What can we learn from sampling the seafloor and how do we do it?

On this cruise, we are studying the processes that modify the seafloor in an area offshore Morro Bay, where a wind farm may be located in the future. The environmental conditions and stability of the seafloor are important factors to consider when evaluating how this infrastructure will impact the seafloor.

Sediment slowly accumulates over geologic time and blankets the seafloor. Seafloor currents also modify these accumulations, eroding sediments in some areas and depositing them in others. Sediment avalanches or submarine landslides can occur as rapid events that change the seafloor considerably, producing scars and channels. In addition, where submarine springs or natural gas vents occur, they may produce depressions called pockmarks over time scales of decades to thousands of years that shape and modify the seafloor morphology. These faster processes need to be considered as potential hazards to any seafloor infrastructure. To better understand them and how they may affect any structures placed on the seafloor, we collect long vertical core samples that give us insight into the processes occurring in this environment.

MBARI’s remotely operated vehicle (ROV) pilots perform the collection of sediment cores. The pilots use their expert skills to carefully operate the ROV manipulator arms to collect two types of cores: push cores and vibracores. Push cores are 25-centimeter-long clear tubes that are pushed into the sediment with the manipulator arm of the ROV.

The vibracores are collected with a vibracoring system mounted on the front of the ROV Doc Ricketts. This device inserts 1.7-meter-long aluminum tubes into the sediment. The insertion is aided by the activation of a powerful vibrating motor atop the tube that induces high-frequency vibrations of the tube walls. These vibrations liquefy the sediment directly around the tube and enable the tube to pass through the sediment with little resistance.

These samples allow us to estimate the frequency of prevailing environmental conditions at the seafloor. Grain size analyses of the sediment cores provide insight into the magnitude of water currents. We can assess how fast sediment accumulates, and understand how quickly the seafloor changes.

We estimate recent sediment accumulation rates using a naturally occurring radioactive isotope of lead (²¹⁰Pb) which is used for sediments that have accumulated over the last 100 years. Longer-term accumulation rates are estimated using a naturally occurring radio-isotope of carbon (¹⁴C), which can be used to date sediments back to 50,000 years ago.

Finally, we extract water samples from the cores. Changes in the chemistry of these waters over the depth within the core can provide information about water and gas flowing up through the seafloor. Is this process the culprit for the formation of the numerous pockmarks found in this area or is another process responsible for their formation? Hopefully, samples collected in this cruise will help us to reach the answer.