The relationship between ostracod distribution and sediment grain size

Marine Micropaleontology, 8 (1983/84) 51--63

51

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

THE RELATIONSHIP BETWEEN OSTRACOD DISTRIBUTION AND SEDIMENT GRAIN SIZE

DENNIS BARKER Middlesex Polytechnic, Queensway, Enfield (Great Britain) (Received November 17, 1981; revised version accepted January 31, 1983)

Abstract Barker, D., 1983. The relation between ostracod distribution and sediment grain size. Mar. Micropaleontol., 8: 51--63. Sediment samples from an area seaward o f the River Crouch in the Thames estuary have been analysed for their particle size distribution and later picked over for ostracods. The sediments were classified into sediment types and three depositional sub~nvironments were recognised, viz. Intertidal Flat, Sandbank and Tidal Channel. Study of the ostracods showed that three distinct faunas characterise the same three environments. Log grain size/probability plots have been used to show a relationship between ostracod distribution and sediment grain size.

Introduction

The top sediment layer of an area seaward of the River Crouch in the Thames estuary was sampled on a grid" of 1 km squares (Fig. 1). The area sampled is occupied by two major channels, the Whittaker Channel and the Ray Sand Channel (Fig. 2). The Whittaker Channel is the seaward extension of the River Crouch. It is bounded by Ray Sand and Buxey Sand to the north and by Foulness Sand to the south. These sandbanks are characteristic of the Thames estuary and are highly mobile in the short term. However, they are persistent geographically as they have remained in approximately the same place for centuries. Between Buxey Sand and the Intertidal Flats lies the Ray Sand Channel. Comparing the 1956 and 1968 Admiralty Charts it would appear that this channel is silting up. Landward of the Intertidal Flats is a great acreage of salt marsh broken by m a n y creeks.

0377-8398/83/$03.00

Also sampled was the East Swin, a channel with deeper water than the other channels in the area studied. All the samples were analysed for their particle size distribution by sieving all the material retained on a BS.200 sieve {3.75 phi). The sedimentation method incorporating the bottom withdrawal of samples from a tube was then applied to the fines {Office of Indian Affairs, et al., 1943). Owing to the number of analyses which had to be undertaken in the time available for the contract, 0.50 phi fractions rather than the more desirable 0.25 phi fractions were used to separate the samples by grain size. The grain-size parameters median, mean, standard deviation, skewness and kurtosis were plotted and contoured. The positions of the samples are shown by the grid references on Figs. 2, 3. In order to establish the reliability of the sampling, 18 stations out of the 146 were double resampled.

© 1983 Elsevier Science Publishers B.V.

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Samples containing less than 5% gravel are separated into the following categories on a percentage sand basis. Sands: samples containing >75% sand. Silty sands: samples containing 45--75% sand. Sandy silts: samples containing 20--45% sand. Clayey silts: samples containing < 20% sand. The gravel size material is mainly shell and comminuted shell, hence the following gravel size terms were adopted: 5-.-30% gravel -- shell gravelly, >30% gravel --shell gravel, >30% gravel -- lithic shell gravel. The latter category includes shell gravel samples containing flint pebbles. The sediment type distribution map (Fig. 3) is based on the various sand percentages indicating the sediment types and the areas

were obtained from a contour map of percentage sand. The sand areas were then subdivided according to their degree of "coarseness" based on central tendency values. Hence descriptions such as "very fine sand" could be assigned based on the Wentworth 1922 scale. The heavy mineral separation was made using b r o m o f o r m with the 2.0 phi and 3.75 phi fractions only. The determination of soluble carbonate was obtained by removing the material from the mud and sand fractions with dilute hydrochloric acid (Molna, 1974). The carbonate in the gravel and coarse sand was removed by hand picking. Three modes of sediment transport, surface creep, saltation and suspension, can be identified by the analysis of grain-size distribution curves (Visher, 1969). Such analysis is based on being able to recognise three sub-populations within the grain-size distribution. These sub-populations are best separated by plotting

54

log of grain size (phi) against cumulative frequency percent on a probability scale. Such grain-size distribution plots are composed of several sub-populations with different means and standard deviations. The proportion of each sub-population is related to the relative importance of each transport process, i.e. creep, saltation and suspension. The distinctive feature of such a grain-size plot is that it normally has two or three straight line segments and the tails of the " s " shaped curves produced tend to be straight lines (Fig. 4). These features make it easy to compare samples and make measurements.

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The breaks between the various subpopulations are called truncation points and the positions of these are important in the interpretation of environments of deposition. The steepness of the slopes indicates sorting, and the dominance of populations is indicated by the lengths of the slopes. Computer printed plots were produced for all the samples studied. Since the project was n o t planned from an ecological view point, the study of ostracods was a secondary consideration and took place after the sedimentological work had been completed. Dry samples from the 1.5 phi and

2.0 phi fractions were picked over and the ostracods sorted and m o u n t e d on slides. Since the study is based on a single collection at each station, there may be some seasonal variations which are not apparent from the present study.

The sub-environments

Intertidal Flats This sub-environment extends seawards from the degraded salt marsh to the Ray Sand Channel. Intertidal Flats have silty sand, sandy silt or clayey silt exposed to the atmosphere at some time during the tidal cycle and are ripple marked in places. The area adjacent to the degraded salt marsh has a mud mound topography with large drainage channels seawards. Shell flats are prevalent over the intertidal area particularly just seaward of the salt marsh and further off shore between the larger drainage channels. The samples from this area contain subangular quartz grains, forams, shell fragments, mica flakes, sponge spicules, dark opaque minerals and coal fragments. Sediments from the Intertidal Flats have from 9--15% total soluble carbonate. Intertidal Flat sediments are characterised by a relatively high percentage of the suspension population which is poorly sorted. The saltation population is well sorted and the proportion increases with the sandier sediments. The creep population is negligible and normally consists of shell fragments and/or organic matter. See Fig. 5 and Table III. Sandbanks The positions of the Sandbanks are shown in Fig. 2. Their general positions have persisted for centuries but each bank is highly mobile thus requiring frequent resurveying. They can be exposed at low tide. The sand size of most sandbank samples falls within the very fine sand range. Sandbank sediments are composed of

55 particles of transparent subangular quartz grains, iron-stained quartz grains, dark opaque minerals, shell fragments, mica flakes and coal fragments. There is some rounding of the quartz grains (from 1.0 phi to 2.5 phi). Bachelors Sand has a more restricted variety, subangular quartz grains both transparent and iron stained, and shell fragments. Buxey Sand has about 8% total soluble carbonate, Foulness Sand 9--14% and Bachelors Sand 20%. Sandbank sediments are characterised by a well-sorted saltation population with a narrow size range. The saltation population forms more than 80% of the total population whereas less than 10% of the total population is a poorly sorted suspension population. The creep population is negligible except for Bachelors Sand in which it is significant and predominantly shell (see Fig. 5 and Table III).

quartz grains, forams, and dark opaque minerals as minor constituents of the sediments. The percentage total soluble carbonate varies from channel to channel. The Outer Channel and the Ray Sand Channel have from 8 to 13% total soluble carbonate and the Whittaker Channel 9--15%. The silty sands from the channels have approximately the same ranges of truncation points and percentages of each sub-population as the Intertidal Flat silty sands. However, the range of the suspension population is wider and can compose up to 30% of the total distribution. The creep population is significant and dominantly shell. The channel sands have more variable percentages of suspension and creep populations than the sand bank sands. The saltation population forms a very high proportion of the total distribution -- 90% or more in some cases (see Fig. 5 and Table III).

Tidal Channels The ostracods The channels shown in Fig. 2 vary in sediment types from silty sand with occasional gravels, through very fine sand to fine sand. The depth of the channel varies from the shallow Ray Sand Channel ( > 2 m) to the deep East Swin (> 12 m ) . . Each channel appears to have a characteristic particle content. The Outer Channel sediment is composed of transparent subangular quartz grains, iron stained quartz, dark opaque minerals, shell fragments, mica flakes, coal fragments. East Swin sediments are predominantly transparent subangular quartz grains. From 1.5 phi to 2.5 phi, rounded and iron-stained quartz grains are also found. Shell fragments, forams and ostracods are also present. The Ray Sand Channel sediments are predominantly transparent subangular quartz grains with forams and both whole and fragmentary shells. Whittaker Channel sediments have a high shell content, both whole and comminuted. Transparent subangular quartz grains predominate, with faecal pellets, mica flakes, coal fragments, rounded and iron-stained

The names used for recent ostracods are normally based on identification of soft parts. However, since dried samples were used, identification is based on the shell structure only as is the case with fossil ostracods. The following species were recovered from the samples studied: Loxoconcha gu ttata (Norman) Brady et al., 1874 Carinocy thereis antiq uata (Baird) Brady et al., 1874 Robertsonites tuberculata (Sars) Sars, 1928 Pontocythere elongata (Brady) Brady, 1868 Loxoconcha elliptica (Brady) Brady, 1868 Hemicythere villosa (Sars) Sars, 1866 Leptothere pellucida (Baird) Baird, 1850 ElofsoneUa concinna (Sars) Sars, 1928 Loxoconcha rhomboidea (Fischer) Fischer, 1855 Heterocythereis albomaculata (Baird) Baird, 1838 Paradoxostoma ensiforme (Brady) Brady, 1868 Eucythere declivis (Norman) Norman, 1865

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59 Cytheropteron latissimum (Norman) Norman, 1865 Cytheropteron p u n c t a t u m (Brady) Brady, 1868 Sars, 1866 Leptocythere castanea (Sars) Cythere lutea (Muller) Muller, 1785 Urocy thereis britannica (Athersuch) Athersuch, 1977 Kilenyi (1969) was able to recognise five stages in the preservation of ostracods dredged from the Thames estuary: 1. Carapace with soft parts 2. Carapace or valve with appendages or traces of cuticle 3. Transparent carapaces or valves 4. Opaque (white, yellow or brown) carapaces 5. Opaque valves filled with various matrices

In the Thames estuary, faunas from distinct ecological environments can be easily displaced and mixed by the strong tidal currents. Therefore, in order to make some sense of the ostracods from the Crouch entrance, identifications were based on stages 1, 2 and 3 only. Due to the rapid transport which occurs in the Thames estuary a biocoenose could quite easily be a thanatocoenose which has not had time to change from transparent to opaque valves and carapaces. In spite of any mixing of faunas and also the difficulty of identifying living populations, the ostracods have been related to the sedimentary environments shown in Table I. A cursory inspection of the faunal lists shows some mixing may have occurred. Table I! shows that ostracods such as

TABLE I To show the sub-environments in which the s p e c i e s

occur

Channel

Tidal Sandbank Flat

•

Loxoconeha guttata Carinocythereis antiquata Robertsonites tuberculata Pontocythere elongata Loxoconcha elliptica Hemicythere villosa Leptocy there pellucida ElofsoneUa coneinna Loxoconcha rhomboidea Heterocythereis albomaculata Paradoxostoma ensiforme Eucythere declivis Cytheropteron latissimum Cytheropteron punctatum Leptocythere castanea Cythere lutes Urocythereis britannica

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60 TABLE II A comparison of the ostraeods from the Crouch entrance with the ecology indicated by the same oatracods in other localities* Algae

L o x o c o n c h a gu ttata Carinocythereis a n t i q u a t a R o b e r t s o n i t e s tuberculata P o n t o c y t h e r e elongata L o x o c o n c h a elliptica H e m i c y t h e r e villosa L e p t o c y t h e r e pellucida Elofsonella c o n c i n n a Loxoconcha rhomboidea Heterocythereis albomaculata P a r a d o x o s t o m a ensiforrne E u c y t h e r e declivis Cytheropteron latissimum Cytheropteron punctatum L e p t o c y t h e r e castanea C y t h e r e lutea U r o c y t h e r e i s britannica

Sand

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*~ The samples of Whatley and Wall were collected by grab and dredge and by hand collection of Laminaria in Cardigan Bay. Hemicythere villosa, Leptocythere pellucida, Loxconcha rhomboidea, Cytheropteron latissimum and C y t h e r e l u t e a can be f o u n d alive

in a variety o f e n v i r o n m e n t s . O t h e r s such as and P a r a d o x be associated with plants and plant debris as well as sand. Thus the vegetation c o n t e n t o f sediments as well as grain size m a y be a f a c t o r in locating a particular species. Kilenyi ( 1 9 6 9 ) f o u n d t h a t t h e r e was a m a j o r d i f f e r e n t i a t i o n b e t w e e n the channels and the s a n d b a n k s and flats. T h a n a t o c o e n o s e in faunas were m o r e p r o m i n e n t in the Channels even u p to 100% whereas on the

Heterocythereis albomaculata ostoma ensiforme appear to

S a n d b a n k s and Flats b i o c o e n o s e was dominant. Kilenyi ( 1 9 7 1 ) suggests t h a t in a highenergy tidal estuary like the T h a m e s , t h r e e basic t y p e s o f post m o r t e m d i s p l a c e m e n t exist: (a) t r a n s p o r t in suspension and mainly associated with limnic/oligohaline ostracods; (b) t r a n s p o r t within s e d i m e n t which is the usual m e t h o d ; (c) t r a n s p o r t by floating seaweeds which he believes is n o t o f great importance. T h e ostracods o f t h e C r o u c h e n t r a n c e are characteristic o f t r a n s p o r t within sediments since t h e y comprise part o f the c r e e p population (1--2 phi) usually very close t o the

61

TABLE

III

Results of sub-population analysis (146 samples)

Environment

Range of truncation Points (phi units)

Range of percentage of each sub-population

suspension/ saltation

saltation/ creep

suspension

saltation

creep

Intertidal Flat Sandbanl¢~

3.80--4.50 3.10--4.25

2.90--3.45 1.10---2.55

Channels

3.00--4.60

1.40 --2.90

9.80--53.00 2.30 -9.20 1.60---28.00

44.3--90.00 77.7--95.00 6.00--98.00

0.00--2.70 0.00--20.00 0.00--53.00

truncation point with saltation. The significance of this may be that a slight increase in velocity of the currents would pick up the shells as a saltation load and redistribute them. An examination of the information from the Intertidal Flats (Tables I, III) shows the saltation/creep truncation points to be from 2.90 to 3.45 phi. These are some way from the grain sizes for ostracods (1--2 phi). Thus the ostracods are well within the limits of the grain size for the creep population and are unlikely to be picked up into the saltation population. The fauna Loxoconcha guttata,

Carinocythereis antiquata, Robertsonites tuberculata, Leptocythere custanea and Cythere lutea therefore might be characteristic of such an environment. The Sandbanks (Tables I, III) have the saltation/creep truncation points in the range 1 . 1 0 - 2 . 5 5 phi which includes part of the size range of ostracods. One would expect, therefore, that carapaces and valves would be taken up into the saltation population. The size distribution is well sorted with 77.7--95% saltation population. This indicates a highly mobile sand-size population. Any light material such as ostracods would be easily winnowed out and redeposited elsewhere when the tide retreats off the sandbank. The fauna of Loxoconcha guttata, Carinocythereis antiquata, Pontocythere elongata and Urocythereis britannica is not large in numbers or diversity. These banks are believed to be strongly mobile but Urocythereis contain soft parts and are obviously living on them. The Channels (Tables I, III) have the salta-

tion/creep truncation points within the range 1.40--2.90 phi which includes part of the size range of ostracod. Thus one would expect carapaces and valves to be taken up into the saltation population. The degree of sorting is variable indicating that ostracods could be picked up from time to time but redeposited when energy reduces. Ostracods could also be deposited here from elsewhere. The ostracods are c o m m o n and diverse, e.g.

Loxoconcha guttata, Carinocythereis antiquata, Robertsonites tuberculata, Pontocypris elongata, Loxoconcha impressa, Hemicythere villosa, Leptocythere pellucida, Elofsonella concinna. This fauna is similar to biofacies III and IV of Kilenyi {1969}. Since this is a detailed examination of a small part of the estuary, a different zonation from Kilenyi (1969) might be expected. He divides the whole of the Thames estuary into seven major biofacies: I. From London Bridge to Cliff; II. From Cliff to Canvey Island; III. From Canvey Island to the inner estuary; IV. The greater part of the outer estuary; V. Area off Margate and Herne Bay; VI. Extreme eastern area of the estuary; VII. The Blackwater estuary and Wallet. The three environments, i.e. Intertidal Flats, Sandbanks and Channels, of the Crouch entrance are included in Kilenyi's biofacies IV. Discussion The faunas characteristic of each environment can be interpreted in a number of ways. Some will say the faunas show evidence of

62 mixing, i.e. species being brought in from littoral and inner shelf areas. However, the condition of the shells and the rare presence of soft parts for most of the species indicate they were live on collection. The faunal lists provided by Kilenyi (1969) indicate that most of these species live in the estuary. An interesting feature is the variation in the number of species in each of the environments indicated. Intertidal Flats have few species and the absence of Pontocypris elongata whereas the Sandbanks have few species and are characterised by Pontocypris elongata. The channels have a larger number of species compared to the other two environments. Since the sampling was not carried out for ecological purposes many parameters of interest in such studies have not been collected. However, the evidence presented earlier for the relationship between ostracod distribution and sediment grain sizes is also reflected in articles by a number of authors. For example, Kilenyi {1969) discussed the distribution of ostracods in the Thames estuary under two groups of factors. (a) Prohibiting factors such as fluid mud, polluting discharges, black mud and the nature of the sediment. These are believed to determine the distribution of the ostracod fauna as a whole and the abundance or lack of individuals. Some of these factors can be due to man's interference with the environment. (b) Factors influencing the composition of the fauna such as the salinity, temperature and depth. The most important was his discussion of the nature of the sediment. He says that the relationship between bottom sediment and the abundance of ostracods is complex and leads to two lines of argument: (1) there is a critical current velocity; or (2) the ostracod has some ability to exist in or on particular substrata. Kilenyi states that the latter is more likely and cites the evidence of Wieser (1959), Williams {1969) and Kornicker (1958, 1961, 1964). Gray and Rieger (1971) made a quantita-

tire study of the meiofauna of an exposed beach at Robin Hoods Bay. The data were compared with results from other beaches in Europe and they concluded that there was a relationship between the meiofauna, grain size and stability of the sediment. Fine-grained sediments produce a richer fauna. Likewise, poor sorting resulting from greater stability produces a richer fauna and greater density of individuals. A complex of sand grade and sorting seems to control the richness and density of the meiofauna. Whereas, other factors such as salinity, temperature, oxygen, microvariations in grades of sediment, water flow, food, etc. will control local spatial and temporal variations. There is, therefore, accumulating evidence to indicate a relationship between ostracod distribution and sediment grain size. This is reflected in the faunas of the three subenvironments, Intertidal Flats, Sandbanks and Channels in the area studied. The use of log grain size/probability plots seems to indicate possibilities for the interpretation of facies in both recent and fossil sediments. It would be interesting to see them applied in a variety of situations.

Acknowledgements During 1973 Middlesex Polytechnic was contracted by the Department of the Environment (Hydraulics Research Station) to study the top bed sediment layer throughout the tidal reaches of the River Crouch, River Roach and an area seaward of the Crouch entrance. The Hydraulics Research Station, Wallingford kindly gave permission for this paper to be published. Dr. T.R.W. Hawkins was the leader of the team at Middlesex Polytechnic which worked on the original project. The team comprised Mr. K. Codd and Mrs. K. Adcock with technical help from Mr. J. Hutcheson, Mr. N. Sanderson and Mrs. J. Gibbs. Mr. K. Codd was responsible for the sedimentological study and his size analyses and log grain size/probability plots have been used with his permission. All concerned have been very helpful during this work.

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T h e a u t h o r w o u l d also like t o t h a n k the following f o r helpful discussions a n d criticism o f the various drafts o f t h e r e p o r t : Dr. R.H. Bate o f t h e British M u s e u m (Nat. Hist.), Dr. M.C. Keen o f the University o f Glasgow, and Dr. E. R o b i n s o n o f University College, London. T h a n k s are also due t o m e m b e r s o f Middlesex P o l y t e c h n i c w h o have h e l p e d t o m a k e this r e p o r t possible. References Athersuch, J., 1977. The genus Urocythereis (Crustacea: Ostracoda) in Europe, with particular reference to Recent Mediterranean species. Bull. Br. Mus. Nat. Hist. (Zool.), 32: 247--283. Baird, W., 1838. The natural history of the British Entomostraea. Mag. Zool. Bot., 1: 35-41, 309-333,514--526; 2: 132--144,400--412. Baird, W., 1850. The natural history of British Entomostraca. The Royal Society of London, 364 pp. Brady, G.S., 1868. Monograph of Recent British Ostracoda. Trans. Linn. Soc. Lond., 26: 353--495. Brady, G.S., Croaakey, H.W. and Robertson, D., 1874. A monograph of the post-Tertiary Entomostraea of Scotland. Pal. Soc. London. Fischer, S., 1855. Beitrag zur Kenntnis der Ostracoden. Abhandl. Math. Phys. KI. Bayer. Akad. Wiss., 7 : 635--666. Gray, J.S. and Rieger, R_M., 1971. A quantitative study of the meiofauna of an exposed sand beach, at Robin Hood's Bay, Yorkshire. J. Mar. Biol. Assoc. UK, 51: 1--19. Hawkins, T.R.W., Adcock, K~M. and Codd, K.C., 1975. Maplin Investigation. Sedimentation in the River Crouch. Hydraulics Research Station, Wallingford, Rep. EX660. Kilenyi, T.I., 1969. The problems of Ostracod ecology in the Thames Estuary. In: The Taxonomy, Morphology and Ecology of Recent Ostracoda, Oliver and Boyd, Edinburgh, pp. 251-265.

Kilenyi, T.I., 1971. Some basic questions in the palaeoecology of ostracods. Bull. Centre Rech. Pau-5NPA, 5: 31--44. Kornicker, L.S., 1958. Ecology and taxonomy of recent marine ostracods in the Bimini area, Great Bahama Bank. Inst. Mar. Sci. Publ. Univ. Texas, 5: 194--300. Kornicker, L.S., 1961. Ecology and taxonomy of recent Bairdiinae (Ostracoda). Micropaleontology, 7 : 55--70. Kornicker, L.S., 1964. Ecology of ostracoda in the northwestern part of the Great Bahama Bank. Publ. Staz. Zool. Napoli, 33: 345-360. Molna, B.F., 1974. A rapid and accurate method for the analysis of calcium carbonate in small samples. J. Sediment. Petrol., 44: 589--590. Muller, O.F., 1785. Entomostraea sen Insecta Testacea, quae i aquis Daniae et Norvegiae reperit. Lipsiae et Havniae, pp. 1--135. Norman, A_M., 1865. Report on the Crustacea (dredged off the coasta of Northumberland and Durham, 1862--64). Trans. Soc. Nat. Hist. Northumberland Durham, 1 : 12--29. Office of Indian Affairs, et al., 1943. A Study of New Methods for Size Analysis of Suspended Sediment Samples. Univ. of Iowa. Sars, G.O., 1866. Oversight of Norges Marine Ostracodes. Forh. Vid. Seisk. Christiania. Sars, G.O., 1928. An Account of the Crustacea of Norway VoI. IX, Ostracoda. Bergen Museum, Norway. Vishner, G.S., 1969. Grain size distributions and depositional processes. J. Sediment. Petrol., 39: 1074--1106. Whatley, R.C. and Wall, D.R., 1975. The relationship between Ostracoda and Algae in littoral and sublittoral marine environments. Bull. Am. Palaeontol., 65: 173--203. Wieser, W., 1959. The effect of grain size on the distribution of small invertebrates inhabiting the beaches of Puget Sand. Limnol. Oceanogr., 4: 181--194. Williams, R., 1969. Ecology of the Ostracoda from selected marine intertidal localities on the coast of Anglesey. In: The Taxonomy, Morphology and Ecology of Recent Ostracoda. Oliver and Boyd, Edinburgh, pp. 299--329.