Origins of pockmarks offshore Big Sur, California

Potential development of floating wind turbines anchored in deep-water offshore of Morro Bay, California, has focused research on this region. Surface-ship multibeam bathymetry surveys completed in 2018 show that one of the potential areas of interest for development is the site of North America’s largest known pockmark field. Here ~5,000 circular pockmarks averaging 4.8 m deep and 175 m across occur in 500-1150 m water depths. Pockmark fields are commonly interpreted to be the result of fluid venting, which would suggest that the seafloor is unstable and that chemosynthetic biological communities may be present. However, it is not clear that pockmarks necessarily indicate the presence of and excavation by fluid venting. To elucidate the origin of these pockmarks, visual inspections with Remotely Operated Vehicle (ROV), and collection of sediment cores, Autonomous Underwater Vehicle mapping surveys, and Chirp profiles were done. ROV dives reveal no clear differences between the seafloor inside and outside pockmarks. Gentle downcore pore-water sulfate gradients within and in between pockmarks indicate anomalously low methane fluxes throughout the pockmark field. Chirp profiles show no seismic blanking indicative of interstitial gas bubbles in the sediment layers underlying pockmarks. The uppermost layer is a ~5 m thick acoustically transparent zone, which sediment cores and ¹⁴C dating show consists of very fine-grained sediment which has been accumulating continuously since the Late Pleistocene. Below it, there is a succession of nearly parallel laterally continuous sub-bottom reflectors which show the pockmark morphology commonly extends down to ≥40 m below seafloor. Profiles also show that many pockmarks have migrated up to 60 m laterally. The reflectors both thin and thicken subtly on the pockmark flanks suggesting the migration occurred through preferential erosion and deposition sometimes in discrete infilling events. Reflectors outlining infill deposits are of enhanced strength, have flat tops, and geometries that suggest the depressions act as sediment ponds during infilling events. The work to date provides no evidence that these pockmarks are active methane venting sites, but does suggest pre-Holocene sediment transport processes played a role in the lateral migration of these pockmarks.