A New Fast, Reliable Technique for the Sampling of Dissolved Inorganic Carbon in Sea Ice

Abstract:

For a long time, sea ice was considered to act as a lid over seawater preventing CO₂ exchange between the atmosphere and ocean. Recent observations suggest that sea ice can be an active source or a sink for CO₂, although its magnitude is not very clear. The direct measurements on CO₂ flux based on the chamber method and eddy covariance often do not agree with each other. It is therefore important to measure the dissolved inorganic carbon (DIC) stock in sea ice precisely in order to better understand the CO₂ flux through sea ice.

The challenges in sea ice DIC sampling is how to melt the ice core without being exposed to the air gaining or losing CO₂. A common practice is to seal the ice core in a self-prepared gas-tight plastic bag and suck the air out of the bag gently using a syringe (together with a needle) through a valve mounted on one side of the bag. However, this method is time consuming (takes up to several minutes to suck the air out) and very often there is large headspace found in the bag after the ice melts due to the imperfect bag-preparation, which might affect the DIC concentration in melt ice-water.

We developed a new technique by using a commercially available plastic bag with a vacuum sealer to seal the ice core. In comparison to syringe-based method, this technique is fast and easy to operate; it takes less than 10 seconds to vacuum and seal the bag all in one button with no headspace left in the bag. Experimental tests with replicate ice cores sealed by those two methods showed that there is no difference in the DIC concentration measured after these two methods, suggesting that there is no loss of DIC during the course of vacuum sealing. In addition, a time series experiment on DIC in melt ice-water stored in the new bag shows that when the samples were not poisoned, the DIC concentration remains unchanged for at least 3 days in the bag; while poisoned by HgCl₂, there is no change in DIC for at least 21 days, indicating that this new bag is impermeable to CO₂. Therefore, this new technique offers a convenient yet reliable method for DIC sampling of sea ice.