The preservation potential of ash layers in the deep-sea: the example of the 1991-Pinatubo ash in the South China Sea

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1 Sedimentology (2009) 56, doi: /j x The preservation potential of ash layers in the deep-sea: the example of the 1991-Pinatubo ash in the South China Sea ANDREAS WETZEL Geologisch-Paläontologisches Institut, Universität Basel, Bernoullistrasse 32, CH-4056 Basel, Switzerland ( Associate Editor: Vern Manville ABSTRACT Following the eruption of Mount Pinatubo on 15 June 1991, volcanic ash was transported westward to the South China Sea in an atmospheric plume, falling out and settling to the sea floor within days and forming an up to 10 cm thick layer on an area > km 2. Immediately after deposition, surviving deepburrowing animals re-opened their connection to the sea floor to obtain water for respiration and/or food take-up. Later, small-sized meiofauna and then macrofauna re-colonized the sea floor, mixing newly deposited organic fluff with the underlying ash. Consequently, ash deposits thinner than 1 mm have not often been observed as a continuous layer when cored six years after the eruption, while ash about 2 mm thick is now patchily bioturbated. In areas covered by ash thicker than 5 mm, mixing by benthic animals is controlled mainly by the adaptation of the burrowing fauna to variations in grain-size, the rate of background sedimentation, the availability of benthic food on and within the sediment and pore water oxygen levels. With respect to these factors, four provinces can be distinguished: (i) Along the Philippines margin run-off from land fuels primary production that, in turn, leads to a high benthic food content. The benthic fauna is adapted to a variable grain-size and rapid sedimentation. Therefore, mixing is intense and the preservation potential of the ash layer is low. (ii) In areas affected by deposition of hyperpycnites and turbidites, i.e. in canyons in front of river mouths and in the Manila Trench, the ash layer is preserved due to rapid burial. (iii) The area to the west to about 116 E receives low amounts of benthic food, benthic mixing is less intense and the preservation potential of the ash is high. (iv) The central South China Sea, where the ash is thinner than 3 cm, is affected by intense wind mixing and upwelling and the benthic food content is high; thus, the chance that the ash will be preserved as a sharp-based layer is low. Consequently, the style of ash preservation has palaeo-environmental significance. Older buried and burrowed event layers provide further information to elucidate the fate of the 1991 Pinatubo ash layer; in general their appearance fits with observations in the Recent. Keywords Benthic food content, benthic mixing, bioturbation, deep sea, environmental disturbance, event layer. INTRODUCTION Volcanic ash layers may occupy wide areas and represent excellent marker beds within marine sediments because of their distinct composition (e.g. Ninkovich & Shackleton, 1975). Therefore, these layers can be correlated from site to site and, in some cases, to their source regions (e.g. Wallrabe-Adams & Lackschewitz, 2003). Marine ash layers form within a short time where ash is 1992 Ó 2009 The Author. Journal compilation Ó 2009 International Association of Sedimentologists

2 Ash layer preservation and benthic mixing 1993 transported rapidly to the sea floor, often by vertical density currents (e.g. Carey, 1997; Manville & Wilson, 2004). The resulting ash distribution reflects the tropospheric to stratospheric wind field rather than oceanic currents (e.g. Wiesner et al., 2004). After deposition an ash layer is affected by benthic organisms that mix it with underlying and newly deposited sediments for as long as the ash resides within the bioturbated zone. For palaeoclimatic or palaeoceanographic analyses, the effects of benthic mixing have been modelled applying the mixed layer concept (Berger & Heath, 1968). The thickness of a uniformly mixed layer resulting from repeated short-distance local displacement of particles (Boudreau, 1986a) is determined from an excess of radioisotopes such as 14 C, 210 Pb or 234 Th (Erlenkeuser, 1980). For modern sediments it has been shown that mixing is affected strongly by environmental factors such as grain-size, sedimentation rate, organic matter flux to the sea floor and rate of organic matter burial (Wheatcroft, 1990; Kuehl et al., 1993; Trauth et al., 1997). In hemipelagic deposits, the mixed layer approach can be applied to predict the fate of an event deposit as long as it is similar to the host sediment in terms of grain-size and organic matter content, for instance muddy tempestites or turbidites (Bentley & Nittrouer, 2003). However, volcanic ash has different properties; it is barren of organic matter, consists of angular to sub-rounded grains and behaves as a granular material. Therefore, ash does not really attract organisms so that bioturbational mixing differs from that of hemipelagites for which mixing parameters have been determined. This deduction is supported by the frequent non-local mixing (Boudreau, 1986b) observed in fossil ash layers when material is displaced over longer distances by the production of deep-penetrating single burrows (Simonsen & Toft, 2006). Non-local mixing prevents the use of the mixed layer model (Boudreau, 1986b; Hughes et al., 2005). Consequently, additional observations are necessary to better understand the mixing of ash. The 1991-Pinatubo ash represents an excellent example, as the environmental conditions in the South China Sea are fairly well-known. In 1991, Mount Pinatubo in the Philippines erupted ash equivalent to about 5Æ5 km 3 of magma that formed an extensive ash layer in the South China Sea (Wiesner et al., 2004) and was deposited in various environmental settings. In general, it shows a decrease in thickness and grain-size with distance from source and presents an ideal case to evaluate the preservation potential of such an event layer. Known environmental factors can be related to the ongoing bioturbational mixing because the burrows contain ash. Consequently, the mode of preservation of a fossil ash layer can be used for palaeoenvironmental reconstruction because burrowing organisms respond sensitively to the environmental conditions (e.g. Wetzel, 1981, 1991; Ekdale et al., 1984; Goldring, 1995). The purpose of this paper was to evaluate the actual mixing in relation to modern environmental factors and to show that short-term observations in the Recent allow prediction of the mode of preservation in the fossil record. AREA OF INVESTIGATION The South China Sea represents a western marginal sea of the Pacific Ocean, surrounded by the South-east Asian mainland in the north and the west and islands to the south and the east (Fig. 1). It includes a prominent, ca 4300 m deep basin between the Philippines and Vietnam. Well-oxygenated bottom waters (ca 2ml O 2 l )1 ; Wyrtki, 1961; Chao et al., 1996) are introduced via the Bashi Channel between Taiwan and Luzon (Philippines) to the South China Sea. 25 N 20 N 15 N 10 N 5 N 0 N 5 S 2 00 South-east Asia Sumatra 2 00 Sunda Shelf Borneo Java Sea Area of Figure South China Sea 2 00 Bashi Channel Philippines Sulu Sea Taiwan Pacific Ocean 2 00 Celebes Sea E 105 E 110 E 1 15 E 120 E 125 E Fig. 1. Location of the study area.

3 1994 A. Wetzel The South China Sea is conditioned by seasonal reversal of the monsoonal wind system (Tomczak & Godfrey, 1994). During summer, north-easterly directed winds affected by a coast-parallel mountain range in Vietnam result in a wind jet at ca 10 N; this leads to pronounced coastal upwelling (Xie et al., 2003). In addition, southwesterly monsoon winds may enhance seasonal primary productivity in the central South China Sea depending on wind stress (Wiesner et al., 1996). During November to March the north-west monsoon reverses the direction of flow and upwelling takes place off the north-west Luzon coast, off the northern Sunda Shelf and in the central South China Sea. The winter upwelling zones have been identified by their anomalously cold subsurface temperatures and high chlorophyll concentrations, rather than sea surface temperatures and, as such, cannot be observed in satellite-derived sea surface temperature fields (e.g. Liu et al., 2002). During the high season of the monsoons particle flux to the sea floor increases by a factor of three to four; about 70% of the total annual organic matter flux is exported to the deep-sea (Wiesner et al., 1996). Following seasonally high primary production, organic-rich fluff (3% to 5% C org ) accumulates on the sea bed where it is consumed within weeks to months by benthic organisms. El Niño events suppress upwelling and lead to significant reduction in particle flux. The modern calcite compensation depth (CCD) is located in about 3500 m water depth, but was several hundred metres shallower during the Early Holocene (e.g. Wang, 1999). Diffuse downslope transport of material derived from the Philippines is indicated by displaced ostracod shells (Zhou & Zhao, 1999). The study area can roughly be subdivided into four provinces with respect to the environmental conditions: Province I comprises the continental slope of the Philippines (Luzon, Calamian Islands, Palawan); Province II is the Manila Trench and lower reaches of canyons; Province III is located west of the Manila Trench where upwelling is subordinate; and Province IV lies in the central South China Sea where upwelling and wind mixing is intense (Fig. 2). Pinatubo 1991-ash On 15 June 1991, Mount Pinatubo in the Philippines erupted about 5Æ5 km 3 (magma equivalent) of volcanic material; winds transported it mainly westward to the South China Sea. There, an up to 10 cm ash layer formed that was studied in detail by Wiesner et al. (2004): their results are summarized briefly here. Ash thicker than 1 mm covers an area of km 2. Three days after the eruption, fine, light ash (median ca 6 F, M z, sorting 1Æ2 to 1Æ8) was recorded in sediment traps 586 km west of Mount Pinatubo, moored in 1190 and 3730 m water depth, respectively. The front of the fine ash was calculated to have settled at a minimum velocity of 1670 m day )1 (Wiesner et al., 1995), suggesting that vertical density currents, as observed in experiments by Carey (1997), were involved in its transport through the water column (Wiesner et al., 2004). Such vertical density currents may result in a faint layering of the ash (cf. McCool & Parsons, 2004). The distribution of the ash therefore mainly reflects the wind fields (Fig. 2). Proximal to Mount Pinatubo, the ash is bipartite having a coarse (median ca 2 to 3 F), dark, salt-and-pepper basal part with phenocrysts (Fig. 3). This dark ash is graded vertically and exhibits a decrease in grain-size from the source to 117 E, where it disappears. Virtually ungraded light ash of uniform grain-size (median ca 6 F; Fig. 3) occurs throughout the whole lobe decreasing in thickness downwind. When the ash settled to the sea floor it stripped out planktic organisms from the water column (Wiesner et al., 1996), but the ash deposits are nearly barren of organic matter (0Æ002% C org ). The ash reduced the oxygen flux into the underlying deposits because of the increased distance to the bottom water and, thus, anoxic conditions established in pre-ash deposits where ash thickness exceeds 2 to 3 cm (Haeckel et al., 2001). The sediment just below the ash is insignificantly enriched in organic carbon compared with the sediments further down. However, the organic matter of the previous, now ash-buried, sediment surface appears to be less refractory as indicated by an increased content in amino-acids and sugars. MATERIALS AND METHODS Samples were collected during cruises 118, 132, and 140 of the German research vessel Sonne to the South China Sea in 1996, 1998, and 1999, respectively (Wiesner et al., 1997, 1998, 1999; Fig. 2). Cores were taken by a cm 2 box corer, which has automatically closing flaps to protect the sediment surface from winnowing during retrieval. To analyse sedimentary structures in detail, 8 mm thick sediment slices were prepared

4 T r e n Ash layer preservation and benthic mixing DA NANG 10 c h i l a M a n Vietnam NHA TRANG MT. PINATUBO MANILA Mindoro Mindoro Calamian Islands Calamian Islands Palawan Luzon Isopachs of Pinatubo ash [mm] Eastern boundary of strong SW monsoon wind stress Eastern boundary of intense upwelling Cores not investigated Nereites ichnofabric Thalassinoides ichnofabric Nereites ichnofabric near-surface, Thalassinoides ichnofabric below Mud-prone deposits Mud-prone deposits overlain by hyperpycnites Sand-prone deposits with large Scolicia Sand-prone deposits with large Scolicia overlain by hyperpycnites Fig. 2. Location of the cores studied and the distribution of ichnofabrics below the Pinatubo 1991-ash within the South China Sea (for details see Wetzel, 2008); ash isopachs after Wiesner et al. (2004, fig. 1). The area in the west is affected by seasonally enhanced organic matter deposition due to upwelling (eastern boundary = stippled line) and seasonally strong SW monsoon winds in the central South China Sea (grey band = eastern boundary of the 100 mpa wind stress field after Wiesner et al., 1996). This boundary and the Manila Trench define the borders of the environmental provinces distinguished in the study which are: (I) Philippines slope; (II) Manila Trench and lower reaches of canyons; (III) area with subordinate wind mixing of surface waters and upwelling; and (IV) area influenced by intense wind mixing of surface waters and upwelling. The numbers refer to cores figured in this study; the first number refers to the cruise, 0=Sonne cruise 140, 2 = Sonne Cruise 132, 4 = Sonne cruise 114, the last two numbers refer to the site (for exact location see references in text).

5 1996 A. Wetzel Fig. 3. Normally graded Pinatubo 1991-ash at site 241 (cruise Sonne 132, station 41; for the grain-size distribution see Wiesner et al., 2004, fig. 3); pas = pre ash sediment, ca = coarse ash, fa = fine ash. for X-ray radiography (Werner, 1967). These slices were irradiated in the laboratory at 30 kv for varying times. For three-dimensional analysis of the sedimentary structures, serial sections in vertical and horizontal directions were prepared. Organic carbon was calculated as the difference between total carbon measured on a Science NA 1500 Elemental Analyzer (Carlo Erba, Cambridge, UK) and carbonate carbon measured on a Carmograph 6 (Woesthoff, Bochum, Germany) using calcite standards for calibration and 2 n H 3 PO 4 for sample acidification. To evaluate the preservation potential of the Pinatubo 1991-ash layer, the newly produced biogenic sedimentary structures, the actual mixing of the ash and the ichnofabrics, have been evaluated. The latter provide information about the long-term, tiered ichnofauna and allow an evaluation of the ecological conditions and the potential for mixing. In addition, already buried event layers were studied for comparison. Ichnological analysis The biogenic sedimentary structures in the studied deposits differ in their fill (composition, grain-size, grain packing and colour) and/or boundary from the surrounding sediment. Two general types are distinguished: Biodeformational structures disturb pre-existing structures, but display no distinct outlines nor a recognizable geometry allowing their classification. Biodeformational structures are formed mainly within the uppermost sediment layer (e.g. Wetzel, 1981). These structures are produced by mega-benthos, macro-benthos and even meiobenthos. Trace fossils exhibit sharp outlines and possess a characteristic geometry which allows their taxonomic classification. This observation was restricted to the ichnogenus level in the present study because the burrows have been studied using sediment slabs too small to enable reconstruction of the total three-dimensional geometry of most burrows. With respect to mixing, nine ichnogenera are of importance (Fig. 4; Table 1). Ecological conditions change with depth in the sediment, due to factors such as oxygenation, organic matter content and consistency (e.g. Gage & Tyler, 1991). Therefore, the sea floor is divided into different habitat intervals (e.g. Jumars & Ekman, 1983; Jumars et al., 1990), with the vertical zonation of animals documented by tiered burrows. Each tier is defined as a distinct depth interval containing co-occurring traces that intersect each other (e.g. Wetzel, 1981; Werner & Wetzel, 1982; Bromley & Ekdale, 1986). The tiering structure can be deciphered by studying cross-cutting relationships. The mixed layer as evaluated on the basis of excess radioisotopes does not really fit to observations based on bioturbational structures (Wheatcroft & Drake, 2003) and the tiering concept. The vertical extent of the mixed layer, as defined by radioisotopes, appears to comprise roughly the uppermost interval homogenized by meiofauna and grazers (Tier I) and at least the intensely bioturbated part of Tier II where distinct burrows are formed. The terms homogenized layer (Tier I) and mixed layer, defined by an almost constant amount of excess radioisotopes, therefore, need to be distinguished sharply. ENVIRONMENT AND ICHNOLOGY The four, environmentally different, provinces (as outlined above) are discussed separately (Fig. 2).

6 Ash layer preservation and benthic mixing 1997 Chondrites Nereites Palaeophycus Phycosiphon Planolites Rhizocorallium Scolicia Thalassinoides Zoophycos Fig. 4. Schematic representation of the observed biogenic sedimentary structures described in Table 1. Table 1. Short description of ichnogenera encountered (shown in Fig. 4). Ichnogenus Chondrites Sternberg 1833 Nereites MacLeay 1839 Palaeophycus Hall 1847 Phycosiphon Fischer-Ooster 1858 Planolites Nicholson 1873 Rhizocorallium Zenker 1836 Scolicia de Quatrefages 1849 Thalassinoides Ehrenberg 1944 Zoophycos Massalongo 1855 Ch Nv, Nh Pa Ph Pl Rh Sc Th Zp Short description From a master shaft, tubes regularly ramify at depth to form a dendritic network. Winding to meandering, central back-filled tunnel enveloped by even to lobate zone of reworked sediment. Branched or unbranched, lined, cylindrical burrow, structureless fill, of the same lithology as host rock. Repeated narrow, U-shaped tubes enclose spreiten at millimetre to centimetre-scale, branching from an axial spreiten. Unlined, rarely branched, straight to tortuous, tubular burrow; structureless fill differs from host rock. Simple U-tube with spreite, generally protrusive, parallel or somewhat oblique to bedding. Large, bilaterally symmetrical, subcylindrical burrow having meniscate lamellae often divided into two concave sets. In traverse cross-section a concentric structure of bilobate lamellae surrounds an eccentric axis. Three-dimensional burrow system composed of smooth, cylindrical tubes branching at Y to T-shaped, enlarged points. U-shaped or J-shaped protrusive elements of variable length and orientation form a spreiten structure. Province I Slope of the Philippines Environment The deposits consist of clastic material delivered from the Philippines and planktic and benthic shells (e.g. Chen, 1978; Wang et al., 1995) and contain a considerable sand fraction. Diffuse transport displaces particles downslope (Zhou & Zhao, 1999). West of Mount Pinatubo, numerous turbidites and post-1991 ash turbidites have been encountered where rivers, such as the Bucao, Tomaso or Pasig-Potero, enter the sea. In addition,

7 1998 A. Wetzel A m P A B paf Ch Sc Pl mpa Sc tu1 C PA Pa bd Sc Pa Ch Pl Sc tu2 D Sc Ph tu Th Fig. 5. Biogenic sedimentary structures and benthic mixing in Philippines slope deposits; dotted lines indicate completely mixed, broken lines indicate incompletely mixed and solid line indicates not mixed. (A) Pinatubo ash mixed (mpa) into large Scolicia (Sc) in sand-prone deposits (site 235, 0 to 25Æ5 cm; X-ray radiograph negative); below, an originally ca 2 cm thick turbidite (tu 1) is completely mixed, further below a ca 10 cm thick turbidite (tu 2) is mixed only from above by the producers of Scolicia (Sc) and Phycosiphon (Ph). (B) Producers of large Scolicia (Sc) and Planolites (Pl) have already mixed originally 2 cm thick Pinatubo 1991-ash completely (mpa) before colonization of the ash by producers of Chondrites (Ch); (paf) post-ash fluff (site 233, 0 to 8 cm; X-ray radiograph negative). (C) Turbidite (tu) in mud-prone pre-ash deposits with Chondrites (Ch) filled with Pinatubo 1991-ash, the ash itself exhibits a sharp base (PA), but is affected by local mixing by the producers of Planolites (Pl); in addition, biodeformational structures (bd) and Palaeophycus (Pa) occur (site 237, 0 to 7 cm; X-ray radiograph negative). (D) Turbidite (tu) in mud-prone deposits with non-local mixing by Thalassinoides forming a vertical tube (Th), in addition Scolicia (Sc) is present only in the sand (site 421, 10 to 2; X-ray radiograph negative). heavy monsoonal precipitation led to the strong erosion from areas covered by the 1991-Pinatubo ash (Janda et al., 1996). Downstream, densitydriven, hyperpycnal flows may have formed at mouths of rivers draining areas covered by pyroclastics. The sedimentation rates within this area have not been determined exactly yet, but data from cores to the south provide values ranging between 5 and 15 cm ka )1 (Thunell et al., 1992; Wei et al., 2003). Organic matter content varies between 0Æ4% and 0Æ8% C org. The redox boundary occurs at sediment depths of less than 5 cm. The sediments are bioturbated completely except for some event layers. Along the slope of the Philippines downwelling water masses may provide additional benthic food (e.g. Canals et al., 2006). Ichnology Numerous mounds on the sea floor demonstrate that deep-burrowing animals survived the ash fall. These organisms have the ability to produce pronounced non-local mixing. Ash has been encountered in all burrow types, but not at all sites. Two different ichnofabrics, each comprising four tiers, have been observed depending on the sediment composition; sand-prone deposits exhibiting granular behaviour and mud-prone sediments with cohesive properties. Sand-prone sediments occur between N and N and along the

8 Ash layer preservation and benthic mixing 1999 continuation of the channel between Mindoro and Luzon. In between and north of N mudprone deposits dominate (Figs 2 and 5). Ichnofabrics in sand-prone deposits below the 1991-ash comprise: Tier I Biodeformational structures document homogenization of a layer as thick as 3 cm. Tier II Scolicia occurs between about 3 and 14 cm depth. Tier III Palaeophycus and Planolites occur between ca 8 to 17 cm depth. Tier IV Thalassinoides occurs between 10 and about 20 cm; also Chondrites may be found. Ichnofabrics in mud-prone deposits below the 1991-ash comprise: Tier I Biodeformational structures normally affect the upper 5 cm. A Tier II Palaeophycus and Planolites occupy an interval between ca 5 to 15 cm; rare, small Scolicia (<2 cm height) are formed between 5 and 10 cm. Tier III Thalassinoides occurs below 10 to >15 cm; in addition Chondrites may be present; a Thalassinoides filled with 1991-ash penetrated as deep as 45 cm. Tier IV Zoophycos has been observed locally. In sand-prone deposits the 1991-ash layer is mixed from below, especially by producers of large Scolicia. At some localities, 1991-ash up to 6 cm thick has already been mixed completely (Fig. 5). If ash thickness exceeded 6 cm it is not burrowed from below. By contrast, in mudprone deposits the production of biodeformational structures only continued where the ash was thinner than ca 2 to 3 cm, otherwise the B Fig. 6. Biogenic sedimentary structures and in mass-flow deposits, turbidites and supposed hyperpycnites. (A) Surface of core 244, note abundant plant debris consisting of charcoal (cc), fresh bamboo fragments (fb) and wood debris (w); in particular the fresh bamboo fragments document direct transport from a terrestrial source and imply a hyperpycnal flow. (B) Vertical section of core 244 (X-ray radiograph, negative); several thin muddy layers of supposed hyperpycnal origin, each base marked by a line lie above the Pinatubo ash (PA); the number of events corresponds to the number of monsoon periods between 1991 and 1998, when the core was retrieved. (C) Pinatubo 1991-ash (PA) covered by several turbidites; the turbidites are sparsely bioturbated by Scolicia (Sc) and biodeformational structures (bd) and small tubes (t) (site 243, 0 to 2; X-ray radiograph negative). (D) Thick Pinatubo ash (PA) covered by several hyperpycnal flow deposits (hy); note small Palaeophycus-like (Pa) and Planolites-like (Pl) burrows (site 242, 0 to 13Æ5 cm; X-ray radiograph negative). (E) Rhizocorallium (Rh) in Pinatubo 1991-ash (PA) burrowing down from overlying hyperpycnal deposits (hy) (site 240, 0 to 8 cm; X-ray radiograph negative). C t w t P A Sc fb bd cc fb fb D E Pl hy PA Rh Pa hy PA PA

9 2000 A. Wetzel organisms occupying the uppermost tier probably were killed by de-oxygenation of the pore water following ash deposition (Haeckel et al., 2001). At many localities the ash is overlain by multiple layers of inferred hyperpycnal deposits. These layers are graded and some tens of millimetres thick; often exhibiting a faint lamination. Plant debris and wood on the surface of one core document a terrestrial source (Fig. 6). These brownish muds contain some organic material (0Æ1% to 0Æ2% C org ) and are characterized by an ichnofauna composed of small burrows (Fig. 6). These muddy deposits form the base for colonization, especially of thick ash deposits. At some localities within thick ash, large Rhizocoralliumlike spreiten and U-shaped burrows, as well as Planolites filled with material from above, have been observed (Fig. 6). Chondrites also occurs in thick fine ash. Already-buried turbidites provide information about the fate of the 1991-ash layer. Turbidites estimated to have been originally <5 cm thick are almost completely mixed, especially by Scolicia producers. Only enrichment of coarse grains of allochthonous provenance classifies them as event layers. Mixing by Scolicia producers was probably fast and intense because the turbidites are mixed from both above and below. However, if turbidite thickness exceeds 5 to 6 cm the basal part is preserved and bioturbation starts from the top only. In such cases non-local mixing can be frequent (Fig. 5). Phycosiphon may occur in the upper 3 to 8 cm of turbidites as traces of a colonizer (see Wetzel & Uchman, 2001). However, the Phycosiphon producers do not really mix the deposits (e.g. Wetzel & Bromley, 1994). Province II Manila Trench Sedimentation within the Manila Trench and the lower reaches of canyons is dominated by sediment gravity flows. Turbidites and hyperpycnites form the bulk of the deposits. Bioturbational A PA B bd bd Rh Th Th tu C Th Th Fig. 7. Biogenic sedimentary structures and benthic mixing in the area west of the Manila trench that is subordinately affected by wind mixing of surface waters and upwelling; dotted lines indicate complete mixing, broken lines indicate incomplete mixing, and solid line indicates no mixing. (A) Approximately thick ash (PA) at top of image, still having a sharp base, but locally mixed into biodeformational structures (bd) and Thalassinoides (Th); in the middle of the image a 5 cm thick turbidite (tu) is affected by non-local mixing, mainly by Thalassinoides producers and mixing from above, in this case by Rhizocorallium (Rh) (site 224, 0 to 20 cm; X-ray radiograph negative). (B) Non-local mixing by Thalassinoides (Th) producers, they have already buried 1991-ash (light) down to about 35 cm depth (site 402, 22 to 35 cm; X-ray radiograph negative). (C) Completely mixed, originally ca 1Æ5 cm thick ash layer (light), note non-local mixing by Thalassinoides (Th) producers (site 247, 0 to 6 cm; X-ray radiograph negative).

10 Ash layer preservation and benthic mixing 2001 structures within these deposits are sparse (Fig. 6). Province III West of the Manila Trench with subordinate upwelling Environment West of the Manila Trench, mainly airborne particles, suspended material and siliceous shells accumulate below the CCD. Over the long-term, this area is not affected by intense upwelling or wind mixing (see above). Primary production in the surface waters is lower than it is further to the west (e.g. Wiesner et al., 1996; Liu et al., 2002). Sediments accumulate slowly: rates of 2 to 5cmka )1 have been determined at two locations (Kuehl et al., 1993; Sarnthein et al., 1994). Sediments just below the 1991-ash contain 0Æ2% to 0Æ4% C org. The deposits are brownish in colour, and black horizons stained by Mn oxides occur at several levels, the uppermost normally at >10 cm sediment depth. Ichnology The sediments are completely bioturbated. Pre ash ichnofabrics display four tiers, all of which were occupied by animals because some 1991-ash was found within the burrows of each. In general, the ichnofabrics exhibit the following tiers (Fig. 7): Tier I Biodeformational structures occupy a 1 to 2 cm thick interval. Tier II Planolites occurs below, down to about 5 to 8 cm depth. Tier III Thalassinoides occurs quite abundantly down to 15 to 20 cm, in addition Chondrites may occur, then cross-cutting Planolites. Tier IV Zoophycos has been found below 20 cm but only locally. The 1991-ash still forms the sediment surface within this province. Accumulation of organic flocs is low. Surface traces have seldom been observed on the ash; where present, they are small and indistinct. Some endobenthic animals have re-opened their connecting tubes to the surface, producing mounds and open holes on the sediment surface. The abundance of both, however, is lower than in the areas where the organic matter accumulation rate is high (Wetzel, 2008). No burrows have been observed within the ash. Benthic mixing activity therefore is inferred to be low in this area. Only non-local mixing effects have been observed where 1991-ash occurs within Thalassinoides burrows (Fig. 7), but the amount of ash displaced downward is low. Consequently, it was not possible to evaluate the influence of the ash thickness on benthic mixing activity. Older buried event layers up to 2 cm thick are completely bioturbated (once again, characterized by enrichment of coarse, allochthonous material), whereas event layers exceeding this thickness are only partially mixed, the coarse base being less or not bioturbated (Fig. 7). In such cases bioturbation starts from the top, and some deep burrowers have penetrated the event layer and disturbed it locally. Event layers thicker than 2 cm exhibit a completely mixed interval up to 3 cm thick, an additional 2 to 3 cm can be partially mixed but, in all cases, the coarse basal interval is preserved almost intact. Province IV Abyssal central South China Sea affected by intense upwelling and wind mixing Environment Below CCD a considerable proportion of the sea bed sediments consist of airborne material (Gerbich, 2001). Near-surface deposits mainly consist of clayey mud (Wang et al., 1995). In addition some material may be delivered from the southeast, namely from the slope of the Calamian Islands and Palawan. Within this part of the South China Sea seasonal upwelling and intense wind mixing of surface waters commonly occurs, decreasing eastwards (see above). Therefore, at a similar water depth the organic matter content of the (now ash-buried) surface sediments decreases from about 0Æ6% to 0Æ4% C org (see also Wetzel, 2002). The redox boundary lies at ca 5to10cm sediment depth. Low sedimentation rates of 2 to 3cmka )1 have been determined (Kuehl et al., 1993; Sarnthein et al., 1994). The deposits are completely bioturbated. Ash is fine-grained throughout and 3 cm thick. Ichnology The pre-1991-ash sediments are completely bioturbated (Fig. 8) with the burrows arranged in tiers, some of which contain 1991-ash. Four tiers have been encountered. Tier I Biodeformational structures occupy a ca thick interval. Tier II Nereites occurs abundantly down to 6 to 8 cm. Tier III Palaeophycus/Planolites are found to 10 cm depth. Tier IV Thalassinoides has been found below 20 cm. Chondrites occurs occasionally.

11 2002 A. Wetzel A PA B mpa Nv Nh Nv Nh Th Nh Pl tu1 C st Zp Th D Zp c st tu2 Xe Fig. 8. Biogenic sedimentary structures and benthic mixing in the central South China Sea region affected by intense wind mixing of surface waters and upwelling; dotted lines indicate completely mixed and solid line indicates not mixed. (A) 1991-ash (PA) still exhibits a sharp base, but is penetrated locally by vertical parts of Nereites (Nv), that become horizontal (Nh) with depth; a 3 cm thick turbidite (tu1) is completely to locally mixed by Planolites (Pl) and Thalassinoides (Th) producers; at the bottom of the image, the upper 3 cm of an originally 7 cm thick turbidite (tu2) are completely mixed from above by producers of Planolites, Thalassinoides and Zoophycos (Zp) (site 003; 0 to 15 cm; X-ray radiograph negative). (B) About thick 1991-ash completely mixed (mpa) mainly by the producers of Nereites, vertical (Nv) as well as horizontal (Nh) parts of Nereites contain ash, in addition to some undetermined burrows (site 211; 4 to 5 cm, X-ray radiograph negative). (C) Sediment surface after a bloom, note the cover with dark organic flocs (fluff) and surface traces (st) (site 216). (D) Sediment surface during non-bloom times, some dark flocs, surface traces (st), a xenophyophore (Xe) and a mound (c) are seen (site 212). Within the 1991-ash itself only a few burrows have been observed, but many penetrate it. Mounds and holes have been found on top of the ash where material from below has been moved upward by endobenthic organisms to maintain an open connection to the sea floor for respiration or feeding. Furthermore, surface traces were seen following a strong upwelling period in The production of surface traces leads to the mixing of organic-rich fluff and ash, forming a habitable surface layer. However, years later these bioturbational structures had disappeared, probably due to intense mixing by meiofauna or other smallsized organisms, coincident with a strong ENSO event in 1998 that suppressed upwelling. A homogenized surface layer has not established yet within this area, due to slow sedimentation, and the ash still exhibits a sharp base (Fig. 8). Sediment feeding and temporary surface feeding of endobenthic organisms is common. High abundance of organisms and their immediate response to surficial food sources will lead to a complete mixing of the 1991-ash when thick by non-local mixing related to vertical movements of Nereites producers (Fig. 8; see Wetzel, 2002, 2008 for details). However, for ash thick a prediction is not yet possible based on the available data, but comparisons with fossil examples might allow some information to be deduced (see below). In addition, surface trail

12 Ash layer preservation and benthic mixing 2003 producers will incorporate organic matter into the barren ash layer reducing its fossilization potential. Nonetheless, once a new surface layer has been formed (in about 1000 years), mixing from above might destroy the stratification of an ash layer as thick as 2 to 3 cm, while already buried thin event layers are completely mixed (Fig. 8). INTERPRETATION The deposition of ash severely disturbs the habitat of the fauna living on and shallowly within the sediments. Several aspects are of major importance. Environmental factors Food availability: The main food source for many organisms is the uppermost sediment layer where organic flocs accumulate. In addition, some organic matter is available deeper below the sediment surface. As ash is barren of organic matter, benthic food availability is restricted to newly arriving flocs and thus fluctuates seasonally. When this organic material is mixed with the underlying sediment, the average organic content of the surface layer is significantly lower compared with that prior to ash deposition. Oxygenation: The deposition of ash 3 cm thick diminishes the oxygen diffusion into the pre-ash sediment by increasing the distance to oxygenated bottom water producing anoxic pore-waters (Haeckel et al., 2001). Consequently, the abundance of organisms respirating only pore water decreases strongly, thus reducing burrowing below the ash. Even ash thinner than 3 cm lowers oxygenation of the pre-ash deposits by increasing the diffusion length. Sediment properties: Ash consists of angular to sub-rounded grains that exhibit a granular behaviour fundamentally different from mud (e.g. Mitchell, 1993). For instance, unlined tubes are not stable and will collapse. To crawl through such deposits requires burrowing techniques and/or adaptations differing from those of organisms living in mud (e.g. Seilacher, 2007). Adaptation of organisms The outlined environmental changes reduce benthic activity and mixing. However, the longterm adaptation of the organisms to such changes is important as evidenced by the following observations. Surface grazers disappear: Surface trails have not been observed for years after ash deposition, regardless of ash thickness, with the first trails appearing in benthic food-rich areas along the Philippines and the central South China Sea (Fig. 8). Therefore, the abundance of epibenthic animals sharply declined in areas where the ash formed a coherent layer. There, seasonal organic matter deposition forms a thin hospitable surface layer only. Surface feeding leads to local mixing of the ash layer. Deep burrowers often survive: Mounds and holes have been observed on the sediment surface shortly after ash deposition as deep, mainly sediment-feeding burrowing animals re-opened their connection to the sea floor to obtain respiration water and possibly temporary food uptake. The uppermost sediment layer develops in different ways: West of the Manila Trench, the ash exhibits a sharp base, but the ash surface is undergoing recolonization, especially by foraminifera and other small-sized organisms (Hess & Kuhnt, 1996). These organisms mix and homogenize newly deposited organic flocs with some of the underlying ash, and material that has been brought up by deep-burrowing organisms. The size of the fauna restricts this type of mixing to fine ash. East of the Manila Trench, in sand-prone deposits, ash is mixed from below by burrowing echinoids that are adapted to sandy, poorly sorted deposits. Echinoderms require sediments low in mud that would otherwise plug their gastrovascular system (e.g. Brett & Seilacher, 1991). Mixing modes Three types of mixing need to be considered to predict the fate of the 1991-ash layer. Mixing by ash-penetrating burrows: Deep burrowers dig through the ash layer, incorporating some ash into their burrows and also displacing some sediment upward. In this way, the ash layer is penetrated and can be disturbed severely by single burrows (non-local mixing) but, in many instances, it is still recognizable as a continuous bed if affected by this type of mixing only.

13 2004 A. Wetzel Mixing from below: In the sand-prone deposits along the Philippines slope the ash is bioturbated from below mainly by Scolicia producers (burrowing echinoids). Observations on the 1991-ash and on already buried turbidites show that intervals as thick as 6 cm can be mixed completely from below. Mixing from above: This type of mixing often matches the normal, commonly described local mixing case and fits to the mixed layer model (e.g. Bentley et al., 2006 and references therein). Crawling macrofauna and megafauna mix the layer in addition to endobenthic macrofauna and meiofauna. Local mixing by meiofauna is efficient because of their high abundance (e.g. Thiel, 1983). Therefore, ash thinner than 1 mm has rarely been observed as a continuous layer when cored six years after the eruption. Ash about 2 mm thick is now patchily bioturbated. This type of mixing has been stated to depend mainly on the organic matter flux to the sea floor (e.g. Trauth et al., 1997) or organic matter burial (e.g. Kuehl et al., 1993), but grain-size may also be of importance (e.g. Wheatcroft, 1990). The effects of mixing from above can be estimated by comparing the thickness of tiers preserved below the ash with the thickness of the ash layer itself. A simple calculation is used for explanation: A 1:1 mixture of newly arriving material and ash has mechanical properties and porosity similar to the background deposits, Ash layer preservation potential Low High Ash thickness Thin 1 Thick Event deposition Rare Frequent 2 Organic matter delivery Seasonality of organic matter deposition Sedimentation rate High 3 High 4 High Low Low Low Grain size Coarse 5 Fine Threshold thickness for preservation of a sharp-based ash layer at intermediate organic matter deposition ~ 3 to 4 cm. >1 event/ 10 years. Surface sediment layer contains >0.3 to 0.4% Corg. Fluctuating organic matter deposition favours temporary surface-feeding and non-local mixing by deep burrowers. Scolicia producers (sea urchins) prefering a granular substrate may mix up to 6 cm thick ash from below if benthic food content is high. The larger the difference in grain size between ash layer and the background sediment, the higher is the preservation potential of the ash layer. Fig. 9. Factors affecting the preservation of the Pinatubo 1991-ash layer in the deep South China Sea. Ash thickness refers to the volume to be mixed to destroy the ash layer; at a given mixing rate the preservation potential increases with increasing ash layer thickness. Event deposition refers to hyperpycnal flow deposits and turbidites that may bury the ash layer; because bioturbational mixing decreases with depth in sediment, frequent events lead to rapid burial and, hence, increase the preservation potential. Organic matter delivery is used as a proxy for benthic food availability; the higher the benthic food content, the more intense is bioturbational mixing and, hence, the destruction of the ash layer. Seasonality of organic matter deposition leads to subsequent vertical movements of endobenthic organisms that, in turn, destroy the ash layer while passing through. Sedimentation rate affects the burial of the ash layer, but also that of organic matter; with respect to abyssal deposits the latter effect is of major importance because organic-rich sediments attract burrowing organisms that may mix the ash layer. Grain-size distribution of the background sediments affects the adaptation of the burrowing organisms to their habitat; thus, the silty to sandy ash layer can be burrowed easily when the difference between background sediment and ash is small, whereas the opposite is true for large differences in grain-size.

14 Ash layer preservation and benthic mixing 2005 because theoretically 35 vol% mud is sufficient to envelop all granular material and produce mudlike mechanical behaviour (Mitchell, 1993). Mixing 3 of ash of 50% interparticle porosity with 3 of freshly deposited material could result in an enlarged porosity of 70% (and not the mean of 62Æ5%), the mixture occupying 2Æ4 cm 3. This value is in the range of the vertical extent of Tier I (2 to 3 cm thick). Therefore, ash thinner than has a very low preservation potential even in Province III, where mixing is low. By contrast, in benthic food rich areas, even thicker ash layers can be mixed as Tier II is also densely populated (see above); this applies for Provinces I and IV. There, a mixture of 2 to 3 cm ash with newly arrived material would result in a 6 to 8 cm thick, new soft surface layer. Preservation potential Taking into account local environmental conditions, the thickness of the Pinatubo 1991-ash and the various modes of mixing, its preservation potential can be estimated (Fig. 9). Along the Philippines slope the ash has a very low preservation potential, because plenty of benthic food is available and the endofauna is adapted to a wide grain-size spectrum. In particular, Scolicia producers will destroy an up to 6 cm thick ash layer from below. Later, when a new surface layer has established, mixing from above will affect the upper 2 to 4 cm of the event layer as evidenced by observations on fossil counterparts (Fig. 5). However, in mud-prone areas the base of ash thicker than 3 cm has a high preservation potential because there is no mixing from below. However, severe mixing by ash-penetrating burrows is possible. Within the Manila Trench and the lower reaches of canyons the ash layer will be preserved while rapidly buried by gravity-flow deposits. West of the Manila Trench in the area with restricted wind mixing of surface waters and upwelling, the preservation potential of the ash is quite high because Tier I is thin and faunal abundance in Tier II appears to be low. Slow sedimentation, relatively low average primary productivity and considerable water depth led to a low burial rate of organic matter. Low organism abundance and burrowing activity do not allow complete bioturbation of an event layer even if it resides for several thousands of years within the bioturbated zone. An ash layer exceeding the thickness of Tier I may be preserved, at least its sharp base. However, locally the base may become diffuse due to some mixing in Tier II and ash-penetrating burrows. Further to the west, in the area affected by strong wind mixing of surface waters and upwelling, the 4 to 6 cm thick ash layer will be mixed by the inhabitants of Tier I and Tier II. Thicker ash may be preserved at least partly, exhibiting a sharp base and a mixed top. DISCUSSION The mixing of an event layer depends on the abundance of organisms, their environmental adaptation, the benthic food content and the properties of the event layer. Critical factors are the proportion of the fauna that survived the depositional event and the speed of recolonization. As long as substrate consistency has not significantly changed, re-establishment of the fauna will take place over short time spans, years for microfauna and meiofauna, and probably several years to tens of years for macrofauna as modern observations show, although some changes in faunal composition may occur (Ingole et al., 2005). Recolonization experiments carried out after physical disturbance of the sea floor by large, plough-like equipment clearly demonstrate the importance of the re-establishment of the uppermost, 2 to 3 cm thick, soft surface layer to its preevent conditions (Borowski & Thiel, 1998). This layer houses the most animals and represents the main food source for benthic animals living on and shallowly below the surface and even sometimes for deep burrowers (Borowski & Thiel, 1998). Mixing of ash with newly arrived material at an assumed 1:1 ratio illustrates the importance of the sedimentation rate: a mixed surface layer about 2Æ5 cm thick that roughly comprises Tier I forms at a sedimentation rate of 2 cm kyr )1 in 500 years, at 4 cm kyr )1 in 250 years and at 10 cm kyr )1 in 100 years, respectively. A mixed surface layer as thick as 7Æ5 cm comprising Tiers I and II requires 1Æ5 kyr, 750 or 300 years, respectively. Assumption of a mixed layer thickness of 7Æ5 cm is justified by the data of Kuehl et al. (1993), who found a mixed interval in the range of 6 to 10 cm, derived from excess radioisotopes within the south-western part of the study area, which is environmentally similar to the slope of the Philippines. Compared with the ichnofabrics, this interval comprises Tiers I and II; such a mixed interval would re-establish within 300 to 500 years at the given sedimentation rates.

15 2006 A. Wetzel To test the above predicted preservation potential, fossil counterparts have been examined. Unfortunately, there are only a few published reports on ash layers. Normally, a sharp base and a bioturbated top of fossil ash layers is recorded, but the initial thickness has not been evaluated (e.g. Wei et al., 1998; Bühring et al., 2000). The same is true for almost completely mixed ash layers, where the ash has been used as a tracer for bioturbational mixing, but the initial thickness has not been calculated (Ruddiman & Glover, 1972, 1982). However, the few available studies support the above deductions. It appears that a low sedimentation rate (<4 cm kyr )1 ) favours non-local mixing. Ash-filled burrows that penetrate the ash and displace ash over considerable distances downward are not uncommon (e.g. Lacasse et al., 1996; Löwemark et al., 2004). These observations support the predicted preservation potential in Province III west of the Manila Trench affected by subordinate wind mixing of surface waters and upwelling. The expected preservation in the other provinces is supported by the observations of Löwemark et al. (2004). Ash layers having a sharp base occur where the sedimentation rate is low; these layers exhibit a slightly mixed top, being 2 cm thick, and frequently non-local mixing by burrows penetrating the ash. By contrast, at high average sedimentation rates (ca 15 cm kyr )1 ), Scolicia producers completely mixed ash layers similar to those in the area close to the Philippines. The influence of sedimentation rate on mixing is, however, indirect: burial of organic matter and, hence, the food source for deposit feeders strongly depends on sedimentation rate (Müller & Suess, 1979). The relationships between organic matter burial, bioturbation and oxygenation are affected by multiple feedback (e.g. Hartnett et al., 1998). CONCLUSIONS Ash differs in its properties from deep-marine deposits mainly with respect to the angularity of grains, the absence of argillaceous material and the lack of organic matter. Therefore, ash layers have a higher potential to become preserved at a given thickness than event layers originating from sediment reworking. The ongoing bioturbational mixing of the Pinatubo 1991-ash layer and the observations on already buried event layers allows prediction of the fate of the Pinatubo 1991-ash layer in the South China Sea. With respect to the modern environmental conditions, four distinct potential preservational provinces are distinguished: (I) Slope of the Philippines. In this area organic matter content is high due to fertilization by fluvial dissolved load. The sand-prone background sediment is poorly sorted and carries an adapted fauna. The Pinatubo 1991-ash, although up to 10 cm thick, has a low preservation potential, because the ash layer is mixed from below by echinoids producing Scolicia, and later from above, when a new, 2 to 3 cm thick surface layer is established. Fossil counterparts suggest that ash up to several centimetres thick can be mixed completely under similar conditions. However, where background sediments are mud-prone and uniformly fine-grained, mixing is similar to that in Province IV. (II) Manila Trench and lower reaches of canyons; as depocentres they receive numerous sediment gravity flows, resulting in rapid burial of the ash: by 1999 by 30 cm thick sediments. The ash layer will be preserved. (III) West of the Manila Trench, where wind mixing of surface waters and upwelling is subordinate, the sedimentation rate and organic matter content is low. These conditions are unfavourable for bioturbational mixing. The ash layer will be preserved if thicker than 1 to 2 cm. Even thicker ash will exhibit a sharp base in the fossil record, but a mixed top. Mixing by animals penetrating the ash is common. (IV) Area affected by intense wind mixing of surface waters and upwelling. An ash layer up to 3 to 4 cm thick will be completely bioturbated. A secondary peak of ash will form along the redox boundary at the base of Tier II inhabited by Nereites producers. The high benthic food content supports an abundant fauna in the upper interval of the sea floor. Ash layer preservation is affected strongly by ecological conditions; conversely, the preservational features of an ash layer provide information about the palaeoenvironment. Mixing from below points to a fauna adapted to poorly sorted sandy deposits and to a high benthic food content. Thick bioturbated ash indicates high benthic food levels. The higher the extent of upward mixing, the higher is the benthic food content. Prevailing non-local mixing documented by distinct burrows implies low benthic food levels and probably slow sedimentation. Thin preserved ash implies very low benthic food. Thin mixed ash has no environmental indication.