Alexander Kohly

Plankton in the Norwegian-Greenland Sea: from living communities to sediment assemblages —an actualistic approach

A synoptic study is carried out to reconstruct the development of the plankton community in the late Quaternary in the Norwegian-Greenland Sea. It comprises quantitative analyses of coccolithophores, dinoflagellate cysts, diatoms and radiolarians. An actualistic approach is applied to evaluate the different fossil records of these plankton groups. The preliminary results of the current investigation are reported here.The composition and distribution of living communities of coccolithophores are presented as an example. A close relationship between the distribution of regional groups and surface water masses is observed. Seasonal vertical fluxes of coccolithophores and radiolarians through the water column show similar patterns within different years. However, diatoms are highly variable, both in absolute fluxes and species composition. The differentiation of sporadic and periodic processes is evident only after several years of observation. During settling and sedimentation biotic and abiotic processes such as grazing, dissolution and lateral transport alter the assemblages.Investigation of death assemblages in surface sediments reveals that in spite of these alteration processes the abundance and species distribution are related to surface water masses. Higher abundances and diversities are usually found in sediments underlying the warm Norwegian Current. Concentrations decrease to the north-west towards the cold polar water masses.The sediment assemblages of all groups are strongly altered relicts of former living communities. They are characterized by distinct changes in species composition and absolute abundances related to palaeo-oceanographic development. Their variation through the sedimentary record is used to distinguish four ecostratigraphic units during the late Weichselian and Holocene.

Development of anoxia during the Holocene fresh-brackish water transition in the Baltic Sea

One of the most pronounced environmental changes during the Holocene Baltic Sea history was the transition from the freshwater Ancylus Lake to the brackish water Litorina Sea. The establishment of ...

Diatom flux and species composition in the Greenland Sea and the Norwegian Sea in 1991–1992

Abstract Vertical diatom fluxes were investigated in sediment trap material from June 1991 to July 1992 from the Greenland Sea at 72 °N, 007 °W and the Norwegian Sea at 70 °N, 000 °E in order to determine seasonal species compositions and alterations that affect settling assemblages. Different hydrographical conditions, including seasonal ice coverage, are reflected in seasonal variations of total diatom flux and species composition. The purpose of this study is to achieve a better understanding of pelagic and sedimentary processes. The quantification of the annual diatom fluxes with respect to accumulation rates of underlying sediments is a suitable approach in reconstructing palaeoceanographic changes and their effects on the productivity of the oceans. The influx of laterally advected material and the alteration of sedimenting assemblages due to silica dissolution are also considered in this approach. A strong pulse-like sedimentation pattern was observed in the Greenland Sea. Highest values of up to 7.7 × 10 6 valves m −2 d −1 were recorded during the summer (second half of July 1991). Allochthonous diatoms, such as benthic and subtropical-boreal, as well as enriched autochthonous species found in the deep water trap indicate the sporadic occurrence of lateral advection and resuspension processes in the Greenland Sea. In the Norwegian Sea, low diatom flux was observed and three distinct sedimentation events could be differentiated at 500 m water depth. At this site, Zooplankton grazing pressure during spring is known to have more influence on the phytoplankton stock than in the Greenland Sea, which causes a delay in diatom accumulation. This is evident from maximum values of 0.3 × 10 6 valves m −2 d −1 during November/December 1991. Significant enrichment of Arctic diatom species suggests a transport from the Greenland Sea into the Norwegian Sea via deep currents. The assemblages of diatom species that are preserved in underlying surface sediments reflect the local oceanographic conditions and are quite similar to the assemblages found in deep water traps from both regions. However, they do not necessarily reflect the composition of the main sedimentation event, but rather the typical post-bloom species composition with stronger silicified diatoms. Despite the absence of weakly silicified species due to silica dissolution, diatom assemblages were useful in reconstructing modern and ancient oceanographic conditions.

Distribution, Export and Alteration of Fossilizable Plankton in the Nordic Seas

The present study gives an overview of recent research results concerning the distribution of living fossilizable plankton groups (coccolithophores, diatoms, dinoflagellate cysts, radiolarians and planktic foraminifers) in the water column of the Nordic Seas. Information is combined with results of sediment trap investigations on fluxes and the species composition of these fossilizable plankton groups in the Nordic Seas and their relation to oceanographic conditions. In order to validate spatial and temporal occurrence patterns of the plankton assemblages, their fluxes were monitored with sediment traps over several years in three different oceanic regions, characterized by the major surface water masses of the Nordic Seas. In addition, the alteration processes on settling plankton assemblages were investigated at three water depths.

Distribution of Calcareous, Siliceous and Organic-Walled Planktic Microfossils in Surface Sediments of the Nordic Seas and their Relation to Surface-Water Masses

The distribution of calcareous, siliceous and organic-walled planktic microfossils has been investigated in surface sediments from the Nordic Seas, Planktic foraminifers, coccolithophores, radiolarians, diatoms and dinoflagellate cysts generally reflect major oceanographic domains in the distribution of sediment assemblages, The occurrence of fossilizable plankton in the water column and in sediments has been compared to describe processes which alter the composition of assemblages.

The Potential of Synoptic Plankton Analyses for Paleoclimatic Investigations: Five Plankton Groups from the Holocene Nordic Seas

Five plankton groups, including diatoms, radiolarians, coccolithophores, foraminifers, and dinoflagellate cysts, were synoptically analyzed in six sediment cores and two sediment traps from the Norwegian-Greenland Sea and the North Atlantic in order to provide more detailed insights into the paleoclimatic and paleoceanographic evolution and the development of plankton assemblages of the northern North Atlantic during the last 15, 000 years. Based on Q-mode factor analyses, cold, warm, transitional, and relict assemblages were calculated for each of the plankton groups. Data from the different plankton groups complement one another, although they are not always consistent. However, the multiple plankton-group data set is able to bridge intervals in which single groups lack preservation or the ability to react to changes. Synoptically interpreted, the results provide a detailed picture of the response of plankton assemblages to environmental changes during the time period investigated, which includes the Bolling/ Allerod interstadial, the Younger Dryas cold spell, Termination IB, and, in all likelihood, also the “8, 200 Event”, and the Hypsithermal (approximately 8-4 14C ky BP).

The Post-Glacial Evolution of the Baltic Sea

Sediment records in the Baltic Sea image the development of late Glacial and Holocene climate in Northwestern Europe, the transgression of global sea level and the flooding of the shelf, and the effects of isostatic uplift of Scandinavia. Changes in salinity of surface and deep waters were pronounced to approximately 5,000 years before present and were mainly caused by sea level rise and sill subsidence. Density stratification between salty deep and fresher surface waters caused the development of anoxic conditions and deposition of laminated sediments on large parts of the sea floor in the Baltic proper. Blooms of cyanobacteria were more intense than today, and the Baltic Sea was a fertile environment that supported diverse and abundant coastal resources nourishing Mesolithic Man. At 5,000 years before Present, the climate deteriorated at the transition from the Atlantic to the Sub-Boreal climatic period. Early Neolithic agricultural communities spread from the South to the North and replaced foragers, initiating the clearing of forests in the Baltic Sea catchment. The salinity of the Baltic Sea decreased, oxygenation of deep seafloors was improved, and fertility of the sea surface was significantly reduced, as seen by low accumulation rates of organic carbon in bioturbated sediments. From approximately AD 1,000 on, salinity rises again and anoxic periods are more common. A clear impact of human activity in the catchment is seen in sediment records of the last 100 years, with increased burial rates of nutrient elements from fertilisers and trace elements. On the other hand, climate continues to govern changes in the salt budget and influences the balance between oxic and anoxic conditions at the seafloor that have a large impact on nutrient budgets in the water column.