Use of SAR to Improve Paleo-Climate Investigations at Lake El’gygytgyn, Siberia

Matt Nolan, University of Alaska Fairbanks

A time-series of more than 450 combined ERS-2, RADARSAT-1, and Landsat-7 scenes acquired between 1998 and 2001 was analyzed to develop a fairly complete picture of lake ice dynamics on arctic Lake El’gygytgyn, NE Siberia (67.5°N, 172°E). This 14 km3 lake partially fills a meteorite impact crater formed 3.6 million years ago and is home to an ongoing paleoenvironmental coring project. The duration of lake ice cover and the onset of lake ice breakup are important both to interpretations of the archived sediment core record and to future drilling projects that will use the ice as a stable platform. Ice formation, snowmelt, and ice breakup were found to occur in late October, mid-May, and early July, respectively (Table 1). These data were used to validate a one-dimensional, energybalance lake-ice model, which can now be used to hindcast paleoclimate based on core proxy information. SAR backscatter from the lake ice also revealed unusual spatial variations in bubble content that were found to indicate the level of biological productivity in the sediments directly beneath the ice, with the highest productivity located in the shallowest (0 – 10 m) as well as the deepest (170 – 175 m) regions of the lake. The fact that large spatial variations in biological productivity exist in the lake has important implications for selecting the locations of future sediment cores. This article gives an overview of how SAR facilitated these findings; the complete analysis can be found in Nolan, et al (2003).

What first caught our interest when analyzing these SAR scenes of the lake was the unexpected area of high backscatter near the center of the lake (Figure 1), reminiscent of a bullseye target. This pattern forms every winter, with only minor annual variations. Our interpretation of this phenomenom is that bubbles accumulate within the lake ice here and act as scatterers, reflecting energy back towards the sensor. As more bubbles accumulate throughout the winter, the backscatter increases, until it finally saturates the signal. Another region of high backscatter exists over the shallow shelves surrounding the deeper lake water. Subsequent field work has confirmed that the pattern of backscatter observed here also matches the distribution and concentration of bubbles within the lake ice. What is causing the bubbles? On the shelves (

A valuable side benefit to the ice bubble dynamic is that it allowed us to pin down the onset of snow melt to within a day or two each year. Figure 2 shows the onset of snow melt in two different years. What’s happening here is that during winter, when the snow is cold and dry, microwaves penetrate through it without much loss, largely reflecting only from the lake ice bubbles below. Once snowmelt begins, however, the liquid water in the snow prevents penetration through it and the backscatter changes remarkably and no longer shows the bullseye pattern representing the lake ice bubbles. We used information like this to construct a history of lake ice formation and decay, which we subsequently used to confirm that the lake-ice model we are using here is accurately representing reality. It seems clear from the biogeochemistry of shallow cores obtained thus far that the lake ice has remained intact for thousands of years at a time in the past. Thus, anything that improves our understanding of lake ice dynamics will aid in core interpretations and SAR has thus far been an important tool towards this end.

Nolan, Matt, et al, 2003. Analysis of Lake Ice Dynamics and Morphology on Lake El’gygytgyn, Siberia, using SAR and Landsat. J. Geophys. Research, 108 (D2) 8062, doi:10.1029/2001JD000934.

Click here to download a copy of the newsletter featuring this article