Copyright  2008, SEPM (Society for Sedimentary Geology) 0883-1351/08/0023-0601/$3.00 PALAIOS, 2008, v. 23, p. 601–615 Research Article DOI: 10.2110/palo.2007.p07-096r RECENT BIOTURBATION IN THE DEEP SOUTH CHINA SEA: A UNIFORMITARIAN ICHNOLOGIC APPROACH ANDREAS WETZEL Geologisch-Pala ¨ontololgisches Institut, Universita ¨t Basel, Bernoullistrasse 32, CH-4056, Basel, Switzerland e-mail: Andreas.Wetzel@unibas.ch ABSTRACT Four environmental provinces are distinguished in the deep South China Sea based on ichna and their relationship to the Pinatubo 1991 ash accumulation. Scolicia ichnofabrics along the Philippine Islands are in sand-prone, greenish, oxygen-deficient deposits to 3500 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, Plan- olites, 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, Thalassi- noides 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 bound- ary. Following blooms, however, temporary surface feeding is docu- mented by 1991 ash in the burrows. The high benthic food content and the vertical movements of the Nereites producers probably pre- vented 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 or- ganic matter reaches the seafloor, surface trails have been observed, especially along the Philippines slope and the area affected by up- welling and intense wind mixing. INTRODUCTION By studying modern analogues, the significance of marine trace fossils, ichnofabrics, and ichnofacies as ecologic proxies and environmental indicators can be improved. The deep South China Sea and its biotur- bators are appropriate for such a uniformitarian approach to the study of organism-sediment interactions because the environmental factors and their variation are well known. Within this context, the eruption of Mount Pinatubo in 1991 is important because a volcanic ash layer formed over a wide area of the South China Sea; burrows contain this 1991 ash and document their actual production. This ash represents a large-scale natural disaster, and the colonization of this ash provides evidence about the restoration of the endobenthos after such environmental perturbations. Biogenic sedimentary structures are autochthonous indicators of en- vironmental conditions and are advantageous for paleoecological analy- ses. Trace fossils have been used for ecological studies for a long time (e.g., Abel, 1935; Ekdale et al., 1984; Pemberton et al., 2001). A biotope can be characterized better by its bioturbation structures than by other paleontologic constituents, especially in the abyss (e.g., Leszczyn ´ski, 1991; Wetzel, 1991; Wetzel and Uchman, 1998a). The ecologic interpretation of trace-fossil communities (ichnocoenoses) is today normally based on ichnofacies, index trace fossils, and ichno- fabrics. Ichnofacies are recurrent trace-fossil associations that characterize the environmental setting in a general sense (Seilacher, 1967). Index trace fossils indicate a specific environmental situation; for instance, some types of Chondrites indicate lowered oxygenation (e.g., Bromley and Ek- dale, 1984).Ichnofabrics result from bioturbation at all scales, and they include all aspects of the texture and internal structures of marine sedi- ments (Bromley and Ekdale, 1986). Crosscutting relationships provide information about tiered bioturbation or a succession of burrows in ich- nofabrics. The latter implies environmental changes to which infaunal organisms respond (e.g., Wetzel and Uchman, 2001). Tiering implies a division of the available ecospace, where various endobenthic organisms occupy different depth levels within the seafloor at the same time (e.g., Wetzel, 1981, 1984; Ausich and Bottjer, 1982). Sediment consistency, food availability, and oxygenation are important (e.g., Ekdale et al., 1984, Bromley, 1996). REGIONAL SETTING The South China Sea represents a western marginal sea of the Pacific Ocean, surrounded by the Southeast Asian mainland in the north and west and the islands of Borneo, Palawan, Luzon, and Taiwan to the south and the east (Fig. 1). The South China Sea has large shelf regions and deep basins, including the prominent 4300-m-deep basin between the Phil- ippines and Vietnam. It is traversed at 15°N by the roughly east-west– trending Scarborough Seamount Chain. To the east, the deep basin of the South China Sea passes into the Manila Trench, which exceeds 5000 m in depth and acts as sediment trap. Further to the east is the Philippines continental margin (Fig. 2). Sed- iment delivered from here is characterized by a smectite-rich clay fraction (Chen, 1978). The Manila Trench roughly marks the western boundary of the smectite-rich sediments. At the steep continental margin of the Philippines diffuse and distinct downslope transport takes place, as in- dicated by displaced ostracode shells (Zhou and Zhao, 1999) and sus- pension currents. In 1991 Mt. Pinatubo on the Philippines erupted and an ash layer covering 400,000 km 2 formed in the South China Sea (Wiesner et al., 1995, 2004). The only connection between the South China Sea and the Pacific Ocean is the Bashi Channel, between Taiwan and Luzon (Philippines), which has a sill depth of 2600 m (Fig. 1). Well-oxygenated bottom waters (2 ml O 2 /l; Wyrtki, 1961) are introduced into the South China Sea via that connection (Chao et al., 1996). The entire region of the South China Sea is under the influence of the monsoon system, and, in the absence of major oceanic inflow, the currents undergo a seasonal reversal of direction (e.g., Tomczak and Godfrey, 1994). During summer, northeasterly directed winds affected by a coast- parallel mountain range in Vietnam result in a wind jet at 10°N. This leads to pronounced coastal upwelling (Xie et al., 2003). In addition, SW monsoon winds may enhance seasonally primary productivity in the cen- tral South China Sea depending on wind stress (Wiesner et al., 1996). During November–March the northwest monsoon reverses the direction of flow, and upwelling takes place off the northwestern Luzon coast, off the northern Sunda Shelf and in the central South China Sea. The winter