Drying active layer in Utqiagvik, Alaska revealed from the GPS-IR-measured seasonal subsidence during 2004–2019

Abstract:

In permafrost areas, the ground surface is subject to uplift/subside seasonally due to the active layer freezing/thawing. The seasonal subsidence can indicate the hydrological and thermal changes of the active layer, as its magnitude mainly depends on the soil moisture content within the thawed soil. In this study, we use GNSS interferometric reflectometry to obtain the seasonal subsidence in each summer during 2004–2019 in Utqiagvik (formerly Barrow), Alaska. We compare them to the square-root of thaw indices (denoted as √DDT) (Fig. 1(a)). We find that the slope of the seasonal subsidence versus √DDT was 0.57 ± 0.08 cm·(°C·day)^-1/2 during 2004–2008, decreasing to 0.32 ± 0.06 cm·(°C·day)^-1/2 during 2009–2019 (Fig. 1(b)), indicating that the seasonal subsidence was more responsive to the air temperature during 2004–2008 than those during 2009–2019. We postulate that this significant change in sensitivity was caused by active layer drying. More specifically, decreasing soil moisture lowers soil thermal conductivity, retarding the heat transfer from air to frozen ground to lead to shallower active layer, consequently resulting in the decrease of sensitivity. From in situ observations, we find that the soil moisture up to 30 cm depth presented a significant decreasing trend from 2004 to 2008, followed by a relatively stable period and then slightly increased (Fig. 1(c)). We also find that the n-factors (i.e., the ratio between DDT derived from ground and air temperature, respectively) were higher during 2004–2008 than those during 2009–2017. These evidences support our hypothesis that active layer drying is the main driver of the sensitivity change. This study provides a new perspective into the usage of surface deformation for studying frozen ground dynamics, by revealing active layer drying from seasonal subsidence. It also implies that using long-term surface deformation to infer permafrost degradation/aggradation should consider the soil property changes in the active layer.