(Palaeo-)ecological significance, transport and preservation of organic-walled dinoflagellate cysts in the Somali Basin, NW Arabian Sea

Abstract

To date, relatively little information is available about factors influencing organic-walled cyst production of tropical dinoflagellates and processes influencing the final burial of cysts in bottom sediments, such as transport and preservation. To extend this information, cyst fluxes were documented for three sediment traps from June 1992 to February 1993 at two sites in the Somali Basin (northwestern Arabian Sea) as well as the cyst association of underlying sediments. By comparing cyst associations of contemporaneously collected trap samples at different depths at one site, information about transport and processes of decay in the water column was obtained. Neither transport nor decay appears to have any detectable influence on cyst association during cyst settlement through the water column. Comparing the trap associations with the underlying sediments indicates that downslope transport appears to have influenced the cyst association on a local scale only. Species-selective decay, probably related to the presence of oxygen in bottom sediments, has influenced the cyst association most pronouncedly at the most offshore site. Relating variations in the trap associations with environmental conditions of the overlying surface waters indicates that highest production of both filled and empty cysts occurs during the SW Monsoon upwelling. Based on this correlation three groups of species can be distinguished: Species with highest fluxes during (1) the first-half of the SW Monsoon (June–August); Bitectatodinium spongium, Echinidinium granulatum, Echindinium transparantum, Echinidnium spp., cysts of Protoperidinium compressum and cysts of Protoperidinium subinerme, (2) the transition between the SW-Monsoon and inter-Monsoon; Spiniferites mirabilis and Spiniferites spp., (3) no particular season; all other species. Cyst associations of all trap samples are dominated by cyst of Protoperidinium species. Cysts with highest fluxes during the SW-Monsoon form about a third of the associations.

Introduction

Organic-walled dinoflagellate cyst assemblages in marine sediments are increasingly used for palaeo-environmental and -oceanolographic reconstructions (e.g. De Vernal and Hillaire-marcel, 1987; Aksu et al., 1992; Marret and Turon, 1994; Versteegh, 1994; Versteegh et al., 1996; Zonneveld et al., 1997a). These reconstructions are based on the environmental affinities of the cyst-producing dinoflagellates. The presently available knowledge of their ecology is mainly derived from studies of temperate and (sub-) arctic regions (e.g. Wall et al., 1977; Harland, 1983; Edwards and Andrle, 1992; Mudie, 1992; Matthiessen, 1994; Dale, 1996; Marret and De Vernal, 1997). Relatively limited information is available of (sub-) tropical associations (Davey and Rogers, 1975; Bradford and Wall, 1984; Marret, 1994; Matsuoka, 1994; Biebow, 1996). In order to refine palaeoenvironmental and -oceanological reconstructions it is essential to enhance the ecological understanding of tropical associations. It is also fundamental to have detailed knowledge about other factors that influence the final cyst recovery in sediments such as lateral transport within the water column of recently produced cysts, relocation of already settled cysts (here indicated as secondary transport), and (species selective) preservation. Unfortunately, information about these factors is extremely limited, and includes mainly the results from laboratory experiments (Dale, 1976; Bradford and Wall, 1984; Turon, 1984; Dale and Dale, 1992; Keafer et al., 1992; Marret, 1993; Versteegh et al., 1996; Zonneveld et al., 1997b).

In this study, we aim to enhance the presently available knowledge on the ecological affinity, transport and preservation of tropical organic-walled dinoflagellate cysts. This is achieved by comparing the cyst content of three sediment traps, collected between June 1992 and February 1993 at two sites in the northern Somali Basin (NW Arabian Sea, Indian Ocean), with that of the underlying surface sediments (Fig. 1a). By studying the cyst content from traps at different water depths at a single site, we can better delineate processes of decay and transport processes within the water column. Information about secondary transport and preservation in the water column is obtained by comparing the cyst associations recovered from the traps with those in the underlying sediments.

The oceanic circulation of the Arabian Sea is strongly influenced by the semi-annual reversal of wind patterns; the Southwest (SW) Monsoon in the boreal summer, and the Northeast (NE) Monsoon in the boreal winter. In summer, differential heating of the continental and oceanic regions leads to low atmospheric pressure above the Asian Plateau and high atmospheric pressure over the relatively cool southern Indian Ocean. This results in the development of a strong low-level jet stream, the Findlater Jet (e.g. Smith et al., 1991). The response of the ocean to these southwestern winds includes the development of the southwestern Somali Boundary Current and upwelling along the Somali coast (e.g. Bruce 1973, Bruce 1974, Bruce 1979; Schott et al., 1990; Brock et al., 1991). This current can contain one or several clockwise-rotating eddies, leading to offshore transport of the upwelled cold and nutrient-rich waters (e.g. Currie et al., 1971; Schott et al., 1990; Bauer et al., 1991; Brock et al., 1991; Jochem et al., 1993; Brummer et al., 2000). Temperature records based on satellite imagery and shipboard data indicate that the Southwest Monsoon begins to appear in the beginning of June as a small band of upwelling developing along the coast of Somali (Keafer et al., 1992; Van Hinte et al., 1995). Subsequently, the area of upwelling expands laterally offshore Ras Hafun along the northern limits of the developing gyres. During the SW Monsoon, intervals with cold and warmer surface waters alternate relative to the position and extension of gyres in the Somali Current. At site MST-8, surface waters are relatively cold from June 2 until about September 13, indicating that surface waters are derivation from upwelled water (Fig. 2). At site MST-9, surface temperatures (SST) drop between June 5 and June 17. At both sites a trend of increasing temperatures can be observed in early September (Fig. 2), suggesting diminished influence of upwelled waters.

During the NE Monsoon (December–February) both wind and surface circulation patterns are reversed (Qasim, 1982; Schott, 1983; Schott et al., 1990; Smith et al., 1991). During the NE Monsoonal and interMonsoonal periods, most of the Arabian Sea is stratified, resulting in reduced primary production (relative to the SW Monsoon; e.g. Hitchcock and Olson, 1992; Owens et al., 1993; Veldhuis et al., 1997).