The accelerating sea-ice, ice sheet and glacial melt associated with climate warming have resulted in important changes in the Arctic region over the past decades. In northern Baffin Bay, the formation of the North Open Water (NOW) polynya, which is intrinsically linked to regional sea-ice conditions and ocean circulation, has become more variable in recent years. To understand how climate-driven changes affect sea-surface conditions in the polynya, we analyzed dinoflagellate cyst assemblages from a sediment core record that covers the past ca. 3800 years, and developed an index based on the locations of modern analogues from a large regional reference dataset. Our results suggest a prolonged open-water season characterized by higher summer sea-surface salinity and temperature between ca. 3800 to 2500 years BP, followed by gradual cooling, freshening and increased sea-ice influence from 2500 to 1500 years BP, and continued sea-surface cooling with a shorter open-water season from 1500 to 156 years BP. The modern warming translates into a rapid turnover in the composition of dinoflagellate cyst assemblages during the last 50 years of our record, unprecedented for at least the past 3800 years studied here. For the uppermost part of our core (ca. 2009 to 2015 CE), the dinoflagellate cyst assemblages suggest increased stratification and sea-surface freshening resulting from increased glacial runoff and Arctic sea-ice export into the NOW region. Arctic climate projections indicate accelerated sea-ice thinning and ice sheet melt in the future, pointing to a shorter polynya season and increased polar inflows leading to fundamental changes within the NOW polynya into future years.

与气候变暖相关的加速海冰、冰盖和冰川融化导致北极地区在过去几十年中发生了重大变化。在巴芬湾北部，与区域海冰条件和海洋环流有着内在联系的北开放水域 (NOW) 冰间湖的形成近年来变得更加多变。为了了解气候驱动的变化如何影响冰穴中的海面条件，我们分析了来自覆盖过去约 3800 年，并根据来自大型区域参考数据集的现代类似物的位置开发了一个索引。我们的研究结果表明，开放水域季节延长，其特征是夏季海面盐度和温度较高。3800年到2500年BP，随后逐渐降温，从 2500 年到 1500 年，海冰的影响逐渐变淡和增加，并且海面持续变冷，从 1500 年到 156 年 BP 的开放水域季节较短。在我们记录的最后 50 年中，现代变暖转化为甲藻囊肿组合的快速转换，至少在过去的 3800 年中是前所未有的。对于我们核心的最上部（大约 2009 年至 2015 年 CE），甲藻囊肿组合表明冰川径流增加和北极海冰出口到 NOW 地区导致分层增加和海面清新。北极气候预测表明，未来海冰变薄和冰盖融化加速，这表明冰湖季节更短，极地流入量增加，导致现在冰湖在未来几年发生根本性变化。