Population age structure techniques and ostracods: Applications in coastal hydrodynamics and paleoenvironmental analysis

Palaeogeography, Palaeoclimatology, Palaeoecology 199 (2003) 51^69 www.elsevier.com/locate/palaeo

Population age structure techniques and ostracods: Applications in coastal hydrodynamics and paleoenvironmental analysis F. Ruiz a;
, M.L. Gonza¤lez-Regalado a , J.M. Mun‹oz b , J.G. Pendo¤n c , A. Rodr|¤guez-Ram|¤rez a , L. Ca¤ceres a , J. Rodr|¤guez Vidal a a b

Departamento de Geodina¤mica y Paleontolog|¤a, Universidad de Huelva, Avenida de las Fuerzas Armadas, s/n, 21071 Huelva, Spain Departamento de Estad|¤stica e Investigacio¤n Operativa, Universidad de Sevilla, Avenida Reina Mercedes, s/n, 41071 Sevilla, Spain c Departamento de Geolog|¤a, Universidad de Huelva, Avenida de las Fuerzas Armadas, s/n, 21071 Huelva, Spain Received 21 June 2002; accepted 2 June 2003

Abstract Ostracods are microcrustaceans that grow by moulting (eight to nine instars in most cases) and, consequently, studies of its populations need the application of some special techniques. The population age structure techniques [Whatley, in: Ostracoda in the Earth Sciences, 1988, pp. 245^256] are a simple statistical method to estimate paleoenvironmental conditions in fossil/Recent populations of ostracods. For its application, it is necessary to determine the percentages of each instar present in the samples studied, in order to analyze the different types of population age structure histograms and their (paleo-)environmental interpretations. Some potential applications are the (paleo-)energy levels or the sedimentation rates. In this paper, these special techniques are applied to Recent populations of the ostracod Pontocythere elongata collected in littoral sediments of southwestern Spain. In this area, the relative percentages of this species and the adult percentages are closely related with depth and consequently this species was used as a bathymetrical tracer in shallow Neogene areas of the Guadalquivir Basin. In addition, a first three-step multivariate analysis permits the definition of three groups, closely related to the theoretical models proposed by Whatley: (a) Group 1 structure or biocoenosis, present either in Recent low- to medium-energy environments located in erosional coastal stretches and Neogene fair-weather conditions; (b) Group 2 structure or high-energy thanatocoenosis, observed either in Recent river mouths with groynes and Neogene storm and post-storm conditions; and (c) Group 3 structure or low-energy thanatocoenosis, characteristic of Recent, progradational coastal areas and present in the Recent^Neogene deeper areas analyzed in this paper (30^40 m depth). This separation is mainly controlled by intrinsic factors (i.e. the natural growth of this species), whereas the extrinsic factors (i.e. the hydrodynamic levels) are only a minor cause of the distribution of this species in the area studied. B 2003 Elsevier B.V. All rights reserved. Keywords: ostracods; population age structures; Recent; Neogene; SW Spain

1. Introduction * Corresponding author. E-mail address: [email protected] (F. Ruiz).

The population age structure techniques are

0031-0182 / 03 / $ ^ see front matter B 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0031-0182(03)00485-1

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mainly based on simple statistical methods requiring the determination of the percentages of the di¡erent ostracod instars collected in each sample, the ratio between adult valves and carapaces and/ or the species diversity (Whatley, 1983, 1988). In recent decades, the application of these statistical techniques to ostracod populations has revealed a very interesting tool in the interpretation of (paleo-)environmental features, such as: (a) sedimentation rates (Pokorny, 1965); (b) petroleum sedimentology (Oertli, 1970); or (c) the hydrodynamic characterization of di¡erent coastal stretches, using either the total population (Brouwers, 1988) or by delimiting sample groups in a single species by multivariate statistical methods (Ruiz et al., 1998a). Most of these investigations have been undertaken using species of fresh-water or littoral environments. In these shallow marine areas, Pontocythere Dubowsky is a well known ostracod genus from the Eocene to the Quaternary (Van Morkhoven, 1963; Carbonel, 1985). Today, living species are widely distributed in littoral sediments of America (Hulings, 1967; Machain-Castillo et al., 1990), Europe (Wagner, 1957; Yassini, 1969; Carbonel, 1973; Athersuch et al., 1989; Pascual, 1990), Africa (Llano, 1981; Lachenal, 1989) and Asia (Bodergat and Ikeya, 1990; Taburi and Nohara, 1990). Along the Atlantic coasts of Europe and North Africa, this genus is represented by the species Pontocythere elongata (Brady), although it may be confused with the Mediterranean species Pontocythere turbida (Mu«ller) (Fig. 1). The main di¡erences between these species have been described by Bonaduce et al. (1975) and Athersuch (1982). The biocoenoses of P. elongata are observed in sandy sediments collected in the outer estuaries and coastal areas (depth down to 30 m in most cases) from Skagerrak to Morocco, although reworked specimens are cited in lagoons and even near the slope (Wagner, 1964; Llano, 1981; Chait et al., 1998). The ostracod assemblage is usually composed of P. elongata, Urocythereis britannica Athersuch, Aurila convexa (Baird) and sometimes Heterocythereis albomaculata (Baird), which may tolerate the wide ranges of salinity (15^35x) and temperature (5^28‡C) present in these £uctuating

environments (Carbonel, 1980; Rodr|¤guez La¤zaro and Pascual, 1985; Ruiz et al., 2000a). In the Huelva province (SW Spain), this species has been found in: (a) Recent ¢ne to very ¢ne sands collected in the inner shelf, at depths down to 20 m (Ruiz et al., 1997); and (b) Zanclean (Early Pliocene) silty sands of the southwestern Guadalquivir Basin (Ruiz and Gonza¤lezRegalado, 1996). The aim of this paper is the population analysis of this species in this sector of the Ca¤diz Gulf, evaluating its potential as a sedimentary^hydrodynamic tracer through multivariate statistics. Results are applied to the paleoenvironmental analysis of the Huelva Sand Formation (Civis et al., 1987), a Neogene stratigraphic unit of the Guadalquivir Basin.

Fig. 1. Distribution of Pontocythere in Western Europe and North Africa.

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Fig. 2. (A) Main hydrodynamic features of the Huelva littoral. (B) Location of the Recent samples.

2. The Huelva littoral The Huelva littoral corresponds to a typical example of siliciclastic sedimentation. In this zone of the Ca¤diz Gulf, the shelf is relatively wide (50 km average width), with bottom sediments constituted by ¢ne to very ¢ne sands in the innermost zone ( 6 20 m depth) and silty clays in the deeper areas (IGME, 1974; Ruiz et al., 1997). The transport processes of these sediments are controlled by four main factors: littoral drift currents induced by the Atlantic surface waters, presence of arti¢cial groynes, £uvial terrigenous sources and tidal £uxes. The coastal drift currents are derived from the North Atlantic circulation and have the greatest e¡ects on the sediment redistribution of this litto-

ral. The net sediment £ow is toward the east, with a high annual transport of sediment (180 000^ 300 000 m3 ) in this direction (CEEPYC, 1979; Cuena, 1991). This natural circulation is interrupted by modern protective groynes, causing either total (Ayamonte, Huelva) or partial (Isla Cristina, Punta Umbr|¤a) obstacles to the sedimentary £uxes. Consequently, two sedimentary units (Fig. 2A, Ayamonte^Punta Umbr|¤a and Mazago¤n^Matalascan‹as) may be delimited in this human-impacted area, directly related to the e¡ects of the main groynes (Ojeda, 1989). Each unit includes three di¡erent zones: (a) beaches a¡ected by erosive processes (Isla Cristina, La Antilla, Punta Umbr|¤a, Mazago¤n, Matalascan‹as); (b) intermediate prograding areas (i.e. El Asperillo) ; and (c) sandy spits (i.e. El Rompido).

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Fig. 3. (A) Stratigraphical synthesis of the Southwestern Guadalquivir Basin. (B) Location and stratigraphy of the Neogene sections studied.

The Guadiana river is the main terrigenous sediment source of the Huelva littoral, with a mean water discharge of 144 m3 s31 . The remaining rivers (Piedras, Tinto and Odiel) have very limited £ows, with low importance in the sedimentary dynamics of the southwestern Spanish coast. In addition, the sediment transport capacity of these rivers has decreased since 1960, with the construction of 65 dams regulating over 75% of the £ow (Ojeda, 1988). Tidal £uxes have important di¡erences between the open sea and the estuarine domains. In the inner shelf, these cyclical currents come from the southeast, with a mean velocity up to 0.65 m s31 . In the estuarine mouths, there is considerable reduction (mean velocity 0.26 m s31 in the Guadiana river) due to the bottom friction (Morales, 1993).

3. Neogene formations from southwestern Spain In the western sector of the Guadalquivir Basin, four Neogene formations are de¢ned, lying unconformably on a Paleozoic^Mesozoic substrate (Fig. 3A): (a) Niebla Formation (Baceta and Pendo¤n, 1999). This unit consists of continental and shallow marine deposits (£uvial conglomerates, littoral sands and beach sandy calcarenites) with a very variable thickness (0^25 m). The paleontological record of these coastal deposits is composed of bivalves (Crassostrea, Chlamys, Pecten), echinoderms (Clypeaster, Echinolampas), red algae (Melobesidae) and nummulitids (Heterostegina). The ostracod association collected in the littoral sands is abundant and diverse ( s 50 individuals/ 100 g), being composed of Aurila zbyszewskii Nas-

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cimento, Cytheretta orthezensis Moyes, Cytheretta simplex Moyes, Falunia spp., Senesia triangularis postriangularis (Oertli) and Xestoleberis paisi Nascimento. This association indicates a Tortonian age sensu latu (Ruiz et al., 2001) for these basal deposits. The upper limit is characterized by a discontinuity with subaerial exposure features. (b) Gibraleo¤n Clay Formation (Civis et al., 1987). This formation comprises a very monotonous lithofacies consisting of gray-blue marls and clays, only interrupted by the presence of a condensed, silty glauconitic layer near the base. Its thickness varies from 20 m (near Bonares) to more than 1000 m below the Ca¤diz Gulf, as it pinches out southeastward. These ¢ne sediments are extremely rich in foraminifera and calcareous nannoplankton, which indicate an Upper Tortonian to Zanclean age (Sierro, 1985) and the presence of upper bathyal (near the base) to circalittoral (near the top) paleoenvironments during this period (Gonza¤lez-Regalado and Ruiz Mun‹oz, 1990). Bivalves (Amussium, Palliollum), echinoderms (Schizaster) and condrychtians (Carcharocles, Isurus) are also frequent. Ostracods are poorly represented in the lowermost part of this Formation, with scarce individuals of Krithe spp., Parakrithe spp., Costa tricostata (Reuss) and Henryhowella asperrima (Reuss). Toward the top, this association is replaced by Acanthocythereis hystrix (Reuss), Cytherella vulgata Ruggieri, Pterigocythereis jonesii (Baird) and Ruggieria tetraptera (Seguenza) (Ruiz and Gonza¤lez-Regalado, 1996). (c) Huelva Sand Formation (Civis et al., 1987). The overlying deposits are composed of sandy silts enclosing a glauconitic layer at 3^4 m from the base. This distinctive bed is utilized for regional correlation and presents a rich fauna of selachians (Ruiz et al., 1998b). In the upper part, this formation consists of several lumachellic layers of mollusc shells interbedded with massive, bioturbated levels. These accumulations have been attributed to the action of storms in shallow to very shallow waters ( 6 40 m depth; Gonza¤lez Delgado et al., 1995). The ostracod diversity is high (see Fig. 8), with rare specimens of P. elongata (Ruiz and Gonza¤lez-Regalado, 1996). (d) Bonares Sand Formation (Mayoral and Pendo¤n, 1986). The lowermost sediments are

55

formed by silty, ¢ne^medium sands with some mollusc-rich beds (Tellinidae, Veneridae) and abundant bioturbation (Mayoral, 1986). Near the top, this facies is replaced by conglomerate beds showing cross-strati¢cation with moderate to poor sorting. These sediments indicate the ¢nal transition from marine to continental paleoenvironments.

4. Material and methods 4.1. Recent sediments The total ostracod population was examined from samples collected at 67 stations located in the Huelva littoral, at depths down to 20 m (Fig. 2B). The surface sediments (500^1000 cm3 ) were obtained with a modi¢ed Van Veen grab, determining the grain-size by standard sieve analysis (Table 1). A ¢xed quantity of dried sediments (200 g) was wet-sieved (500, 250 and 125 Wm mesh) and dried in an oven at 70‡C for the ostracod analysis. If possible, 100 individuals of each fraction were picked from each sample and the count was then recalculated to yield the total number of ostracods in the whole sample (cf. Whatley and Watson, 1988). In addition, six new samples (68^73) were obtained in order to establish an approximation to the bathymetrical distribution of Pontocythere elongata in this sector of the Ca¤diz Gulf. Additional information about the ostracod associations of this area may be consulted in Ruiz et al. (1997, 2000b). The total population of this species was separated, counted and measured. The biocoenoses were separated using a mixture of Rose Bengal (1 g/l) and alcohol. Nevertheless, the rare live individuals observed in most samples ( 6 10) impede a rigorous statistical analysis and, consequently, this paper is centered on the whole population of each sample. The biometric dimensions (length, height) of the di¡erent moults were determined, a simple procedure that determines the percentages of both juvenile instars and adults in each sample. This ontogenetic distribution has been tentatively related to: (a) the percentages of each grain size;

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Table 1 Grain-size analysis (wt%) of the Recent samples Sample

GR

VCS

CS

MS

FS

VFS

S+C

Depth (m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52

1.42 0.03 1.3 0.5 0.43 0.18 1 6.57 4.83 7.36 0.06 0 0.54 2.12 29.33 58.76 0.19 0.1 0.03 0.07 23.11 2.72 0.1 0.63 0.02 16.19 3.88 0.6 2.88 2.81 0.2 2.7 0.24 0.05 0.65 9.69 0.16 42.86 8.49 4.06 0.51 0.6 1.64 0.97 0.16 0.9 1.71 2.68 2.57 7.82 0 3.76

5.15 0.14 3.4 0.98 1.21 0.82 0.3 3.72 3.27 18.51 0.27 0.28 0.51 3.84 13.34 8.15 0.25 0.4 0.05 0.09 5.49 7.61 0.13 0.44 0.15 16.14 6.05 0.2 1.78 3.03 0.2 1.24 0.2 0.59 1.35 6.75 0.32 8.56 4.99 5.12 0.65 0.38 1.3 1.14 0.46 0.51 1.04 2.15 1.91 4.69 0.21 2.63

36.88 2.61 39.73 17.95 6.84 4.78 0.5 5.28 2.33 44.04 3.36 0.96 1.05 11.95 24.53 8.16 1.73 1.5 0.13 0.4 4.87 39.54 0.2 0.91 0.3 20.26 36.2 1.5 11.96 26.1 0.8 2.63 0.49 1.08 4.99 12.32 7.15 16.4 22.47 11.96 2.19 1.28 2.2 3.05 2.18 2.35 3.51 1.41 2.66 4.17 1.9 3.49

40.88 85.93 51.93 77.45 58.59 43.58 1.6 7.64 3.92 23.74 35.4 3.08 4.35 40.66 28.57 6.29 7.74 6.2 1.43 3.49 5.34 41.16 0.9 1.96 4.37 40.44 49.37 7.1 51.52 57.8 5.6 13.5 3.4 1.22 15.76 31.28 34.32 29.01 60.15 38.71 67.36 2.68 13.4 15.86 20.17 15.51 12.63 4.64 11.39 10.2 8.46 9.72

13.69 10.99 3.5 3.01 30.08 44.84 5.5 6.2 8 4.38 54.86 26.58 44.84 26.5 2.67 8.89 30.65 27 34 51.29 14.6 7.89 39.14 57.58 70.62 5.81 3.94 6 13.72 8.35 76.6 54.97 26.22 3.01 9.1 19.51 29.65 3.01 3.83 39.71 25.18 45.13 40 37.76 33.77 25.37 31.43 37.55 21.4 19.16 47.85 34.76

2.09 0.29 0.16 0.07 2.78 5.72 80.2 29.85 64.4 1.19 6.19 65.72 46.49 9.31 1.19 8.64 58.04 61.6 63.5 42.93 41.91 0.88 56.16 36.45 24.3 0.82 0.55 80.5 16.77 1.91 14.3 23.94 8.64 7.96 11.43 7.26 10.47 0.16 0.28 0.6 1.78 42.72 26 34.91 39.64 52.12 41.6 18.75 56.89 50.5 40.56 35.78

0 0.01 0.01 0.03 0.07 0.06 10.6 40.93 1.3 0.67 0.06 3.38 2.22 5.62 0.67 1.12 1.41 3.2 1.6 1.78 4.67 0.2 3.4 1.81 0.24 0.35 0.01 4.1 1.35 0 2.3 1.02 60.81 86.09 56.71 13.19 17.93 0 0 0.1 2.33 7.13 16 6.3 3.62 3.25 8.03 32.83 3.12 3.44 1.01 9.86

5 5 5.5 5.5 6 5.5 11 16 14.6 18 5 5 6 14.6 16 11 6 5.5 5.5 6 10 15 5.5 6 8.2 15 5.5 5.5 6 4 5.5 5.5 12 13 10 8 12.8 12 10.9 7.3 13 12 12.8 16.4 7.3 7.3 13 16.4 7.3 7.3 6 14

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Table 1 (Continued). Sample 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67

GR 0.64 0.17 0.5 0.1 3.17 0 0.01 0.05 1.54 0 0.07 0 0.2 0.26 0.04

VCS 0.51 0.75 1.52 0.7 1.34 0.02 0.01 0.07 0.9 0.08 0.21 0.03 0.06 0.27 0.17

CS 1.87 1.43 5.63 2.3 1.98 0.06 0.22 0.13 2.56 1.23 1.17 0.1 0.1 0.44 0.76

MS

FS

VFS

S+C

Depth (m)

4.54 5.91 63.22 36.3 18.16 0.27 0.71 27 5.81 51.45 3.96 1.2 0.6 1.95 5.57

19.06 26.46 27.04 57.4 43.38 9.45 37.18 67.8 48.14 38.48 13.7 24.4 58.5 12.24 21.4

70.52 62.84 0.99 3 30.42 43.57 60.36 3 39.94 8.66 79.09 72 38.4 82.51 70.3

2.87 2.44 1.1 0.1 1.54 46.64 1.34 2 1.1 0.09 1.81 2.5 2.2 2.34 1.76

9.1 9.1 16.4 16.4 14 14.6 6 5.5 11 5 5.4 14.6 5.5 5.4 14.6

GR, gravel; VCS, very coarse sand; CS, coarse sand; MS, medium sand; FS, ¢ne sand; VFS, very ¢ne sand; S+C, silts and clays.

and (b) the depth. For this purpose, data were standardized, i.e. each variable has a mean of zero, being expressed by units of standard deviation. The Pearson correlation coe⁄cient was calculated between the variables measured (moults, depth and grain sizes) and a principal-component analysis (PCA) was computed. In this case, the two main factors explained 40% of the total variance. In a further step, a multivariate analysis was applied to determine the sample groups based on the percentages computed of the di¡erent instars (Table 2). An initial clustering procedure is applied using a hierarchical agglomerative technique with the application of the Euclidean distance and the Ward linkage. Results are contrasted by discriminant analysis, by determining both the dimensions and variables on which the groups di¡er. In addition, a stepwise selection procedure was computed and the contribution of each of the predictor variables to the overall discrimination was determined. The error rate estimation was obtained by a ¢nal cross-validation method. These statistical techniques were carried out using several subprograms of the Statistical Package for the Social Sciences (SPSS1). Further details of these techniques may be consulted in Dillon and Goldstein (1984) and Seber (1984).

Finally, a two-step multivariate analysis (discriminant analysis and cross-validation method) was carried out in order to determine the possible in£uence of the sediment sorting in the de¢nition of the groups previously obtained. Percentages of silts and clays are eliminated upon not passing the previous test of tolerance (see Fig. 7B). This initial constraint does not imply that P. elongata was not found in these ¢ne-grained sediments, but its contribution to this statistical analysis is scarce in relation to the remaining grain-size classes. 4.2. Neogene sediments Twenty-nine samples (250 g wet sediment) were collected from three Neogene sections (Fig. 3B, Bonares, Lucena, Moguer) belonging to the Huelva Sand Formation in order to compare them with modern samples. This variation in the initial sample weight is necessary to compensate (at least partially) the population losses derived from taphonomic processes. In each sample, a similar procedure was followed by obtaining the total population of Pontocythere elongata. Finally, the two discriminant functions extracted from the Recent populations were applied to the percentages of the di¡erent instars obtained from each Neogene sample. The ostracod associations of the

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Table 2 Percentages of instars in Recent samples Sample 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53

Total 4 1 2 2 8 16 18 3 6 101 40 3 31 60 40 188 82 103 283 2 390 35 976 9 9 112 31 58 59 8 2 4 96 5 19 894 199 20 51 124 145 563 283 23 116 429

Adults

A-1

25 100 50

25

37.5 18.75 5.55

50 50 25

A-2

A-3

A-4

A-5

A-6

50

50 25 43.75

25 6.25

12.5 6.25 16.67

77.78

100

50 10.98 12.5

50 18.81 22.5

21.67 2.5 4.79 2.44 15.53 8.48

10 7.5 2.66 2.44 6.8 24.03 50

5.94 32.5

38.61 2.5

3.22 26.67 12.5 3.72 12.19 10.68 31.8

19.35 10

0.77 20 0.2 55.56 33.33 7.14 12.9

5.71 7.79 22.22 33.33 8.93 3.23

9.23 8.57 31.35 11.11 22.22 13.39 22.58

6.91 9.76 15.53 14.84 50 20.77 2.86 23.46 11.11 11.11 7.14 19.35

25.86 8.47

44.83 20.34

20.69 28.81

8.62 32.2

12.5

12.5 50 25

12.5

37.5 50

50

62.5 20 15.79 1.45 11.56 20 31.37 4.84 5.52 10.48 9.19 4.35 24.14 6.06

20.83 20 36.84 3.47 3.01 10 11.76 26.61 5.52 13.5 14.49 13.04 43.1 7.46

25 8.33

9.55 10 11.76 6.45 11.9 21.74 1.72 14.22

6.25 60 21.05 3.01 30 11.76 21.77 5.52 20.07 12.72 4.35 18.97 11.19

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25.74 20 100

5

5

58.06 6.66 37.5 10.11 8.54 16.50 4.24

19.35

16.15 31.43 37.19

32.31 31.43

20.77

8.93 41.94

26.79

26.68

3.39

3.39

3.39

25 40 66.49 40.24 34.95 16.61

5.32 24.39

25

1:1 1:8.2 1:7 0 0 1:3.6 1:39 1:19.9 1:40 1:5.4 1:10.8 0 1:128.8 1:4 1:495 1:0.8 1:2 1:13 1:6.8 1:2.9 1:10.8

1:7 0 1:3

2.08 10.53 57.05 18.09 30 21.57 33.87 5.52 4.97 42.4 34.78 6.03 54.54

A:J 1:3 1:0 1:1 0 1:1.8 1:4.3 1:17.2 0

15.79 38.03 54.77 11.76 6.45 4.97 21.2 21.74 6.03 6.53

77.93 4.8

1:11 0 0 0 1:9.5 1:9 1:7.5 1:14.5 0 1:7.4 0 1:3.6 1:57.1 1:6

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Table 2 (Continued). Sample

Total

Adults

54 55 56 57 58 59 60 61 62 63 64 65 66 67

232 32 77 382 567 34 31 471 29 117 922 35 39 1519

1.29 9.68 5.19 10.21 5.64 8.82 6.45 7.64 3.45 14.53 0.54 5.71 12.82 4.67

A-1 7.33 14.52 12.99 12.83 7.23 22.58 11.04 51.72 13.67 1.95 20 20.51 5.46

A-2 13.36 4.84 12.99 25.39 33.33 14.71 38.71 37.37 27.59 35.04 3.9 40 41.03 7.77

A-3 19.83 1.61 18.18 25.39 27.69 23.53 12.9 39.28 10.34 20.51 19.85 8.57 20.51 3.88

A-4 39.22 4.84 32.47 22.77 17.46 41.18 9.68 3.82 6.9 13.67 55.31 14.29 2.56 26.07

A-5 18.97 64.52 6.5

A-6

11.69

11.76 9.68 0.85 2.56 18.44 11.43 2.56 26.07

26.07

A:J 1:76.5 1:9.3 1:18.3 1:8.8 1:16.7 1:10.3 1:14.5 1:12.1 1:28 1:5.9 1:184.2 1:16.5 1:6.8 1:20.4

A, adults; J, juveniles.

Huelva Sand Formation are described in Ruiz and Gonza¤lez-Regalado (1996).

eral distribution and no specimens were found in the ¢ne sediments present either in the Tinto^ Odiel junction or at depths up to 15 m, near Isla Cristina.

5. Results: Recent samples 5.2. Ontogenetic evolution, grain size and depth 5.1. Abundance and distribution In the Huelva littoral, the number of individuals per sample shows some consistency in the coastal stretch studied (Fig. 4A). In a west^east transect carried out at depths down to 10 m, a scarcely populated area (1^20 individuals/sample) is followed by a more densely occupied sector (30^1500 individuals/sample). In the deepest samples, two sectors may be distinguished: Isla Cristina^Mazago¤n ( 6 10 individuals/sample in all cases) and Mazago¤n^Matalascan‹as. The highest number of individuals in the area studied was found at depths up to 10 m in this latter area, whereas the adjacent shallower samples present scarce populations of P. elongata. Percentages of Pontocythere elongata are related to either the sampling depth or the grain size. At depths down to 10 m, this species generally represents 5^20% of the total number of ostracods, whereas these quantities diminish between 10 and 20 m ( 6 5% in most cases) and any specimen was found in the six additional, deeper water samples collected (Fig. 4B). The grain-size distribution may locally alter this gen-

An initial, biometric analysis permits the delimitation of the last seven stages of Pontocythere elongata (Fig. 5A). Shell length (L) and shell height (H) are closely correlated (n = 200; R2 = 0.963; P 6 0.0001), with L greater than 2H in most cases. Each instar is 100 Wm longer and 50 Wm taller than the previous one, with a single anomaly between the instars A-5 and A-4 (vLV200 Wm). The Pearson correlation matrix and the PCA indicate a possible relation between grain sizes and instars (Tables 1 and 2), with the establishment of three associations between both groups of variables (Fig. 5B): (a) Association 1: Adults^(A-1)^Medium Sands. Both adults (L = 960^1130 Wm; H = 370^ 460 Wm) and the (A-1) stage (L = 840^930 Wm; H = 345^400 Wm) are positively correlated (P 6 0.01) with the medium sand contents (500^ 1000 Wm diameter). These three variables are negatively correlated with depth (P 6 0.01), indicating a preferential concentration of these last stages in the shallower sediments of the area studied. This association was also found in the

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Fig. 4. Pontocythere elongata. (A) Number of individuals/100 g wet sediment. (B) Percentages of the total ostracod population.

PCA, with high to very high loadings in Factor 1 (Fig. 5C, up to 0.4). (b) Association 2: (A-4)^(A-5)^Fine Sands^ Very Fine Sands. These intermediate stages (A-4: L = 510^625 Wm, H = 220^280 Wm; A-5: L = 360^430 Wm, H = 170^210 Wm) are positively correlated with the very ¢ne sand contents (62^ 125 Wm diameter) and, to a lesser extent, with the ¢ne sand (125^250 Wm diameter) percentages. These four variables are associated with high negative loadings of Factor 1 in the PCA analysis and present a spatial distribution independent of the sampling depth. (c) Association 3: (A-6)^(Silt+Clay). This association is clearer in the PCA analysis than in the Pearson matrix (Fig. 4C). This di¡erence may be due to the preferential concentration of these grain sizes in the deeper samples (P 6 0.01),

whereas this tendency is less evident in the lower instar collected (L = 280^330 Wm; H = 145^175 Wm). These relations may indicate : (a) a sorting control of the instar distribution ; or (b) the preference of each instar to live in a determined grainsize class. Consequently, a new analysis (see Section 5.4) is necessary to de¢ne the relative importance of these two possibilities. 5.3. Statistical analysis : sample groups 5.3.1. Cluster analysis The cluster procedure permits one to separate an outlier (sample 48) characterized by the highest percentages of the (A-6) instar of the area. In the remaining samples, three groups may be delimited (Fig. 6A):

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Fig. 5. (A) Ontogenetic analysis of P. elongata. (B) Pearson correlation matrix between grain sizes and instars; GR, gravel; VCS, very coarse sand; CS, coarse sand; MS, medium sand; FS, ¢ne sand; VFS, very ¢ne sand; S+C, silts and clays. (C) Principalcomponent analysis.

(a) Group 1 (32 samples). This cluster is characterized by very high percentages (Fig. 6B, s 75%) of the last juvenile instars (A-1 to A-4), whereas the adult contents are low to very low ( 6 15% in most cases). These samples are collected along the whole area studied, with a spatial con¢guration closely related to Group 3. In most cases, a dominance of Group 1 at depths down to 10 m coincides with deeper samples belonging to Group 3. This group also includes four samples collected in the outer estuary of the Tinto^Odiel rivers. (b) Group 2 (seven samples). Populations of this cluster consist of very few individuals per

sample ( 6 10 except in sample 31) belonging to both adults and (A-1) specimens (Fig. 6B). These samples are mainly concentrated in the coarse sediments and/or lag deposits located near the Guadiana and Tinto^Odiel mouths. (c) Group 3 (20 samples). These samples are dominated by the intermediate juvenile instars (A-4 and A-5), with variable percentages of the remaining stages. 5.3.2. Discriminant analysis This statistical tool de¢nes two main discriminant functions using standardized values. These functions include ¢ve signi¢cant variables (K:

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Fig. 6. (A) Sample groups de¢ned by multivariate analysis. (B) Mean percentages of the sample groups.

adults ; B: A-1; C: A-4; D: A-5; E: A-6) and a constant: F 1 ¼ 30:12430:42K þ 0:03Bþ 1:267C þ 1:334D þ 1:143E F 2 ¼ 30:331 þ 1:715K þ 0:746B þ 0:813Cþ 0:779D þ 0:935E

The application of these functions to the sample groups de¢ned by cluster analysis indicates a very close coincidence between the results extracted from the two methods. Up to 98% of the samples (58 out of 59) were relocated in the same initial group, whereas the outlier (sample 48) was included in Group 3. The unique exception

(sample 59) was reclassi¢ed from Group 1 to Group 3, due to the coexistence of high percentages of intermediate moults (A-3 and A-4) characteristic of both groups. 5.3.3. Cross-validation In this ¢nal step, up to 96.6% of the samples (Fig. 6A, 57 out of 59) are still included in the same initial cluster, indicating a true separation between the three groups di¡erentiated. A new sample (31) was relocated from Group 2 to Group 1. 5.4. Statistical analysis : sorting versus growth The new discriminant analysis was centered on the relation between the di¡erent grain sizes and

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Fig. 7. Statistical analysis of the relation between the three sample groups de¢ned (see Fig. 6) and the grain-size fractions. GR, gravel; VCS, very coarse sand; CS, coarse sand; MS, medium sand; FS, ¢ne sand; VFS, very ¢ne sand; S+C, silts and clays.

the three groups obtained. The two new discriminant functions included three signi¢cant variables (Fig. 7A: coarse sands, medium sands and very ¢ne sands), whereas the remaining fractions have an undi¡erentiated behavior. Nevertheless, these three variables do not satisfactorily explain the di¡erences between the three groups, which are mixed in the F1 ^F2 diagram (Fig. 7B). In addition, the application of the grain-size variables causes a strong alteration in the group composition, with a very high probability of error in the sample classi¢cation according to the crossvalidation method (Fig. 7C, 78.1% for Group 1; 71.4% for Group 2; 30% for Group 3).

6. Pontocythere elongata in the Huelva Sand Formation (Early Pliocene) 6.1. Abundance and distribution The lower part of the Bonares section (samples B1^B8) is characterized by the presence of four

storm levels with a low ostracod density ( 6 50 individuals/100 g). The ostracod association is composed of Cytherella vulgata, Hiltermannicythere spp., Carinocythereis whitei and Paracypris polita, accompanied by reworked specimens of Cyprideis sp. and Ruggieria tetraptera. In these samples, Pontocythere elongata is represented by scattered individuals (mainly females), constituting as little as 5% of the total assemblage. This scarcity contrasts with the relatively abundant populations of this species observed in the upper four samples, with the appearance of new species (Aurila convexa, Neocytherideis fasciata, Semicytherura sella) and increasing ostracod abundance (Fig. 8). These di¡erences are also recognized in the Lucena section. The basal samples (L1^L5) contain scarce juvenile specimens of P. elongata and show an increasing density (30 to 75^100 individuals/ 100 g) after the ¢rst storm level. Some variations may be observed in the ostracod associations, with higher percentages of C. vulgata, R. tetraptera and Acanthocythereis hystrix and the disap-

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Fig. 8. Distribution of P. elongata and ostracod assemblages in the Neogene sections. N, number of individuals; P, percentage of the total ostracod population.

pearance of the Cyprideis^Miocyprideis group. In the upper samples, a drastic reduction in ostracod abundance ( 6 20 individuals/100 g) was noted after the second storm level, with a subsequent recovery near the top. P. elongata constitutes an important element of the ostracofauna (4^24%), being mainly represented by adults (17^30%) and the (A-1) stage (33^50%). Ostracods are very rare in most of the samples collected in the Moguer section ( 6 10 individuals/ 100 g), with C. vulgata, R. tetraptera and Costa edwardsii as main species. This paucity is only interrupted in sample M5, with numerous instars of the dominant species (A. convexa, S. sella, A. hystrix) in the later sections. P. elongata is present as an accessory species ( 6 5%) in samples M4^

M6, where it is represented by intermediate instars (A-3 to A-5). 6.2. Statistical analysis In the Bonares^Lucena area, most of the storm levels are characterized by the absence of Pontocythere elongata or the presence of Group 1. This species is usually absent in the immediate poststorm levels or is represented by Group 2, whereas the three groups delimited are found during the fair-weather conditions. Finally, the upper samples contain an ostracod population structure belonging to Groups 1 and 2. In the Moguer section, two samples are included in Group 1, although the unique presence

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Fig. 9. (A) Distribution of the sample groups in the Huelva littoral. (B) Tentative model explaining the distribution of P. elongata in shallow areas of southwestern Spain.

of the (A-3) instar shows a partial coincidence with Group 3. The latter group is represented by sample M5, which includes higher percentages of the initial instars in the three sections studied.

7. Discussion 7.1. Pontocythere elongata as an environmental tracer 7.1.1. Depth In the modern samples studied, Pontocythere elongata is restricted to depths down to 20 m, although a local transport seaward must be considered in unsampled, deeper areas (i.e. Mazago¤n^ Matalascan‹as). The relative abundance, as a percentage of the total ostracod community, generally increases in the shallower areas ( 6 10 m depth), where this species is found together with Urocythereis britannica, Aurila convexa, Cytheret-

ta adriatica and Semicytherura spp. (Ruiz et al., 1997). Consequently, increasing percentages of this association may be used as paleoecological indicators of: (a) the last marine conditions in regressive sequences; or (b) the ¢rst marine events in transgressive series. Other bathymetrical data may be extracted from the population analysis. Adults and the (A-1) instar show a negative correlation with depth (adults : P 6 0.01; A-1: P 6 0.05) and are mainly concentrated near the shoreline. Conversely, the lower instars measured are more abundant in samples collected at depths up to 20 m (A-5: mean 17.9%; A-6: mean 6.7%) than in the shallower areas (A-5: mean 7.6%; A-6: mean 2.1%), although this relation was not statistically signi¢cant (Fig. 5B). This tendency was proposed by Whatley (1988) for a theoretical coastal species, indicating a hypothetical transport of these intermediate instars at depths up to 50 m. Most of the species collected in the Neogene

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sections are alive today and consequently its paleobathymetry may be estimated by comparison with the Recent depth range in the same ostracod province. The main association collected in the Bonares^Lucena areas (Cytherella vulgata, Hiltermannicythere spp., C. whitei, P. elongata, A. convexa) is found in the innermost infralittoral areas of France and Spain (Yassini, 1969; Llano, 1981; Carbonel, 1980 ; Ruiz et al., 1997). The presence of Cyprideis and Miocyprideis in the Bonares section is indicative of coastal environments close to the paleoshoreline, whereas deeper conditions are estimated for the Lucena section owing to the abundance of ubiquitous, neritic species such as A. hystrix and Ruggieria tetraptera (Carbonel, 1985). In both sections, a general shallowing may be inferred by the increasing percentages of P. elongata in the upper samples, near the transition to the intertidal/continental paleoenvironments represented by the Bonares Sand Formation (Mayoral and Pendo¤n, 1986). The Moguer section shows a partial substitution of the later, shallow species by a neritic association (sensu latu). In the Ca¤diz Gulf and northern Morocco, Costa edwardsii is a main component of the ostracod association ( s 10%) from depths ranging between 20 and 130 m, although some reworked specimens may be collected near the slope (Llano, 1981 ; Ruiz et al., 2000b). The abundance of this species, the occasional presence of shallower species (A. convexa, P. elongata, Semicytherura sella) and the mollusc associations (Castan‹o et al., 1988) will indicate a deeper position of this section in relation with the Bonares^Lucena area. 7.1.2. Grain size, moult stages and sediment transport Valves and carapaces of P. elongata are an additional component of the bottom sediments and may be subject to sedimentological processes. The ¢rst association described (adults, A-1 and medium sands) is characteristic of the residual deposits located near the main river mouths (Guadiana, Odiel), where the remainder, lower juvenile stages are absent or present very low percentages and the ¢nest grain-size classes are poorly represented. This association is also found in local lag deposits

constituted by high proportions of bioclastic fragments (mainly molluscs) collected near La Antilla and Punta Umbr|¤a. In these areas, the sediment transport is dominated by creep-saltation process transport due to the strong tidal currents ( s 0.5 m s31 ; Barker, 1983; Borrego, 1992). Some intermediate sizes (0.25^0.063 mm diameter) are grouped in Association 2, being mainly transported by saltation processes. Finally, the (A-6) instar and the ¢nest grain-size fraction (silts and clays) are included in Association 3, which characterize the quieter environments of the area studied. They are mainly concentrated in areas situated at the end of the energy gradient, where suspension processes are dominant. In this zone, the ostracod population structure (Group 3) is typical of low-energy thanatocoenosis (Whatley, 1983, 1988). 7.1.3. Extrinsic or intrinsic factors? The distribution of Pontocythere elongata described above suggests an extrinsic, hydrodynamic control on the distribution of the ostracod populations. Consequently, it is necessary to contrast this factor with the intrinsic, natural growth of P. elongata in each sample. The second multivariate analysis (see Section 5.4) indicates that the population structure of P. elongata is not explained by the grain-size variables, although some partial correlation may exist between individual grain-size classes and several instars (see Section 5.2). These data indicate that the instar distribution of this species in an individual sample is better explained by the sub-Recent populations living there before sampling than by the hydrodynamic level of the surrounding environment. 7.2. Rivers, groynes and storms If these three elements are considered as ‘events’, the e¡ects of Recent rivers and groynes may be compared with those caused by the Pliocene storms in two ways. 7.2.1. Immediate e¡ects High-discharge river mouths (which exhibit signi¢cant sediment erosion, transportation and distribution) and storm events may cause similar ef-

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fects on the ostracod populations. In both cases, the bottom sediments are frequently eroded by the £uvial/tidal bottom currents and/or waves. These unstable conditions may be further in£uenced by groynes, causing the deposition of coarser grain sizes transported by the rivers and littoral drift currents in the western side (in this case). Total ostracod density and diversity decrease after these events. These microcrustaceans are represented by adults and the last juvenile instars (Group 1) of selected species with coarse and/or reticulate carapaces (P. elongata, Aurila convexa, Urocythereis britannica) or may even disappear (Ruiz et al., 2000b). 7.2.2. Post-event consequences Temporal e¡ects of storms are stronger than the spatial changes induced by groynes. In most cases, storms cause the disappearance of P. elongata and an important reduction in the ostracod density in the succeeding samples of the shallower sections (Bonares, Lucena). In this area, the later recovery started with ostracod populations belonging to Group 1, which may be considered as an indicator of a (paleo-)biocoenosis (Whatley, 1988). In a further step, Group 2 may appear, mainly in the ¢nal samples of each section, near the transition to intertidal^continental environments represented by the Bonares Sand Formation (Mayoral and Pendo¤n, 1986). In deeper Neogene areas (Moguer: 30^50 m depth), only the lower instars are present, indicating a low-energy thanatocoenosis (Brouwers, 1988; Whatley, 1988). The absence of this species in the remaining samples may indicate possible suspension transport of these immature moults from the latter zone. This transport is usual in Recent neritic areas during and/or after storm periods (Carter, 1988). On the other hand, groynes cause either a local interception of the total sedimentary volume in motion in its western sides and an arti¢cial refraction of the wave trains in the opposite sides. In the latter areas, these wall-like structures cause an important decrease in the drift current velocities and induce the erosion of depleted beaches. In these less energetic environments, the ostracod population structure of P. elongata is represented

67

by Group 1, including a well developed biocoenosis (Ruiz, 1995). Erosion of the western sides nourishes the eastern coastal stretch and causes additional progradation phenomena (Ojeda, 1988). The nourishment was also detected in the ostracod analysis, with the presence of a Group 3 structure due to the suspension transport of lower instars belonging to Group 1 structure initially located to the west. This correlation (erosion^Group 1; progradation^Group 3) was found in the whole area studied at depths down to 10 m, whereas an inversion of this pattern was detected in the deeper water samples (Fig. 9A). This model will indicate a possible relation between well-developed biocoenoses of this species and coastal areas with medium to low energetic levels (Fig. 9B).

8. Conclusions Population analysis of Pontocythere elongata permits an evaluation of its potential as a bathymetrical tracer in Recent samples of southwestern Spain. This species is mainly limited to depths down to 30 m, where its relative percentages are inversely related to depth and the percentages of adults increase landward. These results are applied to the paleoenvironmental analysis of three Neogene sections of the Guadalquivir Basin. By combining three independent, multivariate analyses, three sample groups were delimited: (a) Group 1, present in medium- to low-energy environments and characterized by low percentages of adults ( 6 15% in most cases) and the predominance of the (A-1) to (A-4) instars; (b) Group 2, containing high to very high proportions of adults ( s 40%) and observed mainly in the medium sands located near river mouths; and (c) Group 3, dominated by intermediate stages (A-3 and A-4). This distribution implies a statistical con¢rmation of the basic population age structures de¢ned by Whatley (1988). This distribution is mainly due to intrinsic factors (natural growth of the species), although some partial correlations have been found with extrinsic, hydrodynamic factors. These techniques permit a comparison of the

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e¡ects of the river mouths, groynes and storms over the ostracod populations. These three elements may be considered as high-energy ‘systems’, causing the appearance of Group 2 structures (river mouths and groynes) or even the disappearance of this species (storms). The subsequent fairweather conditions are characterized by: (a) the dominance of Group 1 structures after groynes; or (b) the absence of this species during a variable time interval after storm and the later appearance of Group 1 structures. In both cases, density and diversity increase progressively in these new lowto medium-energy conditions.

Acknowledgements The initial version of this manuscript was much improved by the constructive opinions of Dr. Pierre Carbonel and an anonymous reviewer. The ¢nancial support of the DGYCIT (Projects MAR98-0209 and BTE-2000-1153) and the Andalousia Board (PAI RNM-238) is gratefully acknowledged. This is a contribution to the PICG 396 ‘Continental Shelves in the Quaternary’.

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