Sediment mapping and long-term monitoring of currents and sediment fluxes in pockmarks in the Oslofjord, Norway

• Sediment cores reveal higher sedimentation rates and coarser sediment in a pockmark.
• Sediment traps collected more and coarser-grained material near the pockmark bottom.
• Acoustic backscatter varies in a diurnal cycle and is higher inside the pockmark.
• Fine material is kept in suspension by turbulence and possibly biological activity.

Pockmarks in the Oslofjord, Norway, show no evidence of recent expulsion of gas or water, but have still experienced reduced sedimentation rates through the Holocene. They are therefore suitable model systems for studying the currents and sedimentation in pockmarks. Mapping of sediment in a single pockmark using a large number of precisely positioned cores shows thickening of an unconsolidated, almost flocculent layer inside the pockmark. X-ray fluorescence logging of anthropogenic metals indicates higher sedimentation rates and mass accumulation rates inside the pockmark with respect to the surrounding seafloor. Furthermore, the cores taken near the pockmark centre contained relatively coarser material than those from outside the depression. Sediment traps collected substantially more sediments inside than outside of the pockmark when positioned close to the seafloor. This agrees with resuspension of sediments from the seafloor being enhanced inside the pockmark. Moreover, the traps deployed in the pockmark intercepted relatively coarser sediments. Acoustic backscatter near the seabed varies in a diurnal cycle, not related to the tide. This is interpreted as a probable result of vertical migration of zooplankton or fish. Inside the pockmark there is also a diurnal cycle in current velocity with high values in the daytime, possibly because of swimming fish. The data indicate that pockmarks collect particles, which may be largely due to deposition of sediments transported as bedload. This process should be dominant for pockmarks found in proximity to fluvial coarse-grained sediment input. The suspended fine material, however, is kept in suspension by turbulence and possibly biological activity, and can be transported away before settling. Whether an inactive pockmark will behave as a sediment depocentre or will be maintained by the action of currents, is likely to depend on the relative importance of bedload versus suspended load transport.