Organic-rich aquatic sediments are a significant source of methane to the atmosphere. In situ remote quantifications of gas content in shallow sediment is a complex task due to its large spatiotemporal heterogeneity. The spatial and multiannual changes of free gas (methane) content ( Θ ) in shallow sediments were studied in deep subtropical Lake Kinneret. We implemented recently developed acoustic methodology that allows estimating Θ in sediment based on assessment of sound speed. This method is based on measurement of reflection coefficient of acoustic signal at low-to-mid frequencies from the water-sediment interface using geoacoustic inversion technique. The used approach provides an assessment of the mean gas content over certain bottom area near the sampling location. Analysis of acoustic measurements carried out in 2015–2018 shows distinct changes in Θ with bottom depth along the offshore transects. We found the inverse relationship between Θ and lake level. The observed patterns in sedimentary gas content are supported by previous observations showing (a) changes of acoustic sediment properties with depth and (b) inverse relationship between rate of gas ebullition from bottom and lake level. The obtained absolute values of Θ at different locations were the same order of magnitude as those estimated directly from frozen cores. The validity and reliability of the method and its further development are discussed.

中文翻译：

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浅层沉积物中游离气体含量的时空变异性：以 Kinneret 湖为例

富含有机物的水生沉积物是大气中甲烷的重要来源。由于其巨大的时空异质性，浅层沉积物中气体含量的原位远程量化是一项复杂的任务。研究了深亚热带Kinneret湖浅层沉积物中游离气体（甲烷）含量（ Θ ）的空间和多年变化。我们实施了最近开发的声学方法，该方法允许基于声速评估来估计沉积物中的 Θ。该方法基于使用地声反演技术测量来自水-沉积物界面的中低频声信号的反射系数。所使用的方法提供了对采样位置附近特定底部区域的平均气体含量的评估。对 2015-2018 年进行的声学测量分析显示，θ 随海上横断面的底部深度发生明显变化。我们发现 Θ 与湖泊水位呈反比关系。沉积气体含量的观测模式得到先前观测结果的支持，这些观测结果显示 (a) 声学沉积物特性随深度的变化和 (b) 底部和湖面的气体沸腾速率之间的反比关系。在不同位置获得的 Θ 绝对值与直接从冻结岩心估计的值处于同一数量级。讨论了该方法的有效性和可靠性及其进一步发展。沉积气体含量的观测模式得到先前观测结果的支持，这些观测结果显示 (a) 声学沉积物特性随深度的变化和 (b) 底部和湖面的气体沸腾速率之间的反比关系。在不同位置获得的 Θ 绝对值与直接从冻结岩心估计的值处于同一数量级。讨论了该方法的有效性和可靠性及其进一步发展。沉积气体含量的观测模式得到先前观测结果的支持，这些观测结果显示 (a) 声学沉积物特性随深度的变化和 (b) 底部和湖面的气体沸腾速率之间的反比关系。在不同位置获得的 Θ 绝对值与直接从冻结岩心估计的值处于同一数量级。讨论了该方法的有效性和可靠性及其进一步发展。