Brine flow through sea ice

Metadata record for data from ASAC Project 1060See 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.

本数据集为ASAC项目1060（ASAC Project 1060）的相关数据元记录，有关该项目的公开详情请参阅下方链接。本文数据源自以下已发表文献：海冰的流体输运特性会发生显著转变：当盐度为5‰时，其临界盐水体积分数Pc（critical brine volume fraction）约为5%，对应的临界温度Tc（temperature Tc）约为-5℃。当温度高于Tc时，携带热量与营养盐的盐水可在海冰中运移；而当温度低于Tc时，海冰将变为不透水层。这一转变对海冰的地球物理学、生物学及遥感研究均具有关键意义。可利用渗流理论（percolation theory）解析海冰中的这一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末的相似性，从理论上预测Pc约为5%。南极海冰上方的积雪常会将海冰压至海平面以下，使得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-气热通量（ocean/atmosphere heat flux）。随后，该饱和积雪层或雪浆层会重新冻结形成雪冰（snow-ice），在部分区域其贡献可达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获取该参数。本研究利用非参数统计方法，将钻孔测量获取的海冰与积雪厚度数据，结合常规冰雪特性目视观测结果进行整合，以估算盐水渗透积雪的覆盖范围。1994年7月，研究团队在南极威德尔海（Weddell Sea）开展野外实验期间，将“内森尼尔·B·帕尔默号”（R.V. Nathaniel B. Palmer）科考船系泊于一处漂流浮冰上，收集了海冰与积雪的特性数据。本文报道了新开放冰间水道（lead）内形成的海冰演化过程。在寒冷的大气环境下，冰间水道内最初形成了柱状冰（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淹没程度。被淹没的积雪随后发生冻结；在冰间水道开放后的第5天，所有柱状冰均已融化，同时形成了26厘米厚的雪冰。本研究利用实测的海冰与积雪盐度、厚度数据，以及新形成的冰间水道冰的氧同位素值，计算了向海洋输出的盐通量。尽管海冰最初形成时会向海洋输出盐分，但研究计算结果显示，在5天实验期结束时，上层海洋获得了少量净淡水输入。周边较厚的海冰也发生了类似的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温盐环流（thermohaline circulation），并将深层海洋的热量释放至上层，以融化海冰覆盖层。这类深层对流被认为是威德尔海冰间湖（Weddell polynya）形成的诱因，而该冰间湖仅在20世纪70年代被观测到。本研究结果表明，海冰覆盖层可在未向海洋输出盐分的情况下形成，这一机制可为近年来威德尔海冰间湖的消失现象提供合理的解释。