Pockmark like structures in Lake Constance and their effects on methane emission from large lakes

湖泊在全球甲烷收支中的作用似乎比以前认为的更重要。然而，大型湖泊中这种气候活性气体的来源和汇仍然是相当未知的。除了微生物甲烷产量高导致的扩散通量或沸腾外，另一个潜在的重要甲烷来源是来自较深地圈的流体和气体的排放，例如甲烷渗漏或麻坑。麻坑是沉积物表面的凹陷，通常以流体流动和甲烷释放增强为特征。海洋环境中的许多麻点已被描述，但湖沼系统中的麻点及其相关的地质、化学和生物过程相当未知。在康斯坦茨湖（德国南部），最近发现了类似麻子的结构。在2005年的初步调查中，我们能够观察到甲烷沸腾，并且在这些麻子附近的水柱中甲烷浓度增加。本研究的目的是：(1)定位和描述康斯坦茨湖的麻点区域；(2)确定康斯坦茨湖麻点形成的机制；(3)确定甲烷的来源；(4)量化湖底甲烷通量及其时间变率。

The role of lakes in the global methane budget seems to be more important than previously thought. However, the sources and the sinks of this climate active gas in large lakes are still quite unknown. Beside diffusive flux or ebullition due to high microbiological methane production, another potentially important methane source is the emission of fluids and gas from the deeper geosphere, such as methane seeps or pockmarks. Pockmarks are depressions at the sediment surface, often characterized by fluid flow and sites of enhanced methane release. Numerous pockmarks have been described for the marine environment, but pockmarks in limnic systems are rather unknown, as well as their associated geological, chemical and biological processes. In Lake Constance (southern Germany) pockmark like structures have been discovered recently. In a preliminary survey in 2005, we were able to observe methane ebullition and increased methane concentrations in the adjacent water column at these pockmarks. The objectives of this study are (1) to locate and to describe the pockmark areas in Lake Constance, (2) to identify the mechanisms responsible for the formation of pockmarks in Lake Constance, (3) to identify the sources of methane and (4) to quantify the fluxes of methane from the lake floor and their temporal variability.