canadian shelf of the Beaufort Sea during summer 2009

Biogeosciences Discussions ◽

10.5194/bgd-9-15567-2012 ◽

2012 ◽

Vol 9 (11) ◽

pp. 15567-15602 ◽

Cited By ~ 1

Author(s):

J. Para ◽

B. Charrière ◽

A. Matsuoka ◽

W. L. Miller ◽

J. F. Rontani ◽

...

Keyword(s):

Solar Radiation ◽

Beaufort Sea ◽

The Arctic ◽

Mackenzie Delta ◽

Fluorescence Excitation ◽

North East ◽

The North ◽

Fluorescent Dissolved Organic Matter ◽

Solar Radiation Incident ◽

Mackenzie River

Abstract. Water masses from the Beaufort Sea in the Arctic Ocean were evaluated for dissolved organic carbon (DOC), and optical characteristics including UV and PAR diffuse attenuation (Kd), and chromophoric and fluorescent dissolved organic matter (CDOM and FDOM) as part of the MALINA field campaign (30 July to 27 August). Even with relatively low mean daily solar radiation incident on the sea surface (0.12 ± 0.03, 8.46 ± 1.64 and 18.09 ± 4.20 kJ m−2 for UV-B (305 nm), UV-A (380 nm) and PAR, respectively), we report significant light penetration with 10% irradiance depths (Z10% (λ)) reaching 9.5 m for 340 nm (UV-A) radiation in the Eastern sector and 4.5 m in the Mackenzie River influenced area (Western sector). Spectral absorption coefficients (aCDOM (350 nm) (m−1)) were significantly correlated to both diffuse attenuation coefficients (Kd) in the UV-A and UV-B and to DOC concentrations. This indicates CDOM as the dominant attenuator of UV solar radiation and suggests its use as an optical proxy for DOC concentrations in this region. Extrapolating CDOM to DOC relationships, we estimate that ~ 16% of the DOC in the Mackenzie River does not absorb radiation at 350 nm. DOC and CDOM discharges by the Mackenzie River during the MALINA Cruise are estimated as ~ 0.22 TgC and 0.18 TgC, respectively. Three dissolved fluorescent components (C1–C3) were identified by fluorescence Excitation/Emission Matrix Spectroscopy (EEMS) and PARAFAC analysis. Our results showed an in-situ biological component (C1) that co-dominated with a terrestrial humic-like component (C2) in the Mackenzie Delta sector, whereas the protein-like (C3) component dominated in the saltiest waters of the North East sector.

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Variations in tropospheric submicron particle size distributions across the European continent 2008–2009

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-31197-2013 ◽

2013 ◽

Vol 13 (11) ◽

pp. 31197-31249 ◽

Cited By ~ 2

Author(s):

D. C. S. Beddows ◽

M. Dall'Osto ◽

Roy M. Harrison ◽

M. Kulmala ◽

A. Asmi ◽

...

Keyword(s):

Particle Size ◽

Particle Number ◽

The Arctic ◽

Aerosol Size Distribution ◽

Size Distributions ◽

European Continent ◽

Complex Behaviour ◽

Back Trajectories ◽

Air Masses ◽

Aerosol Aging

Abstract. Cluster analysis of particle number size distributions from background sites across Europe is presented. This generated a total of nine clusters which could be further combined into two main groups, namely: a South to North category (four clusters) and a West to East category (five clusters). The first category was identified as most frequently being detected inside and around Northern Germany and neighbouring countries, showing clear evidence of local afternoon nucleation and growth events that could be linked to movement of air masses from South-to-North arriving ultimately at the Arctic resulting in Arctic Haze. The second group of particle size spectra proved to have narrower size distributions and collectively showed a dependence of modal diameter upon the longitude of the site (West to East) at which they were most frequently detected. These clusters indicated regional nucleation (at the coastal sites) growing to larger modes further inland. The apparent growth rate of the modal diameter was around 1–3 nm h−1. Four specific air mass back trajectories were successively taken as case studies to examine in real time the evolution of aerosol size distributions across Europe. While aerosol growth processes can be observed as aerosol traverses Europe, the processes are often obscured by the addition of aerosol by emissions en route. This study revealed that some of the 24 stations exhibit more complex behaviour than others, especially when impacted by local sources or a variety of different air masses. Overall, the aerosol size distribution clustering analysis greatly simplifies the complex dataset and allows a description of aerosol aging processes, which reflects the longer-term average development of particle number size distributions as air masses advect across Europe.

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Identification of the Aerosol Types over Athens, Greece: The Influence of Air-Mass Transport

Advances in Meteorology ◽

10.1155/2010/168346 ◽

2010 ◽

Vol 2010 ◽

pp. 1-15 ◽

Cited By ~ 8

Author(s):

D. G. Kaskaoutis ◽

P. G. Kosmopoulos ◽

H. D. Kambezidis ◽

P. T. Nastos

Keyword(s):

Winter Season ◽

Saharan Dust ◽

Dust Particles ◽

Hysplit Model ◽

Air Mass ◽

Back Trajectories ◽

Air Masses ◽

Local Pollution ◽

Coarse Mode ◽

Aerosol Types

Aerosol optical depth at 550 nm () and fine-mode (FM) fraction data from Terra-MODIS were obtained over the Greater Athens Area covering the period February 2000–December 2005. Based on both and FM values three main aerosol types have been discriminated corresponding to urban/industrial aerosols, clean maritime conditions, and coarse-mode, probably desert dust, particles. Five main sectors were identified for the classification of the air-mass trajectories, which were further used in the analysis of the ( and FM data for the three aerosol types). The HYSPLIT model was used to compute back trajectories at three altitudes to investigate the relation between -FM and wind sector depending on the altitude. The accumulation of local pollution is favored in spring and corresponds to air masses at lower altitudes originating from Eastern Europe and the Balkan. Clean maritime conditions are rare over Athens, limited in the winter season and associated with air masses from the Western or Northwestern sector. The coarse-mode particles origin seems to be more complicated proportionally to the season. Thus, in summer the Northern sector dominates, while in the other seasons, and especially in spring, the air masses belong to the Southern sector enriched with Saharan dust aerosols.

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Hydrogenetic Fe-Mn crusts from European seas: source of potentially economic cobalt mining.

10.5194/egusphere-egu2020-22091 ◽

2020 ◽

Author(s):

Egidio Marino ◽

Javier González ◽

Teresa Medialdea ◽

Luis Somoza ◽

Rosario Lunar ◽

...

Keyword(s):

Lithium Ion ◽

Essential Elements ◽

Raw Material ◽

The Arctic ◽

World Production ◽

Critical Metals ◽

North West ◽

North East ◽

The North ◽

The World

<p>The world increasing demand of electric vehicles (EVs) that use lithium-ion batteries (LIB), in which cobalt is one of the essential elements, focused the attention on its demand that is calculated will increase of 7-13% annually until 2030. The actual production of cobalt, usually extract as by-product of nickel and copper mine, is reduced to almost 20 countries between which the Democratic Republic of the Congo is the bigger producer with 55% of the world production. In Europe cobalt is produced only in Finland that actually provides 2.300 tonnes, the 2% of the world production. In this way several projects have been promoted by European Union, with the Raw Material Initiative, in order to find and evaluate the sustainable production of important materials in Europe.</p><p>MINDeSEA[1] project is part of the GeoERA and represent the collaboration of 12 national geological institution partners, to characterize marine deposits and their contents in Critical Raw Materials (CRM) and to generate a comprehensive cartography and metallogenic models of them. The first preliminary map produced in 2019 represents the localization and evaluation of cobalt rich deposits in the oceans within the EEZ and ECS of the European countries. &#160;Cobalt deposits are represented essentially by hydrogenetic Fe-Mn crusts located essentially in the Macaronesian area of the north east Atlantic Ocean (in the Portugal and Spain), submarine plateaus, as the Galicia Bank (in the north west Spanish) and in the Arctic Ocean ridges (Norway and Iceland). The report differentiates between occurrences (<0.05 wt. %) and deposits (>0.05 wt. %), with the possibility of more than 200 Mt resources per potential deposit.</p><p>Detailed mineralogical, geochemical and metallogenic studies are being developed in crusts from the Macaronesia. Fe-Mn crusts absorb dissolved elements in seawaters on the surface of the fresh precipitated oxy-hydroxides during their slow growth through millions of years. Several elements are concentrated in Fe-Mn crusts and between them cobalt is one of the most enriched trace metals (average 0.6 wt. %) accompanied by other strategic and critical metals such as nickel, copper, tellurium, molybdenum and rare earth elements plus yttrium (REY) (respectively 3000, 500, 150, 500 and 3500 &#181;g/g). Micro Raman and micro X-Ray diffraction can be used to differentiate the mineralogy in laminae of less than 20 microns. On the other hand, electron probe micro-analyzer (EPMA) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), are useful in order to quantify contents of CRM in the different mineral phases. These are innovative techniques in order to identify critical-elements bearing minerals and thus choose the metallurgic method for a more efficient and sustainable extraction of the interesting elements.</p><p>The evaluation of a seamount as a future mine site has to take into account all these mineralogical and chemical features as well as a proper knowledge of the seamount (morpho-structure, geology, oceanography, ecosystems) and the Fe-Mn crust thickness and extension</p><div><br><div> <p>[1] This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 731166</p> </div> </div>

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Ozone variability and halogen oxidation within the Arctic and sub-Arctic springtime boundary layer

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-10223-2010 ◽

2010 ◽

Vol 10 (21) ◽

pp. 10223-10236 ◽

Cited By ~ 92

Author(s):

J. B. Gilman ◽

J. F. Burkhart ◽

B. M. Lerner ◽

E. J. Williams ◽

W. C. Kuster ◽

...

Keyword(s):

Boundary Layer ◽

North Atlantic ◽

Marine Boundary Layer ◽

Transport Model ◽

Arctic Basin ◽

The Arctic ◽

Arctic Boundary Layer ◽

Air Masses ◽

The North ◽

The North Atlantic

Abstract. The influence of halogen oxidation on the variabilities of ozone (O3) and volatile organic compounds (VOCs) within the Arctic and sub-Arctic atmospheric boundary layer was investigated using field measurements from multiple campaigns conducted in March and April 2008 as part of the POLARCAT project. For the ship-based measurements, a high degree of correlation (r = 0.98 for 544 data points collected north of 68° N) was observed between the acetylene to benzene ratio, used as a marker for chlorine and bromine oxidation, and O3 signifying the vast influence of halogen oxidation throughout the ice-free regions of the North Atlantic. Concurrent airborne and ground-based measurements in the Alaskan Arctic substantiated this correlation and were used to demonstrate that halogen oxidation influenced O3 variability throughout the Arctic boundary layer during these springtime studies. Measurements aboard the R/V Knorr in the North Atlantic and Arctic Oceans provided a unique view of the transport of O3-poor air masses from the Arctic Basin to latitudes as far south as 52° N. FLEXPART, a Lagrangian transport model, was used to quantitatively determine the exposure of air masses encountered by the ship to first-year ice (FYI), multi-year ice (MYI), and total ICE (FYI+MYI). O3 anti-correlated with the modeled total ICE tracer (r = −0.86) indicating that up to 73% of the O3 variability measured in the Arctic marine boundary layer could be related to sea ice exposure.

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Approaching Freshet beneath Landfast Ice in Kugmallit Bay on the Canadian Arctic Shelf: Evidence from Sensor and Ground Truth Data

ARCTIC ◽

10.14430/arctic8 ◽

2009 ◽

Vol 61 (1) ◽

pp. 76 ◽

Cited By ~ 6

Author(s):

Tony R. Walker ◽

Jon Grant ◽

Peter Jarvis

Keyword(s):

Arctic Ocean ◽

Ground Truth ◽

Open Water ◽

Early Spring ◽

The Arctic ◽

Ground Truth Data ◽

The North ◽

Landfast Ice ◽

North American Arctic ◽

Mackenzie River

The Mackenzie River is the largest river in the North American Arctic. Its huge freshwater and sediment load impacts the Canadian Beaufort Shelf, transporting large quantities of sediment and associated organic carbon into the Arctic Ocean. The majority of this sediment transport occurs during the freshet peak flow season (May to June). Mackenzie River-Arctic Ocean coupling has been widely studied during open water seasons, but has rarely been investigated in shallow water under landfast ice in Kugmallit Bay with field-based surveys, except for those using remote sensing. We observed and measured sedimentation rates (51 g m-2 d-1) and the concentrations of chlorophyll a (mean 2.2 ?g L-1) and suspended particulate matter (8.5 mg L-1) and determined the sediment characteristics during early spring, before the breakup of landfast ice in Kugmallit Bay. We then compared these results with comparable data collected from the same site the previous summer. Comparison of organic quality in seston and trapped material demonstrated substantial seasonal differences. The subtle changes in biological and oceanographic variables beneath landfast ice that we measured using sensors and field sampling techniques suggest the onset of a spring melt occurring hundreds of kilometres farther south in the Mackenzie Basin.

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