Image of the Day 11/21

Landsat Images showing Ice Scours in the Caspian Sea

Image 1: This is a Landsat 8 image of the Caspian sea acquired on April 16^th, 2016. The image clearly shows a strange seabed scouring phenomena in the region of Tyuleni Archipelago (the light colored lines that break up the darker green vegetation visible on the seabed). One thing that makes them curious is that groups of scours are often parallel to each other and appear to be formed by objects traveling in the same direction. The initial cause of these features was confusing to analysts, one possible proposal is that they were human in origin - the result of seafloor trawling. It was not until this image was compared to the second image, taken in January, that the origin of the scours was identified as gouging caused by sea ice starting to break up.

Image 2: A Landsat 8 image that shows the same region in winter. In the northeastern portion of the image the ice is starting to break up and it is possible to see the scour marks which occur behind the leading edge of shrinking ice floes. This occurs because the Caspian sea is very shallow and the sea ice that develops there is relatively warm and thin. This means the ice pieces are easily moved by wind and currents and are often pushed together, causing some of them to tilt, forming structures called “hummocks.” In the shallow water, the “keels” of these hummocks can become wedged in the sediment, causing scouring as the ice bed is moved. Once the ice melts, only the scour marks persist in the spring and summer, creating the pattern seen in the first image. The parallel nature of groups of lines in the first image can be explained by several ice pieces all being dragged along a similar path by the same wind/current.