Brine flow through sea ice

Metadata record for data from ASAC Project 1060
See the link below for public details on this project.

Taken from the referenced publications:

Sea ice exhibits a marked transition in its fluid transport properties at a critical brine volume fraction Pc of about 5 percent, or temperature Tc of about -5 degrees Celsius for salinity of 5 parts per thousand. For temperatures warmer than Tc brine carrying heat and nutrients can move through the ice, whereas for colder temperatures the ice is impermeable. This transition plays a key role in the geophysics, biology, and remote sensing of sea ice. Percolation theory can be used to understand this critical behaviour or transport in sea ice. The similarity of sea ice microstructure to compressed powders is used to theoretically predict Pc of about 5 percent.

The snow cover on Antarctic sea ice often depresses the ice below sea level, allowing brine or seawater to infiltrate, or flood the snowpack. This significantly reduces the thermal insulation properties of the snow cover, and increases the ocean/atmosphere heat flux. The subsequent refreezing of this saturated snow or slush layer, to form snow-ice, can account for a significant percentage of the total ice mass in some regions. The extent of saturated snow cannot presently be estimated from satellite remote-sensing data and, because it is often hidden by a layer of dry snow, cannot be estimated from visual observations. Here, we use non-parametric statistics to combine sea-ice and snow thickness data from drillhole measurements with routine visual observations of snow and ice characteristics to estimate the extent of brine-infiltrated snow.

During a field experiment in July 1994, while the R.V. Nathaniel B. Palmer was moored to a drifting ice floe in the Weddell Sea, Antarctica, data were collected on the sea-ice and snow characteristics. We report on the evolution of ice which grew in a newly opened lead. As expected with the cold atmospheric conditions, congelation ice initially formed in the lead. Subsequent snow accumulation and large ocean heat fluxes resulted in melt at the base of the ice, and enhanced flooding of the snow on ice surface. This flooded snow subsequently froze, and, five days after the lead opened, all the congelation ice had melted and twenty-six centimetres of snow ice had formed. We use measured sea-ice and snow salinities, thickness and oxygen isotope values of the newly formed lead ice to calculate the salt flux to the ocean. Although there was a salt flux to the ocean as the ice initially grew, we calculate a small net fresh-water input to the upper ocean by the end of the 5 day period. Similar processes of basal melt and surface snow-ice formation also occurred on the surrounding, thicker sea ice. Oceanographic studies in this region of the Weddell Sea have shown that salt rejection by sea-ice formation may enhance the ocean vertical thermohaline circulation and release heat from the deeper ocean to melt the ice cover. This type of deep convection is thought to initiate the Weddell polynya, which was observed only during the 1970s. Our results, which show than an ice cover can form with no salt input to the ocean, provide a mechanism which may help explain the more recent absence of the Weddell polynya.

ASAC项目1060数据集元数据记录
有关该项目的公开详情，请参阅下方链接。

本数据集源自已发表的相关研究论文：

海冰在临界盐水体积分数Pc（约5%）或盐度为5‰时的临界温度Tc（约-5℃）处，其流体输运特性会发生显著转变。当温度高于Tc时，携带热量与营养盐的盐水可在海冰内部运移；而当温度低于Tc时，海冰将变为不透水介质。该临界转变对海冰的地球物理、生物学及遥感研究均具有关键意义。可利用渗流理论（Percolation theory）阐释海冰中的此类临界行为与输运过程。研究人员通过类比海冰微观结构与压缩粉体的相似性，从理论上预测Pc约为5%。

南极海冰上方的积雪常使冰面沉降至海平面以下，致使盐水或海水渗入并淹没积雪层。这会显著降低积雪层的隔热性能，并增强海-气间的热通量。随后，该饱和积雪或雪浆层会重新冻结形成雪冰（snow-ice），在部分区域中，雪冰的总量可占海冰总质量的可观比例。目前，无法通过卫星遥感数据估算饱和积雪的覆盖范围；且由于其常被干积雪层覆盖，也无法通过目视观测完成估算。本研究采用非参数统计（non-parametric statistics）方法，将钻孔测量得到的海冰与积雪厚度数据，与常规积雪及海冰特性目视观测数据相结合，以估算盐水渗入积雪的覆盖范围。

1994年7月，研究团队在南极威德尔海开展野外实验：R.V. 纳撒尼尔·B·帕尔默号（R.V. Nathaniel B. Palmer）科考船锚泊于一处随流漂移的浮冰之上，期间采集了海冰与积雪的特性数据。本研究针对新开辟冰间水道（lead）内形成的海冰演化过程展开报道。在寒冷大气条件下，冰间水道内最初形成的是凝结海冰（congelation ice）。后续的积雪堆积与强烈的海洋热通量导致海冰底部发生融化，并加剧了冰面积雪的浸水现象。该浸水积雪随后发生冻结；在冰间水道开辟后的第5天，所有凝结海冰均已融化，且形成了26厘米厚的雪冰。本研究利用新形成的冰间水道海冰的实测海冰与积雪盐度、厚度及氧同位素数据，计算了向海洋排放的盐通量。尽管海冰初始形成阶段存在向海洋的盐通量，但在5天实验期结束时，我们计算得到上层海洋的净淡水输入量极小。周边较厚海冰区域同样发生了类似的海冰底部融化与表层雪冰形成过程。

威德尔海该区域的海洋学研究表明，海冰形成过程中排出的盐分可增强海洋垂直热盐环流，并从深海释放热量以融化冰盖。此类深层对流被认为是威德尔海冰间湖（Weddell polynya）的形成诱因，而该冰间湖仅在20世纪70年代被观测到。本研究结果表明，海冰盖形成过程可无需向海洋排放盐分，该机制或有助于解释近期威德尔海冰间湖消失的现象。