Pockmarks off Big Sur, California

A pockmark field was discovered during EM-300 multi-beam bathymetric surveys on the lower continental slope off the Big Sur coast of California. The field contains similar to 1500 pockmarks which are between 130 and 260 in in diameter, and typically are 8-12 in deep located within a 560 km(2) area. To investigate the origin of these features.. piston cores were collected from both the interior and the flanks of the pockmarks, and remotely operated vehicle observation (ROV) video and sampling transects were conducted which passed through 19 of the pockmarks. The water column within and above the pockmarks was sampled for methane concentration. Piston cores and ROV collected push cores show that the pockmark field is composed of monotonous fine silts and clays and the cores within the pockmarks are indistinguishable from those outside the pockmarks. No evidence for either sediment winnowing or diagenetic alteration suggestive of fluid venting was obtained. C-14 measurements of the organic carbon in the sediments indicate continuous sedimentation throughout the time resolution of the radiocarbon technique (similar to45000 yr BP), with a sedimentation rate of similar to 10 cm per 1000 yr both within and between the pockmarks. Concentrations of methane, dissolved inorganic carbon, sulfate, chloride, and ammonium in pore water extracted from within the cores are generally similar in composition to seawater and show little change with depth, suggesting low biogeochemical activity. These pore water chemical gradients indicate that neither significant accumulations of gas are likely to exist in the shallow subsurface (similar to 100 m) nor is active fluid advection occurring within the sampled sediments. Taken together the data indicate that these pockmarks are more than 45 000 yr old, are presently inactive, and contain no indications of earlier fluid or gas venting events. (C) 2002 Elsevier Science B.V. All rights reserved.