Hemipelagic and Pelagic Basin Plains

The bathyal and abyssal environments represent an extent benthic habitat. The deep-sea floor is a dynamic setting affected by fluctuations of organic-matter deposition, sedimentation rate (regulating burial of organic matter), bottom-water oxygenation, etc. In continuously accumulating deposits, the bioturbated zone exhibits tiered burrows; its vertical range increases with benthic-food content, as long as the bottom water is well oxygenated and the substrate is soft. The benthic-food content influences the burrowing activity of the benthic organisms that irrigate the sea-floor sediments. While the various ecologic factors are complexly interrelated, typical end-members are considered with respect to their trace-fossil content: (1) “red” oxidized, slowly accumulating deposits are completely bioturbated by in-average small-sized, shallow-penetrating organisms; (2) grayish-green sediments preserving organic matter are completely bioturbated by in-average large, deep-burrowing organisms; and (3) grayish-black organic-rich sediments are partly bioturbated by a fauna decreasing in size and diversity with decreasing oxygen availability. Examples from modern sediments and their fossil counterparts are given and the occurring trace fossils are briefly described.