Modern Nereites in the South China Sea—Ecological Association with Redox Conditions in the Sediment

RESEARCH REPORTS 507

ANDREAS WETZEL Geologisch-Pala¨ontologisches Institut, Universita¨t Basel, Bernoullistrasse 32 CH-4056 Basel, Switzerland

PALAIOS, 2002, V. 17, p. 507±515 ny and Austria), and Uchman and Demircan (1999: Ter- tiary, Turkey). For some turbidite sequences (flysch) that Nereites ichnofabrics (comprising N. missouriensis) occu- formed during the Alpine tectonic cycle, the trace fossil py the upper 3–7 cm in brown, oxidized, soft to soupy, uni- Helminthoida (now grouped into the ichnogenus Nereites; formly fine-grained sediments that accumulated at a low Uchman, 1995) is eponymous; the so-called Helminthoida rate (Ͻ5–6 cm/kyr) at water depths Ͼ4000 m water in the Flysch occurs in Switzerland, France, and Italy (e.g., central South China Sea. Nereites are nearly vertical close Tru¨ mpy, 1980; Sagri, 1980; Homewood, 1983; Powich- to the sediment surface, become increasingly more inclined rowski, 1989). with depth, and, just above the redox boundary, they show a Because of its frequent occurrence, Seilacher (1967) in- winding, subhorizontal course. The Nereites producers ap- troduced the Nereites ichnofacies as a recurrent type of pear to be guided chemotactically as they maintain a con- trace fossil community in deep-marine, turbidite-affected sistent distance relative to the redox boundary, despite var- settings. Later, Frey and Pemberton (1984, p. 193) defined iation in depth of penetration between studied sites. The the environment for the Nereites ichnofacies as ‘‘bathyal to transition from oxic to anoxic conditions is characterized by abyssal, mostly quiet but oxygenated waters, in places in- high concentrations of microbial biomass on which the Ner- terrupted by down-canyon bottom currents or turbidity eites producers are inferred to feed. Thus, the Nereites tier currents (flysch deposits); or highly stable, very slowly ac- in fossil ichnofabrics may reflect the position of the redox creting substrates. In flysch or flysch-like deposits, pelagic boundary. Ash-filled burrows below the 1991 Pinatubo ash muds typically are bounded above and below by turbidites. layer imply surface-feeding activities of the Nereites-pro- In more distal regions, the record is mainly one of contin- ducing animals. However, the small amount of ash in the uous deposition and bioturbation. (The stable deep-sea burrows demonstrates only subordinate use of surface sed- floor is not universally bioturbated, however, at least not iments, probably during periods of enhanced particle flux equally intensely at every site.)’’ following upwelling. Based on the number of ash-filled bur- The Nereites ichnofacies is typical for deep-marine set- rows and the number of upwelling periods, an average pop- tings. However, trace fossils belonging to the ichnogenus ulation density of the Nereites producers of 5–6 animals Nereites also occur in other environments, such as tidal per m2 is estimated. flats (Mańgano 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- INTRODUCTION tion at a shallow depth within oxygenated sediment (e.g., The suitability of trace fossils and ichnofabrics as eco- Seilacher, 1962; Crimes, 1973; Orr, 1994; Uchman, 1995, logic indicators improves where modern analogs provide 1999; Wetzel and Uchman, 1998), although some deep additional information about the habitat of the fossil coun- penetration has been reported (e.g., Seilacher, 1962). In terparts (e.g., Bromley, 1996). This paper provides new ob- the Recent, the near-surface zone is known for rapid geo- servations about the trace fossil Nereites from modern chemical changes (e.g., Froelich et al., 1979), that cannot deep-sea environments. be reconstructed easily in the fossil record due to subse- Nereites and related trace fossils are very common in the quent diagenesis. Therefore, the specific environmental fossil record (for taxonomic revision of the Nereites-Scalar- conditions affecting the habitat of the Nereites producer ituba-Helminthoida group, see Uchman, 1995). Very old only can be inferred (e.g., Wetzel and Uchman, 1998). Neonereites have been reported from Dalradian (Neopro- terozoic) shallow-water deposits from Scotland by Brasier AREA OF INVESTIGATION and McIlroy (1998) and from Cambrian shallow-water sediments (e.g., Crimes and Anderson, 1985). Nereites is The South China Sea is a western marginal sea of the most commonly reported from turbidite sequences. Ex- Pacific Ocean. It is surrounded by the Southeast Asian amples have been described by Orr (1995: Ordovician-Si- mainland to the north and west (southern part of China, lurian, Wales; 1994: Carboniferous, Spain), Crimes and Vietnam, Thailand, Malaysia) and the islands of Borneo, Crossley (1991: Silurian, Ireland), Chamberlain and Clark Palawan, Luzon, and Taiwan to the south and east (Fig. (1973) and Ekdale and Mason (1988: Pennsylvanian- 1). It includes large shelf regions and deep basins; the Permian, U.S.A, using the name Scalarituba), Seilacher prominent basin between the Philippines and Vietnam is (1962: Cretaceous-Tertiary, Spain), Macsotay (1967: Ter- ϳ 4300 m deep. The major connection between the South tiary, Venezuela), Ksia˛z˙kiewicz (1977: Tertiary, Poland), China Sea and the Pacific Ocean is the Bashi Channel be- Uchman (1995: Tertiary, Italy; 1999: Cretaceous, Germa- tween Taiwan and Luzon, which has a sill depth of about

FIGURE 2ÐIsopachs of the 1991 Pinatubo ash layer (given in mm). Location of map is shown on Figure 1. All dots represent cores used for isopach construction (for details see Wiesner et al., 2002). Large dots represent cores studied by X-ray radiographs for biogenic sedimentary structures. Mt. Pinatubo is marked by a triangle. Depth con- tours are in meters.

production by a factor of 3 to 4 (Antoine et al., 1996), but El Nin˜ o events suppress upwelling and lead to significant re- duction of particle flux (Wiesner, 2000, pers. commun.). FIGURE 1ÐThe South China Sea is a marginal sea of the western Periods of high organic matter fluxes are synchronous Paci®c surrounded mainly by shelf areas, as indicated by the 200 m depth contour. The South China Sea is ventilated via the Bashi Chan- with periods of intensified wind-induced upwelling and nel between the Philippines and Taiwan. The area in¯uenced by up- mixing, whereas weak upwelling leads to low organic mat- welling roughly coincides with that affected by wind-stress exceeding ter flux. Today the CCD (calcite compensation depth) is lo- 25 cm/d (Wiesner et al., 1996) located roughly westward of the stip- cated in about 3500 m water depth (e.g., Wang, 1999). Ma- pled line. Inset represents area shown in Figures 2 and 5. terial derived from the Philippines or the adjacent conti- nental slope is transported to the deep sea by turbidites or indistinct downslope transport (e.g., Zhou and Zhao, 2600 m (Tomczak and Godfrey, 1994). Bottom waters of 1999). the South China Sea are predominantly well oxygenated In 1991, ash erupted by Mt. Pinatubo on the Philippines 2 (ca. 2 ml O2/l; Wyrtki, 1961; Chao et al., 1996) and are in- was deposited over an area of about 400,000 km , forming troduced via the Bashi Channel. a layer up to 10 cm thick in the South China Sea (Wiesner The oceanographic features of the South China Sea are et al., 2002; Fig. 2). The ash is an excellent marker bed. On conditioned by seasonal reversal of the monsoonal wind top of the ash is organic-rich fluff that contains 3–5% Corg system. During the SW-monsoon (June–September), pro- (Wiesner, pers. commun., 2000). nounced upwelling takes place off central south Vietnam. Forced by the general northeasterly directed surface cur- MATERIAL AND METHODS rent flow, the ascending cold and nutrient-rich waters are advected into the central South China Sea (LaFond, 1966). Samples were collected during several cruises of the During the NE-monsoon (November–April), upwelling oc- German research vessel Sonne to the South China Sea curs off northwest Luzon and in the southern central (Sarnthein et al., 1994; Wiesner et al., 1997, 1998, 1999; South China Sea, where deep water ascends to subsurface Fig. 2). They were taken by a 50 x 50-cm2 box corer, which depths of 50–100 m and nutrient entrainment is con- has automatically closing flaps to protect the sediment trolled largely by the intensity of wind-induced mixing. surface from winnowing during retrieval of the corer. To The surface-water circulation is counterclockwise, causing analyse sedimentary structures in detail, 8-mm-thick sed- the Pacific water to intrude through the Bashi Channel iment slices were prepared for X-ray radiography (Wer- and flow along the coasts to SE Vietnam (Shaw et al., ner, 1967). They were radiated in the laboratory at 30 kV 1996). Along with this seasonal incorporation of nutrients for varying times determined automatically. For three-di- into the euphotic zone, the fluxes of particulate matter to mensional analysis of the sedimentary structures, serial the sea floor off Luzon and Vietnam and in the central sections in vertical and horizontal directions were pre- South China Sea maximize at the culmination of the SW- pared. Shear strength was measured by using a GEONOR and NE-monsoon (Wiesner et al., 1996). During both sea- fall-cone apparatus (Hansbo, 1957). Organic carbon was sons, about 70% of the total annual organic matter flux is calculated as the difference between total carbon mea- exported to the deep sea (Wiesner et al., 1996). Upwelling sured on a Erba Science NA 1500 Elemental Analyzer and in the South China Sea normally enhances the primary carbonate carbon measured on a Woesthoff Carmograph 6 MODERN NEREITES ALONG THE REDOX BOUNDARY 509

a zone of reworked sediment as a diagnostic feature (sensu Fu¨ rsich, 1974) of the ichnogenus Nereites. The type of preservation should not be used as an ichnotaxobase at the ichnogeneric level; hence, Scalarituba should be regarded as junior synonym of Nereites. Burrow course (meandering vs. non-meandering), width of the central tunnel and envelope zone, nature of the envelope (including shape of lobes), type of preservation, and selective preservation of morphological elements were regarded by Uch- man (1995) as accessory features to be used for ichnospe- cific assignment. The spatial occurrence of Nereites reflects the sediment type to which the Nereites producers appear to be adapted. Nereites traces are restricted to the central basin of the South China Sea where water depth exceeds 4000 m (Fig. 5; Table 1). Sediments in the basin accumulated below the CCD (e.g., Wang, 1999); hence, the host sediment for the studied Nereites is uniformly fine-grained mud. The sediment accumulated at a rate of about 5 to 6 cm/ky (calculat- FIGURE 3ÐSchematic drawing of Nereites as observed in deep-ma- ed from an accumulation rate of 3.29 g cmϪ2 kyϪ1; Wang, rine sediments in the South China Sea. 1999). The Nereites host sediment contains about 0.2 to

0.6% Corg (Table 1). Porosity of the host sediment varies between 70 and 75%, while undrained shear strength is ap- using calcite standards for calibration and 2N H3PO4 for proximately 0.3 to 1.8 kPa; hence, sediment consistency is sample acidiﬁcation. Organic carbon was measured at the classiﬁed as soft to soupy. Nereites were observed exclu- Institute of Biogeochemistry and Marine Chemistry of the sively in brownish sediments. Pore water at the level of University of Hamburg. the horizontal portion of Nereites burrows displays a low-

ered oxygen content of 25 ␮mol O2/l compared to 125 ␮mol

OBSERVATIONS ON NEREITES O2/l of the bottom water (Haeckel et al., 2001). Nereites occur above a dark-colored zone stained by manganese ox- The near-surface muds in the deep areas of the central ides that rests on top of greenish to gray sediments; the South China Sea display a typical ichnofabric. It consists manganese-stained horizon marks the redox boundary of tunnels filled by less dense, fine-grained material that (e.g., Froelich et al., 1979). The distance between the man- is surrounded by sediment enriched in coarse grains com- ganese-stained horizon and the horizontal part of the bur- pared to the tunnel fill and the host sediment. The coarse rows is fairly constant in the range of about 1 to 2 cm (Fig. envelope displays the characteristics of a reworking halo. 6). Nereites never penetrate into the manganese-stained The outer boundary of the reworking halo is seldom sharp muds or into the underlying gray to greenish muds. (Figs. 3, 4). In vertical sections, from the seafloor down to Nereites was not observed in fairly coarse foraminifera- a depth of 3 to 6 cm, tunnels are nearly vertical to inclined, becoming (sub)horizontal. At this level the tunnels adopt a rich oozes (e.g., cores 419, 420, 955, 956; see Fig. 5) or in winding course (Figs. 3, 4). The burrows do not cross-cut sediments that were reworked by bottom currents (e.g., themselves. On average, the tunnels measure 2 to 4 mm in cores 221, 223, 413; see Fig. 5). The latter are stiff sedi- diameter and the reworking halo is 1 to 2 mm wide. The ments with an undrained shear strength of about Ͼ10 burrows penetrate up to 9 cm, with an average of 4 to 6 cm. kPa, and they normally are deeply oxidized. Furthermore, Based on their expression in X-ray radiographs, burrow Nereites was not found in sediments affected by indistinct morphplogies match that of Nereites missouriensis (Wel- downslope transport from the Philippines (e.g., cores 226, ler, 1899; previously called Scalarituba missouriensis, 228, 418, 421, 422; see Fig. 5). These deposits often are Uchman, 1995). burrowed by shallow-tier echinoids that produce the trace The ichnogenus Nereites comprises a wide variety of Scolicia. All of these cores (except 955, 956) mark the east- winding to meandering traces consisting of an actively ern boundary of the occurrence of Nereites. Nearer to the filled tunnel enveloped by a halo of reworked sediment. Philippines, the above factors become more pronounced This paper follows the taxonomy provided by Uchman and Nereites is absent. (1995), as slightly emended by Mańgano et al. (2000, p. The 1991 Pinatubo ash represents an excellent marker 150), giving the following diagnosis: ‘‘Selectively pre- bed, and it reveals further information about the Nereites served, curved, winding to regularly meandering or spiral- producers. Nereites burrows contain some ash in areas ling, unbranched, predominantly horizontal trails, con- where the ash is thinner than about 2 cm (Fig. 4); there- sisting of a median backfilled tunnel enveloped by an even fore, these burrows had been produced recently and their to lobate zone of reworked sediment.’’ producers were not severely affected by the ash deposi- This diagnosis lumps Helminthoida, Neonereites, and tion. An average of two ash-filled burrows were found per Scalarituba into the ichnogenus Nereites (Uchman, 1995). 450 cm2. If ash thickness exceeds 3 cm, ash was neither ob- However, consensus on this taxonomic issue has not yet served within the Nereites burrows nor was it penetrated been reached (Mańgano et al., 2000). Uchman (1995) em- by the vertical parts of the burrows. This suggests aban- phasized the importance of a central tunnel enveloped by donment of the burrowing activity after ash sedimenta- 510 WETZEL

FIGURE 4ÐNereites in South China Sea sediments and fossil equivalents. (A) horizontal split surface of Permian silt-to-sandstone with Scalarituba (ϭ Nereites); Oquirrh Formation, Utah. (B)-(F) X-ray radiographs (negatives) showing Nereites from the South China Sea. (B) Vertical section. (C)-(F) Horizontal serial sections adjacent to the vertical section shown in (B); C, D, E, and F correspond to 1±2, 2±3, 3±4, and 4±5 cm depth intervals. The visual cross-overs are due to the thickness of the sediment slab that was x-rayed (ca. 1 cm). Burrow segments marked by an ``a'' indicate 1991 Pinatubo ash within Nereites tunnels; Areas marked ``h'' are reworking halos enriched in coarse grains. All photographs are approximately natural size. MODERN NEREITES ALONG THE REDOX BOUNDARY 511

FIGURE 5ÐCore sites of modern sediments in the South China Sea. Triangles indicate cores containing Nereites. Dots indicate other cores examined by x-ray radiography for ichnofabrics but without Nereites. Thick line represents outer boundary of 1991 Pinatubo ash layer. tion. At these sites, the pore water below the ash is oxygen FIGURE 6ÐRelationship between burrowing depth of the Nereites producers and the depth of the redox boundary. free (Haeckel et al., 2001).

INTERPRETATION zontal parts of the burrow are restricted to a well-defined horizon and, hence, they imply chemotactic guidance of The Nereites producers appear to be restricted to oxy- the Nereites-producing animals. Nonetheless, the course of genated sedimentary environments. The burrows that ev- Nereites is not strictly horizontal. This probably is due to idently were produced after 1991 contain ash, and they ex- the undulating nature of the upper boundary of the redox clusively occur above the redox boundary and never below. zone that results from porosity variation due to earlier bio- Where the near-surface pore water is anoxic today owing turbation (e.g., Richardson, 1983) and diffusion within the to the deposition of a thick Pinatubo ash in 1991, Nereites sediment (e.g., Glud et al., 1994). do not display any sign of continuing activity (i.e., there is The occurrence of some ash within Nereites burrows cer- no connection to the sediment surface and no ash within tainly indicates their recent production. The ash infill sug- the burrows). gests episodic excursions of the Nereites producers to the The close spatial relationship of the horizontal parts of sediment surface and ingestion of sediment there. Nereites the Nereites burrows to the redox boundary suggests that producers may take up some additional food, possibly af- the animals fed on microbes that are known to occur there ter arrival of organic matter following an upwelling peri- in high concentrations (e.g., Ko¨ster, 1993). In particular, od. So the question arises—can fairly deep burrowing Ner- the nearly constant distance of the Nereites level to the re- eites producers recognize the arrival of organic matter on dox boundary—independent of the depth in sediment of the seafloor? The chemical composition of pore water re- the latter—supports this interpretation (Fig. 6). The hori- sponds within a few weeks to strongly enhanced organic

TABLE 1ÐCores containing Nereites.

Water Shear Organic Depth of redox depth strength carbon [% boundary below Latitude Longitude [m] [kPa] dry weight] base of ash [cm]

16Њ25,03Ј N 117Њ20,00Ј E 4005 0.9–1.7 0.42 8–9 16Њ06,06Ј N 116Њ59,65Ј E 4122 0.3–1.4 n.d. 7–9 15Њ29,99Ј N 116Њ54,98Ј E 4232 n.d. 0.45 7–9 15Њ25,01Ј N 115Њ05.01Ј E 4248 0.8–1.4 n.d. 6–7 15Њ06,30Ј N 115Њ23,15Ј E 4262 0.2–1.5 0.52 9–10 15Њ03,90Ј N 115Њ45.00Ј E 4220 n.d. n.d. 8–9 14Њ50,14Ј N 116Њ54,50Ј E 4296 n.d. n.d. 6–7 14Њ44,00Ј N 117Њ35,99Ј E 4330 n.d. n.d. 6–7 14Њ39,91Ј N 118Њ05.16Ј E 4251 0.5–0.8 0.46 5–6 14Њ39,89Ј N 118Њ05,21Ј E 4254 0.5–0.8 n.d. 5–6 14Њ33,0Ј N 115Њ08.60Ј E 4307 n.d. 0.40 8–9 14Њ23,01Ј N 116Њ03,99Ј E 4318 0.4–1.4 n.d. 7–8 14Њ14,50Ј N 116Њ51,34Ј E 4321 0.3–0.8 n.d. 6–7 14Њ10,03Ј N 113Њ59,99Ј E 4323 0.4–1.3 0.57 5–6 14Њ03,91Ј N 117Њ43,90Ј E 4152 n.d. 0.60 7–8 13Њ50,04Ј N 116Њ48,34Ј E 4236 0.4–1.2 0.40 6–7 13Њ40,62Ј N 116Њ07,01Ј E 4345 0.3–1.1 n.d. 7–8 12Њ58,27Ј N 116Њ09,90Ј E 4345 0.3–1.4 0.62 9–10 12Њ48,01Ј N 113Њ33,47Ј E 4313 0.5–0.8 0.63 5–6 512 WETZEL

matter accumulation on the sediment surface (Balzer et shallow water. The Nereites producers prefer muds, but al., 1987; Soetaert et al., 1996; Gehlen et al., 1997; Haeckel they also ingested fine 1991 Pinatubo ash. Coarse forami- et al., 2001). Such a chemical signal propagating rapidly niferal oozes, however, were not burrowed by the Nereites into the sediment could be directly received by chemically producers. The critical grain size for the Nereites produc- sensitive burrowing animals or indirectly due to the ers appears to be medium sand. Similarly, in the fossil re- changing position of the redox boundary. Therefore, it is cord, Nereites is described mainly from distal turbidite set- conceivable that the Nereites producers, which exploit the tings, where it normally occurs in muds to fine/medium sediments just above the redox boundary, could receive sands (Seilacher, 1962; Uchman, 1995; Wetzel and Uch- chemical signals propagating down from the sediment man, 1998). Such sediments can be penetrated by the or- surface. If so, the excursions of the Nereites producers to ganisms ingesting or fluidizing the substrate by body the surface suggest that they are feeding on the food-rich movements (e.g., Scha¨fer, 1956). surface sediment. In the case of the South China Sea, The horizontal parts of Nereites suggest a chemotactical some ash was ingested concurrently. guidance of the producers along a fairly sharp redox The ash within the burrows allows an estimation of the boundary. From the fossil record chemotaxis is known to population density of the Nereites-producing organisms, affect burrow patterns (e.g., Bromley, 1996), but guidance assuming that the ash infill results from surface feeding along a redox boundary has not been reported because it is following upwelling periods. Calculations based on the a temporary feature (e.g., Froelich et al., 1979). Jensen studied horizontal sections show that 22 burrows were (1992) described the anthozoan Cerianthus that inhabits produced after the Pinatubo eruption during 7 years per an open burrow system that extends just below the oxic- m2. During this time, high organic matter fluxes were anoxic boundary in muddy bathyal sediments of the Vo¨r- monitored during the 1992, 1994 and 1997 SW-monsoons ing Plateau (1200–1400 m water depth). He suggested in response to stronger than average upwelling, and dur- that the animal used microbes (or their metabolic prod- ing the 1995/1996 NE-monsoon owing to higher than nor- ucts), which occur abundantly along this geochemical mal wind strength (Wiesner, pers. commun., 2001). If all boundary. The polychaete Paraonis fulgens displays a animals grazed the surface after each of the four events change of behavior across the redox boundary in tidal-flat and some consequently produced ash-filled burrows each environments. It makes a thigmo-/chemotactically guided time, then about 5 to 6 organisms would live per m2. A pos- spiral to trap diatoms above the redox burrow and a tun- sible source of error for this estimate is that surface feed- nel system below the redox boundary (Ro¨der, 1971). ing was not systematic. Nonetheless, the estimate pro- Chemotactic guidance of trace producers is not unusual vides a reasonable population density when compared to and has been reported for many taxa (Bromley, 1996). other areas of the deep oceans (e.g., Gage and Tyler, 1991). Therefore, the Nereites producers in the South China Sea The absence of Nereites provides additional information do not represent an extraordinary behavior for endobenth- about the ecology of the trace-producing animals. Nereites ic organisms. Nereites producers appear to be guided by was not observed in sediments affected by bottom cur- chemical changes in pore water along the upper—still ox- rents, where the manganese-stained horizon is underlain ygenated—part of the redox boundary. by brown sediments indicating deep-reaching oxidizing Consequently, the lower boundary of the Nereites tier in conditions. The organic matter content of the sediment in the fossil record would reflect the position of the oxygen- these settings is nearly completely consumed, microbes ated interval within the deposits. Habitats characterized are probably sparse, and thus the sediment is less attrac- by poorly oxygenated pore water and a high benthic food tive for detritus-feeding animals. Furthermore, these sed- content containing Nereites were described, for instance, iments exhibit a high undrained shear strength of about by Ekdale and Mason (1988) and Wetzel and Uchman Ͼ10 kPa and, therefore, are possibly too stiff for the Ner- (1998) from the fossil record. The food content of the sedi- eites producers. Sediment grain size is a major control on ment should be sufficient for the Nereites producers. Ben- the distribution of Nereites producers. This trace is found thic food is too low for the Nereites producers if the redox in muddy to fine sandy sediments, but not in coarse fora- boundary is underlain by previously highly oxidized de- miniferal oozes. In addition, Nereites was not observed in posits. areas affected by diffuse downslope transport. There, sed- Enhanced accumulation of organic matter on the sea- iments are burrowed by Scolicia, which preferentially oc- floor resulting from upwelling lowers the flux of oxygen curs in sandy deposits (e.g., Wetzel, 1984; Fu and Werner, into the sediment (e.g., Gehlen et al., 1997). In response, 2000). The input of organic matter from land supports in- the Nereites producing animals in the oxic zone just above tense bioturbation and affects the redox boundary, which the redox boundary may have moved upward to maintain renders these areas unsuitable for the chemotactically oxygen supply. Seasonal vertical fluctuations of the posi- guided Nereites producers. tion of the redox-boundary may explain the vertical un- dulation of the Nereites burrow (N. missouriensis). It is DISCUSSION possible that, in sediment not affected by strongly seasonal input of organic matter, the Nereites producers strictly Ecologic factors favoring the production of Nereites in- follow a horizontal plain (e.g., N. crassa, N. irregularis, N. clude soft to soupy, oxygenated sediments that display a labyrinthica). well-developed redox boundary near the sediment surface. Surface grazing by subsurface feeders seems to contra- Water depth affects the amount of organic matter oxidized dict the above findings, but in many instances the behav- during settling (e.g., Suess, 1980), but is itself not an im- ior of animals is more complex than previously assumed portant ecologic factor (e.g., Tait, 1971). Conditions for (Kotake, 1989, 1991; Bromley, 1991; Miller, 1998; Miller Nereites production are more favorable in deep than in and Vokes, 1998). Temporary surface feeding by chemical- MODERN NEREITES ALONG THE REDOX BOUNDARY 513 ly sensitive deposit feeders may be an underestimated ad- ments. Furthermore, where the 1991 Pinatubo ash ex- aptation to utilize an additional food source. ceeds 3 cm in thickness, Nereites does not show any sign of In the fossil record, Nereites belongs to the post-deposi- continuing activity. Such activty is documented at other tional suite of trace fossils colonizing turbidites (e.g., Orr, sites covered by Ͻ2 cm ash; where Nereites contain some 1994; Uchman, 1995 and references therein). Seilacher 1991 Pinatubo ash. (1962) described post-depositional Nereites that penetrat- The infill of some 1991 Pinatubo ash implies episodic ed turbidite sands up to 6 cm thick and exploited the mud- surface feeding activity of Nereites producers, which nor- dy interval below. With respect to the findings in the mally explore the sediment along the redox boundary. In South China Sea, the post-depositional penetration of a particular, episodes of intensified upwelling and wind newly deposited turbidite is interpreted as exploration of a mixing enhance the organic matter flux by a factor of 3–4. buried redox zone that is overlain by a still oxygenated Such strong episodic input of organic matter probably re- turbidite before the new redox boundary had formed. This sulted in a geochemical signal propagating into the sedi- is an alternative to adaptation to oxygen-deficient sediment and then received by the burrowing Nereites produc- ments. In fact, a newly deposited turbidite could be oxy- er. Modelling of oxygen flux into the sediment supports genated for some time, as reported by Fo¨llmi and Grimm this hypothesis. This organic matter on the seafloor may (1990). The post-depositional, deep-burrowing Nereites serve as an additional food source for the Nereites trace- producers suggest a slowly re-established redox boundary maker. Alternatively, burrowers may have been forced to after turbidite deposition, probably in a highly oxic, organ- move closer to the surface due to diminished oxygen con- ic-poor sedimentary environment. tents at their living depth in response to pulsed delivery of In the South China Sea sediments the Nereites level organic-rich sediments. marks the upper boundary of the redox zone. Applying Utilization of sediment along the redox boundary by this finding to fossil turbidite sequences, the deepest Ner- Nereites producers implies that the Nereites tier in the fos- eites level would mark the lower boundary of the oxygen- sil record, especially in turbidite sequences (flysch), could ated zone. Several factors that control the depth of the re- reflect the long-term position of the paleo-redox boundary. dox boundary in modern deep-sea sediments have been As the depth of the redox boundary in sediment is influ- discussed. Froelich et al. (1979) envoked the composition enced by many factors, including the sedimentation rate and burial rate of organic matter. The latter, in turn, is and accumulation rate of organic matter, the depth of the controlled by sedimentation rate (Mu¨ ller and Suess, Nereites level potentially could provide a proxy for one or 1979). Similarly, Glud et al. (1994) and Hyacinthe et al. both of these factors. (2001) stated that oxygen penetration into the sediment correlates with organic content of surface sediments. In ACKNOWLEDGMENTS contrast, Henrichs (1992) stated that the accumulation rate of organic matter [g cmϪ2yrϪ1] controls the thickness Captain and crew of RV Sonne are thanked for recover- of the oxygenated zone. The flux of organic matter to the ing the sediment cores, and the participants of RV Sonne seafloor, in turn, affects the activity of burrowing organ- cruises 114, 132 and 140 are thanked for sharing samples isms (e.g., Trauth and Sarnthein, 1997) which enhances and providing enthusiasm during these cruises. M. Wies- the flux of oxygen into the sediment (e.g., Reimers et al., ner (Hamburg, F.R.G.) invited me to participate on these 1986). Mangini et al. (2001) found that the depth of the re- cruises (funded by the German Ministery of Science and dox boundary depends on the flux of organic matter and Technology), contributed unpublished results, provided oxygen into the sediment. Consequently, it appears that background information, and organic carbon data, as well the depth of the redox boundary reflects sedimentation as, critically reading an earlier version of this manuscript. rate or accumulation rate of organic matter. Besides these W. Rehder (Kiel, F.R.G.) x-rayed all radiographs, M. possibilities, it is not clear how other factors, such as the Haeckel (Kiel, F.R.G.) provided unpublished results, and input of manganese (Jakobsson et al., 2000) influence the R. Haese (Utrecht, The Netherlands) provided informa- development of the redox boundary. Where it can be eval- tion about redox conditions in sediments. M. Sarnthein uated in the stratigraphic record, the vertical extent of the (Kiel, F.R.G.) gave access to cores taken during RV Sonne Nereites tier potentially may provide a proxy for sedimen- cruise 95. D. McIllroy (Liverpool, U.K.) and two anony- tation rate or, alternatively, for the accumulation rate of mous journal reviewers improved the text. The Swiss Sci- organic matter. ence Foundation (grant 2100–052256.97) provided finan- cial support. All these contributions are gratefully ac- CONCLUSIONS knowledged. 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