Foraminiferal record of the Holocene development of the marine environment in the southern North Sea

43

Netherlands Journal of Sea Research 31 (1): 43-52 (1993)

FORAMINIFERAL RECORD OF THE HOLOCENE DEVELOPMENT OF THE MARINE E N V I R O N M E N T IN T H E S O U T H E R N N O R T H S E A LEON MOODLEY and TJEERD C.E. VAN WEERING Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands

ABSTRACT Analyses of the benthic foraminifera encountered in a piston core and box core collected from the Frisian Front area in the southern North Sea (53°42'N, 4°30'E) revealed that subtidal conditions began at -6240 B.P. No large scale changes were observed in the foraminiferal compositions since then, but distinct variation in their densities occurred. Lateral transport from the south is effective, as implied by the presence of reworked Cretaceous foraminifera; there is very little long-term Recent sediment accumulation in this area. Fluctuation in the Holocene foraminiferal densities reflects both the variation in local production of tests and the supply of tests from the south.

1. INTRODUCTION

2. HISTORY AND DESCRIPTION OF THE AREA

Benthic foraminifera, being generally abundant, diverse and usually well preserved in marine sediments, are extensively used in palaeoecological studies. Studies on the ecology and distribution of Recent benthic foraminifera (about 5000 extant species) in the different marine ecosystems provide a database with which fossil forms may be compared and interpreted. The majority of benthic taxa have long stratigraphic ranges and, because of their differing ecological requirements, are facies-dependent. Nevertheless, they are extremely useful in biostratigraphic studies, especially within single depositional basins, and they are excellent guides to palaeoecology (MURRAY,1991b). The importance of monitoring ecosystems to understand their natural order has increased over the last years, reflecting the growing concern of the effects of human activities on the environment. Benthic foraminifera preserved in marine sediments can indicate environmental conditions prior to possible anthropogenic changes and are therefore useful tools to study and understand the depositional and ecological evolution of an environment. In this study, the development of the subtidal environment in the southern North Sea is documented by using benthic foraminifera from different depths in a piston core that penetrated the Holocene. For the upper, surficial sediments, a sediment sub-core taken from a box core was analysed.

In the late Weichselian, basal peat was generated by amelioration of the climate and a rising groundwater table as a result of rising sea level (e.g. JELGERSMA, 1961; OELE, 1969, 1971a, 1971b; JELGERSMA et al., 1979). This basal peat therefore predates the Holocene transgression and provides evidence for the progress of the inundation of the pre-Holocene landscape (JELGERSMA, 1961; SIREIF, 1989). Peat was found from Dogger Bank to the Straits of DoverCalais (JELGERSMA, 1961 ; OELE, 1969, 1971 a, 1971 b; BEHRE & MENKE, 1969; KOLP, 1974, 1976; KIRBY & OELE, 1975; BEHRE et aL, 1984). Pollen analysis and radiocarbon dating of the peat indicated a Preboreal and Boreal age. The Holocene sequence in the adjacent southern North Sea can be divided into two parts: 1) a lower clastic part (Elbow deposits, OELE, 1969)) consisting of clays and fine, generally clayey, sands, and 2) an upper clastic part (Dunkerque deposits) consisting of sands deposited in an open marine environment (JELGERSMA et al., 1979). Another recurrent feature of the Holocene sequence is a peak of reworked tidalflat mollusc-fauna encountered in the Dunkerque deposits at various depths below the seafloor (e.g. BEHRE & MENKE, 1969; KOLP, 1974; BEHRE et al., 1984). To the south (53°40'57"N, 4°24'55"E) of our study area the peak of mollusc fauna, predominantly Turritella, has been found at ~64 to 73 cm below the seafloor (Rijks Geologische Dienst, unpubl, data,

44

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north of this latitude (CREUTZBERG et al., 1984). Presently, bottom-water salinity is about 34.0 to 34.8 throughout the year. An annual variation of ~11°C in the mean bottom-water temperatures is found in the vertically mixed area and of 3 to 7°C beneath the stratified surface waters (MURRAY, 1992). Bottom waters are well oxygenated (225 to 275 IxM) throughout the year (VAN RAAPHORSTet al., 1992). 3. MATERIAL AND METHODS A piston core (Core M2, 230 cm long) was collected with a standard piston corer in May 1990 aboard RV 'Aurelia' of the Netherlands Institute for Sea Research. Because part of the surface layers may be lost or compressed during standard piston coring, a shorter sediment sub-core (38 cm, Box M2) was taken from a box core collected at the same site for foraminiferal studies of the undisturbed upper sediment layers. Both samples were collected at 53°42'N, 4°30'E (water depth: 39 m) (Fig. 1). To obtain a stratigraphic framework, 14C ages were determined at selected sediment intervals by Accelerator Mass Spectrometry (AMS), which requires only 1

SOUTHERN NORTH SEA FORAMINIFERAL RECORD mg of organic carbon for accurate 140 determination (VAN DER BORG et al., 1987). The 14C determinations were done on benthic foraminiferal tests (Ammonia and Elphidium) except for two samples (140 cm and 215 cm) (Table 1). The uppermost sample for 14C determination was taken from the box-core sample. Unfortunately, the sedimentary sequences of the piston core and box core did not allow intercore correlation, as the latter was not long enough. Analysis of the particulate organic carbon content (% POC) and sediment characteristics was done on samples taken from the piston core. Due to the lack of additional box-core samples, sediment analysis of the upper surficial layers was also done on the piston core samples. Although these values may not represent the actual surface values, it was not considered critical for this study. POC contents were measured on a Carlo Erba NA 1500-2 elemental analyser following the procedure of VERARDO et al. (1990), after application of sulphurous acid to remove inorganic carbon. The grain size frequency distribution, including the fraction <50 p-m, was determined by dry sieving in an EML sieve analysis shaker. The sediment was first treated with HCI and H 2 0 2 to remove carbonates and organic carbon. Eleven sediment samples for foraminiferal analysis were taken from the box core for the upper 38 cm and 17 samples were taken from the piston core. Foraminifera (>63 p.m) were wet picked from a known wet split, identified, counted and normalized to the number of specimens in 1 cm 3 of sediment. The boxcore samples were stained with Rose Bengal immediately after recovery, so that a distinction could be made between the living and dead. Only the dead assemblages were used to study the evolution of this environment. We used the reciprocal of the BergerParker diversity index (BERGER & PARKER, 1970) and the SANDERS (1960) similarity index to compare the TABLE 1 Radiocarbon dates. sample name

code numbera

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box M2-1 e core M2-1 e

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2410 + 60 4800 -+ 110

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core M2-4 e core M2-5g

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200 d 215 d

8700 +_ 80 8720 _+ 90

"Age in years Before Present from 14C activity after normalization to o313= - 25%0. No correction applied for reservoir age. a Code number of the Van de Graaff Laboratorium, Rijksuniversiteit Utrecht, The Netherlands. b Depth below seafloor, c in box core and d in piston core. e ~4C determination done on benthic foraminiferal tests. f ~4C determination done on concentrated plant debris. g ~4C determination done on peat.

45

assemblages formed under the different conditions. Only the dominant Holocene foraminiferal species are depicted (Plate 1). 4. RESULTS AND DISCUSSION The Holocene sequence recovered by piston core M2 could be broadly divided into two main facies: a lower layer of clay/clayey silt intercalated with silt that overlays the basal peat and an upper layer of fine sand (Fig. 2). The clay/clayey silt layer (200-91 cm) and the sand layer (82-0 cm) were preceded by, respectively, a layer of olive black clay/peaty material (200-213 cm) and a silt layer (91-82 cm) (Fig. 2). The samples taken in the clayey sediment facies were rich in plant debris with decreasing amounts towards the top (Fig. 2). The sand facies contained entire and fragmented shells that increased towards the top with a maximum at the peak of mollusc-fauna (52 to 72 cm below the seafloor), predominantly of Turritella, and then decreased in the uppermost 32 cm. This broad distinction in two units was also evident in the profiles of the % POC and % silt (sediment fraction <63 p-m) (Fig. 2). The transitional level was found at ~91 cm below the seafloor. Extrapolating the 14C dates (Table 1), but excluding the 140-cm sample because it most probably represented reworked older plant material, this transitional level was deposited at ~6240 B.P. The higher POC content below 91 cm is considered to be associated with the plant debris in the clayey sediments. The transitional level at 91 cm also marks a shift in the foraminiferal pattern as the foraminiferal composition changed below this level (Fig. 2). Below, Ammonia beccarii (Linn~) showed a high dominance (Fig. 3). In the clay/peaty layer (200-213 cm core depth) that overlay the peat layer (213-222 cm core depth) significant numbers of foraminifera were encountered. Ammonia beccarii accounted for 82 to 97% of the assemblage with only Elphidium oceanensis (d'Orbigny) (1 to18%) as subsidiary species (Fig. 3). This assemblage indicates a brackish depositional environment (MURRAY, 1971, 1991a). This is in agreement with earlier observations, where a brackish environment of deposition (kind of lagoon, OELE, 1971 b) was indicated by characteristic ostracods and molluscs (JELGERSMA et al., 1979)~ The brackish marine clay layers have been dated as Boreal (JELGERSMA et al., 1979; BEHRE & MENKE, 1969; BEHRE et al., 1984). In addition to the brackish character of this environment of deposition, the shallow (lagoonal) environment (that preceded the intertidal environment) would have experienced major variations in physical conditions and in salinity. The high numbers and dominance of one species also suggests an unstable and variable environment. Diversity is highest in stable, normal marine environments (MURRAY, 1991b). Sex-

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Plate 1. Dominant foraminiferal species encountered in living and dead assemblages. Scale bar = 100 )~m. 1. Ammonia beccarii (Linn6) forma tepida (Cushman) (from 200 cm core depth), a, b. dorsal view. c. ventral view 2. Elphidium oceanensis (d' Orbigny) 3a-d. Ammonia beccarii (Linne) 4. Elphidium excavatum (Terquem) 5. Buccella frigida (Cushman), a. dorsal view. b. ventral view 6. Nonion depressulus (Walker & Jacob) 7. Eggerelloides scabrus (Williamson) 8, Hopkinsina pacifica Cushman 9. Bulimina gibba/elongata Fornasini & d' Orbigny 10. Fursenkoina fusiformis (Williamson).

Fig. 2. Lithology, radiocarbon ages (the ~4C age in brackets is excluded from analysis because it probably represents reworked older plant material), sedimentary characteristics and foraminiferal data of piston and box core M2. Percent similarity is the similarity of the assemblage with the overlying assemblage. See Fig. 3 for legend.

48

L. MOODLEY & TJ.C.E. VAN WEERING

TABLE 2 Comparison of the living and dead assemblage in the upper 5 cm. Numbers per 5 cm3 sediment. Only dominant species are presented.

A. beccarii B. frigida B. gibba/elongata E. scabra E. excavatum E fusiformis N. depressulus H. pacifica species diversity (l/d)* % similarity* "All species included.

living % # 2.90 11.3 4.20 16.4 6.33 24.7 34.16 133.3 23.69 92.4 5.02 19.6 0.0 0.0 8.57 33.4 2.93

dead % # 49.83 5707.5 1.57 179.3 1.26 144.7 2.53 289.5 33.01 3781.4 0.02 1.8 2.98 341.4 0.02 1.8 2.01

33.:28

ual reproduction is the dominant form of reproduction in unfavourable or unstable environments with large seasonal fluctuations (BOLTOVSKOY& WRIGHT, 1976; HALLOCK, 1985). Although no attempt was made to quantitatively distinguish between the different morphotypes of A. beccarii (SCHNITKER, 1974; VI~NECPEYRE, 1983; JORRISEN, 1988; WALTON & SLOAN, 1990), it was conspicuous that A. beccarii, in the 200213 cm layer, was solely of the forma tepida (Cushman), a morphotype in which a high incidence of sexual reproduction was observed (GOLDSTEIN & MOODLEY, 1993). The remainder of the lower part of the Holocene sequence (200-91cm) was formed by tidal deposits (OELE, 1969, 1971a, 1971b; KOLP, 1974; JELGERSMA et al., 1979) with very low foraminiferal numbers (Fig. 2). Above the 91-cm interval, a distinct change in the depositional environment was evident both in the sedimentary characteristics and in foraminiferal data (Fig. 2). The sediment characteristics and associated POC content shifted rapidly to values comparable to values measured under the current subtidal setting (upper surficial layers, Fig. 2) reflecting the onset and persistence of fully marine conditions. Water depth during the deposition of the 91-cm level (~6240 B.P.) was estimated from the curve of sea-level rise (JELGERSMA, 1979) and sediment thickness to have been ~10 m below today's mean sea level. With increasing water depth and environmental stability, additional open-sea species were encountered. Ammonia beccarii remained dominant, but in contrast to the 200 to 213-cm layer several morphotypes of A. beccarii were concurrent. Elphidium excavatum (Terquem) was the second dominant species. Buccella

frigida (Cushman) and Nonion depressulus (Walker & Jacob) were the important subsidiary species. Ammonia beccarii (38 to 67%) and E. excavatum (27 to 47%) determined the general pattern of total foraminiferal numbers and remained the dominant species throughout the upper layers (Fig. 3). Although the number of species and the species diversity increased under full marine conditions, diversity remained low (Fig. 2). Compared to the north of this study area, diversity was also low in the living assemblage of the upper 5 cm but higher than in the south (MOODLEY, 1990). Comparison between the living and dead assemblage of the upper 5 cm revealed that the similarity was very low (33.28%, Table 2). Several factors could account for the major difference between the living and dead: 1) the living fauna was collected only in one season, 2) postmortem transport, 3) mixing with local older or fossil tests, 4) selective removal by predators, 5) differences in shell production, 6) differential mechanical destruction, and 7) differential solubility. There were no signs of dissolution in any of the samples and there are no reports of selective predation on any of the dominant species in this area. The dominant species in the dead assemblage, A. beccarii and E. excavatum are morphologically very similar, so a similar sensitivity to mechanical destruction is expected. Eggerelloides scabrus (Williamson), an agglutinated species that was very dominant in the living assemblage (Table 2), may be more sensitive to postmortem destruction. Differences in shell production are difficult to evaluate but seasonal variation in the living assemblage does occur (MURRAY, 1992; MOODLEY, 1990). CADEE (1984) also found a difference between the living and dead molluscan assemblages and attributed this difference partly to the occurrence of the relict tidal fauna that is mixed with remains of the Recent fauna. In our study area, a tidal deposit with very low foraminiferal numbers (Fig. 2) was present under a layer of younger sands of ~82 cm thickness. Elphidium excavatum was well represented both in the living and dead fauna but A. beccarii, although being extremely dominant in the dead assemblage, formed only 2.9% of the living assemblage (Table 2). Even to the south and north of our sampling site (53°38'N, 4°30'E and 53°45'N, 4°30'E) in June, A. beccariiformed only 6% and 7% of the living assemblage, respectively; whereas E. excavatum formed 69% and 21% of the living assemblage (MOODLEY, 1990). Based on these differences between the living and dead fauna we conclude that postmortem transport is taking place. The loss (disintegration) of the agglutinated species E. scabra is probably another factor causing the large difference between the living and dead fauna. MURRAY (1992) also suggests that post-

Fig. 3. Down-core distribution of dominant foraminiferal species.

SOUTHERN NORTH SEA FORAMINIFERAL RECORD

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mortem transport of tests could take place, especially in the southern part of the southern North Sea. In this area, the direction of the average residual current is to the northeast, running ~ parallel to the 30-40-m isobaths (VAN HAREN & JOORDENS, 1990). Consequently, reworked or additional foraminifera may be derived from the south. This is supported by the occurrence of reworked Cretaceous foraminifera (Fig. 2) of which the Dover-Calais region is the most likely source. The presence of Eocene and Pliocene Bryozoa in Holocene and Recent Dutch coastal sands also demonstrates the northeastward direction of transport (LAGAAIJ, 1968). Cretaceous foraminifera were also found in the Dutch Wadden Sea (tidal flats) and in Zeeland (southern part of the Netherlands) and were suggested to have been derived from the coasts of England and France (VAN VOORTHUYSEN, 1951, 1960). The Cretaceous foraminifera, the most common being Conorboides, Osangularia, Cibicides, Bolivinoides, Heterohelix and Hedbergella, were identified following the descriptions given by JENKINS & MURRAY (1989). Based on the down-core distribution of Cretaceous foraminifera, flooding of the Oyster Grounds through the Straits of Dover-Calais must have begun around 8700 B.P. (Fig. 2), confirming the interpretations of KOLP (1974). Foraminiferal numbers also started to increase at the 91-cm level with maximum numbers above the 82-cm interval (<6000 B.P., water depth <8 m below mean sea level) (Fig. 2). However, the increase in the number of foraminiferal tests cannot be related solely to increase in local production as a result of improvement in the climate and marine production under more stable conditions, because a similar trend is observed in the profile of Cretaceous foraminifera (Fig. 3). Consequently, the larger numbers of Holocene foraminifera (Fig. 2) above the 91cm interval would include those that had been produced locally and those that had been transported from the south. Large fluctuations in the number of foraminifera were also observed in the upper 38 cm (Fig. 2). This is most probably not a result of changes in sedimentation rates, because the sediments showed very little variation (Fig. 2) and Recent sediment accumulation rates are low (see below). Once again, this is considered to reflect both the local production of tests and the supply from the south. Currently, the normal circulation in the North Sea is cyclonic (anti-clockwise), under combined influence of the tides and the mean wind stress, with a net flow from the Straits of Dover to the area studied. This circulation appears to be very sensitive to the wind direction (OTTO et al., 1990), and winds from easterly directions may temporarily reverse the residual circulation from cyclonic to anti-cyclonic. This would affect the transport from the Dover Straits to the study area and in the past, at shallower water depths, the sensitivity to wind changes might have been greater. This could partially

explain the fluctuation in both the Cretaceous and Holocene foraminiferal numbers. Furthermore, one may presume that simultaneous changes in the local hydrographic conditions could have had their effects on the local production. Variations in the rate of sealevel rise (STREIF, 1989) and in the intensity of erosion during the different transgressive phases (JELGERSMA et al., 1979) could have affected the lateral transport. The peak in Cretaceous and Holocene foraminifera at the 74-cm interval (water depth: ~7.5 m below mean sea level) (Fig. 2) probably reflects the increase in water depth and transport energy. In the adjacent area, Recent sediment-accumulation rates, based on 21°pb measurements, were estimated at an average of 4 mm.y -1 (ZUO et al., 1989). However, at our sampling site, based on the age of the 38-cm interval (Table 1), Recent sediment accumulation rates must be extremely low (average ~ 0.16 mm'y~), as also suggested by CADEE (1984). This is confirmed by the 21°-15bmeasurements done in a core collected from the same site in 1991 (G.W. Berger, pers. comm.). Therefore, Recent foraminifera would also be contributing to the older layers which would also, in part, be a condensed fauna. Mixing is probably also enhanced by bioturbation. In spite of these complications and fluctuations in the numbers of Holocene foraminifera, there seems to be a general trend of increasing foraminiferal numbers towards the Recent (Fig. 4). Eutrophication of the southern North Sea has been widely discussed (e.g. FOLKARD & JONES, 1974; VAN BENNEKOM et al., 1975; POSTMA, 1978; VAN DER VEER et al., 1990). Eutrophication results in increased primary production (CADEE, 1986) that would lead to a higher supply of organic material to the benthos. A higher supply of food would support a larger production of foraminifera. But, due to the relatively low sediment-accumulation rates, bioturbation and lateral transport, no conclusions can be drawn Number of tests/cc sediment 500 O"

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SOUTHERN NORTH SEA FORAMINIFERAL RECORD

51

regarding possible eutrophication effects. However, initially identifying the Cretaceous foraminifera. Carbon datA. beccarii and E. excavatum remain the most domi- ings were done by K. van der Borg (University of Utrecht). nant species and the high similarity (74 to 90% simi6. REFERENCES larity) of the foraminiferal assemblages in the entire upper 91 cm of the Holocene sequence (Fig. 2) suggests that no drastic changes have occurred over the BEHRE, J.A. & B. MENKE, 1969. Pollenanalytische Untersuchungen an einem Bohrkern der sSdlichen Doggerlast 6240 years with respect to the supply of organic bank. --Beitr. Meereskunde 24/25:123-129. matter or bottom water oxygenation. BEHRE, J.A., J. D£)RJES & G. IRON, 1984. Ein datierter SediThe sampling site discussed in this study forms mentkern aus dem Holozo~.nder sLidlichen Nordsee.-part of a rich benthic zone (called 'Frisian Front') on Probleme der KOstenforschung im sOdlichen Nordseegebiet 15: 135-148. the transition between the Southern Bight and the muddy Oyster Grounds (CREUTZBERG et al., 1984; BERGER, W.H. & F.L. PARKER, 1970. Diversity of planktonic foraminifera in deep-sea sediments.--Science 168: CREUTZBERG, 1985; CRAMER, 1990). Although this 1345-1347. local enrichment is reflected in several aspects, the enhanced macro-faunal densities, biomass and activ- BOLTOVSKOY,E. & R. WRIGHT, 1976. Recent Foraminifera. W. Junk, The Hague: 1-515. ity are most prominent and persistent when these CADI~E, G.C., 1984. Macrobenthos and macrobenthic characteristics are compared to those observed both remains on the Oyster Grounds, North Sea.--Neth. J. in the south and north of this enriched area (DE GEE et Sea. Res. 18: 160-178. al., 1991). Relatively larger numbers of living benthic - - , 1986. Increased phytoplankton primary production in the Marsdiep area (western Dutch Wadden Sea).-foraminifera were found within this enriched zone Neth. J. Sea Res. 28: 285-290. (MOODLEY, 1990). This general enrichment has been related to the relatively higher organic carbon content CRAMER,A., 1990. Seasonal variation in benthic metabolic activity in a frontal system in North Sea. In: M BARNES& in the sediment as a result of the prevailing hydroR.N. GIBSON.Proc. 24th Europ. Mar. Biol. Symp. Abergraphic conditions (CREUTZBERG et al., 1984; CREUTZdeen University Press: 54-76. BERG, 1985). However, the absence of long-term mud CREUTZBERG,F., 1985. A persistent chlorophyll-a maximum accumulation suggests that there is no continuous coinciding with an enriched benthic zone. In: P.E.GIBBS. accumulation of organic material in this area and that Proc. 19th Europ. Mar. Biol. Syrup., Cambridge Univ. the relatively higher supply of organic material during Press, Cambridge: 97-108. periods of calm weather (in spring and summer) is CREUTZBERG, F. & H. POSTMA, 1979. An experimental approach to the distribution of mud in the southern rapidly utilized by the benthos; this implies that eroNorth Sea.--Neth. J. Sea Res. 13:99-116. sion takes place in autumn and winter, removing much of the sediment. The seasonal variation CREUTZBERG F., P. WAPENAAR, G, DUINEVELD & N. LOPEZLOPEZ, 1984. Distribution and density of the benthic observed in the benthos (e.g. CRAMER, 1990; MOODfauna in the southern North Sea in relation to bottom LEY, 1990) implies that the response to environmental characteristics and hydrographic conditions.--Rapp. stimuli can be rapid. P.-v. Reun. Cons. perm. int. Explor. Mer 183:101-110. DE GEE,A., M.A. BAARS& H.W.VAN DER VEER, 1991. Ecologie 5. GENERAL CONCLUSIONS van her Friese Front (Ecology of the Frisian Front), NIOZ- Report 1991-2: 1-96. --1. Marine incursion at the Oyster Grounds started EISMA, D., W.G. MOOK& C. LABAN, 1981. An early Holocene tidal flat in the Southern Bight.--Spec. Pubis. int. Ass. after 8720 B.P. Sediment. 5: 229-237. --2. Subtidal conditions began ~6240 B.P. --3. Lateral transport is effective from the Channel to FOLKARD,A.R. & P.G.W.JONES, 1974. Distribution of nutrient salts in the southern North Sea during early 1974.the area studied. Mar. Pollut. Bull. 5: 181-185. --4. There is no significant Recent long-term mud GOLDSTEIN,S.T.& L. MOODLEY,1993. Gametogenesisand the accumulation in the Frisian Front area. life cycle of the foraminifer Ammonia beccarii (Linne) --5. No drastic changes are observed in the deposiforma tepida (Cushman).--J. Foram. Res., in press HALLOCK, P., 1985. Why are larger Foraminifera larger?-tional environment over the last 6240 years. Paleobiology 11 : 195-208. Acknowledgements.--We thank J.E. van Hinte (Free Uni- JARKE,J., 1956. Eine neue Bodenkarte der sedlichen Nordsee.--Dt, hydrogr. Z. 9: 1-9. versity, Amsterdam) and G.C. Cadee for critically reviewing the manuscript and for important suggestions. We are JELGERSMA, S., 1961. Holocene sea level changes in the Netherlands.--Meded. Geol. Sticht., C, VI, 7:1-100. greatly indebted to L. Otto for important discussions and C. Laban (NetherlandsGeological Survey) for providing unpub- - - , 1979. Sea-leve! changes in the North Sea basin. In: E. OELE,R.T.E.SCHUTTENHELM& A.J.WlGGERS.The Quaterlished data. Comments by two unknown referees are gratenary history of the North Sea.--Acta Univ. Ups. Symp. fully acknowledged. D. Eisma and G.J. van Noort are Univ. Ups. Ann. Quing. Cel. 2: 234-248. acknowledged for their interest and supplying important literature. K. Booij kindly did the sampling. The authors also JELGERSMA, S, E. OELE & A.J. WlGGERS, 1979. Depositional History and coastal development in the Netherlands wish to thank H.T. Kloosterhuisfor organic-carbon measureand the adjacent North Sea since the Eemian. In: E. ments, A.J.M Gieles for grain-size analysis, S.A. NederOELE,R.T.E.SCHUTTENHELM& A.J.WIGGERS.The Quaterbragt (Free University, Amsterdam) and G.J.A. Brummer for

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nary history of the North Sea.--Acta Univ. Ups. Symp. Univ. Ups. Ann. Quing. Cel. 2: 115-142. JENKINS, D.G. & J.W. MURRAY, 1989. Stratigraphical atlas of fossil foraminifera. Ellis Horwood Limited, Chichester: 1-593. JORRISEN,J.F., 1988. Benthic foraminifera from the Adriatic Sea: principles of phenotypic variation.--Utrecht Micropaleontol. Bull. 37: 1-174. KIRBY,R. & E. OELE,1975. The geological history of the Sandettie-Fairy Bank area, southern North Sea.--Phil. Trans. R. Soc. Lond. (A) 279: 257-267. KOLP, 0., 1974. Submarine Uferterrassen in der s~dlichen Ost- und Nordsee als Marken eines stufenweise erfolgten Meeresspiegelanstieges.--Baltica5:11-40. - - , 1976. Submarine Uferterrassen in der s~dlichen Ostund Nordsee als Marken des Holoz&nen Meeresanstieges und der Oberflutungsphasen der Ostsee.-Petermanns Geogr. Mitt. 120: 1-23. LAGAAU,R., 1968. Fossil Bryozoa reveal long-distance sand transport along the Dutch coast.--Proc. K. Ned. Akad. Wet., Ser. B 71 : 31-50. MOODLEY,L., 1990. Southern North Sea seafloor and subsurface distribution of living benthic foraminifera.--Neth. J. Sea Res. 27: 57-71. MURRAY,J.W., 1971. An atlas of British Recent foraminiferids. Heinemann Educational Books, London: 1-244. , 1991a. Ecology and palaeoecology of benthic foraminifera. Longman Scientific & Technical, London: 1-397. , 1991b. Ecology and distribution of benthic foraminifera. In: J.J. LEE & O.R. ANDERSON.Biology of foraminifera. Academic Press, London, New York: 221-253. ,1992. Distribution and population dynamics of benthic foraminifera from the southern North Sea.--J. Foram. Res. 22: 114-128. OELE,E., 1969. The Quaternary geology of the Dutch part of the North Sea, north of the Frisian Isles.--Geol. Mijnbouw 48: 467-480. , 1971a. Late Quaternary geology of the North Sea south-east of the Dogger Bank.-- Inst. Geol. Sci. Rep. 70/1,5: 25-34. ,1971b. The Quaternary geology of the southern area of the Dutch part of the North Sea.--Geol. Mijnb. 50: 461-474. OTTO, L., J.T.E ZIMMERMAN,G.K. FURNES,M. MORK,R. SAETRE& G. 8ECKER, 1990. Review of the physical oceanography of the North Sea.--Neth. J. Sea Res. 26:161-238. POSTMA, H., 1978. The nutrient contents of the North Sea water, changes in recent years, particularly in the Southern Bight.--Rapp. P.-v. Reun. Cons. perm. int. Explor. Mer 172: 350-357. SANDERS, H.L., 1960. Benthic studies in Buzzards Bay, Ill, the structure of the soft-bottom community.-- Limnol. Oceanogr. 5:138-153.

SCHNITKER,D., 1974. Ecotypic variation in Ammonia beccarii (Linne).--J. Foram. Res. 4: 216-223. SCHOTTENHELM,R.T.E., 1980. The superficial geology of the Dutch sector of the North Sea.--Mar. Geol. :34: M27M37. STREIE H., 1989. Barrier islands, tidal flats, and coastal marshes resulting from a relative rise of sea level in East Frisia on the German North Sea coast. In: Proc. KNGMG Symp. ' Coastal Lowlands, Geology and Geotechnology'. Kluwer Academic Publishers, Dordrecht, The Netherlands: 213-223. VAN BENNEKOM,A.J., W,W.C. GIESKES & S.B. TIJSSEN, 1975. Eutrophication of the Dutch coastal waters.-- Proc. Roy. Soc. (B) 189: 359-374. VAN DER BORG,K., C. ALDERLIESTEN,C.M. HOUSTON,A.F.M. DE JONG & N.A. VAN ZWOL, 1987. Acceleration Mass Spectrometry with 14C and 1°Be in Utrecht.--Nuclear Instr. and Methods B29: 143. VAN DER VEER, H.W., W. VAN RAAPHORST& M.J.N. 8ERGMAN, 1990. Eutrophication of the Dutch Wadden Sea: external nutrient Ioadings of the Marsdiep and Vliestroom basin.--Helgol&nder Meeresunters. 43: 501-516. VAN HAREN,J.J.M. & J.C.A. JOORDENS,1990. Observations of physical and biological parameters at the transition between the southern and central North Sea.--Neth. J. Sea Res. 25:351-364. VAN RAAPHORST,W,, H.T.KLOOSTERHUIS,E.M.BERGHUlS,A.J.M. GIELES,J.F.P.MALSCHAERT& G.J. VAN NOORT,1992. Nitrogen cycling in two types of sediments of the southern North Sea (Frisian Front, Broad Fourteens): field data and mesocosm results.--Neth. J. Sea Res. 28: 293316. VAN VOORTHUYSEN,J.H., 1951. Recent (and derived Upper Cretaeous) foraminifera of the Netherlands Wadden Sea (tidal flats).--Med. Geol. Stichting, N. S. 5: 23-37. - - . , 1960. Die Foraminiferen des Dollart-Ems-Estuarium.--Verh. K. Geol.-Mijnb. Gen. Geol. Ser. 19: 237270. VI~NEC-PEYR(:, M.T., 1983. I~tude de la croissance et de la variabilite chez un foraminif~re benthique littoral, Ammonia beccarii (Linne), en mediterranee occidentale.--Cah, micropal~ontol. 2: 5-31. VERARDO, D.J., PN. FROELICH& A. MCINTYRE, 1990. Determination of organic carbon and nitrogen in sediments using the Carlo Erba Na-1500 analyser.--Deep-Sea Res. 37:157-165. WALTON,W.R. & 8.J. SLOAN, 1990. The genus Ammonia Brennich, 1772: its geographic distribution and morphologic variability.--J. Foram. Res. 20: 128-156. ZUO, Z., D. EISMA& G.W. BERGER,1989. Recent sediment deposition rates in the Oyster Grounds.--Neth. J. Sea Res. 23: 263-269. (received 2 September 1992; accepted 21 January 1993)