During a recent survey of the deep seafloor off Big Sur, MBARI researchers discovered thousands of mysterious holes or pits in the seafloor. Scientists and resource managers want to understand how these pits formed because this area is the site of a proposed wind-energy farm. Researchers Eve Lundsten and Charles Paull describe their discovery this week at the Fall 2019 meeting of the American Geophysical Union in San Francisco.

The researchers found two different sizes of holes. The larger ones, known as pockmarks, average 175 meters (almost 600 feet) across and five meters (16 feet) deep, and are nearly circular and fairly evenly spaced. Some of these pockmarks were initially discovered by MBARI scientists in 1999 during a seafloor survey using ship-mounted sonar. Over the last few years, additional surveys by MBARI and other organizations revealed over 5,200 pockmarks spread out over 1,300 square kilometers (500 square miles), making this area the largest known pockmark field in North America.

More recently, MBARI conducted detailed seafloor surveys using sonar mounted on autonomous underwater vehicles. These surveys revealed thousands of smaller pits, which they termed micro-depressions. The micro-depressions average just 11 meters (36 feet) across and one meter (three feet) deep. They have steeper sides than the pockmarks and are often elongated in one direction.

Map showing the locations of some of the pockmarks and proposed wind-farm areas off Central California. Image: © 2019 MBARI

Seafloor pockmarks have been found elsewhere around the world, and have been associated with releases of methane gas or other fluids from the seafloor. Such methane releases could potentially cause the seafloor to be unstable, which could pose risks for structures such as offshore oil platforms or wind turbines.  However MBARI researchers found no evidence of methane in the sediment or seawater in this region. In fact, sonar data showing layers of seafloor sediments suggest that these pockmarks have been inactive for the last 50,000 years.

In contrast to the pockmarks, the micro-depressions formed in relatively young sediment. In addition, almost all of the micro-depressions contain objects such as rocks, kelp holdfasts, bones, trash, or fishing gear. Many micro-depressions also have “tails” of sediment that probably originated within the depression. In many areas, these tails are all oriented in the same direction.