Lateral variability of ichnological content in muddy contourites: Weak bottom currents affecting organisms’ behavior

Although bioturbation is commonly recognized in contourites, only a few studies have analyzed the ichnological content of these deposits in detail. These studies have mainly focused on meso-scale bigradational sequence (a coarsening upward followed by a fining-upward sequence resulting from variations in current velocity). Here we present data from gravitational cores collected along the NW Iberian Margin showing systematic variation in ichnological content across proximal to distal depocenters within a large-scale elongated contourite drift. Data demonstrate that tracemakers’ behavior varies depending on the distance relative to the bottom current core. Trace fossils are already known to be a useful tool for studying of contouritic deposits and are even used as criterion for differentiating associated facies (e.g., turbidites, debrites), though not without controversy. We propose a mechanism by which the distance to the bottom current core exerts tangible influence on specific macro-benthic tracemaker communities in contourite deposits. This parameter itself reflects other bottom current features, such as hydrodynamic energy, grain size, nutrient transport, etc. Ichnological analysis can thus resolve cryptic features of contourite drift depositional settings.

The role of bottom currents in shaping deep-sea deposits (i.e., contourites) is currently a matter of debate in the scientific community 1,2 . Due to their implications for reconstruction of depositional conditions, contourite deposits have become a critical topic of investigation within the sub-disciplines of paleoceanography, slope-stability, and petroleum exploration. Due to their relative inaccessibility, however, contourites remain somewhat enigmatic. Only few studies have managed to investigate bioturbation and ichnofabrics in contour current settings [3][4][5][6] . Despite ongoing controversies 7,8 , trace fossil content is considered both a criterion for characterizing contouritic deposits and also a proxy for paleoenvironmental conditions 1,5,7 . These records are typically overprinted by bottom current activity 9 . In recent years, detailed ichnological studies conducted on contourite deposits in outcrops and core material have provided new insights into depositional processes, environmental conditions and the influence of bottom currents on tracemakers 5,6,10,11 . Due to lack of detailed records, the ichnological paradigm for contourites remains somewhat tentative. Here we describe unequivocal trace fossils in contouritic deposits from deep-sea gravity cores. Comparison of features reveals distinctive lateral variation with relative to bottom current cores.
The present study investigated core material collected from about 3,000 m water depth 12 during the ForSaGal 09 research cruise around the Galicia Interior Basin (GIB; Fig. 1). The location is known to be affected by northward bottom currents that interact with bathymetry to generate a contouritic drift along the basin. This setting provides a detailed record of Quaternary contourite deposits 13-16 . Contouritic facies appear as massive to coarsely laminated silt to very fine silty sand and, show varying degrees of bioturbation 17 . ichnological content of contourites. Ichnological analysis of contouritic intervals from selected cores revealed an assemblage with relatively low diversity. In order of most to least dominant, contourite deposits contained Thalassinoides, Planolites, Palaeophycus, and Zoophycos (Fig. 2). Thalassinoides is defined as a 3D system of sub-horizontal burrows connected to the surface by sub-vertical shafts. Only limited sections of horizontal burrows appeared in cores analyzed by this study 18,19 . Burrows range from 4 to 18 mm in height and from 10 to 62 mm in length. Planolites are horizontal cylindrical tunnels, actively filled by the tracemaker 20 . These appear as sub-circular cross sections ranging from 3 to 15 mm in diameter. Palaeophycus are sub-horizontal cylindrical burrows characterized by passive filling and a lined wall 20,21 . These appear as lined sub-circular sections, 2-5 mm high  www.nature.com/scientificreports www.nature.com/scientificreports/ and 4-19 mm long, filled by a darker sediment. Finally, Zoophycos is a complex helicoidal structure appearing as horizontal spreiten burrows in vertical sections of several cores 22,23 . The distribution of the ichnotaxa throughout the cores and their facies relationships (pelagic, hemipelagic, contourites and ice rafted debris or 'IRD') exhibit a clear pattern with contouritic intervals dominated by Thalassinoides and Planolites (Fig. 3). Abundant Palaeophycus or Zoophycos appear only occasionally (Fig. 3).
Bioturbated surfaces vary between 0 and 32% within contouritic intervals with a mean value of 6.8%. This value corresponds to a low to moderate BI (0 to 3) (Fig. 3). Contouritic intervals exhibit only minor bioturbation but they show a variable percentage of bioturbated surface, depending on the site. Core material from site FSG09-17 showed less bioturbation (3.7% bioturbated surface average) than that collected from site FSG09-07 (10.8% average) (Fig. 3). Every analyzed contouritic interval showed noteworthy vertical differences in percentages of bioturbated surface and in ichnological composition from bottom to top. At site FSG09-07 both contouritic intervals show increased bioturbation, which exceeded 30% and consist mainly of Thalassinoides at the bottom and Zoophycos at the top. The contouritic intervals from site FSG09-17 likewise record an increase in the bioturbated surface due to abundant Palaeophycus (Fig. 3).
ichnofabrics and paleoenvironmental conditions. The generally mottled background observed in contouritic core material reflects a complete reworking of the uppermost centimeters of the sediment. The degree of bioturbation in turn indicates relatively good environmental conditions for tracemakers working the unconsolidated substrate 24,25 . In this context, vertical changes in contourite ichnological features record significant variations in environmental parameters. Increasing percentages of bioturbated surface together with a shift in the dominant ichnotaxa reveal better conditions for particular tracemakers during the final stage of contourite deposition. Variation in Zoophycos in FSG09-07 and Palaeophycus in FSG09-17 indicates different ecological and depositional settings.
The organisms responsible for Palaeophycus constitute a eurybathic, facies-crossing ichnogenus, which occurs in a wide range of marine and non-marine settings. Palaeophycus itself represents a combined feeding/ www.nature.com/scientificreports www.nature.com/scientificreports/ temporary dwelling burrow made by organisms with filter-, suspensive-and carnivore-feeding behaviors 20,26 . Their presence suggests high rates of organic matter transport to the sediment. Palaeophycus is a horizontal shallow burrow that is typically formed in shallow/middle tiers and remains open while the tracemakers live inside 20 . Dense occurrences of Palaeophycus tubularis, and the absence of other macroburrows, have been interpreted as evidence of ecologically stressful conditions during substrate colonization including salinity fluctuations, oxygen depletion, and turbidity. These features can also indicate a change in sedimentation rate or other conditions 26 .
Zoophycos is a deep tier structure with several ethological interpretations, but consensus favors the interpretation of cache behavior developed by vermiform animals 27,28 . A recent analysis describes relations between deep-marine Zoophycos, sedimentation rate, seasonal primary productivity, and oxygenation 29 . Accordingly, Zoophycos primarily appears in glacial periods with intensive seasonal productivity, reflecting high fluxes and intermediate sedimentation rates from 5 to 20 cm ka −1 . Under these conditions, Zoophycos tracemakers collect nutrients at the sediment surface and transport them to deeper layers within the sediment to prevent oxidation 29 .
As outlined above, established interpretations of ichnological shifts attribute them to variations in sedimentation rate, organic matter availability, and oxygenation. Deposits from Galicia Interior Basin show clear spatiotemporal domains and associated sedimentation processes which themselves document the paleoenvironmental changes 12 . The westernmost FSG09-10 core was collected along the east flank of the Galicia Bank, in the so-called Transitional Zone 30 . This dome-like elevation is strongly influenced by bottom current activity that generates abraded surfaces 31 . Cores FSG09-09 and FSG09-16 were collected from the central part of the basin, between the Transitional Zone 31 and lower slope 12,14 , which are dominated by pelagic and hemipelagic sedimentation 12 . Core FSG09-07, and especially the easternmost core FSG09-17, correspond to a contouritic and hemipelagic depositional setting developed on the lower continental slope 14 . Given their locations within the basin, sedimentation rate, organic matter availability and oxygenation vary with distance to the bottom current core.
Muddy contourites form due to the action of weak bottom currents that transport a substantial volume of organic matter particles 5,32,33 which in turn feed benthic organisms 34,35 . Along the NW Iberian Margin for example, contour currents transport between 2-4 g m −3 of suspended material containing 40-100 mg m −3 of organic matter particles 33 . Distal zones to the core bottom current receive less sediment and organic matter than proximal settings, suggesting potential attendant variations in macrobenthic tracemaker communities. Site FSG09-17 (i.e., distal with respect to the bottom current core) experienced relatively low sedimentation rate and low organic www.nature.com/scientificreports www.nature.com/scientificreports/ matter flux. Organic matter at the sediment surface may be rapidly oxidized, preventing the development of shallow/middle tier structures. Under these conditions, only deep tier structures are produced, by organisms able to store organic matter deeper in the sediment as in the case of the Zoophycos tracemaker. Sediments from site FSG09-07 (i.e., the proximal site) indicate higher sedimentation rate and organic matter flux. Under these conditions, organic matter burial prevents oxidation and allows the development of shallow/middle tier dwelling structures, e.g. Palaeophycus (Fig. 4). The ichnological record, thus, systematically varies within the distal versus the proximal depositional zones,considering distance to core bottom current, of contourite drifts.
conclusions Sedimentation rate, oxygen conditions, and organic matter availability influence the macrobenthic tracemaker communities and resulting trace fossil assemblage in actively forming contourite drifts. This report provides novel evidence of systematic proximal, to core bottom current, versus distal variation in ichnological features from muddy contourites. Varying depositional conditions are interpreted to reflect distance from the bottom current core. Sedimentation rate and organic matter availability are higher in proximal areas where organic matter is rapidly buried. This prevents oxidation and makes organic matter available for shallow tier tracemakers (e.g. Palaeophycus producers). In distal settings, sedimentation rate and organic matter availability is lower. Organic matter is rapidly oxidized at the surface, favoring development of middle and deep tier tracemakers, which transport organic matter to deeper layers of the sediment (e.g., Zoophycos producers). Systematic lateral variation in ichnological content of contourite drifts demonstrates an impactful role for ichnological analysis in contourite research. Trace fossils not only could differentiate contourites from turbidites and associated deposits, they also record proximal to distal deposition within a contourite drift relative to core bottom current features.
Ichnological analysis identified ichnotaxa based on ichnotaxabases (i.e., standard morphological features, wall, filling, etc.), tiering (i.e., vertical distribution of bioturbation structures within the sediment), crosscutting relations, and degree of bioturbation. The degree of bioturbation was determined by calculating the percentage of bioturbated surface using a quantification method based on high resolution digital images 36 , then calculated according to the Bioturbation Index scale 37,38 . These values reflect the spatial extent of discrete trace fossils identified over a common mottled background.
Age was calculated using the age model based on XRF data and AMS-14 C ages for the cores material analyzed 12 .

Data availability
All data analyzed during this study are summarized in this published article. The original datasets are not publicly available due to size restrictions but are available from the corresponding author by request.