On the potential generality of depth-related ecologic structure in cold-seep communities: Evidence from Cenozoic and Mesozoic examples

Faunal communities inhabiting hydrocarbon seeps in the deep sea today show a depth-related change in their ecologic structure: the proportion of obligate (‘endemic’) taxa increases with depth while the abundance of predators and other background taxa decreases. Here the fossil record of seep communities is investigated for similar gradients. The middle Eocene to middle Miocene deep-water cold-seep deposits in western Washington State, USA, are from different depths: those of the middle to late Eocene Humptulips, Bear River and Whiskey Creek sites lived in depths of 500 to 2000 m, those of the late Oligocene to middle Miocene seep communities of the Lincoln Creek and Astoria Formations in depths of 16 to 900 m. These seeps were inhabited by the same taxa that inhabit cold seeps today and like their modern analogs, they show an increasing proportion of obligate taxa and a decreasing abundance of predators and other background taxa with depth. Also among late Mesozoic seep sites a depth-related change in the proportion of obligate taxa is apparent: seep communities in shallow-shelf settings harbor virtually no obligates, while seep communities in outer shelf to slope settings show a high proportion of obligates, although the constituting species have different taxonomic affinities than their Cenozoic and extant counterparts. Unlike in modern examples, differences between seep communities from outer shelf and slope depths were not seen. It is hypothesized that due to the higher Mesozoic sea-levels many seep communities on the continental shelves were located below the photic zone, where an obligate seep fauna could develop. Because an increase in the proportion of obligate taxa with depth can be seen both in the Cenozoic and in the late Mesozoic despite the different taxonomic affinities of the obligate taxa, it is suggested here that this depth-related structure is a general feature of seep faunas independent of the taxa that express this pattern.