For many, Antarctica is out of sight, out of mind. However, the waters that surround the landmass play a major role in the global climate.
The Southern Ocean absorbs and stores a high amount of carbon dioxide, acting as a buffer to slow the rate of climate change. The way these waters form and circulate in the deepest reaches of the ocean is an important control on the ability of these waters to store carbon and act as a climate safeguard.
Here Loose et al. utilize a new technique to understand how the deepest waters in the oceans form. The researchers investigated deep water formation in the Antarctic but also extended these methods to look at the formation of deep waters worldwide.
The authors examined the physical processes that are recorded in noble gases as surface water becomes deep water. These physical processes provide information on sea ice formation, subsurface ice melt of glaciers, and the exchanges between air and sea.
The noble gases neon, argon, krypton, and xenon are unique because they are found primarily in Earth’s atmosphere, whereas helium and radon are naturally produced by radioactive decay in the crust and outer mantle. The helium-3 isotope is also ordinarily found in the mantle and in seawater that emanates from the ocean spreading centers. Therefore, tracing the noble gases and their isotopes can provide insight into water mass origin and past contact with the lithosphere and with sources of geothermal heat.
The concentration of these gases can also provide a record of air-sea interactions. For example, the abundance of noble gases originating in the atmosphere gives insight into wind speed at the time of deep water formation, or the role of air bubbles, which can supersaturate the water. Noble gas concentrations can also provide information on water temperature at the ocean surface—since warmer waters can hold less dissolved gases—or how much sea ice formation and brine rejection occurred at the time of deep water formation.
The scientists focused on the Weddell Sea, a prime location where Antarctic bottom water is known to form. They analyzed samples collected aboard the RRS James Cook in January 2009 and the RRS James Clark Ross in March and April of 2010. The concentrations of noble gases (specifically helium, neon, argon, krypton, and xenon) were determined using a dual mass spectrometer system.
The researchers found that both glacial ice and sea ice govern gas concentrations in these deep water masses. It was already known that salty brine rejection during sea ice formation around Antarctica dramatically alters the density of these deepest waters. This study demonstrates that the same is true for gas concentrations.
The noble gas content found in bottom water, the scientists found, tells a story specific to how this water formed and where it traveled. After it leaves the surface, the water picks up a small fraction of ice melt from glaciers, icebergs, and ice shelves, which further modifies the water’s noble gas concentrations, forming a unique “fingerprint.” Using such fingerprints collected across space and time, it may be possible to reconstruct glacial melt and sea ice production in the past, including during the last major ice age, when ocean properties were distinct from today. (Journal of Geophysical Research: Oceans, doi:10.1002/2016JC011809, 2016)
—Wudan Yan, Freelance Writer