## 5 Results
### TSX Backscatter and NDVI Properties of Radiocarbon-Dated DTLBs (50-10,000 Years since Drainage)  
Analysis of TSX backscatter on 27 August of the radiocarbon-dated DTLBs indicated a significant logarithmic relationship (R\({}^{2}\) = 0.58; p \(<\) 0.05) between basin age and mean basin backscatter (Figure 3). Younger radiocarbon-dated DTLBs showed higher backscatter, while backscatter decreased  
Figure 2: Oblique aerial photographs of DTLBs located on the northern Seward Peninsula at various stages after drainage showing evolution of basin floor characteristics. Photos were collected during field visits in late June 2010 and 2011. **(a)** Early assemblage of highly productive grass vegetation emerging from very shallow water in recently drained modern basin. As the growing season progresses, the graminoid vegetation grows to 50–130 cm tall, densely covering the basin floor. **(b)** A typical characteristic of modern shallow basins after a few decades after drainage is that some parts of the basin get drier and some remain inundated, depending on local topography and hydrological characteristics. A mixed vegetation community is observed in such basin types. **(c)** Dry basin floor with willows growing in a young basin. **(d)** Polygonization and ponding in a medium age basin. for older and ancient basins. However, there was no significant relationship between 09 September backscatter and basin age. Similarly, NDVI from 13 July did not show a significant relation between NDVI of DTLBs and their radiocarbon-dated age. Young DTLBs had relatively high backscatter values that ranged between \(-\)6.1 dB and \(-\)5.5 dB. They had variable but relatively high NDVI values. They largely had not yet developed polygonal networks and had more productive vegetation than older basins (Figure 2(c)), except one that drained 294 years BP situated in the northern coastal lowland. This particular basin was less productive, also indicated by its very low NDVI value, and had developed early stage low-centered polygons. The basins were already characterized by drier surfaces and were affected by frost heave. We observed that DTLBs of medium age (younger than \(\sim\)1,000 years BP) were generally drier and exhibited early low-center ice-wedge polygonal networks (Figure 2(d)). Consequently, backscatter values decreased within the range of \(-\)6.7 dB and \(-\)6 dB for these basins. They generally had similar vegetation cover and NDVI to that of young DTLBs, but with much less variability, indicating homogenization of vegetation cover in the basins with ongoing succession. Only one medium-aged basin dated to 615 years BP in the northern coastal lowland had already developed characteristics of older medium aged DTLBs (older than \(\sim\)1,000 years). Older medium aged DTLBs exhibited lower backscatter. They had fully established low-centered polygonal networks with some wet centers and ponded polygons. Moreover, less productive lowland tundra vegetation had dominated these basin floors. Mean basin backscatter values continued to decrease for old and ancient DTLBs ranging between \(-\)7.1 dB and \(-\)6.2 dB. Old and ancient DTLBs were characterized by fully established ice-wedge polygonal networks either with dry soil surfaces or with a significant number of ponded low-centered polygons in some cases, indicating the potential for coalescence into a new lake. DTLBs with a large  
Figure 3: Plot of mean basin backscatter observed on 27 August 2009 against basin age showing a logarithmic relationship for the period of 0–10,000 years and 50–10,000 years. Basin age class is shown as: I. Modern basins; II. Young basins; III. Medium age basins; IV. Old basins; and V. Ancient basins. topographic gradient within and downstream of their drainage channels often did not appear to pool sufficient surface runoff for forming new ponds and development into a new lake, while basins with a low topographic gradient had often more ponds. We observed emergent vegetation in some of the inundated centers of the polygons.