Water Isotope Tracers Research Articles

Insights into Shallow Freshwater Lakes Hydrology in the Yangtze Floodplain from Stable Water Isotope Tracers

Stable isotopes of lake waters are widely used to identify the relative importance of hydrological processes on the lake water balance across the ungauged landscape via the coupled-isotope tracer model. The isotopic compositions of twenty shallow freshwater lakes across the mid-lower reaches of Yangtze floodplain (MLY) were investigated in January and May of 2018. The lake-specific input water (δI) and evaporation-to-inflow (E/I) ratios were estimated to explore the specific lake hydrology across the MLY. Results showed that distinct isotopic enrichment trends in May compared with those in January, which was indicative of stronger evaporation in May. The δ18OI values of specific lakes exhibited large variability across the MLY, which may be related to the watershed properties, such as watershed area and elevation, and rainfall. The estimated E/I ratios of lakes across the MLY were below 1, which suggested that these lakes (code 1–15) are flood-dominated in the middle reaches of Yangtze River where lakes are susceptible to Three Gorges Dams regulations. By contrast, the relatively lower variability of lake E/I ratios were observed from the Yangtze River Delta (code 17–20) because these lakes with developed river network systems are highly exchanged by artificial regulation. Our investigation of lake types and corresponding isotopic evolution patterns are likely typical of other floodplain landscapes and their identification could be used to better predict hydrological responses to ongoing climate change and artificial regulations by dams.

Evaluation of recharge areas of Arusha Aquifer, Northern Tanzania: application of water isotope tracers

Abstract In Arusha urban, northern Tanzania, groundwater contributes about 80% of the water supply. However, elevated fluoride levels and evidence of anthropogenic pollution have been reported in the groundwater around Mount Meru which is a water source for Arusha urban. This study aims at understanding the recharge areas and flow pathways of groundwater in what has been a poorly monitored area. The study uses the isotopic ratio of oxygen and hydrogen to estimate the groundwater recharge area and flow pathway. The results show the recharge elevation of groundwater is between 1,800 and 3,500 m above mean sea level on the slopes of Mount Meru. The average fluoride contents in the study area are 5.3 ± 0.4 mg/L greater than the limits of 1.5 mg/L set by the World Health Organization (WHO) and Tanzania. The nitrate concentration of 83.9 mg/L at the lower elevation areas (<1,400 m above mean sea level) exceeds the 50 mg/L WHO limit. The relationship of F− with δ18O and NO3− suggests the leaching of fluoride in high altitudes and dilution in lower altitudes.

Delineating extent and magnitude of river flooding to lakes across a northern delta using water isotope tracers

AbstractHydrological monitoring in complex, dynamic northern floodplain landscapes is challenging, but increasingly important as a consequence of multiple stressors. The Peace‐Athabasca Delta in northern Alberta, Canada, is a Ramsar Wetland of International Importance reliant on episodic river ice‐jam flood events to recharge abundant perched lakes and wetlands. Improved and systematic monitoring of landscape‐scale hydrological connectivity among freshwater ecosystems (rivers, channels, wetlands, and lakes) is needed to guide stewardship decisions in the face of climate change and upstream industrial development. Here, we use water isotope compositions, supplemented by measurements of specific conductivity and field observations, from 68 lakes and 9 river sites in May 2018 to delineate the extent and magnitude of spring ice‐jam induced flooding along the Peace and Athabasca rivers. Lake‐specific estimates of input water isotope composition (δI) were modelled after accounting for influence of evaporative isotopic enrichment. Then, using the distinct isotopic signature of input water sources, we develop a set of binary mixing models and estimate the proportion of input to flooded lakes attributable to river floodwater and precipitation (snow or rain). This approach allowed identification of areas and magnitude of flooding that were not captured by other methods, including direct observations from flyovers, and to demarcate flow pathways in the delta. We demonstrate water isotope tracers as an efficient and effective monitoring tool for delineating spatial extent and magnitude of an important hydrological process and elucidating connectivity in the Peace‐Athabasca Delta, an approach that can be readily adopted at other floodplain landscapes.

Hydrologic and water isotope characterization of a regulated Canadian Shield river basin

AbstractStable water isotope surveys have increasingly been integrated into river basins studies, but fewer have used them to evaluate impact of hydropower regulation. This study applies hydrologic and water isotope survey approaches to a Canadian Shield river basin with both regulated and natural flows. Historical streamflow records were used to evaluate the influence of three hydroelectric reservoirs and unregulated portions of the basin on downstream flows and changes in water level management implemented after an extreme flood year (1979). In 2013, water isotope surveys of surface and source waters (e.g., rainfall, groundwater, snowmelt) were conducted to examine spatial and temporal variation in contributions to river flow. Seasonal changes in relative groundwater contribution were assessed using a water‐isotope mass balance approach. Within the basin, two regulated reservoirs exhibited inverted hydrographs with augmented winter flows, whereas a third exhibited a hydrograph dominated by spring snowmelt. In 2013, spatial variation in rain‐on‐snow and air temperatures resulted in a critical lag in snowmelt initiation in the southern and northern portions of the basin resulting in a dispersed, double peak spring hydrograph, contrasting with 1979 when a combination of rain‐on‐snow and coincident snowmelt led to the highest flood on record. Although eastern basin reservoirs become seasonally enriched in δ18O and δ2H values, unregulated western basin flows remain less variable due to groundwater driven baseflow with increasing influence downstream. Combined analysis of historical streamflow (e.g., flood of 1979, drought of 2010) and the 2013 water isotope surveys illustrate extreme meteorological conditions that current management activities are unable to prevent. In this study, the influence of evaporative fractionation on large surface water reservoirs provides important evidence of streamflow partitioning, illustrating the value of stable water isotope tracers for study of larger catchments.

Interpreting Precession‐Driven δ18O Variability in the South Asian Monsoon Region

AbstractSpeleothem records from the South Asian summer monsoon (SASM) region display variability in the ratio of 18O and 16O (δ18O) in calcium carbonate at orbital frequencies. The dominant mode of variability in many of these records reflects cycles of precession. There are several potential explanations for why SASM speleothem records show a strong precession signal, including changes in temperature, precipitation, and circulation. Here we use an Earth system model with water isotope tracers and water‐tagging capability to deconstruct the precession signal found in SASM speleothem records. Our results show that cycles of precession‐eccentricity produce changes in SASM intensity that correlate with local temperature, precipitation, and δ18O. However, neither the amount effect nor temperature differences are responsible for the majority of the SASM δ18O variability. Instead, changes in the relative moisture contributions from different source regions drive much of the SASM δ18O signal, with more nearby moisture sources during Northern Hemisphere summer at aphelion and more distant moisture sources during Northern Hemisphere summer at perihelion. Further, we find that evaporation amplifies the δ18O signal of soil water relative to that of precipitation, providing a better match with the SASM speleothem records. This work helps explain a significant portion of the long‐term variability found in SASM speleothem records.

5. The Effect of Climate Variability on Lake Water Balances and Water Quality in the Apex River Watershed, Baffin Island, Nunavut

Iqaluit, Nunavut is currently facing a water shortage as its population rises, and its drinking water reservoir, Lake Geraldine, is increasingly insufficient. The City of Iqaluit is currently looking into alternative sources of drinking water to supplement this reservoir. This study investigates the effects of inter-annual climate variability on the water chemistry of twenty Arctic lakes in a continuous permafrost region, and the potential implications of these changes on drinking water availability. The twenty lakes are located in the Niaqunguk (Apex) river watershed, Baffin Island, Nunavut (which is adjacent to the Lake Geraldine watershed at Iqaluit) were sampled annually during late July between 2014-2017. Ion concentrations and stable isotopes were measured for each lake annually throughout the study period to compare variability between lakes and between years. Water chemical and physical properties are used to gain insight into the inputs and outputs of each lake, and the changes in estimated water balance between years. This information will be used to determine the impact of different hydrological conditions on water chemistry. Results of water isotope tracers of the twenty lakes indicate that there are important year-to-year changes in the water stable isotopes, indicating inter-annual variations in evaporation rates, suggesting that these lakes are sensitive to changes in summer climatic conditions. Preliminary analyses of the ionic concentrations of the lakes suggest that ion concentrations generally decrease throughout the study period, and largely vary together, signifying the hydrological processes controlling the lake water balances are related to climate variability. This research will provide insights into the hydrologic response of lakes in a continuous permafrost region to long-term climatic change.

Evaluating the Effects of Tracer Choice and End‐Member Definitions on Hydrograph Separation Results Across Nested, Seasonally Cold Watersheds

AbstractIsotope‐based hydrograph separation (IHS) is a widely used method in studies of runoff generation and streamflow partitioning. Challenges in choosing and characterizing appropriate tracers and end‐members have, however led to presumably highly uncertain IHS results. Here we tested the effects of end‐member definitions and tracer choices on IHS results in nested Prairie watersheds of varying size and landscape characteristics. Specifically, the consideration of eight potential “new” water end‐members, eight potential “old” water end‐members, and two stable water isotope tracers led to 80 potential IHS results for each stream water sample. IHS‐related uncertainty was evaluated using a Gaussian error propagation method. Results show that choosing an appropriate “new” water end‐member is most challenging during the freshet: highly variable “old” water fractions associated with high uncertainties were attributed to changing conditions from melting snow only to rain‐on‐snow. In summer and fall, it was rather the choice of an appropriate “old” water end‐member that was most problematic. IHS results obtained using δ18O versus δ2H as a tracer were significantly different except in the flattest and most wind‐sheltered watersheds examined. Overall, δ2H‐based IHS results were more uncertain than their δ18O‐based counterparts. Recommendations are therefore made toward careful selection of a tracer and a sampling strategy aimed at characterizing the most appropriate end‐members for IHS, especially when dealing with seasonally cold watersheds.

Evaluating topography‐based predictions of shallow lateral groundwater discharge zones for a boreal lake‐stream system

AbstractGroundwater discharge along streams exerts an important influence on biogeochemistry and thermal regimes of aquatic ecosystems. A common approach for predicting locations of shallow lateral groundwater discharge is to use digital elevation models (DEMs) combined with upslope contributing area algorithms. We evaluated a topography‐based prediction of subsurface discharge zones along a 1500 m headwater stream reach using temperature and water isotope tracers. We deployed fiber‐optic distributed temperature sensing instrumentation to monitor stream temperature at 0.25 m intervals along the reach. We also collected samples of stream water for the analysis of its water isotope composition at 50 m intervals on five occasions representing distinct streamflow conditions before, during, and after a major rain event. The combined tracer evaluation showed that topography‐predicted locations of groundwater discharge were generally accurate; however, predicted magnitude of groundwater inflows estimated from upslope contributing area did not always agree with tracer estimates. At the catchment scale, lateral inflows were an important source of streamflow at base flow and peak flow during a major rain event; however, water from a headwater lake was the dominant water source during the event hydrograph recession. Overall, this study highlights potential utility and limitations of predicting locations and contributions of lateral groundwater discharge zones using topography‐based approaches in humid boreal regions.

Ecohydrologic considerations for modeling of stable water isotopes in a small intermittent watershed

AbstractNaturally occurring stable water isotope tracers provide useful information for hydrologic model development and calibration. Existing models include varied approaches concerning unsaturated zone percolation mixing (preferential versus matrix flow) and evapotranspiration (ET) partitioning. We assess the impact of unsaturated zone simplifying assumptions when simulating the Shale Hills Watershed, a small (7.9 ha), temperate, forested watershed near Petersburg, Pennsylvania, USA, with a relatively simple model. We found that different model structures/assumptions and parameterizations of unsaturated zone percolation had substantial impacts on the agreement between simulated and observed unsaturated‐zone water isotopic signatures. We show that unsaturated zone percolation mixing primarily affects the unsaturated zone δ18O and δ2H during winter and spring and that percolation was best represented as a combination of both preferential and matrix flow. We evaluate the importance and implications related to the partitioning of ET into evaporation and transpiration and demonstrated that incorporation of a plant growth model for ET partitioning substantially improved reproduction of observed hydrologic isotopic patterns of the unsaturated zone during the spring season. We show that unsaturated zone percolation mixing and ET partitioning approaches do not substantially influence stream δ18O and δ2H and conclude that observed streamflow isotopic data is not always a strong predictor of model performance with respect to intrawatershed processes.

Impact of deep convection on the isotopic amount effect in tropical precipitation

AbstractThe empirical “amount effect” observed in the distribution of stable water isotope ratios in tropical precipitation is used in several studies to reconstruct past precipitation. Recent observations suggest the importance of large‐scale organized convection systems on amount effect. With a series of experiments with Community Atmospheric Model version 3.0 with water isotope tracers, we quantify the sensitivity of amount effect to changes in modeled deep convection. The magnitude of the regression slope between long‐term monthly precipitation amount and isotope ratios in precipitation over tropical ocean reduces by more than 20% with a reduction in mean deep convective precipitation by about 60%, indicating a decline in fractionation efficiency. Reduced condensation in deep convective updrafts results in enrichment of lower level vapor with heavier isotope that causes enrichment in total precipitation. However, consequent increases in stratiform and shallow convective precipitation partially offset the reduction in the slope of amount effect. The net result is a reduced slope of amount effect in tropical regions except the tropical western Pacific, where the effects of enhanced large‐scale ascent and increased stratiform precipitation prevail over the influence of reduced deep convection. We also find that the isotope ratios in precipitation are improved over certain regions in the tropics with reduced deep convection, showing that analyses of isotope ratios in precipitation and water vapor are powerful tools to improve precipitation processes in convective parameterization schemes in climate models. Further, our study suggests that the precipitation types over a region can alter the fractionation efficiency of isotopes with implications for the reconstructions of past precipitation.

Landscape-gradient assessment of thermokarst lake hydrology using water isotope tracers

Thermokarst lakes are widespread in arctic and subarctic regions. In subarctic Québec (Nunavik), they have grown in number and size since the mid-20th century. Recent studies have identified that these lakes are important sources of greenhouse gases. This is mainly due to the supply of catchment-derived dissolved organic carbon that generates anoxic conditions leading to methane production. To assess the potential role of climate-driven changes in hydrological processes to influence greenhouse-gas emissions, we utilized water isotope tracers to characterize the water balance of thermokarst lakes in Nunavik during three consecutive mid- to late summer sampling campaigns (2012–2014). Lake distribution stretches from shrub-tundra overlying discontinuous permafrost in the north to spruce-lichen woodland with sporadic permafrost in the south. Calculation of lake-specific input water isotope compositions (δI) and lake-specific evaporation-to-inflow (E/I) ratios based on an isotope-mass balance model reveal a narrow hydrological gradient regardless of diversity in regional landscape characteristics. Nearly all lakes sampled were predominantly fed by rainfall and/or permafrost meltwater, which suppressed the effects of evaporative loss. Only a few lakes in one of the southern sampling locations, which overly highly degraded sporadic permafrost terrain, appear to be susceptible to evaporative lake-level drawdown. We attribute this lake hydrological resiliency to the strong maritime climate in coastal regions of Nunavik. Predicted climate-driven increases in precipitation and permafrost degradation will likely contribute to persistence and expansion of thermokarst lakes throughout the region. If coupled with an increase in terrestrial carbon inputs to thermokarst lakes from surface runoff, conditions favorable for mineralization and emission of methane, these water bodies may become even more important sources of greenhouse gases.

Simultaneous analysis of (17) O/(16) O, (18) O/(16) O and (2) H/(1) H of gypsum hydration water by cavity ring-down laser spectroscopy.

RationaleThe recent development of cavity ring‐down laser spectroscopy (CRDS) instruments capable of measuring 17O‐excess in water has created new opportunities for studying the hydrologic cycle. Here we apply this new method to studying the triple oxygen (17O/16O, 18O/16O) and hydrogen (2H/1H) isotope ratios of gypsum hydration water (GHW), which can provide information about the conditions under which the mineral formed and subsequent post‐depositional interaction with other fluids.MethodsWe developed a semi‐automated procedure for extracting GHW by slowly heating the sample to 400°C in vacuo and cryogenically trapping the evolved water. The isotopic composition (δ17O, δ18O and δ2H values) of the GHW is subsequently measured by CRDS. The extraction apparatus allows the dehydration of five samples and one standard simultaneously, thereby increasing the long‐term precision and sample throughput compared with previous methods. The apparatus is also useful for distilling brines prior to isotopic analysis. A direct comparison is made between results of 17O‐excess in GHW obtained by CRDS and fluorination followed by isotope ratio mass spectrometry (IRMS) of O2.ResultsThe long‐term analytical precision of our method of extraction and isotopic analysis of GHW by CRDS is ±0.07‰ for δ17O values, ±0.13‰ for δ18O values and ±0.49‰ for δ2H values (all ±1SD), and ±1.1‰ and ±8 per meg for the deuterium‐excess and 17O‐excess, respectively. Accurate measurement of the 17O‐excess values of GHW, of both synthetic and natural samples, requires the use of a micro‐combustion module (MCM). This accessory removes contaminants (VOCs, H2S, etc.) from the water vapour stream that interfere with the wavelengths used for spectroscopic measurement of water isotopologues. CRDS/MCM and IRMS methods yield similar isotopic results for the analysis of both synthetic and natural gypsum samples within analytical error of the two methods.ConclusionsWe demonstrate that precise and simultaneous isotopic measurements of δ17O, δ18O and δ2H values, and the derived deuterium‐excess and 17O‐excess, can be obtained from GHW and brines using a new extraction apparatus and subsequent measurement by CRDS. This method provides new opportunities for the application of water isotope tracers in hydrologic and paleoclimatologic research. © 2015 The Authors. Rapid Communications in Mass Spectrometry Published by John Wiley & Sons Ltd.

Controls on water balance of shallow thermokarst lakes and their relations with catchment characteristics: a multi‐year, landscape‐scale assessment based on water isotope tracers and remote sensing in Old Crow Flats, Yukon (Canada)

AbstractMany northern lake‐rich regions are undergoing pronounced hydrological change, yet inadequate knowledge of the drivers of these landscape‐scale responses hampers our ability to predict future conditions. We address this challenge in the thermokarst landscape of Old Crow Flats (OCF) using a combination of remote sensing imagery and monitoring of stable isotope compositions of lake waters over three thaw seasons (2007–2009). Quantitative analysis confirmed that the hydrological behavior of lakes is strongly influenced by catchment vegetation and physiography. Catchments of snowmelt‐dominated lakes, typically located in southern peripheral areas of OCF, encompass high proportions of woodland/forest and tall shrub vegetation (mean percent land cover = ca. 60%). These land cover types effectively capture snow and generate abundant snowmelt runoff that offsets lake water evaporation. Rainfall‐dominated lakes that are not strongly influenced by evaporation are typically located in eastern and northern OCF where their catchments have higher proportions of dwarf shrub/herbaceous and sparse vegetation (ca. 45%), as well as surface water (ca. 20%). Evaporation‐dominated lakes, are located in the OCF interior where their catchments are distinguished by substantially higher lake area to catchment area ratios (LA/CA = ca. 29%) compared to low evaporation‐influenced rainfall‐dominated (ca. 10%) and snowmelt‐dominated (ca. 4%) lakes. Lakes whose catchments contain >75% combined dwarf shrub/herbaceous vegetation and surface water are most susceptible to evaporative lake‐level drawdown, especially following periods of low precipitation. Findings indicate that multiple hydrological trajectories are probable in response to climate‐driven changes in precipitation amount and seasonality, vegetation composition, and thermokarst processes. These will likely include a shift to greater snowmelt influence in catchments experiencing expansion of tall shrubs, greater influence from evaporation in catchments having higher proportions of surface water, and an increase in the rate of thermokarst lake expansion and probability of drainage. Local observations suggest that some of these changes are already underway.

Avian-Driven Modification of Seasonal Carbon Cycling at a Tundra Pond in the Hudson Bay Lowlands (Northern Manitoba, Canada)

The past ∼40 years have seen a geometric increase (5–7% per year) in the size of the lesser snow goose (LSG; Chen caerulescens caerulescens) population and marked spatial expansion of the area they inhabit within the coastal fen ecotype of Wapusk National Park (Hudson Bay Lowlands, northern Canada), raising concerns and uncertainty about the environmental effects of their activities (grubbing of vegetation, soil disturbance, deposition of feces) on the abundant shallow tundra ponds. In this study, we use conventional limnological measurements as well as water and carbon (C) isotope tracers to explore similarities and differences in seasonal patterns of hydrological, limnological, and biogeochemical conditions of 15 shallow coastal fen ponds that currently have minimal (if any) disturbance from the LSG population with one pond (WAP 20) that is subject to substantial LSG activity. Carbon isotope measurements reveal that C cycling at WAP 20 (LSG-disturbed site) is markedly different compared to the other ponds, whereas only small differences were observed in hydrological conditions and concentrations of major nutrients and chlorophyll a of pond water. A mid-summer decrease in C isotope composition of dissolved inorganic carbon (DIC) occurred at WAP 20, likely as a consequence of high pond-water pH and intense C demand by aquatic productivity. These conditions appear to have promoted “chemically enhanced CO2 invasion,” which causes strong kinetic C isotope fractionation. High C demand at WAP 20 is also suggested by mid-summer 13C enrichment in particulate organic matter. In contrast, the ponds with little to no LSG activity exhibited expected seasonal C isotope behavior (i.e., 13C enrichment of DIC) under conditions of increasing productivity when C is in relatively low demand. Small differences in nutrient concentrations may be due to rapid uptake by the benthic mat at WAP 20. Data from the low disturbance ponds also provide baseline information for future studies assessing potential effects of LSG.

Using Water Isotope Tracers to Develop the Hydrological Component of a Long-Term Aquatic Ecosystem Monitoring Program for a Northern Lake-Rich Landscape

Arctic lake-rich landscapes are vulnerable to climate change, but their remote locations present a challenge to develop effective approaches for monitoring hydroecological status and trends. Here, we structure the hydrological component of an aquatic ecosystem monitoring program that addresses concerns of Parks Canada (Vuntut National Park) and the Vuntut Gwitchin First Nation about changing water levels of Old Crow Flats (OCF), Yukon, Canada, a 5600-km2 thermokarst landscape recognized nationally and internationally for its ecological, historical, and cultural significance. The foundation of the monitoring program is 5 years (2007–2011) of water isotope data from 14 lakes situated in catchments that are representative of the land-cover and hydrological diversity of OCF. Isotopic compositions of input water (δI) and evaporation-to-inflow (E/I) ratios, calculated using the coupled-isotope tracer method, provide key hydrological metrics for each lake over the 5-year sampling interval. From these time series, we identify monitoring lakes that are sensitive to changes in snowmelt, rainfall, and evaporation, and demonstrate the use of the Mann-Kendall test for determining statistically significant trends in the roles of these hydrological processes on lake-water balances. These approaches will serve to identify lake hydrological responses to climate change and variability from ongoing water isotope monitoring by Parks Canada, in partnership with the Vuntut Gwitchin Government, Wilfrid Laurier University, and the University of Waterloo.

Influence of Last Glacial Maximum boundary conditions on the global water isotope distribution in an atmospheric general circulation model

Abstract. To understand the validity of δ18O proxy records as indicators of past temperature change, a series of experiments was conducted using an atmospheric general circulation model fitted with water isotope tracers (Community Atmosphere Model version 3.0, IsoCAM). A pre-industrial simulation was performed as the control experiment, as well as a simulation with all the boundary conditions set to Last Glacial Maximum (LGM) values. Results from the pre-industrial and LGM simulations were compared to experiments in which the influence of individual boundary conditions (greenhouse gases, ice sheet albedo and topography, sea surface temperature (SST), and orbital parameters) were changed each at a time to assess their individual impact. The experiments were designed in order to analyze the spatial variations of the oxygen isotopic composition of precipitation (δ18Oprecip) in response to individual climate factors. The change in topography (due to the change in land ice cover) played a significant role in reducing the surface temperature and δ18Oprecip over North America. Exposed shelf areas and the ice sheet albedo reduced the Northern Hemisphere surface temperature and δ18Oprecip further. A global mean cooling of 4.1 °C was simulated with combined LGM boundary conditions compared to the control simulation, which was in agreement with previous experiments using the fully coupled Community Climate System Model (CCSM3). Large reductions in δ18Oprecip over the LGM ice sheets were strongly linked to the temperature decrease over them. The SST and ice sheet topography changes were responsible for most of the changes in the climate and hence the δ18Oprecip distribution among the simulations.

Characterizing the role of hydrological processes on lake water balances in the Old Crow Flats, Yukon Territory, Canada, using water isotope tracers

We employ water isotope tracers to assess hydrological processes controlling lake water balances in the Old Crow Flats (OCF) landscape, northern Yukon Territory, Canada. Fifty-six lakes were sampled in June and July 2007 and 26 of these were re-sampled in September 2007. Based on patterns of isotopic evolution in δ 18 O– δ 2 H space, calculations of input water compositions ( δ I ) and evaporation-to-inflow ( E / I ) ratios, and field observations we identify snowmelt-dominated , rainfall-dominated , groundwater-influenced , evaporation-dominated and drained lake types, which represent the dominant hydrological process influencing the lake water balance. These results highlight the diversity in lake water balance conditions in the OCF, which are strongly associated with landscape characteristics. Snowmelt-dominated lakes are located where more dense vegetation cover entraps snow transported by prevailing northeasterly winds. Rainfall-dominated lakes occupy areas of sparse tundra vegetation cover where less snow accumulates. Groundwater-influenced oxbow lakes are located along the floodplain of higher-order river and creek channels and receive input throughout the ice-free season from snowmelt-recharged channel fens and sub-surface flow. Only one basin became evaporation-dominated during the 2007 open-water season probably because extremely high precipitation during the preceding late summer, late winter and early spring offset vapour loss. However, rainfall-dominated lakes appear to be more susceptible to evaporative drawdown than snowmelt-dominated and groundwater-influenced lakes, and many would likely evolve to evaporation-dominated during drier summers. Drained lakes are commonly observed throughout the landscape and in most cases likely result from elevated water levels and channel erosion between waterbodies. Unusually high amounts of snowmelt and/or rainfall triggered the drainage of two lakes in early June 2007 in which overflow led to rapid erosion of outlet channels. Our classification of lake types and corresponding isotopic evolution patterns are likely typical of other thermokarst landscapes and their identification could be used to better predict hydrological responses to ongoing climate change.

Multi-year landscape-scale assessment of lakewater balances in the Slave River Delta, NWT, using water isotope tracers

We apply a coupled-isotope tracer model to quantify end-of-thaw-season (fall) lakewater balances in the Slave River Delta (SRD), Canada, during 2003–2005, which effectively differentiates the relative importance of hydrological processes across this complex northern freshwater landscape. The model incorporates Great Slave Lake evaporated vapour to the ambient atmospheric vapour pool and is thus tailored to the hydroclimatic setting of the delta, which experiences onshore winds. Results, expressed as evaporation-to-inflow ratios ( E / I ) for 41 delta lakes, reflect the role of spring break-up flooding and local hydrological setting. Fall E / I ratios for lakes where water balances are dominated by exchange with the Slave River or Great Slave Lake are low (0.06–0.53) and do not vary substantially during the three-year monitoring period. E/I ratios for flood-dominated lakes in the active delta are moderate (0.26–0.98) and have low inter-annual variability, even in the absence of spring flooding. This suggests that annual flooding during the spring break-up period is not necessary to maintain positive ( E / I < 1) water balances in flood-dominated lakes, but multiple years without flooding would clearly lead to greater cumulative evaporation. Fall E / I ratios are generally higher and more variable in evaporation-dominated lakes in the relict delta (0.42 to >1), although greater snowmelt runoff tends to occur in sub-sectors with mature spruce forest and offsets open-water vapour loss. Our results indicate that spring inputs (river flooding and snowmelt runoff) are key components of the hydrological evolution of SRD lakes during the open-water season, and distinguish regions of the delta where expected declines in river discharge and climate warming will likely cause lake-level drawdown. Such findings have particular relevance for informed ecosystem management in the Peace-Athabasca-Slave watershed, where unprecedented industrial development is imposing substantial additional pressure on freshwater resources.

Ensemble, water isotope–enabled, coupled general circulation modeling insights into the 8.2 ka event

Freshwater forcing has long been postulated as a catalyst for abrupt climate change because of its potential to interfere with thermohaline circulation (THC). The most recent example may have occurred about 8.2 ka ago with the sudden drainage of glacial lakes Agassiz and Ojibway into the Hudson Bay. We perform an ensemble of simulations for this freshwater release using the fully coupled atmosphere ocean general circulation model, Goddard Institute for Space Studies ModelE‐R. In all cases, simulated effects include reduced ocean heat transport and enhanced atmospheric heat transport in the Atlantic, increased surface albedo (through greater low cloud and sea ice cover), and local cooling of up to 3°C. Our suite of ensemble experiments allows us to examine the importance of the initial ocean state, in particular the presence or absence of Labrador Sea Water, in controlling the magnitude and length of the climate response. Water isotope tracers included in this model provide an improved means for direct comparisons of the modeled tracer response to water isotope–based, climate proxy data. Comparison of model simulations to data implies that there was an abrupt approximate halving of Atlantic THC, hence providing strong support for the hypothesis that the 8.2 ka event was caused by an abrupt release of fresh water into the North Atlantic.

Spatial and temporal perspectives on spring break‐up flooding in the Slave River Delta, NWT

AbstractSpatial and temporal patterns of spring break‐up flooding in the Slave River Delta (SRD), Northwest Territories, are characterized during three years (2003–2005) using water isotope tracers and total inorganic suspended sediment (TSS) concentrations measured from lakewater samples collected shortly after the spring melt. Strongly contrasting spring melt periods led to a moderate flood in 2003, no flooding in 2004 and widespread flooding in 2005. Flooded lakes have isotopically‐depleted δ18O (δ2H) signatures, ranging between − 19·2‰ (−145‰) and − 17·1‰ (−146‰) and most have high TSS concentrations (&gt;10 mg L−1), while non‐flooded lakes have more isotopically‐enriched δ18O (δ2H) signatures, ranging between − 18·2‰ (−149‰) and − 10·6‰ (−118‰) and low TSS concentrations (&lt;10 mg L−1). These results, in conjunction with the isotopic signatures of Slave River water and snowmelt, are used to estimate the proportion of river‐ or snowmelt‐induced dilution in delta lakes during the spring of each study year. Calculations indicate river flooding caused dilution of ∼70–100% in delta lakes, while snowmelt dilution in the absence of river flooding ranged from ∼0–56%. A positive relationship exists between the spatial extent of spring flooding in the SRD and level and discharge on the Slave River and upstream tributaries, suggesting that upstream flow generation plays a key role in determining the magnitude of spring flooding in the SRD. Parallel variations in the 46‐year instrumental Slave River discharge record and flood stratigraphy in the active delta indicate that there is potential for extending the flood history of the SRD, a development that will contribute to a more robust understanding of the drivers of historic, contemporary and future flood frequency in the delta. Copyright © 2008 John Wiley &amp; Sons, Ltd.