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

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.