Calcareous dinoflagellate cysts analysis of sediment cores from the Equatorial South Atlantic Ocean

An overview is presented of the current state of knowledge on paleo-ecological aspects of calcareous dinoflagellate resting cysts. Apart from literature-based information, a discussion of new results is also provided from Equatorial Atlantic surface plankton samples, surface sediment samples and Late Quaternary sediments from two gravity cores. With the aid of redundancy analysis statistics, variations in the calcareous cyst content of both cores are correlated to variations in total organic carbon (TOC). On a global scale, the calcareous cyst distribution in bottom sediments varies with latitude and inshore-offshore gradients. In the Equatorial Atlantic Ocean, enhanced calcareous cyst production can be observed in regions and time intervals with stratified, oligotrophic conditions in the upper water masses.

Superoxide decay rates during METEOR cruise M83/1

Superoxide is an important transient reactive oxygen species (ROS) in the ocean formed as an intermediate in the redox transformation of oxygen (O2) into hydrogen peroxide (H2O2) and vice versa. This highly reactive and very short-lived radical anion can be produced both via photochemical and biological processes in the ocean. In this paper we examine the decomposition rate of O2- throughout the water column, using new data collected in the Eastern Tropical North Atlantic (ETNA) Ocean. For this approach we applied a semi factorial experimental design, to identify and quantify the pathways of the major identified sinks in the ocean. In this work we occupied 6 stations, 2 on the West African continental shelf and 4 open ocean stations, including the CVOO time series site adjacent to Cape Verde. Our results indicate that in the surface ocean, impacted by Saharan aerosols and sediment resuspension, the main decay pathways for superoxide is via reactions with Mn(||) and organic matter.

Calcareous dinoflagellates in the eastern equatorial Atlantic

Sediments of the Equatorial Atlantic (core GeoB 1105-4) have been investigated for both calcareous dinoflagellates and organic-walled dinoflagellate cysts. In order to determine the ecological affinity of calcareous dinoflagellates the statistical methods of Detrended Correspondence Analysis (DCA) and Redundancy Analysis (RDA) were used. Utilising DCA, distribution patterns of calcareous dinoflagellates have been compared with those of the ecologically much better known organic-walled dinoflagellate cysts. This method was also used to determine which environmental gradients have a major influence on the species composition. By using existing environmental information based on benthic and planktic foraminifera, such as Sea Surface Temperature (SST) and stable oxygen and carbon isotopes, as well as information on the amount of Calcium Carbonate and Total Organic Carbon (TOC) in bottom sediments, these gradients could be interpreted in terms of productivity and glacial-interglacial trends. Using RDA, the direct relationships between the distribution patterns of calcareous dinoflagellates with the above mentioned external variables could be determined. For the studied region and time interval (141-6.7 ka) the calcareous dinoflagellates show enhanced abundances in periods with reduced productivity most probably related to decreased divergence and relatively stratified, oligotrophic oceanic conditions.

(Appendix A) Abundance of calcareous dinoflagellates of sediment core GeoB2204-2

The environmental preferences of calcareous dinoflagellates have been investigated over the last 140 ka by comparing material from two sediment cores: one from the highly productive equatorial divergence of the eastern Atlantic Ocean and the other from the low productivity western tropical Atlantic Ocean. Pronounced differences in palaeoproductivity between the two sediment cores are indicated by high and variable organic carbon accumulation rates in the east, in contrast to relatively constant and low values in the west. Calcareous dinoflagellates show just the opposite pattern: high accumulation rates in the west and lower in the east. At the equatorial divergence, temporal variations of calcareous dinoflagellate and organic carbon accumulation rates show, for the most part, an inverse relationship. High calcareous dinoflagellate content coincides with low organic carbon accumulation rates and vice versa. In the investigated region and time interval, enhanced production of calcareous dinoflagellates can be correlated to periods of reduced palaeoproductivity probably related to relatively stratified conditions of the upper water column.

Dinoflagellates of sediment core GeoB1523-1

Large numbers of calcareous dinoflagellate cysts and the vegetative calcareous coccoid species Thoracosphaera heimii are generally found in sediments underlying oligotrophic and/or stratified (sub)surface water environments. It is difficult to distinguish between the relative importance of these two environmental parameters on calcareous cyst and T. heimii distribution as they usually covary, but this information is essential if we want to apply cysts properly in the reconstruction of palaeoenvironments and past surface water hydrography. In the multi-proxy core GeoB 1523-1 from the Ceará Rise region in the western equatorial Atlantic Ocean (covering the past 155 ka), periods of greatest oligotrophy are not synchronous with periods of greatest stratification (Rühlemann et al., 1996, doi:10.1016/S0025-3227(96)00048-5; Mulitza et al., 1997, doi:10.1130/0091-7613(1997)025<0335:PFAROP>2.3.CO;2; 335-338; Mulitza et al., 1998, doi:10.1016/S0012-821X(98)00012-0), giving us the unique opportunity to differentiate between the effects of both parameters on cyst accumulation. The calcareous cyst record of the core reflects prominent increases in accumulation rate of nearly all observed species only during the nutrient-enriched but more stratified isotopic (sub)stages 5.5, 5.3, 5.1 and 1. In this respect, the distribution trends in the core are more similar to those of the eastern equatorial upwelling region (GeoB 1105-4) than they are to those of the oligotrophic north-eastern Brazilian continental slope (GeoB 2204-2), even though temporal changes in bioproductivity are principally in antiphase between the eastern and western equatorial regions. We conclude that stratification of the upper water column and the presence of a well-developed thermocline are probably the more important factors controlling cyst distribution in the equatorial Atlantic, whereas the state of oligotrophy secondarily influences cyst production within a well-stratified environment.

Abundance of calcareous dinoflagellates of sediment cores from the South Atlantic Ocean

Despite the increasing interest in the South Atlantic Ocean as a key area of the heat exchange between the southern and the northern hemisphere, information about its palaeoceanographic conditions during transitions from glacial to interglacial stages, the so-called Terminations, are not well understood. Herein we attempt to increase this information by studying the calcareous dinoflagellate cysts and the shells of Thoracosphaera heimii (calcareous cysts) of five Late Quaternary South Atlantic Ocean cores. Extremely high accumulation rates of calcareous cysts at the Terminations might be due to a combined effect of increased cyst production and better preservation as result of calm, oligotrophic conditions in the upper water layers. Low relative abundance of Sphaerodinella albatrosiana compared with Sphaerodinella tuberosa in the Cape Basin may be the result of the relatively colder environmental conditions in this region compared with the equatorial Atlantic Ocean with high relative abundance of S. albatrosiana. Furthermore, the predominance of S. tuberosa during glacials and interglacials at the observed site of the western Atlantic Ocean reflects decreased salinity in the upper water layer.