Recent bioturbation in the deep South China Sea : a uniformitarian ichnologic approach

Four environmental provinces are distinguished in the deep South China Sea based on ichna and their relationship to the Pinatubo 1991 ash accummulation. Scolicia ichnofabrics along the Philippine Islands are in sand-prone, greenish, oxygen-deficient deposits to about 3,500 m water depth. Abundance and size of Scolicia appear to be related to quantity and quality of benthic food, respectively. Scolicia producers intensely bioturbate the 1991 ash; below this level, Phycosiphon, Planolites, and Thalassinoides may occupy different tiers. In mud-prone deposits Palaeophycus, Planolites, Thalassinoides, and local Zoophycos (including Spirophyton-like Zoophycos) are present. In the Manila Trench, turbidites are sparsely bioturbated. To the west, Thalassinoides ichnofabrics are found in slowly deposited, deeply oxidized sediments containing little organic matter; Fe- and Mn-oxides lead to their induration. The area west of ~ 118° E is affected by upwelling and intense wind mixing, where the Nereites ichnofabrics are present. The Nereites producers feed preferably just above the redox boundary. Following blooms, however, temporary surface feeding is documented by 1991 ash in the burrows. The high benthic food content and the vertical movements of the Nereites producers probably prevented the production of graphoglyptids. Below this level Planolites, and Thalassinoides are present. The 1991 ash is bioturbated to some degree where hyperpycnites provide a soft surface layer and some benthic food, in particular along the Philippines. Where such deposits are lacking, the 1991 ash is nearly unbioturbated. Where ample organic matter reaches the seafloor, surface trails have been observed, especially along the Philippines slope and the area affected by upwelling and intense wind mixing.