- Email: [email protected]
Changes in coiling direction of Cibicidoides pseudoacutus (Nakkady) across the Cretaceous–Tertiary boundary of Tunisia: palaeoecological and biostratigraphic implications
Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210 www.elsevier.com/locate/palaeo
Changes in coiling direction of Cibicidoides pseudoacutus (Nakkady) across the Cretaceous^Tertiary boundary of Tunisia: palaeoecological and biostratigraphic implications Simone Galeotti , Rodolfo Coccioni Istituto di Geologia and Centro di Palinologia dell’Universita', Campus Scienti¢co, Localita' Crocicchia, 61029 Urbino, Italy Received 10 September 1999; accepted 9 August 2001
Abstract The analysis of coiling direction preference in the benthic foraminifera Cibicidoides pseudoacutus (Nakkady) has been carried out across the Cretaceous^Tertiary boundary (K^T boundary) from four Tunisian sections representing a palaeobathymetric transect from a middle^outer neritic to lower upper bathyal depositional setting. Our study reveals that C. pseudoacutus developed a preference for sinistral coiling in a short time period during the lowermost Danian. The comparison of the coiling ratio (number of sinistral vs. dextral individuals) record with isotope data and dinoflagellate cyst assemblage distribution suggests that the development of sinistrally coiled populations of Cibicidoides pseudoacutus might be related to a short-term cooling of both surface and bottom waters which occurred at the K^T boundary and lasted for some 7 kyr. The continuous occurrence and the high abundance of Cibicidoides pseudoacutus through the K^T boundary in neritic to upper bathyal depositional environments around southern Tethys make identification of the shift in its coiling ratio relatively easy. The development of a sinistrally coiled population in this benthic foraminiferal species is regarded as a potential marker for the base of planktonic foraminiferal Zone P0 in Tethyan shallow water settings. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: benthic foraminifera; coiling ratio; biostratigraphy; palaeoecology; Cretaceous^Tertiary boundary; Tunisia
1. Introduction The use of biota-based sea surface temperature proxies across the Cretaceous^Tertiary boundary (K^T boundary), such as foraminiferal assemblage-based transfer functions, is unfortunately hampered as most calcareous plankton groups below and above the boundary are completely dif-
* Corresponding author. Fax: +39-722-304273. E-mail address: [email protected] (S. Galeotti).
ferent and impossible to compare as a consequence of the mass extinction event. However, the e¡ect of the K^T boundary event on benthic foraminifera was relatively minor with few species extinctions at this chronohorizon (see reviews in Kuhnt and Kaminski, 1996 and Coccioni and Galeotti, 1998) and therefore a comparison of Maastrichtian versus Danian assemblages is possible. Nevertheless, examination of benthic foraminifera as potential stratigraphic and palaeoclimatic indices across the K^T boundary has been very limited possibly also because this group is generally
0031-0182 / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 0 1 ) 0 0 3 9 6 - 0
PALAEO 2755 1-5-02
198
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
believed to be of use only for local correlations due to its facies dependence. An unambiguous correlation between temperature and coiling direction in Recent benthic foraminifera has been claimed by Collins (1990) although this parameter has been suggested to be also related to mode of reproduction (FÖyn, 1936, 1937; Myers, 1940; Lee et al., 1963; Nigam and Rao, 1989; Nigam and Khare, 1992). This parameter has, therefore, two potential applications: data from species in which the coiling ratio is environmentally controlled can be used as a proxy for sea £oor palaeotemperature estimates while inheritable coiling patterns may be useful in establishing phylogenetic relationships among species. In this paper we explore the possibility that variations in the coiling direction preference within the benthic foraminifera Cibicidoides pseudoacutus (Nakkady) (Fig. 1) may be the response to a short-term bottom water temperature change following the K^T boundary event. With this aim four sections have been studied from the shallow water setting of Tunisia where rich Cibicidoides pseudoacutus populations continuously occur across the K^T boundary. Results are then compared to previously published data on the dino£agellate-based sea surface temperature curve (Brinkhuis et al., 1998) and the oxygen isotope record (Keller and Lindinger, 1989) from the El Kef section.
2. Material El Haria Tunisian K^T boundary transition series are among the most complete in marine settings (e.g. Smit and Romein, 1985; Brinkhuis and Zachariasse, 1988; Keller, 1988a,b; Pospichal, 1994; Keller et al., 1995; Smit et al., 1997; Adatte et al., 1998; Dupuis et al., 1998; Stinnesbeck et al., 1998). In particular, the ‘El Kef I section’ was established as the K^T boundary ‘Global Stratotype Section and Point’, the boundary being placed at the base of the so-called boundary clay layer (Cowie et al., 1989). For this study, four sections (Elles, Ain Settara, El Kef, and El Melah; Figs. 2 and 3) from the NW Tunisian Trough have been studied across the K^T boundary which falls within the El Haria Formation. In each of the studied sections the lowermost Danian is characterised by CaCO3 -impoverished lithologies with the deposition of a dark grey clay layer underlain by a thin red layer containing an Ir anomaly, Ni-rich spinels, and shocked quartz which marks the K^T boundary (Smit et al., 1997; Robin et al., 1998; Stinnesbeck et al., 1998). The studied sites represent a north^ south palaeobathymetric transect from the middle^outer neritic Elles section to the lower upper bathyal El Melah section. Palaeobathymetric assessment of the studied sections is based on either literature data (Burollet, 1967; Keller, 1988a,
Fig. 1. Microphotographs of Cibicidoides pseudoacutus (sample K^T boundary +60 cm from the Elles section).
PALAEO 2755 1-5-02
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
199
resented by a 50 cm thick boundary clay layer. Upsection, the lithology changes to black ¢ssile shales rich in organic matter (Stinnesbeck et al., 1998). According to Keller et al. (2002), the studied stratigraphic interval comprises the uppermost part of the planktonic foraminiferal Plummerita hantkeninoides Zone, the entire P0 Zone (28 cm thick), and the lowermost part of Zone P1a (Fig. 4). The Cibicidoides pseudoacutus coiling ratio was calculated from 22 samples collected from 1 m below to 1 m above the K^T boundary. Sampling resolution is higher in proximity to the K^T boundary where samples were collected at 1^3cm intervals. 2.2. Ain Settara K^T boundary transition
Fig. 2. Palaeogeographic sketch of Tunisia at K^T time (redrawn from Adatte et al., 1998) with location of the studied sections indicated by diamonds.
1992; Speijer and van der Zwaan, 1994; Adatte et al., 1998; Stinnesbeck et al., 1998) or new data from the El Kef section which have been interpreted following Sliter (1972), Sliter and Baker (1972); Nyong and Olsson (1983/4), Olsson and Nyong (1984) and van Morkhoven et al. (1986). 2.1. Elles K^T boundary transition The Elles section contains one of the most complete K^T boundary transitions in Tunisia, and is similar to the El Kef stratotype section (Smit et al., 1997; Kouwenhoven et al., 1997). The uppermost Maastrichtian is characterised by grey marls, siltstones and shales deposited in a middle^outer neritic environment (Adatte et al., 1998; Stinnesbeck et al., 1998). The lowermost Danian is rep-
The Ain Settara deposits characterise a transition zone situated between the Tunisian Trough to the north and the emergent Kasserine Island to the south (see Fig. 2). The uppermost Maastrichtian consists of bluish grey marls with burrows in¢lled with black clay at the top. The K^T boundary transition is characterised by a complex succession of CaCO3 -impoverished lithologies from black and grey bioturbated clay to grey carbonate rich silt. Upwards in the sequence there is a return to CaCO3 -enriched facies with the deposition of grey marls. As also reported by Dupuis et al. (1998), 15 cm below the K^T boundary there occurs a concentration of small bivalves, brachiopods, and solitary corals, probably due to an episode of winnowing related to a relative sea level fall. At the K^T boundary, a 60 cm thick clayey layer occurs, which represents the base of the Sidi Nasseur Marl and overlies the Maastrichtian deposits (Dupuis et al., 1998). Our study focused on 23 samples spanning 1 m below to 1 m above the K^T boundary. Samples were taken at 2^10-cm intervals with higher sampling rate close to the K^T boundary. According to Luciani (2002), the studied stratigraphic interval comprises the uppermost part of planktic foraminiferal Plummerita hantkeninoides Zone, the entire Zone P0 zone (2 cm thick), and the lower half of Zone P1a (Fig. 4). It must be mentioned that, according to Molina et al. (1998), ¢rst speci-
PALAEO 2755 1-5-02
200
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
Fig. 3. Palaeodepth of K^T boundary sections in Tunisia. Modi¢ed after Adatte et al. (1998).
mens of Parvularugoglobigerina eugubina occur only some 60 cm above the K^T boundary at Ain Settara, therefore implying that Zone P0 might be much more expanded than reported by Luciani (2002). Such a di¡erence of results can, at least partly, be attributed to di¡erent methodologies as Molina et al. (1998) studied the fraction greater than 63 Wm whereas Luciani (2002) analysed the fraction greater than 45 Wm. 2.3. El Kef K^T boundary transition The El Kef K^T boundary transition is located in a marly section that occupies the relatively steep side of a small stream. Most previous published studies on the El Kef section are in agreement with regard to its o¡shore, outer neritic^ upper bathyal setting at K^T boundary time (e.g. Keller, 1988a,b; Speijer and Van der Zwaan, 1994, 1996; Coccioni and Galeotti, 1998; Galeotti, 1998). Previous studies also indicate that benthic foraminifera are abundant and well-preserved in this section (Keller, 1988b; Speijer and Van der Zwaan, 1994; Coccioni and Galeotti,
1998; Galeotti, 1998) also allowing a comparison with a previously established N18 O record (Keller and Lindinger, 1989). The present study is based on a composite record from El Kef I and El Kef II sections (see Keller et al., 1995 and Smit et al., 1997 for details). For this study, 33 samples (17 from El Kef I section and 16 from El Kef II section) were analysed from 1 m below to 1 m above the K^T boundary. Sampling resolution is higher in proximity to the K^T boundary where samples were collected at 1^2-cm intervals. According to Smit in Brinkhuis et al. (1994), the studied stratigraphic interval comprises the uppermost part of planktonic foraminiferal Abatomphalus mayaroensis Zone, or the Plummerita hantkeninoides Zone in Keller et al. (1995). Zone P0 spans the ¢rst 58 cm of the boundary clay layer according to the latter, but Smit in Brinkhuis et al. (1994) discovered rare tiny Parvularugoglobigerina eugubina 23 cm above the K^T boundary and hence Zone P0 is reduced to the ¢rst 23 cm of the boundary clay layer (Fig. 4). The rest of the studied stratigraphic interval represents the lower third of Zone P1a.
Fig. 4. Variations of Cibicidoides pseudoacutus coiling ratio at Ain Settara, Elles, and El Kef. Planktonic foraminiferal zonation after Smit in Brinkhuis et al. (1994) for El Kef, Luciani (2002) for Ain Settara, and Keller et al. (2002) for Elles.
PALAEO 2755 1-5-02
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
PALAEO 2755 1-5-02
201
202
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
2.4. El Melah K^T boundary transition The uppermost Maastrichtian of the El Melah section is characterised by the deposition of unstructured grey marls. At the K^T boundary, a 15 cm thick boundary clay layer occurs. According to Adatte et al. (1998), Zones P1a and P1b are here very reduced in thickness (1.2 m) as compared to Elles and El Kef ( s 10 m), suggesting a more distal palaeo-location of this site and/or the presence of short hiatuses (erosion and/or condensed intervals). Accordingly, palaeogeographic reconstruction of Tunisia at K^T boundary time by Burollet (1967) and palaeobathymetric interpretation based on benthic foraminiferal assemblages (Adatte et al., 1998) suggest a deeper depositional setting of El Melah when compared to El Kef. The El Melah K^T boundary transition represents, therefore, the deepest depositional setting among the studied sections, most likely having been deposited in the lower part of the upper bathyal environment. Planktonic foraminiferal Zone P0 is also reduced in thickness as compared to the K^T boundary stratotype spanning the lowest 10 cm of the Danian. In this study, only ¢ve samples from a stratigraphic interval spanning the uppermost 47 cm of the Maastrichtian (Zone Plummerita hantkeninoides) have been examined.
3. Methods Fluctuations in coiling direction of planktonic foraminiferal species have been used in stratigraphy and palaeoclimatology since the 1950s (see Kennett, 1976; Vincent and Berger, 1981; Hemleben et al., 1989, and references therein). However, a direct control of temperature on planktonic foraminiferal coiling direction preference has been more recently questioned. According to Brummer and Kroon (1988), variation of the coiling ratio in di¡erent species of planktonic foraminifera is related to water mass organisation and bioprovincialism through reproductive isolation and suppression of gene £ow across water mass boundaries. Examination of benthic foraminiferal coiling di-
rection preference as a proxy for temperature estimation has been more limited and almost exclusively carried out in shallow water settings (e.g. Longinelli and Tongiorgi, 1960; Hallock and Larsen, 1979; Nigam and Rao, 1989; Nigam and Khare, 1992). A few studies have dealt with laboratory experiments on coiling direction preference in benthic foraminifera (FÖyn, 1936, 1937; Lee et al., 1963) while the only study that investigated the relation between coiling preference in modern deep sea benthic foraminifera and environmental conditions (i.e. temperature) is, probably, that of Collins (1990). Such disparities are, at least partly, due to the di⁄culty of sampling oceanic bottom waters compared to the simplicity of collecting planktic samples and measuring chemico-physical properties of surface waters. Although the above-mentioned studies have not de¢nitely established a clear relationship between environmental changes and coiling direction in benthic foraminifera, temperature seems to be the only environmental factor in£uencing the coiling ratio of this group (see review in Boltovskoy et al., 1991). However, the coiling direction preference in benthic foraminifera is also in£uenced by mode of reproduction (FÖyn, 1936, 1937; Myers, 1940; Lee et al., 1963; Nigam and Rao, 1989; Nigam and Khare, 1992). This parameter has, therefore, two potential applications: data from species in which coiling preference is environmentally controlled can be used as a proxy for sea £oor palaeotemperature estimates while heritable coiling patterns may be useful in establishing phylogenetic relationships among species. In this study we explore the possibility that the coiling ratio (number of sinistral versus dextral individuals) in the benthic foraminifer Cibicidoides pseudoacutus can be the response to a rapid change of bottom water temperature following the K^T boundary event. Cibicidoides pseudoacutus was chosen for its continuous distribution and relatively high abundance across the K^T boundary. Other trochospiral taxa, although continuously present across the K^T boundary, do not occur in su⁄cient numbers to allow a thorough evaluation of the coiling ratio record. Sample preparation included gentle crushing, soaking overnight in a peroxide solution, washing
PALAEO 2755 1-5-02
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
203
Table 1 Cibicidoides pseudoacutus coiling ratio values for the studied sections and average values for speci¢c intervals
through a 63-Wm ¢ne mesh and drying at 80‡C. All Cibicidoides pseudoacutus individuals were picked out, glued onto micropalaeontological slides, and counted. As reproductive mode strongly in£uences coiling direction, only smallsized forms (megalospheric, asexually produced) were used in this study to avoid a genetic in£uence in the calculation of the coiling ratio. Data for the studied sections are reported in Table 1 together with average values for speci¢c intervals.
4. Results The Cibicidoides pseudoacutus coiling ratio rec-
ord from the studied sections is shown in Fig. 4 next to lithostratigraphy and planktonic foraminiferal zonation. Mean values in the uppermost Maastrichtian are quite similar in all sections ranging from 0.81 at El Melah to 1.05 at Elles. Rather stable values are observed in the uppermost Maastrichtian of El Kef. In the uppermost Maastrichtian of Elles, Ain Settara, and El Melah sections, the Cibicidoides pseudoacutus coiling ratio record shows larger £uctuations but it is never lower than 0.41 and never exceeds 1.56. At Elles and El Kef, a sharp spike in the proportion of sinistral specimens occurs at the K^T boundary. At El Kef, the coiling ratio in the ¢rst 6.5 cm of Zone P0 averages 1.75 with the highest
PALAEO 2755 1-5-02
204
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
PALAEO 2755 1-5-02
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
205
Fig. 5. Stable isotope and benthic foraminiferal record across K^T boundary (KTB) of El Kef. Planktonic foraminiferal zonation after Smit in Brinkhuis et al. (1994) and Smit in Brinkhuis et al. (1998). Oxygen isotope records from Keller and Lindinger (1989). The N18 O (¡) curve represents the ¢ne fraction ( 6 25 Wm) record while the N18 O (bf) curve represents benthic foraminiferal (Cibicidoides pseudoacutus) values. Faunal density expresses number of benthic foraminifera per g of dry sediment in the fraction greater than 125 Wm.
value (2.67) recorded just above the K^T boundary. A similar pattern is observed at Elles where the highest value (2.45) is recorded in the basal 2 cm of the boundary clay layer and high average values (1.5) are recorded in the lower third of Zone P0. A second, minor spike in the Cibicidoides pseudoacutus coiling ratio is recorded in both the Elles and El Kef sections at 10 cm and 6.5 cm above the K^T boundary, respectively. Upsection, the C. pseudoacutus coiling ratio average values decrease to 0.89 at El Kef and to 1.04 at Elles and therefore revert to uppermost Maastrichtian mean values. The spike in the proportion of sinistral individuals which has been observed at Elles and El Kef is absent in the Ain Settara section where average values are 0.92 in the Maastrichtian and 1.13 in the Danian. In the ¢rst 1 cm of the boundary clay layer, that is within Zone P0, the C. pseudoacutus coiling ratio is 0.96. A comparison with previously published data on benthic foraminifera from El Kef (Coccioni and Galeotti, 1998; Galeotti, 1998) indicates that the shift of the Cibicidoides pseudoacutus coiling ratio is associated with dramatic £uctuations of benthic foraminiferal faunal parameters including changes in the faunal density (i.e. the number of specimens per g of dry sediment), and proportion of epifaunal species (Fig. 5) as well as species diversity and relative abundance of oxygen indices. Semi-quantitative data from Elles indicate a comparable pattern of faunal changes in benthic foraminiferal assemblages across the K^T boundary. An increased relative abundance of C. pseudoacutus above the K^T boundary was observed in all the studied sections. In the El Kef K^T boundary transition C. pseudoacutus forms up to 60% of the whole lowermost Danian benthic foraminiferal assemblage with highest relative abundance recorded some 15 cm above the K^T boundary, therefore above the observed shift in its coiling ratio (Fig. 5).
5. Discussion 5.1. Biostratigraphic implications and completeness of K^T boundary Tunisian transitions Changes observed in benthic foraminiferal assemblages across the K^T boundary in Tunisian sections are comparable to the observations made in neritic to upper bathyal settings such as Brazos River (Keller, 1992), Millers Ferry (Olsson et al., 1996), Nye KlÖv and Stevns Klint (Schmitz et al., 1992; Coccioni and Galeotti, 1998), and Sinai Negev (Keller, 1992). In these areas, the following characteristic succession of benthic foraminiferal assemblages is observed within the K^T boundary interval: (1) late Maastrichtian foraminiferal assemblages are well diversi¢ed and characterised by complex trophic structures ; (2) foraminiferal assemblages within and directly above the boundary clay layer or Fish Clay are dominated by epifaunal species (Cibicidoides, Anomalinoides, and Gavelinella); (3) a slow recovery which is achieved only 200^300 ka after the K^T boundary event occurs in the lowermost Danian. On this basis, Coccioni and Galeotti (1998) distinguished a lowermost Danian shallow water ‘epifaunal domain’ which might be of some use for stratigraphic correlations across the K^T boundary transition. However, this succession of events is only useful for low-resolution correlations. Besides their facies dependence this is the reason why benthic foraminifera are seen as a group of little use for stratigraphic correlations across the K^T boundary, especially if compared to other extinction events (i.e. the Palaeocene^Eocene boundary) The shift in the coiling ratio of Cibicidoides pseudoacutus is a potential high-resolution correlation tool probably representing an ecological event spanning the lower part of Zone P0 which might be traceable over shallow water settings from the Tethys. According to the time scale of
PALAEO 2755 1-5-02
206
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
Berggren et al. (1995) and assuming a constant sedimentation rate of the boundary clay layer, the shift in C. pseudoacutus coiling ratio would, in fact, have lasted for some 7 ka after the K^T boundary event. It is noteworthy that the major spike recorded just above the K^T boundary is followed by a second minor spike occurring at one third of Zone P0 in both Elles and El Kef sections (see Fig. 4). Assuming that this second spike represents the same event in these sections, a further resolution of this event is possible. In particular, the interval where the double spike occurs is slightly thicker at Elles than observed in the stratotype section, suggesting that the basal part of the boundary clay layer is more expanded in the former section. Accordingly, the thickness of Zone P0 is slightly higher at Elles (28 cm) than observed El Kef (23 cm). The lack of a shift in Cibicidoides pseudoacutus coiling ratio in the Ain Settara section suggests that a stratigraphic gap straddling at least the lower third of Zone P0 occurs in this section. Accordingly, Luciani (2002) reports Zone P0 to be restricted to the basal 2 cm of the boundary clay layer at Ain Settara. 5.2. Palaeoecological implications The development of sinistrally coiled dominated Cibicidoides pseudoacutus populations at the K^T boundary is associated with dramatic £uctuations of other benthic foraminiferal faunal parameters. In agreement with Keller (1988a) and Speijer and van der Zwaan (1994, 1996), quantitative data from El Kef (Fig. 5) can be interpreted in terms of decreased oxygenation and organic £uxes to the sea £oor in correspondence to the K^T boundary. At El Kef the response of benthic foraminiferal assemblages to the K^T boundary event mainly occurred in terms of development of an epifaunal-dominated assemblage associated with a drop in the number of species (species richness) and number of specimens per sample (faunal density). The semi-quantitative observation carried out in the Elles section suggests that sudden and marked changes comparable to those observed at El Kef also occurred across the K^T
boundary at this site. In particular, a drastic decrease of species richness and faunal density occurs in response to the K^T boundary event. At Elles and El Kef, such conditions persisted at least up to the top of the studied intervals. On the other hand, according to Keller (1988a) and Speijer and van der Zwaan (1994, 1996), the complete recovery of benthic foraminiferal communities is observed at El Kef only 10 m above the K^ T boundary, that is 200^300 ka after the K^T boundary event. However, the shift in the Cibicidoides pseudoacutus coiling ratio observed at El Kef and Elles straddles the ¢rst 6.5 cm and 10 cm of Zone P0, respectively (Fig. 4). This event therefore terminated well before the end of the series of sea £oor perturbations indicated by the benthic foraminiferal faunal parameter record. This disparity suggests that the development of sinistrally coiled populations of C. pseudoacutus was not a direct response to decreased organic £uxes and/or development of dysaerobic conditions on the sea £oor. In addition, the C. pseudoacutus coiling ratio record does not match the record of the relative abundance of the species on which it is calculated. In the El Kef section, the highest relative abundance of this species is, in fact, recorded 15 cm above the K^T boundary where the coiling ratio has already reverted to pre- K^T boundary values (Fig. 5). It is, therefore, reasonable to assume that the development of sinistrally coiled dominant populations of C. pseudoacutus following the K^ T boundary was independent of changing redox and trophic conditions, and expansion of ecological niches particularly favourable for this species. In Recent and fossil assemblages as well as in laboratory cultures, coiling direction in benthic foraminifera has been shown to be in£uenced by reproductivity mode (FÖyn, 1936, 1937; Lee et al., 1963; Myers, 1940; Nigam and Rao, 1989), bioprovincialism (Hallock and Larsen, 1979) or environmental factors (Longinelli and Tongiorgi, 1960; Hallock and Larsen, 1979). Collins (1990) claimed for the ¢rst time demonstration of an unambiguous correlation between temperature and coiling direction in benthic foraminiferal populations. She reported that populations of predominantly dextrally coiled Bulimina
PALAEO 2755 1-5-02
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
marginata and Bulimina aculeata in two distinct areas (Gulf of Maine to New Jersey and Gulf of Mexico) are strongly associated with warm temperatures. However, the opposite is not true: predominantly sinistrally coiled populations are not associated with cold temperatures. Boltovskoy et al. (1991) reviewed the morphological variations of benthic foraminiferal tests in response to changes in ecological parameters. They concluded that notwithstanding some contradictory evidence, changes in temperature apparently result mostly in test size variation with population of larger individuals associated with lower temperature. However, they continue, it is quite probable, although the evidence is still tenuous and more study is required, that changes in temperature can a¡ect sinistral/dextral ratio in some species. The coiling ratio of some benthic foraminiferal species is therefore a potential proxy for palaeotemperature estimates of bottom waters. The observed shift in the Cibicidoides pseudoacutus coiling ratio can be interpreted in terms of either changing coiling preference within an original population having a V1:1 coiling ratio or (similarly to that suggested for Recent planktic foraminifera) immigration of a sinistral coil-dominated population from a di¡erent bioprovince. Evidence of genotypic isolation has been, in fact, observed in modern species of benthic foraminifera based on nuclear-coded ribosomal DNA sequences (Tsuchiya et al., 1998). On the other hand, as testi¢ed by benthic foraminiferal assemblages, sea £oor environmental changes following the K^T boundary at El Kef were su⁄ciently dramatic and sudden to have hampered the adaptation of an original population of C. pseudoacutus to stressed environmental conditions. Such a mechanism might have allowed the immigration of a sinistral coil-dominated C. pseudoacutus population from a di¡erent bioprovince (possibly from a di¡erent palaeobathymetric setting). In agreement with Keller (1988a) such a hypothesis would imply large sea level £uctuations in correspondence to the K^T boundary at El Kef. However, the presence in the lower upper bathyal El Melah section of C. pseudoacutus populations with a coiling ratio similar (if not lower) to that
207
observed in the shallower depositional setting of El Kef and Elles rules out a mechanism of immigration from a deeper bioprovince. The possibility of a downward bathymetric migration also seems unlikely. In fact, in the outer neritic Elles section, C. pseudoacutus populations have mean Maastrichtian values in coiling ratios similar to that observed in the upper bathyal setting of El Kef. In line with observations made on Recent benthic foraminiferal species, the observed shift in the coiling ratio of Cibicidoides pseudoacutus can be better interpreted as a response to a drastic change of bottom water temperature. A general correlation between benthic N18 O and C. pseudoacutus coiling ratio records cannot be traced in the surveyed interval at Elles and El Kef also because di¡erent sample sets have been used. However, the development of the sinistrally coiled population in C. pseudoacutus is associated with sharp shifts to heavier N18 O values in both the Elles and the El Kef sections and can be regarded as a response to a cooling of bottom waters. In particular, the benthic foraminiferal (C. pseudoacutus) N18 O record at El Kef shows a 0.5x positive excursion in the lowermost Danian, implying a sharp bottom water cooling at the K^T boundary (Keller and Lindinger, 1989; Fig. 5). Moreover, Stueben et al. (1998) have recently reported a 1.5x increase of N18 O benthic values suggesting that bottom waters signi¢cantly cooled during the deposition of the basal 5 cm of the boundary clay layer at Elles. Though not discussed in the text, these authors also show an abrupt shift to heavier values in planktic N18 O ratios at the K^T boundary in their ¢g. 1. By contrast, ¢ne fraction ( 6 25 Wm) oxygen isotope values from El Kef would suggest that a sea surface warming occurred in the lowermost Danian (Keller and Lindinger, 1989; Fig. 5). However, as pointed out by the latter authors, diagenetic and preservational e¡ects are variable across the K^T boundary at El Kef with preservation generally improved in the low-CaCO3 sediments of the boundary clay layer. The negative shift recorded in the ¢ne fraction N18 O ratio record at the K^T boundary of El Kef might, therefore, be due to comparison with underlying and overlying over-estimated, diagenetically altered values. Quantitative changes in
PALAEO 2755 1-5-02
208
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
dino£agellate cyst assemblages indicate that cooler surface water conditions occurred in the ¢rst 10 cm of Zone P0 at El Kef (Brinkhuis et al., 1998). These changes in the dino£agellate cyst assemblages correspond to the same stratigraphic interval that contains the shift in the C. pseudoacutus coiling ratio, indicating that a concomitant cooling of surface and bottom waters occurred in the earliest Danian at El Kef. Such a short-term climatic deterioration would generally agree with model simulation of the primary and secondary e¡ects of an extraterrestrial impact event which postulate that a short-term ‘impact winter’ accompanied the K^T boundary event (e.g. Pope et al., 1994; Ivanov et al., 1996). However, a problem arises when considering the duration of such a ‘cooling event’, the approximation of which represents a crucial point for discriminating between the various scenarios of the climate e¡ects which followed the K^T boundary event. Impact model-derived estimates currently available postulate that the ‘impact winter’ lasted from several months to a decade (Pope et al., 1994) whereas the cooling phase as indicated by the shift in the coiling direction of Cibicidoides pseudoacutus, invasion of boreal dino£agellate cyst, and isotope record would have lasted some 7 ka. Following Brinkhuis et al. (1998), as an alternative to this scenario, we must consider that the sedimentation rate of the boundary clay layer might have been, at least in its lower part, extremely high, rather than the canonical view of being extremely condensed.
6. Conclusions The study of coiling ratios in megalospheric individuals of the benthic foraminifer Cibicidoides pseudoacutus across the K^T boundary from four Tunisian sections reveals that this species developed sinistral coil-dominated populations in the lowermost Danian at El Kef and Elles. Such a shift in coiling direction preference cooccurs with the invasion of cold water (boreal) dino£agellate species and a positive shift in benthic foraminiferal N18 O values. It is, therefore, interpreted as a response to a short-term cooling
of bottom waters which lasted for some 7 ka after the K^T boundary event. Besides its palaeoecological, possibly palaeoclimatic signi¢cance, the development of sinistrally coiled populations in Cibicidoides pseudoacutus is a potential tool to assess the completeness of stratigraphic sequences in Tethyan shallow water settings just above the K^T boundary.
Acknowledgements Alessandro Montanari and Gerta Keller are kindly thanked for reading earlier drafts of the manuscript. Gerta Keller is also thanked for providing samples from the El Melah section. We thank M.B. Hart and an anonymous reviewer for their helpful suggestions which greatly improved the manuscript. This paper benefited from MURST 60% funding to R.C.
Appendix. Taxonomic notes Cibicidoides pseudoacutus (Nakkady) 1950 1988 1994
Anomalina pseudoacuta Nakkady, p. 691, pl. 90, ¢gs. 29^32. Anomalinoides acuta (Plummer), Keller, pl. 2, ¢gs. 9, 10, 12, 13. Cibicidoides pseudoacutus (Nakkady), Speijer and Van der Zwaan, pl. 7, Fig. 6a^c.
Test trochospirally coiled, planoconvex to biconvex, distinctly perforate, periphery subacute. Some specimens show a distinct keel. The umbilical side which is partially involute possesses a pronounced knob surrounded by growths extending up to the periphery. In its original description, Nakkady (1950) reported Cibicidoides pseudoacutus to be very similar to Anomalina acuta (Plummer). However, C. pseudoacutus di¡ers from the latter species in possessing a peripheral aperture which does not extend towards the umbilicus. For its biconvex aspect and partially involute spiral side C. pseudoacutus shows a remarkable a⁄nity to the genus Anomalinoides. However, due to the position and form of the aperture, we assign this species to the genus Cibi-
PALAEO 2755 1-5-02
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
cidoides in agreement with Speijer and Van der Zwaan (1994). References Adatte, T., Keller, G., Liangquan, L., Stinnesbeck, W., Zaghnin-Turky, D., 1998. Climate and sea-level £uctuations across the KT boundary in Tunisia: Warm and humid conditions linked to deccan volcanism? International Workshop on Cretaceous-Tertiary Transition, Volume of Abstracts, 7^ 10. Berggren, W.A., Kent, D.V., Swisher, C.C., III, Aubry, M-.P., 1995. A revised Cenozoic geochronology and chronostratigraphy. SEPM Spec. Publ. 54, 129^213. Boltovskoy, E., Scott, D.B., Medioli, F.S., 1991. Morphological variations of benthic foraminiferal tests in response to changes in ecological parameters: a review. J. Paleontol. 65, 175^185. Brinkhuis, H., Zachariasse, W.J., 1988. Dino£agellate cysts, sea level changes and planktonic foraminifers across the Cretaceous-Tertiary boundary at El Haria, Northwest Tunisia. Mar. Micropaleontol. 13, 153^191. Brinkhuis, H., Romein, A.J.T., Smit, J., Zachariasse, W.J., 1994. Danian-Selandian dino£agellate cysts from lower latitudes with special reference to the El Kef section, NW Tunisia. GFF 116, 46^48. Brinkhuis, H., Bujak, J.P., Smit, J., Versteegh, G.J.M., Visscher, H., 1998. Dino£agellate-based sea surface temperature reconstructions across the Cretaceous-Tertiary boundary. Palaeogeogr. Palaeoclimatol. Palaeoecol. 141, 67^83. Brummer, G.J.A., Kroon, D., 1988. Genetically controlled planktonic foraminiferal coiling ratios as tracers of past ocean dynamics. In: Brummer, G.J.A., Kroon, D. (Eds.), Planktonic Foraminifera as Tracers of Ocean-Climate History: Ontology, Relationships and Preservation of Modern Species and Stable Isotopes, Phenotypes and Assemblage Distribution in Di¡erent Water Masses. Free University Press, Amsterdam, pp. 293^297. Burollet, P.F., 1967. Tertiary Geology of Tunisia. In: Guidebook to the Geology and History of Tunisia. Petroleum Exploration Society, Libya, 9th Annu. Field Conf., pp. 215^225. Coccioni, R., Galeotti, S., 1998. What happened to small benthic foraminifera at the K/T Boundary? Bull. Soc. Ge¤ol. France 169, 271^279. Collins, L.S., 1990. The correspondence between water temperature and coiling direction in Bulimina. Paleoceanography 5, 289^294. Cowie, J.W., Ziegler, W., Remane, J., 1989. Stratigraphic commission accelerates progress 1984^1989. Episodes 112, 79^ 83. Dupuis, C., Steurbaut, E., Matmati, M.F., 1998. The Ain Settara K/T boundary section. Main results and comparison with the El Kef stratotype. International Workshop on Cretaceous-Tertiary Transition, Volume of Abstracts, 67^68.
209
FÖyn, B., 1936. Foraminiferenstudien I. Der Lebenszyklus von Discorbina vilardeboana d’Orbigny. Bergens Mus. Arb. Naturvidensk. Rekke 2, 1^22. FÖyn, B., 1937. Foraminiferenstudien II. Zur Kenntinis der asexuellen Fortp£anzung und Entwicklung der Gamonten von Discorbina vilardeboana d’Orbigny. Bergens Mus. Arb. Naturvidensk. Rekke 2, 1^22. Galeotti, S., 1998. Crisi Biologiche e Foraminiferi Bentonici Attraverso i Limiti Cretaceo/Terziario, Paleocene/Eocene e Eocene/Oligocene, Ph.D. Thesis, University of Parma, 205 pp. Hallock, P., Larsen, A.R., 1979. Coiling direction in Amphistegina. Mar. Micropaleontol. 4, 33^44. Hemleben, C., Spindler, M., Anderson, O.R., 1989. Modern Planktonic Foraminifera. Springer-Verlag, New York, 363 pp. Ivanov, B.A., Badukov, D.D., Yakovlev, O.I., Gerasimov, M.V., Dikov, Y.P., Pope, K.O., Ocampo, A.C., 1996. Degassing of sedimentary rocks due to Chicxulub impact: Hydrocode and Physical simulations. In: Ryder, G., Fastkovsky, D., Gartner, S. (Eds.), The Cretaceous-Tertiary event and other catastrophes in Earth history. Geological Society of America Special Paper 307, pp. 125^140. Keller, G., 1988a. Extinction survivorship and evolution of planktic foraminifera across the Cretaceous-Tertiary boundary at El Kef, Tunisia. Mar. Micropaleontol. 13, 239^263. Keller, G., 1988b. Biotic turnover in benthic foraminifera across the Cretaceous-Tertiary boundary at El Kef, Tunisia. Palaeogeogr. Palaeoclimatol. Palaeoecol. 66, 153^171. Keller, G., 1992. Paleoecological response of Tethyan benthic foraminifera to the Cretaceous-Tertiary boundary transition. In: Takayanagi, Y., Saito, T. (Eds.), Studies in Benthic Foraminifera. Tokai University Press, Tokyo, pp. 77^91. Keller, G., Lindinger, M., 1989. Stable isotope, TOC and CaCO3 record across the Cretaceous/Tertiary boundary at El Kef, Tunisia. Palaeogeogr. Palaeoclimatol. Palaeoecol. 73, 243^265. Keller, G., Li, L., MacLeod, N., 1995. The Cretaceous/Tertiary boundary stratotype section of El Kef, Tunisia: how catastrophic was the mass extinction? Palaeogeogr. Palaeoclimatol. Palaeoecol. 119, 221^254. Keller, G., Adatte, T., Stinnesbeck, W., Luciani, V., Karoui, N., Zaghbib-Turki, D., 2002. Paleoecology of the Cretaceous-Tertiary mass extinction in planktic foraminifera. Palaeogeogr. Palaeoclimatol. Palaeoecol. 178, 257^297. Kennett, J.P., 1976. Phenotypic variation in some recent and Late Cenozoic planktonic foraminifera. In: Hedley, R.H., Adams, C.G. (Eds.), Foraminifera, Volume 2. Academic Press, San Diego, CA, pp. 111^170. Kouwenhoven, T.J., Speijer, R.P., Vanoosterhout, C.W.M., Van der Zwaan, G.J., 1997. Benthic foraminiferal assemblages between two major extinction events ^ the Paleocene El Kef section, Tunisia. Mar. Micropaleontol. 29, 105^ 127. Kuhnt, W., Kaminski, M.A., 1996. The response of benthic foraminifera to the K/T boundary event ^ A review. Ge¤ol. Afr. Atl. Sud Actes Colloq. Angers 1994, 433^442.
PALAEO 2755 1-5-02
210
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
Lee, J.J., Freudenthal, H.D., Muller, W.A., Koosoy, V., Pierce, S., Grossman, R., 1963. Growth and Physiology of Foraminifera in the laboratory. Part 3, Initial studies of Rosalina £oridana. Micropaleontology 9, 449^466. Longinelli, A., Tongiorgi, E., 1960. Frequenza degli individui destrogiri in diverse popolazioni di Rotalia beccarii Linneo. Boll. Soc. Paleontol. Ital. 1, 5^16. Luciani, V., 2002. High-resolution planktonic foraminiferal analysis from the Cretaceous/Tertiary boundary at Ain-Settara (Tunisia). Palaeogeogr. Palaeoclimatol. Palaeoecol. 178, 299^319. Molina, E., Arenillas, I., Arz, J.A., 1998. Mass extinction in planktic foraminifera at the Cretaceous/Tertiary boundary in subtropical and temperate latitudes. Bull. Soc. Ge¤ol. France 169, 351^363. Myers, E.H., 1940. Observation on the origin and fate of £agellated gametes in multiple tests of Discorbis (Foraminifera). J. Mar. Biol. Assoc. UK 24, 201^226. Nakkady, S.E., 1950. A new foraminiferal fauna from the Esna shales and upper Cretaceous Chalk of Egypt. J. Paleontol. 24, 675^692. Nigam, R., Rao, A.S., 1989. The intriguing relationship between coiling direction and reproductive mode in benthic foraminifera. J. Palaeontol. Soc. India 34, 79^82. Nigam, R., Khare, N., 1992. The reciprocity between coiling direction and dimorphic reproduction in benthic foraminifera. J. Micropaleontol. 11, 221^228. Nyong, E.E., Olsson, R.K., 1983/4. A paleoslope model for Campanian to Lower Maestrichtian Foraminifera in the North America Basin and adjacent continental margin. Mar. Micropaleontol. 8, 437^477. Olsson, R.K., Nyong, E.E., 1984. A paleoslope model for Campanian-Lower Maastrichtian Foraminifera of New Jersey and Delaware. J. Foraminifer. Res. 14, 50^68. Olsson, R.K., Liu, C., van Fossen, M., 1996. The Cretaceous/ Tertiary catastrophic event at Millers Ferry, Alabama. In: Ryder, G., Fastkovsky, D., Gartner, S. (Eds.), The Cretaceous-Tertiary Event and Other Catastrophes in Earth History. Geological Society of America Special Paper 307, pp. 263^277. Pope, K.O., Baines, K.H., Ocampo, A.C., Ivanov, B.A., 1994. Impact winter and the Cretaceous/Tertiary extinctions: Results of a Chicxulub asteroid impact model. Earth Planet. Sci. Lett. 128, 719^725. Pospichal, J.J., 1994. Calcareous nannofossil at the K-T boundary, El Kef: No evidence for stepwise, gradual or sequential extinctions. Geology 22, 99^102. Robin, E., Rocchia, R., Lefevre, I., Pierrard, O., Dupuis, C., Smit, J., Zaghib-Turki, D., Karoui, N., Matmati, F., 1998. The compositional Variation of K/T spinel in Tunisia: Evidence for a global Deluge of Projectile debris. International
Workshop on Cretaceous-Tertiary Transition, Volume of Abstracts, 49^50. Schmitz, B., Keller, G., Stenvall, O., 1992. Stable isotope and foraminiferal changes across the Cretaceous-Tertiary boundary at Stevns Klint, Denmark: Arguments for long-term oceanic instability before and after bolide-impact event. Palaeogeogr. Palaeoclimatol. Palaeoecol. 96, 233^260. Sliter, W.V., 1972. Upper Cretaceous planktonic foraminiferal zoogeography and ecology ^ eastern Paci¢c margin. Palaeogeogr. Palaeoclimatol. Palaeoecol. 12, 15^31. Sliter, W.V., Baker, A., 1972. Cretaceous bathymetric distribution of benthic foraminifera. J. Foraminifer. Res. 2, 167^ 183. Smit, J., Romein, A.J.T., 1985. A sequence of events across the Cretaceous-Tertiary boundary. Earth Planet. Sci. Lett. 74, 155^170. Smit, J., Keller, G., Zargouni, F., Razgallah, S., Shimi, M., BenAbdelkader, O., Ben Haj Ali, N., Ben Salem, H., 1997. The El Kef sections and sampling procedures. Mar. Micropaleontol. 29, 67^69. Speijer, R.P., Van der Zwaan, G.J., 1994. Extinction and survivorship patterns in southern Tethyan benthic foraminiferal assemblages across the Cretaceous/Paleogene boundary. Geol. Ultraiect. 124, 19^64. Speijer, R.P., Van der Zwaan, G.J., 1996. Extinction and survivorship of southern Tethyan benthic foraminifera across the Cretaceous/Palaeogene boundary. In: Hart, M.B. (Ed.), Biotic Recovery from Mass Extinction Events. Geological Society Special Publication 102, pp. 343^371. Stinnesbeck, W., Keller, G., Adatte, T., 1998. Lithological Characteristics of the K/T transitions in Tunisia: Evidence of a tsunami event? International Workshop on CretaceousTertiary Transition, Volume of Abstracts, 53^54. Stueben, D., Kramar, U., Berner, Z., Keller, G., 1998. Trace elements and stable isotopes in foraminifera of the Elles K/T pro¢le: Indications for sea level £uctuations and primary productivity. International Workshop on Cretaceous-Tertiary Transition, Volume of Abstracts, 55^56. Tsuchiya, M., Kitazato, H., Pawlowski, J., 1998. Phylogenetic relationships among three genera within the subfamily Glabratellidae from Japan based on ribosomal DNA sequences. FORAMS ’98, International Symposium on Foraminifera, Monterrey, Mexico, July 5^12, Sociedad Mexicana de Paleontologia, A.C., Special Publication 107. van Morkhoven, F.P.C.M., Berggren, W.A., Edwards, S.A., 1986. Cenozoic cosmopolitan deep-water benthic foraminifera. Bull. Cent. Rech. Explor. Prod. Elf-Aquitaine Me¤m. 11, 1^412. Vincent, E., Berger, W.H., 1981. Planktonic foraminifera and their use in paleoceanography. In: Emiliani, C. (Ed.), The Oceanic Lithosphere. Sea 7, 1025^1119.
PALAEO 2755 1-5-02
Changes in coiling direction of Cibicidoides pseudoacutus (Nakkady) across the Cretaceous^Tertiary boundary of Tunisia: palaeoecological and biostratigraphic implications Simone Galeotti , Rodolfo Coccioni Istituto di Geologia and Centro di Palinologia dell’Universita', Campus Scienti¢co, Localita' Crocicchia, 61029 Urbino, Italy Received 10 September 1999; accepted 9 August 2001
Abstract The analysis of coiling direction preference in the benthic foraminifera Cibicidoides pseudoacutus (Nakkady) has been carried out across the Cretaceous^Tertiary boundary (K^T boundary) from four Tunisian sections representing a palaeobathymetric transect from a middle^outer neritic to lower upper bathyal depositional setting. Our study reveals that C. pseudoacutus developed a preference for sinistral coiling in a short time period during the lowermost Danian. The comparison of the coiling ratio (number of sinistral vs. dextral individuals) record with isotope data and dinoflagellate cyst assemblage distribution suggests that the development of sinistrally coiled populations of Cibicidoides pseudoacutus might be related to a short-term cooling of both surface and bottom waters which occurred at the K^T boundary and lasted for some 7 kyr. The continuous occurrence and the high abundance of Cibicidoides pseudoacutus through the K^T boundary in neritic to upper bathyal depositional environments around southern Tethys make identification of the shift in its coiling ratio relatively easy. The development of a sinistrally coiled population in this benthic foraminiferal species is regarded as a potential marker for the base of planktonic foraminiferal Zone P0 in Tethyan shallow water settings. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: benthic foraminifera; coiling ratio; biostratigraphy; palaeoecology; Cretaceous^Tertiary boundary; Tunisia
1. Introduction The use of biota-based sea surface temperature proxies across the Cretaceous^Tertiary boundary (K^T boundary), such as foraminiferal assemblage-based transfer functions, is unfortunately hampered as most calcareous plankton groups below and above the boundary are completely dif-
* Corresponding author. Fax: +39-722-304273. E-mail address: [email protected] (S. Galeotti).
ferent and impossible to compare as a consequence of the mass extinction event. However, the e¡ect of the K^T boundary event on benthic foraminifera was relatively minor with few species extinctions at this chronohorizon (see reviews in Kuhnt and Kaminski, 1996 and Coccioni and Galeotti, 1998) and therefore a comparison of Maastrichtian versus Danian assemblages is possible. Nevertheless, examination of benthic foraminifera as potential stratigraphic and palaeoclimatic indices across the K^T boundary has been very limited possibly also because this group is generally
0031-0182 / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 0 1 ) 0 0 3 9 6 - 0
PALAEO 2755 1-5-02
198
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
believed to be of use only for local correlations due to its facies dependence. An unambiguous correlation between temperature and coiling direction in Recent benthic foraminifera has been claimed by Collins (1990) although this parameter has been suggested to be also related to mode of reproduction (FÖyn, 1936, 1937; Myers, 1940; Lee et al., 1963; Nigam and Rao, 1989; Nigam and Khare, 1992). This parameter has, therefore, two potential applications: data from species in which the coiling ratio is environmentally controlled can be used as a proxy for sea £oor palaeotemperature estimates while inheritable coiling patterns may be useful in establishing phylogenetic relationships among species. In this paper we explore the possibility that variations in the coiling direction preference within the benthic foraminifera Cibicidoides pseudoacutus (Nakkady) (Fig. 1) may be the response to a short-term bottom water temperature change following the K^T boundary event. With this aim four sections have been studied from the shallow water setting of Tunisia where rich Cibicidoides pseudoacutus populations continuously occur across the K^T boundary. Results are then compared to previously published data on the dino£agellate-based sea surface temperature curve (Brinkhuis et al., 1998) and the oxygen isotope record (Keller and Lindinger, 1989) from the El Kef section.
2. Material El Haria Tunisian K^T boundary transition series are among the most complete in marine settings (e.g. Smit and Romein, 1985; Brinkhuis and Zachariasse, 1988; Keller, 1988a,b; Pospichal, 1994; Keller et al., 1995; Smit et al., 1997; Adatte et al., 1998; Dupuis et al., 1998; Stinnesbeck et al., 1998). In particular, the ‘El Kef I section’ was established as the K^T boundary ‘Global Stratotype Section and Point’, the boundary being placed at the base of the so-called boundary clay layer (Cowie et al., 1989). For this study, four sections (Elles, Ain Settara, El Kef, and El Melah; Figs. 2 and 3) from the NW Tunisian Trough have been studied across the K^T boundary which falls within the El Haria Formation. In each of the studied sections the lowermost Danian is characterised by CaCO3 -impoverished lithologies with the deposition of a dark grey clay layer underlain by a thin red layer containing an Ir anomaly, Ni-rich spinels, and shocked quartz which marks the K^T boundary (Smit et al., 1997; Robin et al., 1998; Stinnesbeck et al., 1998). The studied sites represent a north^ south palaeobathymetric transect from the middle^outer neritic Elles section to the lower upper bathyal El Melah section. Palaeobathymetric assessment of the studied sections is based on either literature data (Burollet, 1967; Keller, 1988a,
Fig. 1. Microphotographs of Cibicidoides pseudoacutus (sample K^T boundary +60 cm from the Elles section).
PALAEO 2755 1-5-02
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
199
resented by a 50 cm thick boundary clay layer. Upsection, the lithology changes to black ¢ssile shales rich in organic matter (Stinnesbeck et al., 1998). According to Keller et al. (2002), the studied stratigraphic interval comprises the uppermost part of the planktonic foraminiferal Plummerita hantkeninoides Zone, the entire P0 Zone (28 cm thick), and the lowermost part of Zone P1a (Fig. 4). The Cibicidoides pseudoacutus coiling ratio was calculated from 22 samples collected from 1 m below to 1 m above the K^T boundary. Sampling resolution is higher in proximity to the K^T boundary where samples were collected at 1^3cm intervals. 2.2. Ain Settara K^T boundary transition
Fig. 2. Palaeogeographic sketch of Tunisia at K^T time (redrawn from Adatte et al., 1998) with location of the studied sections indicated by diamonds.
1992; Speijer and van der Zwaan, 1994; Adatte et al., 1998; Stinnesbeck et al., 1998) or new data from the El Kef section which have been interpreted following Sliter (1972), Sliter and Baker (1972); Nyong and Olsson (1983/4), Olsson and Nyong (1984) and van Morkhoven et al. (1986). 2.1. Elles K^T boundary transition The Elles section contains one of the most complete K^T boundary transitions in Tunisia, and is similar to the El Kef stratotype section (Smit et al., 1997; Kouwenhoven et al., 1997). The uppermost Maastrichtian is characterised by grey marls, siltstones and shales deposited in a middle^outer neritic environment (Adatte et al., 1998; Stinnesbeck et al., 1998). The lowermost Danian is rep-
The Ain Settara deposits characterise a transition zone situated between the Tunisian Trough to the north and the emergent Kasserine Island to the south (see Fig. 2). The uppermost Maastrichtian consists of bluish grey marls with burrows in¢lled with black clay at the top. The K^T boundary transition is characterised by a complex succession of CaCO3 -impoverished lithologies from black and grey bioturbated clay to grey carbonate rich silt. Upwards in the sequence there is a return to CaCO3 -enriched facies with the deposition of grey marls. As also reported by Dupuis et al. (1998), 15 cm below the K^T boundary there occurs a concentration of small bivalves, brachiopods, and solitary corals, probably due to an episode of winnowing related to a relative sea level fall. At the K^T boundary, a 60 cm thick clayey layer occurs, which represents the base of the Sidi Nasseur Marl and overlies the Maastrichtian deposits (Dupuis et al., 1998). Our study focused on 23 samples spanning 1 m below to 1 m above the K^T boundary. Samples were taken at 2^10-cm intervals with higher sampling rate close to the K^T boundary. According to Luciani (2002), the studied stratigraphic interval comprises the uppermost part of planktic foraminiferal Plummerita hantkeninoides Zone, the entire Zone P0 zone (2 cm thick), and the lower half of Zone P1a (Fig. 4). It must be mentioned that, according to Molina et al. (1998), ¢rst speci-
PALAEO 2755 1-5-02
200
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
Fig. 3. Palaeodepth of K^T boundary sections in Tunisia. Modi¢ed after Adatte et al. (1998).
mens of Parvularugoglobigerina eugubina occur only some 60 cm above the K^T boundary at Ain Settara, therefore implying that Zone P0 might be much more expanded than reported by Luciani (2002). Such a di¡erence of results can, at least partly, be attributed to di¡erent methodologies as Molina et al. (1998) studied the fraction greater than 63 Wm whereas Luciani (2002) analysed the fraction greater than 45 Wm. 2.3. El Kef K^T boundary transition The El Kef K^T boundary transition is located in a marly section that occupies the relatively steep side of a small stream. Most previous published studies on the El Kef section are in agreement with regard to its o¡shore, outer neritic^ upper bathyal setting at K^T boundary time (e.g. Keller, 1988a,b; Speijer and Van der Zwaan, 1994, 1996; Coccioni and Galeotti, 1998; Galeotti, 1998). Previous studies also indicate that benthic foraminifera are abundant and well-preserved in this section (Keller, 1988b; Speijer and Van der Zwaan, 1994; Coccioni and Galeotti,
1998; Galeotti, 1998) also allowing a comparison with a previously established N18 O record (Keller and Lindinger, 1989). The present study is based on a composite record from El Kef I and El Kef II sections (see Keller et al., 1995 and Smit et al., 1997 for details). For this study, 33 samples (17 from El Kef I section and 16 from El Kef II section) were analysed from 1 m below to 1 m above the K^T boundary. Sampling resolution is higher in proximity to the K^T boundary where samples were collected at 1^2-cm intervals. According to Smit in Brinkhuis et al. (1994), the studied stratigraphic interval comprises the uppermost part of planktonic foraminiferal Abatomphalus mayaroensis Zone, or the Plummerita hantkeninoides Zone in Keller et al. (1995). Zone P0 spans the ¢rst 58 cm of the boundary clay layer according to the latter, but Smit in Brinkhuis et al. (1994) discovered rare tiny Parvularugoglobigerina eugubina 23 cm above the K^T boundary and hence Zone P0 is reduced to the ¢rst 23 cm of the boundary clay layer (Fig. 4). The rest of the studied stratigraphic interval represents the lower third of Zone P1a.
Fig. 4. Variations of Cibicidoides pseudoacutus coiling ratio at Ain Settara, Elles, and El Kef. Planktonic foraminiferal zonation after Smit in Brinkhuis et al. (1994) for El Kef, Luciani (2002) for Ain Settara, and Keller et al. (2002) for Elles.
PALAEO 2755 1-5-02
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
PALAEO 2755 1-5-02
201
202
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
2.4. El Melah K^T boundary transition The uppermost Maastrichtian of the El Melah section is characterised by the deposition of unstructured grey marls. At the K^T boundary, a 15 cm thick boundary clay layer occurs. According to Adatte et al. (1998), Zones P1a and P1b are here very reduced in thickness (1.2 m) as compared to Elles and El Kef ( s 10 m), suggesting a more distal palaeo-location of this site and/or the presence of short hiatuses (erosion and/or condensed intervals). Accordingly, palaeogeographic reconstruction of Tunisia at K^T boundary time by Burollet (1967) and palaeobathymetric interpretation based on benthic foraminiferal assemblages (Adatte et al., 1998) suggest a deeper depositional setting of El Melah when compared to El Kef. The El Melah K^T boundary transition represents, therefore, the deepest depositional setting among the studied sections, most likely having been deposited in the lower part of the upper bathyal environment. Planktonic foraminiferal Zone P0 is also reduced in thickness as compared to the K^T boundary stratotype spanning the lowest 10 cm of the Danian. In this study, only ¢ve samples from a stratigraphic interval spanning the uppermost 47 cm of the Maastrichtian (Zone Plummerita hantkeninoides) have been examined.
3. Methods Fluctuations in coiling direction of planktonic foraminiferal species have been used in stratigraphy and palaeoclimatology since the 1950s (see Kennett, 1976; Vincent and Berger, 1981; Hemleben et al., 1989, and references therein). However, a direct control of temperature on planktonic foraminiferal coiling direction preference has been more recently questioned. According to Brummer and Kroon (1988), variation of the coiling ratio in di¡erent species of planktonic foraminifera is related to water mass organisation and bioprovincialism through reproductive isolation and suppression of gene £ow across water mass boundaries. Examination of benthic foraminiferal coiling di-
rection preference as a proxy for temperature estimation has been more limited and almost exclusively carried out in shallow water settings (e.g. Longinelli and Tongiorgi, 1960; Hallock and Larsen, 1979; Nigam and Rao, 1989; Nigam and Khare, 1992). A few studies have dealt with laboratory experiments on coiling direction preference in benthic foraminifera (FÖyn, 1936, 1937; Lee et al., 1963) while the only study that investigated the relation between coiling preference in modern deep sea benthic foraminifera and environmental conditions (i.e. temperature) is, probably, that of Collins (1990). Such disparities are, at least partly, due to the di⁄culty of sampling oceanic bottom waters compared to the simplicity of collecting planktic samples and measuring chemico-physical properties of surface waters. Although the above-mentioned studies have not de¢nitely established a clear relationship between environmental changes and coiling direction in benthic foraminifera, temperature seems to be the only environmental factor in£uencing the coiling ratio of this group (see review in Boltovskoy et al., 1991). However, the coiling direction preference in benthic foraminifera is also in£uenced by mode of reproduction (FÖyn, 1936, 1937; Myers, 1940; Lee et al., 1963; Nigam and Rao, 1989; Nigam and Khare, 1992). This parameter has, therefore, two potential applications: data from species in which coiling preference is environmentally controlled can be used as a proxy for sea £oor palaeotemperature estimates while heritable coiling patterns may be useful in establishing phylogenetic relationships among species. In this study we explore the possibility that the coiling ratio (number of sinistral versus dextral individuals) in the benthic foraminifer Cibicidoides pseudoacutus can be the response to a rapid change of bottom water temperature following the K^T boundary event. Cibicidoides pseudoacutus was chosen for its continuous distribution and relatively high abundance across the K^T boundary. Other trochospiral taxa, although continuously present across the K^T boundary, do not occur in su⁄cient numbers to allow a thorough evaluation of the coiling ratio record. Sample preparation included gentle crushing, soaking overnight in a peroxide solution, washing
PALAEO 2755 1-5-02
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
203
Table 1 Cibicidoides pseudoacutus coiling ratio values for the studied sections and average values for speci¢c intervals
through a 63-Wm ¢ne mesh and drying at 80‡C. All Cibicidoides pseudoacutus individuals were picked out, glued onto micropalaeontological slides, and counted. As reproductive mode strongly in£uences coiling direction, only smallsized forms (megalospheric, asexually produced) were used in this study to avoid a genetic in£uence in the calculation of the coiling ratio. Data for the studied sections are reported in Table 1 together with average values for speci¢c intervals.
4. Results The Cibicidoides pseudoacutus coiling ratio rec-
ord from the studied sections is shown in Fig. 4 next to lithostratigraphy and planktonic foraminiferal zonation. Mean values in the uppermost Maastrichtian are quite similar in all sections ranging from 0.81 at El Melah to 1.05 at Elles. Rather stable values are observed in the uppermost Maastrichtian of El Kef. In the uppermost Maastrichtian of Elles, Ain Settara, and El Melah sections, the Cibicidoides pseudoacutus coiling ratio record shows larger £uctuations but it is never lower than 0.41 and never exceeds 1.56. At Elles and El Kef, a sharp spike in the proportion of sinistral specimens occurs at the K^T boundary. At El Kef, the coiling ratio in the ¢rst 6.5 cm of Zone P0 averages 1.75 with the highest
PALAEO 2755 1-5-02
204
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
PALAEO 2755 1-5-02
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
205
Fig. 5. Stable isotope and benthic foraminiferal record across K^T boundary (KTB) of El Kef. Planktonic foraminiferal zonation after Smit in Brinkhuis et al. (1994) and Smit in Brinkhuis et al. (1998). Oxygen isotope records from Keller and Lindinger (1989). The N18 O (¡) curve represents the ¢ne fraction ( 6 25 Wm) record while the N18 O (bf) curve represents benthic foraminiferal (Cibicidoides pseudoacutus) values. Faunal density expresses number of benthic foraminifera per g of dry sediment in the fraction greater than 125 Wm.
value (2.67) recorded just above the K^T boundary. A similar pattern is observed at Elles where the highest value (2.45) is recorded in the basal 2 cm of the boundary clay layer and high average values (1.5) are recorded in the lower third of Zone P0. A second, minor spike in the Cibicidoides pseudoacutus coiling ratio is recorded in both the Elles and El Kef sections at 10 cm and 6.5 cm above the K^T boundary, respectively. Upsection, the C. pseudoacutus coiling ratio average values decrease to 0.89 at El Kef and to 1.04 at Elles and therefore revert to uppermost Maastrichtian mean values. The spike in the proportion of sinistral individuals which has been observed at Elles and El Kef is absent in the Ain Settara section where average values are 0.92 in the Maastrichtian and 1.13 in the Danian. In the ¢rst 1 cm of the boundary clay layer, that is within Zone P0, the C. pseudoacutus coiling ratio is 0.96. A comparison with previously published data on benthic foraminifera from El Kef (Coccioni and Galeotti, 1998; Galeotti, 1998) indicates that the shift of the Cibicidoides pseudoacutus coiling ratio is associated with dramatic £uctuations of benthic foraminiferal faunal parameters including changes in the faunal density (i.e. the number of specimens per g of dry sediment), and proportion of epifaunal species (Fig. 5) as well as species diversity and relative abundance of oxygen indices. Semi-quantitative data from Elles indicate a comparable pattern of faunal changes in benthic foraminiferal assemblages across the K^T boundary. An increased relative abundance of C. pseudoacutus above the K^T boundary was observed in all the studied sections. In the El Kef K^T boundary transition C. pseudoacutus forms up to 60% of the whole lowermost Danian benthic foraminiferal assemblage with highest relative abundance recorded some 15 cm above the K^T boundary, therefore above the observed shift in its coiling ratio (Fig. 5).
5. Discussion 5.1. Biostratigraphic implications and completeness of K^T boundary Tunisian transitions Changes observed in benthic foraminiferal assemblages across the K^T boundary in Tunisian sections are comparable to the observations made in neritic to upper bathyal settings such as Brazos River (Keller, 1992), Millers Ferry (Olsson et al., 1996), Nye KlÖv and Stevns Klint (Schmitz et al., 1992; Coccioni and Galeotti, 1998), and Sinai Negev (Keller, 1992). In these areas, the following characteristic succession of benthic foraminiferal assemblages is observed within the K^T boundary interval: (1) late Maastrichtian foraminiferal assemblages are well diversi¢ed and characterised by complex trophic structures ; (2) foraminiferal assemblages within and directly above the boundary clay layer or Fish Clay are dominated by epifaunal species (Cibicidoides, Anomalinoides, and Gavelinella); (3) a slow recovery which is achieved only 200^300 ka after the K^T boundary event occurs in the lowermost Danian. On this basis, Coccioni and Galeotti (1998) distinguished a lowermost Danian shallow water ‘epifaunal domain’ which might be of some use for stratigraphic correlations across the K^T boundary transition. However, this succession of events is only useful for low-resolution correlations. Besides their facies dependence this is the reason why benthic foraminifera are seen as a group of little use for stratigraphic correlations across the K^T boundary, especially if compared to other extinction events (i.e. the Palaeocene^Eocene boundary) The shift in the coiling ratio of Cibicidoides pseudoacutus is a potential high-resolution correlation tool probably representing an ecological event spanning the lower part of Zone P0 which might be traceable over shallow water settings from the Tethys. According to the time scale of
PALAEO 2755 1-5-02
206
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
Berggren et al. (1995) and assuming a constant sedimentation rate of the boundary clay layer, the shift in C. pseudoacutus coiling ratio would, in fact, have lasted for some 7 ka after the K^T boundary event. It is noteworthy that the major spike recorded just above the K^T boundary is followed by a second minor spike occurring at one third of Zone P0 in both Elles and El Kef sections (see Fig. 4). Assuming that this second spike represents the same event in these sections, a further resolution of this event is possible. In particular, the interval where the double spike occurs is slightly thicker at Elles than observed in the stratotype section, suggesting that the basal part of the boundary clay layer is more expanded in the former section. Accordingly, the thickness of Zone P0 is slightly higher at Elles (28 cm) than observed El Kef (23 cm). The lack of a shift in Cibicidoides pseudoacutus coiling ratio in the Ain Settara section suggests that a stratigraphic gap straddling at least the lower third of Zone P0 occurs in this section. Accordingly, Luciani (2002) reports Zone P0 to be restricted to the basal 2 cm of the boundary clay layer at Ain Settara. 5.2. Palaeoecological implications The development of sinistrally coiled dominated Cibicidoides pseudoacutus populations at the K^T boundary is associated with dramatic £uctuations of other benthic foraminiferal faunal parameters. In agreement with Keller (1988a) and Speijer and van der Zwaan (1994, 1996), quantitative data from El Kef (Fig. 5) can be interpreted in terms of decreased oxygenation and organic £uxes to the sea £oor in correspondence to the K^T boundary. At El Kef the response of benthic foraminiferal assemblages to the K^T boundary event mainly occurred in terms of development of an epifaunal-dominated assemblage associated with a drop in the number of species (species richness) and number of specimens per sample (faunal density). The semi-quantitative observation carried out in the Elles section suggests that sudden and marked changes comparable to those observed at El Kef also occurred across the K^T
boundary at this site. In particular, a drastic decrease of species richness and faunal density occurs in response to the K^T boundary event. At Elles and El Kef, such conditions persisted at least up to the top of the studied intervals. On the other hand, according to Keller (1988a) and Speijer and van der Zwaan (1994, 1996), the complete recovery of benthic foraminiferal communities is observed at El Kef only 10 m above the K^ T boundary, that is 200^300 ka after the K^T boundary event. However, the shift in the Cibicidoides pseudoacutus coiling ratio observed at El Kef and Elles straddles the ¢rst 6.5 cm and 10 cm of Zone P0, respectively (Fig. 4). This event therefore terminated well before the end of the series of sea £oor perturbations indicated by the benthic foraminiferal faunal parameter record. This disparity suggests that the development of sinistrally coiled populations of C. pseudoacutus was not a direct response to decreased organic £uxes and/or development of dysaerobic conditions on the sea £oor. In addition, the C. pseudoacutus coiling ratio record does not match the record of the relative abundance of the species on which it is calculated. In the El Kef section, the highest relative abundance of this species is, in fact, recorded 15 cm above the K^T boundary where the coiling ratio has already reverted to pre- K^T boundary values (Fig. 5). It is, therefore, reasonable to assume that the development of sinistrally coiled dominant populations of C. pseudoacutus following the K^ T boundary was independent of changing redox and trophic conditions, and expansion of ecological niches particularly favourable for this species. In Recent and fossil assemblages as well as in laboratory cultures, coiling direction in benthic foraminifera has been shown to be in£uenced by reproductivity mode (FÖyn, 1936, 1937; Lee et al., 1963; Myers, 1940; Nigam and Rao, 1989), bioprovincialism (Hallock and Larsen, 1979) or environmental factors (Longinelli and Tongiorgi, 1960; Hallock and Larsen, 1979). Collins (1990) claimed for the ¢rst time demonstration of an unambiguous correlation between temperature and coiling direction in benthic foraminiferal populations. She reported that populations of predominantly dextrally coiled Bulimina
PALAEO 2755 1-5-02
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
marginata and Bulimina aculeata in two distinct areas (Gulf of Maine to New Jersey and Gulf of Mexico) are strongly associated with warm temperatures. However, the opposite is not true: predominantly sinistrally coiled populations are not associated with cold temperatures. Boltovskoy et al. (1991) reviewed the morphological variations of benthic foraminiferal tests in response to changes in ecological parameters. They concluded that notwithstanding some contradictory evidence, changes in temperature apparently result mostly in test size variation with population of larger individuals associated with lower temperature. However, they continue, it is quite probable, although the evidence is still tenuous and more study is required, that changes in temperature can a¡ect sinistral/dextral ratio in some species. The coiling ratio of some benthic foraminiferal species is therefore a potential proxy for palaeotemperature estimates of bottom waters. The observed shift in the Cibicidoides pseudoacutus coiling ratio can be interpreted in terms of either changing coiling preference within an original population having a V1:1 coiling ratio or (similarly to that suggested for Recent planktic foraminifera) immigration of a sinistral coil-dominated population from a di¡erent bioprovince. Evidence of genotypic isolation has been, in fact, observed in modern species of benthic foraminifera based on nuclear-coded ribosomal DNA sequences (Tsuchiya et al., 1998). On the other hand, as testi¢ed by benthic foraminiferal assemblages, sea £oor environmental changes following the K^T boundary at El Kef were su⁄ciently dramatic and sudden to have hampered the adaptation of an original population of C. pseudoacutus to stressed environmental conditions. Such a mechanism might have allowed the immigration of a sinistral coil-dominated C. pseudoacutus population from a di¡erent bioprovince (possibly from a di¡erent palaeobathymetric setting). In agreement with Keller (1988a) such a hypothesis would imply large sea level £uctuations in correspondence to the K^T boundary at El Kef. However, the presence in the lower upper bathyal El Melah section of C. pseudoacutus populations with a coiling ratio similar (if not lower) to that
207
observed in the shallower depositional setting of El Kef and Elles rules out a mechanism of immigration from a deeper bioprovince. The possibility of a downward bathymetric migration also seems unlikely. In fact, in the outer neritic Elles section, C. pseudoacutus populations have mean Maastrichtian values in coiling ratios similar to that observed in the upper bathyal setting of El Kef. In line with observations made on Recent benthic foraminiferal species, the observed shift in the coiling ratio of Cibicidoides pseudoacutus can be better interpreted as a response to a drastic change of bottom water temperature. A general correlation between benthic N18 O and C. pseudoacutus coiling ratio records cannot be traced in the surveyed interval at Elles and El Kef also because di¡erent sample sets have been used. However, the development of the sinistrally coiled population in C. pseudoacutus is associated with sharp shifts to heavier N18 O values in both the Elles and the El Kef sections and can be regarded as a response to a cooling of bottom waters. In particular, the benthic foraminiferal (C. pseudoacutus) N18 O record at El Kef shows a 0.5x positive excursion in the lowermost Danian, implying a sharp bottom water cooling at the K^T boundary (Keller and Lindinger, 1989; Fig. 5). Moreover, Stueben et al. (1998) have recently reported a 1.5x increase of N18 O benthic values suggesting that bottom waters signi¢cantly cooled during the deposition of the basal 5 cm of the boundary clay layer at Elles. Though not discussed in the text, these authors also show an abrupt shift to heavier values in planktic N18 O ratios at the K^T boundary in their ¢g. 1. By contrast, ¢ne fraction ( 6 25 Wm) oxygen isotope values from El Kef would suggest that a sea surface warming occurred in the lowermost Danian (Keller and Lindinger, 1989; Fig. 5). However, as pointed out by the latter authors, diagenetic and preservational e¡ects are variable across the K^T boundary at El Kef with preservation generally improved in the low-CaCO3 sediments of the boundary clay layer. The negative shift recorded in the ¢ne fraction N18 O ratio record at the K^T boundary of El Kef might, therefore, be due to comparison with underlying and overlying over-estimated, diagenetically altered values. Quantitative changes in
PALAEO 2755 1-5-02
208
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
dino£agellate cyst assemblages indicate that cooler surface water conditions occurred in the ¢rst 10 cm of Zone P0 at El Kef (Brinkhuis et al., 1998). These changes in the dino£agellate cyst assemblages correspond to the same stratigraphic interval that contains the shift in the C. pseudoacutus coiling ratio, indicating that a concomitant cooling of surface and bottom waters occurred in the earliest Danian at El Kef. Such a short-term climatic deterioration would generally agree with model simulation of the primary and secondary e¡ects of an extraterrestrial impact event which postulate that a short-term ‘impact winter’ accompanied the K^T boundary event (e.g. Pope et al., 1994; Ivanov et al., 1996). However, a problem arises when considering the duration of such a ‘cooling event’, the approximation of which represents a crucial point for discriminating between the various scenarios of the climate e¡ects which followed the K^T boundary event. Impact model-derived estimates currently available postulate that the ‘impact winter’ lasted from several months to a decade (Pope et al., 1994) whereas the cooling phase as indicated by the shift in the coiling direction of Cibicidoides pseudoacutus, invasion of boreal dino£agellate cyst, and isotope record would have lasted some 7 ka. Following Brinkhuis et al. (1998), as an alternative to this scenario, we must consider that the sedimentation rate of the boundary clay layer might have been, at least in its lower part, extremely high, rather than the canonical view of being extremely condensed.
6. Conclusions The study of coiling ratios in megalospheric individuals of the benthic foraminifer Cibicidoides pseudoacutus across the K^T boundary from four Tunisian sections reveals that this species developed sinistral coil-dominated populations in the lowermost Danian at El Kef and Elles. Such a shift in coiling direction preference cooccurs with the invasion of cold water (boreal) dino£agellate species and a positive shift in benthic foraminiferal N18 O values. It is, therefore, interpreted as a response to a short-term cooling
of bottom waters which lasted for some 7 ka after the K^T boundary event. Besides its palaeoecological, possibly palaeoclimatic signi¢cance, the development of sinistrally coiled populations in Cibicidoides pseudoacutus is a potential tool to assess the completeness of stratigraphic sequences in Tethyan shallow water settings just above the K^T boundary.
Acknowledgements Alessandro Montanari and Gerta Keller are kindly thanked for reading earlier drafts of the manuscript. Gerta Keller is also thanked for providing samples from the El Melah section. We thank M.B. Hart and an anonymous reviewer for their helpful suggestions which greatly improved the manuscript. This paper benefited from MURST 60% funding to R.C.
Appendix. Taxonomic notes Cibicidoides pseudoacutus (Nakkady) 1950 1988 1994
Anomalina pseudoacuta Nakkady, p. 691, pl. 90, ¢gs. 29^32. Anomalinoides acuta (Plummer), Keller, pl. 2, ¢gs. 9, 10, 12, 13. Cibicidoides pseudoacutus (Nakkady), Speijer and Van der Zwaan, pl. 7, Fig. 6a^c.
Test trochospirally coiled, planoconvex to biconvex, distinctly perforate, periphery subacute. Some specimens show a distinct keel. The umbilical side which is partially involute possesses a pronounced knob surrounded by growths extending up to the periphery. In its original description, Nakkady (1950) reported Cibicidoides pseudoacutus to be very similar to Anomalina acuta (Plummer). However, C. pseudoacutus di¡ers from the latter species in possessing a peripheral aperture which does not extend towards the umbilicus. For its biconvex aspect and partially involute spiral side C. pseudoacutus shows a remarkable a⁄nity to the genus Anomalinoides. However, due to the position and form of the aperture, we assign this species to the genus Cibi-
PALAEO 2755 1-5-02
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
cidoides in agreement with Speijer and Van der Zwaan (1994). References Adatte, T., Keller, G., Liangquan, L., Stinnesbeck, W., Zaghnin-Turky, D., 1998. Climate and sea-level £uctuations across the KT boundary in Tunisia: Warm and humid conditions linked to deccan volcanism? International Workshop on Cretaceous-Tertiary Transition, Volume of Abstracts, 7^ 10. Berggren, W.A., Kent, D.V., Swisher, C.C., III, Aubry, M-.P., 1995. A revised Cenozoic geochronology and chronostratigraphy. SEPM Spec. Publ. 54, 129^213. Boltovskoy, E., Scott, D.B., Medioli, F.S., 1991. Morphological variations of benthic foraminiferal tests in response to changes in ecological parameters: a review. J. Paleontol. 65, 175^185. Brinkhuis, H., Zachariasse, W.J., 1988. Dino£agellate cysts, sea level changes and planktonic foraminifers across the Cretaceous-Tertiary boundary at El Haria, Northwest Tunisia. Mar. Micropaleontol. 13, 153^191. Brinkhuis, H., Romein, A.J.T., Smit, J., Zachariasse, W.J., 1994. Danian-Selandian dino£agellate cysts from lower latitudes with special reference to the El Kef section, NW Tunisia. GFF 116, 46^48. Brinkhuis, H., Bujak, J.P., Smit, J., Versteegh, G.J.M., Visscher, H., 1998. Dino£agellate-based sea surface temperature reconstructions across the Cretaceous-Tertiary boundary. Palaeogeogr. Palaeoclimatol. Palaeoecol. 141, 67^83. Brummer, G.J.A., Kroon, D., 1988. Genetically controlled planktonic foraminiferal coiling ratios as tracers of past ocean dynamics. In: Brummer, G.J.A., Kroon, D. (Eds.), Planktonic Foraminifera as Tracers of Ocean-Climate History: Ontology, Relationships and Preservation of Modern Species and Stable Isotopes, Phenotypes and Assemblage Distribution in Di¡erent Water Masses. Free University Press, Amsterdam, pp. 293^297. Burollet, P.F., 1967. Tertiary Geology of Tunisia. In: Guidebook to the Geology and History of Tunisia. Petroleum Exploration Society, Libya, 9th Annu. Field Conf., pp. 215^225. Coccioni, R., Galeotti, S., 1998. What happened to small benthic foraminifera at the K/T Boundary? Bull. Soc. Ge¤ol. France 169, 271^279. Collins, L.S., 1990. The correspondence between water temperature and coiling direction in Bulimina. Paleoceanography 5, 289^294. Cowie, J.W., Ziegler, W., Remane, J., 1989. Stratigraphic commission accelerates progress 1984^1989. Episodes 112, 79^ 83. Dupuis, C., Steurbaut, E., Matmati, M.F., 1998. The Ain Settara K/T boundary section. Main results and comparison with the El Kef stratotype. International Workshop on Cretaceous-Tertiary Transition, Volume of Abstracts, 67^68.
209
FÖyn, B., 1936. Foraminiferenstudien I. Der Lebenszyklus von Discorbina vilardeboana d’Orbigny. Bergens Mus. Arb. Naturvidensk. Rekke 2, 1^22. FÖyn, B., 1937. Foraminiferenstudien II. Zur Kenntinis der asexuellen Fortp£anzung und Entwicklung der Gamonten von Discorbina vilardeboana d’Orbigny. Bergens Mus. Arb. Naturvidensk. Rekke 2, 1^22. Galeotti, S., 1998. Crisi Biologiche e Foraminiferi Bentonici Attraverso i Limiti Cretaceo/Terziario, Paleocene/Eocene e Eocene/Oligocene, Ph.D. Thesis, University of Parma, 205 pp. Hallock, P., Larsen, A.R., 1979. Coiling direction in Amphistegina. Mar. Micropaleontol. 4, 33^44. Hemleben, C., Spindler, M., Anderson, O.R., 1989. Modern Planktonic Foraminifera. Springer-Verlag, New York, 363 pp. Ivanov, B.A., Badukov, D.D., Yakovlev, O.I., Gerasimov, M.V., Dikov, Y.P., Pope, K.O., Ocampo, A.C., 1996. Degassing of sedimentary rocks due to Chicxulub impact: Hydrocode and Physical simulations. In: Ryder, G., Fastkovsky, D., Gartner, S. (Eds.), The Cretaceous-Tertiary event and other catastrophes in Earth history. Geological Society of America Special Paper 307, pp. 125^140. Keller, G., 1988a. Extinction survivorship and evolution of planktic foraminifera across the Cretaceous-Tertiary boundary at El Kef, Tunisia. Mar. Micropaleontol. 13, 239^263. Keller, G., 1988b. Biotic turnover in benthic foraminifera across the Cretaceous-Tertiary boundary at El Kef, Tunisia. Palaeogeogr. Palaeoclimatol. Palaeoecol. 66, 153^171. Keller, G., 1992. Paleoecological response of Tethyan benthic foraminifera to the Cretaceous-Tertiary boundary transition. In: Takayanagi, Y., Saito, T. (Eds.), Studies in Benthic Foraminifera. Tokai University Press, Tokyo, pp. 77^91. Keller, G., Lindinger, M., 1989. Stable isotope, TOC and CaCO3 record across the Cretaceous/Tertiary boundary at El Kef, Tunisia. Palaeogeogr. Palaeoclimatol. Palaeoecol. 73, 243^265. Keller, G., Li, L., MacLeod, N., 1995. The Cretaceous/Tertiary boundary stratotype section of El Kef, Tunisia: how catastrophic was the mass extinction? Palaeogeogr. Palaeoclimatol. Palaeoecol. 119, 221^254. Keller, G., Adatte, T., Stinnesbeck, W., Luciani, V., Karoui, N., Zaghbib-Turki, D., 2002. Paleoecology of the Cretaceous-Tertiary mass extinction in planktic foraminifera. Palaeogeogr. Palaeoclimatol. Palaeoecol. 178, 257^297. Kennett, J.P., 1976. Phenotypic variation in some recent and Late Cenozoic planktonic foraminifera. In: Hedley, R.H., Adams, C.G. (Eds.), Foraminifera, Volume 2. Academic Press, San Diego, CA, pp. 111^170. Kouwenhoven, T.J., Speijer, R.P., Vanoosterhout, C.W.M., Van der Zwaan, G.J., 1997. Benthic foraminiferal assemblages between two major extinction events ^ the Paleocene El Kef section, Tunisia. Mar. Micropaleontol. 29, 105^ 127. Kuhnt, W., Kaminski, M.A., 1996. The response of benthic foraminifera to the K/T boundary event ^ A review. Ge¤ol. Afr. Atl. Sud Actes Colloq. Angers 1994, 433^442.
PALAEO 2755 1-5-02
210
S. Galeotti, R. Coccioni / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 197^210
Lee, J.J., Freudenthal, H.D., Muller, W.A., Koosoy, V., Pierce, S., Grossman, R., 1963. Growth and Physiology of Foraminifera in the laboratory. Part 3, Initial studies of Rosalina £oridana. Micropaleontology 9, 449^466. Longinelli, A., Tongiorgi, E., 1960. Frequenza degli individui destrogiri in diverse popolazioni di Rotalia beccarii Linneo. Boll. Soc. Paleontol. Ital. 1, 5^16. Luciani, V., 2002. High-resolution planktonic foraminiferal analysis from the Cretaceous/Tertiary boundary at Ain-Settara (Tunisia). Palaeogeogr. Palaeoclimatol. Palaeoecol. 178, 299^319. Molina, E., Arenillas, I., Arz, J.A., 1998. Mass extinction in planktic foraminifera at the Cretaceous/Tertiary boundary in subtropical and temperate latitudes. Bull. Soc. Ge¤ol. France 169, 351^363. Myers, E.H., 1940. Observation on the origin and fate of £agellated gametes in multiple tests of Discorbis (Foraminifera). J. Mar. Biol. Assoc. UK 24, 201^226. Nakkady, S.E., 1950. A new foraminiferal fauna from the Esna shales and upper Cretaceous Chalk of Egypt. J. Paleontol. 24, 675^692. Nigam, R., Rao, A.S., 1989. The intriguing relationship between coiling direction and reproductive mode in benthic foraminifera. J. Palaeontol. Soc. India 34, 79^82. Nigam, R., Khare, N., 1992. The reciprocity between coiling direction and dimorphic reproduction in benthic foraminifera. J. Micropaleontol. 11, 221^228. Nyong, E.E., Olsson, R.K., 1983/4. A paleoslope model for Campanian to Lower Maestrichtian Foraminifera in the North America Basin and adjacent continental margin. Mar. Micropaleontol. 8, 437^477. Olsson, R.K., Nyong, E.E., 1984. A paleoslope model for Campanian-Lower Maastrichtian Foraminifera of New Jersey and Delaware. J. Foraminifer. Res. 14, 50^68. Olsson, R.K., Liu, C., van Fossen, M., 1996. The Cretaceous/ Tertiary catastrophic event at Millers Ferry, Alabama. In: Ryder, G., Fastkovsky, D., Gartner, S. (Eds.), The Cretaceous-Tertiary Event and Other Catastrophes in Earth History. Geological Society of America Special Paper 307, pp. 263^277. Pope, K.O., Baines, K.H., Ocampo, A.C., Ivanov, B.A., 1994. Impact winter and the Cretaceous/Tertiary extinctions: Results of a Chicxulub asteroid impact model. Earth Planet. Sci. Lett. 128, 719^725. Pospichal, J.J., 1994. Calcareous nannofossil at the K-T boundary, El Kef: No evidence for stepwise, gradual or sequential extinctions. Geology 22, 99^102. Robin, E., Rocchia, R., Lefevre, I., Pierrard, O., Dupuis, C., Smit, J., Zaghib-Turki, D., Karoui, N., Matmati, F., 1998. The compositional Variation of K/T spinel in Tunisia: Evidence for a global Deluge of Projectile debris. International
Workshop on Cretaceous-Tertiary Transition, Volume of Abstracts, 49^50. Schmitz, B., Keller, G., Stenvall, O., 1992. Stable isotope and foraminiferal changes across the Cretaceous-Tertiary boundary at Stevns Klint, Denmark: Arguments for long-term oceanic instability before and after bolide-impact event. Palaeogeogr. Palaeoclimatol. Palaeoecol. 96, 233^260. Sliter, W.V., 1972. Upper Cretaceous planktonic foraminiferal zoogeography and ecology ^ eastern Paci¢c margin. Palaeogeogr. Palaeoclimatol. Palaeoecol. 12, 15^31. Sliter, W.V., Baker, A., 1972. Cretaceous bathymetric distribution of benthic foraminifera. J. Foraminifer. Res. 2, 167^ 183. Smit, J., Romein, A.J.T., 1985. A sequence of events across the Cretaceous-Tertiary boundary. Earth Planet. Sci. Lett. 74, 155^170. Smit, J., Keller, G., Zargouni, F., Razgallah, S., Shimi, M., BenAbdelkader, O., Ben Haj Ali, N., Ben Salem, H., 1997. The El Kef sections and sampling procedures. Mar. Micropaleontol. 29, 67^69. Speijer, R.P., Van der Zwaan, G.J., 1994. Extinction and survivorship patterns in southern Tethyan benthic foraminiferal assemblages across the Cretaceous/Paleogene boundary. Geol. Ultraiect. 124, 19^64. Speijer, R.P., Van der Zwaan, G.J., 1996. Extinction and survivorship of southern Tethyan benthic foraminifera across the Cretaceous/Palaeogene boundary. In: Hart, M.B. (Ed.), Biotic Recovery from Mass Extinction Events. Geological Society Special Publication 102, pp. 343^371. Stinnesbeck, W., Keller, G., Adatte, T., 1998. Lithological Characteristics of the K/T transitions in Tunisia: Evidence of a tsunami event? International Workshop on CretaceousTertiary Transition, Volume of Abstracts, 53^54. Stueben, D., Kramar, U., Berner, Z., Keller, G., 1998. Trace elements and stable isotopes in foraminifera of the Elles K/T pro¢le: Indications for sea level £uctuations and primary productivity. International Workshop on Cretaceous-Tertiary Transition, Volume of Abstracts, 55^56. Tsuchiya, M., Kitazato, H., Pawlowski, J., 1998. Phylogenetic relationships among three genera within the subfamily Glabratellidae from Japan based on ribosomal DNA sequences. FORAMS ’98, International Symposium on Foraminifera, Monterrey, Mexico, July 5^12, Sociedad Mexicana de Paleontologia, A.C., Special Publication 107. van Morkhoven, F.P.C.M., Berggren, W.A., Edwards, S.A., 1986. Cenozoic cosmopolitan deep-water benthic foraminifera. Bull. Cent. Rech. Explor. Prod. Elf-Aquitaine Me¤m. 11, 1^412. Vincent, E., Berger, W.H., 1981. Planktonic foraminifera and their use in paleoceanography. In: Emiliani, C. (Ed.), The Oceanic Lithosphere. Sea 7, 1025^1119.
PALAEO 2755 1-5-02