Beschreibung: A sediment core from the West Spitsbergen continental margin was studied to reconstruct climate and paleoceanographic variability during the last ~9 ka in the eastern Fram Strait. Our multiproxy evidence suggests that the establishment of the modern oceanographic configuration in the eastern Fram Strait occurred stepwise, in response to the postglacial sea-level rise and the related onset of modern sea-ice production on the shallow Siberian shelves. The late Early and Mid Holocene interval (9 to 5 ka) was generally characterized by relatively unstable conditions. High abundance of the subpolar planktic foraminifer species Turborotalita quinqueloba implies strong intensity of Atlantic Water (AW) inflow with high productivity and/or high AW temperatures, resulting in a strong heat flux to the Arctic. A series of short-lived cooling events (8.2, 6.9. and 6.1 ka) occurred superimposed on the warm late Early and Mid Holocene conditions. Our proxy data imply that simultaneous to the complete postglacial flooding of Arctic shallow shelves and the initiation of modern sea-ice production, strong advance of polar waters initiated modern oceanographic conditions in the eastern Fram Strait at ~5.2 ka. The Late Holocene was marked by the dominance of the polar planktic foraminifer species Neogloboquadrina pachyderma, a significant expansion of sea ice/icebergs, and strong stratification of the water column. Although planktic foraminiferal assemblages as well as sea surface and subsurface temperatures suggest a return of slightly strengthened advection of subsurface Atlantic Water after 3 ka, a relatively stable cold-water layer prevailed at the sea surface and the study site was probably located within the seasonally fluctuating marginal ice zone during the Neoglacial period.

Beschreibung: Mg to Ca ratios of the epibenthic foraminifer species Cibicidoides wuellerstorfi have been identified to be strongly controlled by temperature and thus to have great potential for reconstructing bottom water temperatures, especially from the lower end of the temperature range (0-6°C; Tisserand et al., 2013). In the Fram Strait, where main water mass exchanges between the Arctic Ocean and the world’s oceans occur, new temperature estimation tools independent from faunal assemblages can help to better understand the complex interaction of different water masses with possible implications to changes in the meridional overturning circulation and the heat flux to the Arctic Ocean. Furthermore, Mg/Ca temperatures can help unravelling the local impact (e.g., of brine-enriched waters) from general trends in bottom water circulation. In order to apply Mg/Ca-derived temperatures to paleo-records from the Fram Strait, a calibration relationship between modern Mg/Ca ratios to bottom water temperatures which fits the environmental conditions of the Fram Strait needs to be developed. We therefore studied Mg/Ca ratios of C. wuellerstorfi in a set of coretop samples from the Fram Strait and the Norwegian margin where bottom temperatures range between -0.5 and -1°C. For the calibration to modern temperatures, we used modern oceanographic data from both existing conductivity-temperature-depth (CTD) casts and the World Ocean Data Base 2013 (Boyer et. al., 2013). Benthic Mg/Ca ratios are relatively high suggesting a preference of C. wuellerstorfi to incorporate Mg at temperatures below 0°C. Although no correlation has been found to existing temperature calibrations using higher temperature ranges (0-6°C), the data are in line with existing Mg/Ca data from C. wuellerstorfi from the Norwegian Sea and the Fram Strait (Martin et al., 2002; Elderfield et al., 2006).While correlation between Mg/Ca ratios to either temperature or salinity is difficult to constrain, better correlation exists to water depth. We therefore consider the carbonate ion effect as one possible explanation for the relatively high Mg/Ca ratios found in coretop samples from the Fram Strait and the Nordic Seas. Despite the difficulties to constrain a temperature calibration for this low temperature range down to -1°C, variations in benthic Mg/Ca ratios investigated in Holocene records from the eastern Fram Strait display trends similar to those found in other benthic proxy indicators. A short-lived decrease in benthic carbon isotopes and sortable silt mean grain size thus seems to correlate to lower Mg/Ca ratios during the 8.2 ka event. Also, a Late Holocene trend towards significantly higher benthic oxygen isotopes may be related to decreasing Mg/Ca ratios. Essential bibliography Boyer, T.P., Antonov, J.I., Baranova, O.K., Coleman, C., Garcia, H.E., Grodsky, A., Johnson, D.R., Locarnini, R.A., Mishonov, A.V., O'Brien, T.D., Paver, C.R., Reagan, J.R., Seidov, D., Smolyar, I.V., Zweng, M.M. 2013. World Ocean Database 2013. Sydney Levitus, Ed., Alexey Mishonov, Technical Ed., NOAA Atlas NESDIS 72. 209 pp. Elderfield, H., Yu, J., Anand, P., Kiefer, T., Nyland, B. 2006. Calibrations for benthic foraminiferal Mg/Ca paleothermometry and the carbonate ion hypothesis. Earth and Planetary Science Letters 250, 633-649. Martin, P.A., Lea, D.W., Rosenthal, Y., Shackleton, N., Sarnthein, M., Papenfuss, T. 2002. Quaternary deep sea temperature histories derived from benthic foraminiferal Mg/Ca. Earth and Planetary Science Letters 198, 193-209. Tisserand, A.A., Dokken, T.M., Waelbroeck, C., Gherardi, J.-M., Scao, V., Fontanier, C., Jorissen, F. 2013. Refining benthic foraminiferal Mg/Ca-temperature calibrations using core-tops from the western tropical Atlantic: Implication for paleotemperature estimation. Geochemistry, Geophysics, Geosystems, 14(4), 929-946.

Beschreibung: A multiproxy data set of an AMS radiocarbon dated 46 cm long sediment core from the continental margin off western Svalbard reveals multidecadal climatic variability during the past two millennia. Investigation of planktic and benthic stable isotopes, planktic foraminiferal fauna, and lithogenic parameters aims to unveil the Atlantic Water advection to the eastern Fram Strait by intensity, temperatures, and salinities. Atlantic Water has been continuously present at the site over the last 2,000 years. Superimposed on the increase in sea ice/icebergs, a strengthened intensity of Atlantic Water inflow and seasonal ice-free conditions were detected at ~ 1000 to 1200 AD, during the well-known Medieval Climate Anomaly (MCA). However, temperatures of the MCA never exceeded those of the 20th century. Since ~ 1400 AD significantly higher portions of ice rafted debris and high planktic foraminifer fluxes suggest that the site was located in the region of a seasonal highly fluctuating sea ice margin. A sharp reduction in planktic foraminifer fluxes around 800 AD and after 1730 AD indicates cool summer conditions with major influence of sea ice/icebergs. High amounts of the subpolar planktic foraminifer species Turborotalia quinqueloba in size fraction 150–250 μm indicate strengthened Atlantic Water inflow to the eastern Fram Strait already after ~ 1860 AD. Nevertheless surface conditions stayed cold well into the 20th century indicated by low planktic foraminiferal fluxes. Most likely at the beginning of the 20th century, cold conditions of the terminating Little Ice Age period persisted at the surface whereas warm and saline Atlantic Water already strengthened, hereby subsiding below the cold upper mixed layer. Surface sediments with high abundances of subpolar planktic foraminifers indicate a strong inflow of Atlantic Water providing seasonal ice-free conditions in the eastern Fram Strait during the last few decades.

Beschreibung: Marine habitats worldwide are increasingly pressurized by climate change, especially along the Antarctic Peninsula. Well-studied areas in front of rapidly retreating tidewater glaciers like Potter Cove are representative for similar coastal environments and, therefore, shed light on habitat formation and development on not only a local but also regional scale. The objective of this study was to provide insights into habitat distribution in Potter Cove, King George Island, Antarctica, and to evaluate the associated environmental processes. Furthermore, an assessment concerning the future development of the habitats is provided. To describe the seafloor habitats in Potter Cove, an acoustic seabed discrimination system (RoxAnn) was used in combination with underwater video images and sediment samples. Due to the absence of wave and current measurements in the study area, bed shear stress estimates served to delineate zones prone to sediment erosion. On the basis of the investigations, two habitat classes were identified in Potter Cove, namely soft-sediment and stone habitats that, besides influences from sediment supply and coastal morphology, are controlled by sediment erosion. A future expansion of the stone habitat is predicted if recent environmental change trends continue. Possible implications for the Potter Cove environment, and other coastal ecosystems under similar pressure, include changes in biomass and species composition.

Beschreibung: Kelp forests represent a major habitat type in coastal waters worldwide and their structure and distribution is predicted to change due to global warming. Despite their ecological and economical importance, there is still a lack of reliable spatial information on their abundance and distribution. In recent years, various hydroacoustic mapping techniques for sublittoral environments evolved. However, in turbid coastal waters, such as off the island of Helgoland (Germany, North Sea), the kelp vegetation is present in shallow water depths normally excluded from hydroacoustic surveys. In this study, single beam survey data consisting of the two seafloor parameters roughness and hardness were obtained with RoxAnn from water depth between 2 and 18 m. Our primary aim was to reliably detect the kelp forest habitat with different densities and distinguish it from other vegetated zones. Five habitat classes were identified using underwater-video and were applied for classification of acoustic signatures. Subsequently, spatial prediction maps were produced via two classification approaches: Linear discriminant analysis (LDA) and manual classification routine (MC). LDA was able to distinguish dense kelp forest from other habitats (i.e. mixed seaweed vegetation, sand, and barren bedrock), but no variances in kelp density. In contrast, MC also provided information on medium dense kelp distribution which is characterized by intermediate roughness and hardness values evoked by reduced kelp abundances. The prediction maps reach accordance levels of 62% (LDA) and 68% (MC). The presence of vegetation (kelp and mixed seaweed vegetation) was determined with higher prediction abilities of 75% (LDA) and 76% (MC). Since the different habitat classes reveal acoustic signatures that strongly overlap, the manual classification method was more appropriate for separating different kelp forest densities and low-lying vegetation. It became evident that the occurrence of kelp in this area is not simply linked to water depth. Moreover, this study shows that the two seafloor parameters collected with RoxAnn are suitable indicators for the discrimination of different densely vegetated seafloor habitats in shallow environments.