An underwater stalactite dripping salt?

The dive team showed us this video that they took recently. It illustrates a couple of really interesting aspects of the relationship between sea ice and seawater. The stalactite-like projection extending down from the bottom of the sea ice is called a brine tube. Brine tubes form when some physical process causes very salty water to drain from porous ice. Since this brine is below the freezing point for relatively fresh seawater when the two come in contact the seawater freezes, forming a tube around the brine. You can still see the brine draining from the bottom of the tube. Anecdotal evidence suggests that these brine tubes are more active on sunny days suggesting that the ice warms up enough on these days to change its porosity. The ice where this video was taken is over 4 feet thick, only the bottom few inches likely warmed enough to drain. A rule of thumb exists for when the ice becomes so porous that the pores actually connect (like a block of very holey Swish cheese), called the “rule of 5s”. For ice that contains .5 % salt, at -5 C, 5 % of the total volume of water contained in the ice is liquid. This is a sufficient volume for the pores to connect and the brine to potentially drain from the ice interior.

This flushing has important implications for sea ice biology and the larger ecosystem. The brine flows out of the sea ice because it is denser than the underlying seawater that replaces it in the sea ice matrix. By now the ice algae have been active for some time and have probably depleted many of the nutrients within the sea ice brine. The incoming seawater replenishes these and allows primary production to continue. Recent research shows that the sea ice algae are not passive players in this process; they surround themselves with a gelatinous mixture of sugars and proteins that alters the physical structure of the ice. Among other things this enhances the porosity of the ice, encouraging an exchange of water.

Brine drainage might even effect have an impact on global climate. In an earlier post I talked about how specific salts form during the sea ice formation process (see “Pass the Salt”). One of the first salts to form, at a relatively warm temperature, is a form of calcium carbonate known as calcite. This is the same mineral that gives coral reefs their structure. It has been suggested that the flushing of sea ice “pumps” calcite into the water column. This pump is in essence a transfer of carbon from the atmosphere (where it exists as carbon dioxide) to the water column.

There’s a lot that we don’t know about brine drainage from sea ice. Here we’re observing it during the spring, but what about in the fall when sea ice is forming? A tremendous amount of salt is rejected from forming sea ice but due to logistical challenges researchers are rarely around to observe it (there have been some notable exceptions). And what about the sea ice bacteria that we study? Are they rejected along with the salt, or do they have some trick to help them stay in the ice?