•Assessment of reactive phases in suspended particulates in a major Arctic river.•Neodymium isotopes show equilibration between Fe-Mn oxides and dissolved loads.•Reactive iron flux from the Mackenzie ...River is large; 0.21(+0.06,−0.05) Tg/yr.•Sedimentary rock weathering provides an important source of labile trace metals.
Riverine suspended particulate matter (SPM) is essential for the delivery of micronutrients such as iron (Fe) to the oceans. SPM is known to consist of multiple phases with differing reactivity, but their role in the delivery of elements to the oceans is poorly constrained. Here we provide new constraints on the source and composition of reactive phases in SPM from the Mackenzie River, the largest sediment source to the Arctic Ocean. Sequential leaching of SPM shows that river sediments contain labile Fe phases. We estimate the labile Fe flux is substantial (0.21(+0.06,−0.05) Tg/yr) by quantifying Fe concentrations in weak leaches of the SPM. The labile Fe phase hosts a considerable amount of rare earth elements (REE), including neodymium (Nd). We demonstrate that the labile Fe phase and dissolved load have radiogenic Nd isotope ratios that are identical within uncertainty, but up to 8 epsilon units distinct from the silicate phase. We interpret this as evidence for dynamic cycling between Fe-oxide phases in SPM and the river water, demonstrating the high reactivity of the labile Fe phase. Nd isotope and elemental molar ratios suggest that a significant amount of labile Fe- and Nd-bearing phases are derived from Fe-oxides within the sedimentary source rock rather than silicate mineral dissolution. Thus, sedimentary rock erosion and weathering provides an important source of labile Fe, manganese (Mn) and by extension potentially other trace metals. Our results imply that both past and future environmental change in the Arctic, such as permafrost thaw, may trigger changes to the supply of reactive trace metals. These results demonstrate that a re-evaluation of sediment reactivity within rivers is required where uplifted sedimentary rocks are present.
We report Li isotopic compositions, for river waters and suspended sediments, of about 40 rivers sampled within the Mackenzie River Basin in northwestern Canada. The aim of this study is to ...characterize the behaviour of Li and its isotopes during weathering at the scale of a large mixed lithology basin. The Mackenzie River waters display systematically heavier Li isotopic compositions relative to source rocks and suspended sediments. The range in δ
7Li is larger in dissolved load (from +9.3‰ to +29.0‰) compared to suspended sediments (from −1.7‰ to +3.2‰), which are not significantly different from δ
7Li values in bedrocks. Our study shows that dissolved Li is essentially derived from the weathering of silicates and that its isotopic composition in the dissolved load is inversely correlated with its relative mobility when compared to Na. The highest enrichment of
7Li in the dissolved load is reported when Li is not or poorly incorporated in secondary phases after its release into solution by mineral dissolution. This counterintuitive observation is interpreted by the mixing of water types derived from two different weathering regimes producing different Li isotopic compositions within the Mackenzie River Basin. The incipient weathering regime characterizing the Rocky Mountains and the Shield areas produces
7Li enrichment in the fluid phase that is most simply explained by the precipitation of oxyhydroxide phases fractionating Li isotopes. The second weathering regime is found in the lowland area and produces the lower δ
7Li waters (but still enriched in
7Li compared to bedrocks) and the most Li-depleted waters (compared to Na). Fractionation factors suggest that the incorporation of Li in clay minerals is the mechanism that explains the isotopic composition of the lowland rivers. The correlation of boron and lithium concentrations found in the dissolved load of the Mackenzie Rivers suggests that precipitation of clay minerals is favoured by the relatively high residence time of water in groundwater. In the Shield and Rocky Mountains, Li isotopes suggest that clay minerals are not forming and that secondary minerals with stronger affinity for
7Li appear.
Although the weathering mechanisms operating in the Mackenzie Basin need to be characterized more precisely, the Li isotope data reported here clearly show the control of Li isotopes by the weathering intensity. The spatial diversity of weathering regimes, resulting from a complex combination of factors such as topography, geology, climate and hydrology explains, in fine, the spatial distribution of Li isotopic ratios in the large drainage basin of the Mackenzie River. There is no simple relationship between Li isotopic composition and chemical denudation fluxes in the Mackenzie River Basin.
Air temperature (Tair) plays an important role in determining how a canopy intercepts snow and apart from event size is the single most important micrometeorological variable found to adequately ...influence interception rates and magnitude. We present results from a 6‐year study on snow‐forest interactions. This data set reveals the central role Tair plays in how a forest intercepts snow and the need to effectively incorporate this within snow process models. Warm temperature events show a higher canopy interception efficiency (CIE) for all sites across the study period, while colder Tair events demonstrated a lower corresponding CIE. Additionally, there is a structural component of the forest itself that plays a role in the ability of canopy to intercept snow. Recognizing the physical vertical structure of a forest as nontrivial, we present a simple canopy interception model that includes a novel three‐dimensional forest metric that captures canopy complexity, Gz, combined with event size and Tair to adequately predict event‐based interception. Low complexity forests decrease interception capacity from the outset, whereas a highly complex or diverse forest increases interception potential and leads to a nonlinear increase in temperature‐based canopy interception due to more surface area able to intercept falling snow. Essentially, forest structure sets the boundary condition of the potential to intercept, while event size and Tair determines the rate or amount of interception. We develop a simple canopy snow interception model from these data and compare modeled output against three commonly used interception models in both a maritime and continental snow climate. Partitioning the relative importance of snow‐forest interactions on canopy interception will help provide the information to accurately model snow‐forest interactions and also aid in our water resources predictive capabilities now and into the future.
Key Points
Snow interception in a maritime forest is driven by event size and air temperature within a boundary condition set by forest structure
We develop a simple canopy snow interception model from measured data across an elevation gradient and under various forest structure
Inclusion of air temperature in snow interception models is essential to adequately capture event‐based canopy interception variability
Mackenzie River discharge and bathymetry effects on sea ice in the Beaufort Sea are examined in 2012 when Arctic sea ice extent hit a record low. Satellite‐derived sea surface temperature revealed ...warmer waters closer to river mouths. By 5 July 2012, Mackenzie warm waters occupied most of an open water area about 316,000 km2. Surface temperature in a common open water area increased by 6.5°C between 14 June and 5 July 2012, before and after the river waters broke through a recurrent landfast ice barrier formed over the shallow seafloor offshore the Mackenzie Delta. In 2012, melting by warm river waters was especially effective when the strong Beaufort Gyre fragmented sea ice into unconsolidated floes. The Mackenzie and other large rivers can transport an enormous amount of heat across immense continental watersheds into the Arctic Ocean, constituting a stark contrast to the Antarctic that has no such rivers to affect sea ice.
Key Points
Warm waters intrusion from the Mackenzie River discharge impacts sea ice melt
Strong Beaufort Gyre fragmented sea ice to precondition andenhance melting
Arctic rivers constitute the starkest contrast to Antarctic without such river
•Carbon isotope ratios of GDGTs are examined in Arctic lakes from the Mackenzie delta.•iGDGTs are produced by lacustrine acetoclastic as well as heterotrophic Archaea.•BrGDGTs are dominated by hexa- ...and penta-methylated 5- and 6-methyl compounds.•Reconstructed summer air temperatures show no warming during the last 200 years.•Oxygen condition, connection time to the river control the distribution of brGDGT.
Glycerol Dialkyl Glycerol Tetraethers (GDGTs) are ubiquitous biomolecules whose structural diversity or isotopic composition is increasingly used to reconstruct environmental changes such as air temperature or pCO2. Isoprenoid GDGTs (iGDGT), in particular GDGT-0, are biosynthesized by a large range of Archaea. To assess the potential of GDGT-0 as a tracer of past methane cycle variations, three sediment cores from the Mackenzie River Delta have been studied for iGDGT and diploptene concentration, distribution and stable carbon signature. The absence of crenarchaeol, high GDGT-0 vs crenarchaeol ratio values, and 13C-enriched carbon signature of GDGT-0 indicate production by acetoclastic methanogens as well as heterotrophic Archaea. The oxidation of methane seems to be dominated by bacteria as indicated by the high abundance of 13C-depleted diploptene. Branched GDGTs (brGDGT), thought to be produced by heterotrophic bacteria, are dominated by hexa- and penta-methylated 5- and 6-methyl compounds. The presence of 5,6-methyl isomer IIIa’’ points towards in situ production of brGDGTs, with only a minor input from soil branched GDGT brought by the Mackenzie River. Carbon isotopic compositions of brGDGTs are in agreement with heterotrophic producers, likely living during summer. The reconstructed temperatures using a global lake calibration reflect recorded summer air temperature (±2 °C) during the last 60 years, and further highlight the absence of warming in summer in this region during the last 200 years. Oxygen availability and connection time to the Mackenzie River also seem to control the distribution of brGDGT with an increase in 6-methyl and 5,6-methyl isomers during periods of increased anoxia.
Climate change in the Arctic is ongoing and causes drastic modification on the ecosystem functioning. In soft‐bottom environments, organic matter remineralization is considered an important ecosystem ...function. Here we provide a large‐scale assessment of the current knowledge on the benthic organic matter remineralization and its potential response to climate change. Sediment oxygen demand (SOD) values (n = 1154), measured throughout the Arctic, were gathered from 30 publications and 16 databases, and nutrient flux values, available in a far lesser extent (n < 80), were also compiled. Generalized additive models were used to estimate the influence of explanatory variables on benthic oxygen fluxes and for interpolating SOD to the whole Arctic region. This first Pan‐Arctic review of the distributions of SOD showed that oxygen fluxes strongly depended on water depth, i.e., followed the general trend observed for other regions, and also on the availability of labile organic matter. The continental shelves (representing ~50% of Arctic Ocean's total area) were characterized by the highest SOD values (10.5 ± 7.9 mmol O2 m−2 d−1), and differences among shelves were observed; SOD values in inflow, interior, and outflow shelves were 11.8 ± 8.0, 6.2 ± 5.6, and 3.9 ± 3.5 mmol O2 m−2 d−1, respectively. Moreover, seasonal variation in SOD changed significantly among areas. The interpolation based on the best fitted model showed high respiration in the inflow and interior shelves. In the inflow shelves, characterized by productive waters, benthic activities replenish bottom water with nutrients which may augment primary productivity, whereas sediments from the interior shelves, e.g., under the direct influence of the Mackenzie River, consume nutrients.
Key Points
The Pan‐Arctic compilation of sediment oxygen demand exhibits spatial and seasonal variations
Oxygen fluxes strongly depend on water depth and food supply, in line with global oceanic patterns
The predicted SOD values (GAM model) show high respiration in the inflow and interior shelves
Meltwater routing and the Younger Dryas Condron, Alan; Winsor, Peter
Proceedings of the National Academy of Sciences - PNAS,
12/2012, Letnik:
109, Številka:
49
Journal Article
Recenzirano
Odprti dostop
The Younger Dryas—the last major cold episode on Earth—is generally considered to have been triggered by a meltwater flood into the North Atlantic. The prevailing hypothesis, proposed by Broecker et ...al. 1989 Nature 341:318–321 more than two decades ago, suggests that an abrupt rerouting of Lake Agassiz overflow through the Great Lakes and St. Lawrence Valley inhibited deep water formation in the subpolar North Atlantic and weakened the strength of the Atlantic Meridional Overturning Circulation (AMOC). More recently, Tarasov and Peltier 2005 Nature 435:662–665 showed that meltwater could have discharged into the Arctic Ocean via the Mackenzie Valley ∼4,000 km northwest of the St. Lawrence outlet. Here we use a sophisticated, high-resolution, ocean sea-ice model to study the delivery of meltwater from the two drainage outlets to the deep water formation regions in the North Atlantic. Unlike the hypothesis of Broecker et al., freshwater from the St. Lawrence Valley advects into the subtropical gyre ∼3,000 km south of the North Atlantic deep water formation regions and weakens the AMOC by <15%. In contrast, narrow coastal boundary currents efficiently deliver meltwater from the Mackenzie Valley to the deep water formation regions of the subpolar North Atlantic and weaken the AMOC by >30%. We conclude that meltwater discharge from the Arctic, rather than the St. Lawrence Valley, was more likely to have triggered the Younger Dryas cooling.
Cold regions provide water resources for half the global population yet face rapid change. Their hydrology is dominated by snow, ice and frozen soils, and climate warming is having profound effects. ...Hydrological models have a key role in predicting changing water resources but are challenged in cold regions. Ground‐based data to quantify meteorological forcing and constrain model parameterization are limited, while hydrological processes are complex, often controlled by phase change energetics. River flows are impacted by poorly quantified human activities. This paper discusses the scientific and technical challenges of the large‐scale modelling of cold region systems and reports recent modelling developments, focussing on MESH, the Canadian community hydrological land surface scheme. New cold region process representations include improved blowing snow transport and sublimation, lateral land‐surface flow, prairie pothole pond storage dynamics, frozen ground infiltration and thermodynamics, and improved glacier modelling. New algorithms to represent water management include multistage reservoir operation. Parameterization has been supported by field observations and remotely sensed data; new methods for parameter identification have been used to evaluate model uncertainty and support regionalization. Additionally, MESH has been linked to broader decision‐support frameworks, including river ice simulation and hydrological forecasting. The paper also reports various applications to the Saskatchewan and Mackenzie River basins in western Canada (0.4 and 1.8 million km2). These basins arise in glaciated mountain headwaters, are partly underlain by permafrost, and include remote and incompletely understood forested, wetland, agricultural and tundra ecoregions. These illustrate the current capabilities and limitations of cold region modelling, and the extraordinary challenges to prediction, including the need to overcoming biases in forcing data sets, which can have disproportionate effects on the simulated hydrology.
This paper reports developments in cold region process representation for hydrological models, based on a Canadian modelling system (MESH) and detailed experimental data. It evaluates model capabilities for cold region environments and presents example applications for large river basins in Canada. It demonstrates that the model is able to address challenges that include modelling the earth system at global and regional scales, planning and management of uncertain water futures, and the forecasting and management of changing flood and drought risk.
Global gaseous nitrogen (N2) fixation rates may be underestimated and data is lacking from many regions without conspicuous diazotrophic cyanobacteria, such as cold oceans. We estimated N2 fixation ...rates at diverse sites in the Canadian Arctic, including the mouth of the Mackenzie River, the offshore Beaufort Sea, Lancaster Sound, Baffin Bay and a river influenced fjord. We also identified potential diazotrophic communities using a targeted survey of the nifH gene. Nitrogen fixation rates ranged from 0.02 nmol N L−1 d−1 in Baffin Bay to 4.45 nmol N L−1 d−1 in the Mackenzie River plume. Sequences recovered from the nifH gene survey belonged mainly to Cluster III, a group of nifH sequences associated with diverse microorganisms, with some α‐ andγ‐proteobacterianifH genes at most sites. Cyanobacteria nifH genes with best matches to Nostocales, which are common in Arctic freshwaters, were recovered from the marine Beaufort Sea. The geographic pattern of N2 fixation rates and nifHgene identities suggest that the Mackenzie River is the source of a diazotrophic community that contributes new nitrogen to the nitrogen‐depleted surface waters of the Beaufort Sea. This first record of N2 fixation at high latitudes refines our understanding of the global nitrogen budget.
Key Points
Nitrogen fixation occurs in coastal Arctic waters
The Mackenzie River supplies an active diazotrophic community to Beaufort Sea
Sequences recovered from nifH gene survey belonged mainly to Cluster III
The Middle to Late Devonian was characterized by the widespread deposition of organic-rich mudstone units and successive biotic crises and anoxic events in the marine realm, the cause of which ...remains debated and requires constraints from associated marine conditions. This study provides an example of the marine nitrogen cycle throughout the late Eifelian to middle Frasnian anoxic pulses. We present new and previously published organic whole-rock N (δ15Nbulk) and carbon (δ13Corg) isotopic datasets from organic-rich mudstone units of the Horn River Group (Canol and Hare Indian Formations) and overlying Imperial Formation in the Central Mackenzie Valley, Northwest Territories, Canada. In the ConocoPhillips Mirror Lake N-20 core, δ13Corg ranges from −31.0 ‰ to −24.3 ‰ with δ15Nbulk from −3.8 ‰ to +1.9 ‰, whereas the Husky Little Bear N-09 core is characterized by δ13Corg from −31.0 ‰ to −27.2 ‰ and δ15Nbulk from −2.0 ‰ to +5.9 ‰. The N isotopic signatures near 0 ‰ and a lack of δ15Nbulk – δ13Corg relationship are characteristic of N2 fixation by primary producers. Regular oscillations in δ15Nbulk are interpreted as the product of episodic, mild oxygenation events. Together, our δ13Corg and δ15Nbulk results suggest that locally, N2 fixation was the dominant source of N for primary producers in the late Eifelian to middle Frasnian, despite fluctuations in δ13Corg and global marine paleoredox. These findings contribute to our understanding of the nitrogen speciation and bioavailability associated with anoxic events, biotic crises, and widespread organic carbon burial in the Eifelian to Frasnian oceans.
•δ15Nbulk and δ13Corg from Middle to Late Devonian mudstones of northern Canada.•N2 was the dominant nitrogen species used by primary producers at this location.•No relationship between δ15Nbulk and δ13Corg in these mudstone units.•No recorded change in nutrient nitrogen speciation despite Devonian marine events.
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