ALBERT

Description: To investigate the potential use of the stable isotope composition of the vegetative cysts of the photosynthetic dinoflagellate Thoracosphaera heimii for quantitative palaeotemperature reconstructions a method has been developed to purify T. heimii cysts from sediment samples. Stable oxygen and carbon isotopes have been measured on T. heimii cysts from 21 surface sediment samples from the equatorial Atlantic and South Atlantic Oceans. Calculated temperatures based on the palaeotemperature equation for inorganic calcite precipitation generally reflect mean annual temperatures of the upper water column, notably of thermocline depths. Although the present results suggest that the isotopic composition of T. heimii shells might be formed in equilibrium with the seawater in which the shells are being formed, future investigations are required to determine possible effects of metabolic and kinetic processes on the fractionation process. This pilot study therefore forms the basis for future investigations on the development of this tool and the determination of a species-specific palaeotemperature equation. The wide geographic and stratigraphic distribution of T. heimii cysts in sediments, the stable position of T. heimii within the water column and the high resistance of its cysts against calcite dissolution underline its potential for a wide usability in palaeotemperature reconstructions.

Description: Laminated sediments spanning the last 20,000 years (though not continuously) in the Shaban Deep, a brine-filled basin in the northern Red Sea, were analyzed microscopically and with backscattered electron imagery in order to determine laminae composition with emphasis on the diatomaceous component. Based on this detailed study, we present schematic models to propose paleoflux scenarios for laminae formation at different time-slices. The investigated core (GeoB 5836-2; 26°12.61'N, 35°21.56'E; water depth 1475 m) shows light and dark alternating laminae that are easily distinguishable in the mid-Holocene and at the end of the deglaciation (13-15 ka) period. Light layers are mainly composed of coccoliths, terrigenous material and diatom fragments, while dark layers consist almost exclusively of diatom frustules (monospecific or mixed assemblages). The regularity in the occurrence of coccolith/diatom couplets points to an annual deposition cycle where contrasting seasons and associated plankton blooms are represented (diatoms-fall/winter deposition, coccoliths-summer signal). We propose that, for the past ~15,000 years, the laminations represent two-season annual varves. Strong dissolution of carbonate, with the concomitant loss of the coccolith-rich layer in sediments older than 15 ka, prevents us from presenting a schematic model of annual deposition. However, the diatomaceous component reveals a marked switch in species composition between Last Glacial Maximum (LGM) sediments (dominated by Chaetoceros resting spores) and sediments somewhat younger (18-19 ka; dominated by Rhizosolenia). We propose that different diatom assemblages reflect changing conditions in stratification in the northern Red Sea: Strong stratification conditions, such as during two meltwater pulses at 14.5 and 11.4 ka, are reflected in the sediment by Rhizosolenia layers, while Chaetoceros-dominated assemblages represent deep convection conditions.

Description: The present investigation was targeted at diatom composition studies in the surface sediments (0-1 cm) sampled in the Sea of Okhotsk and the northwest Pacific in the depth range from 130 to 6110 m. The taxonomic analysis, as well as the quantitative (the diatom cell abundance per sediment dry weight unit) content and ecological group definition, was applied. Ten diatom taxa are the main body (80-100%) of the diatom assemblages: Bacterosira bathyomphala, Chaetoceros spp. (spores), Actinocyclus curvatulus, Thalassiosira latimarginata (group), T. antarctica (spores), Neodenticula seminae, Rhizosolenia hebetata f. hiemalis, Thalassiothrix longissima, Coscinodiscus marginatus, Coscinodiscus oculus iridis. The relative content of these species reflects the sedimentation conditions for different parts of the sea: the shelf, the continental slope, the open sea, and the ocean. The highest diatom content (45.6.3-60.0 mln per g of dry weight) was found for the surface sediments in the central part of the Sea of Okhotsk and the continental slope of western Kamchatka.

Description: Surface sediments (n=39) from the western Fram Strait and across the Yermak Plateau (Arctic Ocean) were investigated by molecular and isotopic organic geochemical methods to determine the composition, distribution and origin of extractable aliphatic lipids (n-alkanes, n-alkanols, fatty acids). Bulk geochemical parameters (TOC-content, d13Corg) were also determined, including additional samples nearby. Enhanced organic carbon contents of up to 1.6% along the western slope of the Yermak Plateau and north off Spitsbergen, corroborated by an average d13Corg value of -22.3 per mil, indicated most of the organic material to be of a marine origin, despite ice-cover. The extractable aliphatic lipids contributed up to 1% of the sedimentary organic carbon and were dominated by fatty acids (0.7-9.1 mg/g TOC), whereas n-alkanes and n-alkanols contributed only minor amounts (0.1-0.4 mg/g TOC). The detailed molecular and carbon isotopic characterisation of the studied aliphatic compounds enabled assignments of most components to three lipid pools, representing: (a) primary production (marine phytoplankton, sea-ice algae), (b) secondary inputs (by feeding of zooplankton, benthic organisms and bacteria on the former) and (c) terrestrial-derived contributions. The first two compound groups dominated, but varied significantly in relation to the environment and were highest at the MIZ (Marginal Ice Zone), and along the permanently ice-covered western flank of the Yermak Plateau. In contrast, compounds attributable to a terrestrial source were of only minor importance in terms of absolute concentrations and less variable, but showed increasing relative proportions from an average of 8-14% at and southwards of the MIZ up to 27-33% on the Yermak Plateau and towards the central Arctic Ocean as a consequence of the weakening signal of primary and secondary production. This study provides further insights into the Arctic Ocean carbon dynamics, but also an example of the impact of ocean-currents on the deposition and composition of organic matter.

Description: Late-summer thickness distributions of large ice floes in the Transpolar Drift between Svalbard and the North Pole in 1991, 1996, 1998, and 2001 are compared. They have been derived from drilling and electromagnetic (EM) sounding. Results show a strong interannual variability, with significantly reduced thickness in 1998 and 2001. The mean thickness decreased by 22.5% from 3.11 m in 1991 to 2.41 m in 2001, and the modal thickness by 22% from 2.50 m in 1991 to 1.95 m in 2001. Since modal thickness represents the thickness of level ice, the observed thinning reflects changes in thermodynamic conditions. Together with additional data from the Laptev Sea obtained in 1993, 1995, and 1996, results are in surprising agreement with recently published thickness anomalies retrieved from satellite radar altimetry for Arctic regions south of 81.5°N. This points to a strong sensitivity of radar altimetry data to level ice thickness.

Description: The environment of the Fram Strait, the only deepwater connection of the Arctic Ocean to the world oceans via the North Atlantic (Fig.7.8.1; see Fig.7.1.9), is influenced by the distribution of sea-ice and two opposing current systems. The northward flowing West Spitsbergen Current (WSC) transports warm, near-surface water (Manley 1995; Rudels et al. 2000) to the Northern Fram Strait. About 22% of the northward flowing Atlantic waters are re-circulated within the RAC (Return Atlantic Current) between 78 and 80°N, west of Svalbard. At 80°N the WSC splits into the Svalbard (ca. 33% of the WSC waters) and the Yermak Branch (ca. 45% of the WSC waters). On the western side of the Fram Strait, the East Greenland Current (EGC) transports cold and low-salinity water southwards along the eastern continental margin of Greenland. (Fig.7.8.1). Primary production in ice-covered areas of western Fram Strait is limited by sea-ice cover, and influenced by the predominant water mass. Productivity in the interior Arctic Ocean is generally low (0.09 gC/m²/day) (Wheeler et al. 1996; see Chapter 3). At marginal ice zones and oceanic fronts in the Fram Strait, however, primary productivity exhibits strong fluctuations and may exceed 1 gC/m²/day (Hirche et al. 1991). The accumulation of organic carbon in sediments depends not only on the supply from primary productivity, but also on selective degradation in sediments. Efficient vertical transport through the water column by formation of aggregations (ballast effect) (Ittekkot et al. 1992; Knies and Stein 1998) and increased lateral transport by strong currents enable a higher preservation of organic carbon in the sediments. In this region, the WSC is capable of transporting large amounts of suspended organic matter to the ice-covered regions in northern Fram Strait (Rutgers van der Loeff et al. 2002). Numerous studies have dealt with paleoceanography and the associated organic carbon accumulation in the sediments of Fram Strait and adjacent regions during the last glacial/interglacial cycle (e.g. Hebbeln 1992; Hebbeln et al. 1994; Elverhoi et al. 1995; Andersen et al. 1996; Hebbeln and Wefer 1997; Hebbeln et al. 1998; Notholt, 1998; Vogt et al. 2001, Taylor et al. 2002). However, in most of the sedimentary records a low temporal resolution prevents the identification of short-term climatic fluctuations, like those reconstructed from high-resolution terrestrial ice-core records. (e.g. GISP2/GRIP; Grootes et al. 1993). Occasionally, short-term events recorded as enhanced organic matter accumulation have been found in cores from the northern Fram Strait/Yermak Plateau region (Knies and Stein 1998; Vogt et al. 2001). These events are caused by a rapid incorporation of organic matter in fine-grained material followed by rapid transfer to the seafloor. Rapidly changing climatic and oceanographic conditions can be recorded exceptionally well by undisturbed deep-sea sediments, particularly in the distribution and variability of organic carbon in sediments. Rapidly changing climatic and oceanographic conditions can be recorded exceptionally well by undisturbed deep-sea sediments, particularly in the distribution and variability of organic carbon in sediments. Yet, there exists little information about the regional response during the last deglaciation and the potential influence of terrigenous material on marine sedimentation of organic carbon in northern Fram Strait. To address this problem, we studied two high-resolution cores spanning the time intervals of the last glacial, the last deglaciation and the Holocene. Here, we present data on the distribution and sources of organic carbon in surface sediments and in long sediment cores. Accumulation rates of total sediment and organic carbon for three different time intervals are calculated and an organic carbon budget for Fram Strait Yermak Plateau is presented for the Holocene.