Acta Geologica Polonica, Vol. 60 (2010), No. 1, pp. 125–138
Deep-sea ichnology: Observations in modern sediments
to interpret fossil counterparts
ANDREAS WETZEL
Geologisch-Paläontologisches Institut der Universität Basel, Bernoullistrasse 32, CH-4056 Basel, Switzerland.
E-mail: [email protected]
ABSTRACT:
WETZEL, A. 2010. Deep-sea ichnology: Observations in modern sediments to interpret fossil counterparts. Acta
Geologica Polonica, 60 (1), 125–138. Warszawa.
Extensive areas of the abyss represent a dynamic environment experiencing seasonally strongly fluctuating organic-matter deposition that in turn affects the oxygen content of the pore water. At high organic-matter deposition, oxygenation of the pore water decreases and forces organisms respiring this water to move upward. Thus,
times of benthic food richness on the seafloor affect the behaviour of endobenthic organisms; aside from deepdeposit feeding, temporary surface feeding (including unselective bulldozing) represents an additional nutritional strategy. This has been shown for the producers of Nereites and Scolicia as well as Thalassinoides and
Zoophycos, the latter two have an open tube. Each of these activities leads to intense sediment mixing and prevents or disturbs the formation of near-surface burrows including graphoglyptids. The distribution of organic
matter in the sediments is reflected by the orientation and geometry of Phycosiphon. Quantity and quality of food
appear to be related to abundance and size of Scolicia. Food selectivity, the ability of selective feeding and organism mobility all appear to be important factors in benthic ecology, however, they are as yet little known. To
use the full potential of uniformitarian studies relying on cores taken in soft sediments, they should be based on
X-ray radiographs, contain information about the timing of burrow production and focus on ichnotaxonomically determinable burrows.
Key words: Deep-sea; Abyssal sediment; Organic matter deposition; Bioturbation;
Trace fossils.
INTRODUCTION
Biogenic sedimentary structures are autochthonous
indicators of environmental conditions. Hence, trace
fossil communities have been the subject of ecological investigations for several decades (e.g. Abel 1935;
Schäfer 1962; Crimes and Harper 1970, 1977; Ekdale
et al. 1984; Curran 1985; Pemberton et al. 2001; Miller
2007). In many cases, a biotope can be characterized
better by its bioturbation structures than by other
palaeontological constituents, especially in the abyss
(e.g. Leszczyński 1991; Wetzel 1991; Wetzel and Uchman 1998).
For at least sixty years, the abyss has been increasingly studied. When the fauna in the deep-sea has been
found to display an extreme high diversity, it was explained by the so-called time-stability hypothesis (Sanders
1968): The deep-sea represents a long-term stable environment and provides optimal conditions for a maximum
subdivision of the habitat into ecological niches that finally resulted in a large number of species. This hypothesis has also been invoked to explain the extremely high
trace fossil diversity in the abyss (e.g. Seilacher 1977).
However, during the last three decades evidence has been
growing that the modern deep-sea floor represents a dynamic habitat experiencing strong fluctuations.
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ANDREAS WETZEL
It is the purpose of this paper to exemplify the dynamics of the abyssal environment by the fluctuating
deposition of organic matter and its effects on the benthic habitat and to describe its implications for the interpretation of trace fossils. Trace fossils are produced
by macro-organisms having two requirements: oxygen
for respiration and organic matter for food. Consequently, most trace fossil producers respond sensitively
to changes of one or both factors. To apply the findings obtained in modern abyssal sediments to interpret
fossil counterparts is not straightforward, because modern sediments are soft, lack diagenetic enhancement of
structures and are studied in small-diameter cores.
These aspects are addressed first.
ABYSSAL SEDIMENTS AS HABITAT
The habitat of seafloor sediments exhibits a vertical subdivision into zones; with respect to bioturbation, geochemical, biological and sedimentological
aspects are distinguished that in turn affect the ichnologic record (Text-fig. 1). Though all these factors are
interrelated, they are addressed separately.
Geochemically – two main intervals can be distinguished; the pore water of near-surface oxic deposits
contains dissolved O2, whereas anoxic deposits below
lack dissolved oxygen. Oxic and anoxic deposits are
separated by the redox boundary (e.g. Froelich et al.
1979). A closer look shows that the redox boundary is
modified by burrowing activity; especially around
burrows and tubes, local redox zones can develop (e.g.
Forster 2006). Within the interval comprising the
redox boundary, the valence states of iron, manganese
and sulphur are also converted repeatedly by microbes, thus, adjacent to the redox boundary the microbial activity is enhanced (e.g. Konhauser 2007).
Biologically – organism abundance and biomass
decrease with depth in sediment (e.g. Rowe 1983).
Therefore, the time required to bioturbate a sedimentary layer increases with depth in sediment, in particular because the endofauna is often patchily
distributed (e.g. Jumars and Ekman 1983). Furthermore, there is no good information about a subdivision of the endobenthic habitat (e.g. Gage and Tyler
1991). Unfortunately, there is only little information
about the sparse deep-burrowing organisms that imprint the ichnofabric.
Sedimentologically – the surface sediments are affected by benthic organisms that mix newly arriving
particles and underlying deposits. For palaeoclimatic
or palaeoceanographic analyses, the effects of benthic
mixing have been modelled applying the “mixed layer
concept” (Berger and Heath 1968). Within the surface
mixed layer, the sediment is churned by organisms so
effectively that even short-lived radio-isotopes exhibit
a roughly constant concentration; below the mixed
layer, the radionuclide concentration decreases (e.g.
Thomson et al. 2000; Text-fig. 1). For modern sediments it has been shown that mixing is strongly affected by grain size, sedimentation rate, organic matter
flux to the seafloor and rate of organic matter burial
(Wheatcroft 1990; Kuehl et al. 1993; Trauth et al.
1997). However, the mixed-layer concept only ad-
Text-fig. 1. Subdivision of the seafloor sediments. 1 – Oxygen content within the seafloor; the redox boundary marks the transition from oxic to anoxic
deposits. 2 – Schematic representation of the biomass within the seafloor. 3 – Concentration profiles of short-lived radio-isotopes suggesting a homogeneously mixed layer and no mixing below. 4 – Bioturbation intensity within a multilayer model using sediment particles as tracers. 5 – Subdivision into tiers based on the crosscutting relationships; traces crosscutting each other belong to one tier, and traces of deeper tier crosscut those of
shallower ones; though deep-burrow production can be too low to affect radio-isotope distribution, in fact it dominates the fabric
DEEP-SEA ICHNOLOGY
dresses the turnover of sedimentary particles, but not
the resulting bioturbational structures, i.e. ichnofabric.
Ichnologically – it appears from crosscutting relationships of the different burrows that they are tiered.
Tiering implies a subdivision of the available ecospace
wherein various endobenthic organisms occupy different depth levels within the seafloor at the same time
(e.g. Wetzel 1981, 1984; Ausich and Bottjer 1982).
The resulting ichnofabrics document bioturbation at
all scales and they include all aspects of the texture
and internal structures of marine sediments (Bromley
and Ekdale 1986).
MODERN SEDIMENTS AND THE ROCK
RECORD
Applying ichnologic findings obtained by investigations of soft sediment cores to fossil counterparts
faces some problems, four of which are of major importance. They are briefly outlined and possible solutions are suggested.
Observational barrier
In fresh sediment cores burrows are often hardly
visible, although some exceptions occur, especially at
or near the redox boundary.
The lack of visibility can be overcome by using Xray radiographs, which exhibit many details that are
otherwise not seen in fresh sediment. Optimal images
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are acquired by X-raying thin, about 1-cm thick sediment slabs (Werner 1967; Wetzel 1981; Löwemark
2007; Text-fig. 2).
Preservational filter
Modern abyssal sediments are normally completely bioturbated when slowly accumulating in oxygenated settings. Endobenthic organisms occupy
different depth intervals within the sediment, thus, the
deeper a burrow is produced below the surface, the
higher its preservation potential. Consequently, surface traces have a very small chance of being preserved. For deep burrows it is always worth asking
whether they document a response to the properties of
the host sediment, to conditions on the seafloor, or
both (see Wetzel 1981; Löwemark 2007).
The preservational filter is switched off by non- or
low-erosive event deposits preventing further bioturbation below, for instance volcanic ash or distal turbidites. However, even below modern event layers, the
probability of detecting surface traces is low. Surface
traces can only be observed on the sediment surfaces
(e.g. Ekdale and Berger 1978; Kitchell et al. 1978;
Gaillard 1991; Wetzel 2008).
Production dilemma
The abundance of burrowing animals in the deep
sea is so low that seldom a trace producer has been
caught in action. Therefore, it is not clear whether a
Text-fig. 2. Comparison between fresh sediment core and X-ray radiograph of a 1 cm thick sediment slab (Institute of Geosciences, Kiel, Germany, core 13239-1, 87–100 cm, 13°53′ N, 18°19′ W, 3156 m water depth, off NW Africa). 1 – fresh sediment core; note the poor visibility of
structures. 2 – X-ray radiograph (negative; sand appears light, mud dark); PP = Planolites reworked by the Phycosiphon producer, Sc = Scolicia,
Z = Zoophycos
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ANDREAS WETZEL
Text-fig. 3. Comparison between mixing profile based on radio-isotopes and ichnofabric off NW Africa. 1 – Pb radio-isotope profile of core
KG 06 (20°34′ N, 18°09′ W; 1019 m water depth; redrawn from Legeleux et al. (1994); 210Pb has a half-life of about 22 years, the actively mixed
zone is about 3 cm thick. 2 – X-ray radiograph (negative) of a nearby core (Institute of Geosciences Kiel, Germany, core 13588-2; 19°37′ N,
17°15′ W; 1000 m water depth). Evidently, the mixed zone (0–3 cm core depth) makes up only the upper tier of the ichnofabric. All this tracefossil producing, deep-burrowing activity does not affect the radio-isotope profile at all. The simple consequence of this comparison is that the
deep-burrowing animals are so sparse that they rarely affect the profile of short-lived radio-isotopes
burrow is in fact produced at the present time or has
been produced in the past. This dilemma becomes evident when mixing profiles and X-ray radiographs are
compared. Often the mixed zone as derived from
radio-isotopes only comprises tier I (homogeneous
layer; Wetzel 1981, 1991) and maybe tier II, although
a higher number of tiers can be deduced from X-ray
radiographs (Text-fig. 3). Infrequently, deep-tier burrowing activity affects the radio-isotope profile in
pelagic settings (e.g. Trauth et al. 1997). Therefore,
deep-burrowing animals must be so sparse as to rarely
affect the profile of short-lived radio-isotopes. Alternatively, deep-burrowing organisms do not transfer
sediment particles vertically. The latter case, however,
is not very likely, because the deep-burrowing organisms need to maintain an open connection to the
seafloor for respiration and there is strong evidence
that they may feed on the sediment surface episodically (see below). In combination with chronometric
age data, the thickness of the homogeneous layer represents several hundreds of years during which no
deep-burrower from below passed by.
The recent production of burrows is evident if they
contain marker particles such as short-lived radionuclides or recently deposited ash. An ideal example for
this purpose is the South China Sea, where an extensive ash layer was deposited after the Pinatubo eruption
of 1991. If ash occurs in a burrow, then it was produced
in modern times and the production of the burrow can
be related to modern environmental conditions.
Taxonomical uncertainty
Important features for classifying burrows taxonomically are largely lacking in soft sediment cores.
The three-dimensional geometry of many burrows remains unclear because the cores are too narrow. Furthermore, the outer boundaries of burrows cannot be
analysed, because the sediments are soft and bedding
planes have not yet developed.
Two concepts will help to overcome these problems: the use of ichnofabrics and AID (Always Ichnotaxonomically Determinable) burrows.
Ichnofabrics comprise all aspects of the texture
and internal structure of a sediment that result from
bioturbation at all scales. Thus, the ichnofabric concept allows comparing “the pattern” of the bioturbated
deposits, modern and ancient. This is especially useful if the deposits are completely bioturbated, as in so
many modern deep-sea deposits. In that case, it is normally impossible to classify all burrows – which is
also true for fossil counterparts (Text-fig. 4). To prove
the similarity between modern ichnofabrics and fossil counterparts, it is not necessary to identify every
DEEP-SEA ICHNOLOGY
129
fresh sediments, such as Gyrolithes, Helicodromites,
Tasselia, Thalassinoides and Zoophycos; and burrows
that are only seen in X-ray radiographs, such as Chondrites, Phycosiphon, Nereites, Scolicia and Teichichnus; thus, the latter burrows are not as often described
from modern sediments.
ORGANIC MATTER DEPOSITION
Text-fig. 4. Ichnofabrics in Jurassic and modern bathyal deposits. 1
– Lower Jurassic Allgäu Beds (Oberjoch, Bavaria, Germany) bs =
biodeformational structures, Ch = Chondrites, P = Planolites, Z =
Zoophycos. 2 – Modern deposits off NW Africa (Institute of Geosciences, Kiel, Germany, core 13239-2, 98–108 cm, 13°53′ N, 18°19′
W, 3156 m water depth); the ichnofabric exhibited by the X-ray radiograph (negative) is similar to the Jurassic deposits with respect to
general pattern as well as tiering although Chondrites is replaced by
Phycosiphon; bs = biodeformational structures, P = Planolites,
PP = Planolites reworked by Phycosiphon producer, Z = Zoophycos
ichnotaxon. Therefore, when doing so, the analysis is
subject to some uncertainty.
Only a few taxa (AID burrows) can always be ichnotaxonomically classified; the most useful are smallsized burrows and those that exhibit a characteristic
geometry or distinct internal structures. Two groups
are distinguished: burrows that can be recognized in
Although considerable research has been done on
the preservation of organic matter, the mechanisms for
its preservation remain a matter of debate (Morse and
Beazley 2008 and references therein). A confounding
consideration is that several possible controlling factors may commonly co-vary so that one factor, such
as oxygen exposure time, incorporates several other
possibly important environmental variables (e.g. Hartnett et al. 1998). Even so, deposition of organic matter on the seafloor and its effects on the benthic
habitats are here considered from source to sink for
slowly accumulating deep-sea deposits not affected by
currents.
For broad areas of the oceans the organic matter
production fluctuates significantly during the year in
response to the seasonality of insolation, wind stress
and circulation; such variations are more or less pronounced (e.g. Antoine et al. 1996; Lutz et al. 2007).
Depending on water depth, the slowly sinking organic
particles reach the seafloor after 2–4 weeks (e.g. Smith
et al. 1996; Wiesner et al. 1996; Text-fig. 5). While
settling, depending on water depth and oxygenation
of the water body, a considerable portion of the organic particles is oxidized (e.g. Suess 1980; Tyson
2001). In this way, the productivity signal becomes attenuated with increasing water depth, but corresponds
nonetheless to primary production. Thus, the organic
matter deposition on the seafloor can fluctuate considerably with time (e.g. Lampitt and Antia 1997; Lutz
et al. 2002). The deposition of phytodetritus is a major
energy source to the benthos and has been linked both
to seasonal patterns of growth and to reproduction
(Tyler 1988; Carney 1989) as well as regional variability in benthic biomass (Thurston et al. 1994). A
surplus of benthic food results in enhanced benthic activity; microbes and other organisms respond immediately (e.g. Gooday and Turley 1990).
Sediment trap data point to a close relationship between organic matter and siliciclastic material settling
through the water column (e.g. Deuser et al. 1983).
This observation led Hedges and Keil (1995) to propose that organic matter becomes adsorbed or bound
onto siliciclastic particles, mainly clay minerals, while
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ANDREAS WETZEL
Text-fig. 5. Timing of organic matter accumulation on the seafloor
after a bloom in primary production and the effects on pore water
oxygenation in sediments in about 4000 to 5000 m water depth,
Northern Hemisphere, outside the mid-ocean gyres; compiled from
different sources, especially Wiesner et al. (1996), Smith et al.
(2000) and unpublished data of M. Wiesner (Hamburg)
settling to the seafloor. This deduction is supported by
the observation that the organic carbon content of settling particles and their size are inversely related
(Oliveira et al. 2007).
On the seafloor, short-term and long-term benthic
response on organic matter deposition can be observed
(Text-fig. 5).
In the short term, following blooms, organic flocs
can cover the seafloor (e.g. Smith et al. 2002, Wetzel
2008). Concomitantly, when oxygen is consumed
within this organic rich layer, the oxygen flux into the
sediment decreases rapidly (e.g. Gehlen et al. 1997).
Measurements in oceanic deposits imply that decrease
in oxygen flux into the sediment may lag behind organic matter deposition by 2–3 weeks (e.g. Smith et
al. 2002). When the oxygen flux decreases, the redox
boundary separating oxic and anoxic deposits moves
upward. In this way, the organic depositional signal
propagates within some tens of days into the sediments (Soetaert et al. 1996) and affects in particular
those organisms that take their respiration water from
the pore space. Locally, around open tubes, the oxygen
flux into the sediment can be enhanced for some hours
to a day when the inhabiting organisms are active
(Forster 1996).
In the long term, in slowly accumulating deep-sea
settings with sedimentation rates less than 5 cm/ky the
organic carbon content of the sediment depends
mainly on sedimentation rate (Müller and Suess 1979;
Tyson 2001); above that rate a dilution effect occurs
(Tyson 2001). The burial velocity of organic matter in
fact influences the exposure time of organic matter to
the oxygen (e.g. Hartnett et al. 1998). A feedback
mechanism, however, complicates this relationship,
because with increasing benthic food content, the
long-term benthic standing stock and the burrowing
activity expressed as benthic mixing rate both increase
(e.g. Legeleux et al. 1994). Also, a surplus of labile
organic matter is mixed downward; using chlorophyll
a as a tracer of fresh phytodetrital biomass, this labile
pigment may penetrate as deep as 5 cm into the sediment and have a long half-life (months to years)
(Mincks et al. 2005).
Increased mixing in turn leads to an enhanced oxygen flux into the sediment (e.g. Reimers et al. 1986).
This finding is supported by the observation that the
thickness of the mixed layer (exhibiting a nearly constant value of excess radio-isotopes, e.g. Boudreau
1986; Text-fig. 1) is closely related to the flux of organic matter to the seafloor determined for the corresponding interval (e.g. Trauth et al. 1997). However,
it needs to be taken into account that a non-negligible
proportion of organic matter is decomposed within the
surface layer of the seafloor as indicated by organic
carbon burial efficiency (Betts and Holland 1991;
Tyson 2001).
ICHNOLOGIC IMPLICATIONS OF ORGANIC
MATTER DEPOSITION
Within oxygenated settings, the delivery of organic
matter provides the primary energy source for benthic
organisms with exception of seep communities and
sulphide- or methane-utilizing organisms (e.g. Dando
et al. 2008). Variation of organic matter deposition on
the seafloor affects oxygenation within the sediments
(see above). The endobenthic organisms respond in
various ways; in particular, the effects of seasonally
fluctuating delivery of organic matter are considered
here with reference to examples from the South China
Sea, where burrows contain Pinatubo 1991 ash.
Hence, they are recently produced and can be closely
tied to modern environmental conditions.
The western part of the South China Sea is affected by seasonal upwelling and wind-induced fertilization of the surface waters, mainly during the
summer monsoon (Liu et al. 2002; Xie et al. 2003).
During upwelling periods, the organic matter flux is
about 3–4 times higher than during other times (e.g.
DEEP-SEA ICHNOLOGY
131
Text-fig. 6. Schematic representation of the effects
of seasonal organic matter deposition, resulting
oxygenation within the sediment, and effects on
the redox boundary, and possible influence on the
Nereites producers
Wiesner, 1996). This area has a value of >0.3 for the
seasonality index of primary net production (= annual
standard deviation of primary production/average primary net production) as determined from satellite images (Lutz et al. 2007). During periods of high
primary production so much material is provided that
the abyssal seafloor is covered with organic flocs,
whereas the nearly clean Pinatubo 1991 ash makes up
the sediment surface during non-bloom times (Wetzel 2008, fig. 9). Because of organic matter deposition, the oxygen flux into the sediment is lowered and
the producers of Nereites respond;
they are quite sensitive to oxygenation
within the sediment while feeding
about 1 cm above the redox boundary
in non-bloom times (Wetzel 2002).
After the arrival of organic matter,
they move upward and feed on the
surface, and displace some surface
sediment downward as evidenced by
1991 ash in these Nereites (Wetzel
2002; Text-fig. 6).
The geometry of different ichnospecies of Nereites (following the
taxonomy of Uchman 1995) can be explained appropriately by these observations: If organic matter deposition fluctuates seasonally, Nereites missouriensis is
produced in response to fluctuating oxygenation.
However, if organic matter supply to the seafloor is
fairly constant, preferably horizontal Nereites irregularis (formerly Helminthoida labyrinthica) is formed
(Text-fig. 7).
Generalising this deduction implies that the vertical undulation of trace fossils produced by organisms
taking their respiration water from the pore space can
Text-fig. 7. Nereites types showing inferred response to organic matter deposition on the seafloor. 1 – Nereites missouriensis (“Scalarituba” type)
in sediments of the South China Sea (RV “Sonne” cruise 132 core 11-2, 13°50′ N, 116°48′ E, 4249 m water depth, central South China Sea), Xray radiograph (negative) vertical section; PA Pinatubo 1991-ash (= seafloor), Nv vertical tube of Nereites filled with 1991-ash, Nt Nereites tube
close to the redox boundary filled with 1991-ash. 2 – Nereites missouriensis, Permian, Oquirrh Basin, Spanish Fork Canyon, Utah; fossil equivalent to specimen shown in 1. 3 – Nereites irregularis (“Helminthoida labyrinthica” type) reflecting an inferred nearly constant organic matter
deposition; Helminthoid Flysch, Simmental, Switzerland
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ANDREAS WETZEL
be used as an indicator for fluctuation of organic matter deposition in a constantly oxygenated setting. If all
ichnogenera indicate a pronounced upward and downward movement of their producers, a marked seasonality of organic matter input is very likely. The other
extreme is that if all burrows, including the near-surface ones, are horizontal, then the organic matter input
may not fluctuate strongly with time.
Pronounced up-and-down movements of endobenthic animals certainly disturb the near-surface
burrows and sometimes their producers. This may explain the fact that, in areas affected by pronounced
seasonality of primary production, graphoglyptids are
sparse or even lacking. In fact, graphoglyptids have
been observed in the modern abyss only where the
seasonality of primary production is low (Table 1).
However, there is convincing evidence that trace
fossil producers having a permanent open connection
to the seafloor also respond sensitively upon episodic
deposition of organic matter. The chronometric age of
Zoophycos spreite fill has been found to be significantly younger than the host sediment, and hence suggests episodic surface feeding of the producers
(Löwemark and Werner 2001; Leuschner et al. 2002;
Löwemark and Grootes 2004). This neatly explains
the ash infill found in Zoophycos spreiten (Kotake
1989, 1991). Similarly, the fill of some Thalassinoides
in the deep South China Sea records at least temporary surface feeding of the producer (Kaminski and
Wetzel 2004).
Regardless, the apparent relation between fluctuating organic matter deposition and up-and-down
movements of the burrowers has to take into account
the mobility of the trace fossil producers. The producers of Scolicia, today mainly echinoids, appear to
respond to seasonal matter deposition along the Philippines slope to the South China Sea. There, primary
production is fuelled by nutrients delivered by runoff
from the Philippines during heavy summer monsoon
precipitation. The seasonality index of net primary
Area
Graphoglyptids
Central Atlantic
Central Atlantic
Paleodictyon
Cosmorhaphe
Paleodictyon
Spirorhaphe
Cosmorhaphe
Spirorhaphe
Paleodictyon
Urohelminthoida
Central Pacific
Southeastern Pacific
E of New Caledonia
production is ~0.4 (Lutz et al. 2007). Marked up-anddown movements of the Scolicia producers are documented as they burrow through even thick Pinatubo
1991 ash (Text-fig. 8; Wetzel 2009). This implies, as
in the case of Nereites producers, additional surface
feeding. The Scolicia producers are certainly affected
by the oxygenation of the sediment because they can
only burrow for a considerably short time in oxygendeficient deposits (e.g. Bromley et al. 1995). Thus, upand-down movements may indicate a seasonal
deposition of organic matter. In contrast, outcrops
show some Scolicia with minimal vertical undulation
(Uchman 1998; Text-fig. 8), which instead imply constant organic matter deposition, though not with complete certainty. Organisms that can burrow and move
rapidly over large distances, such as the Scolicia producers, are not very suitable indicators of constant organic matter deposition. Extended horizontal burrows
can result from selective feeding and the high mobility of the Scolicia producers, which enable them to
rework a wide horizon within a short period of time,
and, hence, without any relation to organic matter deposition.
Selective feeding and high mobility are advantageous to the Scolicia producers. In turn, these trace
fossils provide further hints about the (palaeo)ecologic
setting; it appears that the abundance of Scolicia in
sediments along the Philippines slope is related to the
amount of benthic food, whereas their size is indicative of the food quality (Wetzel 2008). These findings
are supported by observations of echinoids in the
North Sea that show relationships between food quality and organisms’ size on the one hand, and amount
of food and organisms’ abundance on the other (e.g.
Kröncke et al. 2006).
Seasonal or permanent high organic matter availability favours unselective surface feeding as indicated
by the ingestion of volcanic ash; an unselective feeding or even bulldozing nutritional strategy is followed
at least during “foodbank” times when plenty of benSeasonality index *
(0.1 very low
0.5 high seasonality)
<0.1
<0.15
Reference
<0.1
Ekdale (19890)
Ekdale et al. (1984)
Gaillard (1991)
<0.15
Rona and Merrill (1978)
Ekdale (1980)
Ekdale et al. (1984)
* Seasonality index after Lutz et al. (2007): Seasonality Index of Net Primary Production =
(variance of net primary production/average net primary production)
Table 1. Occurrence of graphoglyptids in areas with low seasonality of net primary production
DEEP-SEA ICHNOLOGY
thic food is available (Glover et al. 2008). Then a feeding specialisation is superfluous (e.g. Wigham et al.
2008). The high amounts of labile organic material entering the benthic system may well allow for deposit
feeders to readily consume detrital organic material
without the apparent fine-scale separation of the resource at the alimentation level as observed in the Porcupine Abyssal Plain (Wigham et al. 2003). These
findings support previous observations that, in those
parts of the upwelling area off NW Africa where the
upwelling intensity and organic matter content of the
sediment are highest, only biodeformational structures
Text-fig. 8. Scolicia as indicator of benthic food enrichment.
1 – Scolicia in sediments of the South China Sea (RV “Sonne” cruise
132, core 35-1, 0–13 cm; 13°37’ N, 119°58’ E, 3321 m water depth,
South China Sea off the Philippines); the producers of Scolicia (Sc)
have multiply penetrated the Pinatubo 1991-ash (light material),
probably in response to organic matter deposition on the seafloor;
thus, the 1991-ash layer has been destroyed; x artefact (hole due to
preparation). 2 – Horizontal Scolicia on the sole of a turbidite indicates selective feeding of the producers rather than constant organic
matter deposition on the seafloor because the Scolicia producer is
very mobile (Zumaia, Spain, Eocene, collected from scree along
coast)
133
are present, whereas trace fossils documenting a specialised behaviour are nearly absent (Wetzel 1981,
1983, 1991).
High organic matter deposition – seasonal or not –
favours the establishment of the redox boundary at
shallow depth within the sediment and, hence, may
support alternative nutritional strategies. In the deposits along the Philippines slope, Chondrites has
been found in two variants: as empty tubes within the
1991 ash and as ash-filled tubes below the ash (Textfig. 9). At first sight it appears that the Chondrites producers followed different nutritional strategies.
However, on closer inspection it becomes evident that
Text-fig. 9. Chondrites in and below the Pinatubo 1991-ash reflecting a similar nutritional strategy although differences in host sediment and tube fill (for details see text and Wetzel 2008). X-ray
radiograph, negative (RV “Sonne” cruise 114, core 02-1, 0–18 cm,
13°55′ N, 119°45′ E, 2370 m water depth, Philippines slope to the
South China Sea). Open Chondrites (Ch1) in 1991-ash (PA) that fills
a hole; below biodeformational structures (bs), Thalassinoides
(Th) and Chondrites filled with 1991-ash (Ch2)
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ANDREAS WETZEL
Text-fig. 10. Phycosiphon producer are inferred highly selective deposit feeders. 1 – Phycosiphon formed along thin, fine-grained laminae enriched in organic matter (ODP Leg 116, Site 718 C, core 78X-5, 124–126 cm; 01°01′ S, 81°24′ E, 4730 m water depth; Bay of Bengal). 2 – Phycosiphon producer having reworked pre-existing burrows with a higher organic content than surrounding sediment (Institute of Geosciences,
Kiel, Germany, core 13239-2, 99–101 cm, 13°53′ N, 18°19′ W, 3156 m water depth, off NW Africa). 3 – Phycosiphon with vertically spread spreiten indicating homogeneous distribution of organic matter (RV “Sonne” cruise 132 core 35-1, 27–29 cm, Sea, 13°37′ N, 119°58′ E, 3321 m water
depth, South China Sea off the Philippines)
the differences are small. If ash thickness exceeds 3
cm, the pore water below the ash becomes anoxic because diffusion length to the bottom water increases
so much (Haeckel et al. 2001). Therefore, the ashfilled tubes within the mud collect anoxic pore water
and the open tubes within the ash do likewise. Thus, it
is highly likely that the Chondrites producer utilizes
anoxic pore water for nutrition.
In the above outlined cases, fluctuations in organic
matter are related mainly to seasonal changes. However, episodic events also affect organic matter supply. In continental slope settings, mass flows transport
sedimentary particles and organic matter episodically.
The distribution of organic matter within these deposits depends on the flow type; in dilute currents, organic matter is enriched in thin layers, whereas in
highly concentrated suspension current deposits it is
homogeneously distributed within a considerable interval. The organic matter within these and other event
deposits, such as tempestites, represents benthic food.
Its distribution is documented by trace fossils that
have been formed by selectively feeding organisms.
In particular, the Phycosiphon producers appear to selectively rework intervals enriched in organic rich
matter, such as fine-grained parts of laminae, and finegrained or faecal fill of burrows (Text-fig. 10). In contrast, Phycosiphon shows vertical or inclined spreiten
and undulates where organic matter is uniformly distributed (Text-fig. 10). The overall arrangement of
Phycosiphon is useful to detect sedimentary layering
even where not macroscopically visible. In turn, the
sediment transport process can be inferred from the
occurrence and orientation of Phycosiphon (Stow and
Wetzel 1990; Wetzel and Balson 1992; Wetzel and
Uchman 2001; Wetzel 2008). Feeding selectivity is
suggested as a major advantage of the Phycosiphon
producer.
PERSPECTIVE
Deposition of organic matter and its fluctuation
with time, activity of burrowing organisms, oxygenation of the sediment, sedimentation rate, and
burial rate of organic matter are so complexly interrelated that these processes are not quantitatively understood today (Morse and Beazley 2008). In
addition, food preferences of burrowing organisms
are not really known, in particular for the abyss. Especially in seasonal or long-term, food-restricted settings, benthic organisms develop food selectivity;
recent studies suggest that there is a preference of
different organisms for different organic compounds
(Wigham et al. 2008). This selectivity certainly influences burrowing behaviour and, hence, the ichnologic record. Furthermore, it turns out that in the
modern abyss the mobility of different species plays
an important role in food resource partitioning because of the ability of different species to move between food patches (Uthicke and Karez 1999). These
aspects also apply to burrowing organisms and,
hence, may affect the trace fossil record. So far there
is little hard knowledge about these aspects in detail
for modern benthic habitats. New findings in these
fields will certainly have strong impact on the interpretation of the trace fossil record. In turn, within the
Nereites and Zoophycos ichnofacies, the rock record
shows that large spatial variability in the composition of modern ichnocoenoses is highly likely (e.g.
Wetzel et al. 2007). The study of large surfaces in
outcrops and the spatial distribution of trace fossils in
them may provide a tool to decipher the complex interrelationships and the response of the trace fossil
producers. Large fossil counterparts can provide information that cannot be extracted from the modern
seafloor yet.
DEEP-SEA ICHNOLOGY
CONCLUSIONS
The impact of trace fossils for paleoenvironmental
studies becomes more useful the more new data are
available from modern environments. Uniformitarian
observations are highly valuable if modern environmental conditions can be related to recently produced
burrows. For example, bioturbation structures in sediments in the South China Sea that contain Pinatubo
1991 ash are unusually favourable for uniformitarian
studies. Furthermore, to obtain good ichnologic data
from cores taken in modern soft deep-sea sediments
for comparing to fossil counterparts requires (1) using
X-ray radiographs to observe all structures in soft sediments, (2) focusing on burrows that can be ichnotaxonomically classified with certainty, and (3) being
aware that deep burrows dominate the fossil record.
In contrast to the well-known time-stability hypothesis, wide areas of the abyss represent a dynamic
habitat; in particular, organic matter deposition fluctuates seasonally and over longer time spans. Organic
matter deposition affects the oxygenation of the pore
water and, hence, the infauna dwelling in the upper
tiers with no permanent open connection to the
seafloor, such as the producers of Nereites and Scolicia. They switch between (near-)surface feeding during blooms and deep-deposit feeding between blooms.
Seasonally fluctuating organic matter deposition is
documented by vertical undulation of such burrows.
Vertical movement and temporary surface feeding
may result in destruction or absence of graphoglyptids and other near-surface burrows. In addition, the
producers of burrows maintaining an open connection
to the seafloor switch temporarily to a surface feeding
mode as documented by the fill of some Thalassinoides and Zoophycos.
The distribution of nutritional organic matter
within the sediment is documented by Phycosiphon.
Its producers feed selectively and can rework thin laminae, pre-existing burrows, or even thick intervals.
Patchiness of fauna, mobility of organisms, and
food selectivity are yet to be explored as aspects of
benthic life that certainly affect the distribution of
tracemakers, and hence of trace fossil associations.
Acknowledgements
The present study would not have been possible without
the help and cooperation of many persons and institutions.
M. Wiesner (Hamburg, Germany) invited me to participate
on various cruises of the German research vessel “Sonne”
to the South China Sea funded by the German Ministry of
135
Education, Science and Technology. He also provided unpublished data. W. Rehder (Kiel, Germany) – although already retired – X-rayed all radiograph slabs. A. Reisdorf
(Basel, Switzerland) prepared all figures. R.G. Bromley
(Copenhagen, Denmark) and A.K. Rindsberg (Livingston,
Alabama, U.S.A.) carefully edited the English text. The
Swiss National Science Foundation provided financial support (grants no. 2100-052256.97, 20021-112128). All these
contributions are gratefully acknowledged.
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