Off the coast of central California, there are more than 10,000 depressions, 10 meters (33 feet) deep, known as pockmarks. We do not know how these large depressions were formed, however, we do know they never overlap, are beautifully circular, and roughly 100 meters in diameter. MBARI’s detailed mapping surveys conducted in 2018 show that the pockmark depressions migrate southeast over time. Pockmark depressions are commonly thought to be associated with water or gas venting, but previous MBARI studies show no evidence for this. Evidence suggests that the water inside and outside of the pockmarks is indistinguishable and contains no methane, the only common sediment gas. We are now back at the same site to investigate how these pockmark depressions were formed.

We want to know if the water currents are similar inside and outside of the pockmark depressions. For example, are there swirling eddies inside the pockmark depressions which inhibit sediment accumulation? To test this, we worked with MBARI’s mechanical engineers and remotely operated vehicle (ROV) pilots to create two Benthic Instrument Packages (BIPs). The BIP’s trawl-resistant housing holds a suite of oceanographic sensors. One sensor detects oscillations in the water temperature, allowing scientists to compare conditions inside and outside the pockmarks. Two upward-looking current meters at five and 65 meters water depth vertically capture overlapping conditions in the water column. Once data collection is complete, researchers use an acoustic beacon to relocate and recover the instrument.

This morning we launched two BIPs—one inside a pockmark depression and one outside a pockmark depression. In November we will recover the instruments and analyze the data they have recorded to better understand what processes are creating these unique and mysterious seafloor features.