Frank Peeters

Interpretation of dissolved atmospheric noble gases in natural waters

Several studies have used the temperature dependence of gas solubilities in water to derive paleotemperatures from noble gases in groundwaters. We present a general method to infer environmental parameters from concentrations of dissolved atmospheric noble gases in water. Our approach incorporates statistical methods to quantify uncertainties of the deduced parameter values. The equilibration temperatures of water equilibrated with the atmosphere under controlled conditions could be inferred with a precision and accuracy of 60.28C. The equilibration temperatures of lake and river samples were determined with a similar precision. Most of these samples were in agreement with atmospheric equilibrium at the water temperature. In groundwaters either recharge temperature or altitude could be determined with high precision (60.38C and 660 m, respectively) despite the presence of excess air. However, typical errors increase to 638C and 6700 m if both temperature and altitude are determined at the same time, because the two parameters are correlated. In some groundwater samples the composition of the excess air deviated significantly from atmospheric air, which was modeled by partial reequilibration. In this case the achievable precision of noble gas temperatures was significantly diminished (typical errors of 618C).

Application of k‐ϵ turbulence models to enclosed basins: The role of internal seiches

[1] A numerical model was developed for the prediction of the density stratification of lakes and reservoirs. It combines a buoyancy-extended k-� model with a seiche excitation and damping model to predict the diffusivity below the surface mixed layer. The model was applied to predict the seasonal development of temperature stratification and turbulent diffusivity in two medium-sized lakes over time periods ranging from 3 weeks to 2 years. Depending on the type of boundary condition for temperature, two or three model parameters were optimized to calibrate the model. The agreement between the simulated and the observed temperature distributions is excellent, in particular, if lake surface temperatures were prescribed as surface boundary condition instead of temperature gradients derived from heat fluxes. Comparison of different model variants revealed that inclusion of horizontal pressure gradients and/or stability functions is not required to provide good agreement between model results and data. With the aid of uncertainty analysis it is shown that the depth of the mixed surface layer during the stratified period could be predicted accurately within ±1 m. The sensitivity of the model to several parameters is discussed. INDEX TERMS: 4211 Oceanography: General: Benthic boundary layers; 4255 Oceanography: General: Numerical modeling; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; KEYWORDS: lake, turbulence model, seiche, stratification, simulation, turbulence kinetic energy

Temporal scales of water-level fluctuations in lakes and their ecological implications

Water-level fluctuations (WLF) of lakes have temporal scales ranging from seconds to hundreds of years. Fluctuations in the lake level generated by an unbalanced water budget resulting from meteorological and hydrological processes, such as precipitation, evaporation and inflow and outflow conditions usually have long temporal scales (days to years) and are here classified as long-term WLF. In contrast, WLF generated by hydrodynamic processes, e.g. basin-scale oscillations and travelling surface waves, have periods in the order of seconds to hours and are classified as short-term WLF. The impact of WLF on abiotic and biotic conditions depends on the temporal scale under consideration and is exemplified using data from Lake Issyk-Kul, the Caspian Sea and Lake Constance. Long-term WLF induce a slow shore line displacement of metres to kilometres, but immediate physical stress due to currents associated with long-term WLF is negligible. Large-scale shore line displacements change the habitat availability for organisms adapted to terrestrial and aquatic conditions over long time scales. Short-term WLF, in contrast, do not significantly displace the boundary between the aquatic and the terrestrial habitat, but impose short-term physical stress on organisms living in the littoral zone and on organic and inorganic particles deposited in the top sediment layers. The interaction of WLF acting on different time scales amplifies their overall impact on the ecosystem, because long-term WLF change the habitat exposed to the physical stress resulting from short-term WLF. Specifically, shore morphology and sediment grain size distribution are the result of a continuous interplay between short- and long-term WLF, the former providing the energy for erosion the latter determining the section of the shore exposed to the erosive power.

Boundary versus internal diapycnal mixing in stratified natural waters

Using the fluorescent dye uranin, tracer release experiments to study the contribution of bottom boundary mixing to diapycnal transport in stratified natural waters were performed in Lake Alpnach (central Switzerland) during 1992-1995. A first experiment involved injecting the tracer from a point source into the center of the hypolimnion (that part of the lake below the surface mixed layer). An in situ fluorometer was then employed to detect the horizontal and vertical spreading of the tracer cloud, allowing rates of diapycnal diffusivity to be determined. As long as the tracer was confined to the interior water region, the diapycnal diffusivity was relatively small. However, after the tracer cloud had reached the lake boundary, the diapycnal diffusivity increased by approximately one order of magnitude. In a second experiment, the tracer was released near the sediment-water interface. In this case the dynamics of vertical tracer spreading were opposite. During the first few hours after tracer release, diapycnal diffusivities were large, subsequently decreasing as the tracer cloud drifted away from the lake boundary. Basin-wide diapycnal diffusivities calculated from heat flux measurements based on temperature profiles obtained from thermistor chains or conductivity-temperature-depth casts agreed well with the values obtained from the vertical tracer diffusion after horizontal homogenization. The results of the tracer experiments corroborate the hypothesis that diapycnal fluxes are determined predominantly by mixing in the bottom boundary region.

Improving noble gas based paleoclimate reconstruction and groundwater dating using 20Ne/22Ne ratios

Abstract The interpretation of noble gas concentrations in groundwater with respect to recharge temperature and fractionated excess gas leads to different results on paleo-climatic conditions and on residence times depending on the choice of the gas partitioning model. Two fractionation models for the gas excess are in use, one assuming partial re-equilibration of groundwater supersaturated by excess air (PR-model, Stute et al., 1995) , the other assuming closed-system equilibration of groundwater with entrapped air (CE-model, Aeschbach-Hertig et al., 2000) . In the example of the Continental Terminal aquifers in Niger, PR- and CE- model are both consistent with the data on elemental noble gas concentrations (Ne, Ar, Kr, and Xe). Only by including the isotope ratio 20Ne/22Ne it can be demonstrated that the PR-model has to be rejected and the CE-model should be applied to the data. In dating applications 3He of atmospheric origin (3Heatm) required to calculate 3H-3He water ages is commonly estimated from the Ne excess presuming that gas excess is unfractionated air (UA-model). Including in addition to the Ne concentration the 20Ne/22Ne ratio and the concentration of Ar enables a rigorous distinction between PR-, CE- and UA-model and a reliable determination of 3Heatm and of 3H-3He water ages.

Horizontal mixing in lakes

Horizontal mixing in the upper hypolimnion of lakes far from the boundaries was studied in lake basins with surface areas between 5 and 220 km(2) by observing the growth of the concentration distribution of the fluorescent dye sodium fluorescein (uranin). In each of the eight experiments, between 0.2 and 2 kg of uranin was instantaneously released into the lake in the appropriate depth (between 15 and 25 m) in such a way to keep the initial cloud size as small as possible. The horizontal extension of the cloud was repeatedly determined by integration of numerous vertical profiles. These surveys served to test theoretical models for horizontal mixing. The temporal development of the size and of the variances along the principal axes of the tracer concentration distribution was the main property considered here. The experiments cover a range of cloud sizes between 3 x 10(2) and 3 x 10(5) m(2). None of them support the hypothesis that cloud size grows with elapsed time raised to the power of 3 as predicted by the inertial subrange turbulence model first applied to dispersion by Batchelor [1950]. The shear-diffusion model of Carter and Okubo [1965] was found to provide a good description of the development of cloud size with time. This model also accounts for the fact that the tracer distributions were not radially symmetric. Effective horizontal diffusivities lie between 0.02 and 0.3 m(2) s(-1). Reevaluation of published data from experiments in Lake Ontario [e.g., Murthy, 1976] and in the ocean [e.g., Okubo, 1971] supports both the applicability of the shear-diffusion model and the doubts raised against the appropriateness of the inertial subrange model for scales up to 1000 m.

Distribution of helium and tritium in Lake Baikal

The 3 H- 3 He age of a water mass is a measure of the time that has passed since the water mass was last in contact with the atmosphere. Between 1992 and 1995 a detailed study of 3 H- 3 He ages was conducted in Lake Baikal, the deepest and largest lake by volume on Earth, to investigate deep water renewal in its three major basins. Maximum 3 H- 3 He ages are 14-17 years in the southern basin, 16-18 years in the central basin, and 10-11 years in the northern basin. Rates of renewal of deep water with surface water, deduced from volume-weighted mean 3 H- 3 He ages below 250 m depth, are about 10% yr -1 in the southern and central basins and 15% yr -1 in the northern basin. In the southern basin the mean 3 H- 3 He age below 250 m depth increased steadily from 9.6 years in 1992 to 11 years in 1995, indicating a slight diminution in deep water renewal during this time. Bottom water renewal by large-scale advection was estimated from the mass balance of 3 He in the 200 m thick bottom layer of each basin. Bottom water renewal rates in the northern basin were found to be between 80 and 150 km 3 yr -1 and in the central basin between 10 and 20 km 3 yr -1 , whereas in the southern basin they were practically zero. Correlating oxygen and dissolved helium-4 concentrations with 3 H- 3 He age allowed us to determine the mean hypolimnetic oxygen depletion rate in the water column (4.5 μmol L -1 yr -1 ), as well as mean helium fluxes from the lake bottom (2.8 x 10 11 atoms m -2 s -1 in the northern basin, and 1.3 x 10 11 atoms m -2 s -1 in the central and southern basins). The helium isotope ratio of the terrigenic helium component injected from the lake bottom, determined from measurements of water from hydrothermal springs in the vicinity of the lake, was found to be ∼2.2 x 10 -7 .

Toward a Unified Scaling Relation for Interfacial Fluxes

Interfacial fluxes, that is, gas exchange at the water–atmosphere interface and benthic fluxes at the sediment–water interface, are often parameterized in terms of wind speed or turbulent friction velocity, with numerous empirical relationships obtained from individual experiments. The present study attempts to combine the general outcome of such experiments at both interfaces into a universal scaling relation for the thicknesses of the viscous and diffusive sublayers in terms of the Kolmogorov and Batchelor length scales, respectively. Transfer velocities can then be described in terms of the Schmidt number of the respective tracer and in terms of the turbulence dissipation rate. Applying law-of-the-wall scaling to convert dissipation rates into an appropriate friction velocity estimate results in a mechanistic description of the transfer velocity, which is comparable to common empirical parameterizations. It is hypothesized, however, that the dissipation rate and hence the directly estimated level of turbulence provide a more appropriate variable for the parameterization of interfacial fluxes than wind speed or turbulent friction velocity inferred from law-of-the-wall scaling.

What prevents outgassing of methane to the atmosphere in Lake Tanganyika

[1] Tropical East African Lake Tanganyika hosts the Earth’s largest anoxic freshwater body. The entire water column holds over 23 Tg of the potent greenhouse gas methane (CH4). Methane is formed under sulphate poor conditions via carbon dioxide reduction or fermentation from detritus and relict sediment organic matter. Permanent density stratification supports an accumulation of CH4 below the permanent oxycline. Despite CH4 significance for global climate, anaerobic microbial consumption of CH4 in freshwater is poorly understood. Here we provide evidence for intense methanotrophic activity not only in the oxic but also in the anoxic part of the water column of Lake Tanganyika. We measured CH4, 13 C of dissolved CH4, dissolved oxygen (O2), sulphate (SO4− ), sulphide (HS − ) and the transient tracers chlorofluorocarbon 12 (CFC 12) and tritium ( 3 H). A basic one dimensional model, which considers vertical transport and biogeochemical fluxes and transformations, was used to interpret the vertical distribution of these substances. The results suggest that the anaerobic oxidation of CH4 is an important mechanism limiting CH4 to the anoxic zone of Lake Tanganyika. The important role of the anaerobic oxidation for CH4 concentrations is further supported by high abundances (up to ∼33% of total DAPI stained cells) of single living archaea, identified by fluorescence in situ hybridization.

Vertical turbulent diffusion and upwelling in Lake Baikal estimated by inverse modeling of transient tracers

Vertical turbulent diffusion coefficients, upwelling velocities, and oxygen depletion rates are estimated by inverse modeling of the concentrations of CFC-11 (CCl3F), CFC-12 (CCl2F2), 3H, 3He, and dissolved oxygen for the southern, central, and northern basin of Lake Baikal. A model is developed that considers two regions in each basin of Lake Baikal: (1) a surface mixed layer (SML) 400 m thick and (2) a deepwater column (DWC) below 400 m. The SMLs are assumed to be well mixed. In each of the DWCs, passive tracers are transported by vertical turbulent diffusion and upwelling. Upwelling is generated by a depth-dependent source of water because of density plumes propagating from the SML downward to larger depths. This water is considered to contain the same tracer concentrations as the SML. The tracer concentrations in the SMLs of the three basins are coupled to the atmosphere by gas exchange (including water vapor transport) and precipitation to the catchment by river inflow and outflow and to the neighboring basins via diffusive exchange and advection. SMLs and DWCs of the same basin are connected by vertical turbulent diffusion, density-driven water transport, and upwelling. Beginning at the turn of this century, the tracers CFC-11, CFC-12, 3H and 3He are modeled simultaneously to predict modern concentrations. On the basis of the tracer data the vertical diffusion coefficient K, is determined to be 4.6×10−4 m2 s−1±10% for the southern, 6.3×10−4 m2 s−4±10% for the central, and 1.7×10−4 m2 s−4±25% for the northern basin. The vertical advective flux of water at 400 m water depth is calculated as 110 km3 yr−1 in the southern, 70 km3 yr−1 in the central, and 290 km3 yr−1 in the northern basin. Concentration of dissolved molecular oxygen is modeled by using the estimated transport parameters and by fitting for the unknown consumption rate. Inverse modeling of oxygen suggests that O2 depletion in the DWC can be described by a volume sink of 44±3 mgO2 m−3 yr−1 combined with an areal sink at the sediment water interface of 17000±3000 mgO2 m−2 yr−1.