Spatial and temporal variability of gas content in sediments of Lake Kinneret, North of Israel

Abstract

<p>Shallow gassy aquatic sediments, abundantly found in Israel and worldwide, are a source of major concern for their contribution to destabilization of coastal and marine infrastructure, ecological balance, air pollutions, and global warming. Gas bubbles within sediment change effective sediment properties, including also its geo-acoustic characteristics. Here we study the spatial and temporal variability of free gas (methane) content (<em>θ</em>) in shallow sediments of deep subtropical Lake Kinneret. We implemented a recently developed noninvasive acoustic methodology that allows estimating <em>θ</em> in sediment based on measurements of bottom reflections of sound signals and subsequent assessment of sound speed in the bottom. The experiments were carried out in the lake in April and August 2021. One- and five-second-long pulses in the frequency bands of 200 – 7000 Hz and 200 – 10000 Hz were radiated in April and August, respectively. Preliminary estimated <em>θ</em> at the 21-22 m isobath was 0.02−0.04% and 0.04−0.12% in April and August, respectively. Analysis of acoustic measurements shows distinct changes in <em>θ</em> in comparison to <em>θ</em> assessed in previous acoustic experiments carried out by our team in 2015-2018, when an inverse relationship between <em>θ</em> and lake level was found. Here we discuss other possible mechanisms, which may pre-determine the spatial and temporal variability in <em>θ</em>, such as ebullition of methane at the 21-22 m isobath and variability in deposited organic matter content, which vary both spatially (with seafloor depth) and seasonally.</p>