Andreas Wetzel

Ecologic interpretation of deep-sea trace fossil communities

Abstract Trace fossil types and their vertical arrangement (tiering) are closely related to the biotope, and hence, can provide information on ecological conditions, especially on oxygenation, benthic food content, sedimentation rate, and substrate consistency. However, to use the full potential of trace fossils for environmental analyses, various ichnologic criteria are necessary in addition to trace fossil taxonomy. Based on a taxonomically oriented, ichnologic evaluation of modern deep-sea environments, six ichnologic aspects to ecologic changes have been found to be of major importance: 1. (1) penetration depth of burrows is influenced by oxygenation, benthic food availability and substrate consistency; 2. (2) the number and vertical extension of tiers varies due to environmental changes. A decrease in the vertical extension of tiers (“telescoping”) indicates a decrease in oxygenation, benthic food content and/or sedimentation rate. If shallow-penetrating traces disappear, oxygen may be depleted, and if the deepest tiers disappear, the benthic food content may be lowered; 3. (3) the size (diameter) of specific trace fossils can indicate oxygen and benthic food availability; 4. (4) degree of bioturbation is related to sedimentation rate and benthic food content, and the diversity of the whole ichnocoenoses or within a specific tier is affected by oxygenation, benthic food content, sedimentation rate and substrate consistency; 5. (5) the type of burrow wall (unlined, mucus-lined, pellet-lined) mainly refers to substrate consistency and — subordinately — to pore water oxygenation; 6. (6) the type of sediment displacement by body appendages used for excavation, ingestion, sideward pressing, or outside pumping is related to substrate consistency and grain size.

Morphology and ecological significance of Zoophycos in deep-sea sediments off NW Africa

Abstract Eighty-six large-diameter (15–50 cm) sediment cores taken off the NW African continental slope have been examined for their bioturbation structures. The Zoophycos “spreiten” — burrows appearing in profile as series of curved arcs — were found to be particularly useful for environmental analysis, because they are (1) widely distributed and numerous in Quaternary sediments, (2) easily identifief due to the characteristic structures in vertical and horizontal core sections, and (3) extraordinarily well preserved. Zoophycos is found off NW Africa in water depths > 2,000 m, in sediments with Corg contents of 0.3–1.8%. It is possible to recognize in the core sections all of the morphological spreiten types of Zoophycos which were described from ancient sediments. All of these spreiten are formed with two modifications classified as “U-type” and “J-type”, according to the shape of the basic tube. The distribution of U- and J-types is significantly related to Corg content and probably depends on the available oxygen content in the respiration water. Morphological analysis leads us to assume that sipunculid animals are the creating organisms. Vertical sediment mixing can be measured by using tracer particles derived from single “event” layers. It is explained by excursions of the creating organism and/or by its active movement of the respiration water. Excellent preservation is a typical feature of Zoophycos, which is evident all over the fossil record. This is due to the considerable depth in the sediment at which the organism producing Zoophycos lives. Core sections with maximum frequency of Zoophycos, indicating optimum living conditions, can be correlated over great distances. For these horizons, it is possible to estimate roughly the maximum population density of Zoophycos-creating organisms, yielding 1 animal/100 m2.

The Xigaze forearc basin: evolution and facies architecture (Cretaceous, Tibet)

Abstract The mid-Cretaceous to Eocene flysch deposits of the Xigaze forearc basin in southern Tibet were investigated in a 120 km segment along the Indus-Yarlung suture zone. The basin evolved south of the magmatic arc (Gangdise belt) of the Lhasa block on top of trapped oceanic or transitional crust. Remnants of shelf carbonates are preserved along the northern edge of the basin. The mapped segment was shortened by about 65%; metamorphism reached low-grade conditions along the northern margin of the basin. The flysch sequence reaches a thickness of at least 5 km and consists to a large degree of volcaniclastic (andesitic to dacitic) material shed from the magmatic-arc Gangdise belt. Particularly in the western part of the study area, plutonic and sedimentary rocks from deep erosion levels and/or more distal sources contributed to the basin fill. Rivers from the Lhasa block acted as point sources and fed five major deep-sea channel systems. Turbidity currents in the channels were directed towards the growing accretionary wedge of the subduction zone, thus indicating that the basin was continuously filled up to outer ridge level and gradually shallowing. The forearc flysch is subdivided into at least three megasequences, which begin with wide (up to several km) incised (10 to 50 m), coarse-grained channel fills and their associated fan deposits. The upper parts of the megasequences contain hemipelagic dark shales and marls (deposited above the calcite compensation depth). Lateral channel migration, channel-lobe switching, but also volcanic pulses generated a predominantly fining-upward, high-frequency cyclicity. After continental collision, the marine sedimentation in the forearc basin was replaced by fluvial deposits of the Eocene-Oligocene Qiuwu formation, which is time-equivalent to the Kailas and Indus molasses farther west and rich in coarse gravel derived from the Gangdise belt. Both forearc flysch and Qiuwu formation were deformed simultaneously during the Miocene. We assume that the molasse-type Qiuwu formation represents the final continental facies of the forearc basin filling.

Recent Bioturbation In The Deep South China Sea: A Uniformitarian Ichnologic Approach

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, Planolites, 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, Thalassinoides 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 boundary. Following blooms, however, temporary surface feeding is documented by 1991 ash in the burrows. The high benthic food content and the vertical movements of the Nereites producers probably prevented 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 organic matter reaches the seafloor, surface trails have been observed, especially along the Philippines slope and the area affected by upwelling and intense wind mixing.

Biogenic structures in modern slope to deep-sea sediments in the sulu sea basin (Philippines)

Abstract Slowly accumulated slope and rise sediments (5–20 cm/1000 yr) are totally bioturbated as compared to only 10–30% in rapidly deposited abyssal plain sediments (100–200 cm/1000 yr). In contrast to a decrease in degree of bioturbation, biogenic trace diversity increases with water depth. All ichnocoenoses are clearly influenced by the low oxygen content of the bottom water caused by minimal circulation and restricted water exchange. Oxygen deficiency and relatively high sedimentation rates have created nutrient-rich sediments. Thus, there is no need for optimal fractionation into individual ecological niches and behavioral programs are not highly developed: (1) in slope and rise sediments, animal traces are associated with biodeformational structures; (2) in slope to abyssal plain sediments, trace assemblages are dominated by burrows with open tubes for circulation of respiration water; (3) in abyssal plain sediments, small-diameter biogenic traces common in turbidite sequences (graphoglyptids, e.g., Paleodictyon) are absent. This may be due to rapid sedimentation leading to inefficiency of graphoglyptid behavior. Where graphoglyptids are lacking, ichnocoenoses are dominated by forms like Phycosiphon. In slope and rise sediments, graphoglyptids may have been destroyed by deeper burrowing organisms.

Deep-sea benthic food content recorded by ichnofabrics; a conceptual model based on observations from Paleogene flysch, Carpathians, Poland

Fluctuations in the input of organic matter into a deep-sea environment can be deciphered from ichnofabrics if the continuous processes of hemipelagic sedimentation and bioturbation are episodically interrupted by turbidite deposition. Below turbidites, the bioturbated zone may be preserved almost intact; the benthic food content in such frozen tiers can be interpreted from the ethology, size, penetration depth, and density of trace fossils. Especially high benthic food content is characterized by (1) dark sediment color, (2) complete bioturbation, (3) high density of burrows produced near-surface in several levels, (4) rarity or absence of graphoglyptids, and (5) deep tiers completely bioturbated by feeding burrows having an open connection to the surface. Oxygen-deficiency can be eliminated as a reason for the black color of the sediment; the size, diversity, and penetration depth of the trace fossils are similar to that of modern and fossil counterparts formed under well-oxygenated settings that are not restricted in benthic food. An increase in sedimentation rate enhances the burial of benthic food and is indicated by the downward extension of the bioturbated zone and by increasing burrow density. Our analysis suggests that several other black shale occurrences may result from increased organic matter input rather than from water-column anoxia due to sluggish circulation. Our model is supported by similarities with other deposits that contain many frozen tiers.

Bioturbation in deep-sea fine-grained sediments: influence of sediment texture, turbidite frequency and rates of environmental change

Summary In the deep sea, burrowing infauna can adapt to variation in sediment texture within < 100–300 years. When there is continuous sedimentation, a continuous bioturbation process also takes place and the biogenic trace association found in the fossil record may be dominated by deeply penetrating burrows. A low-diversity trace fossil association (Zoophycos ichnofacies) is typical. On the other hand, when turbidites interrupt the continuous bioturbation process, a highly diverse trace fossil association is often found in the fossil state that also contains highly organized near-surface traces (Nereites ichnofacies). This biotope is usually controlled by the non-turbiditic sedimentation rate and the recurrence time of turbidites. Biogenic trace assemblages are mainly controlled by the nature of sedimentation and other environmental conditions rather than by water depth. Therefore, the widely accepted use of trace fossils as palaeobathymetric indicators in the deep sea may be restricted.

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

Abstract Nereites ichnofabrics (comprising N. missouriensis) occupy the upper 3–7 cm in brown, oxidized, soft to soupy, uniformly 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 appear to be guided chemotactically as they maintain a consistent distance relative to the redox boundary, despite variation 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 Nereites 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-producing animals. However, the small amount of ash in the burrows demonstrates only subordinate use of surface sediments, probably during periods of enhanced particle flux following upwelling. Based on the number of ash-filled burrows and the number of upwelling periods, an average population density of the Nereites producers of 5–6 animals per m2 is estimated.

Deep-Sea Ichnology: The Relationships Between Depositional Environment and Endobenthic Organisms

Trace fossils and bioturbational structures are import ant component of deep-sea sediment fabric. Various ecological factors controlling distribution of their producers can be recognized Therefore, they curry important data on palaeoenvironmental parameters, such as mode of deposition, oxygenation, trophic level, bathymetry, rate of sedimentation, substrate consistency, direction of current flow and others. Trace fossils in turbiditic environment can be grouped in the pre- and post-depositional forms. They belong to the Zoophycos and Nereites ichnofacies. Within the latter, roughly, the Ophiomorpha rudis–Paleodictyon–Nereites ichnosubfacies may express a bathymetric trend from inner to outer deep-sea fan. Trace fossils were subjected to evolutionary processes influencing their producers. This is reflected in composition of trace fossil assemblages, overall changes in diversity and environmental shift of some individual ichnotaxa.

A HIGHLY DIVERSE ICHNOFAUNA IN LATE TRIASSIC DEEP-SEA FAN DEPOSITS OF OMAN

Abstract We encountered a highly diverse ichnofauna within the deep-sea fan deposits of the Upper Triassic Al Ayn Formation in Oman. It comprises 32 ichnogenera: 18 ichnogenera represent predepositional graphoglyptids and other trace fossils that are preserved as casts on turbidite soles, and 14 ichnogenera represent postdepositional trace fossils that penetrate turbidite beds. The relatively large size of the area studied certainly favors encountering a high number of ichnogenera. The diversity we found approximately doubles the value that has often been stated in the literature and contradicts the paradigm that the Triassic represents a time of low ichnodiversity in the deep sea. Although the data are limited, in general the recovery of deep-sea tracemakers has been very slow owing to environmental disturbances that resulted from cold-bottom-water circulation after the Carboniferous–Permian glaciation. The high ichnodiversity in the Al Ayn Formation is explained by its paleogeographic position and locally formed warm bottom waters. The Al Ayn deposits accumulated adjacent to wide evaporitic and carbonate shelves, indicating continuous warm conditions. The Al Ayn clastic system was likely influenced by dense, salt-rich, warm water flowing back to the ocean from the carbonate and evaporitic shelf area. The downwelling water may have reduced the effects of cold water that formed during the Late Paleozoic glaciation and the Permian–Triassic anoxia, and, thus, it may have provided a refuge habitat. Despite the global trend of low-diversity deep-sea ichnocoenoses, refuge habitats may have been established in areas less affected by the otherwise harsh conditions.