FLAIR-POLARIN Cruise, RV Hespérides

The Antarctic margin hosts one of the most complex hydrogeological systems on Earth, where multiple fluid phases—including free gas, groundwater, permafrost, and gas hydrates—interact with the seafloor. The migration, phase change, and seepage of these fluids can profoundly influence seafloor morphology, sediment stability, nutrient fluxes, and benthic ecosystems. While climate-driven fluid–seafloor interactions have been increasingly documented in the Arctic, equivalent observations in Antarctica remain sparse due to logistical constraints and historically limited access to high-resolution geophysical and geochemical data. The seafloor of the northern Western Antarctic Peninsula, particularly the Bransfield Basin and adjacent continental shelves around the South Shetland Islands, represents a natural laboratory for investigating these processes. This region is characterised by active tectonism related to back-arc rifting and subduction of the Phoenix Plate, elevated geothermal gradients, evidence for gas hydrates, and documented methane seepage to the water column. Sedimentary records indicate repeated glacial advances and retreats, with grounded ice extending to the outer shelf during the Last Glacial Maximum and retreating in multiple phases. These glacial dynamics are hypothesised to have emplaced extensive bodies of overpressured groundwater beneath the continental shelf through subglacial melting and drainage networks. The Western Antarctic Peninsula is currently experiencing among the fastest warming rates in the Southern Hemisphere. Despite this rapid environmental change, the response of subsurface fluid systems and their role in shaping seafloor geomorphology, sediment stability, and benthic habitats remain poorly constrained. Features such as pockmarks, slope failures, and chemosynthetic communities observed along the Bransfield Basin flanks suggest that fluid flow has played, and may continue to play, a key role in margin evolution. Understanding these interactions is critical for interpreting past ice–ocean–seafloor coupling and for assessing how ongoing climate warming may modify Antarctic seafloor processes. b. Aims of the cruise The primary objective of the FLAIR expedition was to investigate the interactions between subsurface fluid flow, seafloor geomorphic processes, and benthic habitats in the northern Western Antarctic Peninsula within the context of a rapidly changing climate. The working hypothesis guiding the expedition was that fluid migration and seepage are major drivers of seafloor evolution and chemosynthetic ecosystems in the Bransfield Basin and adjacent shelves, and that these processes may intensify under continued regional warming. The specific objectives of the cruise were to: 1. Characterise marine hydrogeological systems at targeted sites, including the extent, type, and source of fluids, preferential migration pathways, and the spatial distribution and timing of fluid seepage. 2. Assess the influence of past glacial cycles on subsurface fluid systems, with particular emphasis on the role of ice advance, retreat, and subglacial drainage in shaping present-day hydrogeology. 3. Evaluate the impact of fluid flow on seafloor geomorphology and benthic habitats, including the development of pockmarks, sediment instability, and chemosynthetic communities, and to assess potential long-term geomorphic consequences. To address these objectives, the expedition integrated high-resolution geophysical mapping, sediment coring, water-column and pore-water sampling, Remotely Operated Vehicle surveys, and controlled-source electromagnetic measurements, enabling a multidisciplinary assessment of fluid–seafloor interactions across climatically sensitive Antarctic margins.

南极陆缘发育着地球上最复杂的水文地质（hydrogeological）系统之一，多种流体相态——包括游离气、地下水、多年冻土（permafrost）和天然气水合物（gas hydrates）——与海底发生相互作用。这些流体的运移、相变和渗滤，可对海底地貌、沉积物稳定性、营养盐通量以及底栖生态系统产生深远影响。尽管气候驱动的流体-海底相互作用在北极地区的记录日益增多，但受后勤保障限制，加之历史上难以获取高分辨率地球物理（geophysical）与地球化学（geochemical）数据，南极地区的同类观测仍十分匮乏。西南极半岛北部的海底区域，尤其是布兰斯菲尔德盆地（Bransfield Basin）及南设得兰群岛周边的邻近大陆架，是研究此类过程的天然实验场。该区域具有活跃的构造活动特征，包括与弧后张裂（back-arc rifting）及凤凰板块（Phoenix Plate）俯冲相关的构造作用、较高的地热梯度（geothermal gradient）、天然气水合物存在证据，以及已被记录的甲烷向水体渗滤现象。沉积记录显示该区域经历了多次冰进与冰退：末次冰盛期（Last Glacial Maximum）时，接地冰延伸至大陆架外缘，之后经历多阶段退缩。研究推测，此类冰川动力过程通过冰下融水与排水网络，在大陆架下方形成了大面积的超压地下水（overpressured groundwater）储集体。目前，西南极半岛是南半球升温速率最快的区域之一。尽管环境变化如此迅速，地下流体系统的响应及其在塑造海底地貌、沉积物稳定性与底栖生境中的作用，仍鲜有研究能明确限定。布兰斯菲尔德盆地侧翼观测到的海底麻坑（pockmark）、斜坡失稳（slope failure）及化能合成生物群落（chemosynthetic community）等特征表明，流体运移曾在陆缘演化中发挥关键作用，且未来可能持续扮演重要角色。明晰此类相互作用，对于解读过去的冰-海-海底耦合过程，以及评估当前持续的气候变暖将如何改变南极海底过程，均具有至关重要的意义。

b. 科考航次目标
FLAIR科考航次的核心目标，是在气候快速变化的背景下，研究西南极半岛北部的地下流体运移、海底地貌过程与底栖生境之间的相互作用。本航次的核心工作假说为：流体运移与渗滤是布兰斯菲尔德盆地及邻近陆架区域海底演化与化能合成生态系统的主要驱动因素，且随着区域持续变暖，此类过程可能会加剧。本航次的具体目标包括：
1. 对目标站位的海洋水文地质系统开展特征刻画，涵盖流体的分布范围、类型与来源、优势运移通道，以及流体渗滤的空间分布与时间序列。
2. 评估过去冰川旋回对地下流体系统的影响，重点聚焦冰进、冰退及冰下排水作用对现代水文地质条件的塑造作用。
3. 评价流体运移对海底地貌与底栖生境的影响，包括海底麻坑形成、沉积物不稳定性及化能合成生物群落的发育情况，并评估其潜在的长期地貌效应。
为达成上述目标，本航次整合了高分辨率地球物理测绘、沉积物岩芯取样、水体与孔隙水采样、遥控水下机器人（Remotely Operated Vehicle, ROV）调查以及可控源电磁（controlled-source electromagnetic）测量等技术手段，从而可对气候敏感的南极陆缘区域的流体-海底相互作用开展多学科综合评估。