Unravelling the dynamics on the seafloor: Insights into pockmark morphology and ocean dynamics on the Chatham Rise, offshore Te Waipounamu South Island
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Abstract
Seafloor pockmarks have been widely observed on the seafloor around Aotearoa New Zealand, particularly on the Chatham Rise. These pockmarks, predominantly occurring at water depths of ~500-700 m, across the muddy seabed on the western part of the Chatham Rise exhibit considerable variability in diameter (~150-500 m diameter), relief (5-10 m deep), and shape (sub-circular to elongated). This study investigates the distribution of these pockmarks using new high-resolution, densely spaced multi-beam bathymetry and backscatter data from TAN2006 (RV Tangaroa voyage in 2006) and SO226 (RV Sonne voyage SO226), in addition to the regional Te Riu-a-Māui/Zealandia E Tūhura - Explore Zealandia (TEZ) bathymetric datasets. ArcGIS-based tools were used to identify, spatially delineate, and morphometrically characterise over 5,000 pockmarks. This study applied the Benthic Terrain Modeler (BTM) and Random Trees Classification tools to generate geomorphometric outputs, enabling statistical analysis of the pockmark morphological variation and assessment of linkages to geological and oceanographic settings. Integrating these spatial observations with the Regional Ocean Modelling System enables comprehensive interpretations of the formation and modification processes of pockmarks. Pockmarks are interpreted to result from fluid escape processes, potentially related to gas hydrate dissociation and groundwater discharge. However, the lack of geochemical evidence for gas venting events indicates that other geological and oceanographic processes could influence pockmark morphology, such as bottom currents. Observations indicate that most of the 5,778 identified pockmarks are sub-circular, with around 19% displaying an elongated shape. These elongated pockmarks are often located in areas where strong bottom currents create high bottom stress, highlighting the significant role of ocean currents in modifying pockmark morphology. High-speed bottom currents (>0.25 m/s) associated with high-motion bottom stresses (exceeding Soulsby-Shields thresholds: 0.13 and 0.20, respectively), are inferred to erode the muddy seabed, transforming circular pockmarks into elongated features. This study provides insights into the interaction between fluid escape processes and oceanographic dynamics, contributing to the understanding of seafloor morphology on the Chatham Rise. The comprehensive geospatial analysis presented in this research offers a quantitative baseline for future investigations into the geological and hydrodynamic processes shaping the seafloor.