Bottom currents at the Arctic Mid Oceanic Ridge: insights from deployed sensors, biological indicators, and global models

Abstract. Characterizing near-bottom ocean currents at deep-sea topographic features requires observations across multiple scales, from basin-wide circulation models to local in situ measurements. This study addresses this challenge at DeepInsight Hill on the Mohns Ridge by combining eight months of in situ current measurements, global ocean model predictions, and novel ROV-based biological flow indicators. Our findings reveal a complex and highly variable flow regime with a striking decoupling between tidal pressure and near-bottom currents. Spectral analysis shows that the dominant semi-diurnal tidal pressure signal does not translate into coherent near-bottom currents, suggesting conversion of tidal energy into internal waves at the ridge topography. During winter, sustained directional transport occurs simultaneously in opposite directions at the summit and base of DeepInsight Hill (S-SE and N-NW respectively), accounting for most of the effective water displacement over the record. However, foliaceous sponges, whose fan-shaped morphology is known to develop perpendicular to the prevailing current, show consistent W-SW orientations at the base of the hill that do not match the long-term instrumental mean flow direction (N-NW). Instead, these orientations align with global model predictions and the direction of episodic peak currents recorded during summer, suggesting that sponge morphology records specific high-energy flow events rather than time-averaged conditions, possibly amplified by peak summer nutrient supply. Bedforms observed at the same location eight months apart show no significant change, indicating that sediment-mobilizing currents are episodic and may not recur annually. Further data and interdisciplinary research are essential to resolve the governing factors of these systems and to refine environmental impact assessments in topographically complex deep-sea settings.