Variation of Trace Fossils and Ichnofacies in Neogene and Quaternary Pelagic Sediments from the Eastern Equatorial Pacific Ocean (Leg 138)

Abstract

Analysis of the incidence and relative abundance of trace fossils in sediments from the eastern equatorial Pacific Ocean reveals distinct patterns of occurrence. A major control on variation in ichnofacies appears to be the organic carbon content of the sediments. Ichnofacies dominated by Zoophycos are characteristic of sites having diatomaceous sediments that are relatively rich in organic matter (0.5%-1.5%). Isolated Zoophycos burrows occur in darker, diatom-rich interbeds within nannofossil ooze sections. In thick, variably diatomaceous nannofossil oozes having lower C org contents (0.1%-0.3%), but with pyrite or monosulfide, Chondrites is more evident and Zoophycos rarer. Sections characterized by the lowest sedimentation rates and C org concentrations between 0% and 0.1 %, commonly include Skolithos, which is rare or absent in other ichnofacies. A series of large, subvertical, and open burrows, not previously described in deep-sea cores, is suggested to be the product of echiuran worms.

Site U1433

“…This ichnofacies is typical of the uppermost 3-7 cm in the modern subseafloor sediment of the South China Sea, where water depths exceed 4000 m (Wetzel, 2002), although after correcting for the expected thermal subsidence we anticipate that these assemblages must have been formed at shallower water depths, albeit ones at least as deep as typical seafloor spreading ridges (~2500 m). Work on deep-sea cores from the Central Pacific suggests that Zoophycos is more common in sediments with slightly higher organic carbon content (Kemp, 1995), and because this particular trace is relatively rare in our cores, we deduce that the organic carbon content is never particularly high, an observation confirmed by shipboard analysis (see Geochemistry). Conversely, we note that vertical burrows such as Skolithos are almost absent from our cores.…”

“…Conversely, we note that vertical burrows such as Skolithos are almost absent from our cores. Skolithos burrows are commonly associated with particularly low sedimentation rate environments with extremely depleted organic content (Kemp, 1995). Thus, compared to the Central Pacific, the South China Sea does not experience the types of sediment starvation and low productivity associated with these deep-sea desert regions, which is perhaps not surprising given its relative proximity to landmasses in Southeast Asia and the continental runoff associated with them.…”

Site 963

“…The abundance of pyrite indicates anoxic sulfidic diagenesis and the common Zoophycos burrows further suggest the presence of an enriched organic carbon food source (Kemp, 1995). The presence of these darker gray beds in the lower-middle Pleistocene of the sequence suggests that episodes of increased flux and/or preservation of organic matter occurred during this time.…”

Methods

“…An estimate of bioturbation intensity is indicated on the left side of the Structures column. Bioturbation intensity is classified as nonvisible, indicating either the complete absence of bioturbation (0% of the surface area) or a completely bioturbated sediment (100% of the surface area); minor (<30% of the surface area); moderate (30-60% of the surface area); and intense (>60% of the surface area) following Droser and Bottjer (1986) and Kemp (1995). When identifiable, ichnofossils of Zoophycos, Skolithos, Chondrites, and Planolites burrows were reported in the lithologic description; however, during Expedition 320 these were included in the electronic database "comments."…”