We report oxygen and hydrogen isotopic composition of river discharges in the main stem and tributaries in Mackenzie River system, Canada, based upon a synoptic survey and a time-series monitoring ...program undertaken between 2003 and 2006. The water isotope signals in the Mackenzie River system exhibit distinct geographic variations. Isotopic results from the comprehensive synoptic survey offset from meteoric water lines in
δ
18O–
δ
2H space, and suggest a snowmelt-driven discharge regime with mixing between heavy-isotope depleted discharge from Liard River and Mackenzie Mountains, and evaporatively-enriched water from Great Slave Lake catchment. Based upon these isotopic data in river discharge, we propose an evaporative-mixing line for the Mackenzie River system (
δ
2H
=
6.0
×
δ
18O
−
40.8), which highlights the integrated effects of the two most important hydrologic processes (i.e., evaporation and mixing). Time-series investigation of water isotope signatures and river discharge at three key hydrometric stations demonstrates the isotopic response to the strong seasonality of river discharge in northern Canada. In general, isotopic compositions in river discharge tend to be negatively correlated with discharge amount, which is a typical snowmelt-driven isotope-discharge pattern. However, low flow conditions are found to be noisy with a wide range of isotopic variability. More importantly, we also observe that extreme high flow events, especially in the Liard River, appear to be unexpectedly enriched in heavy isotopes, which indicate contributions from evaporated water that are probably derived from wetland surface storage. This is a situation, which produces a reversal in the isotope-discharge pattern and marks important episodes that may impact chemical and riverine/estuarine biotic processes at times of extremely high runoff.
The optical properties of chromophoric dissolved organic matter (CDOM) were investigated in the Canadian Archipelago and coastal Beaufort Sea surface waters using fluorescence spectroscopy coupled ...with parallel factor analysis (PARAFAC). Environmental dynamics of individual components were evaluated and compared to salinity, in situ fluorescence, absorption at 312 nm (a312), dissolved organic carbon, and lignin phenol concentrations. A positive linear relationship between four fluorescent components and lignin phenols suggests a terrestrial origin, whereas two components were unrelated to a river source, suggesting an autochthonous source. Elevated concentrations of terrestrial components were observed in the Mackenzie River plume near the coast of Alaska and decreased as water was transported to the Canadian Archipelago. The two nonterrestrial components exhibited only background levels in concentrations along the transect, suggesting minimal productivity within plume and archipelago surface waters. The relative abundance of terrestrial components in relation to nonterrestrial components allowed us to distinguish water masses including Atlantic, Archipelago, and Mackenzie River plume, respectively. This study illustrates the usefulness of PARAFAC to fingerprint water masses based on the optical characteristics of CDOM and shows promise to improve our understanding of upper Arctic Ocean ventilation.
River discharge has an important influence on the chemistry of Arctic shelf seas and surface waters. In this study, we examined the changes in nutrient, trace metal, dissolved organic matter, ...dissolved inorganic carbon, alkalinity, and radium isotope distributions across the Mackenzie River delta and estuary in June 2016, and estimated trace element and isotopes fluxes from the Mackenzie River to the Western Arctic Ocean. While inorganic nutrient, dissolved inorganic carbon, alkalinity, trace metal, and radium levels remained fairly constant through the delta, dissolved organic matter concentrations were 2–3.5 times higher at the mouth of the river, reflecting inputs from the delta. In the estuarine mixing zone, radium isotope ratios indicated that both desorption from suspended sediments and benthic inputs affected solute concentrations. Similarly, most of the measured trace metals (Ba, Co, Cu, Mn, Ni) exhibited non-conservative addition in the freshwater-saltwater transition zone, while Pb was removed and U was mixed conservatively. Inorganic nutrients were affected by both addition and removal processes in the estuary. Dissolved inorganic carbon and alkalinity exhibited net removal, likely due to a combination of biological uptake, gas exchange, CaCO₃ precipitation, and surface ion exchange processes. Approximately 45% and 60% of dissolved organic carbon and nitrogen, respectively, were removed rapidly at low salinities, in contrast with previous studies suggesting conservative mixing through Arctic estuaries. This study highlights the need to take deltaic and estuarine processes into account when determining the flux of riverine solutes to the coastal ocean.
The Mackenzie River Basin (MRB) delivers large quantities of organic carbon (OC) into the Arctic Ocean, with significant implications for the global C budgets and ocean biogeochemistry. The amount ...and properties of OC in the Mackenzie River's delta have been well monitored in the last decade, but the spatial variability in OC sources transported by its different tributaries is still unclear. Here we present new data on the radiocarbon (14C) content of dissolved and particulate OC (Δ14C‐DOC and Δ14C‐POC) across the mainstem and major tributaries of the MRB, comprising 19 different locations, to identify factors controlling spatial patterns in riverine OC sources. The Δ14C‐DOC and Δ14C‐POC varied across a large range, from −179.9‰ to 62.9‰, and −728.8‰ to −9.0‰, respectively. Our data reveal a positive spatial coupling between the Δ14C of DOC and POC across the MRB, whereby the most 14C‐depleted waters were issued from the mountainous west bank of the MRB. This 14C‐depleted DOC and POC likely originates from a combination of petrogenic sources, connected with the presence of kerogens in the bedrock, and biogenic sources, mobilized by thawing permafrost. Our analysis also reveals intriguing relationships between Δ14C of DOC and POC with turbidity, water stable isotope ratio and catchment elevation, indicating that hydrology and geomorphology are key to understanding riverine OC sources in this landscape. A closer examination of the specific mechanisms giving rise to these relationships is recommended. For now, this study provides a road map of the key OC sources in this rapidly changing river basin.
Key Points
We present a synoptic survey of radiocarbon content of dissolved and particulate organic carbon across the Mackenzie river basin
The data reveal intriguing relationships between radiocarbon content and turbidity, water isotopes, and topography
This study provides a road map of the key sources and controls of riverine OC age in this rapidly changing river basin
•Increasing salinity favored lipoxygenase induction in terrestrial higher plant debris.•Lipoxygenases initiated autoxidative degradation of terrestrial POM in estuaries.•Autoxidative degradation of ...terrestrial POM enhanced at low and high latitude.•In temperate zones autoxidative damage of terrestrial POM more limited.
There exists a substantial amount of research on abiotic (e.g. photochemical) degradation pertaining to organic matter (OM) in the marine realm. While recent research has shown its importance in the degradation of terrestrial particulate OM (TPOM), the mechanisms involved in the induction of autoxidation in estuaries remain unclear. In this study, we propose for the first time the involvement of lipoxygenase (LOX) activity in the induction of autoxidation in mixed waters. The observation of unusual profiles of palmitoleic acid oxidation products and the presence of jasmonic acid in suspended particulate matter (SPM) collected close to the Rhône River, as well as in samples from the Mackenzie and Amazon rivers, is attributed to strong LOX activity. We show the role played by salinity in the induction of this LOX activity and provide an explanation for the differences in estuarine autoxidation level. At high latitude, lower temperatures and irradiance favor photooxidative damage to higher plant debris and, consequently, hydroperoxide production. High hydroperoxide content strongly contributes to LOX activation in mixed waters. The high resulting LOX activity enhances alkoxyl radical production and thus autoxidation. On the contrary, at low latitude, photooxidative effects are limited, and riverine autoxidation is favored. The higher hydroperoxide content of TPOM may, as a consequence, thereby also contribute to a high level of LOX activity and autoxidation in estuaries. In temperate zones, land and riverine photooxidative and autoxidative damage is limited, unlike estuaries where we observed significant LOX-induced and autoxidative damage.
Alpine headwaters collect larger volumes of precipitation per unit area than neighbouring lowlands, recharge regional aquifers, and generate a greater proportion of river discharge than their limited ...extent would suggest. Despite the importance of alpine headwaters, field observations and assessments of source water contributions to streamflow in alpine tundra settings are sparse throughout the subarctic and absent in the Taiga Cordillera ecozone specifically. As such, it remains uncertain how changes to seasonally specific hydrological processes control discharge of the larger rivers to which these alpine headwaters contribute. This study quantified variability in source water contributions and flow paths during the 2019 open water season within a Mackenzie Mountain alpine tundra basin based on measurements of stable water isotopes, specific conductivity (SPC), and water volumes during runoff generating events. During the freshet, large daily snowmelt volumes resulted in the greatest volume of streamflow, which was composed mainly of pre‐event water (~92%). As the summer progressed, evapotranspiration increased, and groundwater flow paths extended, resulting in reduced event water fractions and hydrograph amplitude, and an extended duration of streamflow response. A headwater subbasin within the larger study basin was both hydrologically and isotopically unresponsive to summer rains, and instead was characterized by a delayed hydrograph response and reduced event water fraction. Results indicate this portion of the catchment was regulated by discharge from groundwater springs capable of sustaining streamflow before snowmelt commenced and during the dry summer months. As climate change continues, greater precipitation volumes and a longer open water season will likely result in reduced runoff and stream discharge from alpine basins as greater evapotranspiration and channel bed infiltration occur. This study provides a valuable data set and observations of seasonally distinct runoff generation processes to inform prediction of changes in northern alpine tundra hydrology in response to a warming climate.
An alpine tundra basin had distinct streamflow generation processes and source water contributions throughout the open water season. The streamflow response to snowmelt was highly efficient, but largely composed of pre‐event waters displaced from near surface soils and a shallow unconfined aquifer. During the summer, precipitation declined, basin storage increased, streamflow was less responsive and composed of even smaller event water fractions. The late summer also had evidence of increasing evapotranspiration and extended groundwater flow paths through deeper mineral substrates.
Fast-flowing ice streams occur within modern ice sheets and also operated in Pleistocene ice sheets. The reconstruction of palaeo-ice streams normally relies on the mapping of mega-scale glacial ...lineations (MSGLs) and drumlins composed of soft sediment, mainly till. Analysis of new satellite imagery and digital terrain models, demonstrates the presence of large fields of kilometre-scale glacial lineations comprising rock drumlins, megagrooves and megaridges. In this paper we describe and analyse a number of such ‘hard-bed’ landform systems from the former Laurentide and British–Irish ice sheets, occurring in a variety of palaeo-ice stream settings. These are attributed to erosion of crystalline and sedimentary rock below fast flowing ice streams. Bedrock properties such as hardness, fracture spacing and bedding and their orientation with respect to ice flow have a profound effect on the occurrence and character of elongate rock bedforms. Elongate streamlined forms on hard crystalline rock, as on the Canadian Shield, only form under special circumstances; in contrast, sedimentary strata are highly susceptible to form streamlined hard beds, specifically if bedrock strike is parallel to ice flow. Large-scale elongate rock bedforms are erosional in origin, formed by preferentially focused abrasion or by lateral plucking, depending on bedrock type. Many palaeo-ice stream footprints previously mapped in the Laurentide Ice Sheet on the basis of soft-bed bedforms are shown to be significantly larger, extending up-ice across sedimentary strata and onto Precambrian crystalline rocks. Hard-bed streamlined forms further show that ice streaming does not necessitate a deformable bed, but can equally occur on smooth hard beds.
The Mann–Kendall non-parametric test for trend is used to explore the trend behaviour of nine measures of the timing of runoff. The relationship between trends in timing measures and trends in ...meteorological variables are investigated using partial correlation analysis. The relationships between six climate indices and trends in the timing measures are also examined. The analysis is conducted for 26 streamflow gauging stations from three sub-watersheds of the Mackenzie River Basin in northern Canada. The results reveal that for several of the timing measures, many more trends are identified than can be expected to occur by chance. The spring freshet is observed to be occurring earlier with this timing shift appearing particularly strong in headwater catchments. Based on the partial correlation analysis, it is plausible to attribute some of the observed trends to trends in meteorological variables. Timing of runoff is affected by the Pacific Decadal Oscillation (PDO), the North Atlantic Oscillation (NAO), the North Pacific (NP) index and the Atlantic Multidecadal Oscillation (AMO) but not by the El Niño-Southern Oscillation (ENSO) or the Arctic Oscillation (AO).
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