Palaeoenvironmental changes during the Danian–Selandian boundary interval: The ichnological record at the Sopelana section (Basque Basin, W Pyrenees)

Sedimentary Geology 284–285 (2013) 106–116

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Palaeoenvironmental changes during the Danian–Selandian boundary interval: The ichnological record at the Sopelana section (Basque Basin, W Pyrenees) F.J. Rodríguez-Tovar a,⁎, A. Uchman b, X. Orue-Etxebarria c, E. Apellaniz c a b c

Departamento de Estratigrafía y Paleontología, Facultad de Ciencias, Universidad de Granada, 18002 Granada, Spain Jagiellonian University, Institute of Geological Sciences, Oleandry Str. 2a, PL-30-063 Kraków, Poland Departamento de Estratigrafía y Paleontología, Facultad de Ciencia y Tecnología, Universidad del País Vasco, E-48080 Bilbao, Spain

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Article history: Received 22 September 2012 Received in revised form 29 November 2012 Accepted 30 November 2012 Available online 7 December 2012 Editor: J. Knight Keywords: Ichnology Danian–Selandian boundary Sea-level fluctuations Hyperthermal event Sopelana Basque Basin

a b s t r a c t Ichnological analysis was conducted in the Danian–Selandian (D–S) boundary interval from the Sopelana section (Basque Basin, northern Spain) to improve characterization of the recently defined Global Stratotype Section and Point of the base of the Selandian Stage (Middle Paleocene) in the nearby Zumaia section, and to interpret the Danian–Selandian boundary event with its associated palaeoenvironmental changes. The trace fossil assemblage of the boundary interval is relatively scarce and shows low diversity, consisting of Chondrites, Planolites, Thalassinoides, Trichichnus and Zoophycos, which cross-cut a diffuse, burrow-mottled background, typical of a normal burrowing tiered community. Distribution of trace fossils shows local drops in abundance and diversity just above the D–S boundary and about half a metre upwards into the succeeding Selandian. Generally, the Selandian part of the section has slightly lower trace fossil diversity and abundance. This is interpreted as due to a higher detrital food supply, corresponding to a sea-level fall, in contrast to a decreased food supply during the Selandian sea-level rise. Smaller-scale fluctuations of trace fossil diversity and abundance are also interpreted as due more to food content fluctuations in the sediment than to oxygenation of pore waters. Results reveal the minor influence of an extreme warming event (hyperthermal conditions) at the D–S boundary which affected the whole benthic habitat. Contrarily, a probable major effect of sea-level fluctuations can be envisaged, which determined variations in siliciclastic input and food content. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Ichnological analysis is now seen to be very useful for characterizing and interpreting palaeoenvironmental changes associated with past bio-events, particularly at stratigraphic boundaries, such as the Cenomanian–Turonian (Uchman et al., 2008; Rodríguez-Tovar et al., 2009a,b; Monaco et al., 2012), the Cretaceous–Palaeogene (RodríguezTovar and Uchman, 2004, 2006, 2008; Rodríguez-Tovar et al., 2004, 2006, 2010a; Rodríguez-Tovar, 2005; Kędzierski et al., 2011), the Paleocene–Eocene (Rodríguez-Tovar et al., 2011) and the Ypresian–Lutetian event (Ortiz et al., 2008b; Molina et al., 2011). In order to improve characterization of the Danian–Selandian boundary interval (D–S), and thereby interpret associated palaeoenvironmental changes, ichnological analysis is underway in several sections of the Western Pyrenees, including the Global Stratotype Section and Point (GSSP) Zumaia section (research in progress). Here we present the first ichnological analysis of the Sopelana section, another reference section of the studied area, which displays a complete D–S boundary transition that can be correlated bed-by-bed with the GSSP at Zumaia (Ortiz et al., 2011).

⁎ Corresponding author. E-mail address: [email protected] (F.J. Rodríguez-Tovar). 0037-0738/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sedgeo.2012.11.009

The International Subcommission on Palaeogene Stratigraphy defines the GSSP of the base of the Selandian Stage (Middle Paleocene) in the Zumaia section at San Telmo (North Spain), at the base of the Itzurun Formation (Schmitz et al., 2011). The base of the Selandian is characterized by a marked lithological change from red limestone and limestone–marl couplets (Aitzgorri Limestone Formation) in the uppermost Danian, to red marls (Itzurun Formation) at the base of the Selandian. Detailed micropalaeontological analysis, mainly based on calcareous nannofossil assemblages, revealed the complete and not condensed nature of the D–S transition record at Zumaia, and considerable micropalaeontological changes across the boundary (Bernaola et al., 2009; Schmitz et al., 2011). The D–S transition has been profusely studied in recent years so as to interpret the causes of recorded biotic changes, mostly with reference to sea-level fluctuations and climatic changes associated with the D–S boundary event and the Latest Danian Event (e.g., Speijer, 2003; Clemmensen and Thomsen, 2005; Guasti et al., 2005, 2006; Van Itterbeeck et al., 2007; Steurbaut and Sztrákos, 2008; Obaidalla et al., 2009). The D–S boundary event is, however, less known than other well-characterized nearby boundaries, such as the Cretaceous–Paleogene (K–Pg) event, or the Paleocene–Eocene (P–E) event associated with the P–E thermal maximum (PETM). In this context, further information focusing on the D–S boundary is of special significance to

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improve knowledge of the involved perturbations and the associated palaeoenvironmental changes. The aim of this paper is to evaluate the influence in the D–S boundary at Sopelana of the usually related global perturbations (sea-level fluctuations and extreme warming), on the palaeoenvironmental parameters controlling benthic habitats, based on the integration of ichnological and micropaleontological data. Ichnological information, mainly changes in abundance and diversity, and foraminiferal data, variations in relative abundance, test size and coiling direction, allow the interpretation of the major paleoenvironmental factors affecting benthic environment across the D–S boundary at Sopelana.

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Formation). The minimal magnitude estimated for the sea-level fall during the Danian–Selandian transition is between 80 and 90 m (Schmitz et al., 2011). Baceta et al. (2007) suggest that this major sea-level fall could be biostratigraphically correlated with the largest sea-level fall occurring in the Palaeocene, associated with the 58.5 My sequence boundary (Sel-2) of Haq et al. (1988); on the other hand, we cannot discard the possibility of its being a response to the nearby global sea-level fall at 60 My correlated to the sequence boundary Sel-1 (Haq et al., 1988). 3. Methods

2. Geological setting Mesozoic and Cenozoic sediments are well represented in the Pyrenees by thick lithostratigraphic units, which are well exposed and mapped (Bernaola et al., 2006). The Late Cretaceous and Palaeogene consist of a wide variety of sedimentary rocks, including continental alluvial clastics, shallow marine carbonates and deep-water hemipelagites and turbidites, which accumulated in a broad E–W elongated marine basin opened to the Proto-Bay of Biscay and the North Atlantic (Plaziat, 1981; Baceta, 1996; Pujalte et al., 1998; Baceta et al., 2004; Bernaola et al., 2006). Basinal hemipelagites and turbidites were deposited in the central part of the embayment, the so-called Basque Basin (Pujalte et al., 1998), at depths ranging from 1000 to 1500 m (Fig. 1). The Late Maastrichtian–earliest Ypresian was a phase of relative tectonic calm, characterized by a generally transgressive trend, during which turbidite deposition was reduced and the Basque Basin became dominated by regular alternations of hemipelagic limestones and marls, usually grouped in bedding couplets and bundles as representative of precession and short eccentricity climatic cycles (Dinarès-Turell et al., 2003, 2007). Decreasing siliciclastics facilitated the development of an extensive shallow marine carbonate platform system during the Paleocene and Early Eocene (Bernaola et al., 2006). The Sopelana section is located north of Bilbao at Sopelana Beach, in the northern limb of the Cretaceous–Palaeogene Biscay Synclinorium, around 60 km west of the Zumaia type section (Fig. 1). The Upper Cretaceous–Paleocene basinal succession of the Sopelana section is affected by reverse and strike-slip faults, determining stratigraphic repetition in different outcrops (Orue-Etxebarria, 1983, 1984, 1985; Apellaniz, 1998). The Danian–Selandian boundary interval outcrop (GPS coordinates: N 43°23.260; W 2°59.691), consists of sediments belonging to the Aitzgorri Limestone Formation (uppermost Danian) and the Itzurun Formation (lowermost Selandian), with a significant lithological change at the D–S boundary from a limestone alternating with limestone–marl succession, to red marls. Palaeobathymetry during deposition of the Danian–Selandian sediments from the nearby Zumaia section was estimated on the basis of benthic foraminifera assemblages; abundant organically cemented and calcareous-cemented agglutinated foraminifera (flysch-type taxa typical of relatively quiet terrigenous environments, suggesting a minimum relative water depth of lower–middle bathyal), together with taxa typical of deep-bathyal environments. This leads to the interpretation of deposition in a middle–lower slope, at 900–1100 m water depth (Arenillas et al., 2008), in agreement with previous proposals (Pujalte et al., 1995, 1998; Kuhnt and Kaminski, 1997) (Fig. 1). At the base of the Selandian, a significant discontinuity (the Mid-Paleocene Unconformity of Baceta et al., 2001) is observed in several areas of North Spain (Basque–Cantabrian region) marking the base of the Th-1 (Pujalte et al., 2000; Baceta et al., 2001, 2004, 2005, 2007; Bernaola et al., 2006) or Se/Th-1 (Schmitz et al., 2011) sequence (Fig. 2). This prominent discontinuity is related to a major sea-level fall at the end of the Danian, which is recognized in the whole Pyrenean domain (Pujalte et al., 2000; Baceta et al., 2004, 2005, 2007), and related to an extended sequence boundary (Haq et al., 1988; Hardenbol et al., 1998). This sea-level fall is associated with a lithological change registered in the lowermost Selandian, characterized by the increase in siliciclastics (the marl succession of the Itzurun

Here we conducted ichnological analysis of the 5 m-thick succession from the Danian–Selandian boundary interval, comprising the top 120 cm of the uppermost Danian and 380 cm of the lowermost Selandian, integrated with information from planktonic foraminifera. Trace fossils at the D–S boundary interval were analysed bed by bed, with detailed observations and continuous sampling at the D–S boundary transition, from the uppermost 120 cm of the Danian to the lowermost 80 cm of the Selandian, and more sparsely in the remaining part of the studied lower Selandian (Figs. 3–5). Discrete specimens were studied directly in the outcrop or in the laboratory, but trace fossils and ichnofabric have been mainly observed in variably oriented polished surfaces. Collected hard samples were cut and polished in the laboratory, and then wetted for enhanced observation or photographic contrast. Abundance of trace fossils has been visually estimated as rare, common and abundant. Rare occurrence means finding up to 3 specimens in a few samples, common occurrence means from 4 to a dozen specimens in a few samples, whereas abundant occurrence means more than a dozen specimens. All samples are housed in the Department of Stratigraphy and Palaeontology of the University of Granada (collection So-D/S). Micropaleontological sampling was conducted in the same interval as the ichnological sampling. For this study 15 samples were collected, each weighing between a half and 1 kg, the Danian ones picked up from marly levels, the Selandian ones from the softest levels of the interval. After a preliminary study, 9 of the 15 samples were selected for a high-resolution analysis because of their greater richness and better preservation of the specimens (Fig. 6). Standard micropaleontological procedures were used for the planktonic foraminiferal study. Firstly, samples were disaggregated by using diluted H2O2 for the softer marly samples and a solution of 80% acetic acid for the more lithified ones. After disaggregation, each sample was washed over sieves of 100 μm and 500 μm mesh size and dry-sieved over a sieve of 250 μm mesh size. The fractions 100–250 μm and 250–500 μm were used for the biostratigraphic study. About 300 specimens were determined from each of these fractions in order to obtain a representative spectrum of the foraminiferal association of each sample. However, only one fraction, between 100 μm and 500 μm, was used to establish the proportion of planktonic foraminifers versus total foraminifers. 4. Results 4.1. Trace fossil assemblage The trace fossil assemblage is relatively scarce and has low diversity, with five ichnogenera differentiated, Chondrites, Planolites, Thalassinoides, Trichichnus and Zoophycos, all of them cross-cutting a diffuse, burrowmottled background. Chondrites isp. were recognized as well developed unwalled branched burrow systems on parting surfaces in the outcrop (e.g., So-D/S-13), and in polished cross sections as patches of circular to elliptical spots and short bars (Fig. 4A–C), occasionally branched, representing variable cross sections of branched cylindrical burrow systems. Different colours of the infilling sediment (darker or lighter) from the host sediment allows

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Fig. 1. (A) Geographical location of the Sopelana and Zumaia sections. (B) Geographical location of the D–S section at Sopelana Beach. (C) Danian palaeogeography of the Pyrenean domain, with an indication of Sopelana and Zumaia outcrops. After Baceta et al. (2004), Bernaola et al. (2006).

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Fig. 2. Outcrop views of the Danian–Selandian boundary transition at the Sopelana section.

for a clear characterization. Smaller forms (about 0.5 mm wide), such as those observed on polished sections, could be assigned to Chondrites intricatus (Brongniart, 1823), while larger (1.5–2.5 mm wide) ones, seen on parting surfaces, could belong to Chondrites targionii (Brongniart, 1828). Locally, small Chondrites are observed in the filling of larger structures, probably Planolites and/or Thalassinoides. Chondrites von Sternberg (1833) are a deep-tier trace fossil, the tracemaker probably being a surface ingestor (Kotake, 1991a) living in a wide range of marine environmental conditions, including the aerobic–anoxic interface, as a chemosymbiotic organism (Seilacher, 1990; Fu, 1991). Planolites isp. occurs as horizontal to oblique, straight to slightly winding, simple structures, tubular or flattened cylinders in the outcrops (e.g., So-D/S-2), and as ovate forms in the polished cross sections (Fig. 4C, E), which are 2–5 mm wide. Infilling material is usually lighter in colour than the host sediment (red or dark grey). This is a faciescrossing ichnotaxon, interpreted as a pascichnion, probably associated with a number of tracemakers; see Pemberton and Frey (1982) and Keighley and Pickerill (1995) for discussion. Thalassinoides isp. has been observed as filling branched tubular burrow systems (Fig. 4D), or in polished surfaces as more or less ovate structures considered as cross-sections of branched cylinders (Fig. 4E–F).

However, branches have been only locally recognized. Thalassinoides Ehrenberg, 1944 is related to a wide variety of environments, being interpreted as a domichnial and fodinichnial structure produced by crustaceans, mostly decapods Ehrenberg (1944); see Fürsich (1973), Frey et al. (1984), Ekdale (1992), and Schlirf (2000) for discussion and ichnotaxonomic problems of this ichnogenus. Trichichnus isp. is recorded as straight, differently oriented, thread-like cylinders, 0.2–0.8 mm wide, filled with a ferruginous material and surrounded by a yellowish halo, which is up to 2 mm wide (Fig. 4C, E, G). Trichichnus Frey, 1970 is a eurybathic marine trace fossil interpreted as a domichnion of marine meiofaunal deposit feeders Frey (1970); see Uchman (1999) for taxonomic discussion. Possibly, Trichichnus tracemakers were chemosymbionts (Uchman, 1995), with a more opportunistic character than Chondrites, occurring at a greater depth and in very poorly oxygenated sediments (McBride and Picard, 1991). Zoophycos isp. (Fig. 4H) has been mainly observed on parting surfaces from the outcrop, as horizontal or oblique oval lobes (up to 185 mm wide) and tongues (up to 50 mm wide), filled with spreite laminae encircled by a thin marginal tunnel (2–2.5 mm wide), as part of a helical burrow system. Their near absence in polished cross-section can be attributed to a significant compression of the

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Fig. 3. Depositional sequences and main facies of the Danian–Selandian boundary transition of the SW Pyrenees. After Baceta et al. (2004), Bernaola et al. (2006).

burrow, impeding clear observation. Some dark stripes (Fig. 4F) most likely represent this flattened Zoophycos. Zoophycos s.l. is generally referred to as an unknown deposit-feeder, as sipunculids (Wetzel and Werner, 1981), polychaete annelids, arthropods (Ekdale and Lewis, 1991), or echiuran worms (Kotake, 1992). The precise ethological interpretation of Zoophycos is controversial, probably due to the assignment to Zoophycos s.l. of diverse structures produced by diverse organisms reflecting different behaviours, or a single tracemaker with varied behaviours (Kotake, 1989, 1991b, 1994; Bromley, 1991; Locklair and Savrda, 1998; MacEachern and Burton, 2000; Löwemark et al., 2004; Olivero and Gaillard, 2007). 4.2. Distribution of trace fossils Stratigraphic analysis of the ichnological assemblages shows a similar ichnotaxonomic composition across the D–S boundary interval from the uppermost 120 cm of the Danian to the lowermost 80 cm of the Selandian (Fig. 5), with no appearances or disappearances of ichnotaxa across the boundary, but with fluctuations in diversity from one to four taxa in particular beds. Also abundance of trace fossils fluctuates. The changes in diversity and abundance are not significantly related to lithological changes. For example, a layer of grey marlstone of a few centimetres thick just above the D–S boundary (So-D/S-9) shows a drop in diversity: only Chondrites are present abundantly here against the totally bioturbated background. About half a metre above this level, the dozen centimetre-thick layer So-D/S-12 of red marlstone, contains only Trichichnus in a bioturbated background. Chondrites and Planolites show an almost continuous record across the boundary, but their abundance fluctuates. Zoophycos, Trichichnus and Thalassinoides show a local record. Most notably, Zoophycos shows local but regular appearances (So-D/S-1b, So-D/S-3, So-D/S-5, So-D/S-8, So-D/S-11, So-D/S-14), but it is always rare in the sense of abundance. The upper 320 cm of the section (Selandian), dominated by massive red marlstones (Fig. 5), reveals a diminution of the abundance of trace fossils, together with a drop in diversity of one or two taxa per sample. Only Trichichnus and Planolites are present here. Zoophycos appears only once in the lower part; Chondrites are absent. The pattern of distribution

of the trace fossils in this part of the section is not fully recognized because of the sparse availability of samples; but continuous inspection in the field reveals a clear drop in diversity and abundance. 4.3. Planktonic foraminifera As occurs in the Global Stratotype Section and Point (GSSP) of the base of the Selandian Stage (Middle Paleocene) in the nearby Zumaia section, planktonic foraminifera assemblages from the Danian– Selandian boundary at the Sopelana section are well preserved and abundant, representing, in most of the studied samples, more than 90% of the total foraminifera assemblage (from 82% to 97% in the planktonic/benthic ratio; Fig. 6), even though a clearly decreasing planktonic component (along with the corresponding increase in benthic forms) is recorded at the lower Selandian (Fig. 6). Planktonic foraminiferal diversity is relatively high. The most frequent species include Globanomalina ehrenbergi, Globanomalina imitata, Parasubbotina varianta, Subbotina linaperta, Subbotina velascoensis, Subbotina triloculinoides, Acarinina subsphaerica, Igorina pusilla, Igorina laevigata, Morozovella angulata, Morozovella conicotruncata, Morozovella occlusa, Morozovella velascoensis and Chiloguembelina midwayensis (Fig. 6). From the different genera identified, a generalized scarcity of the specimens belonging to the genera Igorina, Acarinina and Muricoglobigerina is observed in all the studied samples. The composition of the planktonic foraminifera assemblage is similar throughout the studied D–S boundary, with no first appearance or last occurrence of any particular species (Fig. 6); however, their abundance, test size and coiling direction show distinct changes. The changes include: (a) a decrease in abundance of Morozovella, Globanomalina and Chiloguembelina, which is gradual for the latter genus, (b) in general, a significant increase in small-size specimens (decreased test size), and (c) an evident change in the coiling direction in the species of the Morozovella velascoensis group (e.g., occlusa, velascoensis), from variables below the boundary to dextral coiling dominance above. Similar features in planktonic foraminifera are observed in the Zumaia sections (Ortiz et al., 2011).

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Fig. 4. Trace fossils of the Danian–Selandian boundary transition in the Sopelana section. All on totally bioturbated background. A. Chondrites isp., small form, in vertical section of a red marlstone bed, sample So-D/S-2. B. Chondrites isp., large form, on horizontal parting surfaces of two pieces of grey marlstone, sample So-D/S-13. C. Vertical section of grey limestone with Planolites isp. (Pl), Chondrites isp., large form (Ch) and Trichichnus isp. (Tr), sample So-D/S-8. D. Fragments of Thalassinoides isp., fillings extracted from red marlstones. E. Subhorizontal section of grey marlstone with Thalassinoides isp. (Th), Planolites isp. (Pl) and Trichichnus isp. (Tr), sample So-D/S-13. F. Vertical section of grey limestone with Thalassinoides isp. (Th). The black stripe (?Zo) belongs probably to Zoophycos isp. and the light branched burrow (?Th) probably to other preservational variant of Thalassinoides isp. Sample So-D/S-3. G. Trichichnus isp. on parting surface of red marlstone, sample So-D/S-5. H. Zoophycos isp. on parting surface of red marlstone.

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Obaidalla et al., 2009). Foremost are relative sea-level fluctuations and/or extreme warming (hyperthermal conditions, short periods of extreme atmospheric and oceanic warmth), which involved the atmospheric/oceanic system. This relationship between global perturbations, induced environmental changes and the biotic response is still under study, largely due to difficulties for characterization and correlation of the D–S boundary between sections, as well as to the superimposition of local and regional factors (i.e., distance from shore, depth, etc.), upon the global event. Moreover, two events are actually well differentiated in the D–S boundary transition; the Danian–Selandian boundary event, and the Latest Danian Event (LDE). Initially, a warming event, associated with a significant sea-level change was recognized in Egypt around the Danian–Selandian boundary transition (Speijer, 2000, 2003). Subsequently, a similar event was registered in several places in North Africa (e.g., Tunisia; Guasti et al., 2005, 2006; Van Itterbeeck et al., 2007), and Europe (e.g., North Sea Basin; Clemmensen and Thomsen, 2005). Later research revealed that this warming event characterized in North Africa (Egypt and Tunisia), was prior to (several kyr before) to the Danian–Selandian boundary, being referred to as the Latest Danian Event (Bornemann et al., 2009; Sprong et al., 2011, 2012). According to Vandenberghe et al. (2012) the LDE is recognized slightly prior to the Selandian at about 62 Ma in the Tethys, Atlantic and Pacific, close to the base of C26r (Bornemann et al., 2009; Westerhold et al., 2011). In the Zumaia section, the Danian–Selandian transition event is located at the Danian–Selandian boundary (Schmitz et al., 2011), associated with the potential stratigraphic horizon HDS4 of Arenillas et al. (2008), located in the base of CIE-DS2, into C26r. In this section the LDE could be related with the carbon isotope excursion CIE-DS1, recognized about 11 m below the Selandian GSSP, in the lowermost part of C26r, in turn associated with the potential stratigraphic horizon HDS2 (Arenillas et al., 2008; Dinarès-Turell et al., 2010), and correlated with the CIE starting at the top of Chron C27n (TC27N event) characterized in the same section by Dinarès-Turell et al. (2012). However, as pointed by Dinarès-Turell et al. (2012), the lack of high-resolution sampling may lead to misinterpretations (Westerhold et al., 2011). This differentiation, however, is of special interest because for several authors the now differentiated LDE is of higher magnitude than the Danian–Selandian boundary event, with a more extended record worldwide, and probably more important associated environmental changes, and even in the Zumaia section both horizons HDS2 and HDS4 may represent significant global events (Arenillas et al., 2008). 5.2. Interpretation of environmental changes from ichnological and micropaleontological data

Fig. 5. Lithological column of the Danian–Selandian boundary transition in the Sopelana section with sample locations and range of trace fossils.

5. Discussion 5.1. Global paleoenvironmental changes at the D–S boundary interval Several global perturbations affecting biota have been proposed to induce environmental fluctuations across the Danian–Selandian boundary (e.g., Speijer, 2003; Clemmensen and Thomsen, 2005; Guasti et al., 2005, 2006; Van Itterbeeck et al., 2007; Steurbaut and Sztrákos, 2008;

The continuous presence of a burrow-mottled background, which is cross-cut by a low diversity trace fossil assemblage (Chondrites, Planolites, Thalassinoides, Trichichnus and Zoophycos) with only localised changes in abundance and diversity at the Danian–Selandian boundary interval, reveals a normal burrowing tiered community, that developed in sediments with continuously oxygenated pore waters, confirmed by the prevailing red colour of the sediments, and a minor influence of the D–S boundary event on the macrobenthic habitat. The tiering pattern shows fluctuations throughout the section from bed to bed, in which a trend from simple to more complex styles can be seen (Fig. 7). In the absence of oxygenation changes, fluctuations in diversity and tiering pattern could be linked to the fluctuations of food content in the sediment. Concentrations of food near the sediment–water interface promoted activity of burrowers in a thin layer of soft, if not soupy, sediment, thereby reducing the vertical extent of burrowing and the number of tiers. A generalized shortage of food led to reduced diversity and density of burrowers and their burrows. Burrow density and the burial of organic matter would be affected by the rate of sedimentation (Uchman and Wetzel, 2011), but there is no record of the changing sedimentation rate corresponding to fluctuating trace fossil diversity at the small

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Fig. 6. Stratigraphic distribution of planktonic foraminifera species across the Danian–Selandian boundary transition in the Sopelana section with the sample location and planktonic/benthic ratio.

scale, from layer to layer. This mechanism can, however, be invoked for the general changes in the distribution of trace fossils throughout the section. The Danian part refers to the sea–level fall (Fig. 2), during which a lot of food can be supplied to the deep sea. The sedimentation rate is expected to be higher in this part of the section, meaning there is greater burial of organic matter in the sediment as well. This probably enhanced the diversity and density of the burrowing organisms. Thalassinoides, occurring in this part of the section, is common in well-oxygenated environments and soft but fairly cohesive substrates (Bromley and Frey, 1974; Kern and Warme, 1974; Ekdale et al., 1984; Bromley, 1990). Its occurrence therefore points to food in the deeper tiers, where sediments are already slightly dewatered due to early diagenesis. The relationship between sea-level falls determining resedimentation processes, increase in siliciclastic input, higher organic matter content, eutrophic conditions, and biotic fluctuations, have been recognized for the Eocene Pyrenean deep-sea deposits (Payros et al., 2006; Rodríguez-Tovar et al., 2010b). Only very scarce and recent micropalaeoecological analyses have been conducted on the Sopelana section (e.g., Ortiz et al., 2011), as opposed to the extensive information obtained from the GSSP Zumaia section (see Arenillas, 2012, for a recent review). The lithological similarity between sections, bed-by-bed correlation, and comparable changes in the foraminifera assemblage (planktonic and benthic forms) reported recently (Ortiz et al., 2011) favour correlation/comparison with the

micropalaeontological data and interpretations from the well-known Zumaia section. Data from the foraminifera assemblage analysis in the Sopelana section fit with those based on the ichnological research, and on the minor influence of the D–S boundary global event on the microbenthic environment. The foraminifera assemblage across the Danian–Selandian boundary interval shows no significant changes in terms of new appearances or extinctions, merely variations affecting relative abundance, test size and coiling direction (Fig. 6). These changes are similar to those described previously at the Sopelana section, also for the benthic forms (Ortiz et al., 2011), and are related to changes in temperature, salinity and nutrient availability (Ortiz et al., 2011). An increase in benthic opportunistic forms together with a progressive decrease in calcareouscemented agglutinated specimens across the lowermost Selandian was recorded, with the latter associated with the detrital input caused by the sea-level fall at the D–S boundary (Ortiz et al., 2011). In the Zumaia section, the horizon HDS4 at the base of the Selandian is characterized by a slight decrease in planktic and benthic indices, together with a slight decrease in the planktic Morozovella, and the increase in benthic opportunistic trochamminids and Spiroplectammina (Arenillas et al., 2008; Alegret and Ortiz, 2010; Schmitz et al., 2011), and a negative δ13C excursion. The changes in the foraminifera assemblage were attributed to an apparent decrease in the local climatic and surface oceanic

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Fig. 7. Basic tiering pattern schemes of the Danian–Selandian boundary transition at the Sopelana section with an interpretation of palaeoenvironmental conditions in the sediment. Better palaeoenvironmental conditions for macrobenthos in the sediment, mainly related to a higher food content, are associated with the increase in diversity and abundance of trace fossils and tiering complexity, from a simple tiering pattern, with a mottled background cross-cut by Chondrites, to a more complex tiering pattern with a mottled background cross-cut by Planolites, Thalassinoides, Chondrites, Zoophycos and Trichichnus.

temperature, and a possible increase in the bottom seawater oxygenation (Arenillas et al., 2008). The associated negative δ 13C excursion suggests a significant decline in the local productivity or a sea-level fall (Arenillas et al., 2008). However, these faunal changes and the decreasing trend in δ13C show similarities with those recorded across the PETM, that, according to Ortiz et al. (2008a) might point to a hyperthermal event of a lesser magnitude. A comparatively higher effect on the planktonic habitat could be envisaged, as reflected by the major changes in the calcareous nannoplankton assemblage. Calcareous nannofossils show a significant increase in their overall abundance (Bernaola et al., 2006, 2009), with a major global radiation of the fasciculiths (Bernaola et al., 2006, 2009; the so-called second radiation in Schmitz et al. (2011) starting slightly below the top of the Aitzgorri Formation. This second radiation is considered the best event for the global marine correlation of the D–S boundary.

The most evident factor affecting the Basque Basin and the whole Pyrenean domain during the D–S boundary event is the major sea-level fall at the end of the Danian (Pujalte et al., 2000; Baceta et al., 2004, 2005, 2007) (Fig. 2) and associated with an extended sequence boundary (Haq et al., 1988; Hardenbol et al., 1998). Associated with these sea-level changes and detrital input (the marl succession of the Itzurun Formation; Baceta et al., 2004; Bernaola et al., 2006), fluctuations in depositional and ecological conditions are to be expected, as in nutrient availability, oxygenation and/or salinity. According to the interpreted continuously oxygenated pore water sediment (see above), significant fluctuations in oxygenation can be discarded, however. The major influence of the involved depositional and ecological conditions on the planktonic habitat that on the benthic one could be explained by: (a) the fact that controlling factors mostly influenced the water-column and then the planktonic habitat (i.e., water temperature, salinity, oxygenation, among others), and/or (b) local or regional conditions protected the benthic environment. In the first case, changes in temperature, salinity or other factors could bear a minor influence on substrate-related biota, while having a major impact on planktonic assemblages. In the second case, variations in nutrients, probably associated with a significant sea-level fall, could be of minor relevance in a deep-bathyal setting such as at Sopelana. The second alternative is very plausible, taking into account that well-expressed micropalaeontological changes across the Danian–Selandian boundary transition, associated to the Danian– Selandian boundary and the Latest Danian Event, worldwide are mainly typical of comparatively proximal and shallow-water settings (e.g., Speijer, 2003; Clemmensen and Thomsen, 2005; Guasti et al., 2005, 2006; Van Itterbeeck et al., 2007; Obaidalla et al., 2009). All these changes of major importance in proximal, shallow habitats have a relatively limited impact on deeper open marine settings, such as at Sopelana and Zumaia. In this context of a comparatively unaffected macro- and microbenthic communities, we may evoke minor scale fluctuations associated with siliciclastic input, probably in food content, such as those affecting the Thalassinoides tracemaker and determining the increase in benthic opportunistic trochamminids and Spiroplectammina. The continuous presence – without any significant fluctuation – of Chondrites and Trichichnus, produced by opportunistic tracemakers and frequently associated with unfavourable or very poorly oxygenated environments, is compatible with the absence of significant fluctuations in oxygenation. According to this, oxygenation can be discarded as a major environmental factor controlling macrobenthic community, which is probably the major influence of food content. Sea-level fluctuations appear as the most important factor inducing environmental changes during the D–S boundary at the Sopelana section. Given the noteworthy variations in siliciclastic input and the associated food content, a possible hyperthermal event cannot be discarded, but at any rate its impact would be limited. As previously indicated, a possible hyperthermal event at the Danian–Selandian boundary transition is recently associated to the LDE. The contrasted palaeoenvironmental change associated to the PETM was well-characterized in the Zumaia section by significant changes in trace fossil and benthic foraminifera assemblages, revealing that this major environmental perturbation significantly affected the whole benthic habitat (Rodríguez-Tovar et al., 2011). 6. Conclusions Ichnologic analysis of the Danian–Selandian boundary interval at the Sopelana section (Basque Basin) shows a normal burrowing tiered community. This reveals the minor influence of the D–S boundary global event in the macrobenthic habitat, as well as continuously oxygenated pore water sediments, in agreement with that from the microbenthic environment. In a context of a major sea-level fall at the end of the Danian affecting the Basque Basin, environmental changes influencing benthic habitat may be related to minor scale fluctuations associated with siliciclastic

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input, and probably food content. Other worldwide recorded environmental changes (e.g., temperature and salinity, among others), can be of minor influence in a deep-bathyal setting as at Sopelana. From the usually proposed global factors affecting marine environment during the Danian–Selandian boundary, sea-level fluctuations appear as the most important perturbation inducing environmental changes at Sopelana, while a possible extreme warming, hyperthermal event, could be not discarded but in any case may be of a limited impact. Acknowledgements The field research was partially supported by the Basque Country University. Additional support for A.U. was given by the Jagiellonian University (DS funds). This research was supported by projects CGL2008-03007/CLI, CGL2008-00009/BTE, CGL2012-33281, and RNM3715, and the research group RNM-178. Editor Jasper Knight, and the two Sedimentary Geology reviewers (Dr. Robert Speijer and one anonymous) provided useful feedback. 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