Temperature, water level and bathymetry of thermokarst lakes in the continuous permafrost zone of northern Siberia - Lena River Delta, Siberia

Thermokarst lakes are typical features of the northern permafrost ecosystems, and play an important role in the thermal exchange between atmosphere and subsurface. The objective of this study is to describe the main thermal processes of the lakes and to quantify the heat exchange with the underlying sediments. The thermal regimes of five lakes located within the continuous permafrost zone of northern Siberia (Lena River Delta) were investigated using hourly water temperature and water level records covering a 3-year period (2009-2012), together with bathymetric survey data. The lakes included thermokarst lakes located on Holocene river terraces that may be connected to Lena River water during spring flooding, and a thermokarst lake located on deposits of the Pleistocene Ice Complex. Lakes were covered by ice up to 2 m thick that persisted for more than 7 months of the year, from October until about mid-June. Lake-bottom temperatures increased at the start of the ice-covered period due to upward-directed heat flux from the underlying thawed sediment. Prior to ice break-up, solar radiation effectively warmed the water beneath the ice cover and induced convective mixing. Ice break-up started at the beginning of June and lasted until the middle or end of June. Mixing occurred within the entire water column from the start of ice break-up and continued during the ice-free periods, as confirmed by the Wedderburn numbers, a quantitative measure of the balance between wind mixing and stratification that is important for describing the biogeochemical cycles of lakes. The lake thermal regime was modeled numerically using the FLake model. The model demonstrated good agreement with observations with regard to the mean lake temperature, with a good reproduction of the summer stratification during the ice-free period, but poor agreement during the ice-covered period. Modeled sensitivity to lake depth demonstrated that lakes in this climatic zone with mean depths > 5 m develop continuous stratification in summer for at least 1 month. The modeled vertical heat flux across the bottom sediment tends towards an annual mean of zero, with maximum downward fluxes of about 5 W/m**2 in summer and with heat released back into the water column at a rate of less than 1 W/m**2 during the ice-covered period. The lakes are shown to be efficient heat absorbers and effectively distribute the heat through mixing. Monthly bottom water temperatures during the ice-free period range up to 15 °C and are therefore higher than the associated monthly air or ground temperatures in the surrounding frozen permafrost landscape. The investigated lakes remain unfrozen at depth, with mean annual lake-bottom temperatures of between 2.7 and 4 °C.热喀斯特湖（thermokarst lakes）是北方多年冻土生态系统的典型特征，在大气与地下之间的热交换过程中发挥着重要作用。本研究旨在阐明这类湖泊的主要热过程，并量化其与下伏沉积物的热交换。研究针对位于西伯利亚北部连续多年冻土区（勒拿河三角洲）的5个湖泊展开，采用2009至2012年共三年的逐小时水温、水位观测记录，结合水深测量数据开展分析。其中包括全新世河成阶地上的热喀斯特湖——这类湖泊在春季洪水期可与勒拿河水体连通——以及一处位于更新世冰复合体沉积物之上的热喀斯特湖。 这些湖泊被厚度可达2米的冰层覆盖，每年封冰期长达7个多月，从10月延续至次年6月中旬。封冰初期，湖底温度因下伏融化沉积物产生的向上热通量而升高。融冰发生前，太阳辐射有效加热冰下水体并引发对流混合。融冰过程始于6月初，持续至6月中下旬。从融冰开始直至整个无冰期，整个水柱均会发生混合，这一结论可通过韦德伯恩数（Wedderburn numbers）得到验证——该指标是表征风混合与水体分层平衡的定量参数，对湖泊生物地球化学循环的研究具有重要意义。 本研究采用FLake模型（FLake model）对湖泊热状况开展数值模拟。模型结果与观测数据在湖泊平均水温方面吻合度良好，能够较好复现无冰期的夏季水体分层现象，但在封冰期的模拟效果欠佳。针对湖泊水深的模型敏感性分析显示，该气候区内平均水深大于5米的湖泊，夏季会出现至少持续1个月的连续水体分层。模拟得到的穿过湖底沉积物的垂直热通量年平均值趋近于0：夏季最大向下热通量约为5 W/m²，而封冰期返还至水体的热通量不足1 W/m²。 研究表明，这类湖泊是高效的热吸收体，并可通过混合作用有效传递热量。无冰期的月度湖底水温最高可达15 ℃，高于周边多年冻土冰冻景观的同期月均气温或地温。本次研究涉及的湖泊深层始终保持不冻，年平均湖底水温介于2.7 ℃至4 ℃之间。