Biological response to Milankovitch forcing during the Late Albian (Kirchrode I borehole, northwestern Germany)

Palaeogeography, Palaeoclimatology, Palaeoecology 174 (2001) 269±286

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Biological response to Milankovitch forcing during the Late Albian (Kirchrode I borehole, northwestern Germany) M.E. Weber*, J. Fenner, A. Thies, P. Cepek Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, 30655 Hannover, Germany Received 13 May 1997; Revised 4 June 1999; Accepted for publication 17 September 2000

Abstract We studied the biological response to orbital forcing in marine Upper Albian sediments recovered from the 245 m-long Kirchrode I borehole in the Lower Saxony basin in northwestern Germany. Results from quantitative analysis of planktonic and benthic foraminifera, of calcareous nannofossils, and radiolaria were used for this study. Spectral analysis in the depth domain indicates for the high sedimentation rate part of the Upper Albian dominant periods with wavelengths of 10±13 m, 5±6 m, and 2±3 m, which we interpret to represent the biological response to orbital forcing in the Milankovitch frequency bands eccentricity, obliquity, and precession, respectively. In addition, a low amplitude 40±50 m cycle was found, which would represent the long-term eccentricity variation of roughly 400 ka. Microfossil cyclicity does not change signi®cantly within the whole core indicating sedimentation rates of 11±12 cm/ka on an average, with variations between 3.5 and 13 ka). Microfossils show greater variability in their abundance changes than the physical and chemical parameters and also greater power in the higher-frequency bands (obliquity and precession). While most of the planktonic foraminifer species studied are dominated by variations in the obliquity, most benthic foraminifer species show an additional strong in¯uence of precession. These differences in the cyclicity of the abundance changes are interpreted as re¯ecting a stronger in¯uence of low latitude water in the deep waters of the Late Albian Lower Saxony basin than in the shallow waters. This basin was part of a wide, `Boreal' epicontinental sea, which was connected to the Tethys to the south via the Polish strait and via the Paris basin, and which was connected with the North Atlantic and Arctic to the north. In analogy to results from analysis of data from the Late Neogene, strong effects of precession interpreted as being more characteristic for changes/in¯uences triggered in the low latitudes and those of obliquity to be more characteristic for in¯uences from the high latitudes. The presence of a relatively strong eccentricity cycle, not only in the compound parameters, but also in the abundance changes of single species during the Late Albian means that there must have been a non-linear response to orbital forcing and internal feedbacks. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Milankovitch; Microfossils; Biological cycles; Spectral-analytic; Albian

1. Introduction As part of the international ALBICORE project and * Corresponding author. Department of Geodesy and Geomatics Engineering, University of New Brunswick, P.O. Box 4400, Frederickton, NB, Canada E3B 5A3, Fax: 11-506-453-4943. E-mail address: [email protected] (M.E. Weber).

the Special Research Program of the German Science Foundation (DFG) `Cretaceous sedimentation; Global and regional controls on biogenic sedimentation', an interdisciplinary group of geoscientists studied the Upper Albian of the Kirchrode I research borehole drilled in the Kirchrode district of Hannover, Germany (Fig. 1). Continuous coring of this borehole recovered 245 m of Upper Albian sediments with a

0031-0182/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0031-018 2(01) 00297-8

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Fig. 1. Albian palaeogeography of northwestern Europe showing the location of the Kirchrode I borehole. Modi®ed after Schott (1969) and Ziegler (1982).

recovery close to 100%. Investigations focus on the reconstruction of palaeoceanographic and palaeoclimatic evolution during the mid-Cretaceous greenhouse time. The drill site was chosen in the Lower Saxony basin in a rapidly subsiding part of the broad northwest European shelf sea. The palaeo-location was far enough from the Albian coast to prevent input of sand but close enough to guarantee high sedimentation rates from input of ®ne-clastic detritus (Fenner, 2001a,b). These Upper Albian basinal sediments consist of light gray, clayey marlstones and mudstones alternating with darker gray, marly claystones. Cyclic deposition took place above the carbonate compensation depth, and carbonate dissolution has not been recognised as a factor in¯uencing microfossil abundances and preservation. Benthic foraminifera indicate a water depth around or below 100 m; shallow-water species are absent (Fenner et al., 1996). Generally, sediment cycles on time scales of thousands to a few tens of thousands of years re¯ect climatic oscillations that are controlled by variations in the earth's orbital eccentricity, obliquity and precession. These frequencies, called Milankovitch frequencies, affect the global, seasonal and latitudinal distribution of the incoming solar insolation (e.g. Hilgen et al., 1997). The present study focuses on

how such orbitally induced frequency variations are documented by changes in abundance of microfossil groups and species in the Kirchrode I borehole. Cyclic ¯uctuations in the Upper Albian in this borehole are documented by physicochemical parameters, such as gamma-ray logs (Prokoph and Thurow, 2001; Wonik, 2001), calcium carbonate ¯uctuations (Jendrzejewski et al., 2001), Ca/Al ratios (Rachold and Brumsack, 2001), sedimentcolor values (Prokoph and Thurow, 2001), and shown by these authors to re¯ect Milankovitch cyclicity. Whereas these data sets re¯ect composite signals, microfossil data have the advantage of allowing studies of the biological response of species groups and even of single species to orbital forcing. Microfossil data provide palaeoceanographic and palaeoclimatic proxies, which are dif®cult to extract from physical and chemical parameters such as nutrient availability, temperature, current intensity, and information on the origin of water masses occurring in this mid-latitude `Boreal' sea from low (Tethys) and high northern latitudes. Spectral analysis of such data sets is a useful tool, because it can provide a more detailed understanding of the response of the biological system of deep shelf basins to orbital forcing under conditions of extreme warm climate (greenhouse situation) and to ¯uctuations in sea level.

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Fig. 2. Sketch illustrating depth habitat of microfossil groups within the water column.

2. Material and methods 2.1. Microfossil material Benthic and planktonic foraminifera, calcareous nannofossils, and radiolaria were used for this study. These microfossil groups comprise unicellular microfauna and micro¯ora from different habitats (Fig. 2). Calcareous nannofossils, dependent on photosynthesis, live in the euphotic zone. From modern planktonic radiolaria and foraminifera, we know that different species occupy different depth levels in the water column. For their fossil representatives, no accurate depth habitats are known. Water depth models for the Cretaceous (e.g. Hart and Bailey, 1979; Hart, 1980; Carron, 1983; Carron and Homewood, 1983; Leckie, 1987; Leary and Hart, 1989) indicate that deep water (.100 m) planktonic foraminifera species in the Upper Albian of Kirchrode I (Praeglobotruncana delrioensis and Rotalipora appenninica) occur only in the upper part of the Upper Albian deposits, while Hedbergella planispira, H. delrioensis, H. aff. trocoidea, Favusella hiltermanni, F. washitensis and Globigerinelloides bentonensis, which lived in surface water down to the lower part of the mixed surface layer, dominate the assemblages throughout the Upper Albian. Benthic foraminifera represent different habitats of the sea¯oor: epifauna lives on the sediment surface, while infauna reside in the uppermost centimeters of the sediment. Within the benthic realm, foraminifera occupy different niches by utilizing specialized food acquisition strategies, such as epifaunal suspension feeding, grazing and infaunal detritus feeding. Two different modes of test construction exist among inbenthic and epibenthic species, secretion of calcium carbonate shells (calcareous foraminifera), or cement-

271

ing of sediment grains either by organic compounds or calcium carbonate (arenaceous foraminifera). In the Upper Albian of the Kirchrode I borehole, benthic foraminifera are the group with the highest diversity; 234 taxa have been recognised and counted. Of these, we selected for this study those species which are suf®ciently abundant throughout the core and which represent different habitats or ecological conditions. The selected benthic foraminifera taxa and their mode of living are listed in Table 1. Of these benthic taxa, the nine benthic included in Table 2 and listed in detail in Thies (2001) comprise 43±85% of the benthic foraminifer fauna in samples from Kirchrode I (mean is 57%). In addition, we used the ratio of planktonic to benthic foraminifera (P/B), the ratio of calcareous to arenaceous foraminifera, and the number of planktonic and benthic foraminifera shells per gram sediment (Thies, 2001). Of the 134 calcareous nannofossil taxa identi®ed, the relative abundance was determined by counting 300 specimens per sample (Cepek, 1995). For spectral analysis, we chose 13 abundant species: Biscutum constans, B. magnum, Discorhabdus rotatorius, Glaukolithus compactus, G. diplogrammus, G. sp., Lithraphidites carniolensis, Predicosphaera avitus, P. spp., Rhagodiscus asper, Vekshinella stradneri, Watznaueria barnesae and Zeughrabdotus erectus. Together, these species comprise 62±87% of the nannofossil ¯ora (the mean is 74%). They include species of different solubility of their skeletal elements, e.g. Biscutum constans is known to be susceptible to dissolution, while Watznaueria barnesae is relatively resistant. The ratio of these two species can be used to evaluate whether carbonate dissolution has affected abundance changes. There is much less diversity among the planktonic foraminifera and radiolaria. Of the 16 identi®ed planktonic foraminifera species, eight were frequent enough to be used for spectral analysis (Table 3). Discussion will primarily focus on three of these species (Table 2): Hedbergella planispira, H. delrioensis, and Globigerinelloides bentonensis, comprising 78±99% of the total planktonic foraminifera fauna (the mean is 91%). The ®rst two species are known from both the Tethyan and `Boreal' regions, whereas G. bentonensis is considered characteristic of the northern Tethys and Tethys-in¯uenced `Boreal' seas.

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Table 1 Taxa of benthic foraminifera selected for this study, their test construction, habitat, and feeding mode Taxa

Test

Habitat

Feeding strategy

Arenobulimina spp. Ammobaculites spp. Egerella mariae Falsogaudryinella alta Falsogaudryinella n.sp. Gavellinella bertelini Gavellinella cenomanica Haplophragmium aequale

arenaceous arenaceous arenaceous arenaceous arenaceous calcitic perforate calcitic perforate arenaceous

infauna infauna infauna infauna infauna epifauna epifauna shallow infauna

Lenticulina spp. Nodobacularia nodulosa Quinqueloculina antiqua Ramulina spp. Recurvoides spp. Spiroplectinata annectens Textularia conica Valvulineria lotterlei Vaginulina trilobata

calcitic perforate miliolid miliolid calcitic perforate arenaceous arenaceous arenaceous calcitic perforate arenaceous

infauna epifauna (on hard substrates) infauna infauna infauna shallow infauna epifauna (on hard substrates) infauna infauna

detritus feeding detritus feeding detritus feeding detritus feeding detritus feeding suspension feeding suspension feeding detritus feeding? suspension feeding detritus feeding suspension feeding detritus feeding detritus feeding detritus feeding suspension feeding suspension feeding detritus feeding detritus feeding

Radiolaria were encountered in quantities large enough for statistical analysis only in the top 40 m. In this study, we used the percent Nasselaria of the radiolarian assemblage and the number of radiolaria .125 mm per gram sediment. For microfaunal analysis, 300 specimens of each fossil group (radiolaria, benthic and planktonic foraminifera) were counted in the .125 mm fraction of each sample. For calcareous nannofossils, smear slides were prepared from a suspension of the sediment using the method described by Hay (1965) and Cepek (1981). Hence, this study provides (i) absolute abundances (specimens per gram sediment for the microfauna groups and specimens per scanned slide area for the calcareous, planktonic micro¯ora), which can be expected to provide general information, and (ii) relative abundances (percentages of foraminifera and calcareous nannofossil species within these microfossil group assemblages, various ratios), which should re¯ect reactions of individual species to palaeoenvironmental changes. Foraminifera and radiolaria samples were analysed in 1 m increments throughout the core. For calcareous nannofossils, counts at 1 m intervals were made only in the 35±96 m depth interval; above and below this interval samples were analysed in 5 m increments. In

addition, carbonate concentrations measured at 0.5 m increments throughout the core were used (data from Jendrzejewski et al., 2001; Fenner, 2001b). Microfossil counts of this study are taken from Fenner et al. (1996), Weiû (1997), Thies (2001), Fenner (unpublished data), and Cepek (2001). The time frame for this study is provided by biostratigraphic results from ammonites (Wiedmann and Owen, 2001), bivalves (Fenner, 2001b), and calcareous nannofossils (Cepek, 2001). 2.2. Spectral analysis For the Fourier transformation, we used the ARAND software (Philip J. Howell, Brown University, Rhode Island) and the ANALYSERIES software of D. Paillard (1996). We selected the relatively robust Blackman±Tukey method (for details, see Jenkins and Watts, 1968). All microfossil data were analysed in the depth domain using a constant sampling interval of 1 m, as many lags as 30% of the series length, a pre-whitening constant of 0.5 to reduce the low-frequency imprint, a con®dence level of 80%, a frequency increment of 0.005 cycles per sample interval and 101 frequencies. Sediment boundaries described below as well as the availability of data and the presence of hiatuses in the

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Fig. 3. Bulk carbonate record of the Kirchrode I borehole (data from Jendrzejewski et al., 2001; Fenner, 2001b; sampling increment is 0.5 m); biostratigraphy from the ammonite zonation of the Upper Albian according to Wiedmann and Owen (2001).

Upper Albian record led to the decision to study the relationship between orbital forcing and microfossil cyclicity, both throughout the entire record and in several discrete sections. The following discrete sections were chosen: 0±40 m, 40±95 m, 40±132 m, 40±160 m, 133±230 m, 161±245 m, 206±245 m. The results are documented in Table 2 for those species that were abundant enough to provide a gapless record for spectral analysis. The results of these analyses are displayed in Figs. 5±8; the results of analyses for individual species in the core as a whole are displayed in evolutionary spectra (Figs. 9±12), a technique introduced by Mayer et al. (1996). For the evolutionary spectra, we used a 80 m window offset by 5% of the series length (4 m)

from one analysis to another, providing a total of 42 spectrograms for Kirchrode I. 2.3. Stratigraphy and sediment boundaries According to radiometric age determinations the Albian covers a time span of 12±15 Ma (Haq et al., 1987, 1988; Harland et al., 1990; Gradstein et al., 1995). Estimates of the duration of the Late Albian by applying cyclochronometry are about 6 Ma according to Herbert et al. (1995) and, more than 4 Ma, possibly more than 4.8 Ma according to Fenner (2001b). As the Late Albian is within the Cretaceous magnetic quiet zone, no useful, well-dated palaeomagnetic reversals are available to provide known

274 M.E. Weber et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 174 (2001) 269±286 Fig. 4. Downcore variations of selected planktonic and benthic foraminifera, calcareous nannofossils, and radiolaria in the Kirchrode I borehole. Depth intervals studied by spectral analysis are indicated by arrows on the right. P/B is the ratio of planktonic to benthic foraminifera.

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Fig. 5. Spectral analysis of selected planktonic foraminifera species, the plankton/benthos ratio of foraminifera, and the abundance of planktonic foraminifera in the sediment for the depth intervals 40±160 m (left) and 132±230 m (right) in the Kirchrode I borehole. The sampling interval is 1 m for all of the microfossil analyses, lags are 30% of the series length, the pre-whitening constant is 0.5 to reduce the imprint of low frequencies, the 80% con®dence interval (CI) is given as vertical bars on the lower right, BW is bandwidth over which spectral estimates are averaged, frequency increment is 0.005 cycles per sampling interval, and the calculated frequencies are 101. Eccentricity (E), obliquity (T), and precession (P) are interpreted periods of these orbital parameters. Note that most spectral power is in the 5 m cycle (precession band).

datum levels in it. Biostratigraphic data can be used to provide information about the completeness of the studied sequence. The stratigraphy of the Kirchrode I borehole was determined most accurately using ammonites, which were found in suf®cient abundances for taxonomic and stratigraphic analysis (Wiedmann and Owen, 2001). These authors were able to identify all ammonite subzones of the Upper

Albian (Fig. 3). With a thickness of 165 m (64 m to 229 m in the core), the Callihoplites auritus Subzone is well represented (Wiedmann and Owen, 2001). However, all biostratigraphic results agree that not all of the Upper Albian was recovered, that the top of the borehole is just below the Albian/Cenomanian boundary and the bottom is just above the Middle/ Upper Albian boundary. How much of the stratigraphic

Fig. 6. Spectral analysis of selected benthic foraminifera species and genera, and of the abundance of benthic foraminifera in the sediment for the depth intervals 40±160 m (left) and 132±230 m (right) in the Kirchrode I borehole. For method and legend, see Fig. 5. Note that most of the spectral power is in the 2±3 m (precession bands) and 5 m cycles (obliquity band).

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Fig. 7. Spectral analysis of selected calcareous nannofossils from the depth interval 40±95 m in borehole Kirchrode I. For method and legend, see Fig. 5. Note that most of the spectral power is in the 5 m (obliquity band) and 2±3 m cycles (precession bands).

sequence between the top of the recovered Upper Albian and the Albian/Cenomanian boundary is missing cannot be determined by biostratigraphy. However, in this part of the pro®le, faults and slickenslides do not occur, whereas the basal part of the Upper Albian is shortened by a fault (Fenner, 2001b). Intervals for spectral analysis were selected primar-

Fig. 8. Spectral analysis of selected radiolaria and bulk carbonate for the depth interval 0±40 m in borehole Kirchrode I. For method and legend, see Fig. 5. Note that spectral power is not concentrated within speci®c orbital periods.

Fig. 9. Spectral analysis of bulk carbonate contents in selected depth intervals of borehole Kirchrode I (data from Jendrzejewski, 1995). For method and legend, see Fig. 5. The sampling interval is 0.5 m; hence, the frequency increment is 0.025 cycles per interval. Note that spectral power is mainly within the periods of 10±13 m, 5±6 m, and 2±3 m, which are interpreted as the response to variations in orbital eccentricity, obliquity, and precession, respectively. Note that during most intervals, spectral power is in the 10±13 m (eccentricity band) and the 5 m cycles (obliquity band); low power is observed in the 2±3 m cycles (precession bands).

ily on the basis of changes indicated in the Kirchrode I borehole documented by the overall changes in abundance patterns of several microfossil species and microfossil groups (Fig. 4) and by variations in the physical and chemical parameter values with depth (Wonik, 2001; Rachold and Brumsack, 2001). For instance, a rather drastic change in abundances of species and microfossil groups is documented at 40 m core depth. The bulk carbonate content is higher above 40 m than below this depth; the radiolaria are the dominant zooplankton group in the sand fraction .125 mm above this depth. The hiatus at 132 m is marked by a layer of macrofossils with also glauconite and pyrite occurring. It occurs within the depth interval from 150±180 m within which many parameters are changing. The amplitude of the calcium carbonate ¯uctuations changes from high to low between 150±180 m. Moreover, below this depth, planktonic foraminifera and calcareous nannofossils are more abundant than

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Fig. 10. Evolutionary spectra of planktonic (A, left) and benthic (B, right) foraminifera per gram sediment in the size fraction .125 mm. Each plot consists of 42 spectra (horizontal bars in the center) generated as described in Fig. 5. We used a 80 m window shifted by 5% of the series length (4 m) from one analysis to the next. Levels of equal spectral power are contoured at constant intervals (the higher the energy, the darker the color). The power scale is arbitrary. Note that the abundance of planktonic foraminifera shows dominant obliquity and weak precession forcing. The same is true for the abundance of benthic foraminifera, which in addition show weak eccentricity forcing.

above this depth. Also in the species of planktonic foraminifera, abundances change between 125 and 145 m, e.g. Hedbergella delrioensis is dominant below this interval and the ®rst occurrence of Globigerinoides bentonensis is above this interval (Weiû, 1997). At 160 m, there is a change in the gamma-ray logs (Wonik, 2001). 3. Results and interpretation The results of our spectral analyses are documented in Table 2. Important spectrograms for the interval studies are shown in Figs. 5±9 and those for the whole-core studies in Figs. 10±12. While wholecore evolutionary spectra of single species should provide detailed information about the response to orbital forcing, as well as information about potential changes in sedimentation and palaeoenvironment at

boundaries, interval studies are more accurate for a comparison of the response of individual species to orbital forcing. Our study shows that most of the microfossils exhibit cyclic abundance patterns. Cyclicity is best documented in the core between 40±205 m depth. The most prominent periodicity has periods of 5±6 m, followed by periods of 2±3 m and 10±13 m. Many power spectra that contain the 10±13 m cycle also show a weak 40±50 m cycle. In the following discussion, we (i) describe the cyclicity of all of the microfossil groups, (ii) assign the microfossil cycles to orbital cycles, and (iii) evaluate major trends. 3.1. Species variations Variations in the number of benthic foraminifer specimens per gram sediment, which re¯ect conditions at the sea¯oor, have a very distinct 5 m cycle

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Fig. 11. Evolutionary spectra of Hedbergella planispira (A, left) and the ratio of planktonic to benthic foraminifera (P/B ratio, B, right). For method, see Fig. 10. Note that H. planispira shows a strong obliquity signal and weak precession forcing. Moreover, there is indication of weak forcing within the short-term (100 ka) and long-term (400 ka) eccentricity cycles. The P/B ratio shows a strong obliquity signal and weak precession forcing.

throughout the core. In addition, there are 2.2 m cycles in the lower and upper parts of the core; and a 2.8 m and a 10±11 m cycle in the central and upper parts of the core, respectively (Figs. 6 and 10A). The ratio of planktonic to benthic foraminifera varies on the 5 m band almost throughout the core (Fig. 5). Regarding individual species (Table 2, Fig. 6) Lenticulina spp. shows dominant 5±6 m and 2.5 m cycles in the lower and central parts, respectively (Fig. 12B), Gavellinella cenomanica has 10 m cycles in the lower and upper parts, and 5±6 m and 2±3 m cycles in the middle and upper parts, respectively (Fig. 12A), Ammobaculites spp., Textularia conica, and Recurvoides spp., and Valvulineria lotterlei are primarily forced in the 2±3 m bands, whereas Quinqueloculina antiqua and Haplophragmium aequale show a complicated pattern of various forcing frequencies. Spectral power of other species, such as Arenobulimina spp., is lower and can only provide a tentative indication for cyclic variations.

With respect to planktonic foraminifera, H. planispira shows a dominant 5 m cycle almost throughout the core. In addition, it indicates a weak 2.5 m cycle and a weak 10±12 m cycle in the lower and central parts of the core, respectively (Table 2, Figs. 5 and 11A). H. delrioensis also has a dominant 5 m cycle (Fig. 5), whereas G. bentonensis shows dominant 2±3 m cycles and weak 5 m and 10 m cycles (Fig. 5) in the 30±120 m interval, where this species makes up 12±69% (42% in average) of the planktonic foraminifer fauna. The calcareous nannofossil species were analysed in a suf®cient number of samples for spectral analysis (1 m spacing) only in the 40±95 m core interval. In this interval, seven of the 13 species selected for this study show distinct cyclic variations (Table 2, Fig. 7). Clear periodicities are recognizable in the abundance curves for Discorhabdus rotatorius (2.5 m, 5 m, 10 m), Biscutum magnum (2.4 m, 3 m), Watznaueria barnesae (2.9 m, 5 m, 13 m), Glaukolithus spp. (5 m),

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Fig. 12. Evolutionary spectra of the benthic foraminifera Gavellinella cenomanica (A, left) and Lenticulina spp. (B, right). Note that both species show, in addition to eccentricity and obliquity forcing, more spectral power at high frequencies, which is associated with precession forcing and, presumably, non-linear interactions between primary forced responses.

Zeugrhabdotus erectus (2.9 m, 5.5 m), Rhagodiscus asper (2.9 m), Prediscosphaera avitus (2.3 m), and Vekshinella stradneri (2.9 m). Weaker indications of periodicity are recognizable for Prediscosphaera spp. (2.9 m), Glaukolithus compactus (2.4 m), and Lithraphidites carniolensis (2.4 m). Other species, such as Biscutum constans, and Glaukolithus diplogrammus, show no clear cyclic signals. From radiolaria, which occur in suf®cient numbers for performing spectral analysis only in the 0±40 m core interval, the number of skeletons per gram sediment and the relative abundance of Nasselaria show no clear indications of orbital frequencies (Fig. 8). It should be noted that the errors in the results concerning radiolaria and calcareous nannofossils are relatively large because only short sections were available for spectral analysis of their abundance ¯uctuations (Figs. 7 and 8). Hence, our interpretations of these microfossil groups are tentative.

3.2. Milankovitch frequencies For the Upper Neogene sediment cycles documented in open marine environments can be con®dently calibrated to orbital cycles, since the time control is good (e.g. Hilgen et al., 1995; Lourens et al., 1996). Older sediments, especially of shallow marine or continental origin, have some serious drawbacks, e.g. the lack of accurate time control, diagenetic effects, potential occurrence of hiatuses, and autocyclic processes (Hilgen et al., 1997). In spite of such effects not missing from the Kirchrode sequence, the Kirchrode I microfossil record reveals the most prominent spectral peaks at periods of roughly 10±13 m, 5±6 m, and 2±3 m throughout the core, which can be interpreted as the biological response to orbital forcing. The 5±6 m peak is the most prominent cycle. Individual periods may change slightly or be less important from one interval to another (Figs. 5±9), nevertheless, the

280

Species (and other parameters) 0±40 m

40±95 m

Bulk carbonate ± 12±13, 5±5.5, 3 Globigerinelloides bentonensis p 10±12, 5, 2.5 5.5 5, 2.9, 2.2 Hedbergella planispira ² Hedbergella delrioensis 5±5.5, 2.8 5.5, 2.6 Plankt. foram./g sediment ² 4.5, 2.5 4.5, 2.3 P/B ratio ² 2.5 5 12, 5 11, 5, 2.5 Benth. foram./g sediment ² Ammobaculites spp. 2.6 ± Arenobulimina spp. 2.4 3 Gavellinella cenomanica 10, 5 5 Haplophragmium aequale 11±12 5, 2.9 Lenticulina spp. ² 2.3 2.3, 3 Quinqueloculina antiqua 10±11 3 Recurvoides spp. 2.9 2.6 Textularia conica ² 10, 2.5 2.5 Valvulineria lotterlei 2.5 2.5 Biscutum constans p ± Biscutum magnum p 3, 2.4 Discorhabdus rotatorius p 10, 5, 2.5 Glaukolithus compactus p 2.4 Glaukolithus diplogrammus p ± Glaukolithus spp. p 5 Lithraphidites carniolensis p 2.4 Prediscosphaera avitus p 2.3 Prediscosphaera spp. p 2.6 Rhagodiscus asper p 2.9 Vekshinella stradneri p 2.9 Watznaueria barnesae p 13, 5, 2.9 Zeugrhabdotus erectus p 5.5, 2.9 Radiolaria/g sediment ± p Nasselaria ± p

40±132 m

40±160 m

12, 5, 3 11, 5.5, 3, 2.5 11, 5, 2.8, 2.3 11, 4.5, 2.8, 2.3 5, 2.8 4.9, 2.8, 2.2 5, 2.7 4.7, 2.7, 2.2 4.5 4.5, 2.5 5 5 11, 5, 2.4 10, 5, 2.4 ± ± 2.5 4.5, 3, 2.4 5 10, 5.5, 2.4 5, 2.8 2.8 10, 2.3 10, 2.5 2.8 2.7, 2.3 2.6 2.7 2.7 2.7 2.5 4.5, 2.5 p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p

132±230 m

161±245 m

205±245 m

0±245 m

13, 5, 2.5 p 10, 5, 2.5 10, 5, 2.5 5, 2.6, 2.3 5 5, 2.3 ± 2. 4 10 2.5 5.5, 2.6 13, 5, 2.9 2.6 2.7 ±

3, 2.5 p 5, 2.9, 2.5 5, 2.5 5 5, 2.5 5 ± 4.5±5, 2.7 10 2.6 5.5, 2.5 4.5, 3 2.6 5.5, 2.4 2.5 p p p p p p p p p p p p p p p

2.4 11, 5, 3, 2.4 p p 5, 2.5 10, 5, 2.9, 2.5 5, 2.5 10, 5, 2.5 5, 2.5 5, 2.5 5, 2.5 5 5, 2.6 11, 5, 2.6 5.2 2.6 ± 2.8, 2.4 10, 6, 2.5 10, 5±6, 2.5 5.5, 2.7, 2.4 2.6 5.5, 2.5 5.5, 2.5 ± 5 ± 2.6, 2.9 5.5, 2.4 2.7, 2.5 2.5 2.5 p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p

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Table 2 Microfossil cyclicity detected by spectral analysis in the Kirchrode I borehole. The table contains species that are abundant enough for spectral analysis and that show evidence for cyclic variation. Cycles are in m for different depth intervals. Single spectrograms are displayed for species with distinct spectra in Figs. 5±9. ² indicates evolutionary spectra given in Figs. 10±12; pronounced peaks are in bold italics; ± there is no coherency; p there is no data

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Table 3 Palaeogeographic distribution of planktonic foraminifera used in this study Species

Palaeogeographic occurrence

Reference

Biticinella breggiensis Favusella hiltermanni

Tethyan Tethys and Tethys-in¯uenced `Boreal' regions

Masters 1977 Loeblich and Tappan 1961

Favusella washitensis

Globigerinelloides bentonensis

Tethys and Tethys-in¯uenced `Boreal' regions

Hedbergella simplex-amabilis

northern Tethys and Tethysin¯uenced `Boreal' regions Tethyan

Hedbergella delrioensis Hedbergella planispira Praeglobotruncana delrioensis

Tethyan and `Boreal' regions Tethyan and `Boreal' regions Tethyan and `Boreal' regions

general pattern of the evolutionary spectra shows striking similarities (Figs. 10 and 11). The ratio of the observed periods is roughly 1 to 0.4 and 0.2, which corresponds to the ratio of variations in orbital periods of the Milankovitch frequency bands at about 100 ka (eccentricity, E), 41 ka (obliquity or tilt, T), and 23 to 19 ka (precession, P1 and P2). These robust peaks indicate relatively constant sedimentation rates throughout the time span preserved in Kirchrode I. Nevertheless, the results from our study also indicate boundaries at which the amplitudes, and hence the importance of individual frequencies, change or where minor changes in sedimentation rates might occur. The microfossil spectral data allow the core to be divided into three sections on the basis of the biological response to orbital forcing. This division into three units is in accordance with ®ndings from geochemical studies (e.g. Jendrzejewski et al., 2001) and may re¯ect changing sea level. From the bottom of the core up to about 205 m, spectral analysis shows non-systematic amplitude changes and noisy spectra (e.g. Figs. 10±12). These ®ndings can be explained by periods of relatively turbulent bottom-water conditions during sediment deposition and later tectonics related to halokinetic movements in the salt structures Benthe and Lehrte (Fenner, 2001b). This interval can be described as representing unstable conditions and a relatively low sea level. For this interval, which is also recovered in

Michael 1973; Masters 1977 Gorbachik and Kusnetzova 1983 Loeblich and Tappan 1961 Michael 1973; Masters 1977 Gorbachik and Kuznetsova, 1983 Masters 1977 Loeblich and Tappan 1961 Moullade 1966 Masters 1977 Masters 1977 Masters 1977

borehole Kirchrode II, Nebe et al. (1997) observed cycles of 3.5±5 m and 13 m length in the sediment color and performed spectral analysis. He interprets these two spectral peaks as eccentricity variations in the 100 and 400 ka bands. However, we propose an alternative interpretation. We propose that the spectral energy of the 5 m cycle is much higher in this basal part than that at shallower core depth (see Figs. 10±12). The long core interval from 205±230 m to 40 m, which mainly covers the Callihoplites auritus Subzone and part of the Mortoniceras rostratum Subzone (Fig. 3), is relatively stable with respect to the spectral power distribution of many microfossil parameters and CaCO3 ¯uctuations (e.g. Figs. 10± 12). From the microfossil spectral data, the interval from 205±230 m to 40 m is much more uniform than the sediments below and above this interval. Most of the species show dominant and well-developed forcing in the precession and obliquity bands during that time (Table 2, Figs. 10±12). In the 0±40 m interval, slickensides are not present. Neither bulk carbonate (Fig. 9) and gamma-ray logs (Wonik, 2001) nor radiolaria (Fig. 8) in this interval show a clear response to orbital forcing. It must be pointed out that spectral analysis is not reliable, if the tested depth interval is too short (e.g. microfossil data for the intervals 0±40 m, 40±95 m, and 205±245 m). According to Prokoph (1994), strong marine in¯uence

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in these upper 40 m (higher biogenic and lower clastic content) may have led to less in¯uence of orbital forcing on the sedimentation. Our analysis, however, indicates that some microfossil species (e.g. H. planispira, H. delrioensis, Q. antiqua) as well as the abundance of planktonic and benthic foraminifera per gram sediment (Table 2, Figs. 10A,B and 11A) have the same cycle lengths here as at greater depth. Generally, the pattern is not as uniform as in the long stable interval immediately below, but for some components, cyclicity is even better documented in this uppermost interval (e.g. Fig. 10B). 4. Discussion Exact lengths of time of orbital cycles is known only for the Late Neogene until approximately 10 Ma according to Berger (1978), Laskar (1990), Berger and Loutre (1991), and Hilgen et al. (1995). Sedimentary sequences have been successfully tuned to orbital parameters from the present to the Middle Miocene (e.g. Shackleton et al., 1995), and Oligocene studies are in progress (e.g. Curry et al., 1996). Tuning of sedimentary cycles to orbital parameters in older sediments becomes of increasing interest, since this method provides a high-resolution chronology and a detailed insight into the biological response to external forcing (Hilgen et al., 1997). For the Cretaceous, successful tuning has been reported by Fischer et al. (1991), and Herbert (1992) and references therein. According to an astronomical reconstruction of Berger et al. (1989), which covers the past 500 Ma, calculated Milankovitch frequencies tend to be higher the earlier they occurred (e.g. the present obliquity (tilt) cycle is approximately 41 ka and may have been as short as 38 ka during the Cretaceous). Nevertheless, the ratio of the three major frequencies, which tend to appear in two groups during the Cretaceous, is still 1 to 0.4 and 0.2. These calculations may be questioned because extrapolation of periodic events into the past involves several assumptions which may not be true, since the behavior of the solar system is chaotic in a mathematical sense and its precise con®guration cannot be calculated for times older than about 10 Ma (Laskar, 1989). In spectral analysis of orbitally induced climatic cycles, the highest frequency that can be distinguished

in a spectrum, the Nyquist frequency, is half the sampling frequency. For Kirchrode I this is 2 m or 17±19 ka. Therefore, the sedimentary imprint of orbital precession, which in our interpretation is the 2± 3 m cycle in the high sedimentation rate part of the Upper Albian of Kirchrode I, and which in the Late Pleistocene has a periodicity of 19±23 ka can, if present, only be detected close to the high-frequency limit of the analysis. Detailed interpretation of highfrequency variations is thus hampered and would only be possible with a higher sampling resolution. Nevertheless, it is unlikely that the dominant 5 m cycle in the microfossil data of Kirchrode I is associated with orbital periods other than obliquity. Assigning this cycle to variations either in eccentricity or precession, instead of variations in obliquity, would lead to a very long or short time interval represented by this core. Besides, changing the assignment would change the ratio of the major cycles from the characteristic Milankovitch 1 to 0.4 and 0.2 ratios. Additional support for this interpretation arises from the low-amplitude 40±50 m cycle shown by some species (e.g. Figs. 11A and 12A) and which then would re¯ect the long-term eccentricity variation of roughly 400 ka. Moreover, some benthic foraminifer species (e.g. G. belorussica/cenomanica group running in this study under G. cenomanica, and Lenticulina spp; Fig. 12) show a 3±4 m cycle. This cycle may correspond to the 29±31 ka cycle found in Pleistocene deep-sea sediments, where it presumably represents non-linear interaction between primary forced responses (Pisias and Rea, 1988). Further independent support for the above interpretation of the prominent spectral peaks in the microfossil data in the upper part of the Upper Albian comes from spectral analysis of geochemical data analysed every half meter throughout the core (Rachold and Brumsack, 2001; Jendrzejewski et al., 2001) and of the gamma ray measurements performed every 5 cm (Wonik, 2001). Although the calcium carbonate content of the sediment, the gamma ray values, and the sediment color values show a pronounced 9±13 m cyclicity, spectral analysis of these data also leads to the same interpretation of the Milankovitch cyclicity, with the prominent 9±13 m cycles representing forcing by eccentricity (above cited authors and Prokoph and Thurow, 2001). According to Fenner (2001b) sedimentation

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rates in the lower part of the Upper Albian are lower, between 1.5 and 6.5 cm/ka. The presence of a relatively strong eccentricity cycle in abundance ¯uctuations of microfossil groups and species Cretaceous sediments is surprising. Since with respect to insolation changes, variations in orbital eccentricity only provide weaker external forcing than obliquity and precession. Its presence in the Lower Saxony basin must be linked to a non-linear response to orbital forcing associated with a long-term constant ($15 ka) and internal feedbacks (e.g. Imbrie et al., 1993). For the Late Pleistocene, when eccentricity variations triggered major glaciation cycles, this non-linearity is linked to large-scale ice sheets (e.g. Hays et al., 1976). The Cretaceous, with its greenhouse climate, is instead thought to have been free of polar ice sheets. One possible explanation for the ampli®cation of the eccentricity cycles often found in the Mesozoic strata is provided by Fischer et al. (1991) for the Piobbico core. They used a nonlinear iterative model to transfer power from the precession index, which is documented by rhythmic bedding, to the eccentricity band. The ®nal result perfectly matches the eccentricity-dominated carbonate record of the core. Another possible explanation arises from a modelling study of Crowley et al. (1992). They suggested that the low-latitude response of the monsoon system on equatorial land masses produced 100 ka and 400 ka cycles during ice-free periods. This model would account for our observations of both 10±13 m and 40±50 m cycles in some microfossil records in Kirchrode I. Most of the calcareous microfossil species show strong power in the high frequency bands (obliquity and precession), whereas in most of the analysed intervals, bulk carbonate shows signi®cant power on the eccentricity band (Fig. 9), with less or even no spectral power is often documented in the highfrequency precession bands. One possible explanation for this observation would be that the individual species respond, depending e.g. on their depth distribution within the water column and their species speci®c tolerances, sooner or later to changes in the environment. Such small differences in timing, detected by cross-spectral analysis as phase leads and lags, will be integrated by the bulk carbonate record. Consequently, bulk carbonate records will have less high-frequency power than records from

283

individual species. In addition, the Fischer et al. (1991) or the Crowley et al. (1992) models (see above) may also apply to the bulk carbonate record of Kirchrode I. Slight leads or lags between bulk carbonate and terrigenous material will also lead to suppression of high-frequency variations in lithological proxies, which could explain the weak power of high-frequencies in the gamma-ray record of Wonik (2001). From modern continental slope settings, we know that there is strong obliquity forcing on terrigenous sediment supply from land to sea via river runoff (e.g. Curry et al. (1996) for the Amazon fan, Weber (1997) for the Bengal Fan). From the study of Quaternary marine pelagic settings, we know that biogenic carbonate re¯ects excellently the eccentricity and obliquity bands at low latitudes (e.g. in the eastern equatorial Paci®c; Shackleton et al., 1995; Weber et al., 1995). The presence of all Milankovitch frequency bands in the bulk carbonate record in Kirchrode I may indicate that biological responses re¯ecting both continental and open ocean environments are documented at this site. Among planktonic foraminifera, species which according to their palaeogeographic distribution are considered to re¯ect a `Boreal' in¯uence (Table 3; e.g. H. planispira, H, delrioensis) are primarily obliquity forced (Table 2), whereas species that may re¯ect more `Tethyan' in¯uence show additional precession and eccentricity forcing (Table 2, G. bentonensis). This is not surprising, since changes in obliquity have the strongest impact on insolation at high latitudes, whereas low latitudes are more affected by changes in precession (e.g. Imbrie et al., 1992). Moreover, an overall comparison of planktonic and benthic foraminifer species (Figs. 5±8 and 11±12) shows that the former are primarily obliquity forced, whereas the latter are more precession forced. This implies that, as can be expected, the planktonic foraminifer species in the Late Albian Lower Saxony basin are more in¯uenced by the local climatic changes in the mid-latitudes and possibly by in¯uences from higher latitudes, whereas benthic foraminifer species, besides also re¯ecting local changes, in addition responded to low-latitude changes. No major difference could be detected between suspensionfeeding and detritus-feeding benthic foraminifera, indicating that both responded almost the same way

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to orbital forcing. A tethyan in¯uence in the deep water of the Lower Saxony basin can also be deduced from the immigration of the benthic foraminifer species Spiroplectammina annectens which Tyszka and Thies (2001) describe as appearing here ®rst in the late Early Albian. We have divided the Kirchrode I core on the basis of the identi®ed cycles into (i) an unstable interval (borehole base to 230 m or 205 m), which has elevated abundances of benthic foraminifera and suspension feeding benthos, and in which a fault is present, (ii) a long middle zone of stable conditions (from 230±205 m to 40 m) characterised by distinct frequency peaks in the spectral analysis of abundance changes of the analysed microfossil groups, and (iii) an upper interval, of which only 40 m were recovered and which is characterised by elevated abundances of marine planktonic microfossils, of which only a few species show clear cyclicity in their abundance changes. These divisions can be interpreted in terms of the transgression known to have occurred during the Late Albian (Haq et al., 1988) which reached one of the highest sea level stands in the geological record. A lower sea level during the early part of the Late Albian seems to have resulted in a stronger obliquity signal in the bulk sediment parameters, like calcium carbonate content of the sediment, whereas with rising sea level, they show a stronger eccentricity signal. 5. Conclusions Microfossil data provide useful tools for studying the response of the palaeoenvironment to orbital forcing in Cretaceous sediments. Microfossil data have the advantage of great diversity, providing palaeoceanographic and palaeoclimatic information that can be dif®cult to extract from physical and chemical parameters, such as nutrient availability, temperature (insolation), current intensity, and origin of the water masses. A major disadvantage is that counting microfossils is time-consuming. Sample spacing of 1 m throughout the Kirchrode I core provides for the upper part of the Upper Albian a time resolution of 8±15 ka; thus biological response to the precession cycle is close to the high-frequency limit of the spectral analysis and hence cannot be studied in detail. One recommendation for further

investigation is therefore to increase the resolution to at least 5 ka, i.e. 0.5 m in the case of Kirchrode I. Major periodicities in changes in the abundance of microfossil groups and species were found to contain wavelengths of 10±13 m, 5±6 m, and 2±3 m with the 5±6 m cycle as the dominant feature. We feel con®dent interpreting these cycles, which have a ratio of 1 to 0.4 and 0.2, as representing the biological response to variations in orbital eccentricity, obliquity, and precession, respectively. Our interpretation is corroborated by the fact that many physical and chemical parameters vary on the same frequency scale. In addition, species showing a 10±13 m cycle usually also contain a low-amplitude 40±50 m cycle that represents the long-term eccentricity variation centered around 400 ka. Sedimentation rates according to our interpretation are fairly constant throughout the upper part of the Upper Albian (7±13 cm/ka) and below 155 m in Kirchrode I vary generally between 3.5± 12.5 cm ka 21. It is of interest that the cyclicity changes during the Late Albian. In the lower part of the Upper Albian, the eccentricity signal is less pronounced than in the middle part (Callihoplites auritus Ammonite Subzone), whether this change in the cyclicity is solely related to the changing sea level cannot be decided on the basis of the present information. As can be expected, the frequency spectra of the individual species show more power in the higher frequencies (obliquity and precession), whereas the bulk sediment parameters, such as calcium carbonate content and gamma ray values, have more spectral power in the low frequency eccentricity band. In Kirchrode I, the relative importance of individual frequencies may differ, depending on the parameter investigated. For instance, the obliquity cycle of 6±5 m is more distinct in the response of most planktonic foraminifera, and the precession cycle of 3±2 m is more distinct in the response of most benthic foraminifera. The later observation is explained by an admixture of low-latitude water in the deep water of this epicontinental, mid-latitude ocean basin that affects the benthic foraminifera, which live at the sea¯oor, whereas most of the more abundant planktonic foraminifera, which live in the water column above, re¯ect the local mid-latitude climatic changes.

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Acknowledgements We thank the reviewers, L. Lourens, L. Beaufort, and B. Christensen for their helpful and critical suggestions, A. Mix and N. Pisias for their comments on spectral analysis, and W. Weiû for providing counts of planktonic foraminifera species. The English was improved by M.L. Weber and R.C. Newcomb. We especially thank R.C. Newcomb for his intense and engaged proof reading. The project was sponsored by the Deutsche Forschungsgemeinschaft (DFG; grants Fe 240/2 and Ce 35/1 to J.F.) and ®nancing of the drilling of the Kirchrode I borehole was provided by BGR. M.E.W. received ®nancial support from the DFG (grant We 2039/1-1) and the Bundesministerium fuÈr Bildung, Forschung und Technologie (BMBF; grant 03 G 0106B).

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