Atmospheric pressure influencing ebullition and turbidity

AbstractMethane ebullition from lake sediment is an important source of atmospheric methane. Previous studies have suggested that temperature variations, water level changes, atmospheric pressure fluctuations and wind-induced current can affect ebullition. However, most of those studies were conducted during open-water season. There is a lack of observations during ice-cover, despite of the abundance of seasonally ice-covered lakes. In this dataset, we present high-frequency ebullition intensity data, atmospheric pressure data, bottom-water temperature data, and turbidite data from Base Mine Lake (57° 1' N, 111° 37' W in Alberta, Canada) during ice cover. During the study period, the water level in the lake is stable. Also, due to the ice cover, the impact of wind-induced current is negligible. The dataset shows that ebullition during ice cover is regulated by atmospheric pressure variations; the stable bottom-water temperature has on correlation with the ebullition. The dataset also shows that turbidity at depth in the lake increases during ebullition events., MethodsThe ebullition intensity data is collected using a single-beam echosounder (Echologger EA400), which is mounted beneath ice. Bursts of 50 pings over 25 seconds (2Hz) are logged once every hour. The turbidity is collected at 30-minute intervals using a RBRDuo logger with a Seapoint turbidity sensor attached to a mooring chain. The instrument was calibrated by the manufacturer (RBR). To remove spikes, for example, a three-point median can be applied to the raw data . Water temperature data is collected every 10 seconds using a RBRSolo logger, attached to the same mooring chain as turbidity logger. The instrument was calibrated by the manufacturer (RBR). To smooth the short-term variations, for example, a 10-minute moving average can be applied to the raw data. Atmospheric pressure data is collected at the center of lake. The pressure data has been compared with the atmospheric pressure at a nearby airport,. The variations in atmospheric pressure at these two stations were nearly identical, except a constant pressure offset. Notes for echo-sounding data: The echo-sounding data is used to derive the ebullition intensity. The echosounder records the full profiles of the water column beneath it. Every hour, a burst of 50 pings over 25 seconds is set to record the ebullition activities through the water column. Each burst of 50 pings produces an echogram. In the echogram, there are several common elements: rising bubbles, floating reflectors, random noises, a blank after transmit region at the top and the lakebed at the bottom. A filtering methode discribed below is used to remove the interference while preserving the signal from the rising bubbles. This filtering method is designed with consideration of the two main features of the echograms: 1) In each echogram, the rising bubbles are diagonal lines and in general have much higher backscattering intensity than the background noise and the average row intensity. 2) the floating reflectors are time invariant and therefore their intensities are similar to the depth specific time average. The filtering method has three steps. Firstly, a certain range of depth (5.2 m to 8.5 m) is chosen to exclude the depths with interference close to the instrument and to exclude the lakebed. Secondly, the influence of the system noise is excluded, i.e. if the intensity of each element is less than the noise it is set to zero. Thirdly, in every row of the echogram, if the value of each element is less than one fifth the time averaged value, it is set to zero. After applying a filtering method, the intensity of the rising bubbles in the individual echogram is averaged and used to represent the ebullition intensity at that hour. , Usage notesThere are five sheets in the Excel, including ebullition intensity, atmospheric pressure, temperature and turbidity data. The missing data is stored as NaN. There is also an associated README.txt.摘要：湖泊沉积物的冒泡式甲烷排放（ebullition）是大气甲烷的重要来源。既往研究表明，温度变化、水位波动、大气压强波动以及风生流均会对甲烷冒泡排放产生影响。然而，此类研究大多开展于敞水期，而尽管全球季节性结冰湖泊数量众多，但冰盖期的相关观测仍较为匮乏。本数据集收录了加拿大阿尔伯塔省Base Mine Lake（北纬57°1′，西经111°37′）冰盖期内的高频冒泡强度数据、大气压强数据、底层水温数据以及浊度数据。研究时段内，该湖泊水位保持稳定，且因冰盖覆盖，风生流的影响可忽略不计。本数据集显示，冰盖期的甲烷冒泡排放受大气压强波动调控；稳定的底层水温与冒泡排放无相关性。此外，数据集还表明，在甲烷冒泡事件期间，湖泊深部的浊度会出现升高。 研究方法：冒泡强度数据通过安装于冰下的单波束回声测深仪（Echologger EA400）采集。每小时记录一次时长25秒、含50个声脉冲（ping）的脉冲串，采样频率为2Hz。浊度数据通过搭载SeaPoint浊度传感器的RBR Duo记录仪，以30分钟为间隔采集，该传感器安装于锚定链上，且已由制造商RBR完成校准。为去除异常尖峰，可对原始数据采用三点中位数滤波法。水温数据通过与浊度记录仪共用同一锚定链的RBR Solo记录仪采集，采样频率为每10秒一次，该仪器同样由制造商RBR完成校准。为平滑短期波动，可对原始数据采用10分钟移动平均法。大气压强数据采集于湖泊中心，且已与附近机场的大气压强数据进行比对：除存在恒定压强偏移外，两处测点的压强波动趋势基本一致。 关于回声测深数据的说明：回声测深数据用于反演甲烷冒泡强度。该测深仪可记录其下方水柱的完整剖面。每小时启动一次时长25秒、含50个ping的脉冲串，以记录全水柱的甲烷冒泡活动，每次脉冲串会生成一张回声图（echogram）。回声图通常包含以下元素：上升气泡、漂浮反射体、随机噪声、顶部发射区域后的空白带以及底部的湖床。下文所述的滤波方法可在去除干扰的同时保留上升气泡的信号。该滤波方法的设计参考了回声图的两大核心特征：1）在每张回声图中，上升气泡呈现为斜线，且其背向散射强度普遍高于背景噪声与各行平均强度；2）漂浮反射体不随时间变化，其强度与对应深度的时间平均强度相近。滤波方法分为三步：首先，选取特定深度区间（5.2 m至8.5 m），以排除靠近仪器的干扰区域以及湖床区域；其次，去除系统噪声的影响，即若某元素的强度低于噪声阈值，则将其置零；最后，在回声图的每一行中，若某元素的强度低于该时间平均强度的五分之一，则将其置零。完成滤波后，对单张回声图中的上升气泡强度取平均值，以此代表该小时的甲烷冒泡强度。 使用说明：本Excel文件包含五个工作表，分别对应冒泡强度、大气压强、水温及浊度数据，缺失数据以NaN格式存储。本数据集附带关联的README.txt文件。