Mineralogical-sedimentological and chemical investigation of sediments from Cross Bank off Florida

The sediments of a core of.1.55 m length taken on the windward side of the Cross Bank, Florida Bay, are clearly subdivided into two portions, as shown by grain size analysis: silt-sized particles predominate in the relatively homogeneous lower two thirds of the core. This is succeeded abruptly by a thin layer of sand, containing fragments of Halimeda. They indicate a catastrophic event in the Florida Bay region, because Halimeda does not grow within Florida Bay. Above this layer, the amount of sand decreases at first and then continuously increases right to the present sediment-water-interface. The median and skewness increase simultaneously with the increase in the sand and granule portion. We assume that the changing grain size distribution was determined chiefly by the density of the marine flora: during the deposition of the lower two thirds of the core a dense grass cover acted as a sediment catcher for the fine-grained detritus washed out of the shallow basins of the Florida Bay, and simultaneously prohibited renewed reworking. Similar processes go on today on the surface of most mud banks of Florida Bay. The catastrophic event indicated by the sand layer probably changed the morphology of the bank to such an extent that the sampling point was shifted more to the windward side of the bank. This side is characterized by less dense plant growth. Therefore, less detritus could be caught and the material deposited could be reworked. The pronounced increase in skewness in the upper third of the core certainly indicates a strong washing out of the smaller-sized particles. The sediments are predominantly made up of carbonates, averagely 88.14 percent. The average CaCO3-content is 83.87 percent and the average MgCO3-content amounts to 4.27 percent. The chief carbonate mineral is aragonite making up 60.1 percent of the carbonate portion in the average, followed by high-magnesian calcite (33.8 percent) and calcite (6.1 percent). With increasing grain size the aragonite clearly increases at the cost of high-magnesian calcite in the upper third of the core. Chemically, this is shown by an increase of the CaCO3 : MgCO3-ratio. This increase is mainly caused by the more common occurrence of aragonitic fragments of mollusks in the coarse grain fractions. The bulk of the carbonates is made up of mollusks, foraminifera, ostracods, and - to a much lesser extent - of corals, worm-tubes, coccolithophorids, and calcareous algae, as shown by microscopic investigations. The total amount of the carbonate in the sediments is biogenic detritus with the possible exception of a very small amount of aragonite needles in the clay and fine silt fraction. The individual carbonate components of the gravel and sand fraction can be relatively easy identified as members of a particular animal or plant group. This becomes very difficult in the silt and clay fraction. Brownish aggregates are very common in the coarse and medium silt fraction. It was not always possible to clarify their origin (biogenic detritus, faecal pellets or carbonate particles cemented by carbonates or organic slime, etc.). Organic matter (plant fragments, rootlets), quartz, opal (siliceous sponge needles), and feldspar also occur in the sediments, besides carbonates. The lowermost part of the core has an age of 1365 +/- 90 years, as shown by 14C analysis.

Station characteristics and copepod fatty alcohol, fatty acid and lipid composition during MSM02/4 in Fram Strait

The biodiversity of pelagic deep-sea ecosystems has received growing scientific interest in the last decade, especially in the framework of international marine biodiversity initiatives, such as Census of Marine Life (CoML). While a growing number of deep-sea zooplankton species has been identified and genetically characterized, little information is available on the mechanisms minimizing inter-specific competition and thus allowing closely related species to co-occur in the deep-sea pelagic realm. Focussing on the two dominant calanoid copepod families Euchaetidae and Aetideidae in Fram Strait, Arctic Ocean, the present study strives to characterize ecological niches of co-occurring species, with regard to vertical distribution, dietary composition as derived from lipid biomarkers, and trophic level on the basis of stable isotope signatures. Closely related species were usually restricted to different depth layers, resulting in a multi-layered vertical distribution pattern. Thus, vertical partitioning was an important mechanism to avoid inter-specific competition. Species occurring in the same depth strata usually belonged to different genera. They differed in fatty acid composition and trophic level, indicating different food preferences. Herbivorous Calanus represent major prey items for many omnivorous and carnivorous species throughout the water column. The seasonal and ontogenetic vertical migration of Calanus acts as a short-cut in food supply for pelagic deep-sea ecosystems in the Arctic.