As the intricacies of paleoclimate dynamics are explored, it is becoming understood that sea-ice variability can instigate, or contribute to, climate change instabilities commonly described as “tipping points”. Compared to ice sheets and circulating ocean currents, sea-ice is ephemeral and continental-scale changes to sea ice cover occur seasonally. Sea-ice greatly influences polar albedo, atmosphere-ocean gas exchange and vertical mixing of polar ocean masses. Major changes in sea ice distribution and thickness have been invoked as drivers of deglaciations as well as stadial climate variability described in Greenland climate records as “Dansgaard-Oeschger” cycles and described in Antarctic climate records as “Antarctic Isotopic Maxima”.

The role of halogens in polar atmospheric chemistry has been studied intensively over the past few decades. This research has been driven by the role of bromine, primarily as gas-phase bromine monoxide (BrO), which exerts a key control on polar tropospheric ozone concentrations. Initial findings led to the discovery of boundary-layer self-catalyzing heterogeneous bromine reactions fed by sunlight and ozone, known as bromine explosions. First-year sea-ice and blowing snow have been identified as key components for this heterogeneous bromine recycling in the polar boundary layer. This understanding of polar halogen chemistry – supported by an expanding body of observations and modeling – has formed the basis for investigating quantitative links between halogen concentrations in the polar atmospheric boundary layer and sea-ice presence and/or extent.

Despite the clear importance of sea-ice in paleoclimate research, the ice core community lacks a conservative and quantitative proxy for sea-ice extent. The most commonly applied proxy, methanesulphonic acid (MSA), is volatile and has not been demonstrated reliably for ice core records extending beyond the last few centuries. Sodium has also been applied to reconstruct sea-ice extent in a semi-quantitative manner although the effects of meteorological transport noise are significant. Contrary to a priori expectations, the halogens bromine and iodine appear to be stable in polar snow and ice over millennial timescales, addressing the temporal limitations of MSA records. Unfortunately, transport and meteorological variability influence sodium deposition as well as the deposition of halogens and the many other ionic impurities found in ice cores. The atmospheric chemistry of halogens is more complex than those of sodium or MSA due to the mixed-phase (gas and aerosol) nature of halogen photochemistry. Thus the application of halogen records in ice cores to sea-ice reconstruction overcomes some challenges posed by existing proxies, but also opens new challenges specific to halogens. Challenges common to all sea-ice proxies include the deconvolution of changes in emission source locations and changes in transport efficacy, particularly those occurring during climate transitions combining changes in sea-ice and atmospheric circulation, such as stadial/interstadial or glacial/interglacial climate variability.

In this review, we describe the rationale and available evidence for linking the halogens bromine and iodine found in polar snow and ice to sea-ice extent. Reported measurements of bromine and iodine in polar snow and ice samples are critically discussed. We also consider aspects of halogen transport and retention in polar snow and ice that are still poorly understood. Overall, there is a growing body of evidence supporting the application of bromine to sea-ice reconstructions, and the use of iodine to reconstruct marine biological activity mediated in part by sea-ice extent. These halogens complement existing sea-ice proxies but most crucially, offer the capacity to greatly extend the temporal and spatial coverage of ice core-based sea-ice reconstructions. We identify knowledge gaps existing in the current understanding of spatial and temporal variability of halogen distributions in the polar regions. We suggest areas where polar halogen chemistry can contribute to a better understanding of the halogen records recovered from ice cores. Finally, we propose future steps for establishing reliable and constructive sea-ice reconstructions based on bromine and iodine as observed in snow and ice cores.

随着对古气候动力学的复杂性的探索，人们逐渐认识到海冰的变化可以引发或促成通常被称为“临界点”的气候变化不稳定性。与冰盖和循环洋流相比，海冰是短暂的，海冰覆盖的大陆尺度变化是季节性发生的。海冰极大地影响极地反照率、大气-海洋气体交换和极地海洋物质的垂直混合。海冰分布和厚度的重大变化被认为是冰川消融和静态气候变化的驱动因素，在格陵兰气候记录中被描述为“丹斯加德-厄施格”循环，在南极气候记录中被描述为“南极同位素极大值”。

在过去的几十年里，人们对卤素在极地大气化学中的作用进行了深入研究。这项研究是由溴的作用推动的，主要是作为气相一氧化溴 (BrO)，它对极地对流层臭氧浓度发挥关键控制作用。最初的发现导致发现了由阳光和臭氧供给的边界层自催化异质溴反应，称为溴爆炸。第一年的海冰和吹雪已被确定为极地边界层中这种异质溴循环的关键组成部分。这种对极地卤素化学的理解——得到了不断扩大的观测和建模主体的支持——已经形成了研究极地大气边界层中卤素浓度与海冰存在和/或范围之间的定量联系的基础。

尽管海冰在古气候研究中具有明显的重要性，但冰芯群落缺乏对海冰范围的保守和定量代理。最常用的替代物甲磺酸 (MSA) 具有挥发性，并且在过去几个世纪之后的冰芯记录中尚未得到可靠证明。尽管气象传输噪声的影响很大，但钠也已被应用于以半定量方式重建海冰范围。与先验相反正如预期的那样，卤素溴和碘似乎在千年时间尺度内在极地冰雪中保持稳定，从而解决了 MSA 记录的时间限制。不幸的是，运输和气象变化影响钠沉积以及卤素和冰芯中发现的许多其他离子杂质的沉积。由于卤素光化学的混合相（气体和气溶胶）性质，卤素的大气化学比钠或 MSA 的化学更复杂。因此，冰芯中卤素记录在海冰重建中的应用克服了现有代理带来的一些挑战，但也开辟了卤素特有的新挑战。所有海冰替代物的共同挑战包括排放源位置变化的去卷积和运输效率的变化，

在这篇综述中，我们描述了将极地雪和冰中发现的卤素溴和碘与海冰范围联系起来的基本原理和可用证据。对极地冰雪样品中溴和碘的报告测量值进行了批判性讨论。我们还考虑了仍然知之甚少的极地冰雪中卤素运输和保留的方面。总体而言，越来越多的证据支持将溴应用于海冰重建，以及使用碘来重建部分由海冰范围介导的海洋生物活动。这些卤素补充了现有的海冰代理，但最重要的是，提供了极大地扩展基于冰芯的海冰重建的时间和空间覆盖范围的能力。我们确定了当前对极地地区卤素分布的时空变异性的理解中存在的知识差距。我们建议极地卤素化学有助于更好地了解从冰芯中恢复的卤素记录的区域。最后，我们提出了基于在雪和冰芯中观察到的溴和碘建立可靠和建设性的海冰重建的未来步骤。