RESEARCH REPORTS 507 Copyright  2002, SEPM (Society for Sedimentary Geology) 0883-1351/02/0017-0507/$3.00 Modern Nereites in the South China Sea—Ecological Association with Redox Conditions in the Sediment ANDREAS WETZEL Geologisch-Pala ¨ontologisches Institut, Universita ¨t Basel, Bernoullistrasse 32 CH-4056 Basel, Switzerland PALAIOS, 2002, V. 17, p. 507–515 Nereites ichnofabrics (comprising N. missouriensis) occu- py the upper 3–7 cm in brown, oxidized, soft to soupy, uni- formly fine-grained sediments that accumulated at a low rate (5–6 cm/kyr) at water depths 4000 m water in the central South China Sea. Nereites are nearly vertical close to the sediment surface, become increasingly more inclined with depth, and, just above the redox boundary, they show a winding, subhorizontal course. The Nereites producers ap- pear to be guided chemotactically as they maintain a con- sistent distance relative to the redox boundary, despite var- iation in depth of penetration between studied sites. The transition from oxic to anoxic conditions is characterized by high concentrations of microbial biomass on which the Ner- eites producers are inferred to feed. Thus, the Nereites tier in fossil ichnofabrics may reflect the position of the redox boundary. Ash-filled burrows below the 1991 Pinatubo ash layer imply surface-feeding activities of the Nereites-pro- ducing animals. However, the small amount of ash in the burrows demonstrates only subordinate use of surface sed- iments, probably during periods of enhanced particle flux following upwelling. Based on the number of ash-filled bur- rows and the number of upwelling periods, an average pop- ulation density of the Nereites producers of 5–6 animals per m 2 is estimated. INTRODUCTION The suitability of trace fossils and ichnofabrics as eco- logic indicators improves where modern analogs provide additional information about the habitat of the fossil coun- terparts (e.g., Bromley, 1996). This paper provides new ob- servations about the trace fossil Nereites from modern deep-sea environments. Nereites and related trace fossils are very common in the fossil record (for taxonomic revision of the Nereites-Scalar- ituba-Helminthoida group, see Uchman, 1995). Very old Neonereites have been reported from Dalradian (Neopro- terozoic) shallow-water deposits from Scotland by Brasier and McIlroy (1998) and from Cambrian shallow-water sediments (e.g., Crimes and Anderson, 1985). Nereites is most commonly reported from turbidite sequences. Ex- amples have been described by Orr (1995: Ordovician-Si- lurian, Wales; 1994: Carboniferous, Spain), Crimes and Crossley (1991: Silurian, Ireland), Chamberlain and Clark (1973) and Ekdale and Mason (1988: Pennsylvanian- Permian, U.S.A, using the name Scalarituba), Seilacher (1962: Cretaceous-Tertiary, Spain), Macsotay (1967: Ter- tiary, Venezuela), Ksia ˛z ˙kiewicz (1977: Tertiary, Poland), Uchman (1995: Tertiary, Italy; 1999: Cretaceous, Germa- ny and Austria), and Uchman and Demircan (1999: Ter- tiary, Turkey). For some turbidite sequences (flysch) that formed during the Alpine tectonic cycle, the trace fossil Helminthoida (now grouped into the ichnogenus Nereites; Uchman, 1995) is eponymous; the so-called Helminthoida Flysch occurs in Switzerland, France, and Italy (e.g., Tru ¨ mpy, 1980; Sagri, 1980; Homewood, 1983; Powich- rowski, 1989). Because of its frequent occurrence, Seilacher (1967) in- troduced the Nereites ichnofacies as a recurrent type of trace fossil community in deep-marine, turbidite-affected settings. Later, Frey and Pemberton (1984, p. 193) defined the environment for the Nereites ichnofacies as ‘‘bathyal to abyssal, mostly quiet but oxygenated waters, in places in- terrupted by down-canyon bottom currents or turbidity currents (flysch deposits); or highly stable, very slowly ac- creting substrates. In flysch or flysch-like deposits, pelagic muds typically are bounded above and below by turbidites. In more distal regions, the record is mainly one of contin- uous deposition and bioturbation. (The stable deep-sea floor is not universally bioturbated, however, at least not equally intensely at every site.)’’ The Nereites ichnofacies is typical for deep-marine set- tings. However, trace fossils belonging to the ichnogenus Nereites also occur in other environments, such as tidal flats (Ma ´ngano et al., 2000), shallow-marine deposits (Schlirf, 2000), and freshwater lakes (e.g., Hu et al., 1998). In turbidites, Nereites often occurs post-depositionally in the tier just below the surface layer, which implies produc- tion at a shallow depth within oxygenated sediment (e.g., Seilacher, 1962; Crimes, 1973; Orr, 1994; Uchman, 1995, 1999; Wetzel and Uchman, 1998), although some deep penetration has been reported (e.g., Seilacher, 1962). In the Recent, the near-surface zone is known for rapid geo- chemical changes (e.g., Froelich et al., 1979), that cannot be reconstructed easily in the fossil record due to subse- quent diagenesis. Therefore, the specific environmental conditions affecting the habitat of the Nereites producer only can be inferred (e.g., Wetzel and Uchman, 1998). AREA OF INVESTIGATION The South China Sea is a western marginal sea of the Pacific Ocean. It is surrounded by the Southeast Asian mainland to the north and west (southern part of China, Vietnam, Thailand, Malaysia) and the islands of Borneo, Palawan, Luzon, and Taiwan to the south and east (Fig. 1). It includes large shelf regions and deep basins; the prominent basin between the Philippines and Vietnam is  4300 m deep. The major connection between the South China Sea and the Pacific Ocean is the Bashi Channel be- tween Taiwan and Luzon, which has a sill depth of about