New Insights into Gravity Flow Dynamics During Submarine Canyon Flushing Events

Academic Background

Submarine canyons are critical conduits connecting land and the deep ocean, and their formation and evolution have long been a focus of marine geological research. However, due to the destructive and infrequent nature of submarine canyon flushing events, observational data are extremely scarce. These events, often triggered by natural disasters such as earthquakes and landslides, can transport vast amounts of sediment from the nearshore to the deep sea, profoundly impacting submarine geomorphology and ecosystems. Although theoretical models and laboratory experiments have attempted to explain these processes, the lack of high-resolution field observations has limited our understanding of the dynamics of gravity flows, such as debris flows and turbidity currents.

This study aims to reveal the dynamics of gravity flows during the 2016 Kaikōura earthquake-induced submarine canyon flushing event in New Zealand, using high-resolution multibeam bathymetry, side-scan sonar, and seafloor video imagery. The research focuses on the transformation of gravity flows from debris flows to turbidity currents and the dynamic variations of turbidity currents within the canyon, providing new insights into the formation and evolution of submarine canyons.

Source of the Paper

This paper, authored by Marta Ribó (Auckland University of Technology), Joshua J. Mountjoy (National Institute of Water and Atmospheric Research, New Zealand), Neil Mitchell (University of Manchester), and others, was published in the journal Geology (online on October 21, 2024). The research data were collected during the TAN2011 voyage in October 2020 in the Kaikōura Canyon, New Zealand, using an autonomous underwater vehicle (AUV) and a deep-towed imaging system (DTIS) to acquire high-resolution seafloor topography and sediment data.

Research Process and Results

1. Data Collection and Processing

The research team used the Hugin 3000 AUV equipped with a Kongsberg EM2040 multibeam echosounder system and an Edgetech 2205 side-scan sonar system to collect high-resolution ( m) seafloor topography data in the Kaikōura Canyon. Simultaneously, the deep-towed imaging system (DTIS) provided high-definition images and videos of the seafloor substrate. These data were used to generate digital elevation models (DEMs) and analyze erosional and depositional features within the canyon.

2. Analysis of Erosional and Depositional Features

In the upper canyon (water depth ~1245–1400 m), the team identified significant erosional features, including linear grooves (30–130 m long, ~3 m average width, >10 m relief) and erosion scours. These features indicate that the debris flow triggered by the 2016 earthquake caused intense bedrock erosion in the upper canyon and transported large amounts of coarse sediment (e.g., boulders >5 m in diameter).

In the mid-canyon (water depth ~1450 m), the canyon width abruptly doubled, and the dominant features transitioned from erosional to depositional. The team observed large gravel waves (wavelengths 75–100 m, wave heights ~6 m) coexisting with cyclic steps (wavelengths ~210 m, wave heights ~12 m). These depositional features suggest that the debris flow transformed into a turbidity current at this location, depositing coarse-grained material.

In the lower canyon (water depth >1700 m), the wavelength and height of gravel waves further increased (wavelengths ~250 m, wave heights ~20 m), and the deposits included numerous large boulders (up to 5 m in diameter). Additionally, the team observed small-scale dunes (wavelengths 15–30 m, wave heights 1.5–5 m) superimposed on the larger gravel waves, indicating internal stratification within the turbidity current.

3. Analysis of Gravity Flow Dynamics

The team proposed that the gravity flow during the 2016 Kaikōura Canyon flushing event underwent a transformation from a debris flow to a turbidity current. In the upper canyon, the high-density debris flow caused intense bedrock erosion; in the mid-canyon, the debris flow gradually diluted and transformed into a turbidity current due to the increased canyon width and hydraulic jumps; in the lower canyon, dynamic variations in the turbidity current (e.g., flow velocity and sediment concentration) further influenced depositional features.

Conclusions and Significance

This study provides the first detailed description of the dynamics of gravity flows during a submarine canyon flushing event, based on high-resolution field observations. The results demonstrate: 1. The debris flow caused intense bedrock erosion and transported large amounts of coarse sediment in the upper canyon. 2. In the mid-canyon, the debris flow transformed into a turbidity current, depositing coarse-grained material. 3. Dynamic variations in the turbidity current (e.g., flow velocity and sediment concentration) significantly influenced depositional features within the canyon.

These findings deepen our understanding of the formation mechanisms of submarine canyons and provide a scientific basis for predicting the impacts of similar events. Additionally, the high-resolution data collection and analysis methods used in this study offer valuable references for future research.

Research Highlights

1. High-Resolution Data: The study utilized high-resolution data collected by AUV and DTIS, providing the first detailed insights into erosional and depositional features during a submarine canyon flushing event.
2. Gravity Flow Transformation Mechanism: The study validated theoretical models of debris flow transformation into turbidity currents through field observations for the first time.
3. Dynamic Variation Analysis: The study analyzed dynamic variations in turbidity currents within the canyon, revealing the influence of flow velocity and sediment concentration on depositional features.

Additional Valuable Information

The research team also noted that future studies should investigate the dynamics of submarine canyon flushing events in different geological settings to refine theoretical models. Furthermore, the high-resolution data collection and analysis methods used in this study can be applied to other submarine geomorphological research, providing new tools and approaches for advancing marine geology.