Stable isotopes in global lakes integrate catchment and climatic controls on evaporation

Global warming is considered a major threat to Earth’s lakes water budgets and quality. However, flow regulation, over-exploitation, lack of hydrological data, and disparate evaluation methods hamper comparative global estimates of lake vulnerability to evaporation. We have analyzed the stable isotope composition of 1257 global lakes and we find that most lakes depend on precipitation and groundwater recharge subsequently altered by catchment and lake evaporation processes. Isotope mass-balance modeling shows that ca. 20% of water inflow in global lakes is lost through evaporation and ca. 10% of lakes in arid and temperate zones experience extreme evaporative losses >40 % of the total inflow. Precipitation amount, limnicity, wind speed, relative humidity, and solar radiation are predominant controls on lake isotope composition and evaporation, regardless of the climatic zone. The promotion of systematic global isotopic monitoring of Earth’s lakes provides a direct and comparative approach to detect the impacts of climatic and catchment-scale changes on water-balance and evaporation trends.

Quantifying floodwater impacts on a lake water budget via volume-dependent transient stable isotope mass balance

Isotope mass balance models have undergone significant developments in the last decade, demonstrating their utility for assessing the spatial and temporal variability in hydrological processes and revealing significant value for baseline assessment in remote and/or flood-affected settings where direct measurement of surface water fluxes to lakes (i.e. stream gauging) are difficult to perform. In this study, we demonstrate that isotopic mass balance modelling can be used to provide evidence of the relative importance of direct floodwater inputs and temporary subsurface storage of floodwater at ungauged lake systems. A volume-dependent transient isotopic mass balance model was developed for an artificial lake (named lake A) in southern Quebec (Canada). This lake typically receives substantial floodwater inputs during the spring freshet period as an ephemeral hydraulic connection with a 150 000 km2 large watershed is established. First-order water flux estimates to lake A allow for impacts of floodwater inputs to be highlighted within the annual water budget. The isotopic mass balance model has revealed that groundwater and surface water inputs account for 60 %–71 % and 39 %–28 % of the total annual water inputs to lake A, respectively, which demonstrates an inherent dependence of the lake on groundwater. However, when considering the potential temporary subsurface storage of floodwater, the partitioning between groundwater and surface water inputs tends to equalize, and the lake A water budget is found to be more resilient to groundwater quantity and quality changes. Our findings suggest not only that floodwater fluxes to lake A have an impact on its dynamics during springtime but also significantly influence its long-term water balance and help to inform, understand, and predict future water quality variations. From a global perspective, this knowledge is useful for establishing regional-scale management strategies for maintaining water quality at flood-affected lakes, for predicting the response of artificial recharge systems in such settings, and for mitigating impacts due to land use and climate changes.

Rapid coupling between solid earth and ice volume during the Quaternary

The solid earth plays a major role in controlling Earth’s surface climate. Volcanic degassing of carbon dioxide (CO2) and silicate chemical weathering are known to regulate the evolution of climate on a geologic timescale (> 106 yr), but the relationship between the solid earth and the shorter (< 105 yr) fluctuations of Quaternary glacial–interglacial cycles is still under debate. Here we show that the seawater osmium isotope composition (187Os/188Os), a proxy for the solid earth’s response to climate change, has varied during the past 300,000 years in association with glacial–interglacial cycles. Our marine Os isotope mass-balance simulation reveals that the observed 187Os/188Os fluctuation cannot be explained solely by global chemical weathering rate changes corresponding to glacial–interglacial climate changes, but the fluctuation can be reproduced by taking account of short-term inputs of (1) radiogenic Os derived from intense weathering of glacial till during deglacial periods and (2) unradiogenic Os derived from enhanced seafloor hydrothermalism triggered by sea-level falls associated with increases of ice sheet volume. Our results constitute the first evidence that ice sheet recession and expansion during the Quaternary systematically and repetitively caused short-term (< 105 yr) solid earth responses via chemical weathering of glacial till and seafloor magmatism. This finding implies that climatic changes on < 105 yr timescales can provoke rapid feedbacks from the solid earth, a causal relationship that is the reverse of the longer-term (> 106 yr) causality that has been conventionally considered.

The Use of Water Vapor Isotopes to Determine Evapotranspiration Source Contributions in the Natural Environment

Water balance measurements are the simplest and most direct means of estimating evapotranspiration (ET). However, numerous factors relating to climate and terrain characteristics contribute to the variability that makes the assessment of evapotranspiration challenging at the ecosystem or even the plot scale. Alternative methods, such as an isotope mass balance (IMB), can provide evapotranspiration estimates. This paper illustrates two IMB examples of partitioning evaporation and transpiration. The first example demonstrates at the laboratory scale how accurate mass-balance measurements provide a complete validation and refinement of the isotope mass balance methods. The second IMB case uses similar data processing methods for an experimental field design. These methods are further validated by comparison with previous laboratory and field studies. Finally, this paper presents a comparison between partitioned ET ratios from a nearby U.S. Geological Survey (USGS) microclimate site produced using the Flux Variance Similarity (FVS) method. The results suggest the potential of employing these methods to estimate evaporation and transpiration source contributions at various scales. This technique and its further development show IMB methods are an appropriate tool for partitioning evapotranspiration.

Anthropogenic Asian aerosols provide Fe to the North Pacific Ocean

Fossil-fuel emissions may impact phytoplankton primary productivity and carbon cycling by supplying bioavailable Fe to remote areas of the ocean via atmospheric aerosols. However, this pathway has not been confirmed by field observations of anthropogenic Fe in seawater. Here we present high-resolution trace-metal concentrations across the North Pacific Ocean (158°W from 25°to 42°N). A dissolved Fe maximum was observed around 35°N, coincident with high dissolved Pb and Pb isotope ratios matching Asian industrial sources and confirming recent aerosol deposition. Iron-stable isotopes reveal in situ evidence of anthropogenic Fe in seawater, with low δ56Fe (−0.23‰ > δ56Fe > −0.65‰) observed in the region that is most influenced by aerosol deposition. An isotope mass balance suggests that anthropogenic Fe contributes 21–59% of dissolved Fe measured between 35° and 40°N. Thus, anthropogenic aerosol Fe is likely to be an important Fe source to the North Pacific Ocean.