Hydrothermal pits in sediments of the equatorial Pacific

Swath mapping and seismic surveying of the equatorial Pacific sediment mound have revealed the common occurrence of pits in the sediment surface. The pits are sub-circular to elongate with horizontal dimensions of 1 to 4 km and are on the order of 50 100m deep. They often are seen in clusters or in linear trends; and in denser fields of such pits, they may appear to overlap. They were mapped in sediment overlying crust from 15 Ma to 55 Ma in age. Their density in the region is about 1 per 220 km2 and their occurrence shows no relationship to water depth, age of crust, latitude or longitude. They most frequently occur over sedimented basement highs where sediment is 300-350 m thick and do not seem to occur where sediment thickness in adjacent valleys is greater than ~500m or less than ~150m. They are believed to be related to hydrothermally driven discharge through vertical conduits of higher permeability in the sediment. These conduits are associated with faulting and fracturing over basement highs and along basement faults that extend well into the sediment column. The pervasive presence of such features on the seafloor indicates the profound effect that such "old-crust" hydrothermal circulation must have on ocean chemistry, crustal weathering, and ecology of the sub seafloor microbial community.