Evidence indicates that Antarctic bottom water production through the Holocene (last 10,000 years) is not in a steady-state condition; that is to say that bottom water production sites shift along the Antarctic margin over decade to century timescales as conditions for the existence of polynyas change. For example, the calving of the Mertz Glacier, which occurred on 12–13 February 2010, dramatically changed the environment for producing bottom water, reducing export by up to 23% in the region of Adelie Land. Evidence from sediment cores, containing layers of cross-bedded sediments indicating phases of stronger bottom currents, collected on the Mac.Robertson shelf and Adélie Land suggests that they have switched "on" and "off" again as important bottom water production sites over the last several thousand years.