Frost flowers on young Arctic sea ice: The climatic, chemical and microbial significance of an emerging ice type

Recent changes in sea ice type and periodicity are increasing both the spatial and temporal range over which frost flowers can occur, yet their significance to overall ocean-sea ice-atmosphere (OSA) exchange processes remains poorly understood. The climate forcing of the ocean surface, at our winter study site in a polynya-fiord system in NE Greenland, showed that frost flowers form on thin ice when open water becomes available to a cold atmosphere and surface wind conditions are low, allowing for supersaturation of the near-surface boundary layer. The formation of new ice and a frost flower-covered ice surface dramatically changed the photosynthetically available radiation (PAR) and thermal environment of this young OSA interface. A brine skim formed very early, followed almost immediately by clusters of frost flowers. The frost flowers themselves were about 5°C colder than the brine surface, with an approximately linear temperature gradient from the base to the upper tip of the frost flowers. Measured change in 18O values of frost flower vertical dimension and over time indicated that the flowers originated primarily from the surface brine skim, consistent with the model of Style and Worster. Ikaite crystals were observed to form within 1 hour in both frost flowers and thin ice in the artificially opened pond. Average ikaite concentrations in frost flowers were 1013 µmol kg-1 and in the surface slush layer, 1061 µmol kg-1. Chamber flux measurements confirmed an efflux of CO2 at the brine-wetted sea ice surface, in line with expectations of the brine chemistry across this OSA interface. The microbial measurements helped to illuminate the physics of the frost flower formation and how this unique habitat may shape the microbial community. In general, the brinier the frost flower (brine skim or surface slush layer), the higher its concentration of bacteria. Analyses of bacterial densities and dominant members of the community both indicated that a selective process occurs at this OSA interface and also confirmed the general pattern of primary oceanic versus negligible atmospheric deposition. [Submitted to JGR Atmospherest by D. G. Barber, J. K. Ehn1, M. Pucko, S. Rysgaard, J. W. Deming, J. S. Bowman, T. Papakyriakou, R Galley, and D. Sogaard. 2014]海冰类型与周期性的新近变化，正扩大霜花（frost flowers）出现的空间与时间范围，但目前学界对其在整体海洋-海冰-大气（OSA）交换过程中的重要性仍知之甚少。我们在格陵兰东北部冰间湖峡湾系统的冬季研究站点所开展的海洋表面气候强迫研究显示，当开阔水域暴露于寒冷大气且地表风速较低、近地表边界层达到过饱和状态时，霜花会在薄冰表面形成。新冰的形成与覆盖霜花的冰面，极大改变了这一新生OSA界面的光合有效辐射（PAR）与热环境。极早期便会形成盐水浮膜（brine skim），随后几乎即刻便会出现成团分布的霜花。霜花自身较盐水浮膜表面低约5℃，且从霜花基部至顶端存在近似线性的温度梯度。对霜花垂直维度与时间序列下的氧18同位素值变化进行的测量结果显示，霜花主要源自表层盐水浮膜，这与Style与Worster提出的模型相符。在人工开挖的池塘中，研究人员观测到伊卡石（ikaite）晶体可在1小时内于霜花与薄冰中形成。霜花中的伊卡石平均浓度为1013 μmol·kg⁻¹，表层雪泥层中的浓度则为1061 μmol·kg⁻¹。箱室通量测量结果证实，被盐水浸润的海冰表面存在二氧化碳外排，这与该OSA界面的盐水化学特征预期相符。微生物测量结果有助于阐明霜花形成的物理机制，以及这一独特生境如何塑造微生物群落结构。总体而言，霜花（含盐水浮膜或表层雪泥层）的盐度越高，其细菌浓度也越高。对细菌密度与群落优势类群的分析均显示，该OSA界面存在选择性过程，同时证实了“海洋源为主、大气沉积可忽略”的普遍分布模式。[由D. G. Barber、J. K. Ehn₁、M. Pucko、S. Rysgaard、J. W. Deming、J. S. Bowman、T. Papakyriakou、R Galley及D. Sogaard于2014年提交至《地球物理学研究杂志：大气卷》（JGR Atmospheres）]