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Palaeoceanographic and climatic changes during the Albian, summary of the results from the Kirchrode boreholes
Palaeogeography, Palaeoclimatology, Palaeoecology 174 (2001) 287±304
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Palaeoceanographic and climatic changes during the Albian, summary of the results from the Kirchrode boreholes J. Fenner* Bundesanstalt fuÈr Geowissenschaften und Rohstoffe, Stilleweg 2, Hannover 30655, Germany Received 9 October 2000; Accepted for publication 3 April 2001
Abstract Analysis of the Albian sediments from the Kirchrode site by different disciplines yielded information about depth, productivity and oxygenation of the water in the deeper parts of the Lower Saxony basin, one of the mid-latitude shelf sea basins in the northern hemisphere. In addition information was obtained about phases during which in¯ow of water from the neighboring oceans (Tethys, North Atlantic) and circulation within the basin was intensi®ed, about sea level changes, as well as about climate changes occurring during the Albian in this mid-latitude region. This information was derived from observations of the effects the changes in the palaeoenvironment had on the fossil and mineral assemblages, the sediment facies, and the geochemical composition of the sediment. The increase in terrigenous input during the Upper Albian, the relative increase in detrital montmorillonite and silt-sized minerals, e.g. quartz and zircon, re¯ect tectonic movements (e.g. uplift of the Rhenish±Bohemian massif and subsidence of the Lower Saxony basin, locally intensi®ed by halokinetic movements) and long-term regional changes in climate. A change to a more humid climate occurred approximately at the stratigraphic ®rst occurrence of the calcareous nannofossil genus Eiffellithus in this region and at the start of a new transgressional period, and it led to increased runoff from the land. Possibly, a second change, this time to a more arid climate, occurred in the latest Albian. During times of high sedimentation rates of terrigenous components the marine productivity in the shelf sea is also increased as a consequence of the increased input of dissolved and particulate matter. During the latest Albian, upwelling may have been the cause of a further increase in productivity. A global sea level change overlying the effects of these regional and local processes was recognised e.g. in changes in sedimentation rates in the monotonous deep-shelf sediments (decreasing during transgression and highstand times, increasing during regression and lowstand times). Global sea level changes are best documented in changes in the abundances of the main plankton and benthos groups, and changes in the relative abundance of benthos groups with different feeding strategies: suspension-feeders and detritus-feeders. Transgressional periods start with an abundance maximum in species that immigrated from the Tethys. Highstand conditions at lower nutrient levels are documented in the sediment record by a maximum in planktonic foraminifera and calcareous nannoplankton. When nutrient levels under highstand conditions were higher, like at the end of the Albian, calcareous nannofossils and siliceous plankton dominate. Milankovitch cyclicity was identi®ed using spectral analysis of the regularly and closely spaced data sets of sedimentological, palaeontological, and geochemical parameters. The higher abundance of terrigenous plant material in the dark parts of the sedimentary cyles in the Upper Albian suggest a change in humidity was the main climatic change and changes in benthos foraminifera assemblages indicate cyclic productivity changes in at least part of the sequence. Among planktonic foraminifera, species that immigrated from the Tethys have a stronger precessional signal in their abundance ¯uctuations, while the `Boreal' species show a stronger obliquity signal. For the deep part of the shelf basin, the strong precessional signal in the cyclicity of * Tel.: 149-511-643-2510; fax: 149-511-643-2304. E-mail address: [email protected] (J. Fenner). 0031-0182/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0031-018 2(01)00298-X
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benthic foraminifera abundances suggests that conditions were in¯uenced for most of the Upper Albian by processes triggered in the low latitudes. Based on the identi®ed Milankovitch cyclicity, sedimentation rates were found to increase through the Upper Albian from 1.5±6.5 to 7±13 cm/ka. The duration of the Callihoplites auritus Ammonite Subzone was determined to be 2.1 Ma and the minimum duration of the Late Albian 4 Ma (possibly 4.5±4.8 Ma). q 2001 Elsevier Science B.V. All rights reserved. Keywords: Albian; Climate; Sea level; Productivity; Milankovitch cyclicity
1. Introduction As a contribution to the international ALBICORE program (Larson et al., 1993), the Albian sequence at the Kirchrode I and II sites in the Lower Saxony basin (Fig. 1) was drilled in November±December 1991 and November±December 1994. Analysis of the sediments from the deep part of this basin was planned as an exemplary study for the wide belt of shelf seas in the northern hemisphere mid-latitudes, the `Boreal' region. The high sedimentation rates especially in the Upper Albian and the relatively undisturbed basinal, offshore sedimentation make possible detailed studies of: ² the palaeoceanographic changes that occurred in such a setting during the Albian transgression and of ² how and how strong Milankovich cyclicity in¯uenced biogenic sedimentation and input of terrigenous detritus during a period known for its `greenhouse climate'. A site in a mid-latitude shelf sea that was connected with the major surrounding oceans (North Atlantic, Arctic, Tethys) was chosen, because sedimentation in such a setting was expected to have been especially sensitive to palaeoclimatic and palaeoceanographic changes. Our statistical evaluation of the quantitative data to identify climate-driven cyclicity did not have to start from zero, but could build on a large number of geological and qualitative palaeontological studies (for an overview, see Fenner, 2001a). This volume mainly presents results from the Kirchrode I borehole. A few studies include results from both the Kirchrode boreholes. Most of the studies were done quantitatively on samples taken at regular spacing (one meter or half a meter) and for logging and scanning of grey values every 5 or 10 cm in order to obtain a data set suitable for statistical analysis. Rachold and Brumsack (2001), in addition,
chose the sedimentary cycle from 80 to 92 m in Kirchrode I, taking samples at intervals of 10 cm for a higher resolution geochemical study. Several methods were used to identify cyclicity in the abundance curves: direct visual evaluation of the ¯uctuations (Fenner, 2001b; Jendrzejewski et al., 2001) or frequency, spectral, and wavelet or sliding window analysis (Prokoph and Thurow, 2001; Rachold and Brumsack, 2001; Weber et al., 2001; Wonik, 2001). It has to be kept in mind that there are very few comparatively detailed analyses of the mid-Cretaceous, and that we are far from having an amount of quantitative data comparable to that of the Quaternary. Accordingly, there is considerably more space for speculation on climatic and palaeoceanographic conditions during the Albian and on how both were affected by tectonic and magmatic activity. Although our project was intended to reduce the space for speculative interpretation, the lack of well dated, detailed analyses of this period from other palaeogeographic positions sometimes led to completely opposing interpretations by the different researchers within our project, in spite of numerous discussions of the results and in spite of exchange of data and manuscripts. The intention of this summary is to present a brief overview of the results treated in detail in the chapters of this volume. 2. Site location The drill sites were selected within the same basinal structure between the two NNE±SSW-trending salt structures Benthe and Lehrte to the southeast of Hannover city (Fig. 2), where earlier mapping had found high sedimentation rates for the Middle and Upper Albian, and only the lowermost part of the Middle Albian seemed to be missing (Bertram and Kemper, 1971). The two sites Kirchrode I and II were drilled
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Fig. 1. Albian geography and tectonic structures of part of the northern hemisphere, showing the extent of the seas. Three depth categories based on the extent and character of Albian marine sediments are shown (map after Ziegler, 1990, modi®ed). Ki location of Kirchrode boreholes.
880 m from each other. The shelf-sea sediments in this area are of a basinal type that does not include sand-sized terrigenous clastics. For more details on the site selection and the geology of the area, see (Fenner, 2001a,b).
There are dif®culties to identify the northern hemisphere mid-latitude equivalent of the Albian R. praeticinensis Subzone, which was de®ned for the Tethyan region and has been selected as the target interval for the international research program
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Fig. 2. Geological section across the Mesozoic subbasin of the Lower Saxony basin selected for this project (simpli®ed from Baldschuhn et al., 1996). The pro®le runs E±W perpendicular to the trend of the Benthe and Lehrte salt structures. The Wealden facies is included with the Upper Jurassic in this drawing.
ALBICORE (Larson et al., 1993). Because of these dif®culties, as much as possible of the Albian record was drilled in order to increase the probability of ®nding stratigraphic markers or events of a wide geographic effect that are suitable for correlation between low and mid latitude marine records (Fenner, 2001b). The Craelius SK 6L wireline system was chosen for coring, a system best suited to recover undisturbed sediment sequences in claystones and marlstones. Slow and cautious drilling resulted in a recovery of close to 100% from both boreholes (Fenner, 2001b). A 385 m long sequence of the Albian was recovered. Especially well represented in this pro®le was the Upper Albian (285 m recovered). The cores were split lengthwise into an archive half for nondestructive studies and documentation, and a working half from which samples were taken.
3. Results 3.1. Stratigraphy The Kirchrode Albian is dated on the basis of biostratigraphic (mainly ammonites, bivalves, and calcareous nannofossils) and lithostratigraphic criteria established for the Northwest German basin. The numerous, excellent imprints of ammonite shells in the ®ne-grained sediments allowed many specimens to be determined to the species level. As a consequence Wiedmann and Owen (2001) were able to identify all seven ammonite subzones of the
`Boreal' Upper Albian in Kirchrode I. The correlation of these ammonite zones and subzones with the microfossil zones identi®ed in the same pro®le, as well as with the stratigraphic ®rst occurrence of the bivalve species group Actinoceramus sulcatus and the mapping units of Bertram et al. (1971), is shown in Fig. 3. For the Upper Albian, the ammonites provide by far the largest number of stratigraphic subdivisions. The stratigraphic resolution using calcareous nannofossils is lower. The high resolution calcareous nannofossil zonation developed for the `Boreal' region by Jakubowski (1987) and Jeremiah (1996) could not be veri®ed by Cepek (2001) and therefore was not applied to these cores. Cepek (2001) and Cepek and Bruns (2001) consider only two datum levels in the Upper Albian as reliable: the stratigraphic ®rst occurrences of Eiffellithus turriseiffelii and of E. monechiae. These two species have the advantage that they are plankton species with a wide geographical distribution, being common in the low as well as in the mid latitudes. The stratigraphic ®rst occurrence of E. turriseiffelii at the Kirchrode site falls within the Callihoplites auritus Subzone of the Mortoniceras in¯atum Ammonite Zone, which is different from the correlation of this biostratigraphic datum with the ammonite zonation in the Tethyan region. There the stratigraphic ®rst occurrence of E. turriseiffelii does not fall within the M. in¯atum Zone, but instead is observed at the boundary between the M. in¯atum and S. dispar Zones (e.g. Haq et al., 1988). Whether one of these datum levels is coeval in both the `Boreal' and Tethyan regions cannot be tested because no widespread volcanic
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Fig. 3. Biostratigraphy of the composite Albian sequence from the Kirchrode boreholes and its correlation to the mapping units of Bertram and Kemper (1971) and Bertram et al. (1971).
ashfall was identi®ed for this time interval nor magnetic reversals. Depending on which fossil group is used, the Middle±Upper Albian boundary has been placed in a different position. When the ®rst occurrence of the strongly sulcate shells of the bivalve species group Actinoceramus sulcatus is used, this boundary lies between 178 and 180 m in Kirchrode II (Fenner, 2001b). Weiss (in Fenner et al., 1996) using the stratigraphic ®rst occurrence of Ticinella raynaudi, a planktonic foraminifera species that is characteristic for the Tethyan region and for the transitional region to the `Austral' region, places this boundary deeper in the core: at 205 m. The Lower±Middle Albian boundary in the Kirchrode sequence has been provisionally placed on the basis of lithological criteria between the clearly Lower Albian dark claystones (krlu) and the clearly Middle Albian varicolored marlstones (krlm2) higher in the sequence. This is supported by preliminary results from ammonite analysis (Owen, written communication Feb. 2001). His information narrows
the interval, in which the boundary must lie, to between 234.8 and 241.2 m in Kirchrode II. Biostratigraphic analyses and tectonic analysis of the sedimentary structures and the fractures in the rock indicate that the recovered Upper Albian is close to being completely documented. Of the Upper Albian, only the basal part is shortened by faulting (Fenner, 2001b). The Middle and Lower Albian though are shortened by several faults (Fig. 4). 3.2. Sediment composition and compositional changes 3.2.1. Mineralogical and geochemical components The carbonate content of the claystones and marlstones increases through the Albian (Jendrzejewski et al., 2001; Fenner, 2001b; Rinna et al., 2001) starting with cyclic alternations between carbonate-free and carbonate-poor sediments in the dark claystone unit of the Lower Albian, and reaching 20±50% CaCO3 in the Upper Albian marlstones. The inverse correlation of the CaCO3 content and gamma log for the recovered Albian (Fig. 4) demonstrates that clay minerals
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Fig. 4. Changes in CaCO3 content (data from Fenner, 2001b; Jendrzejewski et al., 2001; Rinna et al., 2001), gamma ray intensity (Wonik, 2001) and plankton/benthos ratio in foraminifera (Thies, 2001; Tyszka and Thies, 2001) through the whole of the Albian at the Kirchrode sites. Schematic lithology and positions of the Lower±Middle and the Middle±Upper Albian boundaries after Fenner (2001b); occurrences of admixed ash (Benesch, 1997; KuÈhn et al., 2001; Rachold and Brumsack, 2001); ammonite subzones after Wiedmann and Owen (2001).
and marine, biogenic carbonate are the two main components that control the sediment composition in this deep shelf basin (Fenner, 2001b; Jendrzejewski et al., 2001; KuÈhn et al., 2001; Prokoph and Thurow, 2001; Rachold and Brumsack, 2001; Wonik, 2001).
A depositional environment relatively distant from land is suggested by the ®ne grain size of the sediment, the small size of the inertinite and vitrinite grains (mostly ,50 mm), the absence of shallow water species, and the low t/m index (t terrigenous, m marine) of
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the palynomorphs (Fenner et al., 1996; Fenner, 2001b; Jendrzejewski et al., 2001; Prauss, 2001). Most of the clay minerals are detrital in origin. The different crystallinity types of illite and kaolinite indicate that the clay minerals were derived from rocks of different age and from rocks with a different diagenetic history (KuÈhn et al., 2001). 3.2.2. Origin of the montmorillonite and volcanic activity during the Albian Layers containing volcanic glass were not encountered. But the presence of small amounts of alteration products of volcanic ash is indicated by the elevated content of montmorillonite co-occurring with elevated abundance of the elements Zr and Nb at two depths (at 87.20 m in Kirchrode I and at 221.77 m in Kirchrode II) within the Albian sequence (Fig. 4) (KuÈhn et al., 2001; Rachold and Brumsack, 2001; Benesch, 1997). For the presence of ash in these two horizons Benesch (1997) and KuÈhn et al. (2001) assume eolian transport and in situ weathering. Montmorillonite (a variety with predominantly Ca ions in the interlayer spaces is the most common) is concentrated in the ,2 mm grain size fraction, while kaolinite and illite are more abundant in the 2±20 mm grain size fraction. KuÈhn (1994) and KuÈhn et al. (2001) consider weathered products from explosive volcanism associated with the opening of the North Atlantic to be a possible additional source for the montmorillonite, but most of the montmorillonite they interpret to be detrital. Accordingly, the increase in montmorillonite content in the Upper Albian from 20±35% (including mixed-layer illite/smectite) in the lower part to 45±55% in the upper part, which is correlated with a decrease of kaolinite and to a lesser degree with a decrease of illite, is concluded to re¯ect a compositional change in terrigenous input. This interpretation differs from that of earlier studies by Brockamp (1976), Zimmerle (1979), Kemper and Zimmerle (1982) and Keller et al. (1989), who considered weathering of volcanic particles that were produced by volcanism connected to the opening of the North Atlantic to have been the main source. According to KuÈhn (1994) and KuÈhn et al. (2001) a contribution to the montmorillonite content from pedological processes or lateritic weathering was of subordinate importance because mixed-layer illite/
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smectite minerals do not increase in abundance together with montmorillonite, and because these authors ®nd little similarity between the Upper Albian clay mineral assemblage at Kirchrode and that of the weathering horizon on top of the kaolinised `Hilssandstein' described by Valeton (1958). Also the mineral beidellite, which is indicative of vertisols formed in areas with poor drainage under alternating wet and dry conditions, is not found among the Upper Albian clay minerals. Rachold and Brumsack (2001) also come to the conclusion that weathering of volcanic ashes during the Albian was of minor importance because they do not ®nd enrichment of Zr and Nb in the ,0.63 mm fraction, which they consider a characteristic of all ashes. They argue that because volcanic sources are of subordinate importance, terrestrial weathering under subarid climate conditions must have been a major source of the montmorillonite. 3.2.3. Diagenesis Dissolution of CaCO3 can be observed in the dark claystones of the Lower Albian (Fenner, 2001b), but in the Upper Albian Cepek (2001) does not recognize calcium carbonate dissolution. The good preservation of CaCO3 throughout the Upper Albian of Kirchrode I is indicated ² by the high abundance of the calcareous nannofossil species Biscutum constans, which is relatively easily dissolved (Fig. 5), and ² by the low number of unidenti®able calcareous nannofossils (Cepek, 2001). Further study is needed to determine whether the decrease in the abundance of marine palynomorphs relative to terrigenous ones in the basal part and the top of the Kirchrode I core is related in part to poorer preservation of organic matter, a possibility proposed by Fenner (2001b), as a consequence of decreased sedimentation rates and more intense oxygenation and bacterial decay of the organic matter in these core intervals and bacterial decay. Elevated temperatures de®nitely did not affect the Albian sediments at the Kirchrode site nor thermal cracking of the organic matter. According to Jendrzejewski et al. (2001) low thermal maturity is indicated by ² most Tmax values from Rock±Eval pyrolysis
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Fig. 5. Selected indicators of calcium carbonate dissolution and diagenesis in the sediments of the Kirchrode I borehole: relative abundance ¯uctuations of Biscutum constans within the calcareous nannofossil assemblage, a species considered highly susceptible to dissolution (Cepek, 2001); number of concretions and of thin, siderite-stained layers (Prokoph and Thurow, 2001); Fe/Mn ratio in the sediment (data from Rachold and Brumsack, 2001) and the same ratio in concretions (KuÈhn et al., 2001).
measured on concentrates of organic matter are between 387 and 4208C, and ² the yield of aromatic hydrocarbons in the organic matter extracts is low (ca. 10%), while heterocompounds containing nitrogen, sulphur or oxygen predominate (up to 52%). The most obvious sign that diagenesis has affected the Albian sediments is the numerous presence of sideritic concretions and thin sideritestained layers (KuÈhn et al., 2001; Prokoph and Thurow, 2001; Rachold and Brumsack, 2001). The mineralogical composition of these concretions according to KuÈhn et al. (2001) is a mixture of calcite, siderite, and rhodochrosite, with the Mnrich varieties dominating below 150 m in the Upper Albian of Kirchrode I. More rare are phosphatic concretions of francolite (Ca-¯uorapatite). Isotope analyses of selected concretions show that
they have lower d 13C values than the surrounding sediment matrix (KuÈhn et al., 2001). The authigenic mineral clinoptilolite as well as opal-CT are significant components of the top 7 m of the sediment core, where also still some undissolved opal-A (biogenic silica) is preserved (KuÈhn, 1994). A minor diagenetic component that occurs in low abundance but repeatedly throughout the Upper Albian is pyrite (KuÈhn et al., 2001). In contrast, Prokoph and Thurow (2001) state that it is often common or abundant. Clearly more rare than pyrite are occurrences of glauconite. This mineral is concentrated in thin layers within certain depth intervals and largely con®ned to the fraction ,63 mm (KuÈhn et al., 2001). The only interval with a high abundance of glauconite is around 250 m in Kirchrode II between the dark claystones of the lower part of the Lower Albian and the marlstones of the upper part of the Lower Albian (Fenner, 2001b).
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Fig. 6. Sedimentological and geochemical changes in the Upper Albian of Kirchrode I. Sedimentation rates (Fenner, 2001b); Nb/Al2O3, Zr/Al2O3, TiO2/Al2O3, SiO2/Al2O3, MnO/ Al2O3 ratios and % Al2O3 from Rachold and Brumsack (2001); gamma ray intensity from Wonik (2001); montmorillonite abundance relative to the total clay minerals in the ,20 mm CaCO3-free fraction of the sediment; quartz abundance in the ,20 mm CaCO3-free fraction of the sediment (KuÈhn et al., 2001). Ammonite subzones from Wiedmann and Owen (2001). Levels at which several of the parameters change are indicated by dashed lines.
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3.2.4. Changes in clastic terrigenous input The mineralogical results of KuÈhn et al. (2001), the geochemical results of Rachold and Brumsack (2001), and changes in the sedimentation rates (Fenner, 2001b) for the Upper Albian show three levels at which changes in the composition of the sediments and in the amplitude of the abundance ¯uctuations occur (Fig. 6). Proceeding from older to younger these shifts in Kirchrode I lie ² between 180 and 150 m, i.e. towards the end of the lower Callihoplites auritus Ammonite Subzone, ² at 40 m (near the top of the Mortoniceras rostratum Ammonite Subzone), and ² at 7 m (within the Arraphoceras briacensis Ammonite Subzone). At each of these levels a shift to a smaller proportion of clay minerals occurs. At the lowest of these levels the decrease in clay minerals is associated with a shift to slightly more quartz within the ,20 mm grain-size fraction. It coincides with a shift to slightly higher Si/Al, Ti/Al, Zr/Al, and Sr/Al ratios, and slightly lower Nb/Al, Fe/Al, Co/ Al, and Ni/Al ratios within the bulk sediment, and a shift in the amplitude of the abundance ¯uctuations of some of the other parameters. This ®rst step in the increase in ®ne silt-sized detrital minerals correlates with an increase in sedimentation rates from lower and strongly ¯uctuating values between 3.5 and 12.5 cm/ka to more stable and higher sedimentation rates between 7 and 13 cm/ka above this level (Fenner, 2001b). And among the clay minerals the composition shifts to more montmorillonite. At the next higher level where the clay content drops (at 40 m in Kirchrode I), quartz and Si/Al, Zr/Al again increase, but sedimentation rates slightly decrease. Here, quartz reaches values of .15%. The most drastic increase in quartz (to .20%) and in Si/Al, Ti/Al, and Zr/Al occurs at the third step (at 7 m in Kirchrode I). This stepwise increase in the Si/Al, Ti/Al, and Zr/ Al ratios, which is the strongest in the ®ne silt fraction (2±20 mm), is interpreted as being related to an increase in detrital silt-sized minerals, mainly detrital quartz and heavy minerals (e.g. zircon, anatase and sphene). Different explanations concerning the cause of the increase in montmorillonite and silt-sized terrigenous detritus through the Upper Albian have been proposed
by the different authors in this volume. Rachold and Brumsack (2001) explain the increase in montmorillonite in the lower C. auritus Subzone with a change in the weathering regime caused by a change from a humid to a subarid climate. The later change (above 40 m) to more detrital quartz is interpreted as a further step in this direction, re¯ecting increased input from eolian detritus. KuÈhn et al. (2001) recommend caution in the use of clay minerals as an argument for climatic change. Because of the high abundance of illite (.35%) and the type of crystallinity of the clay minerals, they interpret most of the clay minerals, including the montmorillonite, as recycled from ashrich older Mesozoic and Late Paleozoic sediments. And accordingly they see mainly a change in the eroded source rock being re¯ected in the compositional changes of the sediment. Fenner (2001b) adds that these changes in terrigenous input and in sedimentation rates during the Late Albian are related to a combination of causes: ² local subsidence due to salt migration into the salt structures ¯anking the subbasin in which Kirchrode is located, ² regional subsidence of the region north of the rising Rhenish±Bohemian massif, ² a change from a more arid to a more humid climate in the lower part of the C. auritus subzone (a contrasting interpretation to that of Rachold and Brumsack, 2001), and in the latest Albian possibly a change to more arid conditions, leading to windinduced upwelling along submarine relief in the shelf sea, ² 3rd-order sea level changes, and ² the gradual moving of the main depot center closer to the Kirchrode site. 3.3. Biogenic components Ð ¯uvial input Ð productivity Like the mineral components, the changes in the abundance of several biogenic components in the Upper Albian also occur stepwise and at the same depth levels as the changes in terrigenous input (Fig. 7). Two groups of biogenic components can be distinguished, depending on the cause of the abundance changes. The ®rst group comprises allochthonous as well as redeposited older biogenic components. To this group belongs the TOC content of the sediment, which was
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Fig. 7. Changes in biogenic parameters in the Upper Albian of Kirchrode I. Organic carbon content of the sediment (Jendrzejewski et al., 2001); number of calcareous dino¯agellate cyst species and relative abundance of morphotypes among the Obliquipithonelloideae with coarsely crystalline cysts walls (Keupp, 2001); relative abundance of peridinoid cysts and GV cysts (more or less dorsi-ventrally compressed forms with no or only small antapical horns or protrusions, which are used by Prauss (2001), as an indication of in¯ux from coastal environments) relative to the total organic-walled dino¯agellate cysts; presence of freshwater algae and reworked palynomorphs, and the terrigenous/marine ratio within the palynomorphs (Prauss, 2001); relative abundance of calcareous nannofossils (Cepek, 2001); number of benthic foraminifera .125 mm in the sediment and the relative abundance of specimens of Arenobulimina ssp. within the benthic foraminifera assemblage of the .125 mm fraction (Thies, 2001); number of radiolaria .125 mm and interpreted marine plankton productivity (Fenner, 2001b); P2O5/Al2O3 from Rachold and Brumsack (2001); ammonite subzones (Wiedmann and Owen, 2001); climate interpretation (after Fenner, 2001b; Prauss, 2001). The dashed lines are shown at the same depths as in Fig. 6.
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found to be dominated by terrigenous matter. It includes, especially in the lower part of the Upper Albian, some primary terrigenous matter from higher plants (long-chain n-alkane maximum between n-C25 and n-C29 together with the odd carbon number predominating), but most is reworked older material (Jendrzejewski et al., 2001). It, therefore, is plausible that the TOC ¯uctuations are governed by the same processes as those of the silt-sized terrigenous mineral detritus. The reworked, older material includes rare vitrinite that Jendrzejewski et al. (2001) consider to be reworked Carboniferous 1. Analysis of palynomorphs also encountered reworked, older specimens (mostly acritarchs and dinocycsts). They show a drop in abundance at 150 m in Kirchrode I. Below, they occur rarely, but repeatedly, indicating erosion of older rocks. Above this level, their occurrence is very scarce; instead, rare but repeated occurrence of freshwater algae (Botryococcus and Palambages) is characteristic, which points to a change to a more humid climate and the presence of freshwater bodies on a land mass to the south (Prauss, 2001). Such a climatic change is also indicated by the trends in the different allochthonous palynomorph groups through the Upper Albian: a decrease in abundance of bisaccate pollen and an increase in trilete spores. These opposing trends suggest an increase in the proportion of input via ¯uvial transport and a decrease in input via wind transport (Prauss, 2001). The high Mn concentration in the sediment, which slightly increases in the Upper Albian at Kirchrode (Rachold and Brumsack, 2001), may also support this interpretation. Prauss (2001) goes so far to suggest as a consequence of the increased ¯uvial input the possibility of a change to an estuarine circulation in this shelf basin. Also Prokoph and Thurow (2001), from their results of the changes in the trace fossil assemblages, consider strati®cation of the water column a possibility for the time interval during which sediments belonging to the lower part of the C. auritus Subzone were deposited. The second group of biogenic components that change at the same levels comprises authochthonous biogenic components, which re¯ect increasing marine 1
Erosion of Carboniferous sediments is not supported by the results of Prauss (2001), who did not ®nd any of the characteristic, robust spores of the Carboniferous.
productivity (Fig. 7). This increased productivity is considered to be a result of the change to a more humid climate and increased runoff from land, which led to an increased input of dissolved and particulate nutrients into the shelf sea (Fenner, 2001b). At the lowest level of change (180±150 m in Kirchrode I), the increase in plankton productivity is gradual (e.g. increased abundance of calcareous nannofossils). In the top 40 m in Kirchrode I, radiolaria (CaCO3 or pyrite casts of the shells) are common besides the still abundant calcareous nannofossils, indicating an increase in marine plankton productivity. Productivity probably begins to increase again at 7 m, above which siliceous skeletons of radiolaria and diatoms have increasing abundances and better preservation (Fenner, 2001b), and where KuÈhn et al. (2001) report a further increase in silt-sized terrigenous detritus and Rachold and Brumsack (2001) an increase in Si, Zr, Nb, Ba and P. Benthos productivity may also, in general, have increased through the Upper Albian (Prokoph and Thurow, 2001). Owing to variations in oxygen levels at the sea ¯oor that affect the benthos, the interpretation of benthos productivity resulting from the analysis of bioturbation by Prokoph and Thurow (2001), however, does not correlate with that described above for plankton productivity. The Upper Albian trace fossils at the Kirchrode site change from a diverse assemblage with predominantly Thalassinoides, Planolites, Teichichnus, and Chondrites forms (below 132 m in Kirchrode I) to an assemblage that has a lower diversity, is dominated by Chondrites and Teichichnus, has more frequent Sinusites than below, and shows a lower bioturbation intensity and shallower burrowing depth. This low diversity assemblage of the upper part of the Upper Albian is interpreted by Prokoph and Thurow (2001) to either re¯ect lower oxygen concentrations in the deep water and surface sediment or decreasing productivity. Higher in the sequence, above 90 m in Kirchrode I, the diversity of the ichnofauna and the intensity of bioturbation is found to increase again, re¯ecting an increase in productivity and better oxygenation at the sea ¯oor. This trend is seen by these authors to continue into the top 30±40 m of Kirchrode I, where the diversity as well as the density and depth of bioturbation are further increased (Fig. 9). The indications of a stepwise increase in marine
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299
Fig. 8. Occurrence of ammonite species (Wiedmann and Owen, 2001), plankton foraminifera (Weiss, 1997), and calcareous dino¯agellate cysts that are interpreted to have immigrated from the Tethys (Keupp, 2001); abundance of suspension-feeding bivalves (Fenner, 2001b), and planktonic foraminifera (Thies, 2001), calcareous nannofossils (Cepek, 2001), and the dino¯agellate cyst genus Palaeoperidinium (Prauss, 2001), as well as the interpreted sea levels of Fenner (2001).
productivity during the Late Albian, which is also documented in an increase in sedimentation rates of biogenic carbonate parallel to the increase in sedimentation rates of terrigenous detritus (Fig. 6), suggest that marine productivity was not very low as was argued from the low abundance of TOC by Jendrzejewski et al. (2001). Instead the very low abundance of organic matter derived from marine plankton must be due to oxidation before its burial in the sediment. For the productivity increase in the uppermost Albian wind-induced upwelling (Frieg and Kemper, 1989) of more nutrient-rich deep water along submarine topographic highs (Fenner et al., 1996) or coastal upwelling (Rachold and Brumsack, 2001) has been thought to be a possible explanation. The above-described palynological results (Prauss, 2001) though suggest that increased ¯uvial input also could have been a source for the elevated nutrient levels. At present, it thus remains unsolved whether climate changed to more arid conditions during the latest
Albian in the mid-latitudes around the Lower Saxony basins, as was described for the Western Interior basin by Glancy et al. (1993) and Pratt et al. (1993). 3.4. Oxygenation levels in the water surface sediment Suboxic conditions at the sea ¯oor, at times probably approaching anoxic conditions, are suggested for the time of deposition of the Lower Albian dark claystones, which are characterized by reduced benthic life, a predominance of arenaceous foraminifera, and which exhibit faint bioturbation. Marls were deposited for the remainder of the Albian. These marls are characterized by a rich benthic fauna as well as by remains of nektonic and of planktonic organisms, indicating well oxygenated conditions in the water column and at the sea¯oor (Fenner, 2001b). Oxic conditions at the sea¯oor and in the water column are also indicated by the mineralogical and chemical analyses for the Upper Albian of
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Fig. 9. The four different sea level interpretations for the Upper Albian of Kirchrode I, together with at least one parameter used in the argumentation by the respective author(s). Summary plot of the relative abundance of plankton foraminifera considered to have immigrated from the Tethyan after Weiss (1997), summary plot of the abundance of plankton foraminifera and radiolaria in the .125 mm fraction (Fenner, 2001b; Thies, 2001), plankton/benthos foraminifera ratio (Thies, 2001), CaCO3 content of the sediment (Jendrzejewski et al., 2001), bioturbation intensity and occurrence of glauconite (Prokoph and Thurow, 2001), t/m index of palynomorphs (Prauss, 2001). mf maximum ¯ooding surface. UZA Supercycle set `Upper Zuni A' of Haq et al. (1988).
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301
minimum zone to the south in the direction of the coast is postulated by Rachold and Brumsack (2001) for this same time interval. Such an oxygen minimum zone is considered necessary as an additional Mn source, because their mass balance calculations indicate that the high Mn values cannot be explained by ¯uvial input alone. 3.5. Water depth
Fig. 10. Spectral peaks of CaCO3 in the depth inerval 40±100 m in Kirchrode I (Rachold and Brumsack, 2001).
Kirchrode I: ² the relatively low abundance of pyrite (KuÈhn et al., 2001), together with ² high amounts of reactive iron, as well as by the lack of enrichment of the redox-sensitive trace elements V and Cr (Rachold and Brumsack, 2001), and ² the low organic carbon content of the sediment (0.18±0.6%) in spite of high sedimentation rates (Jendrzejewski et al., 2001). Of the easier oxidizable autochthonous, marine organic matter only a small part is left in the Upper Albian sediments. Most that remained is resedimented organic matter. This is supported by microscopical analysis and detailed chemical analysis of the organic matter of 12 selected samples. The carbon/sulphur ratio (low sulphur contents of the sediments) and the hydrogen-poor kerogen indicated by Rock±Eval pyrolysis (HI values below 100 mg HC/g TOC for all samples, refractory type III/IV kerogen), the high pristane/phytane ratio (a mean of 1.25), and the presence of a peak at n-C21 or n-C23 in the n-alkane distribution, lead to the conclusion that bacterial degradation under aerobic conditions during an early stage of sedimentation rather than sulphate reduction was responsible for the low percentage of marine organic matter (Jendrzejewski et al., 2001). While all agree that oxic conditions characterized the environment at the sea¯oor at the Kirchrode site during the Late Albian, the existence of an oxygen
The benthic foraminifera in the Upper Albian suggest a water depth of at least 100 m (Fenner et al., 1996). The deepening of the shelf sea with global sea level rise and regional and local subsidence is re¯ected in the fact that deep-dwelling Tethyan planktonic foraminifera manage to intrude into the Lower Saxony basin during the late Late Albian. Earlier Ticinella spp. intruded (Weiss, 1997), species that are considered to have lived in near-surface water (e.g. Sliter, 1972; Hart, 1980). Another parameter that suggests increasing water depth is the abundance of benthic foraminifera. It decreases at the lowest level of change (between 180±150 m in Kirchrode I) from an average of 50/g of sediment to only 35/g of sediment above this level, while the plankton/benthos ratio of foraminifera increases (Thies, 2001). Besides an increase in water depth, it may re¯ect a change from more turbulent conditions in the shallower shelf to a deep water situation in which a relatively stable pycnocline developed and nutrients became increasingly recycled within the upper water column. During the more turbulent conditions, food for the benthos was supplied from above as well as by lateral advection, supporting a relatively large population of suspension-feeders (Fenner 2001b). Late in the Albian, the reduced amount of food on the sediment surface and in suspension just above it led to a decrease in benthic foraminifera and an increase in the relative abundance of detritus-feeders, e.g. Arenobulimina spp. (e.g. Jones and Charnock, 1985; Frieg and Kemper, 1989). 3.6. Sea level changes and palaeocirculation The effects of global sea level changes on the deep parts of this shelf basin during the Late Albian are recognized in the systematic, repeated changes in relative abundance of the main plankton and benthos groups, which re¯ect changes in the food availability
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and intensity of oceanic circulation. These effects were found to be largely independent of the changes in sedimentation rate (tectonics and climate) and were found to be overlain on the general long-term trend of a deepening shelf sea environment (Fenner, 2001b). Time intervals with intensi®ed circulation are identi®ed on the basis of abundance maxima of benthos suspension-feeding species and of immigrant plankton species from the Tethyan region (Fig. 8). Among the ammonites, species considered as Tethyan immigrants by Wiedmann and Owen (2001) also occur largely in the same intervals where tethyan immigrants among the planktonic foraminifera occur. The intensi®cation of circulation is shown by Fenner (2001b) to have ®rst affected the upper part of the water column, causing an increase in the abundance of immigrated plankton species. Later, the water directly above the sea ¯oor was also affected. The increased lateral advection of food under the later conditions supported more benthic suspensionfeeders (bivalves and foraminifera). The spreading of the benthic foraminifera genus Spiroplectinata from the Tethys to the `Boreal' seas during the later part of the Early Albian and early part of the Middle Albian (Tyszka and Thies, 2001) can be expected to have been aided by the repeated occurrence of such periods of intensi®ed circulation. These times of intensi®ed circulation occurred during times of rising sea level, if the scheme of third order global sea level changes of Fenner (2001b) is correct (Fig. 8). The maximum ¯ooding according to this model occurred during the same interval as the maxima in TOC determined by Jendrzejewski et al. (2001), and the maxima in zooplankton remains. It is associated with or followed by a maximum in phytoplankton remains. The changes found in the trace fossil assemblages by Prokoph and Thurow (2001) also ®t this model and the observed changes in sedimentation rates (but note that the interpretation of the trace fossil assemblages by Prokoph and Thurow, 2001, is different). The general trend of the interpretation of the global sea level ¯uctuations during the Late Albian by Tyszka and Thies (2001) is similar to that of Fenner (2001b). A very different interpretation of the sedimentary and biogenic changes in terms of global sea level change (Fig. 9). is presented by Prokoph, 1994 and Prokoph and Thurow (2001), whose sea level curve largely follows the changes in sedimenta-
tion rates calculated by Fenner (2001b). Again another sea level interpretation is proposed by Prauss (2001) based largely on changes in the t/m ratio of palynomorphs and abundance changes in selected dino¯agellate groups. This divergence in interpretations illustrates the dif®culty to recognize global sea level changes in the relatively monotonous sedimentary record of deep shelf basins, which do not contain such strong lithological changes so characteristic of shallow water or near-shore marine environments. 3.7. Milankovitch cyclicity In addition to the above-described long-term climatic changes through the Late Albian, regular changes in the calcium carbonate contents of the sediment and in abundance ¯uctuations of geochemical, and biogenic components are easily visible in Kirchrode I and are suggestive of Milankovitch cyclicity. The cyclicity is especially prominent in that part of the Upper Albian, which is characterized by high sedimentation rates (Figs. 6 and 7). That these cycles indeed re¯ect Milankovitch-driven cyclicity is established by spectral analysis of these data in the depth domain (Prokoph and Thurow, 2001; Rachold and Brumsack, 2001; Weber et al., 2001; Wonik, 2001), (Fig. 10) which produces frequency peaks with a characteristic peak ratio of roughly 1:2:5 (Fig. 10), which is also characteristic of the Milankovitch periodicities during the Late Neogene. That the cyclicity documented in the ¯uctuations of the carbonate content of the sediment indeed re¯ect climate changes is shown by the fact that long-chain n-alkanes with a maximum between n-C25 and n-C29 are enriched in the darker part of the cycles. This re¯ects an increased input of terrestrial organic matter during these intervals (Jendrzejewski et al., 2001). Changes in the benthic foraminifera assemblages document for at least part of the Upper Albian that in the marine shelf environment these changes in terrestrial input resulted in changes in the productivity (Fenner et al., 1996; Fenner, 2001b). In the intervals with a high sedimentation rate (above 216 m in Kirchrode I), where sample spacing is close enough to recognise precessional cycles, the ¯uctuations in the abundance of immigrant Tethyan plankton species differ from those of the `Boreal' plankton species. The planktonic foraminifer
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Globigerinella bentonensis, which is considered to have immigrated from Tethyan regions, shows a strong precessional signal in its abundance ¯uctuations. The `Boreal' Hedbergella planispira and H. delrioensis instead have a stronger obliquity signal. If the dominant Milankovitch cycles identi®ed in the abundance of a species or species group indeed indicate changes triggered either in the low latitudes or in the mid or high latitudes, the relatively strong precessional signal in the benthic foraminifera in this high sedimentation rate part of the Upper Albian re¯ects in¯uences from the low latitudes on the living conditions in the basin deeps. The identi®cation of the Milankovitch cyclicity was used to determine sedimentation rates. Using a duration of ca. 95,000 a for the identi®ed shorter periodicity eccentricity cycles, sedimentation rates are found to increase from 1.5 to 6 cm/ka in the lower part of the Upper Albian to values between 4.5 and 12.5 cm/ka in the basal part of the C. auritus Subzone (above 216 m in Kirchrode I), and then to values of 7±13 cm/ka in about the middle of the same zone (above 155 m in Kirchrode I) (Fenner, 2001b). The identi®ed eccentricity cycles also are used by Fenner (2001b) to determine the duration of the Callihoplites auritus Ammonite Subzone of the Upper Albian to be ca. 2.1 Ma, and the Upper Albian (base de®ned by the ®rst occurrence of Actinoceramus sulcatus) to have had a minimum duration of 4 Ma (possibly 4.5±4.8 Ma). Acknowledgements This work was supported by DFG grants Fe 240-2 and Fe 240-4 and by the BGR. I thank A.W. Burger for help with producing the ®gures and R.C. Newcomb for checking up on the English. References Baldschuhn, R., Frisch, U., Kockel, F., 1996. Geotektonischer Atlas von NW-Deutschland 1:300 000. 17 parts. Bundesanstalt fuÈr Geowissenschaften und Rohstoffe, Hannover (in German). Benesch, M., 1997. Mineralogische Untersuchungen von Unterkreide Ð Sedimenten aus dem NiedersaÈchsischen Becken. PhD Thesis, University of GoÈttingen, 88 pages, 8 tables, 45 ®gures. GoÈttingen.
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Bertram, H., Kemper, E., 1971. Das Alb von Hannover. Beih. Ber. Naturh. Ges. 7, 27±47, 3 plates. Bertram, H., Kemper, E., Rhode, P., 1971. Eine neue Karte des mesozoischen Untergrundes im oÈstlichen Stadtgebiet von Hannover. Beih. Berichte Naturh. Ges. 7, 13±26. Brockamp, O., 1976. Nachweis von Vulkanismus in Sedimenten der Unter- und Oberkreide in Norddeutschland. Geol. Rdsch. 65, 162±174. Ï epek, P., 2001. Calcareous nannoplankton from the `Boreal' Late C Albian of NW Germany (Kirchrode I borehole): its biostratigraphy and cyclicity. Palaeogeogr., Palaeoclimatol., Palaeoecol. 174, 181±197. Cepek, P., Bruns, A., 2001. Two new Upper Albian subzones, the Eiffelithus monechiae and Eiffelithus turriseiffelii s.s. Subzones. N. Jb. Geol. Pal., (in press). Fenner, J., 2001a. Middle and Late Albian geography, oceanography, and climate and the setting of the Kirchrode I and II borehole sites. Palaeogeogr., Palaeoclimatol., Palaeoecol. 174, 5±32. Fenner, J., 2001b. The Kirchrode I and II boreholes: technical details and evidence on tectonics and on palaeoceanographic development during the Albian. Palaeogeogr., Palaeoclimatol., Palaeoecol. 174, 33±65. Fenner, J., Bruns, A., Cepek, P., KoÈthe, A., Owen, H.G., Prauss, M., Rigel, W., Thies, A., Tyszka, J., Weiss, W., Wiedmann, J., 1996. Palaeontological results from the Boreal Albian (cores Kirchrode I and II), biostratigraphy, palaeoenvironment and cycle analysis. In: Reitner, J., Neuweiler, F., Gunkel, F. (Editors), Global and Regional Controls on Biogenic Sedimentation. Part II. Cretaceous sedimentation. Research reports. GoÈttinger Arb. Geol. PalaÈont., Sb 3, 5-12. GoÈttingen. Frieg, C., Kemper, E., 1989. MikropalaÈontologische Gliederung und Abgrenzung von Ober-Alb und Unter-Cenoman in Nordwestdeutschland. In: Kemper, E. (Editor), Alb und UnterCenoman in Mittel- und Nordeuropa. Geol. Jb., A 113, 73-193. Hannover. (in German). Glancy Jr, T.J., Arthur, M.A., Barron, E.J., Kauffman, E.G., 1993. A paleoclimatic model for the North American Cretaceous (Cenomanian±Turonian) epicontinental sea. Geol. Ass. Can., Spec. Pap. 39, 219±241. Hart, M.B., 1980. A water depth model for the evolution of planktonic foraminiferida. Nature 286 (5770), 252±254. Haq, B.U., Hardenbol, J., Vail, P., 1988. Mesozoic and Cenozoic chronostratigraphy and eustatic cycles. In: Wilgus, C.K. (Ed.), Sea-level changes: an integrated approach. SEPM Spec. Publ. 42, 71±108 Tulsa, Oklahoma. Jakubowski, M., 1987. A proposed Lower Cretaceous calcareous nannoplankton zonation scheme for the Moray Firth area of the North Sea. Abh. Geol. Bundesanstalt Wien 39, 99±119. Jendrzejewski, L., Littke, R., RullkoÈtter, J., 2001. Organic geochemistry and depositional history of Upper Albian sediments from the Kirchrode I borehole, northern Germany. Palaeogeogr., Palaeoclimatol., Palaeoecol. 174, 107±120. Jeremiah, J., 1996. A proposed Albian to Lower Cenomanian nannofossil biozonation for England and the North Sea Basin. J. Micropal. 15, 97±129. Jones, R.W., Charnock, M.A., 1985. `Morphogroups' of agglutinating
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www.elsevier.com/locate/palaeo
Palaeoceanographic and climatic changes during the Albian, summary of the results from the Kirchrode boreholes J. Fenner* Bundesanstalt fuÈr Geowissenschaften und Rohstoffe, Stilleweg 2, Hannover 30655, Germany Received 9 October 2000; Accepted for publication 3 April 2001
Abstract Analysis of the Albian sediments from the Kirchrode site by different disciplines yielded information about depth, productivity and oxygenation of the water in the deeper parts of the Lower Saxony basin, one of the mid-latitude shelf sea basins in the northern hemisphere. In addition information was obtained about phases during which in¯ow of water from the neighboring oceans (Tethys, North Atlantic) and circulation within the basin was intensi®ed, about sea level changes, as well as about climate changes occurring during the Albian in this mid-latitude region. This information was derived from observations of the effects the changes in the palaeoenvironment had on the fossil and mineral assemblages, the sediment facies, and the geochemical composition of the sediment. The increase in terrigenous input during the Upper Albian, the relative increase in detrital montmorillonite and silt-sized minerals, e.g. quartz and zircon, re¯ect tectonic movements (e.g. uplift of the Rhenish±Bohemian massif and subsidence of the Lower Saxony basin, locally intensi®ed by halokinetic movements) and long-term regional changes in climate. A change to a more humid climate occurred approximately at the stratigraphic ®rst occurrence of the calcareous nannofossil genus Eiffellithus in this region and at the start of a new transgressional period, and it led to increased runoff from the land. Possibly, a second change, this time to a more arid climate, occurred in the latest Albian. During times of high sedimentation rates of terrigenous components the marine productivity in the shelf sea is also increased as a consequence of the increased input of dissolved and particulate matter. During the latest Albian, upwelling may have been the cause of a further increase in productivity. A global sea level change overlying the effects of these regional and local processes was recognised e.g. in changes in sedimentation rates in the monotonous deep-shelf sediments (decreasing during transgression and highstand times, increasing during regression and lowstand times). Global sea level changes are best documented in changes in the abundances of the main plankton and benthos groups, and changes in the relative abundance of benthos groups with different feeding strategies: suspension-feeders and detritus-feeders. Transgressional periods start with an abundance maximum in species that immigrated from the Tethys. Highstand conditions at lower nutrient levels are documented in the sediment record by a maximum in planktonic foraminifera and calcareous nannoplankton. When nutrient levels under highstand conditions were higher, like at the end of the Albian, calcareous nannofossils and siliceous plankton dominate. Milankovitch cyclicity was identi®ed using spectral analysis of the regularly and closely spaced data sets of sedimentological, palaeontological, and geochemical parameters. The higher abundance of terrigenous plant material in the dark parts of the sedimentary cyles in the Upper Albian suggest a change in humidity was the main climatic change and changes in benthos foraminifera assemblages indicate cyclic productivity changes in at least part of the sequence. Among planktonic foraminifera, species that immigrated from the Tethys have a stronger precessional signal in their abundance ¯uctuations, while the `Boreal' species show a stronger obliquity signal. For the deep part of the shelf basin, the strong precessional signal in the cyclicity of * Tel.: 149-511-643-2510; fax: 149-511-643-2304. E-mail address: [email protected] (J. Fenner). 0031-0182/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0031-018 2(01)00298-X
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benthic foraminifera abundances suggests that conditions were in¯uenced for most of the Upper Albian by processes triggered in the low latitudes. Based on the identi®ed Milankovitch cyclicity, sedimentation rates were found to increase through the Upper Albian from 1.5±6.5 to 7±13 cm/ka. The duration of the Callihoplites auritus Ammonite Subzone was determined to be 2.1 Ma and the minimum duration of the Late Albian 4 Ma (possibly 4.5±4.8 Ma). q 2001 Elsevier Science B.V. All rights reserved. Keywords: Albian; Climate; Sea level; Productivity; Milankovitch cyclicity
1. Introduction As a contribution to the international ALBICORE program (Larson et al., 1993), the Albian sequence at the Kirchrode I and II sites in the Lower Saxony basin (Fig. 1) was drilled in November±December 1991 and November±December 1994. Analysis of the sediments from the deep part of this basin was planned as an exemplary study for the wide belt of shelf seas in the northern hemisphere mid-latitudes, the `Boreal' region. The high sedimentation rates especially in the Upper Albian and the relatively undisturbed basinal, offshore sedimentation make possible detailed studies of: ² the palaeoceanographic changes that occurred in such a setting during the Albian transgression and of ² how and how strong Milankovich cyclicity in¯uenced biogenic sedimentation and input of terrigenous detritus during a period known for its `greenhouse climate'. A site in a mid-latitude shelf sea that was connected with the major surrounding oceans (North Atlantic, Arctic, Tethys) was chosen, because sedimentation in such a setting was expected to have been especially sensitive to palaeoclimatic and palaeoceanographic changes. Our statistical evaluation of the quantitative data to identify climate-driven cyclicity did not have to start from zero, but could build on a large number of geological and qualitative palaeontological studies (for an overview, see Fenner, 2001a). This volume mainly presents results from the Kirchrode I borehole. A few studies include results from both the Kirchrode boreholes. Most of the studies were done quantitatively on samples taken at regular spacing (one meter or half a meter) and for logging and scanning of grey values every 5 or 10 cm in order to obtain a data set suitable for statistical analysis. Rachold and Brumsack (2001), in addition,
chose the sedimentary cycle from 80 to 92 m in Kirchrode I, taking samples at intervals of 10 cm for a higher resolution geochemical study. Several methods were used to identify cyclicity in the abundance curves: direct visual evaluation of the ¯uctuations (Fenner, 2001b; Jendrzejewski et al., 2001) or frequency, spectral, and wavelet or sliding window analysis (Prokoph and Thurow, 2001; Rachold and Brumsack, 2001; Weber et al., 2001; Wonik, 2001). It has to be kept in mind that there are very few comparatively detailed analyses of the mid-Cretaceous, and that we are far from having an amount of quantitative data comparable to that of the Quaternary. Accordingly, there is considerably more space for speculation on climatic and palaeoceanographic conditions during the Albian and on how both were affected by tectonic and magmatic activity. Although our project was intended to reduce the space for speculative interpretation, the lack of well dated, detailed analyses of this period from other palaeogeographic positions sometimes led to completely opposing interpretations by the different researchers within our project, in spite of numerous discussions of the results and in spite of exchange of data and manuscripts. The intention of this summary is to present a brief overview of the results treated in detail in the chapters of this volume. 2. Site location The drill sites were selected within the same basinal structure between the two NNE±SSW-trending salt structures Benthe and Lehrte to the southeast of Hannover city (Fig. 2), where earlier mapping had found high sedimentation rates for the Middle and Upper Albian, and only the lowermost part of the Middle Albian seemed to be missing (Bertram and Kemper, 1971). The two sites Kirchrode I and II were drilled
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Fig. 1. Albian geography and tectonic structures of part of the northern hemisphere, showing the extent of the seas. Three depth categories based on the extent and character of Albian marine sediments are shown (map after Ziegler, 1990, modi®ed). Ki location of Kirchrode boreholes.
880 m from each other. The shelf-sea sediments in this area are of a basinal type that does not include sand-sized terrigenous clastics. For more details on the site selection and the geology of the area, see (Fenner, 2001a,b).
There are dif®culties to identify the northern hemisphere mid-latitude equivalent of the Albian R. praeticinensis Subzone, which was de®ned for the Tethyan region and has been selected as the target interval for the international research program
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Fig. 2. Geological section across the Mesozoic subbasin of the Lower Saxony basin selected for this project (simpli®ed from Baldschuhn et al., 1996). The pro®le runs E±W perpendicular to the trend of the Benthe and Lehrte salt structures. The Wealden facies is included with the Upper Jurassic in this drawing.
ALBICORE (Larson et al., 1993). Because of these dif®culties, as much as possible of the Albian record was drilled in order to increase the probability of ®nding stratigraphic markers or events of a wide geographic effect that are suitable for correlation between low and mid latitude marine records (Fenner, 2001b). The Craelius SK 6L wireline system was chosen for coring, a system best suited to recover undisturbed sediment sequences in claystones and marlstones. Slow and cautious drilling resulted in a recovery of close to 100% from both boreholes (Fenner, 2001b). A 385 m long sequence of the Albian was recovered. Especially well represented in this pro®le was the Upper Albian (285 m recovered). The cores were split lengthwise into an archive half for nondestructive studies and documentation, and a working half from which samples were taken.
3. Results 3.1. Stratigraphy The Kirchrode Albian is dated on the basis of biostratigraphic (mainly ammonites, bivalves, and calcareous nannofossils) and lithostratigraphic criteria established for the Northwest German basin. The numerous, excellent imprints of ammonite shells in the ®ne-grained sediments allowed many specimens to be determined to the species level. As a consequence Wiedmann and Owen (2001) were able to identify all seven ammonite subzones of the
`Boreal' Upper Albian in Kirchrode I. The correlation of these ammonite zones and subzones with the microfossil zones identi®ed in the same pro®le, as well as with the stratigraphic ®rst occurrence of the bivalve species group Actinoceramus sulcatus and the mapping units of Bertram et al. (1971), is shown in Fig. 3. For the Upper Albian, the ammonites provide by far the largest number of stratigraphic subdivisions. The stratigraphic resolution using calcareous nannofossils is lower. The high resolution calcareous nannofossil zonation developed for the `Boreal' region by Jakubowski (1987) and Jeremiah (1996) could not be veri®ed by Cepek (2001) and therefore was not applied to these cores. Cepek (2001) and Cepek and Bruns (2001) consider only two datum levels in the Upper Albian as reliable: the stratigraphic ®rst occurrences of Eiffellithus turriseiffelii and of E. monechiae. These two species have the advantage that they are plankton species with a wide geographical distribution, being common in the low as well as in the mid latitudes. The stratigraphic ®rst occurrence of E. turriseiffelii at the Kirchrode site falls within the Callihoplites auritus Subzone of the Mortoniceras in¯atum Ammonite Zone, which is different from the correlation of this biostratigraphic datum with the ammonite zonation in the Tethyan region. There the stratigraphic ®rst occurrence of E. turriseiffelii does not fall within the M. in¯atum Zone, but instead is observed at the boundary between the M. in¯atum and S. dispar Zones (e.g. Haq et al., 1988). Whether one of these datum levels is coeval in both the `Boreal' and Tethyan regions cannot be tested because no widespread volcanic
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Fig. 3. Biostratigraphy of the composite Albian sequence from the Kirchrode boreholes and its correlation to the mapping units of Bertram and Kemper (1971) and Bertram et al. (1971).
ashfall was identi®ed for this time interval nor magnetic reversals. Depending on which fossil group is used, the Middle±Upper Albian boundary has been placed in a different position. When the ®rst occurrence of the strongly sulcate shells of the bivalve species group Actinoceramus sulcatus is used, this boundary lies between 178 and 180 m in Kirchrode II (Fenner, 2001b). Weiss (in Fenner et al., 1996) using the stratigraphic ®rst occurrence of Ticinella raynaudi, a planktonic foraminifera species that is characteristic for the Tethyan region and for the transitional region to the `Austral' region, places this boundary deeper in the core: at 205 m. The Lower±Middle Albian boundary in the Kirchrode sequence has been provisionally placed on the basis of lithological criteria between the clearly Lower Albian dark claystones (krlu) and the clearly Middle Albian varicolored marlstones (krlm2) higher in the sequence. This is supported by preliminary results from ammonite analysis (Owen, written communication Feb. 2001). His information narrows
the interval, in which the boundary must lie, to between 234.8 and 241.2 m in Kirchrode II. Biostratigraphic analyses and tectonic analysis of the sedimentary structures and the fractures in the rock indicate that the recovered Upper Albian is close to being completely documented. Of the Upper Albian, only the basal part is shortened by faulting (Fenner, 2001b). The Middle and Lower Albian though are shortened by several faults (Fig. 4). 3.2. Sediment composition and compositional changes 3.2.1. Mineralogical and geochemical components The carbonate content of the claystones and marlstones increases through the Albian (Jendrzejewski et al., 2001; Fenner, 2001b; Rinna et al., 2001) starting with cyclic alternations between carbonate-free and carbonate-poor sediments in the dark claystone unit of the Lower Albian, and reaching 20±50% CaCO3 in the Upper Albian marlstones. The inverse correlation of the CaCO3 content and gamma log for the recovered Albian (Fig. 4) demonstrates that clay minerals
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Fig. 4. Changes in CaCO3 content (data from Fenner, 2001b; Jendrzejewski et al., 2001; Rinna et al., 2001), gamma ray intensity (Wonik, 2001) and plankton/benthos ratio in foraminifera (Thies, 2001; Tyszka and Thies, 2001) through the whole of the Albian at the Kirchrode sites. Schematic lithology and positions of the Lower±Middle and the Middle±Upper Albian boundaries after Fenner (2001b); occurrences of admixed ash (Benesch, 1997; KuÈhn et al., 2001; Rachold and Brumsack, 2001); ammonite subzones after Wiedmann and Owen (2001).
and marine, biogenic carbonate are the two main components that control the sediment composition in this deep shelf basin (Fenner, 2001b; Jendrzejewski et al., 2001; KuÈhn et al., 2001; Prokoph and Thurow, 2001; Rachold and Brumsack, 2001; Wonik, 2001).
A depositional environment relatively distant from land is suggested by the ®ne grain size of the sediment, the small size of the inertinite and vitrinite grains (mostly ,50 mm), the absence of shallow water species, and the low t/m index (t terrigenous, m marine) of
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the palynomorphs (Fenner et al., 1996; Fenner, 2001b; Jendrzejewski et al., 2001; Prauss, 2001). Most of the clay minerals are detrital in origin. The different crystallinity types of illite and kaolinite indicate that the clay minerals were derived from rocks of different age and from rocks with a different diagenetic history (KuÈhn et al., 2001). 3.2.2. Origin of the montmorillonite and volcanic activity during the Albian Layers containing volcanic glass were not encountered. But the presence of small amounts of alteration products of volcanic ash is indicated by the elevated content of montmorillonite co-occurring with elevated abundance of the elements Zr and Nb at two depths (at 87.20 m in Kirchrode I and at 221.77 m in Kirchrode II) within the Albian sequence (Fig. 4) (KuÈhn et al., 2001; Rachold and Brumsack, 2001; Benesch, 1997). For the presence of ash in these two horizons Benesch (1997) and KuÈhn et al. (2001) assume eolian transport and in situ weathering. Montmorillonite (a variety with predominantly Ca ions in the interlayer spaces is the most common) is concentrated in the ,2 mm grain size fraction, while kaolinite and illite are more abundant in the 2±20 mm grain size fraction. KuÈhn (1994) and KuÈhn et al. (2001) consider weathered products from explosive volcanism associated with the opening of the North Atlantic to be a possible additional source for the montmorillonite, but most of the montmorillonite they interpret to be detrital. Accordingly, the increase in montmorillonite content in the Upper Albian from 20±35% (including mixed-layer illite/smectite) in the lower part to 45±55% in the upper part, which is correlated with a decrease of kaolinite and to a lesser degree with a decrease of illite, is concluded to re¯ect a compositional change in terrigenous input. This interpretation differs from that of earlier studies by Brockamp (1976), Zimmerle (1979), Kemper and Zimmerle (1982) and Keller et al. (1989), who considered weathering of volcanic particles that were produced by volcanism connected to the opening of the North Atlantic to have been the main source. According to KuÈhn (1994) and KuÈhn et al. (2001) a contribution to the montmorillonite content from pedological processes or lateritic weathering was of subordinate importance because mixed-layer illite/
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smectite minerals do not increase in abundance together with montmorillonite, and because these authors ®nd little similarity between the Upper Albian clay mineral assemblage at Kirchrode and that of the weathering horizon on top of the kaolinised `Hilssandstein' described by Valeton (1958). Also the mineral beidellite, which is indicative of vertisols formed in areas with poor drainage under alternating wet and dry conditions, is not found among the Upper Albian clay minerals. Rachold and Brumsack (2001) also come to the conclusion that weathering of volcanic ashes during the Albian was of minor importance because they do not ®nd enrichment of Zr and Nb in the ,0.63 mm fraction, which they consider a characteristic of all ashes. They argue that because volcanic sources are of subordinate importance, terrestrial weathering under subarid climate conditions must have been a major source of the montmorillonite. 3.2.3. Diagenesis Dissolution of CaCO3 can be observed in the dark claystones of the Lower Albian (Fenner, 2001b), but in the Upper Albian Cepek (2001) does not recognize calcium carbonate dissolution. The good preservation of CaCO3 throughout the Upper Albian of Kirchrode I is indicated ² by the high abundance of the calcareous nannofossil species Biscutum constans, which is relatively easily dissolved (Fig. 5), and ² by the low number of unidenti®able calcareous nannofossils (Cepek, 2001). Further study is needed to determine whether the decrease in the abundance of marine palynomorphs relative to terrigenous ones in the basal part and the top of the Kirchrode I core is related in part to poorer preservation of organic matter, a possibility proposed by Fenner (2001b), as a consequence of decreased sedimentation rates and more intense oxygenation and bacterial decay of the organic matter in these core intervals and bacterial decay. Elevated temperatures de®nitely did not affect the Albian sediments at the Kirchrode site nor thermal cracking of the organic matter. According to Jendrzejewski et al. (2001) low thermal maturity is indicated by ² most Tmax values from Rock±Eval pyrolysis
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Fig. 5. Selected indicators of calcium carbonate dissolution and diagenesis in the sediments of the Kirchrode I borehole: relative abundance ¯uctuations of Biscutum constans within the calcareous nannofossil assemblage, a species considered highly susceptible to dissolution (Cepek, 2001); number of concretions and of thin, siderite-stained layers (Prokoph and Thurow, 2001); Fe/Mn ratio in the sediment (data from Rachold and Brumsack, 2001) and the same ratio in concretions (KuÈhn et al., 2001).
measured on concentrates of organic matter are between 387 and 4208C, and ² the yield of aromatic hydrocarbons in the organic matter extracts is low (ca. 10%), while heterocompounds containing nitrogen, sulphur or oxygen predominate (up to 52%). The most obvious sign that diagenesis has affected the Albian sediments is the numerous presence of sideritic concretions and thin sideritestained layers (KuÈhn et al., 2001; Prokoph and Thurow, 2001; Rachold and Brumsack, 2001). The mineralogical composition of these concretions according to KuÈhn et al. (2001) is a mixture of calcite, siderite, and rhodochrosite, with the Mnrich varieties dominating below 150 m in the Upper Albian of Kirchrode I. More rare are phosphatic concretions of francolite (Ca-¯uorapatite). Isotope analyses of selected concretions show that
they have lower d 13C values than the surrounding sediment matrix (KuÈhn et al., 2001). The authigenic mineral clinoptilolite as well as opal-CT are significant components of the top 7 m of the sediment core, where also still some undissolved opal-A (biogenic silica) is preserved (KuÈhn, 1994). A minor diagenetic component that occurs in low abundance but repeatedly throughout the Upper Albian is pyrite (KuÈhn et al., 2001). In contrast, Prokoph and Thurow (2001) state that it is often common or abundant. Clearly more rare than pyrite are occurrences of glauconite. This mineral is concentrated in thin layers within certain depth intervals and largely con®ned to the fraction ,63 mm (KuÈhn et al., 2001). The only interval with a high abundance of glauconite is around 250 m in Kirchrode II between the dark claystones of the lower part of the Lower Albian and the marlstones of the upper part of the Lower Albian (Fenner, 2001b).
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Fig. 6. Sedimentological and geochemical changes in the Upper Albian of Kirchrode I. Sedimentation rates (Fenner, 2001b); Nb/Al2O3, Zr/Al2O3, TiO2/Al2O3, SiO2/Al2O3, MnO/ Al2O3 ratios and % Al2O3 from Rachold and Brumsack (2001); gamma ray intensity from Wonik (2001); montmorillonite abundance relative to the total clay minerals in the ,20 mm CaCO3-free fraction of the sediment; quartz abundance in the ,20 mm CaCO3-free fraction of the sediment (KuÈhn et al., 2001). Ammonite subzones from Wiedmann and Owen (2001). Levels at which several of the parameters change are indicated by dashed lines.
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3.2.4. Changes in clastic terrigenous input The mineralogical results of KuÈhn et al. (2001), the geochemical results of Rachold and Brumsack (2001), and changes in the sedimentation rates (Fenner, 2001b) for the Upper Albian show three levels at which changes in the composition of the sediments and in the amplitude of the abundance ¯uctuations occur (Fig. 6). Proceeding from older to younger these shifts in Kirchrode I lie ² between 180 and 150 m, i.e. towards the end of the lower Callihoplites auritus Ammonite Subzone, ² at 40 m (near the top of the Mortoniceras rostratum Ammonite Subzone), and ² at 7 m (within the Arraphoceras briacensis Ammonite Subzone). At each of these levels a shift to a smaller proportion of clay minerals occurs. At the lowest of these levels the decrease in clay minerals is associated with a shift to slightly more quartz within the ,20 mm grain-size fraction. It coincides with a shift to slightly higher Si/Al, Ti/Al, Zr/Al, and Sr/Al ratios, and slightly lower Nb/Al, Fe/Al, Co/ Al, and Ni/Al ratios within the bulk sediment, and a shift in the amplitude of the abundance ¯uctuations of some of the other parameters. This ®rst step in the increase in ®ne silt-sized detrital minerals correlates with an increase in sedimentation rates from lower and strongly ¯uctuating values between 3.5 and 12.5 cm/ka to more stable and higher sedimentation rates between 7 and 13 cm/ka above this level (Fenner, 2001b). And among the clay minerals the composition shifts to more montmorillonite. At the next higher level where the clay content drops (at 40 m in Kirchrode I), quartz and Si/Al, Zr/Al again increase, but sedimentation rates slightly decrease. Here, quartz reaches values of .15%. The most drastic increase in quartz (to .20%) and in Si/Al, Ti/Al, and Zr/Al occurs at the third step (at 7 m in Kirchrode I). This stepwise increase in the Si/Al, Ti/Al, and Zr/ Al ratios, which is the strongest in the ®ne silt fraction (2±20 mm), is interpreted as being related to an increase in detrital silt-sized minerals, mainly detrital quartz and heavy minerals (e.g. zircon, anatase and sphene). Different explanations concerning the cause of the increase in montmorillonite and silt-sized terrigenous detritus through the Upper Albian have been proposed
by the different authors in this volume. Rachold and Brumsack (2001) explain the increase in montmorillonite in the lower C. auritus Subzone with a change in the weathering regime caused by a change from a humid to a subarid climate. The later change (above 40 m) to more detrital quartz is interpreted as a further step in this direction, re¯ecting increased input from eolian detritus. KuÈhn et al. (2001) recommend caution in the use of clay minerals as an argument for climatic change. Because of the high abundance of illite (.35%) and the type of crystallinity of the clay minerals, they interpret most of the clay minerals, including the montmorillonite, as recycled from ashrich older Mesozoic and Late Paleozoic sediments. And accordingly they see mainly a change in the eroded source rock being re¯ected in the compositional changes of the sediment. Fenner (2001b) adds that these changes in terrigenous input and in sedimentation rates during the Late Albian are related to a combination of causes: ² local subsidence due to salt migration into the salt structures ¯anking the subbasin in which Kirchrode is located, ² regional subsidence of the region north of the rising Rhenish±Bohemian massif, ² a change from a more arid to a more humid climate in the lower part of the C. auritus subzone (a contrasting interpretation to that of Rachold and Brumsack, 2001), and in the latest Albian possibly a change to more arid conditions, leading to windinduced upwelling along submarine relief in the shelf sea, ² 3rd-order sea level changes, and ² the gradual moving of the main depot center closer to the Kirchrode site. 3.3. Biogenic components Ð ¯uvial input Ð productivity Like the mineral components, the changes in the abundance of several biogenic components in the Upper Albian also occur stepwise and at the same depth levels as the changes in terrigenous input (Fig. 7). Two groups of biogenic components can be distinguished, depending on the cause of the abundance changes. The ®rst group comprises allochthonous as well as redeposited older biogenic components. To this group belongs the TOC content of the sediment, which was
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Fig. 7. Changes in biogenic parameters in the Upper Albian of Kirchrode I. Organic carbon content of the sediment (Jendrzejewski et al., 2001); number of calcareous dino¯agellate cyst species and relative abundance of morphotypes among the Obliquipithonelloideae with coarsely crystalline cysts walls (Keupp, 2001); relative abundance of peridinoid cysts and GV cysts (more or less dorsi-ventrally compressed forms with no or only small antapical horns or protrusions, which are used by Prauss (2001), as an indication of in¯ux from coastal environments) relative to the total organic-walled dino¯agellate cysts; presence of freshwater algae and reworked palynomorphs, and the terrigenous/marine ratio within the palynomorphs (Prauss, 2001); relative abundance of calcareous nannofossils (Cepek, 2001); number of benthic foraminifera .125 mm in the sediment and the relative abundance of specimens of Arenobulimina ssp. within the benthic foraminifera assemblage of the .125 mm fraction (Thies, 2001); number of radiolaria .125 mm and interpreted marine plankton productivity (Fenner, 2001b); P2O5/Al2O3 from Rachold and Brumsack (2001); ammonite subzones (Wiedmann and Owen, 2001); climate interpretation (after Fenner, 2001b; Prauss, 2001). The dashed lines are shown at the same depths as in Fig. 6.
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found to be dominated by terrigenous matter. It includes, especially in the lower part of the Upper Albian, some primary terrigenous matter from higher plants (long-chain n-alkane maximum between n-C25 and n-C29 together with the odd carbon number predominating), but most is reworked older material (Jendrzejewski et al., 2001). It, therefore, is plausible that the TOC ¯uctuations are governed by the same processes as those of the silt-sized terrigenous mineral detritus. The reworked, older material includes rare vitrinite that Jendrzejewski et al. (2001) consider to be reworked Carboniferous 1. Analysis of palynomorphs also encountered reworked, older specimens (mostly acritarchs and dinocycsts). They show a drop in abundance at 150 m in Kirchrode I. Below, they occur rarely, but repeatedly, indicating erosion of older rocks. Above this level, their occurrence is very scarce; instead, rare but repeated occurrence of freshwater algae (Botryococcus and Palambages) is characteristic, which points to a change to a more humid climate and the presence of freshwater bodies on a land mass to the south (Prauss, 2001). Such a climatic change is also indicated by the trends in the different allochthonous palynomorph groups through the Upper Albian: a decrease in abundance of bisaccate pollen and an increase in trilete spores. These opposing trends suggest an increase in the proportion of input via ¯uvial transport and a decrease in input via wind transport (Prauss, 2001). The high Mn concentration in the sediment, which slightly increases in the Upper Albian at Kirchrode (Rachold and Brumsack, 2001), may also support this interpretation. Prauss (2001) goes so far to suggest as a consequence of the increased ¯uvial input the possibility of a change to an estuarine circulation in this shelf basin. Also Prokoph and Thurow (2001), from their results of the changes in the trace fossil assemblages, consider strati®cation of the water column a possibility for the time interval during which sediments belonging to the lower part of the C. auritus Subzone were deposited. The second group of biogenic components that change at the same levels comprises authochthonous biogenic components, which re¯ect increasing marine 1
Erosion of Carboniferous sediments is not supported by the results of Prauss (2001), who did not ®nd any of the characteristic, robust spores of the Carboniferous.
productivity (Fig. 7). This increased productivity is considered to be a result of the change to a more humid climate and increased runoff from land, which led to an increased input of dissolved and particulate nutrients into the shelf sea (Fenner, 2001b). At the lowest level of change (180±150 m in Kirchrode I), the increase in plankton productivity is gradual (e.g. increased abundance of calcareous nannofossils). In the top 40 m in Kirchrode I, radiolaria (CaCO3 or pyrite casts of the shells) are common besides the still abundant calcareous nannofossils, indicating an increase in marine plankton productivity. Productivity probably begins to increase again at 7 m, above which siliceous skeletons of radiolaria and diatoms have increasing abundances and better preservation (Fenner, 2001b), and where KuÈhn et al. (2001) report a further increase in silt-sized terrigenous detritus and Rachold and Brumsack (2001) an increase in Si, Zr, Nb, Ba and P. Benthos productivity may also, in general, have increased through the Upper Albian (Prokoph and Thurow, 2001). Owing to variations in oxygen levels at the sea ¯oor that affect the benthos, the interpretation of benthos productivity resulting from the analysis of bioturbation by Prokoph and Thurow (2001), however, does not correlate with that described above for plankton productivity. The Upper Albian trace fossils at the Kirchrode site change from a diverse assemblage with predominantly Thalassinoides, Planolites, Teichichnus, and Chondrites forms (below 132 m in Kirchrode I) to an assemblage that has a lower diversity, is dominated by Chondrites and Teichichnus, has more frequent Sinusites than below, and shows a lower bioturbation intensity and shallower burrowing depth. This low diversity assemblage of the upper part of the Upper Albian is interpreted by Prokoph and Thurow (2001) to either re¯ect lower oxygen concentrations in the deep water and surface sediment or decreasing productivity. Higher in the sequence, above 90 m in Kirchrode I, the diversity of the ichnofauna and the intensity of bioturbation is found to increase again, re¯ecting an increase in productivity and better oxygenation at the sea ¯oor. This trend is seen by these authors to continue into the top 30±40 m of Kirchrode I, where the diversity as well as the density and depth of bioturbation are further increased (Fig. 9). The indications of a stepwise increase in marine
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Fig. 8. Occurrence of ammonite species (Wiedmann and Owen, 2001), plankton foraminifera (Weiss, 1997), and calcareous dino¯agellate cysts that are interpreted to have immigrated from the Tethys (Keupp, 2001); abundance of suspension-feeding bivalves (Fenner, 2001b), and planktonic foraminifera (Thies, 2001), calcareous nannofossils (Cepek, 2001), and the dino¯agellate cyst genus Palaeoperidinium (Prauss, 2001), as well as the interpreted sea levels of Fenner (2001).
productivity during the Late Albian, which is also documented in an increase in sedimentation rates of biogenic carbonate parallel to the increase in sedimentation rates of terrigenous detritus (Fig. 6), suggest that marine productivity was not very low as was argued from the low abundance of TOC by Jendrzejewski et al. (2001). Instead the very low abundance of organic matter derived from marine plankton must be due to oxidation before its burial in the sediment. For the productivity increase in the uppermost Albian wind-induced upwelling (Frieg and Kemper, 1989) of more nutrient-rich deep water along submarine topographic highs (Fenner et al., 1996) or coastal upwelling (Rachold and Brumsack, 2001) has been thought to be a possible explanation. The above-described palynological results (Prauss, 2001) though suggest that increased ¯uvial input also could have been a source for the elevated nutrient levels. At present, it thus remains unsolved whether climate changed to more arid conditions during the latest
Albian in the mid-latitudes around the Lower Saxony basins, as was described for the Western Interior basin by Glancy et al. (1993) and Pratt et al. (1993). 3.4. Oxygenation levels in the water surface sediment Suboxic conditions at the sea ¯oor, at times probably approaching anoxic conditions, are suggested for the time of deposition of the Lower Albian dark claystones, which are characterized by reduced benthic life, a predominance of arenaceous foraminifera, and which exhibit faint bioturbation. Marls were deposited for the remainder of the Albian. These marls are characterized by a rich benthic fauna as well as by remains of nektonic and of planktonic organisms, indicating well oxygenated conditions in the water column and at the sea¯oor (Fenner, 2001b). Oxic conditions at the sea¯oor and in the water column are also indicated by the mineralogical and chemical analyses for the Upper Albian of
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Fig. 9. The four different sea level interpretations for the Upper Albian of Kirchrode I, together with at least one parameter used in the argumentation by the respective author(s). Summary plot of the relative abundance of plankton foraminifera considered to have immigrated from the Tethyan after Weiss (1997), summary plot of the abundance of plankton foraminifera and radiolaria in the .125 mm fraction (Fenner, 2001b; Thies, 2001), plankton/benthos foraminifera ratio (Thies, 2001), CaCO3 content of the sediment (Jendrzejewski et al., 2001), bioturbation intensity and occurrence of glauconite (Prokoph and Thurow, 2001), t/m index of palynomorphs (Prauss, 2001). mf maximum ¯ooding surface. UZA Supercycle set `Upper Zuni A' of Haq et al. (1988).
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301
minimum zone to the south in the direction of the coast is postulated by Rachold and Brumsack (2001) for this same time interval. Such an oxygen minimum zone is considered necessary as an additional Mn source, because their mass balance calculations indicate that the high Mn values cannot be explained by ¯uvial input alone. 3.5. Water depth
Fig. 10. Spectral peaks of CaCO3 in the depth inerval 40±100 m in Kirchrode I (Rachold and Brumsack, 2001).
Kirchrode I: ² the relatively low abundance of pyrite (KuÈhn et al., 2001), together with ² high amounts of reactive iron, as well as by the lack of enrichment of the redox-sensitive trace elements V and Cr (Rachold and Brumsack, 2001), and ² the low organic carbon content of the sediment (0.18±0.6%) in spite of high sedimentation rates (Jendrzejewski et al., 2001). Of the easier oxidizable autochthonous, marine organic matter only a small part is left in the Upper Albian sediments. Most that remained is resedimented organic matter. This is supported by microscopical analysis and detailed chemical analysis of the organic matter of 12 selected samples. The carbon/sulphur ratio (low sulphur contents of the sediments) and the hydrogen-poor kerogen indicated by Rock±Eval pyrolysis (HI values below 100 mg HC/g TOC for all samples, refractory type III/IV kerogen), the high pristane/phytane ratio (a mean of 1.25), and the presence of a peak at n-C21 or n-C23 in the n-alkane distribution, lead to the conclusion that bacterial degradation under aerobic conditions during an early stage of sedimentation rather than sulphate reduction was responsible for the low percentage of marine organic matter (Jendrzejewski et al., 2001). While all agree that oxic conditions characterized the environment at the sea¯oor at the Kirchrode site during the Late Albian, the existence of an oxygen
The benthic foraminifera in the Upper Albian suggest a water depth of at least 100 m (Fenner et al., 1996). The deepening of the shelf sea with global sea level rise and regional and local subsidence is re¯ected in the fact that deep-dwelling Tethyan planktonic foraminifera manage to intrude into the Lower Saxony basin during the late Late Albian. Earlier Ticinella spp. intruded (Weiss, 1997), species that are considered to have lived in near-surface water (e.g. Sliter, 1972; Hart, 1980). Another parameter that suggests increasing water depth is the abundance of benthic foraminifera. It decreases at the lowest level of change (between 180±150 m in Kirchrode I) from an average of 50/g of sediment to only 35/g of sediment above this level, while the plankton/benthos ratio of foraminifera increases (Thies, 2001). Besides an increase in water depth, it may re¯ect a change from more turbulent conditions in the shallower shelf to a deep water situation in which a relatively stable pycnocline developed and nutrients became increasingly recycled within the upper water column. During the more turbulent conditions, food for the benthos was supplied from above as well as by lateral advection, supporting a relatively large population of suspension-feeders (Fenner 2001b). Late in the Albian, the reduced amount of food on the sediment surface and in suspension just above it led to a decrease in benthic foraminifera and an increase in the relative abundance of detritus-feeders, e.g. Arenobulimina spp. (e.g. Jones and Charnock, 1985; Frieg and Kemper, 1989). 3.6. Sea level changes and palaeocirculation The effects of global sea level changes on the deep parts of this shelf basin during the Late Albian are recognized in the systematic, repeated changes in relative abundance of the main plankton and benthos groups, which re¯ect changes in the food availability
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and intensity of oceanic circulation. These effects were found to be largely independent of the changes in sedimentation rate (tectonics and climate) and were found to be overlain on the general long-term trend of a deepening shelf sea environment (Fenner, 2001b). Time intervals with intensi®ed circulation are identi®ed on the basis of abundance maxima of benthos suspension-feeding species and of immigrant plankton species from the Tethyan region (Fig. 8). Among the ammonites, species considered as Tethyan immigrants by Wiedmann and Owen (2001) also occur largely in the same intervals where tethyan immigrants among the planktonic foraminifera occur. The intensi®cation of circulation is shown by Fenner (2001b) to have ®rst affected the upper part of the water column, causing an increase in the abundance of immigrated plankton species. Later, the water directly above the sea ¯oor was also affected. The increased lateral advection of food under the later conditions supported more benthic suspensionfeeders (bivalves and foraminifera). The spreading of the benthic foraminifera genus Spiroplectinata from the Tethys to the `Boreal' seas during the later part of the Early Albian and early part of the Middle Albian (Tyszka and Thies, 2001) can be expected to have been aided by the repeated occurrence of such periods of intensi®ed circulation. These times of intensi®ed circulation occurred during times of rising sea level, if the scheme of third order global sea level changes of Fenner (2001b) is correct (Fig. 8). The maximum ¯ooding according to this model occurred during the same interval as the maxima in TOC determined by Jendrzejewski et al. (2001), and the maxima in zooplankton remains. It is associated with or followed by a maximum in phytoplankton remains. The changes found in the trace fossil assemblages by Prokoph and Thurow (2001) also ®t this model and the observed changes in sedimentation rates (but note that the interpretation of the trace fossil assemblages by Prokoph and Thurow, 2001, is different). The general trend of the interpretation of the global sea level ¯uctuations during the Late Albian by Tyszka and Thies (2001) is similar to that of Fenner (2001b). A very different interpretation of the sedimentary and biogenic changes in terms of global sea level change (Fig. 9). is presented by Prokoph, 1994 and Prokoph and Thurow (2001), whose sea level curve largely follows the changes in sedimenta-
tion rates calculated by Fenner (2001b). Again another sea level interpretation is proposed by Prauss (2001) based largely on changes in the t/m ratio of palynomorphs and abundance changes in selected dino¯agellate groups. This divergence in interpretations illustrates the dif®culty to recognize global sea level changes in the relatively monotonous sedimentary record of deep shelf basins, which do not contain such strong lithological changes so characteristic of shallow water or near-shore marine environments. 3.7. Milankovitch cyclicity In addition to the above-described long-term climatic changes through the Late Albian, regular changes in the calcium carbonate contents of the sediment and in abundance ¯uctuations of geochemical, and biogenic components are easily visible in Kirchrode I and are suggestive of Milankovitch cyclicity. The cyclicity is especially prominent in that part of the Upper Albian, which is characterized by high sedimentation rates (Figs. 6 and 7). That these cycles indeed re¯ect Milankovitch-driven cyclicity is established by spectral analysis of these data in the depth domain (Prokoph and Thurow, 2001; Rachold and Brumsack, 2001; Weber et al., 2001; Wonik, 2001), (Fig. 10) which produces frequency peaks with a characteristic peak ratio of roughly 1:2:5 (Fig. 10), which is also characteristic of the Milankovitch periodicities during the Late Neogene. That the cyclicity documented in the ¯uctuations of the carbonate content of the sediment indeed re¯ect climate changes is shown by the fact that long-chain n-alkanes with a maximum between n-C25 and n-C29 are enriched in the darker part of the cycles. This re¯ects an increased input of terrestrial organic matter during these intervals (Jendrzejewski et al., 2001). Changes in the benthic foraminifera assemblages document for at least part of the Upper Albian that in the marine shelf environment these changes in terrestrial input resulted in changes in the productivity (Fenner et al., 1996; Fenner, 2001b). In the intervals with a high sedimentation rate (above 216 m in Kirchrode I), where sample spacing is close enough to recognise precessional cycles, the ¯uctuations in the abundance of immigrant Tethyan plankton species differ from those of the `Boreal' plankton species. The planktonic foraminifer
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Globigerinella bentonensis, which is considered to have immigrated from Tethyan regions, shows a strong precessional signal in its abundance ¯uctuations. The `Boreal' Hedbergella planispira and H. delrioensis instead have a stronger obliquity signal. If the dominant Milankovitch cycles identi®ed in the abundance of a species or species group indeed indicate changes triggered either in the low latitudes or in the mid or high latitudes, the relatively strong precessional signal in the benthic foraminifera in this high sedimentation rate part of the Upper Albian re¯ects in¯uences from the low latitudes on the living conditions in the basin deeps. The identi®cation of the Milankovitch cyclicity was used to determine sedimentation rates. Using a duration of ca. 95,000 a for the identi®ed shorter periodicity eccentricity cycles, sedimentation rates are found to increase from 1.5 to 6 cm/ka in the lower part of the Upper Albian to values between 4.5 and 12.5 cm/ka in the basal part of the C. auritus Subzone (above 216 m in Kirchrode I), and then to values of 7±13 cm/ka in about the middle of the same zone (above 155 m in Kirchrode I) (Fenner, 2001b). The identi®ed eccentricity cycles also are used by Fenner (2001b) to determine the duration of the Callihoplites auritus Ammonite Subzone of the Upper Albian to be ca. 2.1 Ma, and the Upper Albian (base de®ned by the ®rst occurrence of Actinoceramus sulcatus) to have had a minimum duration of 4 Ma (possibly 4.5±4.8 Ma). Acknowledgements This work was supported by DFG grants Fe 240-2 and Fe 240-4 and by the BGR. I thank A.W. Burger for help with producing the ®gures and R.C. Newcomb for checking up on the English. References Baldschuhn, R., Frisch, U., Kockel, F., 1996. Geotektonischer Atlas von NW-Deutschland 1:300 000. 17 parts. Bundesanstalt fuÈr Geowissenschaften und Rohstoffe, Hannover (in German). Benesch, M., 1997. Mineralogische Untersuchungen von Unterkreide Ð Sedimenten aus dem NiedersaÈchsischen Becken. PhD Thesis, University of GoÈttingen, 88 pages, 8 tables, 45 ®gures. GoÈttingen.
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