Distributions of calcareous dinoflagellate cysts in surface sediments of the western equatorial Atlantic Ocean, and their potential use in palaeoceanography

Only very few studies focus on recent calcareous dinoflagellate cyst divers ity, geographic distribution and ecology, so that information on the distri bution patterns and environmental affinities of individual cyst species is extremely limited. This information is, however, essential if we want to us e calcareous dinoflagellate cysts for palaeoenvironmental reconstruction Su rface sediment samples from the generally oligotrophic western equatorial A tlantic Ocean, offshore northeast Brazil, were therefore quantitatively ana lysed for their calcareous dinoflagellate cyst content, including the calca reous vegetative coccoid Thoracosphaera heimii. Seven calcareous dinoflagel late cyst species/morphotypes and T. heimii were encountered in high concen trations throughout the area. Substantial differences in the distribution p atterns were observed. The highest concentrations of cysts are found in sed iments of the more oligotrophic, oceanic regions, beyond the influence of A mazon River discharge waters. Dinoflagellates producing calcareous cysts th us appear to be capable of surviving low nutrient concentrations and produc e large numbers of cysts in relatively stable and predictable environments affected by minimal seasonality. To test for the environmental affinities o f individual species, distribution patterns in surface sediments were compa red with temperature, salinity, density and stratification gradients within the upper water column (0-100 m) over different times of the year, using p rincipal components analysis and redundancy analysis. T. heimii and four of the seven encountered cyst species (Sphaerodinella? albatrosiana, two morp hotypes of Sphaerodinella? tuberosa and Scrippsiella regalis) relate to the se parameters significantly and the variations in the cyst associations app ear to be associated with the different surface water currents characterisi ng the area. The results imply that calcareous dinoflagellate cyst distribu tions can potentially be used to distinguish between different open oceanic environments and they could, therefore, be useful in tracing water mass mo vements throughout the late Quaternary. (C) 2000 Elsevier Science B.V. All rights reserved.