GLITTERING across the briny surface of newly formed sea ice, frost flowers are as bewitching to polar scientists as Homer鈥檚 sirens 鈥� luring them and their instrument-laden sleds to the treacherous boundary between ice and sea.
These popcorn-sized puffs of ice crystals have long been thought the source of the bromine oxide that triggers the deposition of the mercury from the atmosphere and the depletion of ozone near the poles. But new findings suggest that the ice blooms do not affect the polar atmosphere as expected.
Every spring, ozone levels near the surface of Earth鈥檚 polar regions drop to near zero. At the same time, inert atmospheric mercury is transformed into a form that is more readily absorbed by plants and animals. Models suggest that this seasonal event may dump 90 tonnes of mercury a year in Arctic regions (Tellus, vol 56B, p 397). No one knows how much of this mercury enters the food chain, but it may contribute to the elevated levels in Arctic natives.
Advertisement
Frost flowers are three times as salty as seawater and have a high surface area. Could they be the source of the bromine oxide that gets into the atmosphere and triggers the seasonal phenomena? Finding out is tricky. The blooms appear at the 鈥渓eads鈥� where ice and sea meet. Ice here is thin, and conditions can quickly become treacherous.
This spring, a team led by Matthew Sturm of the Cold Regions Research and Engineering Laboratory in Fort Wainwright, Alaska, braved the leads near Barrow. The group鈥檚 results, presented this week at the American Geophysical Union meeting in San Francisco, suggest that frost flowers are not wholly to blame. Measurements of the air near the leads did not show dramatically more bromine oxide than that in the air further inland.
鈥淓very spring ozone levels near the poles drop to zero and 90 tonnes of mercury are dumped in the Arctic鈥�
鈥淸Frost flowers] certainly could be the initiator of the chemistry,鈥� says team member William Simpson. 鈥淏ut they don鈥檛 appear to be the only thing that鈥檚 activating the [bromine].鈥� He suspects the wind may whip the frost flowers into salty aerosol particles that spread away from the leads and cause cascading reactions that activate bromine from other sources.
Hans-Werner Jacobi of the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany, and his team are also studying frost flowers. But rather than venturing onto thin ice, they succeeded in growing frost flowers in the lab, and found their composition was more variable than earlier studies had reported.
So for now frost flowers remain enigmatic. 鈥淚 don鈥檛 think the jury is in yet on how important frost flowers are as the bromine source,鈥� says Eric Wolff of the British Antarctic Survey in Cambridge, UK.
