Ostracod diversity and sea-level changes in the Late Cretaceous of southern England

Palaeogeography, Palaeoclimatology, Palaeoecology 225 (2005) 266 – 282 www.elsevier.com/locate/palaeo

Ostracod diversity and sea-level changes in the Late Cretaceous of southern England Ian J. Slipper * Medway School of Science, University of Greenwich, Chatham Maritime, Kent ME4 4TB, UK

Abstract The available data of ostracod ranges for the Cenomanian, Campanian and Maastrichtian stages of the Late Cretaceous of the northern part of the Anglo-Paris Basin were examined and combined with new data from the Turonian, Santonian and Coniacian stages. A new cumulative species diversity curve is presented for the Ostracoda of the Late Cretaceous of Britain. The results obtained challenge the method of chronoecologic charts to determine sea-level from diversity. When a more complete data set is applied, and compared with published sea-level curves, the result is the inverse of that previously predicted by employing chronoecologic charts. A model is presented of changing sea-levels in S.E. England from the Cenomanian through to the Santonian, which integrates the new diversity data with published sea-level changes and curves of stable isotopes of oxygen and carbon. In the earliest Cenomanian, low diversity is associated with a deeper water depositional environment and warmer temperatures. The mid-Cenomanian diversity maximum corresponds to a regressive trough and cooler water. Over the Cenomanian–Turonian boundary interval the diversity minimum is correlated with global sea-level and temperature maxima. The proportion of ostracods possessing eye tubercles falls to a minimum over this period. After the diversity crash, the Cenomanian fauna was replaced by the new Turonian fauna; east–west migrations into the Anglo-Paris Basin were facilitated by the sea-level rise overcoming marginal basin highs. The pattern seen in the mid-Cenomanian is also present at the Turonian–Coniacian boundary interval; that of high diversity corresponding with a regressive trough on a long-term regressive trend with cooling conditions. The model for this northern part of the Anglo-Paris Basin then associates high diversity with regressive cooler conditions, and low diversity with deeper and warmer water. D 2005 Elsevier B.V. All rights reserved. Keywords: Ostracods; Late Cretaceous; Diversity; Sea-level changes; Southeast England

1. Introduction There have been many studies of the Late Cretaceous Ostracoda of the British Isles in the last 150 * Fax: +44 208 331 9805. E-mail address: [email protected]. 0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2005.06.014

years, most of which have been taxonomical (Jones, 1849, 1870; Jones and Hinde, 1890; Kaye, 1964; Keen and Siddiqi, 1971; King, 1968; Weaver, 1982; Wilkinson, 1988; Slipper, 1997), others have addressed the stratigraphical distribution (Neale, 1978; Horne et al., 1990; Slipper, 1996; Wilkinson, 1988), while some have used the stratigraphical and taxonomic distribu-

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tions to analyse palaeoenvironments (Horne and Rosenfeld in Jarvis et al., 1988; Slipper, 1998). With the exception of the very early work, the monographic works have concentrated on a limited stratigraphical span, usually bounded by the chronostratigraphic stage boundaries, while the stratigraphical works have concentrated on one interesting aspect within the Late Cretaceous. Neale (1978) presented a synthesis of stratigraphically significant species for the Cretaceous period, but to date there has been no attempt to examine the total Late Cretaceous fauna in terms of the palaeoenvironmental signals which may be present in the distribution of the taxa through time. Ostracoda are well known for their ability to reflect palaeoenvironments in broad terms of fresh, brackish and marine waters, and are particularly sensitive indicators where fluctuating salinity is a significant control. In the fully marine and stable environment that existed for 30 million years and resulted in the deposition of the chalk facies, it is more difficult to determine any changes within the palaeoenvironment from a simple species based investigation. In order to examine the controls on distribution (temperature, salin-

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ity, depth, light, oxygen concentration, food supply, substrate) it is necessary to look at the fauna as a whole, over longer periods of time. With the publication of eustatic sea-level curves (Haq et al., 1988) it is possible to assess the response of the Ostracoda to long-term changes in palaeoenvironment. In more stressful conditions, such as fluctuating salinity or energy regimes, or reduced supply of nutrients or food, organisms decline in number and become less diverse, often leaving very high numbers of fewer species to survive. Conversely under more bountiful conditions, such as stable salinity or energy and abundant nutrients, organisms will thrive with the consequent probability that diversity will increase. By combining the diversity of Ostracoda through the Late Cretaceous with various assessments of depth, light, food supply, oxygen concentration and salinity, it is hoped that a consistent picture will emerge. This then is an attempt to combine the stratigraphic ranges all the currently known Late Cretaceous Ostracoda for southern England together in one dataset and assess the response to changing sea-levels through time.

Fig. 1. Distribution of Upper Cretaceous strata in southern England. Named locations are the sampled sites of Wilkinson (1988), Weaver (1982), Jarvis et al. (1988), Horne et al. (1990), Slipper (1996, 1997, 1998), King (1968) and other sites used by author for this work.

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2. The dataset During the preparation of the Late Cretaceous chapter of the 2nd edition of the Stratigraphical Atlas of British Ostracoda, the author made a compilation of the ranges of all currently known Late Cretaceous ostracods, which forms the basis of the dataset used in the present analysis. The data sources are given below and discussed by stage; all locations are shown on Fig. 1. NGR refers to the National Grid Reference of the locality. The macrofaunal zones within each stage are shown in Fig. 2. Data for the Albian–Cenomanian boundary were obtained from Wilkinson (1988) who described four localities in Cambridgeshire and West Suffolk, where 26 species of Cenomanian age were indicated. The stratigraphical logs (Wilkinson, 1988, text-Fig. 3) clearly show the sampling horizons while the summary stratigraphical diagram (Wilkinson, 1988, text-Fig. 2) relates first appearances and extinctions to the M. mantelli Ammonite Zone and subzones and the benthonic foraminiferal zones of Carter and Hart (1977). Localities include: Grantchester cutting (NGR TL 420562), Grantchester footbridge (NGR TL 425550), Hauxton interchange (NGR TL 436534), Mildenhall borehole (NGR TL 692730). The information for the Cenomanian stage was derived from Weaver (1982) and Wilkinson (1988). Weaver (1982) described and illustrated 117 species and subspecies from 13 localities in southern England (one locality, the Carr’s Glen Shell Bed, the fauna from which was described by Keen and Siddiqi (1971) was omitted from this analysis since it falls outside of the depositional basin to which all other localities belong): Barrington (NGR TL 398509), Pitstone (NGR SP 935148), Bluebell Hill (NGR TQ 737618), Glyndebourne (NGR TQ 442114), Southerham (NGR TQ 431091), Culver Cliff (NGR SZ 630855), Shillingstone (NGR ST 824098), Buckland Newton (NGR ST 703051), Cambridge (NGR TL 4556), Charing (NGR TQ 9549), Didcot (NGR SU 5291), Dover (NGR TR 3342). The stratigraphical logs of Weaver (1982) clearly show the sampling horizons and the first appearances and extinctions of all species related to the benthonic foraminiferal zones of Carter and Hart (1977). The Cenomanian–Turonian boundary was examined by Horne and Rosenfeld in Jarvis et al. (1988)

Fig. 2. Stratigraphy of the Late Cretaceous.

from sections at Dover. That work included the highest Cenomanian and the lowest 20 m of the Turonian. A total of 32 species were presented for this interval, no macrofaunal zone information is given but the range chart presented is clearly tied into the lithostratigraphy. Greater stratigraphical coverage of the Turonian stage was given by Horne et al. (1990) who gave a check list and stratigraphical ranges for 28 species. Although the range chart of Horne et al. (1990, Fig. 2) gives no macrofaunal zone information,

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Fig. 3. Chronoecologic chart of all Late Cretaceous Ostracoda for southern England set against the eustatic sea-level changes of Haq et al. (1988). Data from Wilkinson (1988), Weaver (1982), Jarvis et al. (1988), Horne et al. (1990), Slipper (1996, 1997, 1998), King (1968) and current work. Stratigraphical axis after Haq et al. (1988). Cen=Cenomanian, Tur=Turonian, San=Santonian, Maa=Maastrichtian.

the accurate lithostratigraphy allows the data therein to be transferred onto the standard ammonite zonal scheme. Slipper (1996) revisited the lowest Turonian and gave ranges for 34 species from Abbotts Cliff. The ranges presented by Slipper (1996, Fig. 1) are related only to the assemblage zone of Mytiloides spp. Slipper (1997) gives range charts and a check list of 103 species and subspecies for the complete Turonian stage from three localities at Dover: Abbotts Cliff (NGR TL 268385), Akers Steps (NGR TR 297394) and Langdon Stairs (NGR TR 345425). All ranges therein are related to marker horizons, ammonite zones and macrofaunal assemblage zones. Slipper (1998) presented data for the Turonian– Coniacian boundary from Langdon Stairs at Dover. The ranges for 81 species were given, of which 46 had stratigraphical significance for the boundary interval. Samples used in this work included material from the Coniacian which was prepared by Amnon Rosenfeld (Geological Survey, Israel). The full Coniacian dataset

is at present unpublished, but has been included in Fig. 3. The Santonian stage has not yet been systematically sampled, but three localities supply information from each of the zones within the stage. The author has systematically sampled through the lower socialis Zone at Pinden Quarry (NGR TQ 593695). The author has been supplied with samples from the lower socialis Zone from near Seaford Head (NGR TV 491981) by Dr. D. S. Wray (University of Greenwich) and also from the upper part of the socialis Zone and the lower part of the testudinarius Zone of the Santonian by Dr. A. Bruce from the coast on the Isle of Thanet (NGR TR 384716). Since the completion of this study, there has been recent work (Pyne, 2002; Pyne et al., 2003) on the Coniacian to Maastrichtian Ostracoda of East Anglia; the sixteen new species presented in the latter work have not been integrated into the present dataset. So the data for the Campanian and Maastrictian stages

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have been compiled from the unpublished thesis of King (1968); this was the main source for the range charts of the Upper Cretaceous given by Neale (1978). There are some problems still with the stratigraphical control since King (1968) does not present ostracod range data against the stratigraphy, the charts are simple matrices of the number of specimens of each species against sample number with little attempt to link ostracod occurrence to stratigraphical distribution. However, by a careful analysis of the relative sizes, shapes and distances between the flint bands given in the measured logs of King (1968), it has been possible to make a successful correlation with the lithostratigraphical schemes of Peake and Hancock (1970) and Wood (1988) for ten of King’s localities, which encompass the Upper Campanian and the Lower Maastrichtian. These samples, and therefore the species ranges can then be tied into the belemnite zonal scheme of Christensen (1995, Fig 3), and can then be successfully compared with standard eustatic sea-level curves of Haq et al. (1988). Localities: Wells-next-the Sea (NGR TG 928429), Stiffkey (NGR TF 975428), Drayton (NGR TG 175132), Keswick Lime works (NGR TG 212048), Weybourne Hope (NGR TG 110438), Catton Grove Quarry (NGR TG 229104), Caistor St. Edmunds, (NGR TG 238046), Overstrand (NGR TG 255406), Sidestrand (NGR TG 255404) and Trimingham (NGR TG 298379). A total of 279 species has been compiled from the above data sources. When comparisons of the illustrations of each of the species were made from author to author, it became apparent that a number of synonyms existed. If left, these would distort the data, so I have attempted to apply a consistent taxonomy across all the stages. Due to these taxonomic uncertainties the final number of species in the dataset is therefore lower than the total number of species recorded in the literature. Since the dataset is so large and the number of species which may cause errors is small, I believe that the trends which are displayed are valid.

3. The chronoecologic chart Diversity patterns have been used as chronoecologic tools (Harten and Hinte, 1984) to link diversity to eustatic changes in sea-level. Harten and Hinte (1984)

postulated that by examining the patterns created by range charts calibrated to a linear scale determined from geochronology, that it was possible to infer rising sea-levels from increasing diversity, conversely, reduced diversity is the product of falling sea-levels. The rationale was drawn from the correlation which Pokorny´ (1971) made between diversity and sea-shore movements for the Bohemian Cretaceous Basin; with transgressive movements went niche creation and diversity increase, conversely with regression came niche destruction and diversity reduction. Similar relationships have been pointed out by Lethiers (1983, 1987) and Colin and Lethiers (1988). In addition, the chronoecologic method of presenting range information could distinguish between stressful and stable environments. This approach has been used by others (Flexer et al., 1986; Harten, 1988) to infer sea-level changes from range charts, with the suggestion that as more comprehensive ostracod range charts become available, the global eustatic sea-level curves may be improved upon by using the transgressive–regressive signals obtained from the chronoecologic chart (Harten, 1988). With the compilation of a comprehensive range chart it should be possible to test these ideas. The data presented in Fig. 3 have been ordered chronostratigraphically, in accordance with the method of Harten and Hinte (1984), that is by first appearance, and then by duration. The vertical axis used is the time scale of Haq et al. (1988) in order to facilitate comparison with their putative eustatic sea-level curve. The range data have been calibrated using the dmacrofossil biochronozones (boreal) Great Britain’ of Haq et al. (1988 Fig. 3) and not the stage boundaries, since the base of the coranguinum Zone is shown to be coincident with the base of the Santonian by Haq et al. (1988, Fig. 3). In the sections from which the ostracod data have been taken, the lower coranguinum Zone is within the Upper Coniacian. On examination of the chronoecologic chart of Harten and Hinte (1984, Fig. 4), the Late Cretaceous part of which is redrawn here as Fig. 4, it can be seen that the correlation of regression with extinction is an artifact of the lack of data at that time for the Cenomanian and Turonian stages. The more complete dataset presented here (Fig. 3) shows in the Early and Middle Cenomanian, a series of variable duration ranges with high species turnover which may be

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Fig. 4. Chronoecologic chart of Harten and Hinte (1984, Fig. 4) redrawn to show the Late Cretaceous.

indicative of stress in the environment. A possible cause of this is the change throughout the Cenomanian to a more carbonate rich facies; intermediate between the clays of the Albian and the onset of white chalk in the Turonian. Species originating in the Turonian and later stages are characteristically of longer duration. This would accord with the notion of greater stability within the environment which resulted in the deposition of the White Chalk facies. The dflat topped triangularT pattern predicted by Harten and Hinte (1984) is seen twice in the Late Cretaceous, broadly within the Cenomanian as a whole and secondly from the Turonian to the Maastrichtian. Accordingly, this should indicate two trangressive– regressive cycles. However, the tops of the ranges in the Maastrichtian are an artifact of the strata sampled, and not true stratigraphical tops, therefore no regression should be implied from this. The area of interest then lies in the change in assemblages which occurred around the Cenomanian–Turonian boundary. When the present data are compared with the putative eustatic sea-level curve of Haq et al. (1988), it is seen that the position of the large number of extinctions at the Cenomanian–Turonian boundary corresponds not to a regressive event, but occurs on a rising trend which resulted in the single major sea-

level maximum within the whole of the Cretaceous period. Conversely within the Upper Turonian, where the sea-level is falling to a minimum, there is an increase in species diversity. This level of diversity is maintained through the Coniacian and lower Santonian, but increases again slightly in the Upper Santonian, which corresponds to a short-term sea-level fall. Incomplete data for the Campanian explains the heavily stepped pattern. The position of apparent diversity increase in the Campanian occurs in samples taken from Drayton which lies within the Eaton Chalk (Peake and Hancock, 1970), while the next stratigraphically lower samples taken by King (1968) are from Stiffkey which lies in the higher part of the Gonioteuthis Zone (Peake and Hancock, 1970). The result is that the intervening basal mucronata Zone was not sampled. It is therefore predicted that should this interval be sampled systematically for Ostracoda, the stepped pattern on the chronoecologic chart would disappear. The relationship between diversity and sea-level change is more clearly seen when the range data are abstracted from Fig. 3 and re-presented in terms of species originations, extinctions and the resultant compound diversity, that is the number of species at any one time determined from their total stratigraphi-

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cal range, and not the number of species present in any given sample. This approach compensates for data loss through imperfect preservation and differences in sampling or methodology (Fig. 5). Only information for the Cenomanian, Turonian, Coniacian and Santonian stages are given in Fig. 5, since the data collection is almost complete through this period of time, whereas the Campanian and Maastrichtian data at present are still fragmentary. Within the Cenomanian, the overall trend is one of diversity increase to a Late Cretaceous maximum, followed by reduction to a Late Cretaceous minimum. In detail, there is a rise from 37 species at the Albian– Cenomanian boundary to an initial maximum of 80 species in the mantelli Zone. The diversity reduces very slightly to 79 species in the dixoni Zone, and then rises to the Late Cretaceous maximum of 87 species in the lower rhotomagense Zone. The diversity falls away to a brief minimum in the lower guerangeri Zone, before increasing slightly to 69 species in the upper guerangeri Zone. There is then a steady decline to a Late Cretaceous minimum of 20 species in the juddii Zone just below the Cenomanian–Turonian boundary. Comparison with the eustatic sea-level curve of Haq et al. (1988) shows similar

trends with an inverse correlation; the long-term curve reaches a minimum in the rhotomagense Zone, and a maximum in the Mytiloides Zone of the Lower Turonian. This is matched by the diversity rise to a maximum in the rhotomagense Zone, and fall to a minimum in the Mytiloides Zone. The short-term sealevel changes show a similar pattern to the diversity curve, though the timing of the peaks and troughs does not correspond exactly. The initial short-term low corresponds with the first rise in diversity in the mantelli Zone. As the short-term sea-level rose through the Early Cenomanian the diversity shows a small decline. The diversity increases to its maximum at the point where there is a short-term sea-level maximum, however, this also corresponds to the position of the long-term sea-level low. At the short-term sea-level low in the Middle Cenomanian, the diversity values are in decline. In the later Middle Turonian the short-term high corresponds to a brief diversity minimum, before regaining in the early part of the Late Cenomanian, again here the short-term sea-level curve is falling to a low before rising to the maximum over the Cenomanian–Turonian boundary interval. The correspondence then of the ostracod diversity curve and the short-term sea-level curve is not clear. While

Fig. 5. Number of ostracod species originations, extinctions and compound diversity set against the eustatic sea-level changes of Haq et al. (1988). Stratigraphical axis after Haq et al. (1988). Co=Coniacian.

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the trends are identical the timing of them is not. This may indicate that the Ostracoda are influenced to a greater degree by the global sea-level trends than by the short-term fluctuations. This is also the case as reported by Hilbrecht et al. (1996) who noted that the Cenomanian–Turonian marine biotas of the Regensburg area of southern Germany were influenced more by global changes than local environmental conditions. This is significant since the Regensburg faunas are of shallower water facies, where changes in local conditions would be of greater magnitude than those in the chalk seas of southern England. In the Turonian, the diversity rises stepwise to a maximum of 62 species near the Turonian–Coniacian boundary, at the level of the Kingston Nodular Chalks, which were formed under conditions of lowered sea-level (Hancock, 1990, 1993; Gale, 1996). This corresponds to the eustatic sea-level short-term minimum on a long-term falling trend. During the Coniacian and Santonian, the diversity remains fairly constant between 48 and 61 species with minor peaks in the lower and upper Santonian, which could correspond to short-term sea-level minima. The long-term trend throughout this period is quite stable. This demonstrates the most significant effects in the Late Cretaceous: ! Low global sea-level in the Early Cenomanian corresponds to an ostracod diversity maximum. ! In the Late Cenomanian the rate of species extinctions increased with the rise of global sea-level. Species originations remained low and this resulted in a diversity minimum over the Cenomanian–Turonian boundary interval. ! An increase in species originations through the Turonian coupled with low rates of extinctions just below the Turonian–Coniacian boundary result in rising diversity while the global sea-level is falling. These are in direct opposition to the assertions of Harten and Hinte (1984). An explanation might lie in the fact that the range charts selected by Harten and Hinte (1984), for the Jurassic and Cretaceous, upon which their hypothesis was based, were taken from Sheppard (1978). These charts contained only a partial data set of stratigraphically useful species avail-

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able at that time. When a more complete data set is used, such as is presented here, the interpretation appears to be reversed for the British Late Cretaceous. A second and perhaps more important consideration is the interpretation of the work of Pokorny´ (1971) whose initial studies stimulated the idea of the chronoecologic chart. Harten and Hinte (1984) comment on the fact that Pokorny´ (1971) related movement in sealevels with diversity changes, there being a positive correlation in shallow water and negative with greater depth. An explanation may be found in the relative depths of the basins. In the Bohemian Cretaceous Basin the shallow water facies from the middle Turonian in the Kosˇtice borehole are considered to be from near the wave base, sandy silts with all fines washed out (Pokorny´, 1971); it is these sediments which were shown to have the lowest diversity. Pokorny´ (1971) noted that in the axial part of the basin, in deeper water sediments recovered from the Borek-1 and Vsestary-1 boreholes, the diversity trends were the opposite of those found at Kosˇtice. The Anglo-Paris Basin during the Cretaceous was an area of dominantly coccolith chalk deposition with bands of marls, hardgrounds and diagenetic flints. Given the fine particle size of the sediments, it is unlikely that the water depth would be the equivalent to that found in the sediments from Kosˇtice, but would have perhaps a greater similarity to those of Borek and Vsestary. Therefore, for the position of southern England in the Anglo-Paris Basin, an inverse correlation between diversity and sea-level might be expected following the model of Pokorny´ (1971). This is explored further in the integrated model presented below. It is unlikely that the method of using ostracod ranges to construct chronoecologic charts can be of use to verify and adjust gobal eustatic curves as suggested by van Harten (1988), since it is necessary to first know the position within the basin and the relative water depth.

4. Stratigraphic distribution of Ostracoda The data have been analysed for patterns within higher taxonomic groupings at ordinal, superfamilial and familial levels, and the results are presented on Fig. 6. The analysis was carried out by assessing the number of species per macrofaunal zone from the

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Fig. 6. Number of species present per zone within the orders: Platycopida and Podocopida (scale 0–90 species), and species present per zone within families of the Cytheroidea from the Cenomanian to the Maastrichtian (scale 0–40 species). Co=Coniacian, Sant=Santonian, Camp= Campanian, Maa=Maastrichtian, Brachy=Brachycytheridae, Progono=Progonocytheridae, Proto=Protocytheridae.

Cenomanian to the Maastrichtian, within each of the higher taxa. The two main orders Platycopida and Podocopida are shown to have similar diversity trends, only differing in scale. Both reach an initial maximum in the Cenomanian rhotomagense ZonePlatycopids 12 species and Podocopids 72 species. Both groups then fall to minima in the Upper Cenomanian juddii Zone — Platycopida 6 spp. and Podocopida 12 spp. The diversity increases for both groups through the Turonian stage, a second minimum occurs for both groups in the Upper Santonian testudinarius Zone, before both orders expand to maxima in the Upper Campanian mucronata Zone — Platycopida 11 spp., Podocopida 83 spp. Although Platycopida become relatively more abundant at the Cenomanian– Turonian boundary, it can be seen that, in terms of specific diversity, both groups are diminished at this level. The abundance of species within families of the Cytheroidea is shown on Fig. 6. It is interesting to

note that the only two families with representatives at the Cenomanian–Turonian boundary are the Brachycytheridae (species of Pterygocythere and Pterygocythereis) and Bythocytheridae (Bythoceratina, Monoceratina, Patellacythere). Both of these groups have a deeper water association, Pterygocythere is a present day component of the abyssal assemblage. Another deep water family, the Krithidae, makes its first appearance in the Cretaceous, immediately after the eustatic sea-level maximum. The Cytheruridae, which was very successful in the Cenomanian, suffered a specific diversity crash in the latest Cenomanian, which then took nearly the whole of the remaining Late Cretaceous to regain, suggesting that at that time the Cytheruridae were not adapted to deep water habitat. In addition to the brachycytherids and bythocytherids, members of the Bairdioidea and Cypridoidea also survived through the Cenomanian– Turonian interval. Though the analysis of higher taxa

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by zone may be biased due to differing durations of zones, absence of species in a zone is real since it was derived from a compound species diversity count. Therefore the presence of the brachycytherids and bythocytherids, when all other Cytheroidea are absent, is significant.

5. Ostracod diversity and stable isotope profiles as proxies for sea-level and palaeotemperature The 13C / 12C ratio in planktonic carbonate is influenced by the rapid burial of organic carbon, which is enriched in the lighter isotope 12C. The oceanic waters are then enriched in 13C, from which the carbonate is precipitated by the plankton, and in which a resultant positive shift in d 13C is seen. High rates of organic burial, as seen in the black shales of the North and South Atlantic, have been linked to the large positive carbon isotope excursion at the Cenomanian–Turonian boundary, with the suggestion that the ocean became anoxic at that time (Summerhayes, 1987; Arthur et al., 1987). Support for this has come from the identification of the products of photosynthetic green sulphur bacteria in the southern North Atlantic Ocean which indicate the base of the photic zone was euxinic (Sinninghe Damste´ and Ko¨ster, 1998). Intensification and expansion of the oxygen minimum zone onto the shelf has been suggested as a cause for the reduction in microfaunal diversity (Jarvis et al., 1988). The identification of large numbers of platycopids has been taken to indicate reduced oxygenation of the water (Whatley, 1991; Whatley et al., 1994), and indeed there is an increase in the relative abundance of Cytherella at the Cenomanian–Turonian boundary (Jarvis et al., 1988). The support for this hypothesis was the work by Dingle et al. (1989), who examined the relationship between water masses and modern ostracod faunas off southwestern Africa, and Cronin (1983) who studied the Ostracoda off the Florida coast. Although the depth at which a reduction in the concentration of dissolved oxygen (b 1 ml/ l change) occurred did contain species of Cytherella, Dingle et al. (1989) show that it is the presence of a different water mass, the Antarctic Intermediate Water in this case, which contained high numbers of Krithe and Cytherella. The water mass between 200 and 650 m is characterised by the salinity minimum zone.

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While the difference in salinity between these water masses is not sufficiently large to act as a control on ostracod distribution, it does demonstrate a physically different water mass, with differing circulations which may influence the benthonic fauna. It is evident from the studies of both Dingle et al. (1989) and Cronin (1983), that there is also the simple control of depth on the faunas. Indeed, in the Florida faunas, Cytherella continued its dominance below the oxygen minimum zone. This agrees with the work of Dingle (1980, 1981) for the marine Late Cretaceous South African ostracods, where triangular plots were made of Cytheroidea, Cytherellidae and combined Bairdioidea and Cypridoidea. The deeper water (suggested as 500 m) correlated with positions closer to the apex containing Cytherellidae. Ayress et al. (1997) studied deep water ostracod faunas from five separate localities in the southern oceans; and each had oxygen minimum zones at differing depths, either within or below the Antarctic Intermediate Water. They concluded that although no species were restricted to one watermass, the limits of the species were closely related to the watermass properties of temperature and salinity. All taxa belonged to the Podocopida, and no indication of species reduction in the oxygen minimum zones at these sites was noted. Similar results were obtained from the Late Maastrichtian faunas of the South Atlantic by Majoran et al. (1997), who used principal components analysis to determine the causes controlling faunal density. They discovered that variation in food supply was more critical than variation in oxygen levels. For certain elements of the fauna such as Bythocypris and Krithe, palaeotemperature was found to be the most important control on distribution. So it would appear that the water mass boundary with the consequent parameters of depth, temperature, salinity and food supply are the dominant factors controlling the distribution of the fauna. Jenkyns et al. (1994) have provided curves of the stable isotopes of carbon and oxygen from Eastbourne in the south of England. Carbon isotope profiles have been used as a proxy for sea-level, but as Jenkyns et al. (1994) point out, the major positive excursion could relate to the maximum rate of flooding rather than the time of maximum flooding. The eustatic sea-level maximum of Haq et al. (1988) does lag behind the position of the carbon isotope maximum. There is a strong negative correlation of ostracod diversity with

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d 13C during the Cenomanian and Turonian (Fig. 7), but there is no clear relationship in the Coniacian and Santonian stages. The ostracod diversity minimum corresponds to the carbon isotope maximum (Fig. 7). If this is taken as the onset of flooding, this would indicate that the response of Ostracoda to changing environmental conditions was rapid. A more consistent positive correlation is discovered when the compound diversity curve is compared with the d 18O profile from Eastbourne. An initial rise in d 18O values in the Early Cenomanian is matched by a rise in diversity to the maximum in the rhotomagense Zone. Both sets of data fall to minima at the Cenomanian–Turonian boundary. The d 18O values then rise throughout the succeeding Turonian, Coniacian and Santoian stages, with small peaks in the upper Turonian and Upper Santonian. The diversity values also reach a maximum in the Upper Turonian, the number of species begins to fall slightly and two small diversity peaks occur in the Upper Coniacian and the Upper Santonian. Oxygen isotopes may be used to determine seawater temperatures since the difference in ratio of 18 O / 16O of calcium carbonate and the sea water from which it precipitates is temperature dependant (Hudson and Anderson, 1989). An increase in the ratio corresponds to a decrease in the temperature and vice

versa. The oxygen isotope profile from Eastbourne (Fig. 7) then shows a cooling in the Early Cenomanian (20 8C), a warm event at the Cenomanian–Turonian boundary (28 8C), followed by a steady cooling to the Santonian–Campanian boundary (19 8C), with cool spikes in the Upper Turonian and Upper Santonian (temperatures from Jenkyns et al. (1994)). Voigt (2000) suggests a 2 8C cooling event to 16 8C in the Upper Turonian calculated from oxygen isotopes measured on brachiopods. The data for Eastbourne in Jenkyns et al. (1994) are taken from bulk rock analyses and relate to sea surface temperatures. Voigt (2000) shows both surface values from whole rock and sea floor values from Turonian brachiopods and notes that the oxygen isotope values in the matrix are consistently lower. She suggests that this is due to the bulk samples being more prone to burial diagenesis. The trends of the two signals, however, are the same, which indicate that there was little de-coupling between the surface and bottom waters. It is seen that the ostracod diversity is negatively correlated with temperature, which indicated that changing oceanic circulation together with sea-level changes has resulted in a response from the benthonic fauna. This is supported by other groups such as the inoceramids which show very similar diversity trends (Fig. 8). Voigt (1995) also ascribed the changing frequency in inoceramids to

Fig. 7. Compound ostracod diversity for the Cenomanian to the Santonian set against the d 13C and d 18O curves from Eastbourne. Isotope data and stratigraphical axis after Jenkyns et al. (1994).

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6. Functional morphology

Fig. 8. Compound diversity curves of Ostracoda from southern England and global inoceramids, from the Cenomanian to the Santonian. Inoceramid curve redrawn after Voigt (1995). Stratigraphical axis after Haq et al. (1988).

changes in sea-level, with regressions and lowstands corresponding to a rise in the number of species due to restricted communication between regions, which allows speciation. Abreu et al. (1998) have commented on the positive correlation that exists between the Cretaceous low-frequency oxygen isotope profile and the longterm eustatic sea-level curve, suggesting that since sea-level fall corresponds to cooler palaeotemperatures, that the control on eustacy maybe glacial. Although dropstones are known from the Albian, there is little evidence that any polar ice existed in the Late Cretaceous, so glacio-eustacy remains controversial. Price (1999) discusses the distribution of glacially derived sediments and glendonites throughout the Mesozoic and shows the paucity of evidence of Late Cretaceous glaciation. Voigt (2000) suggests an increase in tectonic activity initiated by rifting of the North Atlantic, which could be a cause for a reconfiguration of oceanic circulation as well as sealevel change. Mortimore and Pomerol (1991) described synsedimentary tectonic movements in the Upper Cretaceous Chalks of the Anglo-Paris Basin, the timing of which appear to be concordant with the transgressive–regressive cycles as seen from the eustatic curve, which argues for a tectonic control on the sea-level.

In studies of ostracods from the shore to the deep sea, those species with well developed eye tubercles are found in the shallower realms. Ayress et al. (1997) show that sighted species in several locations in the southern oceans occur down to approximately 500 m water depth. Below this depth the blind psychrospheric fauna becomes dominant. The requirement of light for sighted species is unlikely to have changed since the Cretaceous, and it is therefore reasonable to suggest that in the Cretaceous, sighted ostracods preferred shallow habitats, while blind species lived in deeper water. Kontrovitz and Meyers (1988) have examined the limit of vision of ostracods using measurements of the ocular structures, and determined that sighted ostracods would have useful vision down to 280 m in clear water. This does not explain why sighted ostracods today are discovered at depths of 500 m. Puckett (1991) took this concept further and determined absolute depths for the Campanian and Maastrichtian oceans in Alabama from the presence of sighted ostracods. This method relies solely on a determination of the turbidity of the water, since the only other variables in the equation used by Kontrovitz and Meyers (1988), the focal length and the aperture of the eye, make no significant difference to the calculated value. It is perhaps safer to use the proportion of sighted to blind species to obtain information on relative water depth. If this is carried out for one area, and systematic changes are discovered, this should indicate the direction of sea-level change. There are some cautions which need to be observed with this method: it is perfectly possible for sighted ostracods to possess no eye tubercle eg. Xestoleberis and some cytherurids, conversely it is unlikely that a blind ostracod would possess one. So by counting species with eye tubercles, there can be an underestimation of sighted species. In the Cretaceous fossil record, however, we have no choice but to use the available functional morphology, in this case the association of eye tubercles with shallow water depth. A count was made by zone from the Cenomanian to the Coniacian of the percentage of species which have eye tubercles as an indicator of depth. Specimens were available for 186 species within this stratigraphical range; 90 of these are in possession of eye

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Fig. 9. Percentage of total number of species of Ostracoda which possess eye tubercles arranged by zone from the Cenomanian to the Coniacian, as an indicator of relative depth of water. Con=Coniacian.

tubercles and 96 are not. Fig. 9 shows a decrease from 62% of the species at the Albian–Cenomanian boundary to 46% in the mantelli Zone. The proportion of sighted species then declines steadily through the Cenomanian to 33% in the geslinianum Zone. A minimum of 10% of species have eye tubercles in the juddii Zone. The proportion then steadily rises through the Turonian to reach a maximum in the cortestudinarium Zone of 41%. This shows a good inverse correlation to the long-term eustatic sea-level curve of Haq et al. (1988). As the sea-level rose, it is supposed that those species possessing eye tubercles would have moved out of the area, upslope to a more marginal position within the basin. With regressive movements, those same species would return to the area, though in the case here, after speciation had taken place. This is a confirmation of the approach of Pokorny´ (1971) that an ostracod optimum zone existed which was partly controlled by light levels.

7. Integrated model A model is developed here (Fig. 10) which links the cumulative species diversity pattern, from the Cenomanian to the Santonian, with the sea-level proxy of stable carbon isotopes, global euastatic sea-level changes, palaeotemperatures derived from the oxygen

stable isotope, and the proportion of sighted species of Ostracoda. Pokorny´ (1971) investigated diversity curves for two cores in the Bohemian Cretaceous Basin and demonstrated two opposing diversity patterns for the same oceanic event. Which pattern emerged depended upon the position within the basin. For a central basinal position, diversity increased with regression and decreased with transgression. Conversely, for a more marginal position, the diversity was shown to increase with transgression and decrease with regression. This was explained by invoking an ostracod optimum zone which was moved shorewards or basinwards depending upon the change in sea-level. Below the optimum zone, the food supply and light conditions become poor, and the diversity decreases, above the optimum zone the energy levels become too high for many species to survive, and again the diversity reduces. By analogy with modern oceanic structure, and a consideration of the changing shape of the oceanic basins, the structure and circulation of the Cretaceous ocean has been surmised by Hay (1995). It was suggested that due to the magnitude of the sea-level rise there was a breakdown in shelf-break fronts at the time of the Cenomanian–Turonian boundary, until the end of the Cretaceous. This allowed a stratified ocean to form on the epicontinental margins, which possessed a three layer system of surface water, central water and intermediate water, each with differing circulatory systems (Hay, 1995, Fig. 12B). I suggest that it is possible to equate the diversity zones of Pokorny´ (1971) and the oceanic layers of Hay (1995) to account for the changes in diversity related to sealevel movement. The surface water, with high energy near to the wave base, is a low diversity zone. The optimum zone is equivalent to the central water, where circulation into the marginal basins brought in nutrient rich waters and resulted in increased diversity. The intermediate water had an opposite circulation, removing nutrients, and this, together with the effect of greater depth, which would be more prohibitive to the shelf fauna, had the effect of reducing the diversity. The model developed here, for that interval, explains the changes in diversity in terms of the position in the basin of the optimum zone being affected by changes in sea-level. Fig. 10 shows a series of time frames from the Cenomanian to the Coniacian for the position of southern England within the Anglo-Paris Basin. Also indicated are the chang-

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Fig. 10. Integrated model of sea-level changes, palaeotemperature, and ostracod diversity for the interval from the Cenomanian to the Coniacian stages of the Late Cretaceous. IW=Intermediate Water (impoverished layer), % e/t=percentage of species which possess eye tubercles, OMZ=oxygen minimum zone. =the relative bathymetric position of southern England in the Anglo-Paris Basin during the Late Cretaceous.

ing oceanic temperatures, and the proportion of sighted ostracod species expressed as a percentage of the total fauna for the time frame.

Interpretation — a lower sea-level brought the optimum zone downshelf to the position occupied by the area allowing diversity to increase.

7.1. Albian–Cenomanian boundary

7.3. Cenomanian–Turonian boundary

Conditions — moderately high sea-level on a falling trend, low to moderate diversity. Interpretation — the moderately high sea-level moved the optimum zone up the shelf slope, such that the area was close to the impoverished zone below, and exhibited low diversity. No shelf break fronts existed.

Conditions — transgressive maximum, temperature maximum, diversity minimum, sighted Ostracoda at a minimum. Interpretation — the high sea-level has two effects, firstly, moving the optimum zone upshelf, away from the area, secondly the breakdown of shelf-break fronts and development of a stratified water column, creating differentiated water masses. Intermediate waters remove nutrients from the basin, and have different physico-chemical characteristics, which are reflected in the fauna. There is also the possibility that the oxygen minimum zone began to impinge upon the

7.2. Mid-Cenomanian Conditions — long-term sea-level trough, warming climate, diversity maximum.

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outer shelf. This has the effect of reducing the diversity to a minimum. At the same time, with high sealevels, it is likely that barriers to migration may have been breached allowing new species to migrate into the area. 7.4. Upper Turonian–Coniacian boundary Conditions — long-term regressive trend, shortterm fall in sea-level, diversity peak. Interpretation — an essentially new fauna, having replaced the Cenomanian fauna, gradually becomes more diversified throughout the Turonian, the lowered sea-level has moved the optimum zone downshelf, which increased the number of species present in southern England.

8. Conclusions A review of published literature and the incorporation of new data have allowed the compilation of a data set which includes all currently known Late Cretaceous Ostracoda for southern England. When the data are arranged in the form of chronoecologic chart, that is, by first appearance, then duration, and calibrated against a chronostratigraphic axis, it is seen that during the Cenomanian, the majority of the ranges are of short duration, indicating some stress in the environment. This may be due to changing climatic conditions which resulted in a changing depositional regime from clays to carbonates. The ranges of species originating in the Turonian and succeeding stages are typically of longer duration, and reflect the more stable nature of the white chalk facies. When compared to putative eustatic sea-level curves, diversity minima correlate with long-term sea-level rise, and diversity maxima correlate with long-term sea-level fall, in opposition to the postulate of Harten and Hinte (1984). The discrepancy is explained by the relative depths of water between the shallow Bohemian Cretaceous Basin and the deeper Anglo-Paris Basin. Species which survive the Cenomanian–Turonian boundary event are members of the Platycopida, Bairdioidea, Cypridoidea, Brachycytheridae, and Bythocytheridae, which have deeper water associations. Ostracod diversity is negatively correlated with the carbon isotope and positively correlated with the ox-

ygen isotope profile, suggesting that diversity rises with regression and cooling events while it reduces with transgression and warming. The proportion of sighted ostracods decreased from 62% to 10% throughout the Cenomanian and increased once more to 40% through the Turonian and Coniacian in response to changing light levels caused by the transgressive and regressive movement of the sea. An integrated model is presented for the south of England from the Cenomanian through to the Coniacian which accounts for the variation in diversity in terms of changing sea-levels and oceanic circulation.

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