Persistent overcut regions dominate the terminus morphology of a rapidly melting tidewater glacier

Frontal ablation, the combination of submarine melting and iceberg calving, changes the geometry of a glacier's terminus, influencing glacier dynamics, the fate of upwelling plumes and the distribution of submarine meltwater input into the ocean. Directly observing frontal ablation and terminus morphology below the waterline is difficult, however, limiting our understanding of these coupled ice-ocean processes. To investigate the evolution of a tidewater glacier's submarine terminus, we combine 3-D multibeam point clouds of the subsurface ice face at LeConte Glacier, Alaska, with concurrent observations of environmental conditions during three field campaigns between 2016 and 2018. We observe terminus morphology that was predominately overcut (52% in August 2016, 63% in May 2017 and 74% in September 2018), accompanied by high multibeam sonar-derived melt rates (4.84 m d(-1) in 2016, 1.13 m d(-1) in 2017 and 1.85 m d(-1) in 2018). We find that periods of high subglacial discharge lead to localized undercut discharge outlets, but adjacent to these outlets the terminus maintains significantly overcut geometry, with an ice ramp that protrudes 75 m into the fjord in 2017 and 125 m in 2018. Our data challenge the assumption that tidewater glacier termini are largely undercut during periods of high submarine melting.

前端消融，即海底融化和冰山崩解的组合，改变了冰川末端的几何形状，影响冰川动力学、上升流羽流的命运以及输入海洋的海底融水的分布。然而，直接观测水线以下的前端消融和末端形态是困难的，这限制了我们对这些冰 - 海耦合过程的理解。为了研究入海冰川海底末端的演变，我们将阿拉斯加勒孔特冰川水下冰面的三维多波束点云与2016年至2018年三次实地考察期间对环境条件的同步观测相结合。我们观察到末端形态主要是上切的（2016年8月为52%，2017年5月为63%，2018年9月为74%），同时有多波束声纳得出的高融化速率（2016年为4.84米/天，2017年为1.13米/天，2018年为1.85米/天）。我们发现，冰下高流量时期会导致局部下切的排放口，但在这些排放口附近，末端保持明显的上切几何形状，2017年有一个冰坡伸入峡湾75米，2018年为125米。我们的数据对入海冰川末端在海底高融化时期主要是下切的这一假设提出了质疑。