Acoustical methodology for determination of gas content in aquatic sediments, with application to Lake Kinneret, Israel, as a case study

Abstract

The spatial distribution of gaseous methane in the top layer of the sediment of Lake Kinneret (the Sea of Galilee), Israel is quantified using a novel acoustical methodology. Measurements were carried out with low‐frequency sound pulses (200 Hz–2 kHz) for different depths of the lake along the offshore transects. The methodology is based on the correlation of gas content with sound speed and, in turn, with the reflection coefficient of sound waves from sediment. Variations in the free gas content in sediments with water depth obtained using the proposed method show a remarkable agreement with the distribution of organic matter content in and methane fluxes from sediment, both revealed by the preceding studies. This spatially variable pattern can be explained by the specific biogeochemical, hydrodynamic, and bathymetric characteristics of Lake Kinneret. The suggested method is especially suitable for characterizing gassy sediments and highlighting the location of potential methane emissions.

A novel freeze corer for characterization of methane bubbles and assessment of coring disturbances

“…The spatial differences in gas content between Cores 1 and 4 are also in line with acoustic measurements of Katsnelson et al (), who found that sediment gas content decreased from the lake center toward its periphery, and the maximum gas content was measured in the deepest part of the lake. However, a direct comparison of the gas contents is not appropriate, as the new acoustic method used by Katsnelson et al () has not yet been validated by direct gas content measurements. In addition, gas content at the same locations can vary over time due to partial sediment degassing at low water levels (Ostrovsky et al ).…”

“…The lake is thermally stratified during April-December, resulting in anoxic conditions in the hypolimnion during May-June. LK sediments are rich in autochthonous organic material (Sobek et al 2011) and contain large amount of gaseous methane (Ostrovsky and Tegowski 2010;Katsnelson et al 2017). Sediments are usually sandy in the shallower zone, and silty-muddy in the pelagic zone ( Ostrovsky and Yacobi 1999).…”

Role of gas ebullition in the methane budget of a deep subtropical lake: What can we learn from process‐based modeling?

“…For example, the presence of 5% gas voids can increase the flux of CH 4 by one order of magnitude. Recent acoustic observations indicate important spatial variability in void fractions in the surface sediments of Lake Kinneret (Katsnelson et al ). Further investigations are required to assess their effects on the average gas fluxes from the sediment.…”

Methane Bubble Escape From Gas Horizon in Muddy Aquatic Sediment Under Periodic Wave Loading

“…This contrasts with the littoral zones of the tropical lakes discussed above. The gaseous CH 4 content in the sediment follows the same pattern (e.g., Katsnelson et al, ). Considering its water depth (~42 m at the lake's deepest), and its wave regime (e.g., high‐frequency internal waves with amplitudes of up to 4 m; Antenucci & Imberger, ), the deepest part of the lake may not demonstrate wave‐induced CH 4 ebullition, due to the relatively low scaled wave amplitude ( Figure ), which is confirmed by field observations (Ostrovsky et al, ).…”